ENERGETICS AND KINETICS OF THE PHYSISORPTION OF HYDROCARBONS ON AU(111)

Helium atom reflectivity has been used to study the adsorption of a se ries of n-alkanes, l-alkenes, and cyclic hydrocarbons on a Au(lll) sur face. Using this technique, both adsorption and desorption could be ob served with high sensitivity under UHV conditions to determine adsorpt ion energies and initial sticking coefficients. For the long-chain n-a lkanes studied (C6H14-C12H26), the physisorption energy increases line arly with the chain length by 6.2 +/- 0.2 kJ/mol per additional methyl ene unit. The physisorption energies of the l-alkenes (C6H12-C11H22) s how a similar linear dependence on chain length but are slightly highe r than those of the corresponding alkanes. A bond-additive model is pr esented which is capable of predicting the adsorption energy of 25 sat urated and unsaturated hydrocarbons on the basis of four fitted parame ters with an average error of 1.9%. Of the molecules considered, 84% o f the calculated adsorption energies differ from the experimental valu e by less than twice the average error. When 10 sulfur-containing comp ounds 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, whic h is defined as the temperature measured in units of the peak desorpti on temperature as observed by temperature programmed desorption. The s ticking coefficient of each species is close to unity at low temperatu res, starts to decrease at T = 0.8, and reaches zero as the crystal t emperature approaches the peak desorption temperature.