Ethylene hydrogenation on evaporated iridium films

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Abstract
The reaction of ethylene with hydrogen to form ethane and methane is studied over evaporated iridium thin film catalysts. The rate law obtained at total pressures up to 1000 μ and temperatures between 110 and 200°K is of the form R=KP0C2H4P1/2H2 exp(−3.3 kcal/mole/RT), where 0 and 1/2 are the reaction orders in ethylene and hydrogen, respectively. Over a wider temperature range (up to 100 °C) the reaction is characterized by the use of 13C labeled ethylene or deuterium in the place of hydrogen. The kinetic results taken in conjunction with the isotope studies are consistent with a Langmuir–Hinshelwood hydrogenation mechanism involving the half‐hydrogenated ethyl species C2H5 (a) first postulated by Polanyi. At temperatures greater than ∼150°K the rate of a surface dehydrogenation reaction becomes significant. The extent of this surface reaction can be determined by monitoring the amount of ethane produced when only ethylene is dosed on a clean Ir film or by observing nonlinearities in the Arrhenius plots when both hydrogen and ethylene are dosed. Production of methane from ethane decomposition at 373°K is poisoned by ethylene adsorption. Film area as measured by both hydrogen adsorption and catalytic activity is proportional to film thickness for average thicknesses of 30 to 200 Å.

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