Raman spectroscopy and thermal desorption of ammonia adsorbed on titania (anatase)-supported vanadia

Abstract

The interaction of NH{sub 3} with TiO{sub 2} (anatase) and a series of TiO{sub 2} (anatase)-supported V{sub 2}O{sub 5} catalysts has been investigated using thermal desorption and in situ Raman spectroscopy. NH{sub 3} adsorbs on TiO{sub 2}(a) by coordinating to Ti{sup 4+} Lewis acid sites. Upon heating, most of the adsorbed NH{sub 3} desorbs intact, with only a small amount of N{sub 2} being produced. At V{sub 2}O{sub 5} loadings well below a monolayer, the dispersed vanadia is present on the surface in the form of monomeric vanadyl and polymeric vanadate species. NH{sub 3} adsorbs on vacant Ti{sup 4+} sites not covered by the vanadia and V{sup {minus}+} sites associated with the monomeric vanadyl species. Temperature-programmed desorption of NH{sub 3} adsorbed on the V{sub 2}O{sub 5} loaded samples indicates that NH{sub 3} 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 NH{sub 3}-surface bond. Desorption studies show that substantial quantities of NO and N{sub 2}O are evolved by oxidation of the NH{sub 3} and reduction of the sample. Raman spectra recorded in the presence ofmore » NH{sub 3} 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 NH{sub 3}. These results suggest that the dissociation of NH{sub 3} is accelerated by the presence of adjacent V=O groups. 32 refs., 13 figs., 1 tab.« less