Formation of reactive Lewis acid sites on Fe/WO₃–ZrO₂ catalysts for higher temperature SCR applications

Tungsten–zirconia (WO3–ZrO2), which oxidises NH3 but shows no NOx-reduction activity, can be converted into an active ammonia-SCR catalyst by impregnation with Fe. The role of Fe in inducing SCR activity has been studied by relating the catalytic performance of tungsten–zirconia materials (containing 0, 0.5, 2, 3 and 10 wt% Fe) to their surface acidity, which has been probed by pyridine adsorption. The most active material, 3 wt% Fe/WO3–ZrO2, reduces NOx by 10-20% at the minimum temperature tested (150 °C), and achieves 80–85% conversion at temperatures between 400 and 550 °C. The performance can be correlated with the formation of new Fe3+ Lewis acid sites that have a pivotal role in the SCR reaction by activating NOx, and which are associated with a characteristic peak shift in the IR spectrum of adsorbed pyridine. The introduction of Fe also has the effect of increasing the strength of the Brønsted acidity, which accounts for the similarity in activity observed between the Fe/WO3–ZrO2 materials and benchmark Fe/beta-zeolite catalysts at higher temperatures.