Kinetic considerations of three-way catalysis in automobile exhaust converters

The activity of three-way catalysts is highly dependent on the reactants present in the automobile exhaust gases (CO, NO_x, HC, O₂, H₂O, CO₂, N₂) as well as their relative concentration. Thus, the influence of each reactant on the kinetic behavior of the whole mixture makes difficult to establish the accurate kinetics of the system.

Activity experiments carried out close to the real operation conditions (GHSV, concentration, etc.) with a Pt/CeO₂/Al₂O₃ catalyst supplied data on the CO and HC oxidation and NO reduction reactions in environments formed by different reactant combinations (from binary mixtures to the whole mixture simulating the real conditions at the automobile converter).

The obtained results have shown notable variations in the oxidation/reduction mechanisms depending on the presence (or absence) of components in the environment. The presence of water always promoted the three-way activity of the catalyst. The compensation effect applied to the CO, NO and HC conversions confirmed that kinetic expressions obtained with partial mixtures (not very close to the real converter environment) have only limited application for determining the whole kinetic scheme occurring in the automobile converters.