Energetics and kinetics of the physisorption of hydrocarbons on Au(111)

Helium atom reflectivity has been used to study the adsorption of a series of n-alkanes, l-alkenes, and cyclic hydrocarbons on a Au(lll) surface. Using this technique, both adsorption and desorption could be observed with high sensitivity under UHV conditions to determine adsorption energies and initial sticking coefficients. For the long-chain n-alkanes studied (C6H14-C12H26), the physisorption energy increases linearly with the chain length by 6.2 +/- 0.2 kJ/mol per additional methylene unit. The physisorption energies of the l-alkenes (C6H12-C11H22) show a similar linear dependence on chain length but are slightly higher than those of the corresponding alkanes. A bond-additive model is presented which is capable of predicting the adsorption energy of 25 saturated and unsaturated hydrocarbons on the basis of four fitted parameters with an average error of 1.9%. Of the molecules considered, 84% of the calculated adsorption energies differ from the experimental value by less than twice the average error. When 10 sulfur-containing compounds and two fitting parameters are added, the average error grows to 2.6%. For all linear hydrocarbons studied, the physisorption sticking coefficient is a function of the reduced surface temperature T*, which is defined as the temperature measured in units of the peak desorption temperature as observed by temperature programmed desorption. The sticking coefficient of each species is close to unity at low temperatures, starts to decrease at T* = 0.8, and reaches zero as the crystal temperature approaches the peak desorption temperature.