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, 1-alkenes, and cyclic hydrocarbons on a Au(111) 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 1-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. Wh...