Recent progress in novel zeolite catalysts for selective catalytic reduction of nitrogen oxides

Abstract

Selective catalytic reduction (SCR) using zeolite catalysts is a key technology for the abatement of nitrogen oxides (NOx) from lean-burn heavy-duty vehicles. With the increasing stringency of emission regulations, researchers are now devoting more efforts to enhancing the efficiency and durability of zeolite catalysts. This paper intends to summarize the very recent (mainly in the last five years) advancements in the development and application of novel zeolites as SCR catalysts. Specifically, we first provide a detailed overview of the synthesis, structure-performance relationship, and potential application of small-pore zeolites with different topologies (CHA, AEI, LTA, AFX, SFW, KFI, ERI/EAB, LEV and RHO) as catalysts in SCR with NH3 as the reductant. Then, advances in the post-synthetic modifications of small-pore zeolites for NH3-SCR are reviewed, followed by an introduction of Cu-free small-pore zeolites as NH3-SCR catalysts. We also summarize the recent mechanistic understandings of NH3-SCR chemistry over small-pore zeolite catalysts, mainly over Cu-SSZ-13, using in situ or operando spectroscopy. The application of zeolites as catalysts in SCR with hydrocarbons (e.g., CH4, CH3OH) or H2 as the reductant is also briefly discussed. Finally, we discuss the potential of several novel small-pore zeolites, such as Cu-SSZ-39, for real application in NH3-SCR and the development of new SCR systems with minimized formation of byproducts, such as the greenhouse gas N2O and the toxic HCN.