Microporous carbons functions

Fig. 32. Reversible capacity of microporous carbon prepared from phenolic resins heated between 940 to 1 I00°C plotted as a function of the X-ray ratio R. R is a parameter which is empirically correlated to the fraction of single-layer graphene sheets in the samples.

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. 

For the accurate characterization of the adsorption phenomena, it is necessary to obtain accurate information on pore stmctures. However, most of ordinary microporous carbons and mesoporous carbons are obtained with amorphous stmctures that are characterized by irregular arrangements of non-uniform pores. X-ray (or electron) diffraction (XRD) techniques are not useful for such carbons because there are no well-defined stmctural factors to correlate with the adsorption behavior. Moreover, porous carbons exhibit wide varieties of the surface functional groups and the thickness of the pore walls, depending on the details of the synthesis conditions. The lack of distinct XRD lines makes it difficult to distinguish stmctural differences between samples which causes many works to depend empirically on specific samples. 

Lastoskie, C.M., Gubbins, K.E. and Quiike, N.J., Pore size distribution analysis of microporous carbons a density functional theory approach. J. Phys. Chem. 97 (1993) 4786. Olivier, J.P., Modeling physical adsorption on porous and nonporous solids using density functional theoiy. J. Porous Materials 2 (1995) 9. 

The well-defined initial state requires to use high-purity Hquids, specially when these are hydrocarbons (well-suited for carbons, which they easily wet) it was early shown indeed, by Harkins and Boyd [3], that very small amounts of a polar impurity Hke water dramatically increases the enthalpy of immersion, since the latter is preferentially and strongly adsorbed on the polar functions of the sur ce. Obtaining a reproducible state of outgassing for a microporous carbon is even more critical, specially for microporous carbons. Before deciding on the outgassing procedure, one should have in mind the following ... 

Stoeckh and Huber ° initiated the development of the theory of volume filling for heterogenous microporous carbons and suggested a refinement in the Izotova and Dubinins approach in collaboration with Dubinin and coworkers. Skoecklis refinement is based on the idea of a continuous distribution of the miaopores as a function of B and can be represented as... 

Lastoskie C, Gubbins KE, QuirkeN Pore size distribution analysis of microporous carbons a density functional theory approach, J Phys Chem 97(18) 4786-4796, 1993a. 

These models were generated to study self-diffusion of fluids within microporous carbons as a function of temperature and porosity. However, their immediate application to microporosity of activated carbons from traditional sources is not that obvious. From this point of... 

The utilization of the proposed correction has been compared to modem calculation methods like Density Functional TTieory (DFT), as this methodology is in principle applicable to both micro- and mesoporous materials. To this end the DFT-pore size distribution of the two non-microporous carbons has been calculated (Kernel CO2 adsorption on Carbon [16]). These pore size distributions however showed significant micioporosity in the range 4-10 A. N2 as adsorptive however showed no contribution in this region. It can therefore be concluded that not only the classical methods fail to describe the contribution of the external surface by CO2 adsorption at low relative pressure but also modem methods cannot correct properly. It is expected that the DFT-method can only take into account the presence of mesopores if these pores have actually been quantified (this requires high pressure CO2 adsorption). 

Pore size distribution in microporous carbons obtained from molecular modeling and density functional theory... 

Sigalov S, Levi MD, Salitra G, Aurbach D, Maier J (2010) EQCM as a unique tool for determination of ionic fluxes in microporous carbons as a function of surface charge distribution. 

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