(399h) Oxygen Dissociation for Selective Oxidation on Gold: Effect of Alloying and Geometrical Structure

Authors: 
Montemore, M. M., Harvard University
Friend, C. M., Harvard University
Madix, R. J., Harvard University
Kaxiras, E., Harvard University
Au is capable of selectively and efficiently catalyzing oxidation reactions, such as methanol coupling to methyl formate. However, many of these reactions require atomic oxygen to be adsorbed on the surface in order to proceed, which is a challenge since Au cannot normally dissociate O2. Here, we use density functional theory and thermodynamic modeling to examine two routes for dissociating O2 while maintaining high catalytic selectivity.

First, we examine AgAu alloys, motivated by the excellent catalytic performance of nanoporous Au, which is a nanostructured material with a small amount (~2%) of Ag. Nanoporous Au has proven capable of dissociating O2, while maintaining high selectivity in oxidation reactions; however, the active site for O2 dissociation remains unknown. Using atomistic thermodynamics, we find that, under oxidation reaction conditions, the surfaces of dilute AgAu alloys have Au terraces and AgAu bimetallic steps. These bimetallic steps are likely responsible for O2 dissociation, and the calculated barrier on the AgAu(211) structure that we find in the thermodynamic calculations has a low activation barrier for O2 dissociation that is in agreement with experiment.

We also show that the reconstruction of Au surfaces that occurs spontaneously plays an important role in preventing O2 dissociation. For example, the barrier for O2 dissociation on an unreconstructed Au(100) surface is lower than Ag(111) and Ag(110), both of which dissociate O2 easily. Therefore, by preventing surface reconstruction, Au surfaces that can dissociate O2 can be created. This may play a role in the activity of small Au nanoparticles.