Physical adsorption nonuniform surface

The notion of nonuniformity of catalytic surfaces has been originally advanced by Langmuir (20) and particularly by Taylor. The physical nature of nonuniformity is insufficiently clarified. To some extent, it results from the difference in properties of crystal faces, from dislocations, and other disturbances of crystal lattice. It is possible that admixtures of some foreign substances is of greater importance. The particles of admixtures change adsorption energy on adjacent surface sites. The model of nonuniform surface probably describes the overall result of the effect of particles of admixtures on adsorbed particles and of the mutual influence of adsorbed particles (i.e., to an approximation the model takes into account not only biographical, but also induced nonuniformity). 

The importance of surface area in colloidal chemistry has spurred many attempts to develop a method of its accurate measurement from physical adsorption processes. All of the methods so far are empirical and attended with difficulty involving surface nonuniformity, polymolecularity, conformational shifts, and multilayer adsorption. Polysaccharide surfaces are seldom... 

Roginskif suggested a simplified method of analysis for processes occurring on a nonuniform surface which made it possible to surmount these mathematical difficulties without excessive distortion of the physical model. The method has general applicability to statistical processes however, its application to adsorption equilibrium only will be discussed here. 

The assumption that, for a nonuniform surface, E increases linearly with increase of coverage is unrealistic from a physical viewpoint it is however, a convenient postulate from a mathematical viewpoint, particularly when it is realized that a surface comprising a small number of homogeneous patches, each patch having different E values on which there may or may not be induced effects, gives rise to an adsorption rate which subscribes well to an Elovich equation this model is an acceptable physical description for adsorbents in the form of powders or evaporated films. Similarly, models comprising uniform surfaces, but with site creation or exclusion, may be analyzed and extended to give conclusions of the same natures as those derived from a variation of E over a nonuniform surface mathematically, however, the extension to, e.g., interaction effects between two different adsorbates is more cumbersome. 

Two different assumptions are generally used for the description of the physical chemistry of the real adsorbed layers either surface sites are different or there is a mutual influence of the adsorbed species. The first case is defined as biographical nonuniformity and the second one - induced nonuniformity. On biographical nonuniform surfaces a certain distribution of properties is considered. Such nonuniformity can be either chaotic, when adsorption energy on a given site is independent on the neighbor site or discrete. Flowever, if... 

Experimental and theoretical studies of H adsorption are mostly performed on idealized surfaces, that is single crystals. Adsorption on real surfaces is a more complex phenomenon, mainly because of the physical and chemical nonuniformity of the surfaces. Surface defects (steps, kinks, grain boundaries) and the presence of impurity atoms strongly affect the adsorption [53]. Surfaces of alloys and intermetal-lic compounds show additional phenomena related to the fact that the chemical composition at the surface may differ from that in the bulk [47]. 

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