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 r.t.: 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 deNOx reactions reported elsewhere.