Investigation of palladium interaction with cerium oxide and its state in catalysts for low-temperature CO oxidation

Palladium catalysts supported on nanosized CeO2 supports were synthesized by different methods. The catalysts showed high low-temperature activity (LTA) in CO oxidation. The synthesized palladium-ceria catalysts for low-temperature CO oxidation were investigated by a complex of physicochemical methods, and their catalytic performance was determined in the light-off regime. It was shown using high-resolution transmission electron microscopy (HRTEM) and EDX microanalysis that the catalysts with high LTA are characterized by exceptionally high dispersity of palladium on the surface of the supports. Two different states of palladium were observed by XPS. They correspond to the surface interaction phases (SIPs) as PdxCeO2-delta and small metal clusters (<10 angstrom). According to diffraction images obtained by HRTEM, the latter have flattened shape due to epitaxial binding between (111) facets of palladium and CeO2. Two types of CO adsorption sites (Pd2+ and Pd-0) were distinguished by FTIR. They can be attributed to SIP (Pd2+) and palladium in flat metal clusters (Pd delta+ and Pd-0). The drop of LTA in CO oxidation is related to the loss of the palladium chemical interaction with the surface of the support and palladium sintering to form PdO nanoparticles. The formation of PdO particles is stimulated by crystallization of CeO2 particle surface due to the calcination of support at temperatures above 600 degrees C. The XPS, HRTEM and FTIR data give reliable evidence that PdO particles are not responsible for LTA in CO oxidation. In this work, the structure of the active sites consisting of two phases: atomically dispersed palladium within the SIP and palladium metal nanoclusters is proposed. The catalyst pretreatment in hydrogen was found to improve significantly its catalytic (LTA) properties. The effect of the hydrogen pretreatment was supposed to be related to the formation of hydroxyl groups and their effect on the electronic and geometrical state of the surface active sites and their possible direct participation in CO oxidation. (C) 2009 Elsevier B.V. All rights reserved.