Effect of grafting sequence on the behavior of titania-supported v 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 depended only marginally on the sequence of grafting for catalysts with low loadings (up to an experimental monolayer, ca. 10μmol V/m2 and 6 μmol W/m2, 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 catalysts with higher loading (> experimental monolayer), the interaction between vanadia and tungsta species was intensified with increasing calcination temperature, affording higher activity and new species with hydroxyl groups characterized by anIR-band at a frequency ≤3600 cm−1. The formation of weaker acid sites from which ammonia desorbed at temperatures <500 K was foundto be correlated to SCR activity. In contrast, no correlation was observed between the activity and the ease of reduction of the catalysts by ammonia. TOF-SIMS measurements indicated that V-O-W connectivities were present on the V2O5-WO3/TiO2 catalysts, indicating strong interaction between vanadia and tungsta species, which results in a higher activity compared to the corresponding titania-supported single oxides. The studies demonstrate that high activity can be achieved with ternary V2O5-WO3/TiO2 catalysts if the total loading exceeds an experimental monolayer and the catalyst is calcined at 773 K, or with catalysts derived from WO3TiO2 calcined at 1023 K before vanadia deposition.