Effect of grafting sequence on the behavior of titania-supported V2O5-WO3 catalysts in the selective reduction of NO by NH3

V2O5-WO3/TiO2 catalysts have been prepared by subsequent, alternating, and simultaneous grafting of vanadia and tungsta onto titania. The performance of these catalysts in the selective catalytic reduction (SCR) of NO by NH3 was compared with corresponding titania-supported single oxides prepared by the same method. For a given composition, the activity of the catalysts de pended only marginally on the sequence of grafting for catalysts with low l oadings (up to an experimental monolayer, ca. 10 mu mol V/m(2) and 6 mu mol W/m(2), respectively). Increase of the calcination temperature from 573 to 773 K decreased the activity of catalysts with low loading. This behavior is attributed to spreading of the vanadia species over the titania surface, resulting in an increase of less active monomeric vanadyl species. For cat alysts with higher loading (>experimental monolayer), the interaction betwe en vanadia and tungsta species was intensified with increasing calcination temperature, affording higher activity and new species with hydroxyl groups characterized by an IR-band at a frequency less than or equal to 3600 cm-l The formation of weaker acid sites from which ammonia desorbed at temperat ures <500 K; was found to be correlated to SCR activity. In contrast, no co rrelation was observed between the activity and the ease of reduction of th e catalysts by ammonia. TOF-SIMS measurements indicated that V-O-W connecti vities were present on the V2O5-WO3/TiO2 catalysts, indicating strong inter action between vanadia and tungsta species, which results in a higher activ ity compared to the corresponding titania-supported single oxides. The stud ies demonstrate that high activity can be achieved with ternary V2O5-WO3/Ti O2 catalysts if the total loading exceeds an experimental monolayer and the catalyst is calcined at 773 K, or with catalysts derived from WO3/TiO2 cal cined at 1023 K before vanadia deposition. (C) 2000 Academic Press.