The role of Pd precursors in the oxidation of carbon monoxide over Pd/Al2O3 and Pd/CeO2/Al2O3 catalysts

The role of palladium precursors (e.g. chloride - PdCl2; acetylacetonate - Pd(acac)(2); nitrate - Pd(NO3)(2)) in the catalytic properties of Pd/Al2O3 and Pd/CeO2/Al2O3 catalysts toward CO oxidation was herein investigated. Th e characterization techniques used for mapping the Pd sites were H-2 and CO chemisorption, infrared spectroscopy (FTIR) of CO adsorbed, and temperatur e-programmed desorption (TPD). Unsteady-state CO oxidation was carried out by temperature-programmed surface reaction (TPSR). The nature of palladium precursors and their interaction with CeO2 affected the metallic dispersion and the site morphologies. Highly dispersed metal particles (d > 50%) were obtained by using palladium chloride and acetylacetonate precursors on Pd/ Al2O3 catalysts. Pd(1 0 0) and Pd(1 1 1) were the major palladium crystalli te orientations in these samples, but a larger amount of low coordination s ites located on Pd(1 0 0) faces was observed for the ex-chloride sample. Th ese sites accounted for the oxidation of CO at very low temperatures on Pd- CI catalyst. In the presence of ceria, the Pd dispersion was a function of the way in which each Pd precursors interacted with CeO2. A two-fold decrea se of dispersion was obtained to ex-chloride sample, while a two-fold incre ase to ex-nitrate and the same dispersion to ex-acetylacetonate samples wer e otherwise observed. The metallic redispersion may be the result of the oc cupancy of ceria oxygen vacancies by the palladium crystallites. Ultimately , the interaction with ceria redispersed Pd crystallites in a more organize d bi-dimensional structure with the predominance of(1 1 1) orientation. Due to the transient conditions of the reaction, ceria did not promote, as exp ected, the oxidation of CO. Ceria reduced species (Ce3+) were not able to h elp CO oxidize at lower temperatures due to lack of oxygen into its lattice . Thus, the highest rates for CO oxidation were only observed at higher tem peratures for the Pd/CeO2/Al2O3 catalysts, a result of a combination of str ong competition for oxygen molecules, which have replenished the ceria latt ice, and the low activity of Pd(1 1 1) sites. (C) 2001 Elsevier Science B.V . All rights reserved.