Effects of metal-support interactions on the chemisorption of H/sub 2/ and CO on Pd/SiO/sub 2/ and Pd/La/sub 2/O/sub 3/

The adsorption of H/sub 2/ and CO was investigated on SiO/sub 2//sup -/ and La/sub 2/O/sub 3/-supported Pd catalysts, and the structure of adsorbed CO was characterized by ir spectroscopy. For each of the Pd/SiO/sub 2/ catalysts, the ratio of adsorbed H atoms, or adsorbed CO, to surface Pd atoms is unity. The stoichiometry for atomic adsorption stoichiometry for CO decreases linearly from 0.6 to 0 as the Pd dispersion decreased from 30 to 8%. The suppression of CO adsorption is attributed to patches of partially reduced support material, LaO/sub x/, transferred to the surface of the Pd crystallites during catalyst preparation. The fraction of the Pd crystallite surface covered by LaO/sub x/ increases with Pd dispersion, in agreement with conclusions based on earlier x-ray photoelectron spectroscopy studies. Ir studies indicate that the structures of CO adsorbed on Pd/La/sub 2/O/sub 3/ and Pd/SiO/sub 2/ are similar, but that the strength of adsorption is weaker for Pd/La/sub 2/O/sub 3/ than for Pd/SiO/sub 2/. This is attributed to a weakening in the sigma-bond component of the Pd-CO bond due to charge transfer from the LaO/sub x/ patches to the Pd crystallites. The absence of any suppression of H/sub 2/ adsorption of Pd/La/submore » 2/O/sub 3/ indicates that H/sub 2/ adsorption occurs both on the exposed Pd surface atoms as well as on the LaO/sub x/ patches covering the balance of the surface Pd atoms. 53 references, 11 figures, 2 tables.« less