Surface properties empirical prediction

There are few systematic guidelines which can be used to predict the properties of AB2 metal hydride electrodes. Alloy formulation is primarily an empirical process where the composition is designed to provide a bulk hydride-forming phase (or phases) which form, in situ, a corrosion— resistance surface of semipassivating oxide (hydroxide) layers. Lattice expansion is usually reduced relative to the ABS hydrides because of a lower VH. Pressure-composition isotherms of complex AB2 electrode materials indicate nonideal behaviour. 

This section discusses the techniques used to characterize the physical properties of solid catalysts. In industrial practice, the chemical engineer who anticipates the use of these catalysts in developing new or improved processes must effectively combine theoretical models, physical measurements, and empirical information on the behavior of catalysts manufactured in similar ways in order to be able to predict how these materials will behave. The complex models are beyond the scope of this text, but the principles involved are readily illustrated by the simplest model. This model requires the specific surface area, the void volume per gram, and the gross geometric properties of the catalyst pellet as input. 

Molecular dynamic studies used in the interpretation of experiments, such as collision processes, require reliable potential energy surfaces (PES) of polyatomic molecules. Ab initio calculations are often not able to provide such PES, at least not for the whole range of nuclear configurations. On the other hand, these surfaces can be constructed to sufficiently good accuracy with semi-empirical models built from carefully chosen diatomic quantities. The electric dipole polarizability tensor is one of the crucial parameters for the construction of such potential energy curves (PEC) or surfaces [23-25]. The dependence of static dipole properties on the internuclear distance in diatomic molecules can be predicted from semi-empirical models [25,26]. However, the results of ab initio calculations for selected values of the internuclear distance are still needed in order to test and justify the reliability of the models. Actually, this work was initiated by F. Pirani, who pointed out the need for ab initio curves of the static dipole polarizability of diatomic molecules for a wide range of internuclear distances. 

Environmental Fate. Experimental data are available regarding the transport and partitioning properties of chloroform in surface waters (Bean et al. 1985 Clark et al. 1982 Class and Ballschmidter 1986 Dilling 1977 Ferrario et al. 1985 Piwoni et al. 1986 Sawhney 1989). Chloroform partitions mainly into the atmosphere and into groundwater. Empirical measurements or model predictions on half-disappearance times in such media as soil could not be identified in the literature. Chloroform can be transported long distances in air. Data are available regarding the degradation of chloroform in the... 

Since liquid does not completely wet the packing and since film thickness varies with radial position, classical film-flow theory does not explain liquid flow behavior, nor does it predict liquid holdup (30). Electrical resistance measurements have been used for liquid holdup, assuming liquid flows as rivulets in the radial direction with little or no axial and transverse movement. These data can then be empirically fit to film-flow, pore-flow, or droplet-flow models (14,19). The real flow behavior is likely a complex combination of these different flow models, that is, a function of the packing used, the operating parameters, and fluid properties. Incorporating calculations for wetted surface area with the film-flow model allows prediction of liquid holdup within 20% of experimental values (18). 

It is the purpose of this review to discuss and illustrate the methods presently employed to obtain potential energy surfaces by approximate, but non-empirical solutions to Schrodinger s electronic equation. In addition to discussing the different levels of approximation employed in these ab initio calculations, we emphasize the type of chemical system (in terms of its electronic structure) to which each level of calculation may be expected to yield usable results, i.e. results with acceptable errors or with predictable bounds on the error. Our interest will be primarily in surfaces which have been determined for the prediction and understanding of chemical reactions. This will include a survey of those calculations which have concentrated on determining the reaction path, and the geometry and properties of the system at points on this path, as well as those in which an essentially complete surface has been determined. The latter type of calculation coupled with either classical or quantal treatments of the nuclear motion on such a surface provides a total theoretical prediction of a chemical reaction. This ultimate objective has been achieved in the case of the H + Ha exchange reaction. 

It is desirable to compare the predictions of the theory presented here with experimental results obtained on some systems in which an independent computation of the gas-surface potential function can be carried out. A calculation of the potential functions for the adsorption of rare gases on solid rare gases involves the least number of unknown parameters. The rare gases crystallize into face-centered cubic solids with known lattice constants. Furthermore, the parameters appearing in the Lennard-Jones potential functions for the gas-gas and the gas-solid atom interaction can be estimated to a good degree of accuracy from experiments on the gas properties as well as from the empirical combining laws for potential parameters. Furthermore, some experimental results have already been reported for these adsorption systems (18, 20). 

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