Analysis of oxidation states of vanadium in vanadia–titania catalysts by the IR spectra of adsorbed NO

Abstract

Adsorption of NO on vanadia–titania samples pre-subjected to different reduction treatments has been studied by FTIR spectroscopy. When the NO adsorption is performed at 85 K on oxidized samples, antisymmetric NONO species, typical for V5+ sites, are detected and characterized by bands at 1779 and 1686 cm−1. At ambient temperature, however, adsorption is negligible and only with time reactive adsorption occurs producing NO+ (2120 cm−1), nitro/nitrato species (bands in the 1650–1100 cm−1 region) and weakly adsorbed NO (broad band at 1915 cm−1). Adsorption of NO at ambient temperature on reduced samples results in the formation of two types of species: (i) V4+(NO)2 dinitrosyls characterized by νs(NO) and νas(NO) at 1903–1880 and 1769–1753 cm−1, respectively, and (ii) V3+(NO)2 complexes, which give rise to νs(NO) at 1834–1822 cm−1 and νas(NO) at 1697–1685 cm−1. At low temperature the dinitrosyls are transformed into species in which more than one (NO)2 dimer is attached to one cationic site. Addition of O2 to NO, preadsorbed on reduced vanadia–titania samples, results in a fast oxidation of the V3+(NO)2 species, whereas the V4+(NO)2 complexes are more stable and do not disappear completely in the presence of oxygen. The results obtained suggest that NO is a convenient probe molecule for the analysis of the oxidation state of vanadium in vanadia–titania catalysts. To prevent oxidation of reduced vanadium sites, low equilibrium pressures of NO and registration of the IR spectrum soon after the NO admission are recommended.