Ru/ZrO2 catalysts I. O-2, CO, and NO adsorption and reactivity

The Ru/ZrO2 catalyst's surface properties have been studied by quantitative chemisorption experiments on carbon monoxide and oxygen at different tempe ratures, by FTIR studies of oxygen, carbon monoxide, and nitric oxide adsor ptions, and by FTIR studies of the GO-NO reaction. On the oxidized sample a t 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 pi bonding, and the coordination of the Ru atoms with the s upport. 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 ind ication that a significant role is played by the coordination properties an d the surface features of the support. The strong increase of the band at 1 008 cm(-1) at temperatures higher than 573 K can be ascribed to the increas e 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 s ample 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 si gnificant modification of the adsorbed CO spectrum, i.e., a reduction of th e intensity of the band assigned to CO adsorbed on Ru-o sites and the appea rence of additional bands that can be assigned to N2O, NO, and carbonate sp ecies. These features indicate that NO is immediately dissociated on Ru alr eady in the presence of preadsorbed CO and gives rise to N2O and CO2 alread y at RT. Upon heating at 373 K the N2O absorptions and the bands ascribed t o CO adsorbed species are depleted and that of CO2 increases, indicating th at the reaction 2CO + 2NO --> N-2 + 2CO(2) may prevail on the catalyst. (C) 2000 Academic Press.