CO oxidation over Pd and Cu catalysts V. Al 2O 3-supported bimetallic PdCu particles

Abstract

Bimetallic Cu-rich (91% Cu) particles dispersed on δ-Al 2O 3 were characterized by chemisorption, in situ IR spectroscopy, and catalytic behavior in the oxidation of CO by O 2. On fresh reduced catalysts hydrogen chemisorption was near zero, and irreversible CO adsorption was low at 300 K but substantially higher at 195 K. Shorter reduction times (3 h) at 573 K produced a strong IR peak near 2120 cm − in the presence of gas-phase CO, which is indicative of CO chemisorbed on an oxygen-covered Cu surface or on Cu 1+ sites, while longer reduction times (18 h) at 573 K shifted the peak to 2105 cm −1, which is near that reported for CO on more open Cu crystal planes. On the reduced bimetallic surface CO bands that could be attributed to multiple coordination Pd sites, i.e., ensembles of two or more Pd atoms, were extremely weak, and the 2070 cm − peak attributable to CO adsorption on singlet Pd sites was either absent or difficult to resolve from the overlap of the 2105 cm −1 band for CO on the Cu atoms. However, under reaction conditions, CO adsorbed on Cu gave a peak between 2115 and 2120 cm −1, and a weaker broadband developed between 1850 and 2000 cm −1 which was attributed to CO multiply coordinated on ensembles of Pd atoms. This behavior clearly showed an interaction between Cu and Pd and demonstrated bimetallic particle formation. The kinetic behavior established that Pd not only was present at the surface but also dominated the reaction at low CO pressures and gave a −0.5 rate dependence on CO. In contrast, the catalytic activity was similar to the Cu-only catalyst at higher CO pressures and the rate dependence on CO shifted to 0.7. Thus, the contribution to the overall rate from each metal component was approximately additive and each was similar to that observed on the single metal catalysts; however, the TOF for Pd atoms in a Cu matrix may be greater due to the higher surface concentration of oxygen provided by the Cu atoms. None of the catalytic behavior appeared to be due to a ligand (or electronic) effect in this bimetallic system.