BASIC CONCEPTS AND AN OVERVIEW - Foundations of Nanotechnology: Pore Size in Carbon-Based Nano-Adsorbents - page 105

depends on the proportion of pores of a particular size. In general highly micropo-
rous carbons are preferred for the adsorption of gases and vapors and for the sepa-
ration of gas molecules of different dimensions if the carbon possesses a suitable
distribution of narrow size pores (molecular sieves) while well-developed meso
and macroporosity is necessary for the adsorption of solutes from solutions [145].
It is the entrance dimension and shape, which controls the adsorption process,
be hexagonal (an appropriate shape), or circular or square. Once the adsorbate
molecule is through the pore entrance, then the characteristics of the adsorbent
take over and the isotherm is created that is not enough, because the processes of
physical and chemical activation have to be understood and to do this, requires
three-dimensional models porosity in carbons. It would be an advantage to have
some idea of the structure of this network include of carbon atoms, in three di-
mensions, in order to understand the extraction (gasification) process. There are
four limitations of importance. First, this maze, of course, is in two dimensions;
second, the lines of the maze are too orientated relative to an x - y axis. Such paral-
lelism is unlikely to exist within a porous carbon; third, this labyrinth is best suit-
ed to a microporous carbon, only and not to microporous carbon fibers. Fourth,
in such a model, rates of diffusion are likely to be too slow and hence there is a
need to consider the location of mesoporosity. The inclusion of mesoporosity is
another matter. Mesoporosity has to promote enhanced adsorption to the interior
of the fiber. As a matter of scaling, although the models that mentioned before
provide an impressive number of adsorption locations, it will require too much of
such models. The human mind cannot cope with this necessity. But, apart from
these limitations the similarities are relevant enough:
1. There is a continuous connection between the lines throughout the laby-
rinth. All carbon atoms form part of a continuous graphene sheet.
2. There is a continuous connection of the routes (spaces) of the labyrinth.
Hence, all adsorption sites are available to the adsorbate molecules.
3. The widths of the routes (spaces) of the labyrinth are not constant. Some
are narrower than other. This is a very relevant point as it demonstrates,
very clearly, the range of porosities, within the definition of microporos-
ity of <2.0 nm, this accounting for molecular sieving effects.
4. Some of the routes of the labyrinth are barely visible (being very close to
each other). This offers the suggestion that it is representative of closed
porosity, that is closed to everything except helium and lithium, noting,
on the way, that the term closed porosity is an imprecise term, meaning
porosity not accessible to a specifically defined adsorbate molecule.
5. The PSD calculating models makes the point that access to the interior of
the labyrinth is available from all external surfaces.
6. A close inspection of the edges of the lines of labyrinth (surfaces of the
carbon surfaces) indicates a lack of smoothness, the edges being rough.
This point is of importance to carbon science because the surfaces of po-