Characterization of Alumina Solid Lewis Superacid-Supported Platinum Catalyst Surface Using Hydrogen and Pyridine as Probe Molecules

Kazunori HONDA and Akimi AYAME*

Department of Applied Chemistry, Muroran Institute of Technology; 27-1 Mizumoto-cho, Muroran-shi 050-8585 Japan

Surface acid properties of alumina solid Lewis superacid-supported platinum catalyst (Pt/AmLSA) were measured by FT-IR spectrometer using pyridine and hydrogen as probe molecules. Preparation of the catalyst was as follows: Pt was supported on AmLSA, prepared by high-temperature chlorination of η-alumina, by sublimation of PtCl2 at 773 K followed by hydrogen reduction above 673 K. On the AmLSA original hydroxy groups dissapeared. On the Pt/AmLSA hydroxy groups appeared again after hydrogen reduction treatment and hydroxy groups came out when adsorbed pyridine was outgassed above 493 K. These hydroxy groups had no Bronsted acidity. However, below 583 K the adsorption peak at 1540--1543 cm−1 due to pyridinium ion was always present on the Pt/AmLSA and pyridine-H5−xDx molecules were formed if deuterium was used. Furthermore, the pyridine molecules coordinated with strong Lewis acid sites were converted to piperidine in the presence of gaseous hydrogen at 473 K. These results indicate that hydrogen atoms remain on the Pt/AmLSA after the reduction of PtCl2; a part of the hydrogen atoms form pyridinium ion, and another part causes hydrogen exchange reaction of the pyridine coordinated to the strong Lewis acid site. Such phenomena are concluded to be so-called “hydrogen spillover”, which are observed only on the Pt/AmLSA.

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