Photoemission and in situ XRD investigations on CuCoZnAl-mixed metal oxide catalysts for the oxidative steam reforming of methanol

Abstract

A series of CuCoZnAl-multicomponent mixed oxide catalysts with various Cu/Co atomic ratios were prepared by the thermal decomposition of CuCoZnAl-hydrotalcite (HT)-like precursors at 450°C in static air atmosphere. The XRD of calcined materials revealed the formation of a mixture of CuO and ZnO, Co₃O₄, and/or ZnCo₂O₄ depending on the chemical compositions. Core-level XPS and X-ray induced Auger electron spectroscopy (AES) revealed unambiguously the existence of Cu²⁺, Co²⁺, and Co³⁺ species in the calcined materials. Upon H₂-reduction at 300°C, the Cu²⁺ was reduced to a mixture of Cu⁺ and Cu⁰, while Co³⁺ was reduced to Co²⁺. This was further supported by the in situ XRD of the reduced samples. Valence band (VB) photoemission studies demonstrated that the overlap between 3d bands of Cu and Co was marginal in calcined materials, however it was very high in the reduced samples. These mixed oxides were tested as catalysts in the oxidative steam reforming of methanol (OSRM) reaction for the production of H₂ in order to understand the effect of substitution of Co in the CuZnAl-oxide system. The Cu-rich catalyst, without Co, offered the highest H₂ production rate of about 200 mmol kg⁻¹ s⁻¹ with about 100% methanol conversion at 290°C. Only trace amounts of CO were noticed. Introduction of Co decreased the H₂ production rate as the Co-containing species favored the hydrogenation of CO/CO₂ to CH₄ under the reaction operating conditions.