Article pubs.acs.org/JPCC
How Does Nanoporous Gold Dissociate Molecular Oxygen? Matthew M. Montemore,†,‡ Robert J. Madix,‡ and Efthimios Kaxiras*,‡,§ †
Department of Chemistry and Chemical Biology, ‡John A. Paulson School of Engineering and Applied Sciences, and §Department of Physics, Harvard University, Cambridge, Massachusetts 02138, United States S Supporting Information *
ABSTRACT: Nanoporous Au and other dilute AgAu alloys are highly active and selective oxidation catalysts. Their ability to dissociate O2 is to a large extent unexplained, given that unsupported Au cannot generally dissociate O2, while large ensembles of Ag atoms (>4) are generally necessary to lower the O2 dissociation barrier significantly. Here, we identify a site on the surface of dilute AgAu alloys that is stable under reaction conditions and has a low O2 dissociation barrier, in agreement with experimental measurements. Although Ag generally prefers to disperse throughout Au, the presence of adsorbed O near surface steps creates sites of high local Ag concentration, where the Ag atoms sit in the rows next to the step Au atoms. O2 adsorbs on the Au step atoms, but the transition state involves significant Ag−O interaction, resulting in a barrier lower than expected from the adsorption energies of either the initial or final state.
■
INTRODUCTION Gold is an active and selective catalyst for many oxidation reactions,1−3 but its utility is limited by its inability to dissociate O2 under most circumstances. In fact, formation of adsorbed atomic O is a prerequisite for oxidation on Au.2 Nanoporous Au (npAu), a dilute AgAu alloy, can dissociate O2,2,4 yet its catalytic performance is similar to that of pure Au that has adsorbed O.1−3,5 For example, npAu can catalyze alcohol oxidation with high selectivity, similarly to pure Au. The reason behind npAu’s ability to dissociate O2 remains unclear. While Au nanoparticles can be highly active for oxidation, this has generally been attributed to the Au−support interface,6,7 to very small particles (
