An infrared spectroscopic study of 2-methylpropene (isobutene) adsorbed on a Pt/SiO₂ catalyst before and after hydrogenation

Abstract

GULERANA SHAHID and NORMAN SHEPPARD Can. J. Chem. 69, 1812 (1991). The infrared spectrum from 2-methylpropene (isobutene) adsorbed at room temperature on a WSi02 catalyst is interpreted, with the help of the metal-surface selection rule, in terms of the presence of a mixture of R-bonded, di-o, and alkylidyne (2-methylpropylidyne) surface species. Heating and evacuation at 100°C retains the original alkylidyne, which occurs together with another surface species probably containing multiple -CH2Pt groups. Heating to 200 and then to 300°C leads to progressive and strong reductions in the absorptions from alkyl-type surface species, which are .replaced by increasing absorptions at ca. 3060 cm-' from unsaturated alkene groups. Hydrogenation at room temperature, after the removal of gas-phase and physically adsorbed 2-methylpropane (isobutane) by evacuation, leaves a methyl-rich spectrum, probably from a partially hydrogenated surface species. Hydrogenation and evacuation at 100°C give a much weaker residual spectrum from saturated hydrocarbon surface species. Subsequent hydrogenation at 200°C gives a spectrum suggesting the presence of a surface n-butyl group, clearly the result of a skeletal surface isomerisation. At 300°C in hydrogen the surface species is from a longer n-alkyl chain and occurs together with gas-phase methane implying the operation of a disproportionation reaction on the surface. Evacuation of hydrogen at 200 or 300°C leads to virtually total removal of the n-alkyl absorptions, which are, however, completely restored by re-addition of Hz. In agreement with a similar study by Avery on a Pd/SiOz catalyst, we conclude that after evacuation C-H bonds are largely replaced by C-Pt bonds at stepped-type surface sites.Key words: 2-methylpropene adsorbed on WSi02, infrared spectrum.GULERANA SHAHID and NORMAN SHEPPARD Can. J. Chem. 69, 1812 (1991).Faisant appel a la rkgle de sklection des surfaces mCtalliques, on interprkte le spectre infrarouge du 2-mtthylpropkne (isobutkne) adsorb6 h la temperature ambiante sur un catalyseur de WSiOz en fonction de la prtsence la surface d'un mClange d'espkces avec des liaisons-R, des adsorptions di-o et d'alkylidyne (2-mkthylpropylidyne). Par chauffage et mise sous vide h 100"C, l'alkylidyne original est conservC avec une autre espkce prCsente h la surface qui contient probablement plusieurs groupes -CH2Pt. Par chauffage 200"C, puis h 300"C, il y a une forte perte progressive des espkces alkyles presentes h la surface; elles sont remplacees par des absorptions croissantes aux environs de 3060 cm-I, causCes par des groupes alcknes. Aprks une hydrogknation a la tempCrature ambiante, suivie d'une elimination de la phase gazeuse et du 2-mCthylpropane (isobutane) adsorbe, on observe un spectre riche en groupes mCthyles qui est probablement dQ h une espkce partiellement hydrogCnCe prCsente a la surface. Une hydrogknation, suivie d'une Climination des espkces par chauffage h 100"C, foumit un spectre contenant, a la surface, un rtsidu contenant une quantitt beaucoup plus faible d...

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