Oxidation of CO and Hydrocarbons in Exhaust Gas Treatments

The present chapter aims to describe the kinetics and mechanisms of CO and HC oxidation in exhaust gas treatments. Attention will be paid to reactions carried out on noble metal catalysts (Pt, Pd, Rh) usually employed in three-way catalysts (spark ignition engines) around stoichiometry. The effect of ceria, usually employed as an oxygen storage material, will also be reviewed. 

Oxidation of CO and hydrocarbons in conditions of exhaust gas conversion (concentrations around 1% for CO and less for HCs) has been widely studied since the implementation of catal5 c converters. Yu Yao was one of the first authors to publish a systematic study of these reactions over Pt, Pd and Rh catalysts in O2 excess. Moreover, the effect of ceria was also investigated, making Yao and Yu Yao s reports a source of important information. Their results will be analyzed and summarized in the first part of this chapter. In the second part, more recent studies will be reviewed with special attention paid to investigation under cychng conditions. 

In TW catalysis, an optimal conversion of all the pollutants (reducers hke CO and HC, and oxidants like NO and NO2) is achieved for an S ratio (defined by Schlatter) close to unity. The S ratio is given in Eq. 1.1, in which chemical formulae represent 

The numerator represents the number of O atoms available in the oxidants (O2 and NOx) while the denominator represents the number of O atoms required for a total oxidation of the reducers CO, HC (alkenes and alkanes) and H2. The Schlatter equation may easily be extended to other HC (aromatics for instance) or oxygenated compounds. However, other gases such as H2O and CO2 are not supposed to react with pollutants, which is not always observed (see Section 1.4). Yu Yao investigated CO and HC oxidation in O2 excess (S = 2). Oxidation reactions were carried out over Pd, Pt and Rh catalysts of different metal loading and dispersions and at different temperamres. In the publications ofYu Yao, the reactions were carried out over bulk metals (wires), alumina-supported catalysts and finally over metals supported on ceria-alumina. Specific rates (per gram of catalyst) were reported as well as activation energies, metal dispersions of supported catalysts or metal area of bulk catalysts. From this information it was possible to calculate turnover frequencies (TOP) extrapolated at a given temperature (the same for every metal catalyst). Metal catalysts are compared on the basis of their TOR 

Oxidation of carbon monoxide (Eq. 1.2) is a reaction which can be catalyzed by all the noble metals usually employed in TW converters but also by many oxides or 

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Compliance with the EuroIII standards (2000) forced the fitting of Diesel oxidation catalysts (DOC) in the exhaust line [for the after-treatment of unburnt hydrocarbons (HC) and carbon monoxide (CO)]. Additionally, the exhaust gas recirculation (EGR) was adapted to reduce the engine-out emissions of nitrogen oxides (NOx). 

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