@article {1331861, title = {Hydrogen Migration at Restructuring Palladium{\textendash}Silver Oxide Boundaries Dramatically Enhances Reduction Rate of Silver Oxide}, journal = {Nat. Comm}, volume = {11}, year = {2020}, abstract = {

Hydrogen migration at restructuring palladium{\textendash}silver oxide boundaries dramatically enhances reduction rate of silver oxide

Nature Communicationsvolume\ 11, Article\ number:\ 1844\ (2020)\ Cite this article

Abstract

Heterogeneous catalysts are complex materials with multiple interfaces. A critical proposition in exploiting bifunctionality in alloy catalysts is to achieve surface migration across interfaces separating functionally dissimilar regions. Herein, we demonstrate the enhancement of more than 104\ in the rate of molecular hydrogen reduction of a silver surface oxide in the presence of palladium oxide compared to pure silver oxide resulting from the transfer of atomic hydrogen from palladium oxide islands onto the surrounding surface formed from oxidation of a palladium{\textendash}silver alloy. The palladium{\textendash}silver interface also dynamically restructures during reduction, resulting in silver{\textendash}palladium intermixing. This study clearly demonstrates the migration of reaction intermediates and catalyst material across surface interfacial boundaries in alloys with a significant effect on surface reactivity, having broad implications for the catalytic function of bimetallic materials.

}, url = {https://www.nature.com/articles/s41467-020-15536-x}, author = {O{\textquoteright}Connor, C.R. and van Spronsen, M.A. and Egle, T. and Xu, F. and Kersell, H.R. and Oliver-Meseguer, J and Karatok, M. and Salmeron, M.B. and Madix, R.J. and Friend, C.M.} } @article {1331860, title = {Enhancing Catalytic Performance of Dilute Metal Alloy Nanomaterials}, journal = {Comm.Chem.}, volume = {3}, year = {2020}, pages = {1-9}, abstract = {Dilute alloys are promising materials for sustainable chemical production; however, their composition and structure affect their performance. Herein, a comprehensive study of the effects of pretreatment conditions on the materials properties of Pd0.04Au0.96\ nanoparticles partially embedded in porous silica is related to the activity for catalytic hydrogenation of 1-hexyne to 1-hexene. A combination of in situ characterization and theoretical calculations provide evidence that changes in palladium surface content are induced by treatment in oxygen, hydrogen and carbon monoxide at various temperatures. In turn, there are changes in hydrogenation activity because surface palladium is necessary for H2\ dissociation. These Pd0.04Au0.96\ nanoparticles in the porous silica remain structurally intact under many cycles of activation and deactivation and are remarkably resistant to sintering, demonstrating that dilute alloy catalysts are highly dynamic systems that can be tuned and maintained in a active state.}, url = {https://www.nature.com/articles/s42004-020-0293-2}, author = {Luneau, M. and Guan, E. and Chen, W. and Foucher, A. and Marcella, N. and Shirman, T. and Verbart, D. M. A. and Aizenberg, J. and Aizenberg, M. and Stach, E. and Madix, R.J. and Frenkel, A. and Friend, C.M.} } @article {1331859, title = {Predicting a dramatic decline in selectivity for catalytic esterification of alcohols due to van der Waals interactions}, journal = {Angew. Chem.}, volume = {132}, year = {2020}, abstract = {Controlling the selectivity of catalytic reactions is a critical aspect of improving energy efficiency in the chemical industry; thus, predictive models are of key importance. Herein the performance of a heterogeneous, nanoporous Au catalyst is predicted for the complex catalytic self-coupling of the series of C2{\textendash}C4\ alkyl alcohols, based solely on the known kinetics of the elementary steps of the catalytic cycle for methanol coupling, using scaling methods augmented by density functional theory. Notably, a sharp decrease in selectivity for ester formation with increasing molecular weight to favor the aldehyde due to van der Waals interactions of reaction intermediates with the surface was predicted and subsequently verified quantitatively by experiment. Further, the agreement between theory and experiment clearly demonstrates the efficacy of this approach for building a predictive model of catalytic behavior for a homologous set of reactants using a small set of experimental information.}, url = {https://onlinelibrary.wiley.com/doi/abs/10.1002/ange.202001576}, author = {Reece, C.G. and Luneau, M. and Friend, C.M. and Madix, R.J.} } @article {1331858, title = {Evolution of Metastable Structures in Bimetallic Catalysts from Microscopy and Machine-Learning Molecular Dynamics}, journal = {ChemRxiv}, year = {2020}, abstract = {Restructuring of interface plays a crucial role in materials science and heterogeneous catalysis. Bimetallic systems, in particular, often adopt very different composition and morphology at surfaces compared to the bulk. For the first time, we reveal a detailed atomistic picture of the long-timescale restructuring of Pd deposited on Ag, using microscopy, spectroscopy, and novel simulation methods. Encapsulation of Pd by Ag always precedes layer-by-layer dissolution of Pd, resulting in significant Ag migration out of the surface and extensive vacancy pits. These metastable structures are of vital catalytic importance, as Ag-encapsulated Pd remains much more accessible to reactants than bulk-dissolved Pd. The underlying mechanisms are uncovered by performing fast and large-scale machine-learning molecular dynamics, followed by our newly developed method for complete characterization of atomic surface restructuring events. Our approach is broadly applicable to other multimetallic systems of interest and enables the previously impractical mechanistic investigation of restructuring dynamics.}, url = {https://chemrxiv.org/articles/Evolution_of_Metastable_Structures_in_Bimetallic_Catalysts_from_Microscopy_and_Machine-Learning_Molecular_Dynamics/11811660/3}, author = {Lim, J.S. and Vandermause, J. and van Spronsen, M.A. and Musaelian, A. and O{\textquoteright}Connor, C.R. and Egle, T. and Xie, Y. and Sun, L. and Molinari, M. and Florian, J. and Duanmu, K. and Madix, R.J. and Sautet, P. and Friend, C.M. and Kozinsky, B.} } @article {1331857, title = {Neural Network Assisted Analysis of Bimetallic Nanocatalysts using X-ray Absorption Near Edge Structure Spectroscopy}, journal = {Phys. Chem. Chem. Phys}, year = {2020}, abstract = {X-ray absorption spectroscopy is a common method for probing the local structure of nanocatalysts. One portion of the X-ray absorption spectrum, the X-ray absorption near edge structure (XANES) is a useful alternative to the commonly used extended X-ray absorption fine structure (EXAFS) for probing three-dimensional geometry around each type of atomic species, especially in those cases when the EXAFS data quality is limited by harsh reaction conditions and low metal loading. A methodology for quantitative determination of bimetallic architectures from their XANES spectra is currently lacking. We have developed a method, based on the artificial neural network, trained on\ ab initio\ site-specific XANES calculations, that enables accurate and rapid reconstruction of the structural descriptors (partial coordination numbers) from the experimental XANES data. We demonstrate the utility of this method on the example of a series of PdAu bimetallic nanoalloys. By validating the neural network-yielded metal{\textendash}metal coordination numbers based on the XANES analysis by previous EXAFS characterization, we obtained new results for\ in situ\ restructuring of dilute (2.6 at\% Pd in Au) PdAu nanoparticles, driven by their gas and temperature treatments.}, url = {https://pubs.rsc.org/en/content/articlelanding/2020/cp/d0cp02098b$\#$!divAbstract}, author = {Marcella, N. and Liu, Y. and Timoshenko, J. and Guan, E. and Luneau, M. and Shirman, T. and Plonka, A.M. and van der Hoeven, J. and Aizenberg, J. and Friend, C.M. and Frenkel, A.} } @article {1331856, title = {Tuning reactivity layer-by-layer: formic acid activation on Ag/Pd(111)}, journal = {Chem. Sci.}, year = {2020}, abstract = {The potential for tuning the electronic structure of materials to control reactivity and selectivity in heterogenous catalysis has driven interest in ultrathin metal films which may differ from their bulk form. Herein, a 1-atomic layer Ag film on Pd(111) (Ag/Pd(111)) is demonstrated to have dramatically different reactivity towards formic acid compared to bulk Ag. Formic acid decomposition is of interest as a source of H2\ for fuel cell applications and modification of Pd by Ag reduces poisoning by CO and increases the selectivity for H2\ formation. Formic acid reacts below room temperature on the 1-atomic layer Ag film, whereas no reaction occurs on pristine bulk Ag. Notably, 2 monolayer films of Ag again become unreactive towards formic acid, indicating a reversion to bulk behavior. A combination of infrared reflection absorption spectroscopy (IRRAS), X-ray photoelectron spectroscopy (XPS) and density functional theory (DFT) was used to establish that the Ag monolayer is continuous and electronically modified compared to bulk Ag. The work establishes a demonstration of the altered electronic structure of Ag monolayers on Pd(111) and an associated change in reactivity. The effect on reactivity only persists for the first layer, demonstrating the need for precise control of materials to exploit the modification in electronic properties.}, url = {https://pubs.rsc.org/en/content/articlelanding/2020/sc/d0sc01461c$\#$!divAbstract}, author = {Karatok, M. and Duanmu, K. and O{\textquoteright}Connor, C.R. and Boscoboinik, J.A. and Sautet, P. and Madix, R.J. and Friend, C.M.} } @article {1331855, title = {Growth and auto-oxidation of Pd on single-layer AgO x/Ag (111)}, journal = {Phys. Chem. Chem. Phys}, volume = {22}, year = {2020}, pages = {6202-6209}, abstract = {We investigated the growth and auto-oxidation of Pd deposited onto a AgOx\ single-layer on Ag(111) using scanning tunneling microscopy (STM) and X-ray photoelectron spectroscopy (XPS). Palladium initially grows as well-dispersed, single-layer clusters that adopt the same triangular shape and orientation of Agn\ units in the underlying AgOx\ layer. Bi-layer clusters preferentially form upon increasing the Pd coverage to \~{}0.30 ML (monolayer) and continue to develop until aggregating and forming a nearly conformal Pd bi-layer at a coverage near 2 ML. Analysis of the STM images provides quantitative evidence of a transition from single to bi-layer Pd growth on the AgOx\ layer, and a continuation of bi-layer growth with increasing Pd coverage from \~{}0.3 to 2 ML. XPS further demonstrates that the AgOx\ layer efficiently transfers oxygen to Pd at 300 K, and that the fraction of Pd that oxidizes is approximately equal to the local oxygen coverage in the AgOx\ layer for Pd coverages up to at least \~{}0.7 ML. Our results show that oxygen in the initial AgOx\ layer mediates the growth and structural properties of Pd on the AgOx/Ag(111) surface, enabling the preparation of model PdAg surfaces with uniformly distributed single or bi-layer Pd clusters. Facile auto-oxidation of Pd by AgOx\ further suggests that oxygen transfer from Ag to Pd could play a role in promoting oxidation chemistry of adsorbed molecules on PdAg surfaces.}, url = {https://pubs.rsc.org/en/content/articlelanding/2020/CP/C9CP06973A$\#$!divAbstract}, author = {Mehar, V. and O{\textquoteright}Connor, C.R. and Egle, T. and Karatok, M. and Madix, R.J. and Friend, C.M. and Weaver, J.F.} } @article {1331854, title = {Dual Lewis site creation for activation of methanol on Fe3O4(111) thin films}, journal = {Chem. Sci.}, volume = {11}, year = {2020}, pages = {2448-2454}, abstract = {Despite a wide application in heterogeneous catalysis, the surface termination of Fe3O4(111) remains controversial. Herein, a surface with both Lewis acid and base sites is created through formation of an Fe3O4(111) film on α-Fe2O3(0001). The dual functionality is generated from a locally nonuniform surface layer of O adatoms and Fetet1\ sites. This reactive layer is reproducibly formed even in oxygen-free environments because of the high mobility of ions in the underlying α-Fe2O3(0001). The atomic structure of the Fe3O4(111) surface was identified by scanning tunneling microscopy (STM) and density functional theory (DFT) using the registry of the overlayers with the surface and the distinct electronic structure of oxygen adatom (Oad) and uncovered lattice Fetet1. The surface is dominated by the interface of Oad\ and Fetet1, a Lewis acid{\textendash}base pair, which favors methanol dissociation at room temperature to form methoxy. Methoxy is further oxidized to yield formaldehyde at 700 K in temperature programmed reaction spectra, corresponding to an approximate activation barrier of 179 kJ mol-1. The surface termination of Fe3O4(111) is fully recovered by rapid heating to 720 K in vacuum, demonstrating the high mobility of ions in this material. The work establishes a clear fundamental understanding of a unique magnetite surface and provides insights into the origin of selective oxidation of alcohols on magnetite-terminated catalysts.}, url = {https://pubs.rsc.org/en/content/articlelanding/2020/sc/c9sc06149e$\#$!divAbstract}, author = {Xu, F. and Chen, W. and Walenta, C.A. and O{\textquoteright}Connor, C.R. and Friend, C.M.} } @article {1331853, title = {Effect of Frustrated Rotations on the Pre-Exponential Factor for Unimolecular Reactions on Surfaces: A Case Study of Alkoxy Dehydrogenation}, journal = {J. of Phys. Chem. C}, volume = {124}, year = {2020}, pages = {1429-1437}, abstract = {If theory is to be able to predict the rates of catalytic reactions over extended ranges of temperature and pressure, it must provide accurate rate constants for elementary reaction steps, including both the activation energy and pre-exponential factor. A standing difficulty with this objective is the treatment of floppy modes in the partition function for the adsorbed species. This issue leads to limited accuracy in the pre-exponential factor computed for realistic systems. Here, we investigate the C{\textendash}H bond breaking for a series of linear-chain alkoxides on Cu(110) using density functional theory because the results can be compared to the experimental data for the rate constants. The structural similarity of these species enables us to understand the systematic effect of the molecular size on the frustrated motions and pre-exponential factor. First, we discuss the complexities of finding the global minimum structure of the adsorbed species and highlight the high dimensionality of the configuration space to be sampled. Then, we analyze the motions of harmonic normal modes, including the motions of the underlying metal atoms, and compute the harmonic pre-exponential factors. To account for periodic frustrated rotations, we use the hindered rotor model: this motion significantly decreases the pre-exponential factor in the C{\textendash}H bond breaking, the effect increasing with the molecular size. We also estimate the anharmonic effect using the Morse treatment of potentials. The activation energy and pre-exponential factor computed for CH3O are in excellent quantitative agreement with the experiment. The trends computed for the homologous series of alcohols are also reflected by the experiment.}, url = {https://pubs.acs.org/doi/abs/10.1021/acs.jpcc.9b10017}, author = {Chen, W. and Sun, L. and Kozinsky, B. and Friend, C.M. and E. Kaxiras and Sautet, P. and Madix, R.J.} } @article {1331852, title = {Achieving High Selectivity for Alkyne Hydrogenation at High Conversions with Compositionally Optimized PdAu Nanoparticle Catalysts in Raspberry Colloid-Templated SiO2}, journal = {ACS Cat.}, volume = {10}, year = {2020}, pages = {441-450 }, abstract = {Improving the selectivity for catalytic hydrogenation of alkynes is a key step in upgrading feedstocks for olefin polymerization. Herein, dilute PdxAu1{\textendash}x\ alloy nanoparticles embedded in raspberry colloid-templated silica (x\ = 0.02, 0.04, and 0.09) are demonstrated to be highly active and selective for the gas-phase hydrogenation of 1-hexyne, exhibiting higher selectivity than pure Pd at high conversion. The conversion of 1-hexyne remains high even for the very low amounts of Pd in Pd0.02Au0.98. These catalysts are highly resistant to sintering{\textemdash}addressing a long-standing challenge in the use of Au-based catalysts. Clear evidence is presented that the addition of the second hydrogen to the half-hydrogenated intermediate is the rate-limiting step and that the stability of the half-hydrogenated intermediate of the alkyne is higher than the half-hydrogenated alkene, which explains the high selectivity even at high conversions. Moreover, of the three compositions investigated, optimum selectivity and activity are observed for the nanoparticles containing 4\% Pd. The apparent activation energy for production of 1-hexene from 1-hexyne is measured to be 38 kJ mol{\textendash}1\ for the Pd0.04Au0.96\ catalysts, which is \~{}14 kJ mol{\textendash}1\ lower than for pure Pd. The hydrogenation is completely, but reversibly, suppressed by adding CO to the reactant mixture, indicating that the Pd centers are the active sites for reaction. The method of templating used in preparation of the catalysts is highly customizable and versatile. This study demonstrates that the composition of the nanoparticles as defined by the dilution ratio of Pd in Au and by the method used to make the supported catalyst is an important tunable parameter that can be used to optimize activity and selectivity of bimetallic systems.}, url = {https://pubs.acs.org/doi/abs/10.1021/acscatal.9b04243}, author = {Luneau, M. and Shirman, T. and Foucher, A. and Duanmu, K. and Verbart, D. M. A. and Sautet, P. and Stach, E. and Aizenberg, J. and Madix, R.J. and Friend, C.M.} } @article {1331851, title = {New Role of Pd Hydride as a Sensor of Surface Pd Distributions in Pd-Au Catalysts}, journal = {ChemCatChem }, volume = {12}, year = {2019}, abstract = {Isolated or contiguous, the surface distributions of Pd atoms in the Pd-Au bimetallic nanoparticle (NP) catalysts often influence activity and selectivity towards specific reactions. In this study, we used a concomitant Pd hydride formation upon H2\ exposure as a probe of presence of contiguous Pd regions in bimetallic NPs. For demonstrating this method, we prepared silica supported monometallic Pd and bimetallic Pd-Au NPs with a Pd/Au ratio of 25/75 (Pd25Au75) and used X-ray absorption spectroscopy, scanning transmission electron microscopy and infrared spectroscopy to detect and quantitatively analyze the Pd hydride regions. This work provides a new approach to characterizing intra-particle heterogeneities within the bimetallic NPs at ambient temperature and pressure.}, url = {https://chemistry-europe.onlinelibrary.wiley.com/doi/full/10.1002/cctc.201901847}, author = {Guan, E. and Foucher, A. and Marcella, N. and Shirman, T. and Luneau, M. and Head, A. and Verbart, D. M. A. and Aizenberg, J. and Friend, C.M. and Stacchiola, D. and Stach, E. and Frenkel, A.} } @article {1331849, title = {Evolution of steady-state material properties during catalysis: Oxidative coupling of methanol over nanoporous Ag0.03Au0.97}, journal = {J. of Cat}, volume = {380}, year = {2019}, pages = {366{\textemdash}374}, abstract = {Activating pretreatments are used to tune surface composition and structure of bimetallic-alloy catalysts. Herein, the activation-induced changes in material properties of a nanoporous Ag0.03Au0.97\ alloy and their subsequent evolution under steady-state CH3OH oxidation conditions are investigated. Activation using O3\ results in AgO and Au2O3, strongly enriching the near-surface region in Ag. These oxides reduce in the O2/CH3OH mixture, yielding CO2\ and producing a highly Ag-enriched surface alloy. At the reaction tem-perature (423 K), Ag realloys gradually with Au but remains enriched (stabilized by surface O) in the top few nanometers, producing methyl formate selectively without significant deactivation. At higher tem-peratures, bulk diffusion induces sintering and Ag redistribution, leading to a loss of activity. These find-ings demonstrate that material properties determining catalytic activity are dynamic and that metastable (kinetically trapped) forms of the material may be responsible for catalysis, providing guiding principles concerning the activation of heterogeneous catalysts for selective oxidation.}, url = {https://www.osti.gov/pages/biblio/1606185}, author = {Zugic, B and Van Spronsen, M and Heine, C and Montemore, MM and Li, Y. and Zakharov, D. N. and Karakalos, S. and Lechner, B.A. and Crumlin, E and Biener, M.M. and Frenkel, A and Biener, J. and Stach, E. and Salmeron, M.B. and E. Kaxiras and Madix, R.J. and Friend, C.M.} } @article {1331847, title = {Dilute Pd/Au Alloy Nanoparticles Embedded in Colloid-Templated Porous SiO2 : Stable Au-Based Oxidation Catalysts}, journal = {Chem. of Materials}, volume = {31}, year = {2019}, pages = {5759-5768}, abstract = {Dilute alloy materials hold great promise for enhancing selectivity in catalytic reactions. A major challenge with such catalytic materials is developing supported alloy nanoparticles (NPs) with a catalytically optimal composition that are stable with respect to sintering. Here, we demonstrate that NP Pd/Au catalysts with minor concentrations of Pd prepared by a colloid-templating approach are active for oxygen dissociation and catalytic oxidation, as exemplified by CO oxidation. Using electron microscopic imaging, infrared spectroscopy, and X-ray photoelectron spectroscopy, we demonstrate that Pd is stabilized within the Au nanoparticle in its metallic state with minor amounts of Pd at the surface. The Pd{\textendash}Au nanoparticles are embedded in the colloidally templated silica and form a thermally stable catalyst at high temperature. Surface analysis by diffuse reflectance infrared spectroscopy indicates that the Pd atomic distribution at the surface varies with the bulk concentration, forming isolated Pd atoms at 2\% Pd and a mixture of Pd monomers and small Pd clusters at 9\% Pd. X-ray absorption fine structure analysis indicates that Pd is well distributed in the bulk of the nanoparticle. In-depth understanding of this new class of catalytic materials opens the way to a wide array of possibilities for crucial enhancement of activity and selectivity in catalysis.}, url = {https://pubs.acs.org/doi/10.1021/acs.chemmater.9b01779}, author = {Luneau, M. and Shirman, T. and Filie, A. and Timoshenko, J. and Chen, W. and Trimpalis, A. and Flytzani-Stephanopoulos, M. and E. Kaxiras and Frenkel, A. and Aizenberg, J. and Friend, C.M. and Madix, R.J.} } @article {1154714, title = {Probing Atomic Distributions in Mono- and Bimetallic Nanoparticles by Supervised Machine Learning}, journal = {Nano Letters}, volume = {19}, year = {2019}, pages = {520-529}, abstract = {Properties of mono- and bimetallic metal nanoparticles (NPs) may depend strongly on their compositional, structural (or geometrical) attributes, and their atomic dynamics, all of which can be efficiently described by a partial radial distribution function (PRDF) of metal atoms. For NPs that are several nanometers in size, finite size effects may play a role in determining crystalline order, interatomic distances, and particle shape. Bimetallic NPs may also have different compositional distributions than bulk materials. These factors all render the determination of PRDFs challenging. Here extended X-ray absorption fine structure (EXAFS) spectroscopy, molecular dynamics simulations, and supervised machine learning (artificial neural-network) method are combined to extract PRDFs directly from experimental data. By applying this method to several systems of Pt and PdAu NPs, we demonstrate the finite size effects on the nearest neighbor distributions, bond dynamics, and alloying motifs in mono- and bimetallic particles and establish the generality of this approach.}, url = {https://pubs.acs.org/doi/10.1021/acs.nanolett.8b04461}, author = {Timoshenko, J and Wrasman, CJ and Luneau, M and Shirman, T and Cargnello, M and Bare, SR and Aizenberg, J and Friend, C. M. and Frenkel, AI} } @article {1154713, title = {Oxygen Adsorption on Spontaneously Reconstructed Au(511)}, journal = {Surface Science}, volume = {679}, year = {2019}, pages = {296-303}, abstract = {The four-fold site on {100} facets, which are potential catalytic sites on high-curvature gold nanoparticles, is difficult to prepare on a gold single crystal due to surface reconstruction. Here, the Au(511) surface with a high density of step edges and a well-maintained {100} local structure was studied by scanning tunneling microscopy (STM), temperature programmed reaction spectroscopy (TPRS), and density functional theory (DFT) calculations. Annealing at 550 K and 870 K induces reconstruction on the Au(511) surface to a mixture of (311) and (711) micro-terraces. At room temperature and low oxygen coverage, oxygen adsorbs at both three- and four-fold sites on the surface, leading to a well-ordered zig-zag structure. Oxygen atoms cause significant orbital hybridization and a slight displacement of the gold atoms to which they bind. High oxygen coverages induce the formation of clusters, which are not active towards oxidation of isopropanol at room temperature and a pressure of 10-10 mbar. Oxygen adsorption saturates at 1 monolayer (ML), and two separate oxygen recombinative desorption peaks were observed at coverages above 0.11 ML. Our characterization of this surface and the O adsorption structure allows future studies of {100} micro-terraces for oxidation reactions, which contributes to studies on gold-based catalysts with an irregular shape.}, url = {https://www.sciencedirect.com/science/article/pii/S0039602818306654}, author = {Xu, F and Montemore, MM and O{\textquoteright}Connor, CR and Muramoto, E and van Spronsen, MA and Madix, R J and Friend, C. M.} } @article {1154711, title = {The Dynamics of Surface Alloys: Rearrangement of Pd/Ag(111) Induced by CO and O2}, journal = {Journal of Physical Chemistry C}, year = {2018}, abstract = {Alloys of Ag and small amounts of Pd are promising as bifunctional catalysts, potentially combining the inherent selectivity of the noble Ag with that of the more reactive Pd. Stable PdAg surface alloys are prepared via evaporation of Pd onto Ag(111) at room temperature followed by annealing at 400 K to create a model system. Using this procedure, the most stable form of the surface alloy under vacuum was determined to be a Ag-capped PdAg surface alloy, on the basis of a combination of X-ray photoelectron spectroscopy (XPS), scanning tunneling microscopy (STM), and density functional theory (DFT). Extensive roughening of the surface was apparent in STM images, characterized by islands of the Ag/PdAg/Ag(111) alloy of several layers thickness. The roughening is attributed to transport of Ag from the Ag(111) surface into the alloy islands. Within these islands, there is a driving force for Pd to be dispersed, surrounded by Ag, on the basis of DFT modeling. Exposure of these Ag/PdAg/Ag(111) islands to CO (0.5 Torr) at 300 K induces migration of Pd to the surface, driven by the energetic stabilization of the Pd{\textendash}CO bond based on ambient-pressure XPS. Once the Pd is drawn to the surface by higher pressures of CO at room temperature, it remains stable even under very low CO partial pressures at temperatures of 300 K and below, on the basis of DFT-modeled phase behavior. Exposure to 1 Torr of O2\ at 400 K also causes Pd to resurface, and the resulting structure persists even at low pressures and temperatures below 300 K. These results establish that the state of the PdAg catalyst surface depends strongly on pretreatment and operational conditions. Hence, exposure of an alloy catalyst to CO or O2\ at moderate temperatures and pressures can lead to catalyst activation by bringing Pd to the surface. Furthermore, these results demonstrate that exposure to CO at room temperature, which is often used as a proxy for evaluating the Pd coordination sites available in a catalyst, changes the surface structure. Therefore, the CO vibrational frequencies measured with diffuse-reflectance infrared Fourier-transform spectroscopy (DRIFTS) on PdAg catalyst materials do not necessarily provide information about their working state, and fundamental understanding of the CO-PdAg alloy is crucial.}, url = {https://pubs.acs.org/doi/10.1021/acs.jpcc.8b08849}, author = {van Spronsen, MA and Duanmu, K and O{\textquoteright}Connor, CR and Egle, T and Kersell, H and Oliver-Meseguer, J and Salmeron, M and Madix, R J and Sautet, P and Friend, C. M.} } @article {1154710, title = {Crossing the great divide between single crystal reactivity and actual catalyst selectivity with pressure transients}, journal = {Nature Catalysis}, volume = {1}, year = {2018}, pages = {852-859}, abstract = {The quantitative prediction of catalyst selectivity is essential to the design of efficient catalytic processes and requires a detailed knowledge of the reaction mechanism and rate constants. Here we present a study that accurately predicts, using the kinetics and a mechanism derived from fundamental studies on single-crystal gold, the product distribution resulting from the complex reaction network that governs the oxidative coupling of methanol, catalysed by nanoporous gold between 360 and 425 K and for a vast range of pressures. Analysis of the transient product responses to micropulses of methanol over nanoporous gold yields a precise understanding of the marked dependence of selectivity on pressure, surface oxygen coverage and temperature. The key to a high selectivity for methyl formate is the surface lifetime and abundance of the methoxy. This successful microkinetic modelling of catalytic reactions across a wide set of reaction conditions is broadly applicable to predicting catalytic selectivity and provides a pathway to designing more efficient catalytic processes.}, url = {https://www.nature.com/articles/s41929-018-0167-5}, author = {Reece, C and Redekop, E and Karakalos, S and Friend, C. M. and Madix, R J} } @article {1154706, title = {Chemistry of Methanol and Ethanol on Ozone-Prepared α-Fe2O30001}, journal = {Journal of Physical Chemistry C}, volume = {122}, year = {2018}, pages = {25404-25410}, abstract = {The preparation of α-Fe2O3(0001) single-crystal surface by argon ion sputtering followed by ozone annealing is demonstrated to achieve a stoichiometric surface termination not attainable with molecular oxygen. Both methanol and ethanol thermally react on the stoichiometric α-Fe2O3(0001), and three different desorption states were found. At temperatures above 600 K a dehydrogenative disproportionation pathway to yield the respective aldehyde was found, while two peaks at lower temperatures were observed. The peak at 380 K is assigned as dissociative adsorption and recombination, while the less strongly bound peak at 285 K is attributed to molecular adsorption. The mechanism is attributed to a dissociative adsorption mediated by extra atomic oxygen species on top of iron surface sites from the ozone preparation. The absence of photochemical reaction for neither methanol nor ethanol shows that no hole-driven chemistry occurs on stoichiometric α-Fe2O3(0001).}, url = {https://pubs.acs.org/doi/abs/10.1021/acs.jpcc.8b07574}, author = {Walenta, C and Crampton, A and Xu, F and Heiz, U and Friend, C. M.} } @article {1154704, title = {Spatially Nonuniform Reaction Rates during Selective Oxidation on Gold}, journal = {Journal of the American Chemical Society}, volume = {140}, year = {2018}, pages = {12210-12215}, abstract = {The nonuniform reactivity of adsorbed oxygen during the selective oxidation of methanol on Au(110)-(1{\texttimes}2) was demonstrated using in situ scanning tunneling microscopy (STM), establishing the importance of both atomic and mesoscale structure in determining reaction kinetics. At coverages above 0.06 ML, oxygen consumption occurs preferentially along [11̅0] direction, creating local regions completely devoid of oxygen between oxygen islands. The directionally specific reactivity is attributed to a combination of the weaker binding of oxygen atoms at chain termini and the release of surface strain induced by O bonding to Au. The generality of this phenomenon is illustrated by analogous, but kinetically contrasting behavior, for reaction of 2-propanol with oxygen covered Au(110)-(1{\texttimes}2). Even at low O coverages, there are structurally related changes in the reactivity for the reaction with methanol. With decreasing O coverage, a slow reaction period is followed by a fast reaction period, the latter starting when oxygen coverage decreases to \~{}0.06 monolayer, independent of the initial coverage. This increase in reactivity is attributed to a sudden destabilization of the island structure. These results demonstrate that both local and mesocale structures can affect reactivity.}, url = {https://pubs.acs.org/doi/10.1021/jacs.8b07438}, author = {Xu, F and Madix, R J and Friend, C. M.} } @article {1154703, title = {Perspectives on the Design of Nanoparticle Systems for Catalysis}, journal = {Faraday Discussions}, volume = {208}, year = {2018}, pages = {595-607}, url = {https://pubs.rsc.org/en/content/articlelanding/2018/fd/c8fd00124c$\#$!divAbstract}, author = {Friend, C. M. and Xu, F} } @article {1154702, title = {Toward Digitally Controlled Catalyst Architectures: Hierarchical Nanoporous Gold via 3D Printing}, journal = {Science Advances}, volume = {4}, year = {2018}, pages = {eaas9459}, abstract = {Monolithic nanoporous metals, derived from dealloying, have a unique bicontinuous solid/void structure that provides both large surface area and high electrical conductivity, making them ideal candidates for various energy applications. However, many of these applications would greatly benefit from the integration of an engineered hierarchical macroporous network structure that increases and directs mass transport. We report on 3D (three-dimensional){\textendash}printed hierarchical nanoporous gold (3DP-hnp-Au) with engineered nonrandom macroarchitectures by combining 3D printing and dealloying. The material exhibits three distinct structural length scales ranging from the digitally controlled macroporous network structure (10 to 1000 μm) to the nanoscale pore/ligament morphology (30 to 500 nm) controlled by dealloying. Supercapacitance, pressure drop, and catalysis measurements reveal that the 3D hierarchical nature of our printed nanoporous metals markedly improves mass transport and reaction rates for both liquids and gases. Our approach can be applied to a variety of alloy systems and has the potential to revolutionize the design of (electro-)chemical plants by changing the scaling relations between volume and catalyst surface area.}, url = {http://advances.sciencemag.org/content/4/8/eaas9459}, author = {Zhu, C and Qi, Z and Beck, VA and Luneau, M and Lattimer, J and W Chen and Worsley, MA and Ye, J and Duoss, EB and Spadaccini, CM and Friend, C. M. and Biener, J.} } @article {1154698, title = {Selective Activation of Methyl C-H Bonds of Toluene by Oxygen on Metallic Gold}, journal = {Catalysis Letters}, volume = {148}, year = {2018}, abstract = {In accord with its gas phase acidity, toluene is activated by atomic oxygen adsorbed on Au(111) at the methyl C{\textendash}H bonds. Titration of this species from the surface with HCOOD yields C6H5CHD2, indicating that much of the toluene has been doubly deprotonated at the methyl carbon. Even at low surface coverages of atomic oxygen the thusly-activated species yields only CO, CO2\ and water as gas phase products when heated above room temperature, indicating the facile formation of an adsorbed benzoate, not simple oxygen insertion into the carbene to form benzyl benzyloxy. At higher O pre-coverages benzoic acid forms between 400 and 500\ K.}, url = {https://link.springer.com/article/10.1007/s10562-018-2407-3}, author = {Rodriguez-Reyes, J-C and Friend, C. M. and Madix, R J} } @article {1154695, title = {Identifying key descriptors in surface binding: interplay of surface anchoring and intermolecular interactions for carboxylates on Au(110)}, journal = {Chemical Science}, volume = {9}, year = {2018}, pages = {3759-3766}, abstract = {The relative stability of carboxylates on Au(110) was investigated as part of a comprehensive study of adsorbate binding on Group IB metals that can be used to predict and understand how to control reactivity in heterogeneous catalysis. The binding efficacy of carboxylates is only weakly dependent on alkyl chain length for relatively short-chain molecules, as demonstrated using quantitative temperature-programmed reaction spectroscopy. Corresponding density functional theory (DFT) calculations demonstrated that the bidentate anchoring geometry is rigid and restricts the amount of additional stabilization through adsorbate-surface van der Waals (vdW) interactions which control stability for alkoxides. A combination of scanning tunneling microscopy (STM) and low-energy electron diffraction (LEED) shows that carboxylates form dense local islands on Au(110). Complementary DFT calculations demonstrate that adsorbate{\textendash}adsorbate interactions provide additional stabilization that increases as a function of alkyl chain length for C2\ and C3\ carboxylates. Hence, overall stability is generally a function of the anchoring group to the surface and the inter-adsorbate interaction. This study demonstrates the importance of these two important factors in describing binding of key catalytic intermediates.}, url = {https://pubs.rsc.org/en/content/articlelanding/2018/sc/c7sc05313d$\#$!divAbstract}, author = {O{\textquoteright}Connor, CR and Hiebel, F and W Chen and E. Kaxiras and Madix, R J and Friend, C. M.} } @article {1154675, title = {General Effect of van der Waals Interactions on the Stability of Alkoxy Intermediates on Metal Surfaces}, journal = {Journal of Physical Chemistry B}, volume = {122}, year = {2018}, pages = {555-560}, abstract = {The critical role of noncovalent van der Waals (vdW) interactions in determining the relative thermodynamic stability of alkoxy intermediates has been demonstrated for the Cu(110) surface using a combination of experiment and theory. The results may be significant for the selectivity control of copper-based reactions of alcohols. Previous examination of this effect on Au(110) was also extended to include higher molecular weight alcohols; on Cu(110) and Au(110) the hierarchy for the strength of binding of the alkoxys was found to be the same within experimental accuracy, with alkoxy species of greater chain length being more stable. The equilibrium constants governing the competition of alcohol pairs for binding sites of the alkoxys are also similar on the two surfaces. These results reveal the generality of such vdW effects. This work expands the understanding of the role of vdW interactions on the binding efficacy of key reactive intermediates on metal surfaces, a key factor in the rational design of complex and selective catalytic processes.}, url = {https://pubs.acs.org/doi/10.1021/acs.jpcb.7b04952}, author = {Xu, Y. and W Chen and E. Kaxiras and Friend, C. M. and Madix, R J} } @article {1127038, title = {A Comparative Ab Initio Study of Anhydrous Dehydrogenation of Linear-Chain Alcohols on Cu(110) }, journal = {Journal of Physical Chemistry C}, volume = {122}, year = {2018}, pages = {7806-7815}, abstract = {The catalytic behavior of Cu surfaces in the anhydrous production of aldehydes from alcohols, a process of industrial significance, is puzzling: the two simplest alcohols (methanol and ethanol) show dramatically different decomposition behavior on Cu. Here, we study the thermodynamic and kinetic processes involved in the anhydrous dehydrogenation of linear-chain alcohols including methanol, ethanol, 1-propanol, and 1-butanol on the Cu(110) surface using multiscale approaches. First, we obtain the adsorption structures and energies of the reaction intermediates, in which van der Waals (vdW) interactions play a crucial role. Then, we determine the kinetic barriers for the two dehydrogenation steps, namely, the O{\textendash}H and the subsequent C{\textendash}H bond-breaking on Cu. The reaction of methoxy-to-formaldehyde has a rather high-energy transition state, in contrast to that of alkoxide-to-aldehyde in the longer-chain systems. This difference qualitatively explains the lower production efficiency of formaldehyde on Cu. Finally, we simulate the production rates of aldehydes \ based on which we optimize reaction conditions and propose possible avenues for enhancing the production of anhydrous formaldehyde using Cu-based catalysts.}, url = {https://pubs.acs.org/doi/10.1021/acs.jpcc.8b01698}, author = {W Chen and Cubuk, E D and Montemore, MM and Reece, C and Madix, R J and Friend, C. M. and Kaxiras E} } @article {1117783, title = {Water facilitates oxygen migration on gold surfaces.}, journal = {Physical Chemistry Chemical Physics}, volume = {December}, year = {2017}, pages = {2196-2204}, author = {Xu, F and Fampiou, I and O{\textquoteright}Connor, CR and Karakalos, K and Hiebel, F and E. Kaxiras and Madix, R J and Friend, C. M.} } @article {1117780, title = {Selective oxygen-assisted reactions of alcohols and amines catalyzed by metallic gold: Paradigms for the design of catalytic processes. ACS Catalysis}, journal = {ACS Catalysis}, volume = {December}, year = {2017}, pages = {965{\textendash}985}, author = {Personick, ML and Madix, R J and Friend, C. M.} } @article {1117778, title = {First-Principles Study of Alkoxides Absorbed on Au(111) and Au(110) Surfaces: Assessing the Roles of Non-Covalent Interactions and Molecular Structures in Catalysis.}, journal = {The Journal of Physical Chemistry}, volume = {November: 121}, year = {2017}, pages = {27905{\textendash}27914}, author = {Cheenicode Kabeer, F and W Chen and Madix, R and Friend, C and Tkatchenko, A} } @article {1117776, title = {O2 Activation by Metal Surfaces: Implications for Bonding and Reactivity on Heterogeneous Catalysts.}, journal = {Chemical Reviews}, volume = {November}, year = {2017}, pages = {10.1021/acs.chemrev.7b00217.}, author = {Montemore, MM and Van Spronsen, M and Madix, R J and Friend, C.M.} } @article {1117774, title = {New architectures for designed catalysts: Selective oxidation using AgAu nanoparticles on colloid-templated silica}, journal = {Chemistry: A European Journal}, volume = {September}, year = {2017}, pages = {10.1002/chem.201704552.}, author = {Shirman, T and Lattimer, J and Luneau, M and Shirman, E and Reece, C and Aizenberg, M and Madix, R J and Aizenberg, J and Friend, C. M.} } @article {1117771, title = {Structural differentiation of the reactivity of alcohols with active oxygen on Au(110).}, journal = {Topics in Catalysis}, volume = {September}, year = {2017}, pages = {10.1007/s11244-017-0855-4.}, author = {Hiebel, F and Karakalos, S and Xu, Y. and Friend, C. M. and Madix, R J} } @article {1117769, title = {Thermally Activated Formation Of Reactive Lattice Oxygen In Titania On Nanoporous Gold.}, journal = {The Journal of Physical Chemistry}, volume = {Sept:121}, year = {2017}, pages = {10.1021/acs.jpcc.7b06316.}, author = {Ewers, BW and Crampton, AS and Biener, M M and Friend, C. M.} } @article {1117766, title = {Hydride-Based Solid Oxide Fuel Cell-Battery Hybrid Electrochemical System.}, journal = {Energy Technology}, volume = {August: 5}, year = {2017}, pages = {616-622}, author = {Guan, X. and Jiang, J. and Lattimer, J and Tsuchiya, M and Friend, C. M. and Ramanathan, S} } @article {1117615, title = {Structure of the Clean and Oxygen-Covered Cu(100) Surface at Room Temperature in the Presence of Methanol Vapor in the 10 to 200 mTorr Pressure Range.}, journal = {The Journal of Physical Chemistry}, volume = {July, 27 (122)}, year = {2017}, pages = {548-554}, author = {Eren, B and Kersell, H and Weatherup, R and Heine, C and Crumlin, E and Friend, C and Salmeron, M.} } @article {1117614, title = {General Effect of van der Waals Interactions on the Stability of Alkoxy Intermediates on Metal Surfaces.}, journal = {The Journal of Physical Chemistry}, volume = {July (122)}, year = {2017}, pages = {555-560.}, author = {Xu, Y. and W Chen and E. Kaxiras and Friend, C. M. and Madix, R J} } @article {1117613, title = {Multiscale Morphology of Nanoporous Copper Made from Intermetallic Phases.}, journal = {ACS Applied Materials \& Interfaces}, volume = {July, 11 (9)}, year = {2017}, pages = {25615-25622.}, author = {Egle, T and Barroo, C and Janvelyan, N and Baumgaerte, AC and Akey, AJ and Biener, M M and Friend, C. M. and D.C., B and Biener, J.} } @article {1117612, title = {Accurate formation energies of charged defects in solids: A systematic approach.}, journal = {Physical Review}, volume = {June: 95}, year = {2017}, pages = {https://doi.org/10.1103/PhysRevB.95.235310.}, author = {Vinichenko, D and Sensoy, MG and Friend, C. M. and E. Kaxiras} } @article {1117611, title = {Surface Structure Dependence of the Dry Dehydrogenation of Alcohols on Cu(111) and Cu(110).}, journal = {The Journal of Physical Chemistry}, volume = {May 2017}, year = {2017}, pages = {12800-12806.}, author = {Wang, Z-T and Xu, Y. and Madix, R J and Friend, C. M. and Sykes, ECH} } @article {1117610, title = {Selective non-oxidative dehydrogenation of ethanol to acetaldehyde and hydrogen on highly dilute NiCu alloys.}, journal = {Applied Catalysis B: Environmental.}, volume = {May 2017}, year = {2017}, pages = {541-550}, author = {Shan, J and Janvelyan, N and Friend, C. M. and Biener, J. and Flytzani-Stephanopoulos, M.} } @article {1117604, title = {Methanol Photo-Oxidation on Rutile TiO2 Nanowires: Probing Reaction Pathways on Complex Materials.}, journal = {The Journal of Physical Chemistry }, volume = {April:121}, year = {2017}, pages = {9910{\textendash}9919.}, author = {Crampton, AS and L., C and N., J and X., Z and Friend, C. M.} } @article {1117601, title = {Heterogeneous Catalysis - A Central Science for a Sustainable Future.}, journal = {Accounts of Chemical Research}, volume = {March: 21-50 }, year = {2017}, pages = {517-521.}, author = {Friend, C. M. and Bingjun, X.} } @article {1117597, title = {Strain effects on the behavior of isolated and paired sulfur vacancy defects in monolayer MoS2.}, journal = {Physical Review}, volume = {January: 95}, year = {2017}, pages = {10.1103/PhysRevB.95.014106.}, author = {Sensoy, MG and Vinichenko, D and W Chen and Friend, C. M. and E. Kaxiras} } @article {1117799, title = {Dynamic Restructuring Drives Catalytic Activity on Nanoporous Gold-Silver Alloy Catalysts.}, journal = {Nature Materials}, year = {2016}, pages = {558-564}, author = {Zugic, B and Wang, L-C and Heine, C and Zakharov, DN and Lechner, BAJ and Stach, E.A. and Biener, J. and Salmeron, M and Madix, R J and Friend, C. M.} } @article {1117028, title = {The creation of microscopic surface structures by interfacial diffusion of Au and Ag on Ag(110): A XPS and STM study.}, journal = {Surface Science}, volume = {2016}, year = {2016}, pages = {36-44}, author = {Schmid, M. and Madix, R.J. and Friend, C.M.} } @article {1117025, title = {Catalyst design for enhanced sustainability through fundamental surface chemistry.}, journal = {Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences}, volume = {374}, year = {2016}, pages = {Article Number: 20150077}, author = {Personick, M.L. and Montemore, M.M., and E. Kaxiras and Madix, R.J. and Biener, J., and Friend, C.} } @article {1117022, title = {Continuous Catalytic Production of Methyl Acrylates from Unsaturated Alcohols by Gold: The Strong Effect of C=C Unsaturation on Reaction Selectivity.}, journal = {ACS Catalysis 2016}, volume = {6}, year = {2016}, pages = {1833-1839}, author = {Zugic, B. and Karakalos, S. and Stowers, K.J. and Biener, M.M. and Biener, J. and Madix, R.J. and Friend, C.M.} } @article {1117020, title = {Hydride-based solid oxide fuel cell-battery hybrid electrochemical system}, journal = {Energy Technology}, volume = {2016}, year = {2016}, pages = {DOI: 10.1002/ente.201600406.}, author = {Guan, X. and Jiang, J. and Lattimer, J. and Tsuchiya, M. and Friend, C.M. and Ramanathan, S.} } @article {1117019, title = {Designing for selectivity: Weak interactions and the competition for reactive sites on gold catalysts.}, journal = {Faraday Discussions}, volume = {2016}, year = {2016}, pages = {355-368}, author = {Siler, C.G.F and Madix, R.J. and Friend, C.M.} } @article {1117016, title = {Active site densities, oxygen activation and adsorbed reactive oxygen in alcohol activation on npAu catalysts.}, journal = {Faraday Discussions}, volume = {2016}, year = {2016}, pages = {57-67}, author = {Wang, L-C. and Friend, C.M. and Fushimi, R. and Madix, R.J.} } @article {1117013, title = {Experimental investigation into tungsten carbide thin films as solid oxide fuel cell anodes}, journal = {Journal of Materials Research}, volume = {2016}, year = {2016}, pages = {JMR-2016-0382.R1. }, author = {Jiang, J. and Guan, X. and Lattimer, J. and Friend, C.M. and Verma, A. and Tsuchiya, M. and Ramanathan, S.} } @article {1117010, title = {Active Sites for Methanol Partial Oxidation on Nanoporous Gold Catalysts}, journal = {The Journal of Catalysis}, volume = {2016}, year = {2016}, author = {Madix, R.J. and Wang, L. and Personick, M. and Karakalos, S. and Fushimi, R. and Friend, C.M.} } @article {1117009, title = {Direct visualization of quasi-ordered oxygen chain structures on Au(110)-(1x2).}, journal = {Surface Science}, volume = {650}, year = {2016}, pages = {5-10}, author = {Hiebel, F. and Montermore, M.M. and Kaxras, E. and Friend, C.M.} } @article {1117008, title = {Catalytic production of methyl acrylates by gold-mediated cross coupling of unsaturated aldehydes with methanol.}, journal = {Surface Science}, year = {2016}, pages = {58-66}, author = {Karakalos, S. and Zugic, B. and Stowers, K.J. and Biener, M.M. and Biener, J. and Friend, C.M. and Madix, R.J.} } @article {1117007, title = {Controlling O coverage and stability by alloying Au and Ag.}, journal = {Physical Chemistry Chemical Physics}, volume = {18}, year = {2016}, pages = {26844-26853}, author = {Montemore, M.M. and Cubick, E. D. and Klobas, J.E. and Schmid, M. and Madix, R.J. and Friend, C.M. and E. Kaxiras} } @article {1117006, title = {Selective non-oxidative dehydrogenation of ethanol to acetaldehyde and hydrogen on highly dilute NiCu alloys}, volume = {205}, year = {2016}, pages = {541{\textendash}550}, author = {Shan, J. and Janvelyan, N. and Li, H. and Liu, J. and Egle, T.M. and Ye, J. and Biener, M.N. and Biener, J. and Friend, C.M. and Flytzani-Stephanopoulos, M.} } @article {1117005, title = {Self-assembly of acetate adsorbates drives atomic rearrangement on the Au(110) surface.}, journal = {Nature Communications}, volume = {7}, year = {2016}, pages = {DOI: 10.1038/ncomms13139.}, author = {Hiebel, F. and Shong, B.G. and Chen, W. and Madix, R.J. and E. Kaxiras and Friend, C.M.} } @article {1117004, title = {Non-covalent bonding controls selectivity in heterogeneous catalysis: Coupling reactions on Au.}, journal = {Journal of the American Chemical Society}, volume = {138 }, year = {2016}, pages = {15243{\textendash}15250}, author = {Karakalos, S. and Xu, Y. and Cheenicode Kabeer, F. and Chen, W. and Rodriguez-Reyes, J.-C. and Tkatchenko, A. and E. Kaxiras and Madix, R.J. and Friend, C.M.} } @article {1116955, title = {Dynamic restructuring drives catalytic activity on nanoporous gold{\textendash}silver alloy catalysts.}, journal = {Nature Materials}, volume = {2016}, year = {2016}, pages = {DOI:10.1038/nmat4824.}, author = {Zugic, B. and Wang, L. and Heine, C. and Zakharov, D. N. and Lechner, B.A.J. and Stach, E.A. and Biener, J. and Salmeron, M. and Madix, R.J. and Friend, C.M.} } @article {1117042, title = {Methyl Ester Synthesis Catalyzed by Nanoporous Gold: from 10-9 Torr to 1 atm}, journal = {Catalysis Science \& Technology}, volume = {2015}, year = {2015}, pages = {1299-1306}, author = {Wang, L. and Stowers, K.J. and Zugic, B. and Biener, M.M. and Biener, J. and Friend, C.M. and Madix, R.J.} } @article {1117041, title = {Anatomy of the Photochemical Reaction: Excited-State Dynamics Reveals the C{\textendash}H Acidity Mechanism of Methoxy Photo-oxidation on Titania}, journal = {The Journal of Physical Chemistry Letters}, volume = {6}, year = {2015}, pages = {1624-1627}, author = {Kolesov, G. and Vinichenko, D. and Tritsaris, G. and Friend, C.M. and E. Kaxiras} } @article {1117039, title = {Exploiting Basic Principles to Control the Selectivity of the Vapor Phase Catalytic Oxidative Cross-Coupling of Primary Alcohols over Nanoporous Gold Catalysts}, journal = {The Journal of Catalysis}, volume = {2015}, year = {2015}, pages = {78-86}, author = {Wang, L. and Stowers, K. and Zugic, B. and Personick, M. and Biener, M. and Biener, J. and Friend, C.M. and Madix, R.J.} } @article {1117037, title = {Ozone-Activated Nanoporous Gold: A Stable and Storable Material for Catalytic Oxidation.}, journal = {ACS Catalysis}, volume = {5}, year = {2015}, pages = {4237-4241}, author = {Personick, M. and Zugic, B. and Biener, M. and Biener, J. and Madix, R.J. and Friend, C.M.} } @article {1117035, title = {Nanoporous Gold: Understanding the Origin of the Reactivity of a 21st Century Catalyst Made by Pre-Columbian Technology}, journal = {ACS Catalysis}, volume = {5}, year = {2015}, pages = {6263-6270}, author = {Biener, J. and Biener, M. and Madix, R.J. and Friend, C.M.} } @article {1117031, title = {Achieving Selective and Efficient Electrocatalytic Activity for CO2 Reduction Using Immobilized Silver Nanoparticles.}, journal = {Journal of the American Chemical Society}, volume = {137}, year = {2015}, pages = {12844-13850}, author = {Kim, C. and Jeon, H. S. and Eom, T. and Jee, M. S. and Kim, H. and Friend, C.M. and Min, B. H. and Hwang, Y. J.} } @article {1117029, title = {Direct Visualization of Quasi-Ordered Oxygen Chain Structures on Au(110)-(1x2)}, journal = {Surface Science}, volume = {2015}, year = {2015}, pages = {DOI: 10.1016/j.susc.2015.09.018}, author = {Hiebel, F. and Montemore, M.M. and E. Kaxiras and Friend, C.M.} } @article {ISI:000337637400019, title = {Defects in Surface Chemistry-Reductive coupling of Benzaldehyde on Rutile TiO2(110)}, journal = {Chemistry-a European Journal}, volume = {{20}}, number = {{25}}, year = {2014}, month = {jun}, pages = {{7665{\textendash}7669}}, author = {Clawin, Peter M and Friend, Cynthia M and Al-Shamery, Katharina} } @article {Friend:2014gi, title = {Gold catalysis.}, journal = {Accounts of Chemical Research}, volume = {47}, number = {3}, year = {2014}, pages = {729{\textendash}730}, author = {Friend, Cynthia M and Hashmi, A Stephen K} } @article {Friend:2014ii, title = {Perspectives on Heterogeneous Photochemistry.}, journal = {Chemical record (New York, N.Y.)}, year = {2014}, author = {Friend, Cynthia M} } @article {ISI:000333235800005, title = {Predicting Gold-Mediated Catalytic Oxidative-Coupling Reactions from Single Crystal Studies}, journal = {Accounts of Chemical Research}, volume = {{47}}, number = {{3}}, year = {2014}, month = {mar}, pages = {{761{\textendash}772}}, author = {Xu, Bingjun and Madix, Robert J and Friend, Cynthia M} } @article {Siler:2014cj, title = {Switching Selectivity in Oxidation Reactions on Gold: The Mechanism of C{\textendash}C vs C{\textendash}H Bond Activation in the Acetate Intermediate on Au(111)}, journal = {ACS Catalysis}, volume = {4}, number = {9}, year = {2014}, month = {sep}, pages = {3281{\textendash}3288}, author = {Siler, Cassandra G F and Cremer, Till and Rodriguez-Reyes, Juan Carlos F and Friend, Cynthia M and Madix, Robert J} } @article {ISI:000333947700013, title = {Tuning the Stability of Surface Intermediates Using Adsorbed Oxygen: Acetate on Au(111)}, journal = {Journal of the Physical Chemistry Letters}, volume = {{5}}, number = {{7}}, year = {2014}, pages = {{1126{\textendash}1130}}, author = {Cremer, Till and Siler, Cassandra G F and Rodriguez-Reyes, Juan Carlos F and Friend, Cynthia M and Madix, Robert J} } @article {RodriguezReyes:2014kp, title = {Van der waals interactions determine selectivity in catalysis by metallic gold.}, journal = {Journal of the American Chemical Society}, volume = {136}, number = {38}, year = {2014}, pages = {13333{\textendash}13340}, author = {Rodriguez-Reyes, Juan Carlos F and Siler, Cassandra G F and Liu, Wei and Tkatchenko, Alexandre and Friend, Cynthia M and Madix, Robert J} } @article {239711, title = {Ag/Au Mixed Sites Promote Oxidative Coupling of Methanol on the Alloy Surface.}, journal = {Chemistry-a European Journal}, year = {2014}, month = {Apr 13}, abstract = {

Nanoporous gold, a dilute alloy of Ag in Au, activates molecular oxygen and promotes the oxygen-assisted catalytic coupling of methanol. Because this trace amount of Ag inherent to nanoporous gold has been proposed as the source of oxygen activation, a thin film Ag/Au alloy surface was studied as a model system for probing the origin of this reactivity. Thin alloy layers of Agx Au1-x , with 0.15<=x<=0.40, were examined for dioxygen activation and methanol self-coupling. These alloy surfaces recombine atomic oxygen at different temperatures depending on the alloy composition. Total conversion of methanol to selective oxidation products, that is, formaldehyde and methyl formate, was achieved at low initial oxygen coverage and at low temperature. Reaction channels for methyl formate formation occurred on both Au and Au/Ag mixed sites with a ratio, as was predicted from the local 2-dimensional composition.

}, author = {Xu, Bingjun and Siler, Cassandra G F and Madix, Robert J and Friend, Cynthia M} } @article {239731, title = {

From model studies on Au(111) to working conditions with unsupported nanoporous gold catalysis: Oxygen-assisted coupling reactions

}, journal = {Journal of Catalysis}, volume = {308}, year = {2013}, pages = {131-141}, abstract = {The fundamental principles derived from model studies on metallic gold single crystals for oxygen-assisted reactions provides a framework for understanding nanoporous gold{\textendash}silver alloy catalysts under working catalytic conditions (ambient pressure, moderate temperature). The mechanistic principles for reactivity obtained from fundamental studies guide the understanding of reactivity and selectivity for oxygen-assisted self-coupling and cross-coupling reactions of alcohols on the catalytic materials. Furthermore, current results coupled with recent reports regarding the activation of nanoporous gold provide insight into the activation/deactivation of these catalysts and the challenge of maintaining sustainable reactivity. Finally, new fabrication methods and developments for gold-based nanoporous alloys with Cu, Pd, and Pt show that complementary reactivity and new properties are possible through these materials.}, author = {Stowers, K.J. and Madix, R J and Friend, C.J.} } @article {239736, title = {

The dynamic roles of interstitial and surface defects in oxidation and reduction reactions on titania.

}, journal = {Topics in catalysis}, volume = {56}, year = {2013}, pages = {1377-1388}, abstract = {Defect sites in titania have a substantial effect on many thermal and photochemical reactions. Three common types of defects are titanium interstitials, oxygen vacancies and oxygen adatoms{\textemdash}all of which can react with organic molecules adsorbed on the surface. We look broadly at thermal reduction and photochemical oxidation reactions of oxygenates. In particular, we focus on the reductive coupling of benzaldehyde, the photo-oxidation of butyrophenone, the photostability of benzoic acid, and the photo-oxidative coupling of methanol to methyl formate. Methods used include temperature programmed reaction spectroscopy, scanning tunneling microscopy, and density functional theory.}, author = {Jensen, S.C. and Friend, C. M.} } @article {239726, title = {Alkyl groups as synthetic vehicles in gold-mediated oxidative coupling reactions.}, journal = {Phys. Chem.}, volume = {15}, number = {9}, year = {2013}, month = {Apr 07}, pages = {3179-3185}, abstract = {

The use of surface-bound alkyl and phenyl groups as synthetic vehicles in coupling reactions on oxygen-activated Au(111) is demonstrated by the formation of ethers via alkyl and phenyl iodides. Ethers are formed by successive additions of surface-bound alkyl groups to adsorbed atomic oxygen to form first the alkoxy and then the ether. The addition of the ethyl group to adsorbed oxygen on Au(111) is the rate-limiting step leading to diethyl ether formation. Alkyl groups also add to adsorbed alkoxy groups formed from alcohols. An unusual feature of the alkyl iodide reactions on Au is that oxygen is not required for the activation step; hence, opening new potential reactive pathways on metallic Au.

}, author = {Xu, Bingjun and Madix, Robert J and Friend, Cynthia M} } @article {239716, title = {Norrish Type I surface photochemistry for butyrophenone on TiO2(110).}, journal = {Phys. Chem.}, volume = {15}, number = {14}, year = {2013}, month = {May 14}, pages = {5193-5201}, abstract = {

The photofragmentation of butyrophenone yields benzoate and a propyl radical on oxidized TiO2(110). Oxygen dissociates in native oxygen vacancies to produce reactive oxygen adatoms which react with butyrophenone to create photoactive butyrophenone-O complexes that are sensitive to hole oxidation created upon UV illumination. The same O adatoms also trap one of the primary photoproducts, phenyl-CO, to produce benzoate. The reaction proceeds via a Norrish Type I like process involving α-CC cleavage on the surface, in contrast to the gas phase where a Norrish Type II pathway predominates. The mechanism is probed using mass spectrometry and, for the first time, scanning tunneling microscopy (STM). Our STM experiments show that there is a 1-to-1 correspondence between the immobile butyrophenone-O complex and formation of a benzoate on the surface. We also demonstrate that the benzoate species is in close proximity to the original butyrophenone complex, indicating that benzoate is produced on a time scale more rapid than diffusion of the photoproducts. While the photoproducts of butyrophenone decomposition are similar to ketone oxidation reported previously, butyrophenone reacts via a different starting ground state, based on STM and density functional theory studies. Specifically, butyrophenone does not produce a dioxyalkylene species, which has been proposed to be the photoactive state for other ketones. Based on a combination of STM experiments and density functional theory, we propose that a peroxy-like configuration where the oxygen adatom stabilizes the butyrophenone through its carbonyl oxygen is the surface intermediate that photodecomposes. These results demonstrate the importance of the excited state in determining the photochemistry of ketones on surfaces.

}, author = {Jensen, Stephen C and Phillips, Katherine R and Baron, Martin and Landis, Elizabeth C and Friend, Cynthia M} } @article {239721, title = {Sequential Photo-oxidation of Methanol to Methyl Formate on TiO2(110).}, journal = {J. Am. Chem. Soc. A}, volume = {135}, number = {2}, year = {2013}, month = {Feb 16}, pages = {574-577}, type = {Article}, abstract = {

Methyl formate is produced from the photo-oxidation of methanol on preoxidized TiO(2)(110). We demonstrate that two consecutive photo-oxidation steps lead to methyl formate using mass spectrometry and scanning tunneling microscopy. The first step in methanol oxidation is formation of methoxy by the thermal dissociation of the O-H bond to yield adsorbed CH(3)O and water. Formaldehyde is produced via hole-mediated oxidation of adsorbed methoxy in the first photochemical step. Next, transient HCO is made photochemically from formaldehyde. The HCO couples with residual methoxy on the surface to yield methyl formate. Exposure of the titania surface to O(2) is required for these photo-oxidation steps in order to heal surface and near-surface defects that can serve as hole traps. Notably, residual O adatoms are not required for photochemical production of methyl formate or formaldehyde. All O adatoms react thermally with methanol to form methoxy and gaseous water at rt, leaving a surface devoid of O adatoms. The mechanism provides insight into the photochemistry of TiO(2) and suggests general synthetic pathways that are the result of the ability to activate both alkoxides and aldehydes using photons.

}, author = {Phillips, Katherine R and Jensen, Stephen C and Baron, Martin and Li, Shao-Chun and Friend, Cynthia M} } @article {239751, title = {Butyrophenone on O-TiO2(110): one-dimensional motion in a weakly confined potential well.}, journal = {ACS nano}, volume = {6}, number = {4}, year = {2012}, month = {May 24}, pages = {2925-2930}, abstract = {

We demonstrate the one-dimensional confinement of weakly bound butyrophenone molecules between strongly bound complexes formed via reaction with oxygen on TiO(2)(110). Butyrophenone weakly bound to Ti rows through the carbonyl oxygen diffuses freely in one dimension along the rows even at 55 K, persisting for many minutes before hopping out of the 1-D well. Quantitative analysis yields an estimate of the migration barrier of 0.11 eV and a frequency factor of 6.5 {\texttimes} 10(9) Hz. These studies demonstrate that weakly bound organic molecules can be confined on a surface by creating molecular barriers, potentially altering their assembly.

}, author = {Jensen, Stephen C and Shank, Alex and Madix, Robert J and Friend, Cynthia M} } @article {239756, title = {Dual-function of alcohols in gold-mediated selective coupling of amines and alcohols.}, journal = {Chemistry-a European Journal}, volume = {18}, number = {8}, year = {2012}, month = {Mar 20}, pages = {2313-2318}, abstract = {

Oxidative coupling of alcohols (methanol and ethanol) and dimethylamine on atomic-oxygen-activated Au(111) occurs entirely on the surface to form the corresponding amides when the alkoxy of the alcohol and the amide derived from the amine are co-adsorbed. For effective oxygen-assisted coupling the formation of the amide requires excess methanol. Mechanistic studies reveal that molecularly adsorbed methanol removes excess adsorbed atomic oxygen efficiently, precluding either secondary oxidation or oxidative dehydrogenation of dimethylamide to the imine. The adsorbed amide then can react with the aldehyde produced by β-hydride elimination from the alkoxy to form the hemiaminal, the reactive intermediate leading to coupling. The selectivity for formamide production can be increased to nearly 100 \% in excess methanol.

}, author = {Xu, Bingjun and Madix, Robert J and Friend, Cynthia M} } @article {239761, title = {Oxygen-mediated coupling of alcohols over nanoporous gold catalysts at ambient pressures.}, journal = {Angewandte Chemie (International ed. in English)}, volume = {51}, number = {7}, year = {2012}, month = {Mar 13}, pages = {1698-1701}, author = {Kosuda, Kathryn M and Wittstock, Arne and Friend, Cynthia M and B{\"a}umer, Marcus} } @article {239746, title = {Oxygen-Mediated Coupling of Alcohols over Nanoporous Gold Catalysts at Ambient Pressures}, journal = {Angewandte Chemie {\textellipsis}}, year = {2012}, abstract = {

... This work was supported in part by the NSF through Harvard NSEC (PHY-0646094) and by MRSEC (DMR-0820484). ... 2d X. Liu, B. Xu, J. Haubrich, RJ Madix, CM Friend , J. Am. Chem. Soc. ... 2e R. Meyer, C . Lemire, SK Shaikhutdinov, H. Freund, Gold Bull. 2004, 37, 72{\textendash}124. ...

}, author = {Kosuda, K M and Wittstock, A and Friend, C. M.} } @article {239741, title = {Role of surface-bound intermediates in the oxygen-assisted synthesis of amides by metallic silver and gold.}, journal = {Journal of the American Chemical Society}, volume = {134}, number = {30}, year = {2012}, month = {Aug 01}, pages = {12604-12610}, abstract = {

A general mechanism for the oxygen-assisted synthesis of amides over metallic gold and silver surfaces has been derived from the study of acetaldehyde and dimethylamine in combination with previous work, allowing detailed comparison of the two surfaces{\textquoteright} reactivities. Facile acetylation of dimethylamine by acetaldehyde occurs with high selectivity on oxygen-covered silver and gold (111) crystals via a common overall mechanism with different rate-limiting steps on the two metals. Adsorbed atomic oxygen activates the N-H bond of the amine leading to the formation of an adsorbed amide, which attacks the carbonyl carbon of the aldehyde, forming an adsorbed hemiaminal. Because aldehydes are known to form readily from partial oxidation of alcohols, our mechanism also provides insight into the related catalytic coupling of alcohols and amines. The hemiaminal β-H eliminates to form the coupled amide product. On silver, β-H elimination from the hemiaminal is rate-limiting, whereas on gold desorption of the amide is the slow step. Silver exhibits high selectivity for the coupling reaction for adsorbed oxygen concentrations between 0.01 and 0.1 monolayer, whereas gold exhibits selectivity more strongly dependent on oxygen coverage, approaching 100\% at 0.03 monolayer. The selectivity trends and difference in rate-limiting steps are likely due to the influence of the relative stability of the adsorbed hydroxyl groups on the two surfaces. Low surface coverages of oxygen lead to the highest selectivity. This study provides a general framework for the oxygen-assisted coupling of alcohols and aldehydes with amines over gold- and silver-based catalysts in either the vapor or the liquid phase.

}, author = {Siler, Cassandra G F and Xu, Bingjun and Madix, Robert J and Friend, Cynthia M} } @article {239806, title = {

Oxygen-Mediated Coupling of Alcohols over Nanoporous Gold Catalysts at Ambient Pressures

}, journal = {Ang. Chem. Int. Ed.}, volume = {51}, year = {2011}, pages = {1698-1701}, abstract = {Unsupported catalyst for green chemistry: The aerobic oxidation and coupling of primary alcohols (methanol, ethanol, n-butanol) to give the corresponding aldehydes and esters at temperatures below 100 {\textdegree}C is facilitated by unsupported nanoporous gold catalysts. The experimental conditions employed allow a close comparison to model experiments and open the door to a molecular-level understanding of the reaction.}, author = {Kosuda, Kathryn M and Wittstock, Arne and Friend, Cynthia M and B{\"a}umer, Marcus} } @article {239776, title = {

Interfacial chemistry: Gold{\textquoteright}s enigmatic surface

}, journal = {Nature}, volume = {479}, number = {7374}, year = {2011}, month = {Nov 23}, pages = {482-483}, publisher = {Nature Publishing Group}, type = {News \& Views}, abstract = {Gold is not as inert as was believed{\textemdash}it can promote molecular synthesis. A study uses scanning tunnelling microscopy to catch gold in the act as it guides the formation of one- dimensional polymers from saturated hydrocarbons.}, author = {Madix, Robert J and Friend, Cynthia M} } @article {239801, title = {

Molecular Imaging of Reductive Coupling Reactions: Interstitial-Mediated Coupling of Benzaldehyde on Reduced TiO2(110)

}, journal = {ACS Nano}, volume = {9999}, year = {2011}, abstract = {We report the first visualization of a reactive intermediate formed from coupling two molecules on a surface{\^\i}—{\c\ }a diolate formed from benzaldehyde coupling on TiO2(110). The diolate, imaged using scanning tunneling microscopy (STM), is reduced to gaseous stilbene upon heating to \~{}400 K, leaving behind two oxygen atoms that react with reduced Ti interstitials that migrate to the surface, contrary to the popular expectation that strong bonds in oxygenated molecules react only with oxygen vacancies at the surface. Our work further provides both experimental and theoretical evidence that Ti interstitials drive the formation of diolate intermediates. Initially mobile monomers migrate together to form paired features, identified as diolates that bond over two adjacent five-coordiante Ti atoms on the surface. Our work is of broad importance because it demonstrates the possibility of imaging the distribution and bonding configurations of reactant species on a molecular scale, which is a critical part of understanding surface reactions and the development of surface morphology during the course of reaction.}, author = {Benz, Lauren and Haubrich, Jan and Jensen, Stephen C and Friend, Cynthia M} } @article {239791, title = {Activated metallic gold as an agent for direct methoxycarbonylation.}, journal = {Journal of the American Chemical Society}, volume = {133}, number = {50}, year = {2011}, month = {Dec 21}, pages = {20378-20383}, abstract = {

We have discovered that metallic gold is a highly effective vehicle for the low-temperature vapor-phase carbonylation of methanol by insertion of CO into the O-H bond to form methoxycarbonyl. This reaction contrasts sharply to the carbonylation pathway well known for homogeneously catalyzed carbonylation reactions, such as the synthesis of acetic acid. The methoxycarbonyl intermediate can be further employed in a variety of methoxycarbonylation reactions, without the use or production of toxic chemicals. More generally we observe facile, selective methoxycarbonylation of alkyl and aryl alcohols and secondary amines on metallic gold well below room temperature. A specific example is the synthesis of dimethyl carbonate, which has extensive use in organic synthesis. This work establishes a unique framework for using oxygen-activated metallic gold as a catalyst for energy-efficient, environmentally benign production of key synthetic chemical agents.

}, author = {Xu, Bingjun and Madix, Robert J and Friend, Cynthia M} } @article {239781, title = {Carbonyl coupling: defects and O2 make or break the essential reaction intermediate on titanium dioxide.}, journal = {Chemistry-a European Journal}, volume = {17}, number = {30}, year = {2011}, month = {Jul 18}, pages = {8309-8312}, author = {Jensen, Stephen C and Haubrich, Jan and Shank, Alex and Friend, Cynthia M} } @article {239786, title = {The mystery of gold{\textquoteright}s chemical activity: local bonding, morphology and reactivity of atomic oxygen.}, journal = {Phys. Chem. }, volume = {13}, number = {1}, year = {2011}, month = {Feb 07}, pages = {34-46}, abstract = {

Recently, gold has been intensely studied as a catalyst for key synthetic reactions. Gold is an attractive catalyst because, surprisingly, it is highly active and very selective for partial oxidation processes suggesting promise for energy-efficient "green" chemistry. The underlying origin of the high activity of Au is a controversial subject since metallic gold is commonly thought to be inert. Herein, we establish that one origin of the high activity for gold catalysis is the extremely reactive nature of atomic oxygen bound in 3-fold coordination sites on metallic gold. This is the predominant form of O at low concentrations on the surface, which is a strong indication that it is most relevant to catalytic conditions. Atomic oxygen bound to metallic Au in 3-fold sites has high activity for CO oxidation, oxidation of olefins, and oxidative transformations of alcohols and amines. Among the factors identified as important in Au-O interaction are the morphology of the surface, the local binding site of oxygen, and the degree of order of the oxygen overlayer. In this Perspective, we present an overview of both theory and experiments that identify the reactive forms of O and their associated charge density distributions and bond strengths. We also analyze and model the release of Au atoms induced by O binding to the surface. This rough surface also has the potential for O(2) dissociation, which is a critical step if Au is to be activated catalytically. We further show the strong parallels between product distributions and reactivity for O-covered Au at low pressure (ultrahigh vacuum) and for nanoporous Au catalysts operating at atmospheric pressure as evidence that atomic O is the active species under working catalytic conditions when metallic Au is present. We briefly discuss the possible contributions of oxidants that may contain intact O-O bonds and of the Au-metal oxide support interface in Au catalysis. Finally, the challenges and future directions for fully understanding the activity of gold are considered.

}, author = {Baker, Thomas A and Liu, Xiaoying and Friend, Cynthia M} } @article {239796, title = {Oxidative coupling of alcohols on gold: Insights from experiments and theory}, journal = {Faraday Discuss.}, volume = {152}, year = {2011}, pages = {307-320}, publisher = {Royal Society of Chemistry}, abstract = {

... Bingjun Xu a and Cynthia M. Friend *ab. a Department of Chemistry and Chemical Biology, Harvard University, Cambridge, MA 02138, USA. ... DFT results predict that methoxy is able to activate C {\textendash}H bond in a nearby methoxy , which is confirmed by the isotopic labeling ...

}, author = {Xu, Bingjun and Friend, Cynthia M} } @article {239766, title = {A paradigm for predicting selective oxidation on noble metals: oxidative catalytic coupling of amines and aldehydes on metallic gold.}, journal = {Faraday Discuss.}, volume = {152}, year = {2011}, pages = {241-52- discussion 293-306}, author = {Xu, Bingjun and Friend, Cynthia M and Madix, Robert J} } @article {239771, title = {Theoretical Study of O-Assisted Selective Coupling of Methanol on Au (111)}, journal = {Journal of Physical Chemistry C}, volume = {115}, year = {2011}, pages = {3703-3708}, abstract = {

ABSTRACT: We report the first systematic theoretical study of the oxidative self-coupling of methanol to form the ester, methylformate, on atomic-oxygen-covered Au (111) using density functional theory calculations. The first step in the process{\textemdash}dissociation of the OrH bond ...

}, author = {Xu, Bingjun and Baker, T A and E. Kaxiras and Friend, C. M.} } @article {234911, title = {The Role of Surface and Subsurface Point Defects for Chemical Model Studies on TiO2: A First-Principles Theoretical Study of Formaldehyde Bonding on Rutile TiO2(110)}, journal = {Chemistry-a European Journal}, volume = {17}, number = {16}, year = {2011}, month = {Apr 23}, pages = {4496-4506}, abstract = {

... Cynthia M. Friend (cfriend@deas.harvard.edu ... I. Stoichiometric TiO 2 (110): Two configurations with very similar energies were identified for an η 1 -top geometry, in which the O of the H 2 C O is bound to a five-coordinate Ti ion ({\textquotedblleft}Ti 5c {\textquotedblright}) on a stoichiometric TiO 2 (110) slab (Table ...

}, author = {Haubrich, Jan and Kaxiras, Efthimios and Friend, Cynthia M} } @article {239811, title = {Achieving optimum selectivity in oxygen assisted alcohol cross-coupling on gold.}, journal = {Journal of the American Chemical Society}, volume = {132}, number = {46}, year = {2010}, month = {Nov 24}, pages = {16571-16580}, abstract = {

Oxidative coupling of alcohols is of great practical importance due to the wide range of synthetic applications using esters. Controlling the selectivity toward production of specific esters in cross-coupling is a long-sought goal in designing efficient synthetic routes. We report a quantitative study of the factors that determine the esterification selectivity in the oxidative cross-coupling of alcohols mediated by oxygen-covered Au(111). The high reactivity of gold is attributed to the activity of atomic oxygen bound to Au particles in forming surface-bound alkoxy species. The relative surface concentrations of alkoxys and the ease of the β-H elimination play critical roles in determining the product distribution. For a given reactant composition the surface concentration of alkoxys is skewed toward the higher molecular weight alcohol in the mixture exposed to the surface. Vibrational spectroscopic studies reveal that the relative stabilities of alkoxys parallel the gas phase acidities of the corresponding alcohols, in agreement with other coinage metal surfaces. Further, the activation energies for β-H cleavage of alkoxys are found to follow the descending order: E(methoxy) \> E(ethoxy) \> E(butoxy). The combination of these factors optimizes cross-coupling in a reactant mixture containing an excess of the lower molecular weight alcohol. The excellent match of product distribution between our low-pressure, single-crystal study and that observed for atmospheric pressure, liquid-phase reactions over supported gold catalysts provides strong evidence that the mechanistic insights gained from fundamental surface science study can serve as guiding principles in controlling selectivity under realistic catalytic conditions.

}, author = {Xu, Bingjun and Madix, Robert J and Friend, Cynthia M} } @article {239831, title = {Direct selective oxygen-assisted acylation of amines driven by metallic silver surfaces: dimethylamine with formaldehyde}, journal = {Chemical Communications}, volume = {46}, number = {5}, year = {2010}, pages = {704-706}, publisher = {Royal Society of Chemistry}, abstract = {

Facile, direct acylation of dimethylamine with formaldehyde to N, N-dimethylformamide proceeds with a selectivity approaching 100\% at low oxygen concentrations on metallic silver surfaces; the reaction proceeds via nucleophilic attack of adsorbed dimethylamide ...

}, author = {Zhou, Ling and Freyschlag, Cassandra G and Xu, Bingjun and Friend, Cynthia M and Madix, Robert J} } @article {239836, title = {Effect of molecular structure on epoxidation of allylic olefins by atomic oxygen on Au}, journal = {Dalton Transactions}, volume = {39}, number = {36}, year = {2010}, pages = {8521-8526}, publisher = {Royal Society of Chemistry}, abstract = {

The reactions of three isomers of phenyl-substituted propene, trans-b-methylstyrene, a- methylstyrene, and allylbenzene, with atomic oxygen adsorbed on Au (111) have been investigated in order to probe the effect of molecular structure on the selectivity for ...

}, author = {Liu, Xiaoying and Baker, Thomas A and Friend, Cynthia M} } @article {239841, title = {Highly selective acylation of dimethylamine mediated by oxygen atoms on metallic gold surfaces.}, journal = {Angewandte Chemie (International ed. in English)}, volume = {49}, number = {2}, year = {2010}, pages = {394-398}, author = {Xu, Bingjun and Zhou, Ling and Madix, Robert J and Friend, Cynthia M} } @article {239821, title = {In situ ambient pressure studies of the chemistry of NO2 and water on rutile TiO2(110).}, journal = {Langmuir}, volume = {26}, number = {4}, year = {2010}, month = {Mar 16}, pages = {2445-2451}, abstract = {

The adsorption of NO(2) on the rutile TiO(2)(110) surface has been studied at room temperature in the pressure range from approximately 10(-8) torr to 200 mtorr using ambient pressure X-ray photoelectron spectroscopy (AP-XPS). Atomic nitrogen, chemisorbed NO(2), and NO(3) were formed, each of which saturates at pressures below approximately 10(-6) torr NO(2). Atomic nitrogen originates from decomposition of the NO(x) species. For pressures of up to 10(-3) torr, no significant change in the NO(x) surface species occurred, suggesting that environmentally relevant conditions with typical NO(2) partial pressures in the 1-100 ppb range can be modeled by ultrahigh vacuum (UHV) studies. The chemisorbed surface species can be removed by in situ annealing in UHV: all of the NO(x) species disappear around 400 K, whereas the N 1s signal associated with atomic nitrogen diminishes around 580 K. At higher pressures of NO(2) (p(NO(2)) \> or = 10(-6) torr), physisorbed NO(2) and adsorbed water, which was likely due to displacement from the chamber walls, appeared. The water coverage grew significantly above approximately 10(-3) torr. Concurrently with co-condensation of water and NO(2), the population of NO(3) species grew strongly. From this, we conclude that the presence of NO(2) and water leads to the formation of multilayers of nitric acid. In contrast, pure water exposure after saturation of the surface with 200 mtorr NO(2) did not lead to a growth of the NO(3) signals, implying that HNO(3) formation requires weakly adsorbed NO(2) species. These findings have important implications for environmental processes, since they confirm that oxides may facilitate nitric acid formation under ambient humidity conditions encountered in the atmosphere.

}, author = {Haubrich, Jan and Quiller, Ryan G and Benz, Lauren and Liu, Zhi and Friend, Cynthia M} } @article {239826, title = {Mechanistic insights into selectivity control for heterogeneous olefin oxidation: styrene oxidation on Au(111).}, journal = {Chemistry, an Asian journal}, volume = {5}, number = {1}, year = {2010}, month = {Feb 04}, pages = {78-86}, abstract = {

We demonstrate that intermolecular interactions, controlled by both oxygen and styrene coverage, alter reaction selectivity for styrene oxidation on oxygen-covered Au(111). Several partial oxidation products are formed--styrene oxide, acetophenone, benzoic acid, benzeneacetic acid, and phenylketene--in competition with combustion. The maximum ratio of the yields of styrene oxide to the total CO(2) produced is obtained for the maximum styrene coverage for the first two layers (0.28 ML) adsorbed on Au(111) precovered with 0.2 ML of O. Furthermore, our reactivity and infrared studies support a mechanism whereby styrene oxidation proceeds via two oxametallacycle intermediates which, under oxygen-lean conditions, lead to the formation of styrene oxide, acetophenone, and phenylketene. Benzoate, identified on the basis of infrared reflection absorption spectroscopy, is converted into benzoic acid during temperature-programmed reaction. These results demonstrate the ability to tune the epoxidation selectivity using reactant coverages and provide important mechanistic insight into styrene oxidation reactions.

}, author = {Quiller, Ryan G and Liu, Xiaoying and Friend, Cynthia M} } @article {239816, title = {Selective oxidation of cyclohexanol and 2-cyclohexen-1-ol on O/Au(111): the effect of molecular structure.}, journal = {Langmuir}, volume = {26}, number = {21}, year = {2010}, month = {Nov 02}, pages = {16552-16557}, abstract = {

We combine reactivity studies with infrared reflection absorption spectroscopy to provide molecular-scale insights into the oxidation of two cyclic alcohols, cyclohexanol and 2-cyclohexen-1-ol, by atomic oxygen adsorbed on Au(111). The two alcohols share common features in their reaction pathways: they are both activated by Br{\o}nsted acid-base reactions with adsorbed oxygen. Cyclic ketones, cyclohexanone and 2-cyclohexen-1-one, are the major products, formed from cyclohexanol and 2-cyclohexen-1-ol, respectively. These ketones also undergo secondary ring C-H bond activation. The product distributions reflect a substantial difference in the secondary reactions for these two ketones, which correlate with their gas-phase acidity. The allylic alcohol (2-cylohexen-1-ol) has a greater degree of ring C-H activation, yielding the diketone (2-cyclohexene-1,4-dione) and phenol. Our results provide clear evidence for the importance of C=C functionalities in determining the reactivity of molecules in heterogeneous oxidative transformations promoted on Au-based materials.

}, author = {Liu, Xiaoying and Friend, Cynthia M} } @article {239846, title = {McMurry chemistry on TiO(2)(110): Reductive C=C coupling of benzaldehyde driven by titanium interstitials.}, journal = {Journal of the American Chemical Society}, volume = {131}, number = {41}, year = {2009}, month = {Oct 21}, pages = {15026-15031}, abstract = {

Selective reductive coupling of benzaldehyde to stilbene is driven by subsurface Ti interstitials on vacuum-reduced TiO(2)(110). A combination of temperature-programmed reaction spectroscopy and scanning tunneling microscopy (STM) provides chemical and structural information which together reveal the dependence of this surface reaction on bulk titanium interstitials. Benzaldehyde reductively couples to stilbene with 100\% selectivity and conversions of up to 28\% of the adsorbed monolayer in temperature programmed reaction experiments. The activity for coupling was sustained for at least 20 reaction cycles, which indicates that there is a reservoir of Ti interstitials available for reaction and that surface O vacancies alone do not account for the coupling. Reactivity was unchanged after predosing with water so as to fill surface oxygen vacancies, which are not solely responsible for the coupling reaction. The reaction is nearly quenched if O(2) is adsorbed first-a procedure that both fills defects and reacts with Ti interstitials as they migrate to the surface. New titania islands form after reductive coupling of benzaldehyde, based on scanning tunneling microscope images obtained after exposure of TiO(2)(110) to benzaldehyde followed by annealing, providing direct evidence for migration of subsurface Ti interstitials to create reactive sites. The reliance of the benzaldehyde coupling on subsurface defects, and not surface vacancies, over reduced TiO(2)(110), may be general for other reductive processes induced by reducible oxides. The possible role of subsurface, reduced Ti interstitials has broad significance in modeling oxide-based catalysis with reduced crystals.

}, author = {Benz, Lauren and Haubrich, Jan and Quiller, Ryan G and Jensen, Stephen C and Friend, Cynthia M} } @article {239866, title = {Selectivity control in gold-mediated esterification of methanol.}, journal = {Angewandte Chemie (International ed. in English)}, volume = {48}, number = {23}, year = {2009}, pages = {4206-4209}, abstract = {

The Midas touch: The low-temperature transformation of methanol to methyl formate, formaldehyde, and formic acid is promoted by atomic oxygen adsorbed on metallic gold (see picture). The reactions occur with O-containing Au nanoparticles formed on Au(111) upon oxidation with ozone at 200 K; the facile esterification to methyl formate occurs well below room temperature.

}, author = {Xu, Bingjun and Liu, Xiaoying and Haubrich, Jan and Madix, Robert J and Friend, Cynthia M} } @article {239856, title = {Surface-mediated self-coupling of ethanol on gold.}, journal = {Journal of the American Chemical Society}, volume = {131}, number = {16}, year = {2009}, month = {May 29}, pages = {5757-5759}, abstract = {

The transformation of ethanol to its carbonyl compounds, namely acetaldehyde, ethyl acetate, acetic acid, and ketene, occurs on Au(111) with O-containing Au nanoparticles formed as a result of Au atom release upon ozone exposure. The product distribution strongly depends on the surface oxygen coverage. Ethoxy and acetate are identified as two key reaction intermediates during the oxidation of ethanol. The formation of acetaldehyde is due to the deprotonation of ethoxy, which can be further oxidized into acetate. The low-temperature formation of the ester, ethyl acetate, proceeds via the coupling of acetaldehyde with excess surface ethoxy. These reaction pathways appear relevant to heterogeneous processes catalyzed by supported gold nanoparticles, thus providing further insight into the mechanistic origin of gold-mediated oxidation of alcohols.

}, author = {Liu, Xiaoying and Xu, Bingjun and Haubrich, Jan and Madix, Robert J and Friend, Cynthia M} } @article {239861, title = {Tuning Reactivity and Selectivity for Olefin Oxidation through Local O Bonding on Au}, journal = {ChemCatChem}, volume = {1}, number = {1}, year = {2009}, month = {Aug 24}, pages = {116-121}, abstract = {

Abstract The reactivity and selectivity for the oxidation of propene and acrolein on oxidized Au nanoparticles on Au (111) depend on the local bonding, the particle size, and the degree of order. The local bonding of O and the particle size distribution and order are controlled ...

}, author = {Min, Byoung Koun and Deng, Xingyi and Liu, Xiaoying and Friend, Cynthia M and Alemozafar, Ali Reza} } @article {239851, title = {Vapour-phase gold-surface-mediated coupling of aldehydes with methanol.}, journal = {Nature chemistry}, volume = {2}, number = {1}, year = {2009}, month = {Nov 29}, pages = {61-65}, abstract = {

Selective coupling of oxygenates is critical to many synthetic processes, including those necessary for the development of alternative fuels. We report a general process for selective coupling of aldehydes and methanol as a route to ester synthesis. All steps are mediated by oxygen-covered metallic gold nanoparticles on Au(111). Remarkably, cross-coupling of methanol with formaldehyde, acetaldehyde, benzaldehyde and benzeneacetaldehyde to methyl esters is promoted by oxygen-covered Au(111) below room temperature with high selectivity. The high selectivity is attributed to the ease of nucleophilic attack of the aldehydes by the methoxy intermediate-formed from methanol on the surface-which yields the methyl esters. The competing combustion occurs via attack of both methanol and the aldehydes by oxygen. The mechanistic model constructed in this study provides insight into factors that control selectivity and clearly elucidates the crucial role of Au nanoparticles as active species in the catalytic oxidation of alcohols, even in solution.

}, author = {Xu, Bingjun and Liu, Xiaoying and Haubrich, Jan and Friend, Cynthia M} } @article {234916, title = {Effects of chlorine and oxygen coverage on the structure of the Au(111) surface.}, journal = {J.Chem.Phys.}, volume = {130}, number = {8}, year = {2009}, month = {Mar 28}, pages = {084701}, abstract = {

We investigate the effects of Cl and O coverage on the atomic structure of the Au(111) surface using density functional theory calculations. We find that the release and incorporation of gold atoms in the adsorbate layer becomes energetically favorable only at high coverages of either O or Cl (\>0.66 ML (monolayer) for O and \>0.33 ML for Cl), whereas adsorption without the incorporation of gold is favorable at lower coverages. The bonding between the adsorbate and gold substrate changes significantly with coverage, becoming more covalent (less ionic) at higher Cl and O coverage. This is based on the fact that at higher coverages there is less ionic charge transfer to the adsorbate, while the electron density in the region between the adsorbate and a surface gold atom is increased. Our results illustrate that the O and Cl coverage on Au(111) can dramatically affect its structure and bonding, which are important features in any application of gold involving these adsorbates.

}, author = {Baker, Thomas A and Friend, Cynthia M and Kaxiras, Efthimios} } @article {239871, title = {Selectivity switch for nitrogen functionalization of styrene on Au (111)}, journal = {Surface Science}, year = {2008}, abstract = {

... University, 12 Oxford Street, Cambridge, MA 02138, USA; b Division of Engineering and Applied Sciences, Harvard University, 12 ... were detected in TPRS, ruling out the possibility of forming CH 4 , C 2 H 2 , C 2 H ... 6]; BK Min, AR Alemozafar, D. Pinnaduwage, X. Deng, CM Friend ; ...

}, author = {Deng, X and Friend, C. M.} } @article {239881, title = {Transient hydroxyl formation from water on oxygen-covered Au(111).}, journal = {Journal of Chemical Physics}, volume = {129}, number = {6}, year = {2008}, month = {Aug 14}, pages = {064702}, abstract = {

We present evidence for the formation of transient hydroxyls from the reaction of water with atomic oxygen on Au(111) and investigate the effect of adsorbed oxygen on the hydrogen bonding of water. Water is evolved in peaks at 175 and 195 K in temperature programed reaction experiments following adsorption of water on oxygen-covered Au(111). The peak at 175 K is ascribed to sublimation of multilayers of water, whereas the peak at 195 K is associated with oxygen-stabilized water or a water-hydroxyl surface complex. Infrared reflection absorption spectra are consistent with the presence of molecular water over the entire range of coverages studied, indicating that isolated stable hydroxyls are not formed. Isotopic exchange of adsorbed (16)O with H(2)(18)O following adsorption and subsequent temperature programed reaction, however, indicates that transient OH species are formed. The extent of oxygen exchange was considerable--up to 70\%. The degree of oxygen exchange depends on the initial coverage of oxygen, the surface temperature when preparing oxygen adatoms, and the H(2)(18)O coverage. The hydroxyls are short-lived, forming and disproportionating multiple times before water desorption during temperature programed reaction. It was also found that chemisorbed oxygen is critical in the formation of hydroxyls and stabilizing water, whereas gold oxide does not contribute to these effects. These results identify transient hydroxyls as species that could play a critical role in oxidative chemical reactions on gold, especially in ambient water vapor. The crystallinity of adsorbed water also depended on the degree of surface ordering and chemical modification based on scanning tunneling microscopy and infrared spectra. These results demonstrate that oxidation of interfaces has a major impact on their interaction with water.

}, author = {Quiller, R G and Baker, T A and Deng, X and Colling, M E and Min, B K and Friend, C. M.} } @article {239876, title = {Unraveling molecular transformations on surfaces: a critical comparison of oxidation reactions on coinage metals.}, journal = {Chemical Society Reviews}, volume = {37}, number = {10}, year = {2008}, month = {Oct}, pages = {2243-2261}, abstract = {

The coinage metals, copper, silver, and gold, have unique characteristics for selective oxidation catalysis, particularly for partial oxidation of alcohols and olefins. A basic understanding of surface chemistry at the molecular level can help facilitate the improvement of current catalytic processes and the designing of new catalytic systems. In this critical review, the current state of knowledge of these reactions is reviewed. First, both the experimental and theoretical methods necessary for understanding surface reactivity are discussed with a specific set of examples directly related to these reactions. Next the state of understanding of the surface chemistry of the oxidation reactions of alcohols and olefins on these three coinage metals is reviewed and the reaction pathways are compared. Clear relationships between the low pressure surface science studies and more practical catalytic conditions are illustrated. Finally, recent theoretical advances in this area are discussed as well as possible future directions in this field (132 references).

}, author = {Liu, Xiaoying and Madix, Robert J and Friend, Cynthia M} } @article {234921, title = {Anticipating catalytic oxidation reactions on gold at high pressure (including liquid phase) from ultrahigh vacuum studies}, journal = {J. Cat.}, year = {2008}, abstract = {

... b Department of Chemistry and Chemical Biology, Harvard University, 12 Oxford Street, Cambridge, MA 02138, USA. Received 20 February 2008 Revised 22 June 2008 Accepted ... 16]; A. Andreasen, H. Lynggaard, C . Stegelmann, P ... Full Text via CrossRef. [20]; BK Min, CM Friend ; ...

}, author = {Madix, R J and Friend, C. M. and Liu, X.} } @article {234936, title = {Chlorine adsorption on Au(111): chlorine overlayer or surface chloride?}, journal = {Journal of the American Chemical Society}, volume = {130}, number = {11}, year = {2008}, month = {Apr 19}, pages = {3560-3565}, abstract = {

We report the first scanning tunneling microscope (STM) investigation, combined with density functional theory calculations, to resolve controversy regarding the bonding and structure of chlorine adsorbed on Au(111). STM experiments are carried out at 120 K to overcome instability caused by mobile species upon chlorine adsorption at room temperature. Chlorine adsorption initially lifts the herringbone reconstruction. At low coverages (\<0.33 ML), chlorine binds to the top of Au(111)-(1 x 1) surface and leads to formation of an overlayer with (square root(3) x square root(3))R30 degree structure at 0.33 ML. At higher coverages, packing chlorine into an overlayer structure is no longer favored. Gold atoms incorporate into a complex superlattice of a Au-Cl surface compound.

}, author = {Gao, Weiwei and Baker, Thomas A and Zhou, Ling and Pinnaduwage, Dilini S and Kaxiras, Efthimios and Friend, Cynthia M} } @article {234926, title = {Chlorine interaction with defects on the Au(111) surface: a first-principles theoretical investigation.}, journal = {Journal of Chemical Physics}, volume = {129}, number = {10}, year = {2008}, month = {Sep 14}, pages = {104702}, abstract = {

Chlorine is an important element in promoting oxidation on noble metal surfaces. Here, we report a comprehensive theoretical study of chlorine interaction with defects on the Au(111) surface, using density functional theory calculations and periodic slabs to model the surface. We find that chlorine binds preferentially on steps, vacancies, and gold adatoms. The increase in binding energy per chlorine atom, compared to binding on the flat, defect-free surface, is 0.29 eV when the chlorine atom is on top of a gold adatom, 0.38 eV when it is at the edge of a step, and 0.19 eV when it is next to a single surface vacancy. An extensive study of chlorine interaction with different numbers of surface gold vacancies revealed that chlorine interacts the strongest with three vacancies.

}, author = {Baker, Thomas A and Friend, Cynthia M and Kaxiras, Efthimios} } @article {234931, title = {Nature of Cl bonding on the Au(111) surface: evidence of a mainly covalent interaction.}, journal = {Journal of the American Chemical Society}, volume = {130}, number = {12}, year = {2008}, month = {Apr 26}, pages = {3720-3721}, abstract = {

We report theoretical evidence from first-principles density functional theory (DFT) calculations that the bonding between Cl and the Au(111) surface is primarily covalent in character, which is in contrast to the generally held view that the bonding of halogens to metal surfaces is ionic. We observe the transfer of charge density into the region between interacting Au and Cl atoms, which would not be expected in the case of Cl- anion formation (symmetric charge accumulation on Cl). Importantly, we also find a clear directionality of dz2 orbitals of the Au atoms pointing to the adsorbed Cl and the mixing of electronic states between the gold surface and the adsorbed chlorine.

}, author = {Baker, Thomas A and Friend, Cynthia M and Kaxiras, Efthimios} } @article {239901, title = {Chalcogen doping of silicon via intense femtosecond-laser irradiation}, journal = {Materials Science and {\textellipsis}}, year = {2007}, abstract = {

... b Division of Engineering and Applied Sciences, Harvard University, 9 Oxford Street, Cambridge, MA 02138, United ... Dopant element, Bulk diffusivity in c -Si at T = 775 K (cm 2 s -1 ... Eric Mazur and Cynthia M. Friend supervised the research and contributed to the development of ...

}, author = {Sheehy, M A and Tull, B R and Friend, C. M. and Mazur, E.} } @article {239891, title = {Chlorine promotion of styrene epoxidation on Au(111).}, journal = {JACS}, volume = {129}, number = {7}, year = {2007}, month = {Mar 21}, pages = {1872-1873}, author = {Pinnaduwage, Dilini S and Zhou, Ling and Gao, Weiwei and Friend, Cynthia M} } @article {239886, title = {Cyclopropylmethyl/cyclobutyl rearrangements on surfaces: evidence for transient cation formation near O-covered Mo(110).}, journal = {JACS}, volume = {129}, number = {15}, year = {2007}, month = {May 18}, pages = {4666-4677}, abstract = {

Rearrangement reactions of C4-alkoxides on O-covered Mo(110) have been studied using temperature-programmed reaction spectroscopy and reflection-absorption infrared spectroscopy. Cyclobutoxide on Mo(110), prepared from the corresponding alcohol or bromide, is described for the first time in detail. Several reaction mechanisms are considered for the ring-opening rearrangement of cyclopropylmethoxide during high-temperature annealing. In light of compelling new data, previous results are reinterpreted to support the formation of transient cations near O-covered Mo(110). For the first time, we present strong evidence for clean, heterolytic bond cleavage reactions over a metal surface. Our revised reactivity model is based on spectroscopic and reactivity data that show the rearrangement of cyclopropylmethyl groups to cyclobutyl groups and vice versa. Selectively deuterated 1,1-D2-cyclopropylmethanol was studied as a test of mechanism and as a probe for the lifetime of reactive intermediates. Isotopic scrambling observed for this substrate is consistent with the formation of a relatively long-lived carbocation during rearrangement. The intermediacy of transient cations is further invoked to explain the rearrangements that are now recognized to occur as alkyl bromides are transformed into alkoxides on Mo(110)-(1 x 6)-O. The observed ring expansion/contraction reactions are characteristic of a cationic process; carbon-centered radicals are not known to rearrange in this manner. However, in none of the cases discussed could contributions from radical pathways be completely ruled out. Our results are compared to analogous reactions in the vapor and solution phases. General trends governing rearrangement mechanisms on Mo(110) are presented with respect to metal-surface coverage, heteroatom incorporation, and temperature. Trends are discussed in the context of heterogeneous hydrocarbon oxidation.

}, author = {Wiedemann, Sean H and Kang, Dae-Hyuk and Bergman, Robert G and Friend, Cynthia M} } @article {239896, title = {Heterogeneous gold-based catalysis for green chemistry: low-temperature CO oxidation and propene oxidation.}, journal = {Chemical Reviews}, volume = {107}, number = {6}, year = {2007}, month = {Jul}, pages = {2709-2724}, type = {Review}, author = {Min, Byoung Koun and Friend, Cynthia M} } @article {234941, title = {Structure of incommensurate gold sulfide monolayer on Au(111).}, journal = {J.Chem.Phys.}, volume = {127}, number = {10}, year = {2007}, month = {Sep 14}, pages = {104704}, abstract = {

We develop an atomic-scale model for an ordered incommensurate gold sulfide (AuS) adlayer which has previously been demonstrated to exist on the Au(111) surface, following sulfur deposition and annealing to 450 K. Our model reproduces experimental scanning tunneling microscopy images. Using state-of-the-art Wannier-function-based techniques, we analyze the nature of bonding in this structure and provide an interpretation of the unusual stoichiometry of the gold sulfide layer. The proposed structure and its chemistry have implications for related S-Au interfaces, as in those involved in self-assembled monolayers of thiols on Au substrates.

}, author = {Quek, Su Ying and Biener, Monika M and Biener, Juergen and Bhattacharjee, Joydeep and Friend, Cynthia M and Waghmare, Umesh V and Kaxiras, Efthimios} } @article {234946, title = {Sulfur-induced mobilization of Au surface atoms on Au (111) studied by real-time STM}, journal = {Surface Science}, year = {2007}, abstract = {

... c Center for Imaging and Mesoscale Structures, Harvard University, 12 Oxford Street, Cambridge, MA 02138, USA; d Nanoscale Synthesis and Characterization Laboratory, Lawrence Livermore National Laboratory, 7000 East Ave, Livermore, CA 94550, USA. ...

}, author = {Biener, M M and Biener, J. and Friend, C. M.} } @article {239921, title = {Active role of buried ultrathin oxide layers in adsorption of O\< sub\> 2\</sub\> on Au films}, journal = {Surface Science}, year = {2006}, abstract = {

... c Department of Physics, Harvard University, Cambridge MA 02138, USA. Received 17 February 2006 Accepted 23 May 2006 Available online 27 June 2006. Abstract. Ultrathin oxide layers can exhibit special behavior by enabling ...

}, author = {Quek, S Y and Friend, C. M. and E. Kaxiras} } @article {239906, title = {Efficient CO oxidation at low temperature on Au(111).}, journal = {J. Phys. Chem. B}, volume = {110}, number = {40}, year = {2006}, month = {Oct 12}, pages = {19833-19838}, abstract = {

The rate of CO oxidation to CO2 depends strongly on the reaction temperature and characteristics of the oxygen overlayer on Au(111). The factors that contribute to the temperature dependence in the oxidation rate are (1) the residence time of CO on the surface, (2) the island size containing Au-O complexes, and (3) the local properties, including the degree of order of the oxygen layer. Three different types of oxygen - defined as chemisorbed oxygen, a surface oxide, and a bulk oxide - are identified and shown to have different reactivity. The relative populations of the various oxygen species depend on the preparation temperature and the oxygen coverage. The highest rate of CO oxidation was observed for an initial oxygen coverage of 0.5 monolayers that was deposited at 200 K where the density of chemisorbed oxygen is maximized. The rate decreases when two-dimensional islands of the surface oxide are populated and further decreases when three-dimensional bulk gold oxide forms. Our results are significant for designing catalytic processes that use Au for CO oxidation, because they suggest that the most efficient oxidation of CO occurs at low temperature - even below room temperature - as long as oxygen could be adsorbed on the surface.

}, author = {Min, B K and Alemozafar, A R and Pinnaduwage, D and Deng, X and Friend, C. M.} } @article {239926, title = {Formation of silicon nanoparticles and web-like aggregates by femtosecond laser ablation in a background gas}, journal = {Applied Physics A}, year = {2006}, abstract = {

... br tull1,u je carey1 ma sheehy2 c . friend1,2 e. mazur1 Formation of silicon nanoparticles and web-like aggregates by femtosecond laser ablation in a background gas 1 Division of Engineering and Applied Sciences, Harvard University, 9 Oxford Street, Cambridge ...

}, author = {Tull, B R and J.E. Carey and Sheehy, M A and Friend, C and Mazur, E.} } @article {239916, title = {Partial oxidation of propene on oxygen-covered Au(111)}, journal = {Journal of Physical Chemistry B}, volume = {110}, number = {32}, year = {2006}, month = {Aug 17}, pages = {15982-15987}, abstract = {

Partial oxidation of propene is promoted by Au following deposition of atomic oxygen (0.3 ML) via O-3 decomposition on Au(111) at 200 K. Several partial oxidation productssacrolein, acrylic acid, and carbon suboxide (O=C=C=C=O)-are produced in competition with combustion to CO2 and H2O. Acrolein is the primary partial oxidation product, and it is further oxidized to the other products by excess oxygen. We propose that acrolein is derived from allyloxy intermediate that is formed via insertion of oxygen into the allylic C-H bond. While no propene epoxide formation is detected from oxidation of C3H6, a small amount of epoxidation is observed during reaction of C3D6 and CD3CH=CH2. These results are strong indications that small changes in the energy required for allylic C-H activation, in this case due to a kinetic isotope effect, may dramatically change the selectivity; thus, small modifications of the properties of oxygen on Au may lead to the more desirable epoxidation process. Our results are discussed in the context of the origin of activity of Au-based catalysts.

}, author = {Deng, X Y and Min, B K and Liu, X Y and Friend, C. M.} } @article {239931, title = {A pathway for NH addition to styrene promoted by gold.}, journal = {Angewandte Chemie (International ed. in English)}, volume = {45}, number = {42}, year = {2006}, month = {Oct 27}, pages = {7075-7078}, author = {Deng, Xingyi and Baker, Thomas A and Friend, Cynthia M} } @article {239911, title = {Surface-mediated NH and N addition to styrene on Ag(110).}, journal = {JACS}, volume = {128}, number = {44}, year = {2006}, month = {Nov 08}, pages = {14266-14267}, abstract = {

In this paper, we report the investigation of the reaction of adsorbed NH and N with styrene on Ag(110) using temperature-programmed reaction spectroscopy. Using O2 and NH3 as the starting reagents, NH and N species were deposited on a Ag surface. The reaction of styrene on NH- and N-covered Ag surface appears to yield 2-phenylaziridine and benzonitrile, with additional products HCN and NH3. The formation of aziridine, the nitrogenous analogue of styrene epoxide, is proposed to be due to the cycloaddition of adsorbed NH to the carbon-carbon double bond. These results suggest that Ag-based heterogeneous catalysts may be useful for the aziridination of olefins.

}, author = {Liu, Xiaoying and Madix, Robert J and Friend, Cynthia M} } @article {234956, title = {On the Role of Oxygen in Stabilizing Low-Coordinated Au Atoms}, journal = {{\textellipsis}{\textellipsis}}, year = {2006}, abstract = {... JB is grateful for a fellowship of the Hanse-Wissenschaftskolleg, Germany. CMF gratefully acknowledges the support of the Harvard University MRSEC funded by NSF NEED GRANT NO. ... C . Parker, DE Starr, Science 2002, 298, 811. ... CM Friend , Surf. Sci. 2005, 590, L259. ... }, author = {Nowitzki, T and Hamza, A.V. and Friend, C. M.} } @article {234951, title = {Rich coordination chemistry of Au adatoms in gold sulfide monolayer on Au (111)}, journal = {The Journal of {\textellipsis}}, year = {2006}, abstract = {

... Acknowledgment. We gratefully acknowledge support under the auspices of the Harvard NSEC which is funded by NSF Grant No. ... There is no corresponding record for this reference. (11) Vericat, C .; Vela, ME ... (12) Min, BK; Alemozafar, AR; Biener, MM; Biener, J.; Friend , CM Top. ...

}, author = {Biener, J. and Bhattacharjee, J and Friend, C. M.} } @article {239946, title = {Enhancement of O2 dissociation on Au111 by adsorbed oxygen: implications for oxidation catalysis.}, journal = {JACS}, volume = {127}, number = {25}, year = {2005}, month = {Jul 29}, pages = {9267-9270}, abstract = {

We show that the dissociation probability of O2 on the reconstructed, Au111-herringbone surface is dramatically increased by the presence of some atomic oxygen on the surface. Specifically, at 400 K the dissociation probability of O2 on oxygen precovered Au111 is on the order of 10(-3), whereas there is no measurable dissociation on clean Au111, establishing an upper bound for the dissociation probability of 10(-6). Atomic oxygen was deposited on the clean reconstructed Au111-herringbone surface using electron bombardment of condensed NO2 at 100 K. The dissociation probability for dioxygen was measured by exposing the surface to 18O2. Temperature programmed desorption (TPD) was used to quantify the amount of oxygen dissociation and to study the stability of the oxygen in all cases. Oxygen desorbs as O2 in a peak centered at 550 K with pseudo-first-order kinetics; i.e., the desorption peak does not shift with coverage. Our interpretation is that the coverage dependence of the activation energy for dissociation (deltaE(dis)) and/or preexponential factor (nu(d)) may be responsible for the unusual desorption kinetics, implying a possible energy barrier for O2 dissociation on Au111. These results are discussed in the context of Au oxidation chemistry and the relationship to supported Au nanoparticles.

}, author = {Deng, Xingyi and Min, Byoung Koun and Guloy, Amado and Friend, Cynthia M} } @article {239966, title = {Heteroepitaxial nucleation and oriented growth of manganese oxide islands on carbonate minerals under aqueous conditions.}, journal = {Environmental Science \& Technology}, volume = {39}, number = {5}, year = {2005}, month = {Apr 01}, pages = {1239-1249}, abstract = {

Manganese redox cycling and the accompanying dissolution and precipitation reactions are important processes in natural waters. In the present study, Mn2+ (aq) is reacted with O2(aq) at circumneutral pH to form manganese oxide islands on the (1014) surface of MnCO3. The islands grow heteroepitaxially. The effects of the substrate surface morphology, the substrate atomic structure, and the aqueous concentration of Mn2+ are investigated. On terraces, rhombohedral oxide islands form with 90 degrees rotation relative to the crystallographic axis of the underlying carbonate substrate. Although the island heights self-limit between 2 and 3 nm depending on reaction conditions, the islands grow laterally to several square microns before separate islands collide and coalesce. The islands do not grow over substrate steps or down dissolution-pit edges. Comparison studies done with MgCO3 and CaCO3 show that the former also promotes heteroepitaxial growth whereas the latter does not. This difference is explained by the relative bond length mismatch between the structures of the carbonate substrates and the atomic structures of manganese oxides. A free energy model is also presented to explain why the heights of the manganese oxide islands self-limit. Our results provide an improved basis both for the development of predictive models of contaminant fate and transport and for the modeling of hydraulic flow through carbonate aquifers.

}, author = {Jun, Young-Shin and Kendall, Treavor A and Martin, Scot T and Friend, Cynthia M and Joost J. Vlassak} } @article {239951, title = {Low-temperature reduction of NO(2) on oxidized Mo(110).}, journal = {Journal of Physical Chemistry B}, volume = {109}, number = {26}, year = {2005}, month = {Jul 07}, pages = {12826-12831}, abstract = {

The reactions of nitrogen dioxide (NO(2)) were investigated on oxidized Mo(110) containing both chemisorbed oxygen and a thin film oxide. NO(2) reacts on both oxidized Mo(110) surfaces via a combination of reversible adsorption and reduction to NO, N(2), and trace amounts of N(2)O below 200 K. On the surface containing chemisorbed O, there is some complete dissociation of NO(2) to yield N(a) and O(a). N(2) forms at high temperatures through atom combination. On both surfaces, NO is the predominant product of NO(2) reduction. However, the chemisorbed layer which has a low oxidation state, and hence a greater capacity to accept oxygen, more effectively reduces NO(2). The selectivity for N(2) formation over N(2)O is greater for NO(2) as compared with NO on both surfaces studied. The selectivity changes are largely attributed to an increase in the concentration of Mo=O species and a change in the distribution of oxygen on the surface. Notably, more oxygen, in particular Mo=O moieties, is deposited by NO(2) reaction than by O(2) reaction, indicating that NO(2) is a stronger oxidant. The fact that there are several N-containing species on the surface at low temperatures may also affect the product distribution. On both surfaces, N(2)O(4), NO(2), and NO are identified by infrared spectroscopy upon adsorption at 100 K. All N(2)O(4) desorbs by 200 K, leaving only NO(2) and NO on the surface. Infrared spectroscopy of NO(2) on (18)O-labeled surfaces provides evidence for oxygen transfer or exchange between different types of sites even at low temperatures.

}, author = {Deiner, L J and Kang, D H and Friend, C. M.} } @article {239956, title = {Novel synthesis of two-dimensional TiS2 nanocrystallites on Au(111).}, journal = {J.Chem.Phys.}, volume = {122}, number = {3}, year = {2005}, month = {Feb 15}, pages = {34706}, abstract = {

We describe a novel approach to synthesize two-dimensional nanocrystalline TiS2 islands on Au111. Ti is deposited by physical vapor deposition at room temperature on AuS-covered Au111 surfaces. Subsequent annealing to temperatures between 670 K and 800 K leads to the formation of single-layer, triangular TiS2 islands. These TiS2 nanocrystallites reflect the structure of bulk TiS2, and are composed of S-Ti-S stacking units with hexagonally close-packed layers of sulfur atoms and titanium occupying the octahedral sites in between. The lattice constant of the hexagonal unit cell is 3.45 A. A superlattice with a repeat distance of 17.3 A results from the coincidence of five TiS2 units with six Au atoms and is observed in scanning tunneling microscopy and low energy electron diffraction. The triangular shape of the islands indicates a preference for one of the two possible edge terminations. The observation of two island orientations rotated by 60 degrees with respect to each other can be attributed to the formation of twin-related TiS2 domains. The population of the two different island orientations changes during annealing at 800 K indicating a thermodynamic preference for one of the possible stacking sequences.

}, author = {Biener, Monika M and Biener, Juergen and Friend, Cynthia M} } @article {239981, title = {Oxygen-induced restructuring with release of gold atoms from Au (111)}, journal = {Physical Review B}, year = {2005}, abstract = {

... Electronic address: cfriend@deas. harvard .edu 1 M. Haruta, Catal. Today 36, 153 (1997). ... Sci. 272, 318 (1992). 15 X. Deng, BK Min, A. Guloy, and CM Friend ., J. Am. ... 19 L. Piccolo, D. Loffreda, FJ Cadete Santos Aires, C . Deranlot, Y. Jugnet, P. Sautet, and JC Bertolini, Surf. Sci. ...

}, author = {Deng, X and Pinnaduwage, D and Schalek, R and Friend, C. M.} } @article {239941, title = {Promotion of formaldehyde production from adsorbed methoxy by oxidation of Mo(110) with NO2.}, journal = {Journal of Physical Chemistry B}, volume = {109}, number = {16}, year = {2005}, month = {May 28}, pages = {8085-8090}, abstract = {

The reaction of methoxy (OCH3) in the presence of NO2 is studied on a thin-film oxide of Mo(110) for its relevance to the alkane-assisted reduction of NO(x). Temperature-programmed reaction indicates that oxygen deposited by NO2 dissociation promotes formaldehyde evolution. This pathway is not observed in any appreciable amount for methoxy on the thin-film oxide of Mo(110), nor for the reaction of methoxy in the presence of NO or O2. No new intermediates, in particular those containing C-N bonds, are detected after NO2 is exposed to the surface containing methoxy. Furthermore, infrared spectra provide evidence that the presence of NO2 does not significantly perturb the methoxy intermediate. These results indicate that surface oxidation rather than intermolecular complexation is the most likely mechanism by which NO2 promotes the evolution of oxygenates. In addition, the presence of methoxy decreases the capacity of the Mo surface to reduce NO2. No N2 is produced, and molecular desorption predominates. There are also no NO(x) species present after heating to 500 K when NO2 reacts in the presence of methoxy, whereas monomeric NO and dinitrosyl are present when NO2 reacts alone. These results are discussed in the context of CH4-assisted NO(x) reduction.

}, author = {Deiner, L J and Kang, D H and Friend, C. M.} } @article {239971, title = {Revisiting the S-Au(111) interaction: static or dynamic?}, journal = {Langmuir}, volume = {21}, number = {5}, year = {2005}, month = {Apr 01}, pages = {1668-1671}, abstract = {

The Au-S interaction is probably the most intensively studied interaction of Au surfaces with nonmetals, as, for example, it plays an important role in Au ore formation(1) and controls the structure and dynamics of thiol-based self-assembled monolayers (SAMs). Various S-induced surface structures on Au(111) were recently reported for different conditions and predominantly interpreted in terms of a static Au surface. Here, we demonstrate that the Au(111) surface exhibits a very dynamic character upon interaction with adsorbed sulfur: large-scale surface restructuring and incorporation of Au atoms into a growing 2D AuS phase were observed in situ. These results provide new insight into the Au-S surface chemistry.

}, author = {Biener, Monika M and Biener, Juergen and Friend, Cynthia M} } @article {239961, title = {Selective oxidation of styrene on an oxygen-covered Au(111).}, journal = {JACS}, volume = {127}, number = {49}, year = {2005}, month = {Dec 14}, pages = {17178-17179}, abstract = {

For the first time, we demonstrate olefin epoxidation promoted by an extended Au surface. The oxidation of styrene to styrene epoxide, benzoic acid, and benzeneacetic acid is promoted on Au(111) covered with 0.2 ML of oxygen atoms. The estimated selectivity for styrene epoxide formation is approximately 53\%. Total combustion to CO2 accounts for approximately 20\% of the styrene reaction. We propose that styrene epoxide, benzoic acid, and benzeneacetic acid are produced via two possible oxametallacycle intermediates. Our work demonstrates that extended Au is an effective material for olefin oxidation, which has implications for understanding the activity of nanoscale Au catalysts.

}, author = {Deng, Xingyi and Friend, Cynthia M} } @article {239976, title = {Synthesis of TiO2 nanoparticles on the Au(111) surface.}, journal = {J.Chem.Phys.}, volume = {123}, number = {9}, year = {2005}, month = {Sep 01}, pages = {94705}, abstract = {

The growth of titanium oxide nanoparticles on reconstructed Au(111) was investigated by scanning tunneling microscopy and x-ray photoelectron spectroscopy. Ti was deposited by physical-vapor deposition at 300 K. Regular arrays of titanium nanoparticles form by preferential nucleation of Ti at the elbow sites of the herringbone reconstruction. The titanium oxide nanoclusters were synthesized by subsequent exposure to O(2) at 300 K. Two-and three-dimensional titanium oxide nanocrystallites form during annealing in the temperature range from 600 to 900 K. At the same time, the Au(111) surface assumes a serrated 110-oriented step-edge morphology suggesting step-edge pinning by titanium oxide nanoparticles. The oxidation state of the titanium oxide nanoparticles varies with annealing temperature. Specifically, annealing to 900 K results in the formation of stoichiometric TiO(2) nanocrystals as judged by the Ti(2p) binding energies measured in the x-ray photoelectron data. The nanodispersed TiO(2) on Au(111) is an ideal system to test the various models proposed for the enhanced catalytic reactivity of supported Au nanoparticles.

}, author = {Biener, J{\"u}rgen and Farfan-Arribas, Enrique and Biener, Monika and Friend, Cynthia M and Madix, Robert J} } @article {239936, title = {Water dissociation associated with NO2 coadsorption on Mo(110)-(1 x 6)-O: effect of coverage and electronic properties of oxygen.}, journal = {Journal of Physical Chemistry B}, volume = {109}, number = {43}, year = {2005}, month = {Nov 03}, pages = {20463-20468}, abstract = {

Water dissociation on an oxygen-covered Mo(110) surface was investigated using temperature-programmed reaction spectroscopy (TPRS) and infrared reflectance absorbance spectroscopy (IRAS). Adsorbed hydroxyl formation is enhanced by increasing the coverage of chemisorbed oxygen prior to exposure to water up to saturation (0.66 ML). Additional oxidation of the surface using NO(2) suppresses the formation of hydroxyl species (OH). There is no detectable change in the reaction of NO(2) on Mo(110)-(1 x 6)-O when either the water or hydroxyl is adsorbed on the Mo(110)-(1 x 6)-O surface prior to NO(2) adsorption. In contrast, NO(2) induces the displacement of water into the gas phase and the conversion of hydroxyl species to molecular water. Infrared spectra show that the dissociation of NO(2) populates three types of terminal oxygen sites on Mo(110)-(1 x 6)-O, and the population of the terminal oxygen at step sites increases with respect to the amount of NO(2) deposited. Overall, these results suggest that the oxidic property of oxygen results in a lack of activity for the water dissociation.

}, author = {Min, B K and Quiller, R G and Deiner, L J and Friend, C. M.} } @article {234961, title = {Enhanced transient reactivity of an O-sputtered Au (111) surface}, journal = {Surface Science}, year = {2005}, abstract = {

... Phys., 116 (2001), p. 4698. [6]; AR Alemozafar, X.- C . Guo, RJ Madix, N. Hartmann, J. Wang; Surf. ... Soc., in press. [17]; BK Min, X. Deng, D. Pinnaduwage, CM Friend , Phys. Rev. ... Address: Department of Chemistry, Harvard University, 12 Oxford Street, Cambridge, MA 02138, USA. ...

}, author = {Biener, M M and Biener, J. and Friend, C. M.} } @article {234966, title = {Tuning electronic properties of novel metal oxide nanocrystals using interface interactions: MoO\< sub\> 3\</sub\> monolayers on Au (111)}, journal = {Surface Science}, year = {2005}, abstract = {

... The experimental work was supported by the National Science Foundation, under the Harvard Nanoscale Science and Engineering Center, PHY-011-7795, and the Harvard Materials Research Science and Engineering Center, DMR ... 5]; C . Schlenker; ... [6]; MM Biener, CM Friend ; ...

}, author = {Quek, S Y and Biener, M M and Biener, J. and Friend, C. M. and E. Kaxiras} } @article {239986, title = {Growth of nanocrystalline MoO3 on Au(111) studied by in situ scanning tunneling microscopy.}, journal = {J.Chem.Phys.}, volume = {121}, number = {23}, year = {2004}, month = {Dec 15}, pages = {12010-12016}, abstract = {

The growth of nanocrystalline MoO3 islands on Au(111) using physical vapor deposition of Mo has been studied by scanning tunneling microscopy and low energy electron diffraction. The growth conditions affect the shape and distribution of the MoO3 nanostructures, providing a means of preparing materials with different percentages of edge sites that may have different chemical and physical properties than atoms in the interior of the nanostructures. MoO3 islands were prepared by physical vapor deposition of Mo and subsequent oxidation by NO2 exposure at temperatures between 450 K and 600 K. They exhibit a crystalline structure with a c(4 x 2) periodicity relative to unreconstructed Au(111). While the atomic-scale structure is identical to that of MoO3 islands prepared by chemical vapor deposition, we demonstrate that the distribution of MoO3 islands on the Au(111) surface reflects the distribution of Mo clusters prior to oxidation although the growth of MoO3 involves long-range mass transport via volatile MoO3 precursor species. The island morphology is kinetically controlled at 450 K, whereas an equilibrium shape is approached at higher preparation temperatures or after prolonged annealing at the elevated temperature. Mo deposition at or above 525 K leads to the formation of a Mo-Au surface alloy as indicated by the observation of embedded MoO3 islands after oxidation by NO2. Au vacancy islands, formed when Mo and Au dealloy to produce vacancies, are observed for these growth conditions.

}, author = {Biener, Monika M and Biener, Juergen and Schalek, Richard and Friend, Cynthia M} } @article {239991, title = {Heteroepitaxial growth of novel MoO3 nanostructures on Au (111)}, journal = {Surface Science}, year = {2004}, abstract = {

... Acknowledgments. We gratefully acknowledge support of this work by the National Science Foundation under support for the Harvard Nanoscale Science and Engineering Center, grant no. ... 19]; A Chan, B Richter, C Friend , M Aziz, J Hrbek, Z Song; Nano Letters (2003). ...

}, author = {Biener, M M and Friend, C. M.} } @article {239996, title = {Mineral-assisted pathways in prebiotic synthesis: photoelectrochemical reduction of carbon(+IV) by manganese sulfide.}, journal = {JACS}, volume = {126}, number = {36}, year = {2004}, month = {Sep 15}, pages = {11247-11253}, abstract = {

Photoelectrochemistry on mineral surfaces has the potential to play a central role in the prebiotic syntheses of building blocks for biomolecules. In this study, photoreduction of C(+IV) as bicarbonate is used as a probe to investigate the photoelectrochemical properties of alabandite (MnS) colloidal particles. Our experimental results show that photoreduction occurs and that formate is the initial photoproduct. A quantum efficiency of 4.2\% is obtained (pH = 7.5). The quantum efficiency is temperature-independent from 298 to 328 K. In addition to formate, longer chain carbon products are also produced. Ion chromatography shows the presence of acetate and propionate. Infrared spectroscopy and mass spectrometry indicate the formation of longer chain organic molecules that contain oxygenated functional groups. Our results suggest that some prebiotic syntheses could have occurred via photoelectrochemical reactions on semiconducting minerals.

}, author = {Zhang, Xiang V and Martin, Scot T and Friend, Cynthia M and Schoonen, Martin A A and Holland, Heinrich D} } @article {240001, title = {Sensitivity of methyl thiolate desulfurization selectivity to reaction temperature and hydroxyl coverage.}, journal = {Langmuir}, volume = {20}, number = {26}, year = {2004}, month = {Dec 21}, pages = {11443-11449}, abstract = {

The adsorption and reaction of methanethiol (CH3SH) and dimethyl disulfide (CH3SSCH3) on Mo(110)-(1 x 6)-O have been studied using temperature-programmed reaction spectroscopy and reflection-absorption infrared spectroscopy over the temperature range of 110-550 K. The S-H bond is broken upon adsorption to form adsorbed OH, water, and methyl thiolate (CH3S-) at low temperature. Water is evolved at 210 and 310 K via molecular desorption and disproportionation of OH, respectively. Some hydroxyl remains on the surface up to 350 K. Methyl thiolate is also formed from CH3SSCH3 on Mo(110)-(1 x 6)-O. Methyl thiolate undergoes C-S cleavage above 300 K, yielding methane and methyl radicals. There is also a minor amount of nonselective decomposition leading to the formation of carbon and hydrogen. Methane production is promoted by adsorbed hydroxyl. As the hydroxyl coverage increases, the yield of methyl radicals relative to methane diminishes. Accordingly, there is more methane produced from methanethiol reaction than from dimethyl disulfide, since S-H dissociation in CH3SH produces OH. The maximum coverage of the thiolate is approximately 0.5 monolayers, based on the amount of sulfur remaining after reaction measured by Auger electron spectroscopy. In contrast to cyclopropylmethanethiol (c-C3H5CH2SH), for which alkyl transfer from sulfur to oxygen is observed, there is no evidence for transfer of the methyl group of methyl thiolate to oxygen on the surface. Specifically, there is no evidence for methoxy (CH3O-) in infrared spectroscopy or temperature-programmed reaction experiments.

}, author = {Kang, D H and Friend, C. M.} } @article {240011, title = {Dinitrosyl formation as an intermediate stage of the reduction of NO in the presence of MoO3}, journal = {The Journal of Chemical Physics}, year = {2003}, abstract = {

... of Engineering and Applied Sciences, Harvard University, Cambridge, Massachusetts 02138 Melvin Chen and Cynthia M. Friend Department of Chemistry, Harvard University, Cambridge ... 14 and to experimental data obtained from c the atomic positions given by Wyckoff Ref. ...

}, author = {E. Kaxiras and Chen, M and Friend, C. M.} } @article {240016, title = {Formation of regular arrays of silicon microspikes by femtosecond laser irradiation through a mask}, journal = {Applied Physics Letters}, year = {2003}, abstract = {

... and Applied Sciences, Harvard University, Cambridge, Massachusetts 02138 M. Sheehy and CM Friend Department of ... This work was supported by the Harvard MRSEC NSF/ DMR-98-09363 and the Department of ... 3 T.-H. Her, RJ Finlay, C . Wu, S. Deliwala, and E. Mazur, Appl. ...

}, author = {Younkin, R and Mazur, E. and Sheehy, M and Friend, C. M.} } @article {240021, title = {Infrared absorption by conical silicon microstructures made in a variety of background gases using femtosecond-laser pulses}, journal = {Journal of Applied }, year = {2003}, abstract = {

... and Applied Science, 9 Oxford Street, Cambridge, Massachusetts, 02138 JA Levinson and CM Friend Harvard University, Department ... a)Electronic mail: mazur@physics. harvard .edu ... of conical silicon microstructures formed in a background gas of (a) SF6 , (b) Cl2 , ( c ) N2 , and ...

}, author = {Mazur, E. and Levinson, J A and Friend, C. M.} } @article {240006, title = {Insight into the partial oxidation of propene: the reactions of 2-propen-1-ol on clean and O-covered Mo(110).}, journal = {JACS}, volume = {125}, number = {43}, year = {2003}, month = {Oct 29}, pages = {13252-13257}, abstract = {

The reactions of 2-propen-1-ol (allyl alcohol) were studied on clean and O-covered Mo(110) to understand the effect of resonance stabilization and the presence of surface oxygen on reaction selectivity. Propene is the only gaseous hydrocarbon product evolved from allyl alcohol reaction on O-covered Mo(110). Water and dihydrogen are also produced, along with a small amount of adsorbed carbon. We estimated, using X-ray photoelectron spectroscopy, that approximately 70\% of the 0.11 ML of 2-propen-1-ol that reacts forms propene. In contrast, the dominant reaction pathway on the clean surface is nonselective decomposition to adsorbed carbon and hydrogen, leading to a 23\% selectivity for propene formation. On both clean and O-covered Mo(110), X-ray photoelectron spectroscopy and infrared spectroscopy identify allyloxy as the reaction intermediate yielding propene. These results are discussed in the context of propene oxidation and periodic trends in reactivity.

}, author = {Deiner, L J and Serafin, J G and Friend, C. M. and Weller, S G and Levinson, J A and Palmer, R E} } @article {240026, title = {A novel growth mode of Mo on Au (111) from a Mo (CO) 6 precursor: An STM study}, journal = {The Journal of Physical Chemistry B}, year = {2003}, abstract = {

... At Harvard , the same dosing procedure was used, with the exception that the dosed surface was briefly heated to 600 K. A Pt(10\%)-Ir tip ... are consistent with our previous photoemission study which showed that Mo(CO) 6 decomposed on Au(111) above 400 K. 15 No C or O ...

}, author = {Hrbek, J and Chan, ASY and Friend, C. M.} } @article {240031, title = {The chemical nature of surface point defects on MoO3(010): adsorption of hydrogen and methyl.}, journal = {JACS}, volume = {123}, number = {10}, year = {2001}, month = {Apr 14}, pages = {2224-2230}, abstract = {

We report density functional theory calculations using the Adaptive Coordinate Real-space Electronic Structure (ACRES) method of the terminal oxygen vacancy on the (010) surface of MoO3, within a (2 x 2) ordered array of vacancies on the surface. Analysis of the electronic structure of this surface shows that there are unoccupied dangling d(xz) and d(z)2 orbitals perpendicular to the surface that are created by the removal of terminal oxygen. The Mo-oxygen bonds surrounding the vacancy contract; however, the overall morphology of the surface is not drastically distorted. The vacancies alter the chemical character of the surface, as shown by studies of hydrogen and methyl binding. On both the "perfect" and vacancy surfaces, hydrogen was most strongly adsorbed over the terminal oxygen and most weakly bound over the symmetric bridging oxygen. Hydrogen is bound over the Mo atom, with a slightly smaller binding energy than hydrogen over the asymmetric bridging oxygen. The most favorable binding site for methyl on the vacancy surface is over the Mo atom exposed by removal of a terminal oxygen, whereas methyl bound to terminal oxygen is most stable on the perfect surface. There is no local minimum for adsorption over the symmetric bridging oxygen; instead, a methyl placed over this site moves toward the terminal oxygen vacancy. Analysis of the bonding shows that methyl is bound more strongly than hydrogen over the Mo atom because the C 2p orbital has better overlap with the Mo d(z)2 orbital than the hydrogen 1s. In addition, the steric repulsion observed for methyl over the perfect MoO3(010) surface is more easily relieved with the presence of the terminal oxygen vacancy.

}, author = {Chen, M and Friend, C. M. and E. Kaxiras} } @article {240036, title = {Site-selective surface reactions: nitric oxide reduction on Mo(110).}, journal = {Chemphyschem}, volume = {1}, number = {3}, year = {2000}, month = {Nov 03}, pages = {116-125}, abstract = {

The catalytic reduction of NO(x) compounds formed in combustion processes is a critical factor in maintaining a clean environment. The introduction of the "catalytic converter" has been extremely effective in reducing these pollutants in automobile exhaust over the last two decades. Nevertheless, new environmental regulations have necessitated the development of processes that operate over a wider range of conditions and that are more efficient, so that NOx emissions can be reduced further. The need for new catalysts and processes has motivated a considerable number of studies of NO reduction using metal oxides as catalysts. In order to better understand the mechanisms for NO reduction on oxides, we have systematically studied the reactions of NO on thin-film oxides grown on Mo(110). By using a thin-film oxide, we are able to change the type of coordination sites that are available for NO binding and to use surface-sensitive spectroscopies to identify intermediates on the surface. We specifically explore the role of low-temperature NO coupling to a dinitrosyl species in our work and contrast this reaction to the higher temperature process, NO dissociation followed by nitrogen atom coupling.

}, author = {Queeney, K and Friend, C. M.} } @article {240051, title = {The adsorption of methyl on Rh10clusters: selective C{\textendash}H bond weakening}, journal = {Catalysis Today}, year = {1999}, abstract = {

... C .-M. Chiang, CJ Jenks et al. J. Catal., 147 (1994), p. 250. ... Full Text via CrossRef. Corresponding author contact information Corresponding author contact information Corresponding author. Tel.: +1-617-495-4052; fax: +1-617-496-8410; e-mail: friend @chemistry. harvard .edu. ...

}, author = {Chen, M and Friend, C. M. and van Santen, R A} } @article {240041, title = {Resolving discrepancies between LEED and STM through ab initio calculations: Surface and bonding of sulfur on Mo (110)}, journal = {Physical Review B}, year = {1999}, abstract = {

... M. Chen, PG Clark, Jr., T. Mueller, and CM Friend * Department of Chemistry, Harvard University ... Department of Physics and Division of Engineering and Applied Sciences, Harvard University, Cambridge, Massachusetts ... The adsorption of sulfur at 0.5 ML in both c (22) and 1 ...

}, author = {Chen, M and Clark, Jr, P G and Mueller, T and Friend, C. M. and E. Kaxiras} } @article {240046, title = {Structure and reactivity of thin-film oxides and metals}, journal = {Applied Surface Science}, year = {1999}, abstract = {

... Harvard University, Department of Chemistry, 12 Oxford St. Cambridge, MA 02138 USA. Available online 28 April 1999. Abstract. ... C {\textendash}O bond scission to produce atomic carbon and oxygen also accounted for \~{}54\% of the total methanol reaction. ... 14; X. Xu, CM Friend ; J. Am. Chem. ...

}, author = {Friend, C. M. and Queeney, K T and Chen, D A} } @article {240056, title = {Passivation of Fe (110) via phosphorus deposition: the reactions of trimethylphosphite}, journal = {Surface Science}, year = {1998}, abstract = {

... Department of Chemistry, Harvard University, 12 Oxford Street, Cambridge, MA 02138, USA. Received 10 November 1997 Accepted 12 January ... The fact that H 2 C O is only formed from methoxy formed from methanol at high phosphorus ... 4; TS Rufael, JD Batteas, CM Friend ; ...

}, author = {Batteas, J D and Wong-Foy, A and Rufael, T S and Friend, C. M.} } @article {234971, title = {Desulfurization of benzenethiol on Co Mo (110) phases}, journal = {Surface Science}, year = {1998}, abstract = {

... Harvard University, Department of Chemistry, 12 Oxford Street ... This difference may be due to hydrogen-induced structural transformations of the Co layer or to hydrogen-assisted C S bond breaking. Keywords. ... 3]; DA Chen, CM Friend , H. Xu; Langmuir, 12 (1996), p. 1528. ...

}, author = {Chen, D A and Friend, C. M. and Xu, H.} } @article {240061, title = {Adsorbate-induced structural changes of metal thin films: cobalt-oxygen and cobalt-sulfur overlayers on Mo (110)}, journal = {Surface Science}, year = {1997}, abstract = {

... Department of Chemistry, Harvard University, 12 Oxford Street, Cambridge, MA 02138, USA. Received 23 February 1996 Accepted 28 ... 26]; P. Uvdal, BC Wiegand, JG Serafin, CM Friend ; ... The relative sensitivities to the O 1s and S 2p signals were determined from the C :S intensity ...

}, author = {Chen, D A and Friend, C. M.} } @article {240071, title = {Control of oxidation selectivity by alkyl substitution: the reactions of t-butyl iodide and t-butanethiol on Rh (111)-p (2{\texttimes} 1) O}, journal = {Surface Science}, year = {1996}, abstract = {

... Department of Chemistry, Harvard University, MA 02138 Cambridge, MA, USA. Received 9 January 1996 Accepted 15 April 1996 Available online ... temperatures correlate with the C I and C S bond strengths, indicating that homolytic C I and C S bond ... 5]; CWJ Bol, CM Friend ; ...

}, author = {Bol, CWJ and Friend, C. M.} } @article {240076, title = {Effect of surface modification on the structure and reactivity of cyclohexane on Mo (110)}, journal = {Surface Science}, year = {1996}, abstract = {

... a Department of Chemistry, Harvard University, Cambridge ... The relative sensitivities of the mass spectrometer to H 2 and C 6 H 12 , for a given calibration, were determined by using an ion gauge to estimate the quantity of each ... [15]; MK Weldon, P. Uvdal, CM Friend , JG Serafin; ...

}, author = {Weldon, M K and Uvdal, P and Friend, C. M. and Wiegand, B C} } @article {240066, title = {Probing Surface Reaction Mechanisms Using Chemical and Vibrational Methods: Alkyl Oxidation and Reactivity of Alcohols on Transitions Metal Surfaces.}, journal = {Chemical Reviews}, volume = {96}, number = {4}, year = {1996}, month = {Jul 20}, pages = {1391-1412}, author = {Weldon, Marcus K and Friend, Cynthia M} } @article {240096, title = {CO Bond Formation by Direct Addition of Methyl Radicals to Surface Oxygen on Rh (111)}, journal = {Journal of the American Chemical Society}, year = {1995}, abstract = {

... Ph.6. Thesis, Harvard University, Cambridge, 1991. (9) Xu, X.; Friend , CMJ Phys. Chem. 1989, 93, 8072. (10)Xu. X.; Friend , CMJ Am. Chem. SOC. 1991, 113, 6779. (11)Bol, CWJ; Friend , CM J. Am. Chem. Soc. 1995, 117, 5351- 5358. (12) Houtman, C .; Barteau, M. A. Langmuir ...

}, author = {Bol, CWJ and Friend, C. M.} } @article {240101, title = {Effects of Oxygen Coverage on the Partial Oxidation of Methylene: Reactions of Diiodomethane on Oxygen-Covered Rh (111)}, journal = {Journal of the American Chemical Society}, year = {1995}, abstract = {

... Reactions on Mo(1 lo), Harvard University, Cambridge, 1991. (18) Wiegand, BCPh.D. Thesis: Model Studies of Desulfurization (19) Liu, A. C .; Friend , CM Rev. Sci. Instrum. 1986,57, 1519-1522. (20)Xu, X.; Friend , CM J. Phys. Chem. 1989, 93, 8072-8080. 240 C1*02 mu48 ...

}, author = {Bol, CWJ and Friend, C. M.} } @article {240081, title = {The Effects of Oxygen on Selectivity: The Reactions of 2-Propanethiolate on Oxygen-Covered Rh (111)}, journal = {Journal of the American Chemical Society}, year = {1995}, abstract = {

... (11)Bol, C . WJ; Friend , CM Surf: Sci. Left. 195, 332, 1271. (12) Wiegand, BC Model Studies of Desulfurization Reactions on Mo- (13) Xu, X.; Friend , C . M. J. Phys. Chem. 1989, 93, 8072. (110), Ph.D. Thesis, Harvard University: Cambridge, 1991. ...

}, author = {Bol, CWJ and Friend, C. M.} } @article {240086, title = {Partial oxidation of C\< sub\> 1\</sub\>-hydrocarbons on oxygen-covered Rh (111): formaldehyde from methylene}, journal = {Surface Science}, year = {1995}, abstract = {

... on Mo(110), Harvard University (1991). [10]; X. Xu, CM Friend ; J. Phys. Chem., 93 (1989), p. 8072. ... [11]; X. Xu, CM Friend ; J. Am. Chem. Soc., 113 (1991), p. 6779. Full Text via CrossRef. [12]; MK Weldon, CM Friend ; Surf. Sci., 321 (1995), p. L202. [13]; J. Lin, C . Chiang, CJ Jenks, MX ...

}, author = {Bol, CWJ and Friend, C. M.} } @article {240091, title = {Surface-induced alteration of adsorbate electronic structure and intramolecular vibrational coupling: The vibrational spectrum of 2-propoxide on Mo (110) as {\textellipsis}}, journal = {Physical Review B}, year = {1995}, abstract = {

... B. C . \%{\textquoteright}iegand and CM Friend Department of Chemistry, Harvard University, Cambridge, Massachusetts 02138 (Received 6 September 1994) Interactions with the molybdenum surface have a strong influence on the vibrational spectrum of 2- propoxide on Mo(110). ...

}, author = {Uvdal, P and Mackerell, A D and Wiegand, B C and Friend, C. M.} } @article {240106, title = {The isomerization and tautomerization of azomethane on Mo (110)}, journal = {Surface Science}, year = {1994}, abstract = {

... Department of Chemistry, Harvard University, Cambridge, MA 02138, USA. Received 24 August 1993 Accepted 27 December 1993 Available online 27 September 2002. Abstract. ... and CM Friend , unpublished results. [17]; C . Brecher, RS Halford; J. Chem. Phys., 35 (1961), p. 1109 ...

}, author = {Weldon, M K and Friend, C. M.} } @article {240111, title = {Isotopic labeling as a tool to establish intramolecular vibrational coupling: The reaction of 2-propanol on Mo (110)}, journal = {The Journal of Chemical Physics}, year = {1992}, abstract = {

... on Mo(110) P. Uvdal, Benjamin C . Wiegand, J. G. Serafin, and CM Friend Department 0/ Chemistry, Harvard University, Cambridge, Massachusetts 02138 (Received 30 July 1991; accepted 24 August 1992) The reactions of ...

}, author = {Wiegand, B C and Serafin, J G and Friend, C. M.} } @article {240116, title = {The effect of rhodium environment on oxidation mechanism: conversion of norbornene to norbornanone on Rh (111)-p (2. times. 1)-O}, journal = {Journal of the American Chemical Society}, year = {1991}, abstract = {

... The Effect of Rhodium Environment on Oxidation Mechanism: Conversion of Norbornene to Norbornanone on Rh(lll)-p(SXl)-O Xueping Xu and C . M. Friend * Department of Chemistry, Harvard University Cambridge, Massachusetts 02138 Received July I, 1991 ...

}, author = {X. Xu and Friend, C. M.} } @article {240121, title = {Thermal and photoinduced decomposition of W (CO)6 on W (110)}, journal = {Surface Science}, year = {1991}, abstract = {

... of Chemistry, Harvard University, Cambridge, MA 02138, USA. Received 17 July 1990 Accepted 18 September 1990 Available online 18 September 2002. Abstract. ... 23]; C . Kohrt, R. Gomer; Surf. Sci., 24 (1971), p. 77. ... JT Roberts, CM Friend ; J. Am. Chem. Soc., 108 (1986), p. 7204 ...

}, author = {Flitsch, F A and Swanson, J R and Friend, C. M.} } @article {240126, title = {Evidence for photochemical quenching by a surface: Laser induced decomposition of W (CO)6 on Si (111)-(7{\texttimes} 7)}, journal = {Surface Science}, year = {1990}, abstract = {

... Department of Chemistry, Harvard University, Cambridge, MA 02138, USA. Received 19 June 1989 Accepted 22 September 1989 Available online 18 September 2002. Abstract. ... 38]; JR Swanson, CM Friend , YJ Chabal; J. Chem. Phys., 87 (1987), p. 5028. ...

}, author = {Swanson, J R and Flitsch, F A and Friend, C. M.} } @article {240131, title = {The orientation of phenoxide on oxygen-precovered Mo (110)}, journal = {Surface Science}, year = {1990}, abstract = {

... Department of Chemistry, Harvard University, Cambridge ... The orientation of phenoxide ( C 6 H 5 O) adsorbed on Mo(110) precovered with 0.33 monolayers of oxygen was determined using the near edge X-ray absorption fine structure (NEXAFS) method ... [1]; JG Serafin, CM Friend ...

}, author = {Liu, A C and Friend, C. M. and Stohr, J} } @article {240151, title = {Alkane activation: The adsorption and reaction of ethane on W (100)}, journal = {Surface Science}, year = {1989}, abstract = {

... Harvard University, Cambridge, MA 02138, USA. Received 4 November 1988 Accepted 18 January 1989 Available online 18 September 2002. Abstract. ... Friend ; J. Chem. Phys., 87 (1987), p. 4975. Full Text via CrossRef. [23]; JT Yates Jr., TE Madey; Surface Sci., 28 (1971), p. 437 ...

}, author = {Liu, A C and Friend, C. M.} } @article {240136, title = {Carbon-oxygen bond strength as a control of reaction kinetics: Phenol on Mo (110)}, journal = {Surface Science}, year = {1989}, abstract = {

... Harvard University, Cambridge, MA 02138, USA. Received 6 October 1988 Accepted 17 November 1988 Available online 6 September 2002. Abstract. ... Friend , J. Phys. ... [13]; U. Gelius, PF Heden, J. Hedman, BJ Lindberg, R. Manne, R. Nordberg, C . Nordling, K. Siegbahn; Phys. ...

}, author = {Serafin, J G and Friend, C. M.} } @article {240141, title = {Inhibition of CH and CO bond activation by surface oxygen: stabilization of surface phenoxide in the reaction of phenol on oxygen-precovered Mo (110)}, journal = {Journal of the American Chemical Society}, year = {1989}, abstract = {

... Auteur(s) / Author(s). SERAFIN JG (1) ; FRIEND CM ; ... (1) Harvard univ., dep. ... Temperature- programmed reaction and X-ray photoelectron spectroscopies have been used to study the reactions of phenol ( C 6 H 5 OH) on oxygen-precovered Mo(110) Revue / Journal Title. ...

}, author = {Serafin, J G and Friend, C. M.} } @article {240146, title = {Low energy electron induced decomposition on surfaces: W (CO)6 on Si (111)-(7 x 7)}, journal = {Surface Science Letters}, year = {1989}, abstract = {

... DECOMPOSITION ON SURFACES: W(CO)6 ON Si(111)-(7 x 7) JR SWANSON, FA FLITSCH and CM FRIEND Department of Chemistry, Harvard University, Cambridge ... the new 13 CO peak is clearly attributable to decomposition of tungsten carbonyl N w {\textquoteright}O \< ~ V ow {\textquoteright} C CL .N ...

}, author = {Swanson, J R and Flitsch, F A and Friend, C. M.} } @article {240156, title = {Adsorbate structural determination of CO and HCNH using C K-edge NEXAFS on W (100)-(5{\texttimes} 1)-C}, journal = {Surface Science}, year = {1988}, abstract = {

... structural determination of CO and HCNH using C K-edge NEXAFS on W(100)-(5 {\texttimes} 1)- C . ... Departments of Applied Physics and Chemical Engineering, Stanford University, Stanford, CA 94305-5025, USA. CM Friend . Department of Chemistry, Harvard University, Cambridge, MA ...

}, author = {Stevens, P A and Madix, R J and Friend, C. M.} } @article {240161, title = {Desulfurization of ethylene sulfide on Mo (100): The roles of ring size and strain in adsorbate reaction selectivity}, journal = {Surface Science}, year = {1988}, abstract = {

... Department of Chemistry, Harvard University, Cambridge ... The reactions, under ultrahigh vacuum, of ethylene sulfide ( c - C 2 H 4 S) on Mo(110) have been examined, by isothermal reaction spectroscopy, temperature programmed ... [2]; JT Roberts and CM Friend , in preparation. ...

}, author = {Roberts, J T and Friend, C. M.} } @article {240166, title = {Vibrational studies of 1-propanethiol on Mo (110): Spectroscopic evidence for low temperature thiolate formation}, journal = {Surface Science}, year = {1988}, abstract = {

... Department of Chemistry, Harvard University, Cambridge ... Competitive with hydrocarbon evolution is the formation of a surface hydrocarbon fragment of unknown stoichiometry, detected by the presence of a C H stretch, v( C H), at 2950 cm -1 . The ... [2]; JT Roberts, CM Friend ; ...

}, author = {Roberts, J T and Friend, C. M.} } @article {240171, title = {Bonding and adsorption structure of CO on W (100)-(5{\texttimes} 1){\textendash}C}, journal = {The Journal of Chemical Physics}, year = {1987}, abstract = {

... c . M. Friend , J. G. Serafin, and EK Baldwin Department of Chemistry. Harvard University. Cambridge. Massachusetts 02138 PA Stevens and R. J. Madix Departments of Chemical Engineering and Applied Physics. Stanford University. Stanford. ...

}, author = {Friend, C. M. and Serafin, J G and Baldwin, E K} } @article {240176, title = {Laser-Assisted Deposition of Fe and W: Photodecomposition of Fe (CO) 5, and W (CO) 6 on Si (111)-(7{\texttimes} 7)}, journal = {MRS Proceedings}, year = {1987}, abstract = {

... Jon R. Swanson, CM Friend and YJ Chabal * Harvard University, Dept. ... this work was supported by the NSF ($\#$CHE-83-09455) and ($\#$CHE-84-51307) and the Harvard University Materials ... The C -0 stretching region obtained after: a) exposure of the Si(111)- (7x7) surface to 0.5 L ...

}, author = {Swanson, J R and Friend, C. M. and Chabal, Y J} } @article {240181, title = {The reactions of thiophene on Mo (110) and Mo (110)-p (2{\texttimes} 2)-S}, journal = {Surface Science}, year = {1987}, abstract = {

... Department of Chemistry, Harvard University, Cambridge, MA 02138, USA. Received 3 November 1986 Accepted 24 February 1987 Available online 29 July 2006. ... 28]; RM Wexler, M.- C . Tsai, CM Friend , EL Muetterties; J. Am. Chem. Soc., 104 (1982), p. 2034. ...

}, author = {Roberts, J T and Friend, C. M.} } @article {240191, title = {Alteration in surface reaction selectivity of nitriles. 1. Hydrogen cyanide on tungsten (100) and tungsten (100)-(5. times. 1)-carbon}, journal = {The Journal of Physical Chemistry}, year = {1986}, abstract = {

... Chem. 1986, 90,4341-4343 4341 Alteration in Surface Reaction Selectivity of Nitrlles. 1. HCN on W(100) and W(l00)-(5X1)- C K. A. Pearlstine and CM Friend * Department of Chemistry, Harvard University, Cambridge, Massachusetts 021 38 (Received: February 14, 1986) ...

}, author = {Pearlstine, K A and Friend, C. M.} } @article {240186, title = {Surface chemistry of alkylamines. 1. Ethylamine and triethylamine on W (100), W (100)-(5. times. 1)-C, and W (100)-(2. times. 1)-O}, journal = {Journal of the American Chemical Society}, year = {1986}, abstract = {

... 1. Ethylamine and Triethylamine on W( loo), W( 100)-(5 X 1)- C , and W( 100)-(2 x 1)-0 K. A. Pearlstine and C . M. Friend * Contribution from the Department of Chemistry, Harvard University, Cambridge Massachusetts 02138. Received March 25, 1986 ...

}, author = {Pearlstine, K A and Friend, C. M.} } @article {240196, title = {Carbon-carbon bond activation: cycloheptatriene reaction on tungsten (100), tungsten (100)-(5. times. 1)-C, and tungsten (100)-(2. times. 1)-O}, journal = {Journal of the American Chemical Society}, year = {1985}, abstract = {

... CC Bond Activation: Cycloheptatriene Reaction on W( loo), W( 100)-(5Xl)- C , and W( 100)-(2Xl)-O K. A. Pearlstine and CM Friend * Contribution from the Department of Chemistry. Harvard University, Cambridge, Massachusetts 021 38. Received February 28, 1985 ...

}, author = {Pearlstine, K A and Friend, C. M.} } @article {240201, title = {Reaction of NO on W (100), W (100)-p (2{\texttimes} 2)-O, and W (100)-p (2{\texttimes} 1)-O surfaces: a nondissociative pathway for NN bond formation}, journal = {Journal of Physical Chemistry}, year = {1985}, abstract = {

... 100)-p (2{\texttimes}1)-O: voie non dissociative de la formation de la liaison NN. Auteur(s) / Author(s). BALDWIN EK ; FRIEND CM ; Affiliation(s) du ou des auteurs / Author(s) Affiliation(s). Harvard univ., dep. chemistry R{\'e}sum{\'e} / Abstract. ...

}, author = {Baldwin, E K and Friend, C. M.} }