for dissociative adsorption can be greater than zero, which means endothermic
chemisorption, although uncommon, is possible.
When a gas or vapor is brought into contact with a solid, part of it is taken up by the solid. The molecules that dissappear from the gas either enter the inside of the solid, or remain on the outside attached to the surface. The former phenomenon is termed absorption (or dissolution) and the latter adsorption. When the phenomena occur simultaneously, the process is termed sorption. The phenomenon of adsorption was disovered over two centuries ago. The uptake of gases by charcoal was studied by C. W. Scheele in 1773 and by the F. Fontana in 1777. In 1785, charcoal was found to decolorize' solutions by a surface adsorption mechanism. Since these processes have such a long history, we will make little attempt to be fair to the early pathfinders in this field nor to the historical development of the science.
The solid that takes up the gas is called the adsorbent, and the gas or vapor taken up on the surface is called the adsorbate. It is not always easy to tell whether the gas is inside the solid or merely at the surface because most practical' absorbents are very porous bodies with large internal' surfaces. It is not possible to determine the surface areas of such materials by optical or electron microscopy because of the size and complexity of the pores and channels of the material. The gas adsorption itself, however, can be used to determine the accesible surface area of most absorbents.
Gas adsorption is of practical consequence to engineers and chemists in many ways. It can provide a convenient, cheap and reusable method for fluid purification and purification. Gas masks used in WWI (and even in present day) are an example of the utility of charcoal as an absorbent. More significantly, perhaps, the phenomenon of surface adsorption has been used to modify the rates of product yields of chemical reactions through heterogeneous catalysis. For a catalyst to be useful, it must have a large surface area, bind the reactants quickly and effectively, stabilize the activated complex, and release the products of the reaction. Thus the attraction of various molecules on the surface, as well as the total surface area of the catalyst, are extremely important properties of potential catalytic materials.
Gas adsorption has been studied theoretically for most of this century and the simplist of the resulting theories provide the insight needed for most applications. We will investigate two such treatments, one attributed to Langmuir and one to Brunauer, Emmett and Teller (BET) and apply their equations to our experimental data. We will investigate the adsorption of N2 at cryogenic temperatures on common high area supports such as alumina. We will use this information to test simple adsorption theory, determine the specific area of the absorbent, and estimate the heat of adsorption of N2.
Additional information on experiments of this type may be obtained from many
sources including:
. Because translational degrees of
freedom of the gas phase adsorbate are lost upon deposition onto the substrate
is negative for the process.
Since = 
- T ,
for physisorption must be exothermic.
In chemisorption (chemical adsorption), the adsorbate sticks to the solid by the
formation of a chemical bond with the surface. This interaction is much stronger than
physisorption, and, in general, chemisorption has more stringent requirements for the
compatibility of adsorbate and surface site than physisorption. The chemisorption may
be stronger than the bonds internal to the free adsorbate which can result in the
dissociation of the adsorbate upon adsorption (dissociative adsorption). In some cases
for dissociative adsorption can be greater than zero, which means endothermic
chemisorption, although uncommon, is possible.
The energetics of adsorption depend on the extent to which the available surface
is covered with adsorbate molecules. This is because the adsorbates can interact with
each other when they lie upon the surface (in general they would be expected to repel
each other). The fractional coverage of a surface is defined by the quantity 
:
At any temperature, the adsorbate and the surface come to a dynamic equilibrium, that
is, the chemical potentials of the free adsorbate and the surface bound adsorbate are
equal. The chemical potential of the free adsorbate depends on the pressure of the gas,
and the chemical potential of the bound adsorbate depends on the coverage .
Thus
the coverage at a given temperature is a function of the applied adsorbate pressure.
The variation of with p at a given T is called an adsorption isotherm.
Several adsorption isotherms have proven useful in understanding the process of
adsorption. The simplest isotherm is attributed to a pioneer in the study of surface
processes, Langmuir, and is called the Langmuir isotherm. If one assumes:
then one can derive a simple formula for an adsorption isotherm [I. Langmuir, J. Amer. Chem. Soc., 40, 1361 (1918); I. Langmuir, J. Amer. Chem. Soc., 54, 2798 (1932); I. Langmuir, Nobel Lecture, 1932]. Consider the equilibrium
eq. 2
where A is the free adsorbate, S is the free surface, and A.S is the substrate bound to the surface. The rate of adsorption will be proportional to the pressure of the gas and the number of vacant sites for adsorption. If the total number of sites on the surface is N, then the rate of change of the surface coverage due to adsorption is:
eq. 3
The rate of change of the coverage due to the adsorbate leaving the surface (desorption) is proportional to the number of adsorbed species:
eq. 4
In these equations, ka and kd are the rate constants for adsorption and desorption
respectively and p is the pressure of the adsorbate gas.
At equilibrium, the coverage is
independent of time and thus the adsorption and desorption rates are equal.
The solution
to this condition gives us a relation for :
eq. 5
where K = ka / kd.
Note that because K is an equilibrium constant, the value of K at various
temperatures determined from the Langmuir isotherm allows for the evaluation of the
enthalpy of adsorption, ads, through the van't Hoff equation:
eq. 6
The Langmuir isotherm gives us a wonderfully simple picture of adsorption at low coverage and is applicable in some situations. At high adsorbate pressures and thus high coverage, this simple isotherm fails to predict experimental results and thus cannot provide a correct explanation of adsorption in these conditions. What is missing in the Langmuir treatment is the possibility of the initial overlayer of adsorbate acting as a substrate surface itself, allowing for more adsorption beyond a saturated (monolayer) coverage. This possibility has been treated by Brunauer, Emmett, and Teller [J. Amer. Chem. Soc., 60, 309 (1938)] and the result is named the BET isotherm. This isotherm is useful in cases where multilayer adsorption must be considered. The form of this isotherm is:
eq. 7
where n/nmono is the ratio of the moles adsorbed to the moles adsorbed in a single monolayer, and z = p/p0, where p0 is the vapor pressure of the pure condensed adsorbate. The n/nmono ratio represents a generalized coverage' because its value can exceed unity. The constant c represents the relative strengths of adsorption to the surface and condensation of the pure adsorbate. Simple theory predicts an approximate value of this constant as:
eq. 8
The BET isotherm predicts that the amount of adsorption increases indefinitely as the
pressure is increased since there is no limit to the amount of condensation of the
adsorbate. In the limit that adsorption to the surface is much 'stronger' than the
condensation to a liquid (such as for the adsorption of unreactive gases onto polar
substrates) the BET isotherm simplifies to the form (c=
):
eq. 9
The Langmuir isotherm is found to be useful only at very small coverages (sub-monolayer) but is generally applied to all cases involving chemisorption. This would correspond to the limiting case of c approaching infinity in the BET formalism, and no insight is provided by BET below one monolayer in this limit.
The BET isotherm is found to describe adequately the physisorption at
intermediate
coverage ( = 0.8 - 2.0) but fails to
represent observations at low or high coverage.
The BET isotherm is reasonably valid around =1.0,
however, and this is useful
in characterizing
the area of the absorbent. If one can determine experimentally the number of moles of
adsorbate required to give = 1.0 (i.e. a monolayer),
one can determine the specific
area of the absorbent:
eq. 10
Practically, one measures the number of moles adsorbed as a function of equilibrium
pressure, i.e. one does not directly measure This implies that over the range where the BET isotherm is valid
a plot of z/n(1-z) vs z will be linear.
The slope and intercept of this line will allow the determination of nmono and c.
The specific area of the sample is simply:
In our studies, The adsorption process is generally taken as completely reversible, but,
under some
conditions the isotherm may exhibit a different shape upon desorption as compared to
absorption. This is called hysteresis. Sometimes hysteresis data can be used to
determine
the structure and size of pores in the absorbent. We will therefore need to generate an
isotherm for both absorption and desorption.
Figure 3 We will perform the adsorption measurements in a commercial vacuum manifold
called the Omnisorb 360, manufactured by Omnicron. A schematic diagram of the
relevant portion of the vacuum system is shown in Figure 4.
Although this system is designed for semi-automated use, we will use the
equipment
manually. The numbers on the above schematic represent some of the numbered valves
on the system and the diagram above is similar to the layout of the valve controls
on the
machine itself. Each valve is pneumatically controlled by a numbered push button
switch.
Simply push the button to open any valve; an open valve is indicated by the switch lamp
on. To close, hit the button again.
Several portions of the system require some discussion. The adsorbtive gases
enter
the system through valves 10(N2),13(CO2, not used in this experiment), and 12(He). The
inlet is the portion of the system between these valves, valve 11, the flow
controller, and
valve 8. The manifold is the portion of the system between valves 7,8, and the sample
port valves 3,4,5 and 6. The manifold is evacuated by a vacuum pump when valves 9
and 7 are open. The vaccum pump pressure is monitored by a Pirani gauge on the upper
left of the console. The manifold pressure is monitored by two (0-1000 torr, 0-10 torr)
capacitance manometers (Baratrons). (Please note where the pressure measurement is
made on the manifold in Figure 3. Since there is a slight change in the manifold volume
depending on the state of valves 3 - 8, all pressure readings should be made with these
valves closed whenever possible).
Figure 4 Weigh (tare) a sample bulb with valve. Introduce less than one gram of
sample into
the bulb. Degas the sample (Degassing is accomplished in the furnace at the left bottom
of the instrument). Determine as accurately as possible the degassed weight of the
sample plus bulb and thus the sample mass. Attach the bulb to sample port #3 or #6
and fill the Dewar with LN2 to a level about 2" above the bulb. Record the atmospheric
pressure (barometer) and manifold temperature (displayed on the upper center of the
control panel in oC).
You will find the system with the mechanical and diffusion pumps already on.
Evacuate the inlet and manifold by opening valves 9,7,8. Close the Flow controller by
setting the flow rate to 0.00. Evacuate the sample and known volume by opening the
appropriate sample port valves. Note the base pressure indications on all three
pressure
gauges (One Pirani, and the two Baratrons) and record these in your notebook. As the
system will base at a lower pressure than the manometers can indicate, the Baratron
readings you take now will be used as a zero offset and will be used to correct all
subsequent readings.
Close the sample port valves and flush the inlet and manifold with the gas to be
used (initially He). Gas is introduced into the manifold by the following procedure:
Close
valve 8 and momentarily open (open, then close) the inlet valve for the desired gas.
The
inlet is now charged. Now open and close valve 8. A slug of gas has been introduced
into
the manifold and you should see the pressure rise (to about 300 torr at this point).
Now
pump out the manifold and inlet by opening the appropriate valves. Repeat this
procedure a couple of times each time you change gases.
Only one volume in the system is known at the onset, Vk, the volume of the
bulb on
port #5. We will use it to determine the manifold volume, Vm, as well as the
free volume
in our sample bulb, Vs.
Fill the manifold and known volume bulb to about 700 torr with He and record the
pressure. This may take two 'charges' from the inlet. Evacuate the inlet and manifold
but
leave the known volume pressurized, and record the value. Now fill the manifold
with gas
from the known volume bulb. Record the pressure again. The reduction in pressure will
allow you to calculate the ratio of the manifold volume to the known volume through the
ideal gas relation. Close valve #5, evacuate the manifold, and refill from the known
volume again. This will give you another determination of the manifold volume. Repeat
this procedure several times to obtain an average value of Vm.
Now that the manifold volume is known, determine the sample void volume by the
same procedure as above but fill the manifold from the sample bulb instead of the bulb
on port #5. Helium is used for volume determinations because it does not adsorb to
anything at 77K.
Evacuate the sample and close the valve connecting it to the manifold. Close
valve #5 and isolate the known volume from the manifold for the remainder of the
experiment. Flush the inlet and manifold with N2. Fill the manifold with N2
to about 40 torr
and record the pressure. Since one 'charge' from the inlet will produce more pressure
than
this, you will have to reduce the pressure by closing #8, evacuating the manifold,
momentarily opening #8, and repeating until the manifold pressure is about the desired
value. With all valves closed open the sample port valve and expose the sample to the
sealed off manifold. The pressure will drop when the sample bulb is opened to
the manifold
filled with nitrogen but it will drop significantly more more than would occur if
the gas
in the manifold were He. This is because some of the nitrogen gas is
adsorbing to the sample surface. Allow the sample and adsorbate to equilibrate at
77K.
NOTE: The approach to equilibrium is perceptably slow, especially at high coverage.
It would in principle take an infinite time for equilibrium to be exactly established.
The nature of the isotherm and the required precision of the measurement suggests
that equilibrium pressures need be known only to a few percent. Take a few point
allowing a long time for equilibrium to be achieved and plot the approach to
equilibrium as a function of time. Is this a first order process in adsorbate?
Estimate the time it takes to get to within 5% of the equilibrium pressure and us this
as a cutoff time for data taken at similar coverage. Correct for the incomplete
equilibration in working up the data. In summary, use your judgement as to how long
to wait at each pressure. Analyse and exploit kinetic information about the approach
to equilibrium. Don't get old waiting and not thinking.
Record the equilibrium pressure.
This procedure generates the data you need to allow you to determine the number
of moles adsorbed on the solid as a function of (equilibrium) pressure.
Close the valve to the sample tube and refill the manifold.
'Dose' the sample again
and record the pressures. You now have the next point on the isotherm.
Repeat the above with ca. 50 torr in the manifold until the equilibrium pressure
reaches ca. 20 torr. Then increase the initial manifold pressure to 300 torr
and repeat until
the equilibrium pressure reaches about 200 torr. Then increase the initial
manifold pressure
to ca. 700 torr until the equilibrium pressure reaches ca. 500 torr. This completes
the data
acquisition needed to generate the adsorption isotherm.
The desorption measurement is performed to see if there is any hysterisis
or non-equilibrium effects in the adsorption/desorption cycle.
This is basically performed in reverse
of the procedure above. You should already have the sample and manifold in
equilibrium at some pressure ca. 450 torr from the adsorption run. Close the sample off
from the manifold and evacuate the manifold. Open the sample to the manifold and
allow equilibrium to be achieved. Record this pressure. You now know how many moles
of gas have been removed from the sample and the new equilibrium pressure --- so this
is the first point on the desorption isotherm.
Repeat this procedure until the equilibrium
pressure reaches ca. 5 torr.
If time permits, perform adsorption and desorption studies for all three solids:
alumina, silica, and charcoal.
How does the BET theory fail in comparison to the actual data at low and high
coverage? Why does this behaviour occur? Does the Langmuir Theory work for your
physisorption data at low (sub monolayer) coverage?
.
Algebraic rearrangement of the BET
isotherm to produce a linear equation is usually applied to experimental data.
, the molecular area of the
absorbate N2 is taken to be 15.8
Angstoms2, NA is
Avogadro's number, and m is the mass of the sample.
Experimental Procedure
Sample preparation (this may be performed for you)
System Setup
Volume Determinations
Manifold Volume
Sample Void Volume
Adsorption Studies
Desorption Studies
Data Analysis
Experimental Data taken on the Omnisorp 360 for a ca. 0.3 g sample of silica
Additional Food for thought
REFERENCES
Have a look at a sample papers on this
experiment
Andrew Knight 15.11.1996
PJ Brucat || University
of Florida || Chemistry Dept