Ionic dispersion of Pt and Pd on CeO₂ by combustion method: effect of metal-ceria interaction on catalytic activities for NO reduction and CO and hydrocarbon oxidation

Abstract

Ceria-supported Pt and Pd catalysts have been synthesized by the combustion method. The catalysts are characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), and X-ray photoelectron spectroscopy (XPS). Pt and Pd metals are ionically dispersed on the CeO₂ surface of crystallite sizes in the range of 15-20 nm. In 1% Pt/CeO₂ Pt is found to be in the +2 and +4 oxidation states whereas Pd is in the +2 state in 1% Pd/CeO2. Catalytic activities for NO reduction by CO, NH₃, CH₄, and C₃H₈ and CO, CH₄, and C₃H₈ oxidation by O₂ have been investigated over all these catalysts using the temperature-programmed reaction technique. The results are compared with Pt and Pd metals dispersed on α-Al²O₃ support prepared by the combustion technique. Both oxidation and reduction reactions occur at much lower temperatures over M/CeO₂ compared to those over M/Al₂O₃ (M=Pt, Pd). The rate and turnover frequency of NO+CO and CO+O₂ reactions over M/CeO₂ are higher than those over M/Al₂O₃. The observed enhanced catalytic activity of M/CeO₂ has been attributed to the ionic dispersion of Pt and Pd on CeO₂ leading to a strong metal-ceria interaction in the form of solid solution, Ce_1-xM_xO_2-(4-n)x/2, having linkages of the type -O^2--Ce⁴⁺-O^2--Mⁿ⁺-O^2-- (n=2, 4) on the CeO₂ surface.