RAMAN-SPECTROSCOPY AND THERMAL-DESORPTION OF NH3 ADSORBED ON TIO2 (ANATASE)-SUPPORTED V2O5

The interaction of NH3 with TiO2 (antase) and a series of TiO2 (antase)-supported V2O5 catalysts has been investigated using thermal desorption and in situ Raman spectroscopy. NH3 adsorbs on TiO2(a) by coordinating to Ti4+ Lewis acid sites. Upon heating, most of the adsorbed NH3 desorbs intact, with only a small amount of N2 being produced. At V2O5 loadings well below a monolayer, the dispersed vanadia is present on the form of monomeric vandayl and polymeric vanadate species. NH3 adsorbs on vacant Ti4+ sites not covered by the vanadia and V5+ sites associated with the monomeric vanadyl species. Temperature-programmed desorption of NH3 adsorbed on the V2O5 loaded samples indicates that NH3 is more strongly bound to the surface of these materials and displays a higher tendency to dissociate prior to desorbing. Increasing the coverage of the vanadia species to a monolayer results in a weaker NH3-surface bond. Desorption studies show that substantial quantities of NO and N2O are evolved by oxidation of the NH3 and reduction of the sample. Raman spectra recorded in the presence of NH3 at high temperatures indicate that the terminal oxygen atoms of the polyvanadate species and clustered monomeric species are removed preferentially, resulting in the reduction of these species. On the other hand, isolated monomeric vanadyls are not reduced by the adsorbing NH3. These results suggest that the dissociation of NH3 is accelerated by the presence of adjacent V = O groups.