Infrared spectroscopic study of ammonia and nitric oxide adsorption and reactivity on a Ru/TiO2 catalyst: effect of oxo-reducing treatments

Abstract

The adsorption and the surface reactions of ammonia and nitric oxide on differently pretreated Ru/TiO₂ samples have been investigated by means of FTIR spectroscopy. On all the samples, both oxidized and reduced, the ammonia is adsorbed at room temperature in an undissociated form on different Ti⁴⁺ surface ions, (at coordinatively unsaturated sites), as on pure TiO₂. By heating at 623 K, water and hydroxyls are formed on the oxidized catalyst and some reduction also occurs; on the reduced sample at 623 K, ammonia decomposes with formation of hydrogen atoms. On all the samples NO is adsorbed on Ru atoms or ions, producing bands in the 1750–1900 cm^–1 range; at the same time, on the reduced samples, NO is dissociated giving N₂O species. Oxidation of ammonia occurs at 573–673 K, producing nitrosylic ruthenium species and nitrates on titania: the latter are transformed to the former by heating at T 473 K. The reduction of NO occurs at 473 K, giving adsorbed ammonia and water on titania. The data suggest a mobility of the adsorbed species between the two phases present in the catalyst.