XPS, XANES and EXAFS investigations of CuO/ZnO/Al₂O₃/ZrO₂ mixed oxide catalysts

Abstract

Systematic X-ray photoelectron spectroscopy (XPS), X-ray induced Auger electron spectroscopy (AES), X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) studies were undertaken to investigate the electronic structure, chemical states and local geometry of the active species in the CuO/ZnO/Al₂O₃/ZrO₂ multicomponent mixed oxide catalysts employed in the oxidative steam reforming of methanol (OSRM) reaction for H₂ production. The core level XPS and AES indicated the existence of CuO and ZnO-like species. Two kinds of zirconium species, one similar to that of ZrO₂ and another with relatively higher electron density were noticed from the Zr 3d core level XPS of Zr- containing catalysts. The valence band (VB) XPS studies revealed that for Zr-containing catalysts, the Cu 3d anti-bonding orbital splits from the main VB and shifts toward lower binding energy (BE). The surface Cu/(Al+Zr) ratios were found to be close to those in the bulk while segregation of Zn at the surface was evidenced in all samples. The XANES and EXAFS results also indicated the existence of CuO and ZnO-like species, whose local environments are modified with respect to the chemical composition. The EXAFS study of the Zr-containing catalysts indicated the existence of a "Cu-O-Zr" bonding with a Cu-Zr distance in the range 3.5 to 3.9 Å. The results indicated the existence of a Cu-Zr synergistic interaction in these catalysts which improved the catalytic performance in the OSRM reaction.