Effect of the Catalyst Redox State on NO2 Adsorption

It was also found that the initial catalyst red-ox state influences the amount of nitrates adsorbed on the catalyst surface, the prereduced sample exhibiting a greater storage capacity (see Fig. 9.3c, d). In line with the literature reports [13], the greater amount of stored nitrates observed on the prereduced samples of both catalysts can be justified invoking the redispersion of Fe and Cu ions during the reduction phase, which thus provides additional sites for nitrates storage. 

Another interesting finding from this study was that, in the case of the Fe-zeoHte, the NO2 adsorption/desorption dynamics over the preoxidized catalyst were essentially unaffected by the presence of gaseous oxygen (see Fig. 9.3a-d). 

A different situation was noted in the case of the Cu-zeolite catalyst, for which a significant effect of the oxygen on the NO2 adsorption dynamics was apparent. Indeed, when oxygen was present in the feed stream together with NO2, a lower amount of NO was produced upon NO2 step feed with respect to what observed 

it seems now well established that the presence of NO2 in the NH3-SCR reacting system involves the formation of nitrates adspecies in significant amounts over both Fe- and Cu-promoted zeolites catalysts. The similarity with the chemistry of NOx storage onto Pt-Ba/Al203 Lean NOx Traps (LNT) has been noted in this respect [16]. At this point, the question is whether such species are just spectators or rather participate actively in the NH3-SCR mechanism. We address this important issue in the next paragraph. 

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