Infrared studies on SnO2 and Pd/SnO2

The peculiarities of the IR studies of adsorption on semiconducting oxides are reviewed with special attention to SnO2-Absorption by free electrons is very strong in reducing atmospheres and well-defined conditions are required to get vibrational information on the nature of surface species. CO adsorption on SnO2 at room temperature (r.t.) gives rise to CO-Sn2+ and CO-Sn4+ end-on species and to various carbonate entities. NO adsorption leads to the formation of mononitrosyls, nitrite and nitrate species. Pd/SnO2 catalysts are prepared according to original procedures: photodeposition and grafting of a molecular complex; the grafting method leads to highly dispersed PdO which is reduced to well-dispersed Pd upon CO adsorption at r.t., whereas NO gives rise to nitrosyl species mainly adsorbed on ionic Pd. The specificity of SnO2 is evidenced upon interaction of a stoichiometric (CO + NO) mixture at rt.: formation of N2O on SnO2 alone, generation of N2O and isocyanate on PdO/SnO2. These results are explained by the presence of oxygen vacancies and may be related to the low-temperature catalytic activity of SnO2-supported palladium in deNO(x) reactions reported elsewhere. (C) 2001 Elsevier Science B.V. All rights reserved.