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 O2. Recent theoretical studies provided evidence that Ag impurities can facilitate the adsorption and dissociation of O2 on np-Au. However, these studies predicted quite a high activation barrier for O2 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 O2 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 O2 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 O2 dissociation; nevertheless, the "dry" mechanism, in which CO directly reacts with O2, 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.