Dissecting the steps of CO2 reduction: 2. The interaction of CO and CO2 with Pd/γ-Al2O3: an in situ FTIR study

Abstract

Alumina supported Pd catalysts with metal loadings of 0.5, 2.5 and 10 wt% were investigated by in situ FTIR spectroscopy in order to understand the nature of adsorbed species formed during their exposure to CO₂ and CO. Exposing the annealed samples to CO₂ at 295 K resulted in the formation of alumina support-bound surface species only: linear adsorbed CO₂, bidentate carbonates and bicarbonates. Room temperature exposure of all three samples to CO produced IR features characteristic of both ionic and metallic Pd, as well as bands we observed upon CO₂ adsorption (alumina support-bound species). Low temperature (100 K) adsorption of CO on the three samples provided information about the state of Pd after oxidation and reduction. Oxidized samples contained exclusively ionic Pd, while mostly metallic Pd was present in the reduced samples. Subsequent annealing of the CO-saturated samples revealed the facile (low temperature) reduction of PdO_x species by adsorbed CO. This process was evidenced by the variations in IR bands characteristic of ionic and metallic Pd-bound CO, as well as by the appearance of IR bands associated with CO₂ adsorption as a function of annealing temperature. Samples containing oxidized Pd species (oxidized, annealed or reduced) always produced CO₂ upon their exposure to CO, while no CO₂-related surface entities were observed on samples having only fully reduced (metallic) Pd.