The poisoning effects of calcium on V₂O₅-WO₃/TiO₂ catalyst for the SCR reaction: Comparison of different forms of calcium

The deactivation of the V₂O₅-WO₃/TiO₂ catalyst for selective catalytic reduction of NO_x with NH₃ (NH₃-SCR) by calcium was investigated via doping of CaO, CaSO₄ and CaCO₃. It is found that the degree of the poisoning for the three kinds of deactivated catalysts follows the order of CaCO₃ > CaO > CaSO₄. For a detailed understanding of the deactivation mechanism, the fresh and poisoned catalysts were analyzed and compared by using XRD, Raman, XPS, H₂-TPR, NH₃-TPD and in situ DRIFTS. The results manifest that the introduction of Ca species not only decreases the BET surface area but also leads to CaWO₄ formation on the surface. CaO and CaCO₃ significantly decrease the surface chemisorbed oxygen, the reducibility of vanadium species, the NH₃ chemisorbed amount, and the intensity of surface acid sites, all important factors leading to lower SCR activity. In contrast, the sulfate species can enhance the concentrations of the surface active oxygen and the Brönsted acid sites, thus partially offsets the negative effect in the loss of active sites by CaO. However, due to its higher reducibility, the surface SO₄²⁻ species also leads to more N₂O production.