Energetics and Kinetics of the Physisorption of Hydrocarbons on Au(111)

Abstract

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. 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.

Effects of conformational isomerism on the desorption kinetics of n-alkanes from graphite

“…24 The desorption of n-alkanes (Nϭ6 to 12͒ from the Au͑111͒ surface was studied by Wetterer et al using helium atom reflectivity. 25 Their conclusions from this study indicate that ⌬E des ‡ (N) increased incrementally by 6.2Ϯ0.2 kJ/mole per methylene unit. In these cases the range of alkyl chain lengths has been limited to Nр12.…”

“…It is important to realize that physically this parameter may include both interaction with the graphite surface and the interactions between adsorbed chains. Previous measurements of the interaction of much shorter alcohols and alkanes with metals such as Ag; 24 Cu, 42,43 and Au 25 have yielded values in the range 5 -6 kJ/mole/CH 2 group. A recent study by Bishop et al has reported segment-surface interaction energies of ϳ8 kJ/mole/CH 2 group for n-alkanes (Nϭ6 to 10͒ adsorbed on Pt͑111͒.…”

Two-Dimensional Crystals of Alkanes Formed on Au(111) Surface in Neat Liquid: Structural Investigation by Scanning Tunneling Microscopy

“…[6][7][8][9][10][11][12] The desorption energy was proportional to numbers of the methylene, ca. 6.5 kJ/mol per methylene unit.…”

Electronic structure of octane on Cu(1 1 1) and Ni(1 1 1) studied by near edge X-ray absorption fine structure

“…The interaction between alkanes and metals has attracted wide attention [1][2][3][4][5]. This is due to the fact that many catalytic processes involve the conversion of saturated hydrocarbons to various products, and also because n-alkane is the simplest molecule among the saturated hydrocarbons.…”

“…This is due to the fact that many catalytic processes involve the conversion of saturated hydrocarbons to various products, and also because n-alkane is the simplest molecule among the saturated hydrocarbons. The adsorption energies of n-alkane molecules on metal substrates are rather small (of the order of 10 kJ/mol/CH 2 chain) [2], and the n-alkane-metal interaction has been categorized to be typical of physical adsorption. However, a softening of the C-H vibration has been observed for the n-alkane/metal system with vibrational spectroscopy [3,4].…”