Fourier transform-infrared study of the adsorption and coadsorption of nitric oxide, nitrogen dioxide and ammonia on vanadia-titania and mechanism of selective catalytic reduction

Abstract

The adsorption of nitric oxide, nitrogen dioxide and ammonia and their coadsorption on vanadia-titania have been studied by FT-IR spectroscopy. Upon nitric oxide adsorption, a surface nitrosyl species is formed rapidly and nitrates are formed slowly by oxidation. Nitrogen dioxide adsorption forms nitrate species by oxidation and nitric oxide. Lewis-bonded molecular species and ammonium ions are formed upon ammonia adsorption. Coordinated ammonia is thermally more stable than ammonium ions and can lose an hydrogen atom to give an amide species. Adsorption on a water-covered sample shows that ammonia displaces water from Lewis sites. Experiments of nitric oxide adsorption on ammonia-covered vanadia-titania show that ammonia poisons the nitric oxide adsorption sites and that NO 3- species are formed by nitric oxide oxidation on vanadyl sites. By heating the ammonia-covered sample in the presence of gaseous nitric oxide coordinated ammonia reacts via the amide species, while ammonium ions do not. The following reaction mechanism is proposed: VO 2+ + NH 3 = [HO-V-NH 2] 2+ NO + [HO-V-NH 2] 2+ = [HO-V-NH 2-NO] 2+ [HO-V-NH 2-NO] 2+ = N 2 + H 2O + [VOH] 2+ [VOH] 2+ +1/4 O 2 = VO 2+ +1/2 H 2O This reaction sequence is believed to operate during the selective catalytic reduction of NO x over vanadia-titania based catalysts.