From three-way to deNOx catalysis: a general model

A general model is proposed for investigating catalytic sequences from “three-way catalysis” (TWC) to “deNOx” reactions, including plasma-assisted deNOx processes. This presentation classifies the various NOx reduction processes, according to the amount of oxygen in the feed (stoichiometric or lean conditions) and, consequently, to the nature of active sites. The model deals with both zero-valent (M0), mainly for TWC, and cationic (Mx+) transition metal (TM) active sites, for both TWC and deNOx reactions. For anyone of the above processes, the basis of the model assumes that scissions of nitrogen–oxygen bonds occur, leading to the formation of N2, and leaving oxygen species strongly adsorbed on the active site. In the case of TWC and stoichiometric conditions, a catalytic coupling occurs between NO decomposition and CO oxidation in the same catalytic cycle, over both M0 and Mx+. In lean conditions, the deNOx process in the presence of TM cations needs three kinetically uncoupled functions and, therefore, three catalytic cycles which have to turn over simultaneously. The formation and role of organonitro compounds, in one of the three catalytic sequences, seem to be obvious, according to the model. From TWC to deNOx processes, N2O is the surface intermediate leading to dinitrogen.