Propylene on Pt(111)

The adsorption of propylene on Pt(111) single-crystal surfaces was characterized by reflection-absorption infra-red spectroscopy (RAIRS). The uptake of propylene on the surface at 90 K results in the development of at least four adsorption species as a function of coverage. Significant rehybridization of the CC double bond of propylene takes place at low coverages, so the molecule primarily interacts with the metal via two σ metalcarbon bonds. Below half-saturation, the molecule mainly bonds through the central carbon atom, but at higher coverage, the CC bond becomes flat, and the terminal methyl group tilts towards a more vertical orientation. Further dosing of propylene after saturation of the di-σ state leads to the build-up of a flat π-bonded second layer. Ultimately, a layer of condensed propylene could be grown on the surface under the vacuum conditions of the experiment as long as the temperature was kept below 80 K. Annealing of the low-temperature propylene-saturated Pt(111) surface first induces the desorption of the weakly held π species, and later, between 230 and 250 K, to the dehydrogenation and rearrangement of the remaining di-σ species to propylidyne (Pt3CCH2CH3). The details of the conversion of propylene to propylidyne change somewhat with the conditions under which this transformation is carried out, and appear to involve a stable and identifiable intermediate [2-propyl, CH3CH(Pt)CH3, and/or propylidene, Pt2CHCH2CH3]. Propylene π-bonding is also possible on propylidyne-saturated Pt(111) surfaces under vacuum.