Infrared spectroscopic comparison of the chemisorbed species from ethene, propene, but-1-ene and cis- and trans-but-2-ene on Pt(111) and on a platinum/silica catalyst

Abstract

The chemisorption of ethene, propene, but-1-ene, cis- and trans-but-2-ene and but-2-yne has been studied on a Pt(111) surface using reflection absorption infrared spectroscopy (RAIRS). The results have been compared with transmission infrared spectra of the alkenes adsorbed on a finely divided impregnated Pt/SiO₂ catalyst.

On the Pt(111) surface ethene, propene and but-1-ene are adsorbed in the corresponding n-alkylidyne form CH₃(CH₂)_nCPt₃(n= 0, 1, 2) at room temperature. The cis- and trans-but-2-enes are adsorbed in the form of but-2-yne bonded to the surface in a di-σ/π fashion, as was confirmed by adsorption of but-2-yne itself.

The room-temperature spectra of ethene, propene and but-1-ene on Pt/SiO₂ all exhibited prominent absorptions from the n-alkylidyne, but other adsorbed species also contributed to the spectra, considered to be predominantly the appropriate di-σ species PtCH₂CH(R)Pt (R = H, CH₃, C₂H₅) together with smaller amounts of the relevant π-complexes. In the case of the cis- and trans-but-2-enes, virtually identical spectra were obtained on Pt/SiO₂ but with many of the same prominent features as from but-1-ene. Clearly the finely divided catalyst had sites which led to double-bond isomerization. A smaller proportion of di-σ/π bonded but-2-yne was also present.

The infrared spectra proved to be very effective at delineating the similarities and differences between the species chemisorbed on Pt(111) and on the Pt/SiO₂ catalyst.