815.    --------------.  [ELOVICH, S. Y., AND ZHABROVA, G. M.]  [Mechanism of the Catalytic Hydrogenation of Ethylene on Nickel.  II.  Role of the Activated Adsorption of Ethylene and Hydrogen in the Hydrogenation Process.  General Scheme of the Process.]  Jour. Phys. Chem. (U.S.S.R.), vol. 13, 1939, pp. 1775-1786; Chem. Abs., vol. 35, 1941, p. 373.

     Within the temperature region in which hydrogenation occurs, both H₂ and C₂H₄ undergo activated adsorption on a Ni catalyst prepared by reduction of NiO.  The equation dq/dt=ac-^=aq holds for all 3 processes.  The energy of activation for the activated adsorption for H₂ is 16±1 cal. and for C₂H₄, 9 cal.  The initial hydrogenation velocity is 40-60 times as great as the rate of activated adsorption.  When the reaction is carried out stepwise, that is, adsorption of H₂ and then of C₂H₄, or the reverse, no hydrogenation takes place.  The experimental results can be explained by the following mechanism:  H₂+Ni=H₂Ni (molecular adsorption); C₂H₄+Ni=C₂H₄Ni (molecular adsorption); C₂H₄Ni=C₂H₄Ni* (active state); C₂H₄Ni*=C₂H₄Ni (activated adsorption); C₂H₄Ni*+H₂Ni=C₂H₆+Ni (reaction).  As the temperature rises, the rate of activated adsorption of C₂H₄ increases, relatively, more rapidly than the rate of hydrogenation, until finally its absolute rate is greater also; poisoning of the surface results, and above a certain temperature of maximum rate of hydrogenation the latter decreases.  The temperature for this maximum rate of hydrogenation is lower the more active the catalyst, with respect to activated adsorption and is characteristic for the given catalyst.