The chemistry of ethylene on MoAl alloy thin films formed on dehydroxylated alumina: Hydrogenation, dehydrogenation and H-D exchange reactions

The chemistry of ethylene adsorbed on a thin MoAl layer grown in ultrahigh vacuum on a thin alumina film is studied using a combination of temperature-programmed desorption and X-ray, Auger and reflection absorption infrared spectroscopies. Both di-σ-bonded and a small proportion of π-bonded ethylene are found, where the di-σ-bonded ethylene has a σ/π parameter of ∼0.8 and a heat of adsorption of ∼70 kJ/mol. The ethylene self-hydrogenates to yield ethane and a small amount of methane is detected. The surface hydrogenation activation energy of di-σ-bonded ethylene is ∼65 kJ/mol, while the π-bonded species hydrogenates more easily. Adsorbed ethyl species grafted onto the surface by decomposing ethyl iodide predominantly undergo β-hydride elimination to yield ethylene. Ethyl species hydrogenate to ethane at a lower temperature than does di-σ-bonded ethylene implying that addition of hydrogen to adsorbed ethylene is slower than the rate of ethyl hydrogenation.