Ca Doping Effect on the Competition of NH3–SCR and NH3 Oxidation Reactions over Vanadium-Based Catalysts

Abstract

The Ca poisoning effect constrains vanadium-based catalysts from further application in high-calcium content flue gas, and the deactivation effect of CaO on V2O5–WO3/TiO2 (VWT) and V2O5/TiO2 (VT) catalysts has been investigated from a new perspective. As the NH3 selective catalytic reduction (NH3–SCR) results showed, the NO conversion at 400 °C declined by 71.7% for VT and by 34.8% for VWT after CaO doping, and the decrease in NO conversion for NH3–SCR at temperatures greater than 300 °C was mainly caused by the competition of the NH3 oxidation reaction. The characterization of catalysts by NH3–SCR, XRD, Raman spectroscopy, H2-TPR, NH3-TPD, and TEM showed that the number of surface acid sites decreased and that the oxidation and reduction properties of the catalysts all deteriorated after CaO doping, which is the primary reason for the decrease in NO conversion. CaO also affected NH3 oxidation, and the L-NH3 adsorbed on CaO favored the NH3 oxidation reaction, as DRIFTS showed. NH3 reaction pathways changed by CaO doping because NH3 oxidation becomes more competitive at temperatures greater than 300 °C. Density functional theory (DFT) calculations confirmed the NO formation pathway on CaO, and the competition mechanism of NH3–SCR and NH3 oxidation reactions has been described, which has hardly been reported.