The chemisorption of ethene, propene, but-1-ene, cisand trans-but-2-ene and but-2-yne has been studied on a Pt(ll1) 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/Si02 catalyst.On the Pt(ll1) surface ethene, propene and but-1-ene are adsorbed in the corresponding n-alkylidyne form CH,(CH,),CPt, (n = 0, 1, 2) at room temperature. The cisand trans-but-2-enes are adsorbed in the form of but-2-yne bonded to the surface in a di-o/x fashion, as was confirmed by adsorption of but-2-yne itself.The room-temperature spectra of ethene, propene and but-1-ene on Pt/SiO, all exhibited prominent absorptions from the n-alkylidyne, but other adsorbed species also contributed to the spectra, considered to be predominantly the appropriate di-o species PtCH,CH(R)Pt (R = H, CH, , C, H, ) together with smaller amounts of the relevant n-complexes. In the case of the cis-and trans-but-2-enes, virtually identical spectra were obtained on Pt/Si02 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-o/n 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(ll1) and on the Pt/SiO, catalyst.
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