Decomposition of saturated hydrocarbons adsorbed on Ni(755): Comparison of decomposition starting temperatures among cyclic and straight-chain hydrocarbons

The surface chemistry of saturated hydrocarbons (cyclic and straight-chain ones between C₅ and C₈) adsorbed on a stepped Ni(755) {Ni(S)[6(111)×(100)]} has been investigated mainly with temperature-programmed desorption (TPD). Coadsorbed CO shows several significant effects on decomposition of hydrocarbons (e.g., promoting effect on decomposition of `low-reactivity' hydrocarbons). By using these effects of CO, we have determined desorption energy, decomposition fraction, ratio of desorption-limited H₂ peak area to reaction-limited one, and decomposition starting temperature of hydrocarbons. These quantities are very dependent on molecular structure. The decomposition starting temperature of straight-chain hydrocarbons increases only slightly with increase of carbon atom numbers, suggesting that activation energy for decomposition is similar for all the straight-chain hydrocarbons and only part of the chain reorients regardless of chain length by rotation about C-C bond in a transition state for decomposition. Cyclohexane shows a much higher decomposition starting temperature and lower decomposition fraction than other cyclic hydrocarbons, which can be explained on the basis of the difference in conformational energy to attain eclipsed C-H bonds on adjacent carbons in a transition state. Relating decomposition starting temperature with desorption energy, we have discussed the decomposition mechanism of hydrocarbons and proposed some candidates for transition state of decomposition.