Microscopic insights into the partial oxidation of methanol on gold using Au(332) as a model system

Nanoporous gold (np-Au) is a pure metal-based catalyst, which forms methyl formate very selectively in the partial oxidation of methanol. Although several model studies under ultrahigh vacuum as well as quantum mechanical calculations attempted to provide insight into the structural and catalytic properties of np-Au, the understanding of the factors leading to the high activity and selectivity remain incomplete. In this work, a single crystalline Au(332) surface was used to model structural properties like low index (111) facets as well as low coordinated Au atoms. The experiments were conducted under well-defined UHV conditions by pulsed, isothermal molecular beam (MB) experiments combined with in situ infrared reflection adsorption spectroscopy (IRAS) as well as temperature programmed reaction (TPR) experiments. Using CO as a probe molecule applying isotopically diluted gas mixtures to prevent dipolar coupling between CO molecules, the several adsorption sites on Au(332) were spectroscopically evidenced and assigned by a combination with DFT calculations. In isothermal MB experiments on the partial methanol oxidation to methyl formate, varying the surface temperature as well as the methanol and oxygen fluxes revealed two distinct surface deactivation processes for methyl formate formation due to the formation of formate and due to impurities in the used methanol. Low coordinated sites were found to form methyl formate at a higher rate compared to other sites on the Au(332), while other reaction steps, e.g. formation of methoxy proceed also effectively on terrace sites and with gold-oxygen (AuOx) phases. TPR results demonstrated unwanted oxidation of methyl formate, the desired partial oxidation product in methanol oxidation, to occur on Au(332) already at low temperatures and even for low oxygen coverages, as expected for typical reaction conditions on np-Au. Three different reaction mechanisms for CO2 formation from methyl formate were identified by isotopic labeling experiments, which were connected to specific oxygen species on minority sites. Due to its dependence on oxygen at special sites, the methyl formate oxidation is slow under isothermal conditions compared to the oxidation of methanol which provides a microscopic understanding of the high selectivity of np-Au on the formation of methyl formate