Experimental and numerical studies on the hydrothermal aging stability and catalytic performance of three-way catalytic converters with different precious metal ratios and loading amounts

Abstract

To understand the influence mechanism of precious metal ratios and loading amounts on three-way catalytic converters (TWCs), hydrothermal aging and catalytic characteristics at an aging temperature of 650–950 °C and a water vapor content (WVC) of 5–20 vol% were investigated using a catalyst sample testing device and micro-characterization tests. The impacting laws of precious metal component parameters on TWC catalytic performance were revealed using one-dimensional zonal coating reactor models. Results indicated that the increasing hydrothermal aging temperature resulted in the aggregation and sintering of active materials, which induced a decrease in the specific surface area and an increase in the pore size and grain size of Zr_0.5Ce_0.5O₂. Oxygen storage ability and metal dispersion homogeneity deteriorated. Thus, high-temperature aging lowered catalytic efficiency. The proper content of water vapor in a hydrothermal aging atmosphere could enhance catalytic activity. However, the influence degree of aging atmosphere on TWC performance was less than that of aging temperature. The increase in precious metal loading amount improved catalytic performance in the fresh state, whereas the specific surface area and pore volume decreased with an increase in loading amount in the hydrothermal aging state. The increasing loading amount intensified active coating accumulation after hydrothermal aging, which lowered hydrothermal aging stability and catalytic efficiency. The existence of Rh improved the structural parameters of samples and the Rh³⁺ content, which enhanced anti-hydrothermal aging ability and NO catalytic conversion. The presence of Pd strengthened oxygen storage and release capacity and anti-sintering ability, which promoted the catalytic conversion of non-methane hydrocarbons (NMHCs). The influence degree of noble metal loading amount on hydrothermal aging stability was higher than that of the noble metal ratio. Numerical results revealed that 10/1 was the best Pd/Rh ratio and loading amount ratio of front and rear zones for improving the catalytic performance. In actual application of TWCs, the selection of loading amount must achieve a relative balance between catalytic performance and the cost of precious metals. The results of this research are helpful to develop high-efficiency, long-life cycle and low-cost TWC after-treatment systems.