Noble metal ionic catalysts: correlation of increase in COoxidation activity with increasing effective charge on Pd ion in Pd ion substituted Ce_1−xM_xO_2−δ (M=Ti, Zr and Hf)

Abstract

Pd ion substituted Ce_1−xM_xO_2−δ (M=Ti, Zr, Hf) have been prepared by a single step solution combustion method. Two atom% Pd ion substitution in the title compounds is confirmed by X-ray diffraction (XRD) and Pd ion charge state and redox properties have been determined by X-ray photoelectron spectroscopy (XPS) and H₂/TPR studies. While Pd ion in CeO₂ (Ce_0.98Pd_0.02O_2−δ) showed higher catalytic activity for COoxidation than Pd metal impregnated over CeO₂, further increase in the catalytic activity is observed with Pd ion in Ce_1−xTi(Hf)_xO₂ and a decrease in the activity with Pd ion in Ce_1−xZr_xO₂. Effective charge on Pd ion could be varied by its substitution in these solid solutions Ce_1−xM_xO₂ (M=Ti, Zr & Hf) and also in TiO₂ compared to Pd ion in PdO. Effective positive charge on Pd ion is determined from the core level binding energy shift of Pd(3d_5/2) peak with respect to Pd metal. Rate of COoxidation increased and activation energy decreased with increase in effective charge on the Pd ion in the Pd ion substituted fluorites.