Construction of a Highly Active Rh/CeO₂-ZrO₂-Al₂O₃ Catalyst Based on Rh Micro-Chemical State Regulation and Its Three-Way Catalytic Activity

Abstract

With the increasing number of automobile vehicles, the exhaust emitted poses a severe menace to the environment and human health, necessitating the purification of exhaust pollutants. Meanwhile, the high price of noble metals and their limited supply require a decrease in noble metal loading to reduce the costs of three-way catalysts (TWCs). Therefore, improving the utilization efficiency of noble metals and their catalytic behavior is critical for the development of next-generation TWCs with low noble metal loading. Herein, the Rh micro-chemical state was modulated using the liquid-phase reduction method combined with atmospheric heat treatment to enhance the catalytic behavior of Rh-based catalysts with low Rh loading. The catalyst was characterized using X-ray diffraction (XRD), hydrogen temperature programmed reduction (H2-TPR), CO chemisorption, X-ray photoelectron spectroscopy (XPS), the FTIR spectroscopy of chemisorbed CO (CO-FTIR), transmission electron microscopy (TEM), and in situ diffuse reflectance IR (in-situ DRIFTS) to illustrate the relationship between Rh micro-chemical state (including valence state ratio and dispersion) and catalytic activity. The asprepared catalyst re-Rh/CeO2-ZrO2-Al2O3-H2 (re-Rh/CZA-H2) exhibited better catalytic activity and a wider air/fuel ratio (λ) operating window with T90 values 30-73 °C and 51-86 °C lower than those of the catalysts synthesized by liquid-phase reduction and traditional impregnation method, respectively. In addition, aged samples prepared by the combined scheme also exhibited excellent activity and stability, where the T50 and T90 values were lower than the fresh catalyst. Structureactivity relationship results demonstrated that the better catalytic activity of re-Rh/CZA-H2 could be attributed to the optimal valence state ratio and highly dispersed Rh species, which increased the number of effective active sites. The considerable stability was attributed to the stable structure of the CeO2-ZrO2-Al2O3 (CZA) support, improved dispersion, and the high contents of active Rh species, which exposed more active sites to promote reactant conversion. In addition, the synergistic effect between the metallic Rh and oxygen vacancies could facilitate the anchoring of Rh nanoparticles to inhibit Rh sintering. Therefore, adjusting the micro-chemical state of noble metals by the combinatorial scheme developed herein provides a novel route for improving the catalytic activity, high-temperature stability, and air/fuel operating window of these catalysts.