A general model for both three-way and deNO(x) catalysis: dissociative or associative nitric oxide adsorption, and its assisted decomposition in the presence of a reductant Part I. Nitric oxide decomposition assisted by CO over reduced or oxidized rhodium species supported on ceria

A selective overview of recent studies on both three-way and deNO(x) cataly sis (we shall note 'deNO(x)', the removal of NO in the presence of an exces s of oxygen) leads to an unique and general model of these reactions, based on kinetic concepts. Two kinds of active sites have to be first defined: c ationic and zero-valent metal ones. The first type can form either through the reduction of the support ((v))-C e3+-((v)) or (ii) a strong metal-support interaction ((v))-Rh+-((v))/CeO2, both linked to adjacent oxygen vacancies ((v)) of the reducible support, or (iii) a surface transition metal (TM) complex (TM in zeolite for instance) . The second kind of sites are accessible supported-zero-valent noble metal atoms. These two kinds of sites are involved in three-way Catalysts (TWC), whereas only the cationic ones concern deNO(x) reactions. Therefore, nitri c oxide chemisorption can be either 'associative' on the first kind of site s - leading to dinitrosyl or hyponitrite species - or 'dissociative', on th e second ones, leading to oxygen and nitrogen atoms adsorbed on the sites. Two different catalytic sequences of elementary steps can then be defined. On cationic sites, successive N-O bond scissions of dinitrosyl or hyponitri te species occur, potentially able to produce intermediate N2O, and in all cases leaving oxygen atoms adsorbed on the active site, and inhibiting a fu rther adsorption of NO. A reductant is then necessary to remove these oxyge n atoms and permit the reaction to proceed further. On zero-valent metallic sites, at the temperature of reaction, NO suffers a dissociative chemisorp tion leading again to surface oxygen atoms. Again, a reductant is necessary to remove these oxygen species and permit the reaction to proceed again. I n this first paper, TWC are considered and the reductant is CO. Two catalyt ic cycles are considered based on our results on temperature-programmed des orption and surface reactions of NO in a stoichiometric CO/NO/O-2 mixture. (C) 2000 Elsevier Science B.V. All rights reserved.