EFFECTS OF THE HYDROGEN-INDUCED (1X2) SURFACE RECONSTRUCTION ON THE KINETICS OF ETHYLENE HYDROGENATION ON NI(110)

We show that deuterium in the (1 x 2)-reconstructed phase on Ni(110) is highly active for ethylene hydrogenation, whereas the (1 x 1) surface is active only for decomposition of adsorbed ethylene in UHV. When ethylene-d4 is adsorbed on the deuterium-saturated, (1 x 2)-reconstructed Ni(110) surface, decomposition and hydrogenation proceed concurrently as ethane-d6 is evolved near 180 K in TPD. A detailed analysis of the competing rates of these processes at constant temperature gives an effective activation energy of 16 +/- 2 kcal/mol and a preexponential factor of 5 x 10(18 +/- 2) ML-1 s-1 for hydrogenation, as compared to 12 +/- 1 kcal/mol and 3 x 10(13 +/- 1) ML-1 s-1 for decomposition. The relative magnitudes of the preexponential factors suggest that a significant entropy increase accompanies the conversion of surface sites from (1 x 2) to (1 x 1) symmetry, and this entropic contribution provides a driving force for hydrogenation that outweighs the unfavorable activation energy and ultimately determines the selectivity between the competing reaction pathways.