Adsorption Science & Technology

The experimental and theoretical investigation of adsorption of a number of binary gas mixtures on graphitized thermal black (GTB), on black TG-10 and on silochrome S-80 was carried out to discover the thermodynamic characteristics of the adsorbed substance, when the surface concentration of component 1 was very small and the concentration of component 2 was varied over rather wide limits. The adsorption was measured by the gas chromatographic method. The dependence of the Henry constant, K₁, of the component 1 on the degree of coverage, θ₂, of the adsorbent surface by the adsorbed component 2 was determined. For the theoretical description of the adsorption data obtained on the homogeneous GTB surface three models were used: (a) two-dimensional lattice model of adsrption from gas mixture in the Bragg–Williams approximation; (b) the well-known model of non-localized (mobile) adsorption on the basis of two-dimensional van der Waals' equation of state; (c) unidimensional lattice model of adsorbing surface which takes into account the interaction between the neighbouring adsorbed molecules. For the last model the exact solution was obtained by the matrix method. The numerical calculations with the suitably chosen parameters were carried out using the limiting forms of the isotherm equations for the dependence of K₁ on θ₂. The results of calculations were compared with experimental data.

The retention volumes for small (zero) sample size (Henry constants) of hydrogen-treated graphitized thermal carbon black (HTGTCB) in five- or six-membered mono- and bicyclic nitrogen-containing compounds have been determined. Comparison of Henry constants K₁ and initial heats of adsorption q̄₁ of nitrogen-containing heterocyclic compounds with those of hydrocarbons of a similar structure has shown that adsorption of the studied heterocyclic compounds on HTGTCB depends on the number and position of nitrogen atoms in their molecules. An increase in the number of nitrogen atoms in flat molecules of azoles (pyrrole, pyrazole, 1,2,4-triazole) brings about the growth of K₁ and q̄₁. In six-membered cycles the growth of K₁ and q̄₁ is observed only during the transition from pyridine to 1,2-diazine (pyridazine), whereas in transition to 1,3- and 1,4-diazines (pyrimidine and pyrazine), and further on to s-triazine, the retention greatly reduces. In azanaphthalenes (quinoline, cinnoline, quinoxaline) and benzazoles (indole, indazole) the growing number of nitrogen atoms also results in weaker retention.

Infrared spectra of species adsorbed on supported metal catalysts are subject to dipole coupling effects which have been extensively studied on single crystal substrates. Coupling interactions influence spectra in three ways: frequency shifts, changes in absolute molecular absorption intensities, and transfers of intensity from one absorption band to another. The magnitude of frequency shifts depends on the size of the adlayer, and may consequently vary with the size of the catalyst particles, while depolarization effects can reduce molecular absorption intensities by a factor of three or four at high adsorbate densities. The most striking influence of coupling on spectra from supported catalysts, however, arises from intensity transfers between molecules adsorbed on dissimilar sites. At high surface coverages the apparent importance of a small number of defect sites can be greatly enhanced by this effect, and it is even possible for a strongly bound minority species to be wrongly identified as a weakly bound majority one. Such errors can be avoided if spectra from isotopic mixtures are recorded.

The adsorption process of benzene + cyclohexane mixtures on silica gel with modified surface has been studied. Silica gel was thermally modified at 473 K, 823 K and 1023 K. Another set of modified silicas was prepared by adsorption of different amounts of water on the surface. The characteristics of the silicas were established on the basis of adsorption isotherms of nitrogen at 78 K and thermogravimetric measurements. The stability of the adsorbed layer of water in the adsorption process from the liquid phase was examined by using the Karl-Fischer method. Excess adsorption isotherms of benzene from cyclohexane were discussed in terms of a theoretical formulation concerning the adsorption on heterogeneous solid surfaces. It has been found that energetical homogeneity of the silica gel surface increases from the silica gel activated at 473 K through silicas deactivated at 823 K and 1023 K to silica gels with physically adsorbed water.