It was shown that deuterium in the (1 x 2)-reconstructed phase on Ni(110) was highly active for ethylene hydrogenation, whereas the (1 x 1) surface was active only for decomposition of adsorbed ethylene in ultra-high vacuum. When ethylene-d4 was adsorbed on the deuterium-saturated, (1 x 2)-reconstructed Ni(110) surface, decomposition and hydrogenation proceeded concurrently as ethane-d6 was evolved near 180K in TPD. A detailed analysis of the competing rates of these processes at constant temperature gave an effective activation energy of 16kcal/mol and a pre-exponential factor of 5 x 1018/ML for hydrogenation, as compared with 12kcal/mol and 3 x 1013/ML for decomposition. The relative magnitudes of the pre-exponential factors and a significant entropy increase accompanied 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 out-weighed the unfavourable activation energy and ultimately determined the selectivity between the competing reaction pathways.

Effects of the Hydrogen-Induced (1 x 2) Surface Reconstruction on the Kinetics of Ethylene Hydrogenation on Ni(110). A.M.Glines, A.B.Anton: Surface Science, 1993, 286[1-2], 122-38