Abstract
The adsorption of H 2 and CO was investigated on SiO 2- and La 2O 3-supported Pd catalysts, and the structure of adsorbed CO was characterized by infrared spectroscopy. For each of the Pd SiO 2 catalysts, the ratio of adsorbed H atoms, or adsorbed CO, to surface Pd atoms is unity. The stoichiometry for atomic adsorption of H 2 on Pd La 2O 3 is also unity, independent of Pd dispersion. By contrast, the 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 x , transferred to the surface of the Pd crystallites during catalyst preparation. The fraction of the Pd crystallite surface covered by LaO x increases with Pd dispersion, in agreement with conclusions based on earlier XPS studies. Infrared studies indicate that the structures of CO adsorbed on Pd La 2O 3 and Pd SiO 2 are similar, but that the strength of adsorption is weaker for Pd La 2O 3 than for Pd SiO 2 . This is attributed to a weakening in the σ-bond component of the PdCO bond due to charge transfer from the LaO x patches to the Pd crystallites. The absence of any suppression of H 2 adsorption of Pd La 2O 3 indicates that H 2 adsorption occurs both on the exposed Pd surface atoms as well as on the LaO x patches covering the balance of the surface Pd atoms.
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