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

Abstract

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.