N-15 NMR study of the adsorption of NO and NH3 on titania-supported vanadia catalysts

The adsorption of nitric oxide (NO) by ammonia over a series of V2O5/TiO2 catalysts used for selective catalytic reduction (SCR) has been investigated by solid-state N-15 NMR. The chemistry and the surface acidity are found to vary dramatically as the weight loading of vanadia is increased from 0 to 10%. NO reacts over evacuated TiO2 and V2O5/TiO2 at room temperature to form nitrous oxide (N2O). Over the V2O5/TiO2 catalysts N-2 is also produced upon room temperature adsorption. The chemical shift of the terminal nitrogen of N2O adsorbed on the catalyst is an indicator of the Lewis acid character of the surface. The Lewis acidity of the surface is found to decrease with the loading of V2O5 on the titania support. Ammonia adsorbs in two forms on evacuated TiO2 and V2O5/TiO2. These two forms differ in their surface mobility. The reaction of NO with NH3 over the catalysts proceeds to a measurable degree at room temperature. N-2 is the predominant product; however, N2O is also produced by the partial reduction of NO. Following preadsorption of ammonia, the chemical shift of the terminal nitrogen of N2O is similar to that of samples with a high V2O5 loading regardless of the actual vanadia loading. This indicates that ammonia is adsorbed predominately on the Lewis acid sites of the catalyst, blocking the access of N2O to these sites. (C) 1996 Academic Press, Inc.