Influence of cerium precursor on the size and solid solubility of transition metals in combustion synthesized Ce_1-xM_xO_2-d (M = Cu, Pd) nano crystallites: enhancement of redox properties and catalytic activity

Abstract

Two cerium precursors have been employed to synthesize Ce_1-xM_xO_2-d (M = Cu, Pd) nano-crystalline catalysts by solution combustion method. These nano materials have been characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM) and BET surface area measurement. Redox behaviour of the catalysts was investigated by temperature-programmed reduction (H₂ TPR). Use of ceric ammonium nitrate (precursor A) gives ceria crystallites of size 30-45 nm while cerrous nitrate (precursor B) gives 10-15 nm sized crystallites indicating three fold decrease in crystallite size of CeO₂. Solid solubility of copper in CeO₂ matrix is between 5-10 atom % with precursor A and it increases to 10-15 atom % with precursor B. The crystallite size is between 15-20 nm using B precursor compared to 40-50 nm crystallites with precursor A when Pd was substituted for Ce indicating approximately a two-fold decrease. With precursor A, solid solubility of Pd in ceria is between 3-5 atom % which increases to ~10 atom % with precursor B. Structural study shows M²⁺ ion substitution into CeO₂ matrix. The enhancement of solid solubility coupled with the decrease in crystallite size is associated with an increase in oxygen storage capacity and catalytic activity for CO oxidation by O₂ and NO reduction by CO. Such an approach to synthesis of transition metal ion substituted nano CeO₂ is leading to catalysts with higher catalytic activity.