Why has to be residual Na as co-cation on Cu/SSZ-13?

2020 
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, that is, residual, post-ion-exchanged and impregnated. Si/Al = 14, similar Cu and Na content were controlled to examine the effect of differently imported Na ions on the active Cu ions and catalytic performance. It is proved that Na ions locate 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 six-member-ring sites of zeolites, which results in the enrichment of hydrothermally-vulnerable [CuOH]+ species in CHA cages. Impregnated Na ions dramatically reduced the number of Cu ions, forming CuOx species and tiny Cu-aluminate-like species, and consequently varying the rate-limiting step of the SCR reaction at low temperature. Surprisingly, 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. To the view of applications, remaining part of residual Na ions is the best way to tune the acid sites to achieve high catalytic activity and high hydrothermal stability.
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