Comparative study of three-way catalytic performance over Pd/ CeO2-ZrO2-Al2O3 and Pd/La-Al2O3 catalysts: New insights into microstructure and thermal stability

Series of Pd/CeO2-ZrO2-Al2O3 (Pd/CZA) with different pore structures and lanthanum modified Pd/La-Al2O3 (Pd/La-A) were prepared via coprecipitation or hydrothermal method. The obtained catalysts were subjected to TWC activity evaluation, thermal stability tests as well as systematic characterizations including TEM, BET, Rietveld XRD, XPS, H-2-TPR, O2-TPSR, and in situ DRIFTS. Results show that the TWC catalytic performance of Pd/Al2O3 catalyst can be obviously improved by CeO2-ZrO2 modification compared to La2O3 modification, due to the strong interaction between PdOx species and CeO2-ZrO2 support. The excellent thermal stability of Pd/ CZA-2 catalyst is related to its different microstructure compared to Pd/CZA-1, because the abundant dissociated Zr4+ due to phase separation in Pd/CZA-2 could hinder the theta-Al2O3 to alpha-Al2O3 phase transition and retain small sized PdOx species with excellent reoxidation ability. Besides, DRIFTS studies reveal clear structure-activity correlations between the adsorbed reaction intermediates and TWC catalytic performance, i.e. The formation of NO-Pd0 species on fresh Pd/CZA-1 is beneficial for NO conversion, while the rapid adsorption/decomposition of HC and carbonate species on Pd/CZA also contributes to better HC/CO oxidation. Nevertheless, the active sites poisoning by HC strong adsorption along with temperature-stable nitrate species on Pd/La-A cost its inferior catalytic behaviour regardless of thermal ageing.

Automated summary

Through evaluation and characterization of Pd/CZA catalysts and Pd/La-A catalysts, it was found that CeO2-ZrO2 modification can significantly improve the TWC catalytic performance of Pd/Al2O3 catalysts, while Pd/CZA-2 has a different microstructure compared to Pd/CZA-1, which can retain small sized PdOx species with excellent reoxidation ability. In addition, there are clear structure-activity correlations between the adsorbed reaction intermediates and TWC catalytic performance.