Density Functional Theory Investigation of Oxidation Intermediates on Gold and Gold–Silver Surfaces

Abstract

Gold and gold−silver alloys can be active and selective oxidation catalysts. Previous work has suggested that O 2 dissociation occurs at bimetallic step sites on gold−silver alloys, but the site responsible for the rest of the reaction steps has not been studied. As a first step in gaining insight into this issue, we investigated the adsorption of oxygen and other oxidation intermediates on the ( 111) and ( 211) facets of gold−silver alloys using density functional theory. Oxygen and silver coverage effects were analyzed, and different model structures were compared. We also examined the energy barriers for the diffusion of atomic oxygen to gain insight into O migration and spillover. On (111) surfaces, O adsorption is much stronger at low O coverage (less than 0.22 ML), while on (211) surfaces O is strongly bound at both high and low O coverage. O diffusion across the step is faster than diffusion along the step. Ag stabilizes O, both when directly bound to it and when in an adjacent site. Ag also reduces repulsive O−O interactions at low O coverage. Our calculated reaction barriers for O-assisted CH 3 O dehydrogenation suggest that reaction is faster on steps than on terraces. Overall, our findings suggest that spillover of O from Ag-rich steps to Au-rich terraces does not occur and that oxidation reactions on gold−silver alloys occur on step sites. More specifically, oxidation likely occurs either on Ag-rich step sites or on Au-rich step sites that are adjacent to Ag-rich step sites.

Dilute Alloys Based on Au, Ag, or Cu for Efficient Catalysis: From Synthesis to Active Sites

“…174 DFT studies also suggest that subsequent oxidation steps after O 2 dissociation occur on or near step sites, primarily because O spillover onto pure Au terraces is unfavorable. 175 Comprehensive computational studies employing DFT, cluster expansion, and ab initio MD have found similar conclusions for AgAu(321) as were found for AgAu (211). 176 Without O, the surface is predicted to be pure Au.…”

“…Subsequent experimental studies have supported the finding that reaction conditions induce Ag segregation to the surface and have shown that adding Ag to Au(111) results in no oxidation activity, but adding Ag to curved Au surfaces with steps results in activity . DFT studies also suggest that subsequent oxidation steps after O 2 dissociation occur on or near step sites, primarily because O spillover onto pure Au terraces is unfavorable …”

Reinvestigating oxygen adsorption on Ag(111) by using strongly constrained and appropriately normed semi-local density functional with the revised Vydrov van Voorhis van der Waals force correction

“…Therefore, screening an alternative with a much lower price is imperative. Silver has been proposed as an alternative for electrocatalytic ORR in fuel cells as it has many similar properties to Pt and is more abundant [10][11][12][13] . Moreover, oxygen adsorption is also essential in many disciplines including epoxidation/oxidation for organic synthesis, reducing harmful substances and energy production/storage 10,[13][14][15] .…”

Amorphous Copper‐modified Gold Interface Promotes Selective CO2 Electroreduction to CO

“…The active sites for the rate‐limiting step in electroreduction of CO 2 to CO have been widely studied, while the active sites for subsequent reaction steps after *COOH adsorption step are rarely considered [37] . Hence, the effect of Cu component in AuCu foam was analyzed by comparing the CO intermediates adsorption energy of AuCu foam with pure Au [38] . The calculated results show that the CO intermediates adsorption energy on Au surface was −1.74 eV, of which on AuCu foam surface was −1.37 eV, suggesting that the adsorption structure on Au surface is more stable than that on AuCu foam [39] .…”

“…[37] Hence, the effect of Cu component in AuCu foam was analyzed by comparing the CO intermediates adsorption energy of AuCu foam with pure Au. [38] The calculated results show that the CO intermediates adsorption energy on Au surface was À 1.74 eV, of which on AuCu foam surface was À 1.37 eV, suggesting that the adsorption structure on Au surface is more stable than that on AuCu foam. [39] Therefore, CO is more favorable to desorb from the surface of AuCu foam.…”