Methanol oxidation over model cobalt catalysts: Influence of the cobalt oxidation state on the reactivity

X-ray photoelectron and absorption spectroscopies (XPS and XAS) combined with on-line mass spectrometry were applied under working catalytic conditions to investigate the methanol oxidation on cobalt. Two cobalt oxidation states (Co3O4 and CoO) were prepared and investigated as regards their influence on the catalytic activity and selectivity. In addition adsorbed species were monitored in the transition of the catalyst from the non-active to the active state. It was unequivocally shown that the surface oxidation state of cobalt is readily adapted to the oxygen chemical potential in the CH3OH/O2 reaction mixture. In particular, even in rich to oxygen mixtures the Co3O4 surface is partially reduced, while the degree of surface reduction is higher as the methanol concentration in the mixture increases. The reaction selectivity depends on the cobalt oxidation state with the more reduced samples favouring the partial oxidation of methanol to formaldehyde. In the absence of oxygen, methanol is effectively reducing cobalt to the metallic state, promoting also hydrogen and CO production. Direct evidence of methoxy and formate species adsorbed on the surface upon reaction was found by analysing the O 1s and C 1s photoelectron spectra. However, the surface coverage of those species was not proportional to the catalytic activity, indicating that in the absence of surface oxygen, these species might act also as reaction inhibitors.