Dynamic Change of Rh Oxidation State During Lean-Rich Perturbation and Light-Off of Three-Way Catalysis Analysed using In Situ Diffuse Reflectance UV-vis Spectroscopy

This study investigated the redox behaviour of Rh in three-way catalysts (TWC) using time-resolved in situ diffuse reflectance UV-vis spectroscopy in different modes with air-to-fuel ratio (A/F) step, A/F perturbation, and temperature ramping. When Rh/ZrO2-CeO2 (Rh/ZC) and Rh/ZrO2 (Rh/Z) containing fully oxidised Rh were treated at 400 & DEG;C under simulated TWC conditions with A/F stepping from 15.0 (fuel-lean) to 14.1 (fuel-rich), a steep increase in the metallic fraction (Rh-0/Rh) and a simultaneous increase in Kubelka-Munk function (& UDelta;KM) at & lambda;=450 nm occurred near the stoichiometric point (A/F=14.6). Metallic Rh became abundant until A/F=14.1, where the largest & UDelta;KM was attained. In the A/F perturbation mode, the Rh oxidation state fluctuated with repetitious switching of rich and lean gas feeds, but the & UDelta;KM oscillation amplitude was lessened for Rh/ZC compared to Rh/Z because of the oxygen-scavenging function of ZC. Furthermore, the changes in Rh oxidation state during TWC light-off were analysed in the temperature-ramping mode. Under rich conditions, Rh-0/Rh and & UDelta;KM increased after 200 & DEG;C, where the formation of Rh metal led to efficient conversions of CO and NO. However, metallic Rh did not appear during light-off under the lean condition, where NO conversion was negligible.

Automated summary

This study investigated the redox behaviour of Rh in three-way catalysts using time-resolved in situ diffuse reflectance UV-vis spectroscopy. The results showed that the metallic fraction of Rh increased with changing air-to-fuel ratio, and the Kubelka-Munk function also increased near the stoichiometric point. This study provides insights into the oxidation state of Rh in TWC and its impact on catalytic performance.