Impact of toluene poisoning on MnCe/HZSM-5 SCR catalyst

Abstract

Increasing attentions have been drawn towards the simultaneous removal of toluene in current existing NH3-SCR unit. The development of dual-functional catalysts and the understanding of impact induced by toluene addition are momentous but lacking. Herein, a well-reported MnOx-CeO2 catalyst was chosen as active phase to assess the role of HZSM-5 in this simultaneous removal process. HZSM-5 was found to supply more acid sites especially Lewis acid on the MnOx-CeO2 catalyst. A MnCe/HZSM-5 catalyst with a SiO2/Al2O3 molar ratio of 25 delivered a broad active temperature window of 200–400 °C with NO conversion > 80% and possessed a stable N2 selectivity > 96% in NH3-SCR reaction. Also, it was able to maintain above 60% of toluene conversion in the range of 200–400 °C. More Lewis acid sites, together with abundant Mn4+, Ce3+ and active surface oxygen that formed via the interaction between Mn4+/Mn3+ and Ce4+/Ce3+ (Mn3+ + Ce4+ ↔ Mn4+ + Ce3+), contributed to the superior catalytic performance of MnCe/HZSM-5 with lower SiO2/Al2O3 ratio. A reversible catalyst deactivation in NH3-SCR induced by toluene was found. Toluene was prone to reacting with NH3 forming nitriles, which infringed the active sites and decreased the available coordinated NH3 on Lewis acid sites. In addition, the suppression of NO oxidation to NO2 that decreasing the percentage of “fast SCR” process, and the inhibition of NH3 activation to NH2 species also contributed to the lower SCR activity in the presence of toluene over MnCe/HZSM-5.