VANADIA-TITANIA CATALYSTS FOR SELECTIVE CATALYTIC REDUCTION (SCR) OF NITRIC-OXIDE BY AMMONIA .1. COMBINED TEMPERATURE-PROGRAMMED IN-SITU FTIR AND ONLINE MASS-SPECTROSCOPY STUDIES

Combined in situ FTIR and on-line mass spectrometric studies have prov ided simultaneous information of the surface adsorbed species on vanad ia/titania catalysts and the composition of reaction products during t he selective catalytic reduction (SCR) of NO. The experiments were car ried 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 ma inly ammonia desorption and some ammonia oxidation at high temperature s. 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 latte r case. It was found that NH3 reduces the V=O species and subsequent r eaction with NO results in the formation of reduced V-OH species. The results showed that the NO reduction reaction involves the ammonia spe cies adsorbed on V-OH Bronsted acid sites. Evidence for the importance of redox reactions was also found. Separate temperature programmed re duction (TPR) studies in H-2 showed that the surface vanadia layer bre aks up while re-exposing TiOH groups. Subsequent temperature programme d oxidation (TPO) studies in O-2 showed this phenomenon to be complete ly reversible, thus providing direct evidence for spreading/redispersi on 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.