Structure, reactivity, and mobility of carbonaceous overlayers during olefin hydrogenation on platinum and rhodium single crystal surfaces

The hydrogenation of olefins (ethylene, propylene, and isobutene) and a cyclic olefin (cyclohexene) has been characterized on platinum and rhodium single crystal surfaces under conditions ranging from ultrahigh vacuum (UHV) to elevated pressures. A carbonaceous overlayer, formed by C-H bond activation, exists on the metal surface during catalytic hydrogenation, and the structure of this overlayer has been characterized using sum frequency generation (SFG) vibrational spectroscopy. The dehydrogenated carbonaceous species are unreactive even in the presence of excess hydrogen, while the intermediates that turnover are weakly bonded to the metal surface. The formation of this carbonaceous overlayer is accompanied by a restructuring of the metal surface. The overlayer is mobile on the surface during hydrogenation, as shown by high pressure scanning tunneling microscopy (HP-STM) results. Coadsorbed CO induces ordered surface structures, and as a consequence poisons the reaction.