A novel supported Mn-Ce catalyst with meso-microporous structure for low temperature synergistic removal of nitrogen oxides and toluene: structure–activity relationship and synergistic mechanism

The design and preparation of highly stable and active bifunctional catalysts that function well over a wide temperature range and resist poisoning is a major obstacle in the dual objective of eliminating nitrogen oxides and toluene from flue gas. In this reported work, a series of supported Mn-Ce catalysts with different molecular sieves support (microporous MnCe/ZSM-5, mesoporous MnCe/AlMCM-41 and micro-mesoporous MnCe/ZSM-5@AlMCM-41) were developed and their reaction performance was evaluated for the process of simultaneously removal of NOx and toluene. Of the catalysts prepared, the MnCe/ZSM-5@AlMCM-41 catalyst appeared to exhibit excellent catalytic activity and good poisoning resistance. The catalysts' structure–activity relationships were studied using a series of characterizations. These characterization test results revealed that the outstanding catalytic activity and poisoning resistance of the MnCe/ZSM-5@AlMCM-41 catalyst were associated with its suitable micro-mesopore proportion, high ratio of Mn4+, Ce3+ and the abundance of various active oxygen groups on the catalyst surface. Further investigation revealed the details of the synergistic mechanism of the catalysts that were responsible for catalytic removal of NOx and toluene. The results of this work showed that 1) appropriate proportions of mesopores in the catalysts mitigated the reaction of toluene or oxidation intermediates with NH3 and NO to form nitrogen-containing intermediate species (benzonitrile and benzamide). This reduced the population of these N-containing intermediate species present at the catalyst's active sites; 2) appropriate proportions of mesopores in the catalysts helped to reduce the reaction between the nitrite/nitrate groups with toluene, thus promoting the utilization of activated nitrate/nitrite species in the NH3-SCR reaction.