Theoretical and Experimental Studies of the Nature of the Catalytic Activity of VOx/TiO2 Systems

Abstract

The states of supported vanadium and the nature of activation of ammonia adsorbed on vanadium sites of VO x /TiO 2 catalysts are studied by 5 1 V NMR spectroscopy and diffuse-reflectance IR Fourier-transform (DRIFT) spectroscopy using cluster quantum chemical calculations of NH 3 adsorption. We employ the VO x /TiO 2 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 NH 3 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, NH 3 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-NH 2 group and a water molecule coordinated by a V 5 + ion. It is likely that, in the case of the catalyst containing bulk vanadium, this reaction occurs with the predominant participation of ammonium ions.