Diesel NO x aftertreatment catalytic technologies: Analogies in LNT and SCR catalytic chemistry

Abstract

This paper reviews the main results of wide investigations dedicated to the understanding of the chemistry and the reaction pathways operating in the reduction of NO x in LNT and SCR processes for the aftertreatment of NO x in diesel exhausts. In particular, similarities and differences between the two processes will be highlighted. The reactions involved in the NH 3-NO/NO 2-SCR reacting system were investigated by an extensive set of various unsteady-state experiments performed over both vanadium-based and zeolite-based commercial catalysts: the bulk of results led to the proposal of an original global mechanistic scheme of the complete NH 3-NO/NO 2-SCR reacting system. In such a scheme, a key role is played by nitrite and nitrate species, which are formed by NO 2 disproportion onto catalyst surfaces. Nitrites are readily reduced by ammonia to dinitrogen, whereas the rate limiting step is the reduction of surface nitrates, performed both by NO at lower temperatures and by NH 3 at higher temperatures possibly via formation of nitrites as intermediates in both cases. The systematic study of LNT processes showed that during the lean phase NO x is stored onto the catalyst in the form of nitrite and nitrate species. It was also shown that during the subsequent reduction phase NO x ad-species are reduced to dinitrogen through two consecutive steps in which NH 3 is formed as an intermediate upon reaction of nitrates with H 2, and further reacts with nitrates to selectively produce N 2. Accordingly, in both LNT and SCR chemistries the reduction of NO x involves nitrite and nitrate surface species, which are selectively reduced to nitrogen by ammonia, either formed as an intermediate or supplied as a reactant.