A comparison of gas-phase and electrochemical hydrogenation of ethylene at platinum surfaces

Rates of hydrogenation of ethylene to ethane under gas-solid (G-S) and liquid-solid (L-S) (i.e.electrochemical) conditions at well-defined Pt(lll) and smooth polycrystalline Pt surfaces are reported.The activation energies are 5.9 kcal/mole for the L-S reaction and 10.8 kcal/mole for the G-S reaction.Comparison of the rate laws under appropriate conditions shows that the hydrogenation proceeds by different reaction ~echanisms at the two different interfaces.We have used surface science techniques (low-energy electron diffraction, Auger electron spectroscopy, highresolution electron energy loss spectroscopy, temperature programmed desorption), and electrochemistry (a combination of solution and ultra-high vacuum procedures) to characterize the adsorbed species formed under G-S and L-S reaction conditions and to gain insight into the•reaction mechanisms.We propose that in hydrogenation at the L-S interface, ethylene is reduced on the Pt surface by adsorbed H atoms, while during hydrogenation at the G-S interface, H atoms must be transferred from the Pt surface through a layer of irreversibly adsorbed ethylene to ethylene that is adsorbed on top of this layer.