Solid-state NMR studies of vanadium pentoxide/titanium dioxide catalysts for the selective catalytic reduction of nitrogen oxides with ammonia

Nitrogen oxides (NO_x gases) are detrimental to the environment and their removal from such sources as smokestacks and diesel engines is of great concern. The Selective Catalytic Reduction (SCR) of NO_x gases to nitrogen with ammonia is one method of remediation. Titania-supported vanadium oxides (V₂O₅/TiO₂) are commercially viable deNO_x catalysts.; 2H solid state NMR probed the reaction dynamics of adsorbed ammonia on the surface. Ammonia adsorbs onto the surface of the V₂O ₅/TiO₂ catalyst in two ways. Evidence was found for an ammonia species bound to a Lewis site (undergoing rapid reorientation about the C ₃ axis) and a mobile ammonium ion adsorbed at a Brønsted site.; Heating under evacuation of samples loaded with ammonia resulted in the disappearance of 2H quadrupolar echo NMR and 15N single pulse NMR signal. When such samples were exposed to water vapor, the NMR signals returned. This indicated the presence of an ammonia decomposition species on the surface after heating. This species contained both hydrogen and nitrogen.; 51V NMR was used to characterize the surface of the catalyst as a function of vanadia content and indicated two sites on the surface of the catalyst: 4-fold and 6-fold (distorted octahedra) vanadium. As the V ₂O₅ coverage was increased, there was an increase in 6-fold coordination over 4-fold coordination. The decreased amount of the 4-fold coordinate vanadium (a possible Lewis site) correlates to the redistribution of adsorbed ammonia from a Lewis site adsorbed species to a more mobile ammonium species as the V₂O₅ coverage increased.