New insight into vanadium phosphorus oxides (VPO/TiO2) catalysts for NOx removal with NH3-SCR at low temperature: Relationship between non-VOx phases and SO2&H2O resistance

Abstract

Enhancing SO2&H2O resistance has important environmental implications for the NH3-selective catalytic reduction (NH3-SCR) catalysts in NOx removal. Herein, vanadium phosphorus oxides (VPO) were synthesized as active phases for NH3-SCR catalysts. Non-VOx phases containing (VO)2P2O7 and δ-VOPO4 were generated by adjusting the P/V molar ratio and the atmosphere properties of the activation and calcination. The NOx conversion of the VPO-1.5/TiO2 sample did not decrease (about 90%) after 6 h exposure to 180 °C, 500 ppm SO2 and 6% H2O. The abundant acid content (especially increased content of the weak Brønsted acids) corresponding to the non-VOx phases significantly inhibited stable SO2 adsorption on the catalyst surface. In-situ DRIFTS results showed that SO2 + O2 pre-adsorption on the catalyst surface had no significant influence on the adsorbed NH3 species. Density functional theory calculation also proves that SO2 has low competitive adsorption activity with NH3 and NO in the non-VOx phases. Under more severe conditions, the non-VOx phases still minimized the influence of SO2&H2O poisoning on surface acidity by inhibiting the accumulation of ammonium bisulfate. This work proposes a mechanism for enhancing SO2&H2O resistance with non-VOx phases, which is essential for extending the application of VPO/TiO2 catalysts from alkane selective oxidation to NH3-SCR.