K. Hadjiivanov et al., Analysis of oxidation states of vanadium in vanadia-titania catalysts by the IR spectra of adsorbed NO, TOP CATAL, 11(1-4), 2000, pp. 123-130
Adsorption of NO on vanadia-titania samples pre-subjected to different redu
ction treatments has been studied by FTIR spectroscopy. When the NO adsorpt
ion is performed at 85 K on oxidized samples, antisymmetric NONO species, t
ypical for V5+ sites, are detected and characterized by bands at 1779 and 1
686 cm(-1). At ambient temperature, however, adsorption is negligible and o
nly 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 a
nd 1769-1753 cm(-1), respectively, and (ii) V3+(NO)(2) complexes, which giv
e 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 m
ore 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 m
ore stable and do not disappear completely in the presence of oxygen. The r
esults obtained suggest that NO is a convenient probe molecule for the anal
ysis of the oxidation state of vanadium in vanadia-titania catalysts. To pr
event oxidation of reduced vanadium sites, low equilibrium pressures of NO
and registration of the IR spectrum soon after the NO admission are recomme
nded.