哈工大高等热力学文献阅读报告(2)

Paper #1

Entropies of Adsorbed Molecules Exceed

Expectations

Jason F.Weaver

Department of Chemical Engineering, University of Florida

Description of the problem

The ability to accurately predict the rates of chemical reactions at surfaces is essential to improving technological applications that rely on molecule-surface interactions, such as designing new catalysts for use in chemical synthesis and power-generation applications. Such accurate predictions depend on knowledge of both the entropies and enthalpies of the reacting species. Although advances in molecular modeling have increased the accuracy for the enthalpies of adsorbed molecules, reliable methods for calculating the entropies of adsorbed molecules have been lacking; even estimating their magnitudes has remained elusive.

The mathematical model/method/approach

Any description of a surface reaction needs to consider how molecules bind to the surface, and a common way to probe the kinetics of this process is thermal desorption—heating the surface in vacuum and monitoring the desorption rate. The rate coefficient for desorption k is typically described by an Arrhenius relation

k = A exp(–E/RT),

where A is the kinetic prefactor, E is the activation energy for reaction, R is the gas constant, and T is absolute temperature. The prefactor scales exponentially with the difference in entropy between the initial reactant molecule and a transition structure that the molecule adopts along the reaction path.

Adsorption is often a nonactivated process; that is, the molecule encounters no potential barrier as it moves toward the surface. Thus, the transition structure for desorption may be taken as the gaseous molecule moving away from the surface. To predict a desorption prefactor for this typical case, both the entropy of the gas-phase molecule and that of the adsorbed molecule are needed. The former is readily calculated, but estimating the entropy of an adsorbed molecule is difficult because the

center-of-mass motions of the molecule within the potential field of the surface are not

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