Unraveling the poisoning effects of different K salts exerted on phosphotungstic acid-modified MnOx for NH3-SCR of NOx

The inevitable presence of various K species in real flue gas may deactivate catalysts used for NH3-SCR of NOx. Herein, the poisoning effects of several K salts (KNO3, K2SO4, KCl) on catalytic performance of phosphotungstic acid-modified MnOx (Mn-HPW) were studied. The extent of deactivation follows the sequence of KC/Mn-HPW > KN/Mn-HPW ≈ KS/Mn-HPW. Especially, KCl decreases significantly SCR activity of Mn-HPW at 90–300 °C, whereas K2O or K2SO4 exerts the negative effect only below 210 °C. The characterization results show that the presence of K species leads to worse reducibility as well as fewer amounts of Mn4+, Mn3+, and chemisorbed oxygen of the Mn-HPW catalyst. The decrease in the quantity and reactivity of active NH3-Lewis and active bridged nitrate is another critical reason for inhibiting NOx elimination at low temperatures on poisoned catalysts. Especially, SO42− in KS/Mn-HPW acts as the spectators in the reaction between active NH3-Lewis with NOx or between active bridged nitrate with NH3, so de-NOx activity of KS/Mn-HPW is comparable to that of KN/Mn-HPW. For KCl/Mn-HPW, the reducibility and relative concentrations of active sites as well as chemisorbed oxygen are the worst. Furthermore, Cl− further restrains the adsorption of active NH3-Lewis, undermining the NOx removal by E-R route; Cl− also hinders the formation of active bridged nitrate, cutting off the reaction through the L-H route. Therefore, KCl exerts the most serious deactivation of Mn-HPW.