Nitrogen Oxides Removal with Supported Metals and Oxides

Abstract

Abstract The literature on nontransition/transition metal oxides and platinum group metal (PGM) catalysts for NO x reduction by hydrocarbons under lean‐burn conditions is critically evaluated. Alumina and silver‐promoted alumina catalysts are described in detail with respect to the various reaction mechanisms that have been proposed. The effects of the nature of the reducing agent, and the preparation and structure of the catalysts are also discussed and rationalized for several other systems. For these metal oxides strongly adsorbed species such as nitrates/nitrites and acetates are thought to be key intermediates in the formation of various reduced and oxidized species of nitrogen, whose reaction leads eventually to the formation of molecular nitrogen. For the PGMs a number of different mechanisms have been proposed in the literature and these are critically assessed. It is concluded that although there is indirect evidence for various reaction intermediates it is difficult to confirm that any of these are involved in a critical mechanistic step. A simple mechanism that involves the dissociation of NO on a reduced metal surface to give N (ads) and O (ads), with subsequent desorption of N 2 and N 2 O and removal of O (ads) by the reductant, can explain many of the results with the PGM catalysts.