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 wi th vanadia/titania. It was found that the replacement of Ti-6c by V-6c is t he 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 dist ortions, and no tetrahedrally coordinated V species can be formed. Ammonia fully covers the catalyst surface either through the associative interactio n 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 oct ahedra and on the V=O group can proceed with the highest gain in energy. Th e formation of an adsorbed ammonium ion was found to be an energetically hi ghly unfavorable process. The V=O group is no longer expected to play a maj or role in the stabilization of the surface ammonium ion. NO can be activat ed on the Ti-5c site of the catalyst containing predissociated ammonia on t he 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 th e desorbed NH2 group with NO in the gas phase. (C) 2001 John Wiley & Sons, Inc.