Impact of toluene poisoning on MnCe/HZSM-5 SCR catalyst
Increasing attentions have been drawn towards the simultaneous removal of toluene in current existing NH 3 -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 MnO x -CeO 2 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 MnO x -CeO 2 catalyst. A MnCe/HZSM-5 catalyst with a SiO 2 /Al 2 O 3 molar ratio of 25 delivered a broad active temperature window of 200–400 °C with NO conversion > 80% and possessed a stable N 2 selectivity > 96% in NH 3 -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 Mn 4+ , Ce 3+ and active surface oxygen that formed via the interaction between Mn 4+ /Mn 3+ and Ce 4+ /Ce 3+ (Mn 3+ + Ce 4+ ↔ Mn 4+ + Ce 3+ ), contributed to the superior catalytic performance of MnCe/HZSM-5 with lower SiO 2 /Al 2 O 3 ratio. A reversible catalyst deactivation in NH 3 -SCR induced by toluene was found. Toluene was prone to reacting with NH 3 forming nitriles, which infringed the active sites and decreased the available coordinated NH 3 on Lewis acid sites. In addition, the suppression of NO oxidation to NO 2 that decreasing the percentage of “fast SCR” process, and the inhibition of NH 3 activation to NH 2 species also contributed to the lower SCR activity in the presence of toluene over MnCe/HZSM-5.
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