K⁺ deactivation of V₂O₅-WO_3/TiO₂ catalyst during selective catalytic reduction of NO with NH₃ : Effect of vanadium content

The effect of vanadium content on the resistance to K⁺ deactivation of V₂O₅-WO₃/TiO₂ SCR catalyst in biomass-fired flue gas was investigated. Catalytic activity and ammonia oxidation were measured, and the properties of fresh and K⁺-deactivated catalysts were characterized by XRD, N₂ physisorption, H₂-TPR, NH₃-TPD and NH₃-DRIFT. The BET surface area decreases with increased vanadium content for both fresh and K⁺-poisoned samples, but it is not responsible for catalyst deactivation. Ammonia oxidation starts from 300 °C and becomes more important with increasing vanadium content and at higher temperature. K⁺ can inhibit ammonia oxidation, but inactivates the newly generated NO₂ to be reduced. The increase of vanadium content reduces mainly the Lewis acid sites, while the amount of BrØnsted acid sites increases. Monomeric and polymeric vanadium are the dominant species on the TiO₂ support, and the amount of isolated vanadyl (V = O) species decreases with V₂O₅ content while the amount of V-OH species in polymeric vanadia increases. Isolated vanadyl species are advantageous to high-temperature catalytic activity while polymeric vanadia species increase ammonia and K⁺ adsorption. The (3 wt% V₂O₅)-WO₃/TiO₂ catalyst shows the best performance for both NO reduction and K⁺ resistance due to it containing both monomeric and polymeric vanadia species (or V = O and V-OH). Catalysts with 3 wt% V₂O₅ are preferable for flue gases with high alkali metal contents. Finally, the mechanism of reaction for different vanadium contents and corresponding K⁺-poisoning are also discussed.