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

Abstract

AbstractThis 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 °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 (Rh0/Rh) and a simultaneous increase in Kubelka–Munk function (ΔKM) at λ=450 nm occurred near the stoichiometric point (A/F=14.6). Metallic Rh became abundant until A/F=14.1, where the largest ΔKM was attained. In the A/F perturbation mode, the Rh oxidation state fluctuated with repetitious switching of rich and lean gas feeds, but the ΔKM 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, Rh0/Rh and ΔKM increased after 200 °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.