Diffusion and flow of gases and vapours through micropores III. Surface diffusion coefficients and activation energies
Diffusion and flow of gases and vapours through micropores III. Surface diffusion coefficients and activation energies
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DOI：
10.1098/rspa.1951.0186
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发表时间：
1951-10
期刊：
影响因子：
--
通讯作者：
P. Carman;F. Raal
中科院分区：
文献类型：
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作者：
P. Carman;F. Raal
当存在少于完整的单层时，表面扩散系数随着覆盖范围的减小而迅速减小。这是由于吸附剂表面吸附热的变化造成的。扩散是由于结合最松散的分子造成的，随着覆盖范围的减少，这些分子往往首先消失。在给定的系统中只能遵循有限的覆盖范围，并且在此范围内，活化能没有显示出明显的变化。如果需要高活化能的分子在表面扩散中发挥相对较小的作用，则应该遵循这一点。迄今为止，在研究的系统中，活化能还没有小到足以支持“自由移动”或“类气体”吸附层的存在。在低覆盖率下不会达到类气体状态，因为在这些条件下吸附的分子实际上是固定的。根据分子大小、形状和极性特征的差异，在恒定温度和恒定覆盖范围下比较的表面扩散系数对于各种分子来说是不同的。当单层完成并形成多层时，扩散系数几乎不依赖于覆盖范围，并且虽然可能高于液态下的扩散，但具有相似的数量级。毛细管冷凝，至少在初始阶段，会导致表面扩散系数迅速上升。该区域的测量结果的再现性有限。强调了孔隙结构对吸附等温线形状的重要性。它对表面扩散系数的影响已被指出，但尚未详细说明。
When less than a complete monolayer is present, surface-diffusion coefficients decrease rapidly with decreasing coverage. This is attributed to variation in heats of adsorption over the adsorbent surface. Diffusion is due to the most loosely bound molecules, and these tend to disappear first as coverage decreases. Only a limited range of coverage could be followed in a given system and, within this, activation energies showed no marked variation. This should follow if molecules which require high activation energies play relatively little part in surface diffusion. In no system so far studied have activation energies been small enough to support the existence of ‘freely mobile’ or ‘gas-like’ adsorbed layers. A gas-like state is not approached at low coverages, since molecules adsorbed under these conditions are practically immobile. Surface diffusion coefficients compared at constant temperature and constant coverage differ for various molecules according to differences in molecular size, shape and polar character. When the monolayer is complete and multilayers are formed, diffusion coefficients are little dependent on coverage, and, though perhaps higher than for diffusion in the liquid state, are of a similar order of magnitude. Capillary condensation, in at least the initial stages, produces a rapid rise in surface-diffusion coefficients. Measurements in this region have only limited reproducibility. The importance of pore structure on the shape of adsorption isotherms is stressed. Its effect on surface-diffusion coefficients is indicated, but has not been followed in detail.