Structure Sensitivity in the Hydrogenation of Hindered Hydrocarbons

Hydrogenation of the following compounds has been investigated in a batch reactor at 20 °C on a series of Pt/Al₂O₃ and Pt/SiO₂ catalysts with dispersions ranging from 0.3 to 100%: cyclopentene, di-tert-butylacetylene (the absorption of which should involve serious adsorbate-surface interactions), 3-hexyne, and cis-di-tert-butylethylene, Product compositions during hydrogenation were determined for the last three. The unusually large value of the ratio, isomerization/hydrogenation of the cis-ene, 2.5 to 9, is presumably related to the large heat of isomerization of the cis to the trans isomer. The turnover number of cyclopentene varied by less than a factor of 2 on the catalysts and exhibited no trend with dispersion. The ratio of the turnover number of the acetylene to that of cyclopentene varied from 0.3 to 0.9 and increased with increasing dispersion. Much larger effects of structure sensitivity appeared in the competitive ratio of the acetylene to the cis olefin. The value of the ratio appeared to result from the relative sticking coefficients at free sites on platinum, it varied from 1 to > 50, increased with increasing dispersion, and was larger for Pt/Al₂O₃ than for Pt/SiO₂. Apparently, the acetylene adsorbs more readily at edge atoms than at atoms in faces. The activity of highly dispersed Pt/Al₂O₃ increases during hydrogenation of 3-hexyne which promotes rearrangement of platinum particles to ones of higher activity. Since the acetylene when alone hydrogenates readily on catalysts of low dispersion, we suggest that semiextractive adsorption is involved, i.e., that the atoms of platinum involved in bidding are elevated somewhat above the surface crystal planes.