Reactivity of V_2O_5-WO_3/TiO_2 catalysts in the selective catalytic reduction of nitric oxide by ammonia

摘要

The physico-chemical characteristics and the reactivity of sub-monolayer V_2O_5-WO_3/TiO_2 deNO_x catalysts is investigated in this work by EPR, FT-IR and reactivity tests under transient conditions. EPR indicates that tetravalent vanadium ions both in magnetically isolated form and in clustered, magnetically interacting form are present over the TiO_2 surface. The presence of tungsten oxide stabilizes the surface V~(IV) and modifies the redox properties of V_2O_5/TiO_2 samples. Ammonia adsorbs on the catalysts surface in the form of molecularly coordinated species and of ammonium ions. Upon heating, activation of ammonia via an amide species is apparent. V_2O_5-WO_3/TiO_2 catalysts exhibits higher activity than the binary V_2O_5/TiO_2 and WO_3/TiO_2 reference sample. This is related to both higher redox properties and higher surface acidity of the ternary catalysts. Results suggest that the catalyst redox properties control the reactivity of the samples at low temperatures whereas the surface acidity plays an important role in the adsorption and activation of ammonia at high temperatures.