Study of the NO 2-NH 3 Interaction on a Titania (ANATASE) Supported Vanadia Catalyst

Abstract

The adsorption and coadsorption of nitrogen dioxide and ammonia on an anatase-supported vanadia catalyst prepared by ion exchange from an acidic aqueous solution of ammonium vanadate are investigated by IR spectroscopy and ESR. The sequence of formation of adsorbed species and the thermal stability of the surface complexes are studied. It is established that the active sites for selective catalytic reduction (SCR) of NO 2 by NH 3 on vanadiatitania catalysts are (i) the V 5+O groups, which interact with NO 2, and, as a result, vanadium is reduced to V 4+, while the nitrogen dioxide is oxidized to a nitrate coordinated to the vanadium (IV); and (ii) the V 5+-OH groups, which react with NO 2, thus giving rise to a strong Brønsted acidity. Ammonia is adsorbed with the participation of the induced Brønsted acid sites as NH + 4, whereas NH 3 coordinatively bound to the Lewis acid sites is displaced by NO 2. The presence of delocalized protons on the sample surface is registered. It is found that these protons play an essential role in the occurrence of the SCR process. Some conclusions are drawn concerning the mechanism of SCR of NO x by NH 3. A scheme is proposed according to which the reaction takes place between a NO − 3 ion, having a more pronounced symmetry, and a NH + 4 ion, both coadsorbed on the catalyst active phase, with NH + 4 NO − 2 appearing as an intermediate.