Ru/ZrO2 Catalysts: I. O2, CO, and NO Adsorption and Reactivity

Abstract

The Ru/ZrO2 catalyst's surface properties have been studied by quantitative chemisorption experiments on carbon monoxide and oxygen at different temperatures, by FTIR studies of oxygen, carbon monoxide, and nitric oxide adsorptions, and by FTIR studies of the CO–NO reaction. On the oxidized sample at least three FTIR absorptions are evident at 1008, 980, and 950–940 cm−1. All the observed bands can be assigned to Ru–O surface species, differing in the oxidation state of ruthenium directly bonded to oxygen, the degree of metal–oxygen π bonding, and the coordination of the Ru atoms with the support. The larger heterogeneity of the bands observed on Ru/ZrO2 catalyst, with respect to that previously detected on Ru/ZnO, can be taken as an indication that a significant role is played by the coordination properties and the surface features of the support. The strong increase of the band at 1008 cm−1 at temperatures higher than 573 K can be ascribed to the increase of the surface concentration of Ru–O surface species bonded to an Ru atom with an oxidation state higher than 4. The CO adsorption on the oxidized sample evidences the full reactivity of Ru–O species toward CO between room temperature (RT) and 473. CO and NO chemisorptions on reduced samples show that the two molecules are both molecularly and dissociatively adsorbed on the catalysts. The NO admission on preadsorbed CO produces immediately a significant modification of the adsorbed CO spectrum, i.e., a reduction of the intensity of the band assigned to CO adsorbed on Ru° sites and the appearence of additional bands that can be assigned to N2O, NO, and carbonate species. These features indicate that NO is immediately dissociated on Ru already in the presence of preadsorbed CO and gives rise to N2O and CO2 already at RT. Upon heating at 373 K the N2O absorptions and the bands ascribed to CO adsorbed species are depleted and that of CO2 increases, indicating that the reaction 2CO+2NO→N2+2CO2 may prevail on the catalyst.