Preparation and characterization of alumina supported Pd-W and Pd-Mo catalysts photogenerated from group VI metal carbonyls

Abstract

Tungsten and molybdenum were introduced in γ-Al2O3 and Pd/γ-Al2O3 using metalcarbonyls [W(CO)6 and Mo(CO)6] photoactivation. DRIFTS FT-IR spectra of the surfaces prior to thermal decomposition reveal the presence of unstable metalsubcarbonyl species, in contrast with the ones found previously, when the photoactivation was performed in the presence of triphenylphosphine. The ν(CO) band intensities did not reflect the tungsten and molybdenum uptakes determined by AAS, the loadings being higher in the presence of palladium. After thermal treatment, the catalysts were further characterized by hydrogen chemisorption and FT-IR spectra of adsorbed CO and NO. The results showed that a reaction between the hexacarbonyls and the supported palladium particles takes place leading to bimetallic catalysts, with clear palladium–tungsten and palladium–molybdenum interactions. Pd+Mo(CO)6⇒uvPd+Mo(CO)6−x⇒Pd-Mo(CO)6−x⇒uvPd-Mo. Both group VI metals strongly modify the chemisorption properties of palladium, while molybdenum seems to be present in a reduced state in close association with palladium.