Density functional theory pore size distribution results

Comparison of the pore size distribution determined by the present method with that from the classical methods such as the BJH, the Broekhoff-de Boer and the Saito-Foley methods is shown in Figure 4. Figure 5 shows a close resemblance of the results of our method with those from the recent NLDFT of Niemark et al. [16], and XRD pore diameter for their sample AMI. The results clearly indicate the utility of our method and accuracy comparable to the much more computationally demanding density functional theory. There are several other methods published recently (e. g. [21]), however space limitations do not permit comparison with these results here. It is hoped to discuss these in a future publication. 

The Horvath-Kawazoe (HK) method is capable of generating model isotherms more efficiently than either molecular simulation (MS) or density functional theory (DFT) to characterize the pore size distribution (PSD) of microporous solids. A two-stage HK method is introduced that accounts for monolayer adsorption in mesopores prior to capillary condensation. PSD analysis results from the original and two-stage HK models are evaluated. 

Others already employed local adsorption isotherms obtained from density functional theory in their calculations of pore size distributions for MCM-41 [16-17]. However, desorption data were used, which imposes two severe limitations on the results of calculations. 

J. P. Olivier, W.B. Conklin and M.V. Szombathely, Determination of pore size distribution from density functional theory a comparison of nitrogen and argon results, in J. Rouquerol, F. Rodriguez-Reinoso, K.S.W. Sing and K.K. Unger (Eds.), Characterization of Porous Solids III, Studies in Surface Science and Catalysis Vol. 87, Proc. of the lUPAC Symposium (COPS III), Marseille, France, May 1993, Elsevier Amsterdam, 1994, pp. 81-90. 

Olivier J P, Conklin W B, Vonszombathely M (1994) Determination of Pore-Size Distribution from Density-Functional Theory - a Comparison of Nitrogen and Argon Results. Characterization of Porous Solids III 87 81-89... 

Olivier, Conklin, Szombathley, Determination of Pore Size distribution from density functional theory - a comparison on Nitrogen and Argon results Studies in surface science and catalysts (Characterization of Porous Solids 3) Elsevier, Amsterdam 1994 pp. 81—87. 

We test three theories for adsorption and capillary condensation in pores against computer simulation results. They are the Kelvin equation, and two forms of density functional theory, the local density approximation (LDA) and the (nonlocal) smoothed density approximation (SDA) all three theories are of potential use in determining pore size distributions for raesoporous solids, while the LDA and SDA can also be applied to mlcroporous materials and to surface area determination. The SDA is found to be the most accurate theory, and has a much wider range of validity than the other two. The SDA is used to study the adsorption of methane and methane-ethane mixtures on models of porous carbon in which the pores are slit-shaped. We find that an optimum pore size and gas pressure exists that maximizes the excess adsorption for methane. For methane-ethane mixtures we show the variation of selectivity with pore size and temperature. 

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