Diesel NOx aftertreatment catalytic technologies: Analogies in LNT and SCR catalytic chemistry

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 NOx in LNT and SCR processes for the aftertreatment of NOx in diesel exhausts. In particular, similarities and differences between the two processes will be highlighted.The reactions involved in the NH3-NO/NO2-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 NH3-NO/NO2-SCR reacting system. In such a scheme, a key role is played by nitrite and nitrate species, which are formed by NO2 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 NH3 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 NOx is stored onto the catalyst in the form of nitrite and nitrate species. It was also shown that during the subsequent reduction phase NOx ad-species are reduced to dinitrogen through two consecutive steps in which NH3 is formed as an intermediate upon reaction of nitrates with H2, and further reacts with nitrates to selectively produce N2.Accordingly, in both LNT and SCR chemistries the reduction of NOx involves nitrite and nitrate surface species, which are selectively reduced to nitrogen by ammonia, either formed as an intermediate or supplied as a reactant.