Theoretical and experimental studies of the nature of the catalytic activity of VOx/TiO2 systems

The states of supported vanadium and the nature of activation of ammonia adsorbed on vanadium sites of VOx/TiO2 catalysts are studied by V-51 NMR spectroscopy and diffuse-reflectance IR Fourier-transform (DRIFT) spectroscopy using cluster quantum chemical calculations of NH3 adsorption. We employ the VOx/TiO2 catalyst of two types: the monolayer catalyst in which vanadium is located on the surface of well-crystallized anatase and the catalyst in which vanadium embedded in the anatase lattice at a rather great depth. It is shown that ammonia is predominantly adsorbed on Lewis acid sites of the monolayer catalyst, whereas most of NH3 adsorbed on the catalyst containing bulk vanadium is in the form of ammonium ions. Analysis of experimental and calculated data suggests that in the monolayer catalyst, NH3 Molecules in the selective reduction of nitrogen oxides are activated on Lewis acid sites. Ammonia activation involves the dissociation of the N-H bond in a coordinated molecule, which results in the formation of the amide V-NH2 group and a water molecule coordinated by a V5+ ion. It is likely that, in the case of the catalyst containing bulk vanadium, this reaction occurs with the predominant participation of ammonium ions.