Temperature-programmed desorption/reaction and in situ spectroscopic studies of vanadia/titania for catalytic reduction of nitric oxide

Abstract

Temperature-programmed desorption (TPD) and temperature-programmed reaction (TPR) studies were conducted to probe the catalytic chemistry of nitric oxide reduction by ammonia on thin films of titania, vanadia, and vanadia/titania at vacuum conditions, as well as on powdered catalysts under flow conditions. The activation energies and enthalpies of ammonia desorption estimated in vacuum and powder TPD studies, respectively, are comparable, consistent with nonactivated ammonia adsorption. The enthalpies of ammonia desorption range from about 18 to 26 kcal/mol over vanadia, vanadia/titania, and titania, with the enthalpy increasing with decreasing vanadia content. In situ IR measurements of the surface ammonia coverages under reaction conditions for the selective catalytic reduction (SCR) of nitric oxide suggest that ammonia adsorption on Lewis acid sites is stronger than on Brønsted acid sites and ammonia adsorption is not rate determining for the SCR reaction. Significant amounts of adsorbed NO are not present on vanadia/titania surfaces under SCR reaction conditions. Vacuum and powder TPR studies indicate that the activation energy for the formation of N2 from NO and NH 3 is about 20 kcal/mol, independent of whether the NO species is strongly adsorbed (under vacuum TPR conditions) or weakly adsorbed (under powder TPR conditions).