O'Connor CR, van Spronsen MA, Egle T, Xu F, Kersell HR, Oliver-Meseguer J, Karatok M, Salmeron MB, Madix RJ, Friend CM. Hydrogen Migration at Restructuring Palladium–Silver Oxide Boundaries Dramatically Enhances Reduction Rate of Silver Oxide. Nat. Comm. 2020;11 (1844). Publisher's VersionAbstract
Hydrogen migration at restructuring palladium–silver oxide boundaries dramatically enhances reduction rate of silver oxide

Nature Communications volume 11, Article number: 1844 (2020) Cite this article


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–silver alloy. The palladium–silver interface also dynamically restructures during reduction, resulting in silver–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.

Luneau M, Guan E, Chen W, Foucher A, Marcella N, Shirman T, Verbart DMA, Aizenberg J, Aizenberg M, Stach E, et al. Enhancing Catalytic Performance of Dilute Metal Alloy Nanomaterials. Comm.Chem. 2020;3 (46) :1-9. Publisher's VersionAbstract
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.
Reece CG, Luneau M, Friend CM, Madix RJ. Predicting a dramatic decline in selectivity for catalytic esterification of alcohols due to van der Waals interactions. Angew. Chem. 2020;132 (27). Publisher's VersionAbstract
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–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.
Lim JS, Vandermause J, van Spronsen MA, Musaelian A, O'Connor CR, Egle T, Xie Y, Sun L, Molinari M, Florian J, et al. Evolution of Metastable Structures in Bimetallic Catalysts from Microscopy and Machine-Learning Molecular Dynamics. ChemRxiv. 2020. Publisher's VersionAbstract
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.
Marcella N, Liu Y, Timoshenko J, Guan E, Luneau M, Shirman T, Plonka AM, van der Hoeven J, Aizenberg J, Friend CM, et al. Neural Network Assisted Analysis of Bimetallic Nanocatalysts using X-ray Absorption Near Edge Structure Spectroscopy. Phys. Chem. Chem. Phys. 2020. Publisher's VersionAbstract
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–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.
Karatok M, Duanmu K, O'Connor CR, Boscoboinik JA, Sautet P, Madix RJ, Friend CM. Tuning reactivity layer-by-layer: formic acid activation on Ag/Pd(111). Chem. Sci. 2020;(25). Publisher's VersionAbstract
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.
Mehar V, O'Connor CR, Egle T, Karatok M, Madix RJ, Friend CM, Weaver JF. Growth and auto-oxidation of Pd on single-layer AgO x/Ag (111). Phys. Chem. Chem. Phys. 2020;22 (11) :6202-6209. Publisher's VersionAbstract
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.
Xu F, Chen W, Walenta CA, O'Connor CR, Friend CM. Dual Lewis site creation for activation of methanol on Fe3O4(111) thin films. Chem. Sci. 2020;11 (9) :2448-2454. Publisher's VersionAbstract
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–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.
Chen W, Sun L, Kozinsky B, Friend CM, Kaxiras E, Sautet P, Madix RJ. Effect of Frustrated Rotations on the Pre-Exponential Factor for Unimolecular Reactions on Surfaces: A Case Study of Alkoxy Dehydrogenation. J. of Phys. Chem. C. 2020;124 (2) :1429-1437. Publisher's VersionAbstract
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–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–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.
Luneau M, Shirman T, Foucher A, Duanmu K, Verbart DMA, Sautet P, Stach E, Aizenberg J, Madix RJ, Friend CM. Achieving High Selectivity for Alkyne Hydrogenation at High Conversions with Compositionally Optimized PdAu Nanoparticle Catalysts in Raspberry Colloid-Templated SiO2. ACS Cat. 2020;10 (1) :441-450 . Publisher's VersionAbstract
Improving the selectivity for catalytic hydrogenation of alkynes is a key step in upgrading feedstocks for olefin polymerization. Herein, dilute PdxAu1–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—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–1 for the Pd0.04Au0.96 catalysts, which is ∼14 kJ mol–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.
Guan E, Foucher A, Marcella N, Shirman T, Luneau M, Head A, Verbart DMA, Aizenberg J, Friend CM, Stacchiola D, et al. New Role of Pd Hydride as a Sensor of Surface Pd Distributions in Pd-Au Catalysts. ChemCatChem . 2019;12 (3). Publisher's VersionAbstract
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.
Zugic B, Van Spronsen M, Heine C, Montemore MM, Li Y, Zakharov DN, Karakalos S, Lechner BA, Crumlin E, Biener MM, et al. Evolution of steady-state material properties during catalysis: Oxidative coupling of methanol over nanoporous Ag0.03Au0.97. J. of Cat. 2019;380 :366—374. Publisher's VersionAbstract
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.
Luneau M, Shirman T, Filie A, Timoshenko J, Chen W, Trimpalis A, Flytzani-Stephanopoulos M, Kaxiras E, Frenkel A, Aizenberg J, et al. Dilute Pd/Au Alloy Nanoparticles Embedded in Colloid-Templated Porous SiO2 : Stable Au-Based Oxidation Catalysts. Chem. of Materials. 2019;31 :5759-5768. Publisher's VersionAbstract
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–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.
Timoshenko J, Wrasman CJ, Luneau M, Shirman T, Cargnello M, Bare SR, Aizenberg J, Friend CM, Frenkel AI. Probing Atomic Distributions in Mono- and Bimetallic Nanoparticles by Supervised Machine Learning. Nano Letters. 2019;19 :520-529. Publisher's VersionAbstract
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.
Xu F, Montemore MM, O'Connor CR, Muramoto E, van Spronsen MA, Madix RJ, Friend CM. Oxygen Adsorption on Spontaneously Reconstructed Au(511). Surface Science. 2019;679 :296-303. Publisher's VersionAbstract
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.
van Spronsen MA, Duanmu K, O'Connor CR, Egle T, Kersell H, Oliver-Meseguer J, Salmeron M, Madix RJ, Sautet P, Friend CM. The Dynamics of Surface Alloys: Rearrangement of Pd/Ag(111) Induced by CO and O2. Journal of Physical Chemistry C. 2018;(part of the Hans-Joachim Freund and Joachim Sauer Festschrift special issue). Publisher's VersionAbstract
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–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.
Reece C, Redekop E, Karakalos S, Friend CM, Madix RJ. Crossing the great divide between single crystal reactivity and actual catalyst selectivity with pressure transients. Nature Catalysis. 2018;1 (11) :852-859. Publisher's VersionAbstract
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.
Walenta C, Crampton A, Xu F, Heiz U, Friend CM. Chemistry of Methanol and Ethanol on Ozone-Prepared α-Fe2O30001. Journal of Physical Chemistry C. 2018;122 (44) :25404-25410. Publisher's VersionAbstract
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).
Xu F, Madix RJ, Friend CM. Spatially Nonuniform Reaction Rates during Selective Oxidation on Gold. Journal of the American Chemical Society. 2018;140 (38) :12210-12215. Publisher's VersionAbstract
The nonuniform reactivity of adsorbed oxygen during the selective oxidation of methanol on Au(110)-(1×2) 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)-(1×2). 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.
Friend CM, Xu F. Perspectives on the Design of Nanoparticle Systems for Catalysis. Faraday Discussions. 2018;208 :595-607. Publisher's Version