Adsorption properties, calculation

Binding energy Adsorption properties calculation and zero point energy correction ... 

Tarazona A, Kreisig S, Koglin E and Schwuger M J 1997 Adsorption properties of two cationic surfactant classes on silver surfaces studied by means of SERS spectroscopy and ab initio calculations Prog. Colloid Polym. Sol. 103 181-92... 

The dried polyoxazoline-modified silica gel was immersed into distilled water. The adsorption property of the resulting gel was estimated by the water content. The water uptake was calculated from an expression of (W -W)jW, where Wis the weight of dried gel and W is the weight of water-absorbed gel. The modified gel showed a higher water-adsorption property than that of untreated silica gel, which absorbed 10.8 multiples of water. The water uptake of modified gel was up to 13.7 multiples of the weight of dried gel. Thus, silica gel has been made more hydrophilic by a polyoxazoline segment. 

Selective oxidation of p-xylene to terephthaldehyde (TPAL) on W-Sb oxide catalysts was studied. While WO3 was active in p-xylene conversion but non-selective for TPAL formation, addition of Sb decreased the activity in p-xylene conversion but increased TPAL selectivity significantly. Structure change was also induced by Sb addition. Evidences from various characterization techniques and theoretical calculation suggest that Sb may exist as various forms, which have different p-xylene adsorption property, reactivity toward p-xylene and TPAL selectivity. Relative population of each species depends on Sb content. 

The effects of calcium on polymer-solvent and polymer-surface interactions are dependent on polymer ionicity a maximum intrinsic viscosity and a minimum adsorption density as a function of polymer ionicity are obtained. For xanthan, on the other hand, no influence of specific polymer-calcium interaction is detected either on solution or on adsorption properties, and the increase in adsorption due to calcium addition is mainly due to reduction in electrostatic repulsion. The maximum adsorption density of xanthan is also found to be independent of the nature of the adsorbent surface, and the value is close to that calculated for a closely-packed monolayer of aligned molecules. 

The computed values of WQ lie near those calculated from crystallographic data for synthetic zeolites. The value Wo = 0.195 cm3/gram estimated here for zeolite 5A also compares favorably with the mean value Wo = 0.20 cm3/gram previously obtained 21) for the adsorption of various adsorbates on the same adsorbent, without reference to any isotherm equation. For the synthetic zeolite, the preparation method may lead to variations in adsorption properties, and this may explain the difference between values of W0 shown in the Table I. Finally, for Cecalite, where no theoretical value is known, the values obtained here for W0 with two different adsorbates are consistent with each other. Thus, the proposed method gives realistic values for W0. 

Using a Flory-Huggins and a UNIFAC model for the calculation of the solid- and liquid-phase activities, respectively, it was possible to determine the adsorptive properties as a function of the ion-exchange capacity of the resin [20],... 

The transport and adsorption properties of hydrocarbons on microporous zeolites have been of practical interest due to the important properties of zeolites as shape-selective adsorbents and catalysts. The system of benzene adsorbed on synthetic faujasite-type zeolites has been thoroughly studied because benzene is an ideal probe molecule and the related role of aromatics in zeolitic catalysts for alkylation and cracking reactions. For instance, its mobility and thermodynamic properties have been studied by conventional diffusion 1-6) and adsorption 7-9) techniques. Moreover, the adsorbate-zeolite interactions and related motion and location of the adsorbate molecules within the zeolite cavities have been investigated by theoretical calculations 10-15) and by various spectroscopic methods such as UV (16, 17), IR 17-23), neutron 24-27), Raman 28), and NMR 29-39). 

Most of the calculations support the use of the surface molecule concept. The features predicted for metals such as Ni and Pb and nonmetals such as C are in accord with what is expected from experimental measurements. One exception is the graphite surface where different adsorption properties were found, depending on whether a finite or infinite surface model was employed. 

To examine the adsorption properties, we have first optimized the geometric structures of all the clean tubes under consideration. Secondly, for each tube we have found the optimal adsorption distance d(J between the H atom and the tube. To do this, we have calculated the total energy I(1 [BNNT + 2 H] of the hydrogen-terminated BNNTs with the adsorbed H atom on the top of the B or N atom as a function of the distance d between the adatom and the tube. As an example, in... 

If the elements 112 and 114 have a noble-gas like character [62], then, in a fictitious solid state, they would form non conducting colorless crystals. A physisorptive type of adsorption may occur and their adsorption properties, for example on quartz, can be calculated with this method [61], see Table 3. For physisorbed noble gas atoms a roughly uniform distance to different surfaces of about 2.47 0.2 A was deduced from experimental results [63]. 

Tables I and II list major typical physical and adsorptive properties of the powdered active carbon. Effective surface area, measured by the BET method using a Digisorb 2500, is consistently in the range of 3000 to 3400 m /gm. This exceeds the theoretical area of about 2600 m /gm as calculated by the area of one gram of a graphitic plane because of multilayer adsorption and pore filling in a highly microporous structure.

The reader can find in the literature several calculations showing that in the very closest surroundings of a modifier (sulphur, promoter) the local density of states is dramatically changed, so that a modification of adsorption properties on such sites should be expected. However, the most dramatically changed sites cannot be reached by adsorbing molecules since they are under the van der Waals (repulsive) radius of the modifier atom. 

The results presented in the previous sections demonstrate the importance of point defects at the surface of oxide materials in determining the chemical activity of deposited metal atoms or clusters. A single Pd atom in fact is not a good catalyst of the cyclization reaction of acetylene to benzene except when it is deposited on a defect site of the MgO(lOO) surface. A detailed analysis of the reaction mechanism, based on the calculation of the activation barriers for the various steps of the reaction, and of a study of the preferred site for Pd binding, based on the MgO/Pd/CO adsorption properties, has shown that the defects which are most likely involved in the chemical activation of Pd are the oxygen vacancies, or F centers, located at the terraces of the MgO surface and populated by two (neutral F centers) or one (charged paramagnetic F centers) electrons. 

This proposal was based on adsorption properties of Ti sites, on the reactivity behaviour of organic and inorganic peroxides and on the smooth formation of cyclic structures in hydroperoxides and other compounds. In the alternative species, based on quantum chemical studies, the hydroperoxide group was T] coordinated and did not require stabilization (Figure 18.1b) [72]. In another proposal, also based on DFT calculations, coordination and hydrogen bonding with a protic molecule coexist in the same structure (Figure 18.1c) [73, 74]. 

Calculation of Some Adsorption Properties of Zeolites by Statistical Methods... 

TJecause of their regular structure and the presence of more or less separate cavities, the zeolites offer a very convenient system for the application of statistical methods. In this work, an attempt has been made to calculate some adsorption properties of zeolites using statistical thermodynamics. 

Activity coefficients for nonideal mixtures have been reported. The error associated with the use of IAS theory is shown in Figure 5. The solid lines are the experimental data and the dashed lines were calculated from Equations (13)-(18). The comparison of the IAS prediction with experimental data in Figure 5 raises the following question is it possible to predict activity coefficients Correlations of activity coefficients with single-gas adsorptive properties suggest that such predictions are possible, and reliable methods may be discovered in the future. 

As soon as the expressions and constants in Eq. (4) are fixed one may proceed with calculation of the adsorption energy at a given point of adsorption space. However, to make such a calculation possible one has to know the positions of all the atoms of adsorbent relative to the given point. In other words, one has to know exactly the atomic structure of the adsorbent. This is what is in fact unknown for amorphous oxides. Although one can simulate the atomic structure at the surface of an amorphous oxide as described above, the reliability of the result can at present only be checked by comparison of prediction of adsorption properties with experimental data. But the calculation of adsorption properties (described below) includes, generally speaking, two unknowns the atomic structure of an adsorbent and the adsorption potential. This is the reason why the computer simulation of physical adsorption on amorphous oxides should be preceded by similar simulations on oxides with well defined crystalline structures. 

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