CARBON-MONOXIDE RESTRUCTURING OF PALLADIUM CRYSTALLITE SURFACES

Six palladium on alumina catalysts were prepared with dispersions between 3 and 86%. Over several cycles of carbon monoxide exposure at 25°C and reduction at 300°C, the carbon monoxide, oxygen, and hydrogen adsorption capacities decreased by 20 to 50%. The loss of surface sites was higher on the small crystallites. The adsorption capacity recovered with oxidation at 550°C and reduction at 300°C. The carbon monoxide and oxygen treatments did not affect the hydrogen solubility in the palladium particles. However, infrared spectroscopy of adsorbed carbon monoxide revealed that the surface structure changed with the gas treatments. When the catalyst was converted from the carbon monoxide-exposed state to the oxidized state, the band at 1970 cm-1 for bridge-bonded carbon monoxide on (100) facets decreased, while the band at 2095 cm-1 for linearly bonded carbon monoxide on low-coordination sites increased. In some instances, the bands at 1995 and 1885 cm-1 increased after oxidation. These latter features are tentatively assigned to bridged and threefold-bridged bonded carbon monoxide adjacent to linearly bonded carbon monoxide on low coordination sites. A model is presented to explain these results: cycles of carbon monoxide exposure and reduction smooth the crystallite surfaces into well-ordered facets, whereas cycles of oxidation and reduction roughen the surfaces. © 1990.