TRAPPING-MEDIATED DISSOCIATIVE CHEMISORPTION OF ETHANE AND PROPANE ONRU(001)

The initial probabilities of precursor-mediated dissociative chemisorp tion of (C2H6)-C-13, C2D6, C3H8, CH3CD2CH3 and C3D8 have been measured on the hexagonally close packed Ru(001) surface. The activation energ ies for C-H bond cleavage are 9310 cal mol(-1) for (C2H6)-C-13 and 990 0 cal mol(-1) for C2D6 with respect to the bottom of the physically ad sorbed well of the ethane molecule. For each isotopomer of propane, th ese measured activation energies are 10 430, 10 620 and 10 910 kcal mo l(-1) for C3H8, CH3CD2CH3 and C3D8, respectively. For each alkane inve stigated the ratio of the preexponential factor for desorption relativ e to that of reaction is similar to 100 due to the larger phase space available to the molecule for desorption relative to reaction. An exam ination of the selective activation of primary (1 degrees) versus seco ndary (2 degrees) C-H bonds in propane reveals that 2 degrees C-H bond cleavage is favored energetically by 240 cal mol(-1) and entropically by a factor of 2.1 over 1 degrees bond cleavage. However, the formati on of 1 degrees Ru-propyl intermediates is favored over the formation of 2 degrees Ru-propyl intermediates at all investigated temperatures due to the greater number of 1 degrees C-H bonds in propane compared t o 2 degrees C-H bonds. The measured differences in C-H versus C-D bond activation for both ethane and propane are attributed to zero-point e nergy differences between each isotopomer and point to classical over the barrier reaction dynamics as the reaction pathway.