An operando DRIFTS study of the active sites and the active intermediates of the NO-SCR reaction by methane over In,H- and In.Pd.H-zeolite catalysts

摘要

Zeolites ln,H-ZSM-5 (Si/Al = 29.7,1.7 wt% In) and In,H-mordenite (In,H-M, Si/Al = 6.7,3.5 wt% In) were prepared by reductive solid state ion exchange (RSSIE) method and studied in the selective catalytic reduction of NO (NO-SCR) by methane.The results suggested that the methane oxidation reactions proceed by redox type mechanisms over In~+/InO~+ sites. The NO reduction selectivity was shown to be related to the relative rates of In~+ oxidation by NO and O2. Regarding the relative rates, the In~+ density of the zeolite was the most important. Above about 6731< the In,H-ZSM-5 (T-atom/In = 102) had higher NO reduction selectivity than the In,H-mordenite (T-atom/In = 46). The operando DRIFTS examinations suggested that NO~+ and NO3~- surface species were formed simultaneously on InO~+Z~- sites, and were consumed together in the NO-SCR reaction with methane. The reduction of the NO3~- by methane gave an activated N-containing intermediate, which further reacted with the NO~+ species to give N2. The NO-SCR properties could be significantly improved by adding small amount of Pd to the ln,H-zeoIite catalyst. The promoting effect of Pd was interpreted as a concerted action of InQ~+ and the Pd~(n+) sites. The interplay between these sites is twofold: the Pd speeds up the equilibration of the NO/O2 mixture, thereby, increases the formation rate and the steady state concentration of the activated nitrate species, whereas the In~+/lnO~+ sites prevent the transformation of Pd-nitrosyls to less reactive isocyanate and nitrile species.