Development of the Chemical Reaction Mechanism for Oxide Film Formation and Decomposition in Rh-Based Three-Way Catalysts

A full-hybrid system incorporating a spark-ignition engine is regarded as essential for future passenger car powertrains. To achieve effective exhaust-gas purification in such vehicles, it is necessary to elucidate the characteristics of three-way catalysts and to develop both phenomenon-based models and numerical prediction techniques. Previous studies examined the influence of surface oxidation—caused by exposure to oxygen-rich conditions during fuel cut-off in deceleration and engine stop in HEVs—on the purification performance of Rh, one of the most active precious metals used in three-way catalysts. CO–O₂ light-off tests showed that the light-off curve shifted to higher temperatures after lean pretreatment compared with rich pretreatment. XPS analysis further indicated the formation of a reversible oxide film under lean conditions. In this study, the CO–NO light-off test was conducted three times following lean-atmosphere pretreatment. The second and third light-off curves rose markedly faster than the first, yet did not coincide with the curve obtained after hydrogen reduction. Moreover, the second and third curves were nearly identical, suggesting that only part of the oxide film formed under lean conditions may have been decomposed during the CO–NO light-off process. Previous work has also emphasized the importance of maintaining thermodynamic consistency when modeling chemical reactions occurring on catalyst surfaces. By incorporating NO-related surface reactions into the previously developed model and revising parts of the reaction scheme to ensure thermodynamic consistency, the simulation was able to reproduce the formation and decomposition behavior of oxide films and their influence on CO–NO purification performance.