No abstract

La-loaded Al 2 O 3 (La/Al 2 O 3 ) is a practical support for three-way catalysis (TWC) reactions. Although it has been reported that the addition of La to Al 2 O 3 results in improved thermal stability to retain high specific surface areas, its effect on the catalytic reduction of NO x (DeNO x ) has not been studied systematically. Herein, we describe the role of La in La/Al 2 O 3 -supported Pd catalysts (Pd/La/Al 2 O 3 ) for TWC reactions. For that purpose, we employed various in situ spectroscopic studies, including IR, XAFS, and near-ambient-pressure XPS (NAP-XPS), in combination with DFT calculations. The obtained results revealed that Pd 0 species supported on La/Al 2 O 3 are more electron deficient compared to those on pristine Al 2 O 3 without La (Pd/Al 2 O 3 ). Kinetic studies using powdered catalysts revealed that the addition of La suppresses the poisoning effect by CO during the DeNO x 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/Al 2 O 3 exhibits higher DeNO x activity than Pd/Al 2 O 3 . 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/Al 2 O 3 supports. Our investigations showed that an increased La loading (15 wt%) is even more effective for the DeNO x 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)/Al 2 O 3 has also been tested in a commercial vehicle and been evaluated on a practical driving mode test cycle (LA-4; city cycle of US Federal and California), where it showed a better catalytic performance than the conventionally used Pd/La(3-5)/Al 2 O 3 catalysts. Our study suggests that the loading amount of La in Pd/La/Al 2 O 3 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.

The surface chemical states of Pd(100) during CO oxidation were investigated using ambient pressure x-ray photoelectron spectroscopy and mass spectroscopy. Under the reactant ratio of CO/O2 = 0.1, i.e. an oxygen-rich reaction condition, the formation of surface oxides was observed with the onset of CO oxidation reaction at T = 525 K. As the reactant ratio (CO/O2) increased from 0.1 to 1.0, ∼ 90 % surface oxides remains on surface during the reaction. Upon the formation of surface oxides, the core level shift of oxygen gas phase peak was observed, indicating that change of surface work function. As CO oxidation takes places, i.e. making a transition from CO covered surface to the oxidic surface, the work functions of surface oxide on Pd(100) and Pt(110) display opposite behavior.