IR spectra and the structures of the chemisorbed species resulting from the adsorption of the linear butenes on a Pt/SiO₂catalyst. Part 2.—Spectra of the hydrogenated surface species as a function of temperature

Abstract

An IR spectroscopic study of the hydrogenation of the adsorbed species from but-1-ene and (Z)-but-2-ene has been carried out from room temperature to ca. 300 °C. Room-temperature hydrogenation leads to the removal of a major fraction of the initially adsorbed species as butane and leaves residually adsorbed butyl and possibly sec-butyl species.Hydrogenation at 100 °C or above leads to less removal as butane and to spectra from residual alkyl groups with lengthened chains. The spectra from the residual alkyl groups occur in the presence of hydrogen but are virtually completely removed on evacuation. This remarkable phenomenon, previously observed at room temperature on Ni/SiO2 and Pd/SiO2, is suggested to result from the formation of surface polyenes, (—CHCH—)n[or poly-ynes, (—CC—)n], parallel to the surface whose intrinsically weak spectra are further weakened by the operation of the metal surface selection rule for adsorption on the large Pt particles, diameters ca. 5–15 nm. Some hydrogenolysis to methane occurs at ca. 300 °C.