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

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 nu(s)(NO) and nu(as)(NO) at 1903-1880 and 1769-1753 cm(-1), respectively, and (ii) V3+(NO)(2) complexes, which give rise to nu(s)(NO) at 1834-1822 cm(-1) and nu(as)(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 O-2 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.