Understanding the three-way catalytic reaction on Pd/CeO2 by tuning the chemical state of Pd

Abstract

To investigate the correlation of activity and chemical state of Pd species in the three-way catalytic reaction, the supported Pd catalysts were prepared by using CeO2 calcined at different temperatures as supports. It turned out that the chemical state of Pd could severely affect the catalytic activity of Pd/CeO2, which depended on the CeO2 properties and the interaction between Pd and CeO2. With increasing CeO2 calcination temperature from 500 to 1200 °C, the state of Pd gradually transferred from Pd4+ as PdxCe1-xO2-σ to Pd2+ as PdO due to the decreased interaction between Pd and CeO2. Meanwhile, the TOFs of C3H8 oxidation linearly increased and the TOFs of CO oxidation showed a reversed trend, demonstrating the active states of Pd in C3H8 and CO oxidation were different. NO conversion was correlated to the activity of CO oxidation in low-temperature zone and C3H8 oxidation in high-temperature zone during the three-way catalytic reaction. Among them, Pd/CeO2(500) showed the highest CO oxidation activity and low-temperature NO conversion due to the enhanced adsorption ability of CO and NO. The existence of PdO on the surface of high-temperature calcined CeO2 facilitated C3H8 activation and reaction with surface nitrate/nitrite, which promoted NO conversion in high temperature.