Current Understanding of Cu-Exchanged Chabazite Molecular Sieves for Use as Commercial Diesel Engine DeNOx Catalysts
Selective catalytic reduction (SCR) of NOx with ammonia using metal-exchanged molecular sieves with a chabazite (CHA) structure has recently been commercialized on diesel vehicles. One of the commercialized catalysts, i.e., Cu-SSZ-13, has received much attention for both practical and fundamental studies. For the latter, the particularly well-defined structure of this zeolite is allowing long-standing issues of the catalytically active site for SCR in metal-exchanged zeolites to be addressed. In this review, recent progress is summarized with a focus on two areas. First, the technical significance of Cu-SSZ-13 as compared to other Cu-ion exchanged zeolites (e.g., Cu-ZSM-5 and Cu-beta) is highlighted. Specifically, the much enhanced hydrothermal stability for Cu-SSZ-13 compared to other zeolite catalysts is addressed via performance measurements and catalyst characterization using several techniques. The enhanced stability of Cu-SSZ-13 is rationalized in terms of the unique small pore structure of this zeolite catalyst. Second, the fundamentals of the catalytically active center; i.e., the chemical nature and locations within the SSZ-13 framework are presented with an emphasis on understanding structure-function relationships. For the SCR reaction, traditional kinetic studies are complicated by intra-particle diffusion limitations. However, a major side reaction, nonselective ammonia oxidation by oxygen, does not suffer from mass-transfer limitations at relatively low temperatures due to significantly lower reaction rates. This allows structure-function relationships that are rather well understood in terms of Cu ion locations and redox properties. Finally, some aspects of the SCR reaction mechanism are addressed on the basis of in-situ spectroscopic studies.