Regulating Oxygen Activity of Perovskites to Promote Activity and Selectivity for Methanol Oxidation to Formaldehyde

Rational designs of metal oxides are needed to selectively oxidize methanol (CH3OH) to valuable chemicals like formaldehyde (HCHO) and avoid complete oxidation to CO2. Herein, we show that a descriptor based on the oxide electronic structure of catalysts, defined as the surface O 2p-band center relative to the Fermi level, plays an important role in dictating methanol oxidation activity and selectivity toward HCHO for cobalt-based perovskites. Using a combination of ambient-pressure X-ray photoelectron spectroscopy (AP-XPS) and density functional theory (DFT), CH3OH adsorption with the O−H bond scission on surface cobalt sites was found to be more favorable than the O− H bond scission on surface oxygen sites or oxygen vacancy sites. As the O 2p-band center moved closer to the Fermi level in cobalt-based perovskites with greater Sr substitution, the rate-limiting step for CH3OH oxidation kinetics was changed from the adsorption of CH3OH on surface cobalt sites on LaCoO3 (LCO) to O2 adsorption and desorption of H2O on SrCoO3 (SCO). Oxides with intermediate surface oxygen activity such as La0.6Sr0.4CoO3 (LSC64) were shown to provide the highest activity for CH3OH oxidation to HCHO. Such a tuning of oxide surface electronic structure characteristics and speciation can be extended to design catalysts for selective oxidation of other small molecules such as acetic acid, 2-propanol, and propane.