POROSITY AND SORPTION BEHAVIOUR
understanding of the processes involved in adsorption are vital to gaining
an insight of the mechanisms involved. The principles of adsorption onto
porous adsorbents, in particular activated carbons, are considered with
reference to gas and vapour adsorption processes.
Principles of Adsorption
The term adsorption is said to have been first used by Kayser 
in 1881 in order to explain the condensation of gases on surfaces, in
contrast to gas absorption in which gas molecules penetrate the bulk phase
of the absorbing solid. The term ‘sorption’ was proposed by
McBain  as a complete description of mass transport
into a solid, encompassing surface adsorption, absorption by penetration
into the solid and condensation within pores.
Adsorption is described as the enrichment of one or more components in
the interfacial layer, i.e. an excess of molecules
exists at the adsorbate/adsorbent interface, upon exposure of an adsorbing
solid to a gas or vapour. It is the selective collection and concentration
onto solid surfaces of certain molecules contained in a vapour or gas
stream. Hence, vapours or gases referred to as adsorbates when adsorbed,
even of mixed systems and at low concentrations, may be captured, often
selectively, and removed from the effluent stream using a material of
the category of adsorbents.
Adsorption is divided into the two sub-categories of physical adsorption
(physisorption) or van der Waals adsorption and chemical adsorption (chemisorption)
and the adsorption process can be determined whether chemical bonds are
formed during the process. Physisorption is applicable to all adsorbate-adsorbent
systems provided the conditions of pressure and temperature are suitable
whereas chemisorption may only occur if the system is capable of making
a chemical bond.
Physical Adsorption (Physisorption)
The process is a dynamic one where an equilibrium state exists with molecules
and the interaction between the adsorbate and adsorbent.
No chemical bonds are formed during physical adsorption; attraction between
the adsorbate and adsorbent exists by the formation of intermolecular
electrostatic, such as London dispersion forces, or van der Waals forces
from induced dipole-dipole interactions, or may be dependent on the physical
configuration of the adsorbent such as the porosity of activated carbons.
Dispersion forces are the result of rapid fluctuations in the electronic
density of one adsorbent molecule inducing an electrical moment in a second
atom. If the adsorbate possesses a permanent dipole,
or even a multipole, then additional interactions may occur, as charge
distributions are induced in the adsorbent and interactions of these moments
with any permanent field of the solid.
The process is a very general one and is analogous with that of condensation.
Physisorption occurs to varying extents for all adsorbates, gases and
vapours, with all adsorbing solids and the effect increases with decreasing
temperature or increasing pressure. Physical adsorption is based on certain
basic considerations and adsorption on a heterogeneous surface, that is
a surface on which the sites are different, occurs at the sites of highest
adsorption potential. The process of physical adsorption into the microporous
structure of activated carbon follows the theory of Dubinin.[5-7]
The mechanism of adsorption is dependent upon the size of the admolecule
in comparison with the pore width due to the energetic interactions between
the chosen adsorbate and the pores. Admolecules initially adsorb into
the pores with the highest energy, ignoring activated diffusion effects,
then adsorption proceeds via filling of progressively larger, or decreasing
energy, porosity. Some pores are capable of accommodating two or three
admolecules and, therefore, may undergo co-operative adsorption effects
by reducing the volume element thus increasing the energy and adsorptive
potential of the pore.
The process of adsorption is always exothermic due to the increased ordering
of the adsorbate on the adsorbent surface, reducing the entropy, as:
= DH - TDS
the amount adsorbed should decrease with increasing temperature as a reduction
in the thermal energy supplied to the process, by Le Chatelier’s
principle, favours the exothermic process of adsorption increasing the
equilibrium uptake, except in the case of activated diffusion.
It has been proposed by Lamond and Marsh, by the interpretation
of data for physical adsorption of nitrogen on both polar and non-polar
surfaces that physical adsorption is independent of the surface chemistry
of the adsorbent.
Chemical Adsorption (Chemisorption)
Chemisorption involves the transfer of electrons between the adsorbent
and the adsorbate with the formation of chemical bonds, by chemical reaction,
between the two species causing adhesion of the adsorbate molecules. Chemical
adsorption is far less common than physical adsorption and due to the
chemical bonds formed regeneration of the adsorbent for subsequent re-use
is often difficult or impossible.
Due to the fact that chemical bonds are formed during the adsorption process,
desorption of the adsorbed phase may yield products which are chemically
different to the original adsorbate. For example oxygen may chemically
bond to the surface of a carbon, which upon desorption may evolve CO and
CO2 as products.
Characteristics Associated with Physical/Chemical Adsorption 
of Adsorption / kJmol-1
c.f. heats of liquefaction
80 c.f. bulk-phase chemical reactions
Dependence of Uptake (with Increasing T)
by reduced pressure or increased temperature
- high temperature required to break bonds
be different to original adsorptive
or multilayer condition dependent
The word pore comes
from the Greek word ‘poros’ which
means passage. This indicates the role of a pore acting as a passage between
the external and the internal surfaces of a solid, allowing material,
such as gases and vapours, to pass into, through or out of the solid.
Almost all adsorbents that are used in catalysis or for purification/separation
purposes possess porosity, and this is the only practical method of introducing
greatly enhanced surface areas into a solid.
were the source of several observations made by Bussy.
He reported that porosity, although he could not measure it directly,
was extremely important and that the decolourising power of a carbon was
dependent on the parent precursor and the processes used in its manufacture.
Porosity has been
studied extensively by the process of adsorption 
where exposure of a porous solid to an adsorbate creates a concentration
of the adsorbate at the surface, within molecular or atomic distance,
greater than the adsorptive concentration in the gas phase. An excess
of molecules exists dynamically at the interface in the process called
There are four terms
generally used to describe the accessibility of porosity:
pore’ - a pore which is connected to the external surface of a solid
and allows the passage of an adsorbate through the solid
pore’ - a void within the solid which is not connected to the external
surface and hence is isolated
pores’ - these connect different parts of the external surface of
the solid to the inner microporosity
pores’ - are connected to transport pores but do not lead to any
other pore or surface
It may be useful
to explain the terms of internal and external surfaces:
surface’ - composed of the area which surrounds the closed pores
as well as all fissures and cracks which penetrate deeply into the interior
of the adsorbent which are deeper than they are wide, and
surface’ - consists of the protrusions and superficial cracks which
are wider than they are deep.
The total surface
of a carbon material is predominantly composed of internal surface and
activated carbons owe their adsorptive properties to the internal surface
area and pore size distribution with the external surface area and functional
groups playing a comparatively minor role.
below shows the different components of a typical mass transport system
of a solid. The total pore volume is essentially the sum of two fractions:
= Vo + Vc
where Vt is the total
pore volume, Vo is the pore volume due to the open porosity in the solid,
including transport and blind pores, and Vc is the closed pore volume.
Types of Porosity
The total pore volume
of a carbon is independent of the adsorbate, provided the amount adsorbed
is expressed as a liquid volume and excluding selective effects, according
to the Gurvitsch rule. An exception to this rule
is water as it is assumed that water molecules can not hydrogen bond effectively
in the narrow micropores effectively reducing the density of the adsorbed
phase hence reducing the pore volume. This phenomenon has been frequently
observed in adsorption studies.[14-16] Unfortunately
this rule does not take size exclusion effects into account as is often
the case with CMS but it can be assumed that this rule is applicable ignoring
all other effects.
Meaning of Porosity
Pores are minute openings
in solids that are accessible to vapours and gases. Porosity is a collective
term for these pores and their distribution in the structure of a solid.
Volume element is another term used to describe pores which arises from
the theoretical model of carbon structure where pores are created by the
irregular packing of the lattice. It is probable that pores are irregular
in shape and they may also be interconnected, completely closed from the
particle exterior and other porosity or terminated at some point within
the particle depending on the arrangement of the surrounding lamellae.
There is a common misconception that porosity in carbon is a series of
interconnecting tubes or slits  and the tree-branch
model may also be misleading. A more realistic approach is the view that
the carbon atoms of a porous carbon form a covalently bonded three-dimensional
network within which imperfect lamellar type arrangements may be recognised,
where interstices between carbon sheets give rise to the porous structure.[18-22]
Interconnections do join parts of the porosity but in doing so must be
classed as part of the porosity themselves forming part of both the adsorption
system and the transportation system.
The internal surface
area of a porous solid is composed of several classes of pores and may
often be in excess of the external surface area. The volume is accessible
to gases and vapours with which reactions may occur and it is not the
porosity itself, which interacts directly with the adsorbate but the internal
surface of the adsorbent.
Classification of pores
Porosity has a classification system as defined by IUPAC,
which gives a guideline of pore widths applicable to all forms of porosity.
Distinctions in porosity class are not rigorous and they may often overlap
in size and definition. The widely accepted I.U.P.A.C. classification
is as follows:
less than 2 nm
between 2 and 50 nm
Width greater than 50 nm
then be subdivided into three subsequent categories:
less than 0.5 nm
between 0.5 - 1.4 nm
between 1.4 - 2.0 nm
Origin of Porosity
The total classification
of porosity is tri-modal, as outlined previously, although not all adsorbents
will contain all classes of pores, and the three types of pores are formed
in different ways. The porosity in any one carbon is unique being created
by the constituent basic structural units composed of carbon lamellae,
which by definition are dependent on the precursor hence the porosity
is parent material dependent. Poor alignment of the lamellae in three
dimensions produces extensive internal porosity.
The pores are not uniformly shaped but are irregular shapes and sizes
and these factors as well as the accessibility of the pores is dependent
on the proximity of the constituent structural units.
The micropores are
formed as the result of imperfect stacking of constituent molecules and
packing arrangements of the bulk material, producing a lack of crystallite
alignment, and small pseudo-graphitic crystallites.
The easiest way to imagine the microporosity, in keeping with the model
outlined for general porosity, is that it exists as a series of interconnecting
volume elements, rather like zeolites, but with each volume element of
a different size and shape, connected in a completely random manner, with
exception to precursor retention, three-dimensional structure. No two
micropores will ever be identical and up to, although often much less
than, 90% of adsorption in activated carbons takes place in the micropores.
Their shape has been shown, by TEM studies  to be
either slit-like or convoluted in shape.
The class of micropores
may be subdivided into three separate groups:
i) ultramicroporosity (diameter < 0.5 nm) - usually
responsible for activated diffusion the pore diameter is comparable to
that of the admolecule.
ii) microporosity (diameter ~ 0.5 –1.4 nm)
- fill quickly, within the first few minutes of adsorption. Overlap of
the pore wall potentials results in a pore filling mechanism for adsorption
iii) supermicroporosity (size - 1.4 - 2.0 nm) - co-operative
pore filling is usually associated with a ratio of approximately 4:1 adsorbate
molecule to pore diameter, as is typically found in the supermicropores.
Monolayer formation occurs and the pore diameter is effectively reduced
enhancing the adsorption potential of the pore hence increasing adsorption
and completing the pore filling at low relative pressure. This class of
porosity was proposed by Dubinin  who originally
proposed the classification system.
The micropores provide
sites of maximum adsorption potential for an admolecule/atom and within
the pore an admolecule is usually influenced by approximately twelve adsorbent
atoms. Due to the close proximity of the walls of micropores an interaction
of the Polyani potentials, the result of overlapping of the dispersion
fields, may occur resulting in a relatively deep potential energy well
and enhanced adsorption at a given pressure. Hence, diffusion into the
ultramicropores has a significant activation energy associated with it.
During the adsorption
process an admolecule will wait until the vibration sequence of the pore
wall opens up the porosity and then it will move ‘into’ the
pore. Once inside the pore the admolecule has the ability to move ‘out’
but concentration gradients established within the porosity during the
adsorption process should prevent such a situation.
The pore filling process
may be divided into three steps:
ii) pore filling
by co-operative effects
of the pore filling process.
Adsorption into micropores
is completely reversible and no hysteresis loop is observed because of
the inability for the adsorbate to condense in such narrow volume elements.
Mesopores are the
result of major defects in the structure of a solid and serve as passages,
providing a transport system, to the micropores. These are the pores which
give rise to the phenomenon of capillary condensation with the adsorption/desorption
isotherm, which is observed by the existence of an inherent hysteresis
loop. The mesopores fill in the final stages of the isotherm by multilayer
formation and a cylindrical meniscus. The pore diameters, greater than
2 nm but less than 50 nm according to IUPAC definition, are so large that
at low relative pressures monolayer coverage occurs followed by further
layers and the adsorbed film acts as a nucleus upon which capillary condensation
may take place. This mechanism gives rise to hysteresis, as there are
differences in the pore filling and emptying processes.
Determination of the
volume and distribution of mesopores is via vapour adsorption, the subsequent
evaluation performed is based on capillary condensation.
The micropores are
considered important in the process of adsorption whereas the meso- and
macropores primarily act as transport pores.[22,31,32]
Major lattice structure defects, such as racks, fissures and etching channels,
within a solid lead to the formation of macropores which may be treated
as an open surface. It is possible to observe macroporosity by optical
microscope and scanning electron microscopy as they are of the order 50
nm and greater. There is no actual upper limit to the diameter of the
pores but it is usually 1 – 2 mm.
Adsorption is via
the layer by layer mechanism typical of non-porous solids and they act
as transport pores allowing access to the internal surfaces and microporosity.
These pores do not contribute considerably to the surface area of the
adsorbent, typically less than 2 m2g-1.
The distribution of pore sizes may be described in terms of micro-,
meso- and macropores but are not necessarily tri-modal, exhibiting three
peaks of varying ratios, this result formed the basis of Dubinin’s
A complete distribution is not always present in a carbon and some species
may exhibit only some types of porosity. Distinct peaks are not always
observed and some systems exhibit wide distributions over all pore diameters
or a constant decrease/increase in pore frequency with variation in pore
of the tri-modal pore-size distribution found in many carbons
Classification of Adsorption Isotherms
should conventionally be plotted on the basis of relative pressure, p/po
(x-axis) versus amount adsorbed expressed as a molar quantity (y-axis)
in mmolg-1, to allow comparisons to be made. The experimental procedure
involves the use of partial pressure, where the actual pressure is expressed
with respect to the saturation vapour pressure at a constant temperature
of adsorption, hence the process is isothermal. Adsorption data may alternatively
be expressed in terms of an isobar, the variation in uptake with temperature
at constant pressure, or an isostere, the change in temperature with pressure
at a constant surface coverage. Isotherms provide a significant amount
of information about the adsorbent used and the interaction with the adsorbate
in the system, including:
i) assessment of the
surface chemistry and fundamentals involved in the adsorption process;
of the surface area, pore volume and pore size distribution;
profiles for carbons used in industrial processes.
of adsorption isotherms can yield a large amount of information about
the processes involved, as outlined above, but this is only possible upon
careful analysis of the data obtained and this can often lead to confusion
in the interpretations made. An understanding of the adsorption mechanisms
involved in the different classes of porosity is essential to explain
the shapes observed.
The extent of adsorption
on a surface, usually denoted n, is generally a function of the temperature,
pressure and nature of the adsorbent and adsorbate:
n = f(P, T, adsorbate, adsorbent)
For isothermal adsorption of a particular system this simplifies to
n = f(P)T, adsorbent, adsorbate
By working within the limits of vacuum and the saturation vapour pressure,
the pressure may be expressed in terms of relative vapour pressures, i.e.
n = f(p/po)T, adsorbent,
a universal basis upon which all isotherms may be displayed in order to
make the comparison of results easier. The above relationship may be used
to graphically represent uptake profiles in the form of an ‘adsorption
Representation of Isotherm Classification
All adsorption isotherms should fit at least one, or at least a combination
of two or more, of the six recognised types classified by Brunauer, Deming,
Deming and Teller  (B.D.D.T. system). The figure
above shows the possible shapes and information which may be drawn from
them is outlined below:
Type I Isotherm - these are typical of adsorbents with a predominantly
microporous structure, as the majority of micropore filling will occur
at relative pressures below 0.1. The adsorption process is usually complete
at a partial pressure of ~0.5. Examples include the adsorption of nitrogen
on carbon at 77K and ammonia on charcoal at 273K.
Type II Isotherm - physical adsorption of gases by non-porous solids is
typified by this class of isotherm. Monolayer coverage is followed by
multilayering at high relative pressures. Carbons with mixed micro- and
meso-porosity produce Type II isotherms.
Type III Isotherm - the plot obtained is convex to the relative
pressure axis. This class of isotherm is characteristic of weak adsorbate-adsorbent
interactions  and is most commonly associated with
both non-porous and microporous adsorbents. The weak interactions between
the adsorbate and the adsorbent lead to low uptakes at low relative pressures.
However, once a molecule has become adsorbed at a primary adsorption site,
the adsorbate-adsorbate interaction, which is much stronger, becomes the
driving force of the adsorption process, resulting in accelerated uptakes
at higher relative pressure. This co-operative type of adsorption at high
partial pressures is known as cluster theory and examples include the
adsorption of water molecules on carbon where the primary adsorption sites
are oxygen based.
Type IV Isotherm - A hysteresis loop, which is commonly associated
with the presence of mesoporosity, is a common feature of Type IV isotherms,
the shape of which is unique to each adsorption system. Capillary condensation
gives rise to a hysteresis loop and these isotherms
also exhibit a limited uptake at high relative pressures.
Type V Isotherm - these isotherms are convex to the relative
pressure axis and are characteristic of weak adsorbate-adsorbent interactions.
These isotherms are indicative of microporous or mesoporous solids. The
reasons behind the shape of this class of isotherm are the same as those
for Type III and again water adsorption on carbon may exhibit a Type V
Type VI Isotherm
- introduced primarily as a hypothetical isotherm, the shape is due to
the complete formation of monomolecular layers before progression to a
subsequent layer. It has been proposed, by Halsey,
that the isotherms arise from adsorption on extremely homogeneous, non-porous
surfaces where the monolayer capacity corresponds to the step height.
One example known to exist is the adsorption of krypton on carbon black
(graphitised at 3000 K) at 90 K.
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