La-loaded Al2O3 (La/Al2O3) is a practical support for three-way catalysis (TWC) reactions. Although it has been reported that the addition of La to Al2O3 results in improved thermal stability to retain high specific surface areas, its effect on the catalytic reduction of NOx (DeNOx) has not been studied systematically. Herein, we describe the role of La in La/Al2O3-supported Pd catalysts (Pd/La/Al2O3) for TWC reactions. For that purpose, we employed various in situ spectroscopic studies, including infrared (IR), X-ray absorption fine structure (XAFS), and near-ambient-pressure X-ray photoelectron spectroscopy (NAP-XPS) in combination with density functional theory (DFT) calculations. The obtained results revealed that Pd0 species supported on La/Al2O3 are more electron-deficient compared to those on pristine Al2O3 without La(Pd/Al2O3). Kinetic studies using powdered catalysts revealed that the addition of La suppresses the poisoning effect by CO during the DeNOx reactions. In addition to the catalytic tests with powdered catalysts, monolithic honeycomb forms of the catalysts were prepared and employed for TWC reactions, which showed that Pd/La/Al2O3 exhibits higher DeNOx activity than Pd/Al2O3. In this study, we also reexamined the effective loading amount of La, which has traditionally been ∼3-5 wt % of La for TWC processes in order to retain the high specific surface area of the La/Al2O3 supports. Our investigations showed that an increased La loading (15 wt %) is even more effective for the DeNOx reactions tested in this study due to the higher reactivity toward NO and the greater suppression of the poisoning effect of CO. The developed catalyst Pd/La(15)/Al2O3 has also been tested in a commercial vehicle and has been evaluated on a practical driving mode test cycle (LA-4; city cycle of U.S. Federal and California), where it showed a better catalytic performance than the conventionally used Pd/La(3-5)/Al2O3 catalysts. Our study suggests that the loading amount of La in Pd/La/Al2O3 catalysts needs to be adjusted depending on the application systems, considering not only the support stability (surface areas) but also the promotional effect in the TWC process.
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