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

The surface chemistry of saturated hydrocarbons (cyclic and straight-chain ones between Cg and Cs) adsorbed on a stepped Ni(755) {Ni(S)[6(111) x (100) ]} has been investigated mainly with temperature-programmed desorption (TPD ). Coadsorbed CO shows several significant effects on decomposition of hydr ocarbons (e.g., promoting effect on decomposition of "low-reactivity" hydro carbons). By using these effects of CO, we have determined desorption energ y, decomposition fraction, ratio of desorption-limited H-2 peak area to rea ction-limited one, and decomposition starting temperature of hydrocarbons. These quantities are very dependent on molecular structure. The decompositi on starting temperature of straight-chain hydrocarbons increases only sligh tly with increase of carbon atom numbers, suggesting that activation energy for decomposition is similar for all the straight-chain hydrocarbons and o nly part of the chain reorients regardless of chain length by rotation abou t C-C bond in a transition state for decomposition. Cyclohexane shows a muc h higher decomposition starting temperature and lower decomposition fractio n than other cyclic hydrocarbons, which can be explained on the basis of th e difference in conformational energy to attain eclipsed C-II bonds on adja cent carbons in a transition state. Relating decomposition starting tempera ture with desorption energy, we have discussed the decomposition mechanism of hydrocarbons and proposed some candidates for transition state of decomp osition.