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A study has been made of the degree of selectivity of ethylene formation in the hydrogenation of acetylene catalyzed by rhodium, palladium, platinum and some palladium + silver mixtures supported on a-Al203. Analyses were performed by gasliquid chromatography.Palladium is the most active of the pure metals : prolonged exposure to hydrogen at room temperature decreases its activity for acetylene hydrogenation and completely destroys its activity for ethylene hydrogenation. The selectivity, before the onset of the generally more rapid ethylene hydrogenation, falls in the order Pd > Rh > Pt.Palladium + silver catalysts containing 10 to 30 % silver show notably higher initial selectivities than pure palladium and are also highly active.Thermodynamic and mechanistic factors in selectivity are discussed, and the probable role of dissolved hydrogen in determining the selectivity of palladium catalysts is outlined.The selective hydrogenation of acetylene to ethylene and the selective removal of small amounts of acetylene from gas streams containing large excesses of ethylene constitute problems of considerable technical interest. The relevant literature has recently been reviewed ; 1 previous research has, however, been largely of an applied nature, and the fundamental causes underlying selectivity in this system are as yet not well understood.Of the metals which catalyze the hydrogenation of acetylene, palladium has an outstanding ability to give a highly selective production of ethylene.1 Industrial experience shows that the selectivity of palladium catalysts increases with use, and the present work has provided a possible explanation of this. The behaviour of platinum, rhodium, palladium and some silver + palladium mixtures supported on a-Al203 has been studied with emphasis on determining the factors governing the extent of ethane formation from acetylene in the absence of added ethylene. E X P E R I M E N T A L APPARATUS, MATERIALS AND PROCEDUREReactions were carried out in a cylindrical Pyrex vessel (capacity 125 ml), the catalyst granules resting on the bottom of the vessel. " Oxygen-free " hydrogen was purified by passage through a cooled charcoal trap : cylinder acetylene was degassed and purified by repcated fractional distillation from a solid C02 + methanol bath.Each new catalyst sample (normally 0.25 to 0.30 g) was evacuated while the temperature was raised to 200°C and kept at this temperature until a hard vacuum was obtained (15-30 min). Hydrogen (-100 mm) was then admitted and the temperature adjusted to that required for the first experiment after 30min. The hydrogen was pumped off and the reactants admitted in sequence, acetylene normally being added first. The progress of the reaction was followed manometrically, and after the desired conversion the contents of the reaction vessel were transferred to the analytical section of the apparatus.

Analysis of the " electronic factor " in heterogeneous catalysis suggests that activity of Ni-Fe alloys in multiple bond hydrogenation should increase rapidly in the region where the energy density of electron levels a t the Fermi surface rises ; moreover, that reactions controlled by the rate of transfer of an electron from metal t o substrate should, in the Ni-Cu alloys, decrease in rate as the 3d-band begins t o empty. Experiments on styrene hydrogenation over Ni-Fe catalysts and hydrogen peroxide decomposition on Ni-Cu alloy foils provide general confirmation of these predictions. Methanol and formic acid decompositions on the Ni-Cu alloys decrease in speed as the 3d-band

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