Selective catalytic reduction of nitric oxide with ammonia: A theoretical ab initio study

Ab initio quantum chemical studies at the HF/Lanl2dz level were carried out to investigate the interaction of ammonia, NO, and a mixture of the two with vanadia/titania. It was found that the replacement of Ti-6c by V-6c is the only feasible way to form highly dispersed vanadia over the titania. The V=O species thus formed will be in octahedral symmetry with the axial distortions, and no tetrahedrally coordinated V species can be formed. Ammonia fully covers the catalyst surface either through the associative interaction with the Lewis acid site of Ti-5c or the dissociative adsorption channels. The dissociation of ammonia on the O site bridging the Ti-6c and V-6c octahedra and on the V=O group can proceed with the highest gain in energy. The formation of an adsorbed ammonium ion was found to be an energetically highly unfavorable process. The V=O group is no longer expected to play a major role in the stabilization of the surface ammonium ion. NO can be activated on the Ti-5c site of the catalyst containing predissociated ammonia on the bridging O site and V=O group. It can be expected that the SCR reaction products are formed through the reactions of both adsorbed NO and NH2 or the desorbed NH2 group with NO in the gas phase. (C) 2001 John Wiley & Sons, Inc.