Oxygen-covered Au Research Articles

Temperature-Programmed Reactions of Aromatic Compounds on Au(111) and on a Model Gold Catalyst

Aromatic compounds are often employed as solvents and probe molecules in gold-mediated catalytic processes, and therefore, their interaction with gold surfaces is relevant to improve the design of catalysts and processes. Herein, we compare the interaction of various aromatic molecules (benzene, toluene, phenylacetylene, phenol, o-benzenediol, benzyl alcohol, benzaldehyde, and benzenethiol) with clean and oxygen-covered Au(111) surfaces through temperature-programmed reaction/mass spectrometry. While the clean surface is important from a fundamental point of view, the oxygen-covered Au surface is a model of a working heterogeneous gold catalyst. Condensation experiments demonstrate that, with respect to benzene, the functionalities increase significantly the adsorption energies, opening pathways for decomposition and surface poisoning. Over the O/Au surface, there is a clear trend in Brønsted reactivity, following the order benzenethiol > phenylacetylene > benzenediol > benzyl alcohol > phenol > benzaldehyde > toluene, benzene. This trend has been confirmed by performing displacement reactions of the type B + AH = BH + A, where B is a dehydrogenated species and AH is a molecule with a sufficient acidity to regenerate the original molecule BH. Finally, based on the structure of the B species, we find as potential pathways (a) further dehydrogenation (e.g., benzyl alcohol to benzaldehyde), (b) coupling (e.g., benzenethiol to dibenzene disulfide), or (c) surface decomposition (e.g., phenol to residual carbon). Since the extent to which each of these reactions take place is related to the molecular structure of the aromatic molecule, this study provides relevant information for designing catalytic routes for aromatic molecules.

Oxidation of formic acid on stepped Au(997) surface

The adsorption and reaction of formic acid (HCOOH) on clean and atomic oxygen-covered Au(997) surfaces were studied by temperature-programmed desorption/reaction spectroscopy (TPRS) and X-ray photoelectron spectroscopy (XPS). At 105 K, HCOOH molecularly adsorbs on clean Au(997) and interacts more strongly with low-coordinated Au atoms at (111) step sites than with those at (111) terrace sites. On an atomic oxygen-covered Au(997) surface, HCOOH reacts with oxygen atoms to form HCOO and OH at 105 K. Upon subsequent heating, surface reactions occur among adsorbed HCOO, OH, and atomic oxygen and produce CO2, H2O, and HCOOH between 250 and 400 K. The Au(111) steps bind surface adsorbates more strongly than the Au(111) terraces and exhibit larger barriers for HCOO(a) oxidation reactions. The surface reactions also depend on the relative coverages of co-existing surface species. Our results elucidate the elementary surface reactions between formic acid and oxygen adatoms on Au surfaces and highlight the effects of the coordination number of the Au atoms on the Au catalysis.

Reaction heat-driven CO2 desorption during CO oxidation on Au(997) at low temperatures

Adsorption and reaction of CO and CO2 were studied on oxygen-covered Au(997) surfaces by means of temperature- programmed desorption/reaction spectroscopy. Oxygen atoms (O(a)) on Au(997) enhances the CO2 adsorption and stabilizes the adsorbed CO2(a), and the stabilization effect also depends on the CO2(a) coverage and involved Au sites. CO2(a) desorption is the rate-limiting step for the CO+O(a) reaction to produce CO2 on Au(997) at 105 K and exhibits complex behaviors, including the desorption of CO2(a) upon CO exposures at 105 K and the desorption of O(a)-stabilized CO2(a) at elevated temperatures. The desorption of CO2(a) from the surface upon CO exposures at 105 K to produce gaseous CO2 depends on the surface reaction extent and involves the reaction heat-driven CO2(a) desorption channel. CO+O(a) reaction proceeds more easily with weakly-bound oxygen adatoms at the (111) terraces than strongly-bound oxygen adatoms at the (111) steps. These results reveal complex rate-limiting CO2(a) desorption behaviors during CO+O(a) reaction on Au surfaces at low temperatures which provide novel information on the fundamental understanding of Au catalysis.

Decomposition of H2O on clean and oxygen-covered Au (1 0 0) surface: A DFT study

Employing density functional theory (DFT) together with periodic slab models, the adsorption and dehydrogenation of H2O on clean and oxygen-covered Au (1 0 0) have been investigated systematically. On the basis of the theoretical analysis, the favorable adsorption sites and stable configurations of all species were clarified. H2O was predicted to adsorb weakly on the top, bridge, hollow sites, with the top site preferred. Whereas OH and atomic O prefer to adsorb on the hollow site and H occupies the bridge site. What's more, this work displayed the optimum configurations for the relevant co-adsorption groups. The results elucidated that co-adsorption is apt to impair the interaction of adsorbate-substrate due to the joining of oxygen atom except for H2O molecule. Eventually, the interrelated transition states and activation energies were calculated to explore the dehydrogenation mechanism of H2O. A feasible mechanism on oxygen pre-covered surfaces for complete dehydrogenation of H2O was also presented. It was illuminated that atomic oxygen could diminish the barrier energy substantially of the first dissociation step and play a pivotal role in the decomposition of H2O molecule.

Selective oxidation of propylamine on oxygen-covered Au(111): a DFT study

The reaction mechanism for selective oxidation of propylamine on oxygen-covered gold has been studied by the density functional theory (DFT) and generalized gradient approximation (GGA) with slab model. Our calculation results indicated that the adsorption energy of propylamine decreases with the increasing oxygen coverage, that is -0.38, -0.20 and -0.10 eV on clean, 2/9 monolayer (ML) and 2/3 monolayer (ML) oxygen, respectively. The adsorption energies of the intermediates also have the trend of the gradual lower. The present work also indicated that the final product distribution depends on the oxygen coverage: propylamine undergoes N-H bond and C-H bond cleavage to produce propionitrile and water at low-oxygen-coverage (θ(o) = 2/9 ML), and to yield propionitrile, propionaldehyde and water at high-oxygen-coverage (θ(o) = 2/3 ML). The energy barrier of the first step of propyamine oxidation (CH(3)CH(2)CH(2)NH(2) → CH(3)CH(2)CH(2)NH) is 0.16 eV (θ(o) = 2/9 ML) and 0.38 eV (θ(o) = 2/3 ML). On the second step, the barrier energy is 0.16 (θ(o) = 2/9 ML) and 0.25 (θ(o) = 2/3 ML) eV of CH(3)CH(2)CH(2)NH → CH(3)CH(2)CH(2)N, next both C-H breakage and the barrier energy is 0.20 eV (CH(3)CH(2)CH(2)N → CH(3)CH(2)CHN) and 0.25 eV (CH(3)CH(2)CHN → CH(3)CH(2)CN) on low oxygen coverage, and 0.15 eV (CH(3)CH(2)CH(2)N → CH(3)CH(2)CHN) and 0.26 eV(CH(3)CH(2)CHN → CH(3)CH(2)CN) on the high oxygen coverage. The additional reaction step of CH(3)CH(2)CHN → CH(3)CH(2)CHO occurs on the high oxygen coverage, and the associated barrier is 0.41 eV. The calculation results show that the oxidation of propylamine can occur at room temperature due to the lower energy barrier. Furthermore, it was found that the energy barrier for the possible reaction steps at the low oxygen coverage is generally smaller than that on high oxygen coverage, which agrees with the experimental results.

Achieving Optimum Selectivity in Oxygen Assisted Alcohol Cross-Coupling on Gold

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.

Mechanistic Insights into Selectivity Control for Heterogeneous Olefin Oxidation: Styrene Oxidation on Au(111)

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.

Local Bonding Effects in the Oxidation of CO on Oxygen-Covered Au(111) from Ab Initio Molecular Dynamics Simulations.

A fully dynamical approach using ab initio molecular dynamics (AIMD) simulations is applied to the investigation of CO oxidation on O-covered Au(111). We investigate how the activity of gold depends upon temperature, oxygen coverage, and surface structure. On clean Au(111) at 500 K, CO binds transiently on top of Au atoms, spending a small fraction (∼7%) of the total simulation time adsorbed on the surface. The presence of O on the surface increases the residence time for CO by more than 4 times on a surface containing 0.22 ML of O. On the other hand, the probability for CO adsorption decreases with oxygen coverage from 31% at 0.22 ML of oxygen to 15% at 0.55 ML of oxygen. Our simulations show that the activity for CO reaction with O to yield CO2 decreases with increasing oxygen coverage. We attribute this decrease of activity to (1) the decrease in the CO adsorption probability as the oxygen coverage increases and (2) the decreasing amount of reactive chemisorbed oxygen (oxygen bound in a 3-fold site) with increasing total oxygen coverage. We show that oxygen bound in sites of local 3-fold coordination (chemisorbed oxygen) is nearly 2 times more reactive than other oxygen species observed on the surface, namely, surface and subsurface oxide. Our work demonstrates the value and feasibility of using AIMD to study surface reactions.

Vapour-phase gold-surface-mediated coupling of aldehydes with methanol

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.

Transient hydroxyl formation from water on oxygen-covered Au(111)

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.

Water-Enhanced Low-Temperature CO Oxidation and Isotope Effects on Atomic Oxygen-Covered Au(111)

Water-oxygen interactions and CO oxidation by water on the oxygen-precovered Au(111) surface were studied by using molecular beam scattering techniques, temperature-programmed desorption (TPD), and density functional theory (DFT) calculations. Water thermally desorbs from the clean Au(111) surface with a peak temperature of approximately 155 K; however, on a surface with preadsorbed atomic oxygen, a second water desorption peak appears at approximately 175 K. DFT calculations suggest that hydroxyl formation and recombination are responsible for this higher temperature desorption feature. TPD spectra support this interpretation by showing oxygen scrambling between water and adsorbed oxygen adatoms upon heating the surface. In further support of these experimental findings, DFT calculations indicate rapid diffusion of surface hydroxyl groups at temperatures as low as 75 K. Regarding the oxidation of carbon monoxide, if a C (16)O beam impinges on a Au(111) surface covered with both atomic oxygen ( (16)O) and isotopically labeled water (H 2 (18)O), both C (16)O (16)O and C (16)O (18)O are produced, even at surface temperatures as low as 77 K. Similar experiments performed by impinging a C (16)O beam on a Au(111) surface covered with isotopic oxygen ( (18)O) and deuterated water (D 2 (16)O) also produce both C (16)O (16)O and C (16)O (18)O but less than that produced by using (16)O and H 2 (18)O. These results unambiguously show the direct involvement and promoting role of water in CO oxidation on oxygen-covered Au(111) at low temperatures. On the basis of our experimental results and DFT calculations, we propose that water dissociates to form hydroxyls (OH and OD), and these hydroxyls react with CO to produce CO 2. Differences in water-oxygen interactions and oxygen scrambling were observed between (18)O/H 2 (16)O and (18)O/D 2 (16)O, the latter producing less scrambling. Similar differences were also observed in water reactivity toward CO oxidation, in which less CO 2 was produced with (16)O/D 2 (16)O than with (16)O/H 2 (16)O. These differences are likely due to primary kinetic isotope effects due to the differences in O-H and O-D bond energies.

CO Adsorption and Reaction on Clean and Oxygen-Covered Au(211) Surfaces

We have used primarily temperature-programmed desorption (TPD) and infrared reflection-absorption spectroscopy (IRAS) to investigate CO adsorption on a Au(211) stepped single-crystal surface. The Au(211) surface can be described as a step-terrace structure consisting of three-atom-wide terraces of (111) orientation and a monatomic step with a (100) orientation, or 3(111) x (100) in microfacet notation. CO was only weakly adsorbed but was more strongly bound at step sites (12 kcal mol(-1)) than at terrace sites (6.5-9 kcal mol(-1)). The sticking coefficient of CO on the Au(211) surface was also higher ( approximately 5x) during occupation of step sites compared to populating terrace sites at higher coverages. The nu(CO) stretching band energy in IRAS spectra indicated that CO was adsorbed at atop sites at all coverages and conditions. A small red shift of nu(CO) from 2126 to 2112 cm(-1) occurred with increasing CO coverage on the surface. We conclude that the presence of these particular step sites at the Au(211) surface imparts stronger CO bonding and a higher reactivity than on the flat Au(111) surface, but these changes are not remarkable compared to chemistry on other more reactive crystal planes or other stepped Au surfaces. Thus, it is unlikely that the presence or absence of this particular crystal plane alone at the surface of supported Au nanoparticles has much to do with the remarkable properties of highly active Au catalysts.

Partial Oxidation of Propene on Oxygen-Covered Au(111)

Partial oxidation of propene is promoted by Au following deposition of atomic oxygen (0.3 ML) via O3 decomposition on Au(111) at 200 K. Several partial oxidation products--acrolein, 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.

Selective Oxidation of Styrene on an Oxygen-Covered Au(111)

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.

Propene Adsorption on Clean and Oxygen-Covered Au(111) and Au(100) Surfaces

The adsorption of propene on Au(111) and Au(100) was investigated using temperature-programmed desorption (TPD) and high-resolution energy loss spectroscopy (HREELS). A desorption activation energy of 9.4 kcal/mol and very small shifts of the vibrational frequencies from their gas-phase values indicate that the interaction of propene with the surface is weak. Energy loss spectra suggest that propene adsorbs with its molecular plane tilted slightly with respect to the surface plane. Atomic oxygen, chemisorbed on the gold surfaces, was characterized using TPD and low-energy electron diffraction (LEED), and its reaction with propene investigated. Propene desorption from the oxygen-covered surface occurs at 150 K with a shoulder at 200 K. Only propene and its total oxidation products desorbed from a one monolayer (1 ML) oxygen-covered surface. At an oxygen coverage of ∼0.4 ML, the higher temperature propene desorption feature at 200 K was maximized. HREEL spectra of propene giving rise to the 200 K desorption feature show shifts in the intensities and frequencies of the −CH2 related vibrational features. Small amounts of product with masses 56 and 58 amu were observed for propene adsorbed onto a 0.4 ML oxygen-covered surface.