Recent progress of metal-exchanged zeolites for selective catalytic reduction of NOx with NH3 in diesel exhaust

In recent years, selective catalytic reduction (SCR) with NH3 technology has been extensively applied in the removal of nitrogen oxide (NOx) from diesel exhaust due to its high efficiency and low cost. Catalysts are integral in the NH3-SCR reaction, and metal-exchanged zeolites have been studied in-depth owing to the significant SCR performance and high stability. In the present review, the developing progress of zeolite catalysts adopted in the SCR reaction is summarized, including Cu, Fe, Mn and other metal-exchanged zeolites. The review begins with a brief introduction of reaction mechanism, hydrothermal ability and poisoning mechanism of SO2/H2O, alkali metals, alkali earth metals, and phosphorus. The active sites of different metal-exchanged zeolites in the NH3-SCR reaction are discussed. In addition, the role of second metals and metal oxides, and effects of preparation methods and topologies are emphasized for each type of catalysts. Finally, the challenges and further perspectives of the metal-exchanged zeolites in the NH3-SCR reaction are proposed to promote the SCR efficiency of removing NOx.