In situ XPS study of methanol oxidation over a copper catalyst derived from layered double hydroxides

Abstract

Copper nanoparticles supported on alumina have been synthesized from CuAl-layered double hydroxide by heat treatment at 450 °C and have been characterized by thermal analysis, X-ray diffraction, transmission electron microscopy, and X-ray photoelectron spectroscopy. Methanol oxidation at different molar ratios of the components of the reaction feed (CH₃OH : O₂ = 1 : 1, 3 : 1, 6 : 1 and 1 : 3, P_total = 0.012 mbar) over the prepared CuAlO_x catalyst has been studied by in situ X-ray photoelectron spectroscopy and mass-spectrometry. It was revealed that methanol oxidation takes place at temperatures higher than 250 °C. The conversion of methanol and selectivities towards CH₂O and CO₂ were found to depend on the molar ratio of the reaction mixture components and reaction temperature. A quantitative estimation of the surface composition for the CuAlO_x catalyst under different experimental conditions was performed based on the deconvolution of copper Auger spectra using a linear combination of individual components. It was shown that treatment of the as-loaded CuAlO_x catalyst in hydrogen at 300 °C led to a partial reduction of Cu⁺ to Cu⁰. However, there were only Cu⁰ species on the surface in the CH₃OH : O₂ = 1 : 1 reaction mixture at 400 °C indicating the formation of active sites directly during the catalytic reaction. Metallic copper was shown to be an active component for the production of formaldehyde under the conditions of methanol excess in the reaction mixture.