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

Abstract

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 A mLSA, p repared by high-temperature chlorination of ii-alumina, by sublimation of PtCl₂ at 773 K followed by hydrogen reduction above 673 K. O n the AmL SA 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 outgas sed above 493 K. These hydroxy groups had no Bronsted acidity. However, below 583 K the a dsorption peak at 1540-1543cm^-1 due to pyridinium ion was always present on the Pt/AmLSA and pyridine-H_5-xD_x molecules were formed if deuterium was used. Furthermore, the pyridine molecules coordinated with strong Lewis acid sites were converted to piperidine in t he 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 o f the pyridine coordinated to the strong Lewis acid site. Such phenomena are concluded to be so-called “hydroge n spillover”, which are observed only on the Pt/AmLSA.