Promotional effects of phosphotungstic acid on the alkali metals poisoning resistance of MnOx catalyst for NH3-SCR

Abstract

One of the main challenges for NH₃-SCR catalysts in industrial applications is the deactivation caused by alkali metals in the flue gas. In the study, the influences of various alkali metals (Li, Na, K) on the NH₃-SCR activity of Mn-based catalysts were investigated. The findings reveal that the deactivation of Mn-based catalyst decreased in the following order of Na > K > Li after the same mass of alkali metals was deposited. The incorporation of phosphotungstic acid (HPW) significantly enhances the resistance of MnO_x to alkali metals at temperatures below 180 °C. Notably, the SCR performance of HPW modified MnO_x (Mn-HPW) was nearly unaffected by alkali metals within the range of 180–300 °C, converting over 90 % NO_x on poisoned Mn-HPW at 150–270 °C. HPW increases the specific surface area of the poisoned MnO_x catalyst, thereby promoting the adsorption and activation of the reactants on the catalyst surface. Additionally, HPW mitigates the destruction of the acid sites on MnO_x surface by alkali metal species and weakens the redox property of the poisoned MnO_x catalyst, which helps prevent over-oxidation of NH₃ and facilitates NH₃ species involvement in the SCR reaction. Furthermore, HPW enables the surface of the poisoned MnO_x-based catalyst to retain the active nitrate intermediate, contributing to NO_x conversion at low temperatures.