The poisoning effect of alkali metals doping over nano V2O5–WO3/TiO2 catalysts on selective catalytic reduction of NOx by NH3

The nano TiO2 supported V2O5–WO3/TiO2 catalysts (VWTi) were prepared by impregnation method assisted with ultrasonic energy. Their poisoning on activity by alkali metals (Na, K, Ca and Mg) was investigated. The degree of the poisoning effect for all the alkali metal doped catalysts was shown as follows: K > Na > Ca > Mg. Further investigations were carried by NH3-TPD, DRIFTS, XPS and H2-TPR characterization. Na and K could decrease the amount and stability of the Brönsted acid sites to a greater extent than Mg and Ca. Surface chemisorbed oxygen could also be reduced and the downward trend was in good agreement with SCR activity. In addition, the reducibility of vanadium species on the Ca and Mg doped catalysts are higher than that on the Na and K doped catalysts with the same alkali metal oxides loadings. Furthermore, Na and K ions could also affect the reduction degree of tungsten species, while no obvious changes have happened for the tungsten species over Ca- and Mg-VWTi catalysts. Consequently, the different poisoning effects of these four alkali metals doped nano V2O5–WO3/TiO2 catalysts are correlated not only to the surface acidity but also to the reducibility of vanadium and tungsten species.