Understanding Oxygen Activation on Nanoporous Gold

Nanoporous gold (np-Au) is a catalytically highly active material, prepared by selectively dealloying silver from a gold-silver alloy. It can promote aerobic CO oxidation and a range of other oxidation reactions. It has been debated whether the remarkable catalytic properties of np-Au are mainly due to its structural features or whether the residual Ag remaining in the material after dealloying is decisive for the activity, especially for the activation of O-2. Recent theoretical studies provided evidence that Ag impurities can facilitate the adsorption and dissociation of O-2 on np-Au. However, these studies predicted quite a high activation barrier for O-2 dissociation on Au-Ag alloy catalysts, whereas experimentally reported activation energies are much lower. In this work we use the stepped Au(321) surface with Ag impurities, which is arguably a realistic model for np-Au material as well as for Au-Ag catalysts in general. We present alternative routes for O-2 activation via its direct reaction with adsorbed CO or H2O. In all of the reactions considered, surface atomic O is generated via a sequence of elementary steps with calculated low activation energies of <0.4 eV with respect to coadsorbed reactants. Ag impurities are shown to increase the adsorption energy of O-2 and hence the probability of a surface-mediated reaction versus desorption. We considered four possible mechanisms of CO oxidation in dry and humid environments in a microkinetic modeling study. We show that via the proposed mechanisms water indeed promotes O-2 dissociation; nevertheless, the dry mechanism, in which CO directly reacts with O-2, is by far the fastest route of CO2 formation on pure Au and on Au with Ag impurities. Ag impurities lead to significantly higher turnover rates; thus, calculations point to the key role of Ag in promoting the catalytic activity of Au-Ag alloy systems.