Catalysis Science of NOx Selective Catalytic Reduction With Ammonia Over Cu-SSZ-13 and Cu-SAPO-34

Copper-exchanged, small-pore chabazite (CHA) zeolites were commercialized in 2009 for the selective catalytic reduction (SCR) of NOx compounds with ammonia, as an emissions control strategy in diesel automotive exhaust aftertreatment. Here, we review the fundamental scientific advances that have since been made in the molecular-level understanding of the active sites and mechanisms responsible for NOx SCR with NH3 on Cu-CHA catalysts. A large body of experimental and theoretical characterization has identified that these "single-site" catalysts contain Cu sites of different local coordinations and structures, influenced by synthetic and environmental factors. The speciation of isolated Cu ions is inextricably linked to the support composition and the conditions of exposure. We make new and unifying connections among the seemingly disparate findings of experimental investigations of Cu-CHA catalysts that differ in origin and treatment history, using ex situ and in situ characterizations, and operando characterization during catalysis. We discuss theory-based studies, in conjunction with multiple experimental spectroscopic methods performed on model Cu-CHA catalysts, that provide precise molecular assignments across a wide range of catalysts and conditions. We highlight how molecular-level descriptions of the active sites and mechanisms can provide insight into the chemical factors that influence practical SCR performance and behavior, including the onset of low-temperature NOx conversion, and the critical role of NH3 solvation of Cu active sites for low-temperature activity. Finally, we describe how the fundamental heterogeneous catalysis science approaches used to interrogate Cu-CHA catalysts used for NOx SCR with NH3 can be used to elucidate the molecular-level details of chemistry that occurs on other single-site catalysts.