Approaching rational design of Cu/CHA SCR catalysts

Feng  Gao, PNNL

Using metal-exchanged molecular sieves with a Chabazite structure as catalysts, the NOx selective catalytic reduction (SCR) process with ammonia has recently been commercialized on diesel vehicles. This remarkable achievement has led to a surge of research interest in various aspects of these catalysts. With detailed understanding of the CHA structure, acidity and Cu ion locations, rational design of newer generations of catalysts with even better performance becomes possible. In this presentation, we show that by varying Si/Al and Cu/Al ratios, it is possible to synthesize model Cu/CHA catalysts with only one type of catalytic active sites; i.e., Cu2+ sites that interact stronger with the CHA framework, or [Cu(OH)]+ sites that interact more weakly with the CHA framework. From detailed kinetics studies of NH3-SCR, NO and NH3 oxidation reactions, we are able to confirm that both sites are SCR active. Furthermore, while Cu2+ sites maintain excellent SCR selectivities within a wide temperature range, [Cu(OH)]+ sites lose SCR selectivities above 400 C. From such molecular level understanding of the catalytic chemistries, together with the hydrothermal stability requirements for these catalysts, it is now possible to provide guidelines on Si/Al and Cu/Al ratios for the synthesis of Cu/CHA catalysts with optimized activity, selectivity and hydrothermal stability. Using these guidelines, we further show in a case study that, by adding alkali and alkaline earth cocations to low-Cu loaded Cu/CHA, it is possible to create catalysts with excellent SCR selectivities at elevated reaction temperatures (≥ 500 C) even after harsh hydrothermal aging.

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