Active States of Pd and Cu in Carbon-Supported Wacker-Type Catalysts for Low-Temperature CO Oxidation

Abstract

Carbon-supported Wacker-type catalysts that showed high activities in CO oxidation at low temperatures were characterized with X-ray diffraction (XRD) and X-ray absorption fine structure (XAFS) techniques in order to correlate the catalytic activity with the structures and electronic states of Pd and Cu. The active states of palladium and copper were found to be a molecular Pd(II) species coordinated with chlorine ions and Cu(II) compounds, respectively, with solid Cu2Cl(OH)3 being the particularly effective copper species. A larger than stoichiometric amount of CuCl2 was required for high activity over the carbon support. The presence of chlorine ion was essential to sustain these active palladium and copper species. The solid copper phase of Cu2Cl(OH)3 was superior to traditional CuCl2·2H2O for reoxidizing the reduced palladium species via Wacker chemistry. The chemical environment of copper appeared to have crucial effects in sustaining the active Pd(II) species, which is required for high activities i...