哈工大高等热力学文献阅读报告(4)

(111) crystal face of platinum, and graphite surfaces. The desorption prefactors computed from the Campbell-Sellers equation reduce the maximum error to 50. The main conclusion

1) The large slope of the Campbell-Sellers relation reveals that the entropies of adsorbed molecules are quite high and approach those of two-dimensional gas molecules. The implication is that the adsorbed molecules move nearly freely within the surface plane at temperatures where desorption first becomes important.

2) The Campbell-Sellers relation establishes that, in general, adsorbed molecules readily overcome in-plane barriers to motion once the molecules acquire nearly enough energy to surmount the larger energy barrier for desorption .

3) Although this finding is physically reasonable and perhaps obvious in retrospect, Campbell and Sellers have actually demonstrated and quantified the large entropies of adsorbed molecules. Thus, modeling the in-plane, center-of-mass motions as localized vibrations substantially overestimates desorption prefactors for many adsorbed molecules.

Personal evaluation of the paper with future thinking

Theoretical work may be able to provide a general framework from which to calculate entropies on the basis of molecule and surface properties. The Campbell-Sellers equation will provide essential guidance to such efforts, as it reveals that the molecule-surface potential continues to slightly hinder the motions of adsorbed molecules even as desorption becomes important. Key challenges in theoretical modeling will be to develop methods that properly and efficiently describe the weakly hindered motions of various types of molecules adsorbed on different surfaces. Although more challenging, this situation is analogous to determining the torsional states available to many polyatomic molecules. The findings of Campbell and Sellers indeed represent an important advance in the understanding and quantification of the entropies of a wide range of adsorbed molecules, and may well be broadly applicable.

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