Selective catalytic reduction of NOx with CH4 over zeolite catalysts: Research progress, challenges and perspectives

The elimination of NOx attracts much attention due to the negative effect on the environment, and the selective catalytic reduction of NOx with CH4 (CH4-SCR) was considered as an alternative technology to remove the NOx and CH4 simultaneously. This review summarized the reaction mechanism and SO2/H2O poisoning in the beginning, and discussed the importance of the active sites, oxygen and intermediate species. The highly dispersed metal ions were the main active sites in the CH4-SCR reaction, and the formation of NO2 were the first step of the reaction. The oxygen participated in the production of the NO3- from NO2, which further reacted with CH4 to generate CH3NO2 intermediate species. The CH3NO2 reacted with NO+ to form N2 finally. In addition, the aggregation of CH3NO2 would form CN/NCO, which could also form N2. Subsequently, a detailed review of the represent metal-exchanged zeolite catalysts in the CH4-SCR reaction, including Co-, In-, and other metal-exchanged zeolite catalysts with different structures, were given. The challenges and opportunities for each catalyst were reported, and the efficient strategies were revalued for the enhancement of the SO2/H2O tolerance and low-temperature catalytic activity through introduction of second metal and reducing agent. At last, some suggestions on the future studies could be concentrated on were proposed.