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 NOx 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, H-2-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 Bronsted 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. (C) 2017 Elsevier B.V. All rights reserved.