CHARACTERIZATION OF A PT-PD COMBUSTION CATALYST ON AN ALUMINA WASHCOAT, WITH AND WITHOUT PRIOR HYDROTHERMAL TREATMENT OF THE WASHCOAT

Two Pt/Pd catalysts on cordierite monoliths were prepared by impregnat ing two differently treated alumina washcoats with 10-mu-mol[Pt + Pd] per gram catalyst in the atomic ratio Pt/Pd = 4.0. Both washcoats were first thermally treated, calcined, for 2 h at 550-degrees-C in air an d one of them was additionally treated, hydrothermally, in 100% steam for 2 h at 814-degrees-C. The hydrothermally treated catalyst was more active for complete oxidation of xylene in air: its light-off tempera ture was 232-degrees-C, compared to 259-degrees-C for the sample calci ned only. To explain this higher activity, both catalysts were charact erized by BET surface area, pore-size distribution, hydrogen chemisorp tion, X-ray diffraction, TEM/STEM/EDS and low-energy ion scattering sp ectroscopy (LEIS). The catalyst with a hydrothermally treated washcoat had 30% lower surface area, larger alumina crystal size, higher degre e of crystallization of alumina and larger average catalyst pore size (11 nm vs. 6 nm), than the one with the washcoat, treated only thermal ly. The LEIS results indicated a surface enrichment of Pd on both cata lysts. The Pt signal in LEIS was higher for the hydrothermally treated sample.