An Infrared Spectroscopic Study of the Hydrogenation and Dehydrogenation of the Complexes of Aromatic Compounds and of Fullerene C60 with Silica-Supported Platinum

Abstract

An infrared spectroscopic study has been made of the surface complexes from the chemisorption of the aromatic compounds benzene, toluene, para-xylene (1,4-dimethylbenzene), mesitylene (1,3,5-trimethylbenzene), naphthalene, and anthracene, and also of fullerene C60, on silica-supported platinum catalysts, and of their subsequent hydrogenation and dehydrogenation reactions. The complexes of the initial adsorbates with platinum can be hydrogenated under relatively low pressures of hydrogen (<1 atm) and then dehydrogenated to considerable degrees by evacuation even at ambient temperatures. In the case of benzene the hydrogenated cyclohexane-like complex gives very extensive reversion to adsorbed benzene on evacuation. The surface complexes gradually diminish in amount during successive cycles of hydrogenation and dehydrogenation, with the generation of gas-phase cyclohexane. For the other compounds, except for C60, the hydrogenation is also virtually (or actually) complete but the dehydrogenation is only partial and the surface complexes remain present during multiple cycles of hydrogenation and dehydrogenation. The room temperature spectra from the methyl-substituted benzenes under vacuum support chemisorption via methyl-group dissociation. For naphthalene and anthracene the spectra of stable partially dehydrogenated species are consistent with the rearomatization of only one end ring. For fullerene C60 the spectrum of the hydrogenated species suggests that a five-membered ring is in selective contact with the surface but that some adjacent six-membered rings are also hydrogenated. The hydrogenation and dehydrogenation processes are both slow but the rates, together with the extent of hydrogenation, increase with temperature. The hydrogenated spectra of the fullerene-based complex differ from those reported earlier for C60H18 and C60H36.