Atomistic insights into the partial oxidation of methanol using gold single crystal surfaces

Abstract

The partial oxidation of methanol to methyl formate (MeFo) proceeds with high selectivity at high conversion on nanoporous gold (np-Au) catalysts. To obtain fundamental insights into the surface processes required for a rational improvement of this catalyst, a surface science approach investigating simplified model catalysts under well-defined single-collision conditions is used. Pulsed molecular beam (MB) experiments allow to study the transient and steady state kinetics under isothermal conditions and thereby for comparison to the applied systems. As np-Au catalysts contain, next to terraces, a high number of low-coordinated surface sites (LCS), the reactivity of flat Au(111) was compared to stepped Au(332) exhibiting (111)-terraces separated by monoatomic steps and thus, a notable number of LCS. These LCS enhance at high temperatures MeFo formation and lower overoxidation, as desired for an ideal partial oxidation catalyst. For low coverage conditions preferential adsorption of reactants at LCS enhances the selectivity for the coupling product, whereas conditions with higher surface coverage, e.g. at low temperatures, lower the MeFo selectivity, as adsorbates can act as obstacles for successful reactant encounters required for the coupling reaction. In addition to LCS, accumulated AuxOy-phases play a crucial role for understanding the observed chemistry. If formed at step sites density functional theory calculations from our collaborators reveal increased barriers for unwanted overoxidation explaining the experimentally observed differences in selectivity. Moreover, MeFo formation may require or benefit from AuxOy-phases, especially at step sites, as indicated by the experimental transient kinetics. As water is an oxidation product and a common (methanol) feed impurity, it may affect under applied multi collision conditions the methanol oxidation on np-Au catalysts. MB experiments on model catalysts conducted with and without added water demonstrated that in case of oxygen rich conditions water has a detrimental effect on MeFo formation rationalized by hydrogen-bonding with methanol as well as by reaction with adsorbed oxygen e.g. affecting the formation of AuxOy-phases and thus their beneficial effect at steps in suppressing overoxidation. Yet, under oxygen-poor, low coverage conditions, the negative effect of water decreases and almost disappears for Au(332) exhibiting a high number of LCS. This allows to propose a rationale for diverse findings for the impact of water on methanol oxidation in liquid and gas phases on np- Au, as these results provide insights into conditions critical for high selectivity in partial oxidation.