Why does there have to be a residual Na ion as a co-cation on Cu/SSZ-13?

Abstract

To reveal the nature of Na ions in Cu/SSZ-13 catalysts from synthesis to on-road use, three industrial related introduction methods were simulated: residual, post-ion-exchanged and impregnated. With Si/Al = 14, similar Cu and Na contents were controlled to examine the effect of differently imported Na ions on the active Cu ions and their catalytic performances. It was proved that Na ions were located on the ion exchange sites of the SSZ-13 zeolites for all Na-contained samples. However, they performed very differently. Post-ion-exchanged Na ions mainly locate on the six-membered ring sites of zeolites, which results in the enrichment of hydrothermally vulnerable [CuOH]⁺ species in the CHA cages. Impregnated Na ions dramatically reduced the number of Cu ions, forming CuO_x species and tiny Cu-aluminate-like species, and consequently varying the rate-limiting step of the SCR reaction at low temperatures. Surprisingly, the residual Na ions not only promoted the hydrothermal stability of Cu/SSZ-13 by preserving Si–OH–Al bonds, but also had the least influence on the active sites. This contributed to the large number of Cu ions and the high standard SCR activities after hydrothermal aging. From an application point of view, the remaining part of the residual Na ions is the best way to tune the acid sites to achieve high catalytic activity and high hydrothermal stability.