Combined in situ FTIR and on-line mass spectrometric studies have provided simultaneous information of the surface adsorbed species on vanadia/titania catalysts and the composition of reaction products during the selective catalytic reduction (SCR) of NO. The experiments were carried out as temperature programmed surface reaction (TPSR) studies by exposing catalysts with preadsorbed ammonia to either pure NO, pure O-2, Or a mixture of NO and O-2. This allowed detailed information to be obtained concerning the changes in the concentrations and the nature of the surface V=O and V-OH species. The TPSR studies in O-2 showed mainly ammonia desorption and some ammonia oxidation at high temperatures. The SCR reaction was observed to take place during the TPSR studies in both NO and NO + O-2, but a greater rate was observed in the latter case. It was found that NH3 reduces the V=O species and subsequent reaction with NO results in the formation of reduced V-OH species. The results showed that the NO reduction reaction involves the ammonia species adsorbed on V-OH Bronsted acid sites. Evidence for the importance of redox reactions was also found. Separate temperature programmed reduction (TPR) studies in H-2 showed that the surface vanadia layer breaks up while re-exposing TiOH groups. Subsequent temperature programmed oxidation (TPO) studies in O-2 showed this phenomenon to be completely reversible, thus providing direct evidence for spreading/redispersion of vanadia on titania. The TPR/TPO studies also indicated that the Bronsted acid sites essential for the deNO(x) reaction are associated with V5+-OH surface sites. (C) l995 Academic Press, Inc.