Reactivity and Morphology of Oxygen-Modified Au Surfaces (original) (raw)
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Physical Review B, 2007
We perform density-functional theory calculations to investigate the adsorption of oxygen at the Au͑111͒ surface, including on-surface, subsurface, and surface oxide formation. We find that atomic oxygen adsorbs weakly on the surface and is barely stable with respect to molecular oxygen, while pure subsurface adsorption is only metastable. Interestingly, however, we find that the most favorable structure investigated involves a thin surface-oxide-like configuration, where the oxygen atoms are quasithreefold-coordinated to gold atoms, and the gold atoms of the surface layer are twofold, linearly coordinated to oxygen atoms. By including the effect of temperature and oxygen pressure through the description of ab initio atomistic thermodynamics, we find that this configuration is the most stable for realistic catalytic temperatures and pressures, e.g., for low-temperature oxidation reactions, and is predicted to be stable up to temperatures of around 420 K at atmospheric pressure. This gives support to the notion that oxidized Au, or surface-oxide-like regions, could play a role in the behavior of oxide-supported nanogold catalysts.
Chemisorbed Oxygen on the Au(321) Surface Alloyed with Silver: A First-Principles Investigation
Journal of Physical Chemistry C, 2015
The adsorption of oxygen on kinked Au(321) slabs is investigated theoretically on the basis of density functional theory. On-surface, subsurface, and surface-oxide forms of O are analyzed and compared on pure gold and on the surfaces containing silver atoms. At low O coverage (0.1 ML) subsurface O species are shown to be unstable both thermodynamically and kinetically due to a low barrier for conversion to stronger bound on-surface chemisorbed oxygen. The presence of Ag in the near-surface region was shown to increase the binding strength of on-surface as well as subsurface O but the activation barrier for releasing subsurface O to the surface remains essentially unaffected by the presence of Ag. At oxygen coverage 0.2 ML or higher, the most stable surface arrangements of O atoms are chain-like structures consisting of linear-O-Au-O-fragments. Subsurface O atoms being a part of such chains are significantly stabilized. We examine phase transitions between the clean surface and possible stable oxidized surface structures as a function of temperature and O 2 partial pressure. Ag atoms replacing Au on the Au(321) surface are shown to stabilize the Ocovered surface with respect to the clean surface. Pre-existent chemisorbed atomic oxygen is predicted to facilitate the dissociation of molecular oxygen on the pure and alloyed gold surfaces.
Surface Science, 2006
Nanosized gold particles supported on reducible metal oxides have been reported to show high catalytic activity toward CO oxidation at low temperature. This has generated great scientific and technological interest, and there have been many proposals to explain this unusual activity. One intriguing explanation that can be tested is that of Nørskov and coworkers [Catal. Lett. 64 (2000) 101] who suggested that the ''unusually large catalytic activity of highly-dispersed Au particles may in part be due to high step densities on the small particles and/or strain effects due to the mismatch at the Au-support interface''. In particular, their calculations indicated that the Au(2 1 1) stepped surface would be much more reactive towards O 2 dissociative adsorption and CO adsorption than the Au(1 1 1) surface. We have now studied the adsorption of O 2 and O 3 (ozone) on an Au(2 1 1) stepped surface. We find that molecular oxygen (O 2 ) was not activated to dissociate and produce oxygen adatoms on the stepped Au(2 1 1) surface even under high-pressure (700 Torr) conditions with the sample at 300-450 K.
Noble and Precious Metals - Properties, Nanoscale Effects and Applications, 2018
In this chapter, experimental and theoretical studies on surface segregation in Ag-Au systems, including our own thermodynamic studies and molecular dynamics simulations of surface restructuring, on the basis of density functional theory are reviewed. The restructuring processes are triggered by adsorbed atomic O, which is supplied and consumed during catalysis. Experimental evidence points to the essential role of Ag impurities in nanoporous gold for activating O 2. At the same time, increasing Ag concentration may be detrimental for the selectivity of partial oxidation. Understanding the role of silver requires a knowledge on its chemical state and distribution in the material. Recent studies using electron microscopy and photoelectron spectroscopy shed new light on this issue revealing a non-uniform distribution of residual Ag and coexistence of different chemical forms of Ag. We conclude by presenting an outlook on electromechanical coupling at Ag-Au surfaces, which shows a way to systematically tune the catalytic activity of bimetallic surfaces.
Atomic oxygen adsorption on Au (100) and bimetallic Au/M (M=Pt and Cu) surfaces
Computational and Theoretical Chemistry, 2012
Periodic slab calculations in generalized gradient approximation density-functional theory (GGA-DFT) have been used to demonstrate how the existence of second metals can modify the atomic oxygen adsorption on Au (1 0 0) surfaces. The computed adsorption energies for atomic oxygen adsorbed at 0.125 ML (monolayer) surface coverage on the Au (1 0 0) and bimetallic Au-Pt (1 0 0), Au-Cu (1 0 0), Au-Pt-Au (1 0 0) and Au-Cu-Au (1 0 0) surfaces are 275.60, 417.96, 427.79, 283.38, and 275.55 kJ/mol, respectively. In all coverages, the adsorption energies E O ad for Au-Pt and Au-Cu surfaces are higher than that for Au-Pt-Au, Au and Au-Cu-Au. In low coverage (h < 0.5), the adsorption energy E O ad for Au-Cu is larger than that for Au-Pt. As the coverage rises, E O ad reduces and E O ad for Au-Cu becomes smaller than that for Au-Pt. In Au-Cu-Au (1 0 0) surface, the largest shift in the d-band center away from the Fermi level results in the weakest oxygen binding energy.
Tuning the Stability of Surface Intermediates Using Adsorbed Oxygen: Acetate on Au(111)
The Journal of Physical Chemistry Letters, 2014
Selective oxidative reactions promoted by gold depend critically on controlling the coverage and stability of adsorbed intermediates, as well as promoting specific bond activations of those intermediates. We demonstrate that acetate, a common intermediate in the oxidation of olefins, aldehydes, and alcohols, is destabilized by 7−10 kcal/mol by coadsorbed oxygen relative to its stability on the clean gold surface. The amount of destabilization depends on the oxygen coverage. Peak temperatures of products indicative of oxygen-assisted and clean-surface bond activation differ by up to 130 K. Experiments with d 3acetate show a kinetic isotope effect of 6.9 at 400 K, indicating that the rate-limiting step of the low temperature oxygen-assisted reaction is γ-CH bond breaking. This clearly demonstrates that coadsorbed oxygen activates γ-CH bonds on gold and suggests that an oxygen-assisted activation may also occur for β-CH bonds crucial in oxygen-assisted alcohol coupling on metallic gold catalysts, as predicted by theory.
Journal of the American Chemical Society, 2002
Gold-based catalysts have been of intense interests in recent years, being regarded as a new generation of catalysts due to their unusually high catalytic performance. For example, CO oxidation on Au/TiO2 has been found to occur at a temperature as low as 200 K. Despite extensive studies in the field, the microscopic mechanism of CO oxidation on Au-based catalysts remains controversial. Aiming to provide insight into the catalytic roles of Au, we have performed extensive density functional theory calculations for the elementary steps in CO oxidation on Au surfaces. O atom adsorption, CO adsorption, O 2 dissociation, and CO oxidation on a series of Au surfaces, including flat surfaces, defects and small clusters, have been investigated in detail. Many transition states involved are located, and the lowest energy pathways are determined. We find the following: (i) the most stable site for O atom on Au is the bridge site of step edge, not a kink site; (ii) O 2 dissociation on Au (O2f2Oad) is hindered by high barriers with the lowest barrier being 0.93 eV on a step edge; (iii) CO can react with atomic O with a substantially lower barrier, 0.25 eV, on Au steps where CO can adsorb; (iv) CO can react with molecular O 2 on Au steps with a low barrier of 0.46 eV, which features an unsymmetrical four-center intermediate state (O-O-CO); and (v) O2 can adsorb on the interface of Au/TiO2 with a reasonable chemisorption energy. On the basis of our calculations, we suggest that (i) O2 dissociation on Au surfaces including particles cannot occur at low temperatures; (ii) CO oxidation on Au/inactive-materials occurs on Au steps via a two-step mechanism: CO+O2fCO2+O, and CO+OfCO2; and (iii) CO oxidation on Au/active-materials also follows the two-step mechanism with reactions occurring at the interface.
Chemical Physics Letters, 2012
The reaction routes between co-adsorbed CO and O on kinked Au(321) slabs are analyzed theoretically. Complexes of vicinal type react most easily with calculated barriers from 0.05 to 0.3 eV, whereas the more strongly co-adsorbed geminal structures do not react directly unless surface oxygen is present in excess. Generally, the activation energy of CO 2 formation from vicinal complexes remains low and that from geminal complexes increases, when Ag impurities are introduced. Our results can be generalized to other rough gold surfaces, particularly, helping to understand the formation of CO 2 above 200 K, as observed in the temperature programmed desorption studies of nanoporous gold, a monolithic nanostructured gold catalyst.
Interaction of Molecular Oxygen with a Hexagonally Reconstructed Au(001) Surface
The Journal of Physical Chemistry C, 2016
Kinetics of molecular oxygen / Au (001) surface interaction has been studied at high temperature and near atmospheric pressures of O 2 gas with in situ x-ray scattering measurements. We find that the hexagonal reconstruction (hex) of Au (001) surface lifts to (1×1) in the presence of O 2 gas, indicating that the (1×1) is more favored when some oxygen atoms present on the surface. The measured lifting rate constant vs. temperature is found to be highest at intermediate temperature exhibiting a 'volcano'-type behavior. At low temperature, the hex-to-(1×1) activation barrier (E act = 1.3(3) eV) limits the lifting. At high temperature, oxygen adsorption energy (E ads = 1.6(2) eV) limits the lifting. The (1×1)-to-hex activation barrier (E hex = 0.41(14) eV) is also obtained from hex recovery kinetics. The pressure-temperature (PT)