Unveiling the temperature-dependent effect of Zn on phosphotungstic acid-modified MnOx catalyst for selective catalytic reduction of NOx: A poison at < 180 °C or a promoter at > 180 °C

MnOx-based catalysts are highly efficient for selective catalytic reduction (SCR) of NOx with NH3 at low temperatures, but Zn contained in industrial flue gas may poison conventional SCR catalysts, and MnOx-based catalysts usually have low Zn tolerance. Herein, phosphotungstic acid-modified MnOx (denoted as Mn-HPW) was prepared to study the Zn tolerance. It was found that the deposition of Zn on Mn-HPW decreases the SCR activity at < 180 °C, but both NOx conversion and N2 selectivity are enhanced at 180–300 °C, and > 95% NOx conversion can be achieved on Zn/Mn-HPW. Relevant catalysts were characterized and the results reveal that the decline in the activity at < 180 °C is related to the decrease in the quantities of Mn4+, Mn3+, and chemisorbed oxygen as well as the decreased capacities for adsorbing and activating NH3 and NOx. The unusual improvement of SCR performance on Zn/Mn-HPW at 180–300 °C is mainly attributed to two aspects: (i) HPW can protect effectively the surface acid sites from Zn poisoning; (ii) The decrease in oxidation properties alleviates the over-oxidation of NH3, promoting the participation of more NH3 in SCR.