Adsorption nonlocalized model

Adsorbed ions, specifically, 886 Adsorbent, 969 Adsorption, 971 contact, 959 electrical field, 929 and equation of state, 931 ionic, summary, 964 irreversible, 969, 970 lattice gas models of, 965 nonlocalized, 928, 958 organic and inorganic, 972 of intermediates, 1192... 

The formalism of nonlocal functional density theory provides an attractive way to describe the physical adsorption process at the fluid - solid interface.65 In particular, the ability to model adsorption in a pore of slit - like or cylindrical geometry has led to useful methods for extracting pore size distribution information from experimental adsorption isotherms. At the moment the model has only been tested for microporous carbons and slit - shaped materials.66,67 It is expected that the model will soon be implemented for silica surfaces. 

The model just described is further illustrated in Fig. la, for adsorption of nonlocalizing molecules of solute X or mobile phase M within the adsorbed monolayer. Discrete adsorption sites are shown (asterisks in Fig. 1), but adsorbed molecules are assumed not to prefer positions over these sites i.e., the adsorbent surface can be considered as homogeneous. This model appears to work well for less polar solvents and solutes, but for many LSC systems it begins to fail as adsorbate polarity increases. 

The Ne adsorption isotherms on model AIPO4-5 micropores were calculated from the Tarazona s version of the nonlocal density functional theory [34,35] which has beer actually applied to the study on micropore filling [36,37]. The necessary parameters were obtained fram the adsorption isotherms of Ne on AIPO4-S at 27K and 30K in a lov pressure range. 

Pan et al. [34] used the nonlocal density functional theory (DFT) [35] and the three-process Langmuir model (TPLM) [36] to predict the adsorption heats of propane and butane on carbon and compared these results with experimental data determined from isotherms measured on BAX-activated carbon (Westvaco) in the 297—333 K temperature interval. Both models agreed in showing that the adsorption heat for butane was c. 10 kj/mol higher than that of propane at the same loading. The satisfactory agreement found prompted the authors to propose the use of the DFT method as it requires only one experimental isotherm in contrast with the numerous isotherms required by the classic technique. 

Figure 7.5 (a) A comparison of experimental data for nitrogen adsorbed at 77 K on Vulcan 3-G(2700) (points) with the fit given by the modified nonlocal density functional theory (MNLDFT) models (line), (b) The adsorptive potential distribution for the Vulcan 3 graphite. 

An overall adsorption equation derived within the framework of DFT (Do et al. 2001) improved to be used for different pore models (Gun ko et al. 2007f) and nonlocal DFT (NLDFT Quantachrome Instruments software, version 2.02) with the model of cylindrical pores in silica was applied to calculate the PSDs (PoSD). The differential distribution functions (j/v(l p)6 l p Vp) were converted to incremental PoSDs (IPSDs, 0() p) at EO ( p) Vp) (Figure 1.189). 

The use of nonlocal density functional theory (NLDFT) for modeling adsorption isotherms of Lennard-Jones (LJ) fluids in porous materials is now well-established [1-5], and is central to modem characterization of nanoporous carbons as well as a variety of other adsorbent materials [1-3]. The principal concept here is that in confined spaces the potential energy is related to the size of the pore [6], thereby permitting a pore size distribution (PSD) to be extracted by fitting adsorption isotherm data. For carbons the slit pore model is now well established, and known to be applicable to a variety of nanoporous carbon forms, where the underlying micro structure comprises a disordered aggregate of crystallites. Such slit width distributions are then useful in predicting the equilibrium [1-5] and transport behavior [7,8] of other fluids in the same carbon. 

Experimental results have elearly shown that during the formation of the monolayer, a change from nonlocalized to locahzed adsorption occurs. Several theoretical studies have been made of the so-called partially mobile or partially localized adsorption models [11,60,161,164,228,230]. These theories must explain the phase transitions in the adsorbed monolayers and may also be useful in describing surface transport phenomena [11]. 

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. 

---