Promoted V₂O₅/TiO₂ catalysts for selective catalytic reduction of NO with NH₃ at low temperatures

The influence of varying the V₂O₅ content (3–6 wt.%) was studied for the selective catalytic reduction (SCR) of nitrogen oxides by ammonia on heteropoly acid (HPA)- and tungsten oxide (WO₃)-promoted V₂O₅/TiO₂ catalysts. The SCR activity and alkali deactivation resistance of HPA-promoted V2O5/TiO2 catalysts was found to be much higher than for WO₃-promoted catalysts. By increasing the vanadium content from 3 to 5 wt.% the catalysts displayed a two fold increase in activity at 225 °C and retained their initial activity after alkali doping at a molar K/V ratio of 0.181. Furthermore, the catalysts were characterized by N₂ physisorption, XRPD, NH₃-TPD, H₂-TPR, Raman, FTIR and EPR spectroscopy to investigate the properties of the catalysts. XRPD, Raman and FTIR showed that promotion with 15 wt.% HPA does not cause V₂O₅ to be present in crystalline form, also at a loading of 5 wt.% V₂O₅. Hence, use of HPAs does not cause increased N₂O formation or unselective oxidation of NH₃. NH₃-TPD showed that promotion by HPA instead of WO₃ causes the catalysts to possess a higher number of acid sites, both in fresh and alkali poisoned form, which might explain their higher potassium tolerance. Ex-situ EPR spectroscopy revealed that HPA-promoted catalysts have higher V⁴⁺/V_total ratios than their WO₃-promoted counterparts. H₂-TPR suggests that HPAs do not have a beneficial effect on the V⁵⁺-V³⁺ redox system, relative to WO₃.