BASIC CONCEPTS AND AN OVERVIEW - Foundations of Nanotechnology: Pore Size in Carbon-Based Nano-Adsorbents - page 139

of three wall layers. The wall atoms are arranged on a quadratic grid. This allowed
us to continuously vary the inner pore radius (r = 13.2 A-40 A, measured from
the center of the wall atoms) in a prototype system, broadly characteristic of real
adsorbents, but without being constrained to, the radii of channels in known zeo-
lite structures. The model pores consisted of three coaxial cylinders built by rings
forming a quadratic arid with a distance of 1.6 A between individual wall atoms.
The surface density of these model Pores was chosen to be 38 atoms/nm 2 , a value
close to the value of graphite (note, however, that the volume density is larger than
in graphite, as the lavers were only 1.6 A apart). In DCV-GCMD simulations, the
Dore is divided into two control volumes and a flow region, performing a number
of GCMC insertions and deletions in the control volumes, the chemical Potential
is individually controlled in each control volume and a gradient in the chemical
Potential can thus be established. The standard acceptance rules for GCMC inser-
tions and deletions are used. The movement of the molecules is described by MD
moves where Newton's equation of motion re integrated in order to get the trajec-
tory. Then the system is frozen and a new DCV-GCMD cycle starts by performing
GCMC steps. It was found that in the Pores two different diffusion mechanisms
occur independently surface diffusion in the layer adsorbed to the pore wall and a
combination of Knudsen and molecular diffusion in the center of the Pore. There-
fore, the Pores were divided in two regions: a wall region and an inner region,
and the diffusivities were calculated separately in each region. In the wall region
formed by the first adsorbed layer, 52 to 88% of the fluid molecules are located
(the Proportion depends on the Dore radius and is larger in smaller Pores as here
the influence of the Pore wall is larger) but only 5 to 19% of the flux takes Place.
The diffusivities in the wall region are about two orders of magnitude smaller
than in the inner region and are of the order of magnitude of liquid-phase diffu-
sion coefficients. The simulations at different temperatures revealed that in the
wall region surface diffusion-an activated Process is taking Place. The simulated
activation energy of 10.7 kJ/mol for CH 4 is in the order of magnitude of experi-
mental results. For the inner region, where only 12 to 48% of the fluid molecules
are located but81 to 95% of the flux takes Place, the investigations of the radius
and the temperature dependence showed that diffusion is taking place in the mo-
lecular diffusion regime or in the transition regime. For pores with radius larger
than approximately 23 A, the diffusivity is no longer a function of the pore radius
and diffusion is taking place in the molecular diffusion regime. The pore radius
for which molecular diffusion starts to dominate is smaller for CF 4 than for CH 4 as
the mean free path of CF 4 is smaller (CF 4 is the larger molecule). Our simulations
demonstrate that is possible to use DCV-GCMD simulations to study in detail the
complex diffusion processes that take place in different regions within the pore
space of a porous solid.