The role of Pd precursors in the oxidation of carbon monoxide over Pd/Al 2O 3 and Pd/CeO 2/Al 2O 3 catalysts

Abstract

The role of palladium precursors (e.g. chloride — PdCl 2; acetylacetonate — Pd(acac) 2; nitrate — Pd(NO 3) 2) in the catalytic properties of Pd/Al 2O 3 and Pd/CeO 2/Al 2O 3 catalysts toward CO oxidation was herein investigated. The characterization techniques used for mapping the Pd sites were H 2 and CO chemisorption, infrared spectroscopy (FTIR) of CO adsorbed, and temperature-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 CeO 2 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/Al 2O 3 catalysts. Pd(1 0 0) and Pd(1 1 1) were the major palladium crystallite orientations in these samples, but a larger amount of low coordination sites located on Pd(1 0 0) faces was observed for the ex-chloride sample. These sites accounted for the oxidation of CO at very low temperatures on Pd-Cl catalyst. In the presence of ceria, the Pd dispersion was a function of the way in which each Pd precursors interacted with CeO 2. A two-fold decrease of dispersion was obtained to ex-chloride sample, while a two-fold increase to ex-nitrate and the same dispersion to ex-acetylacetonate samples were otherwise observed. The metallic redispersion may be the result of the occupancy of ceria oxygen vacancies by the palladium crystallites. Ultimately, the interaction with ceria redispersed Pd crystallites in a more organized bi-dimensional structure with the predominance of (1 1 1) orientation. Due to the transient conditions of the reaction, ceria did not promote, as expected, the oxidation of CO. Ceria reduced species (Ce 3+) were not able to help 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 temperatures for the Pd/CeO 2/Al 2O 3 catalysts, a result of a combination of strong competition for oxygen molecules, which have replenished the ceria lattice, and the low activity of Pd(1 1 1) sites.