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

Abstract

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, H2-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 θ-Al2O3 to α-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.