FTIR Study of Carbon Monoxide Oxidation and Scrambling at Room Temperature over Copper Supported on ZnO and TiO₂. 1

Abstract

An FTIR and quadrupole mass spectroscopic study of CO adsorption and oxidation with 16O2 and 18O2 on copper supported on ZnO and TiO2 is presented. The experimental results indicate that CO is adsorbed on the metallic particles dispersed on both oxides on two kinds of sites, on the normal terrace sites and on sites at the borderline of the particles. Moreover, on titania, a band at 2126 cm-1, assigned to CO adsorbed on isolated Cu atoms and/or on two-dimensional small clusters, is detected. A frequency shift of the bands of CO adsorbed on the metallic particles observed in the CO−O2 coadsorption experiments and the occurrence of a scrambling reaction between CO and 18O2 reveal that on all these samples, there are metallic sites which are able to adsorb at the same time oxygen atoms and carbon monoxide. Carbon dioxide and carbonate-like species are formed: the asymmetric stretching frequencies of CO2 and the quadrupole mass spectroscopic analysis reveal that with 18O2, different isotopic molecular CO2's are formed, while the carbonate-like species have the same frequencies of those produced in 16O2. Moreover, these species are completely lacking in the absence of oxygen in the gas phase. The experimental results indicate that there are, on these samples, two independent pathways for the CO oxidation, a direct oxidation of CO at the surface of the metallic particles and an induced oxidation with the surface lattice oxygen species of the supports.