Abstract: The dissolved Al~(3+) amounts of various alumina (Ketjen CK 300, Degussa Alu minium Oxid C, Tonerdo γ-Al_2O_3 and Changling Al_3O_3) in different concentrations of acid (HCl, H_2PtCl_6) and the saturated adsorption amounts of H_2PtCl_6 on the above alumina were determined by EDTA complexometric method and by spectro photometry using SnCl_2. It was found that various alumina showed different Al~(3+) amounts in above acids, the more the dissolved Al~(3+) amounts of alumina are the higher the saturated adsorption amounts of H_2PtCl_6 on alumina used, followed the sequence: Degussa=Tonerdo<Changling<CK-300. The adsorption isotherms of H_2PtCl_6 on the alumina mentioned above all followed Langmuir adsorption isotherm.
The catalysts tested Temperature Programmed Reduction (TPR) and Desorption (TPD) were prepared by wet impregnation of the aforementioned alumina with H_2PtCl_6, followd by precalcination in a fix-bed reactor under air flow for 1 hr at 500 ℃. The TPR and TPD measurements of the catalysts were performed “in situ”. According to the TPR profiles, the appearance of high temperature reduction peak (at ~650 ℃) depends on the nature of alumina used. It seemed namely that there was a close correlation between the dissolved Al~(3+) amounts and the high temperature reduction peak (HTPR). The CK 300 supported Pt catalyst, possessed the highest dissolved Al~(3+) amounts, showed a distinct HTRP, but no HTRP appeared for the lowest one e.g. Pt/Degussa, except Pt/Tonerdo (Tonerdo Al_2O_3 itself showed a HTRP at 685 ℃). The HTRP may be ascribed to the existence of irreducible Pt (soluble platinum) which was created by the adsorption of Pt within the lattice defect of alumina, formed during impregnation with H_2PtCl_6, and by subsequent deposit of Al~(3+) during evaporation.
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