Becker Kelly E, Fichthorn Kristen A
Department of Chemical Engineering, The Pennsylvania State University, University Park, Pennsylvania 16802, USA.
J Chem Phys. 2006 Nov 14;125(18):184706. doi: 10.1063/1.2364894.
We utilize accelerated molecular dynamics to simulate alkane desorption from the basal plane of graphite. Eight different molecules, ranging from n-pentane to n-hexadecane, are studied in the low coverage limit. Acceleration of the molecular dynamics simulations is achieved using two different methods: temperature acceleration and a compensating potential scheme. We find that the activation energy for desorption increases with increasing chain length. The desorption prefactor increases with chain length for molecules ranging from pentane to decane. This increase subsides and the value of the preexponential factor fluctuates about an apparently constant value for decane, dodecane, tetradecane, and hexadecane. These trends are consistent with data obtained in experimental temperature-programed desorption (TPD) studies. We explain the dependence of the preexponential factor on alkane chain length by examining conformational changes within the alkane molecules. For the shorter molecules, torsional motion is not activated over experimental temperature ranges. These molecules can be treated as rigid rods and their partial loss in translational and rotational entropies upon adsorption increases as chain length increases, leading to an increasing preexponential factor. At their typical TPD peak temperatures, torsions are activated in the longer adsorbed chain molecules to a significant extent which increases with increasing chain length, increasing the entropy of the adsorbed molecule. This increase counteracts the decrease in entropy due to a loss of translation and rotation, leading to a virtually constant prefactor.
我们利用加速分子动力学来模拟烷烃从石墨基面的脱附过程。在低覆盖度极限下研究了从正戊烷到正十六烷的八种不同分子。分子动力学模拟的加速通过两种不同方法实现：温度加速和补偿势方案。我们发现脱附活化能随链长增加而增大。对于从戊烷到癸烷的分子，脱附前因子随链长增加。这种增加在癸烷、十二烷、十四烷和十六烷时减弱，且指前因子的值在一个明显恒定的值附近波动。这些趋势与实验程序升温脱附（TPD）研究中获得的数据一致。我们通过研究烷烃分子内的构象变化来解释指前因子对烷烃链长的依赖性。对于较短的分子，在实验温度范围内扭转运动未被激活。这些分子可视为刚性棒，其吸附时平移和旋转熵的部分损失随链长增加而增大，导致指前因子增大。在其典型的TPD峰值温度下，较长吸附链分子中的扭转在很大程度上被激活，且随链长增加而增加，增加了吸附分子的熵。这种增加抵消了由于平移和旋转损失导致的熵的减少，导致指前因子实际上保持恒定。
