Single Surface Area Analysis

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Single Surface Area Measurements

(FlowSorb II---BET)

Louis Scudiero

Washington State University

Instrument description

The FlowSorb II instrument shown below in figure 1 permits measurements of (1)

single point surface area and (2) multipoint surface area measurements, (3) total pore

volume, and (4) distribution of pore wall area and pore volume as a function of pore size.

31

(4)

(5)

(6)

(2)

(7)

(8)

Figure 1 FlowSorb II

The instrument consists of a flowmeter (1), a heating mantle (2), a surface area display

with zero adjustment knobs (3), 2 U shape tube (mounted at degas and test) (4), a cold

trap with Dewar (5), a toggle switch for gas selection (6) and a Dewar for cooling (gas

adsorption) (7) and a beaker containing room temperature water (8) for heating (gas

desorption) experiments.

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Theory taken from Appendix 1 [Micromeritics instruction manual for Flowsorb II

2300 (1986)]

One form of BET equation that describes the adsorption of a gas upon a solid surface is

( ) ( )00

0

11

1P

P

CV

CCV

P

PV

P

P

m

m

+

=

(1)

Where V is the volume of gas adsorbed at pressure P (under std temperature and

pressure, STP), P0the saturation pressure which is the vapor pressure of liquefied gas at

the adsorbing temperature, Vm the volume of gas (STP) required to form an adsorbed

monomolecular layer, and C a constant related to the energy of adsorption.

The surface area S of the sample giving the monolayer adsorbed gas volume Vmis thencalculated from

M

ANVS

m= (2)

where A is Avogadros number (in number of gas molecules in a mole of gas at STP), M

the molar volume of the gas, and N the area of each adsorbed gas molecule.

In this experiment the single point surface area measurement will be used. Therefore the

constant C in equation 1 is typically a large number (C >> 1). The equation 1 reduces to

+=

0

0

011

1 P

P

CV

P

PV

P

P

m

(3)

Now if P/P0>> 1/C Eq. 3 can be further simplified

0

0

01

1 P

P

V

P

PV

P

P

m

=

(4)

Finally Eq. 4 can be rearranged to give

=

0

1P

PVV

m (5)

Substituting Eq. 5 into Eq. 2 we get to the final expression

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degassing alumina. This will take between 15 to 30 min. In the meantime you need to

calibrate the FlowSorb II following the procedure outlined below.

Sample glass tubes

Mantle and

Thermocouple outlets

2 position hinged tray

Dewar support

Figure 3: U-shape tube with sample in the DEGAS position

2.

Calibration

This is done by injecting 1 mL of a gas mixture into the instrument through the

septum. The resulting volume measured should be equal to the calibration value which is

calculated using the following expression.

=

SatP

atmPN

V

NNAtmP

RmTvS

m

A

100

%1

760

15.273 2 (7)

Where v is the volume of gas injected (1 mL), NA = 6.023x1023molecule/gram mole is

the Avogadro number, N = 16.2x10-20

m2is the accepted value for area of a solid surface

occupied by an adsorbed N2molecule and Vm= 22.414 L/gram mole is the molar volume

of a gas at standard conditions.

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The saturated pressure of liquid nitrogen is typically 15 mmHg greater than the

atmospheric pressure.

Example: For Patm= 702 mmHg, RmT = 23 C and %N2= 20 the value of S will be equal

to (8)

S v273.15

296.15

Patm

760

NA N

Vm

1

20

100 Patm

Psat

:=

S 2.982m2

=

This calculated value of S will be used to calibrate the surface area as described below.

a) Fill the 1 mL syringe with 1mL of gas (N2 from cylinder or from LN2). If LN2 is

used allow 2 to 3 min for the gas temperature to reach room temperature.

b) Set on SHORT PATH unless a large volume of gas will be adsorbed.

c)

Dewar tray (TEST) is in the down position.

d) Adjust the gas flow in the flowmeter to have the ball at the orange mark or slightly

lower. (A gas flow around 35 40 will allow easy adjustment of the calibration value for

the surface area)

e) Depress DET and X1 pushbuttons.

f) After equilibrium, zero the Digital DISPLAY using COARSE/FINE ZERO.

g) Depress SURFACE AREA pushbutton.

h) CLEAR SA DISPLAY.

i) Inject slowly the 1 mL of gas.

j) THRESHOLD light will begin to flash after 1 2 minutes (counting). When

counting ceases, use the CALIBRATION knob to adjust the SA value to the S value as

calculated above (20% N2, S = 2.982 m2) if needed.

Figure 4: Clear SA display and Threshold

pushbuttonsFigure 5: Display meter and the different

reading functions.

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k) Repeat from step (e) to confirm calibration. The surface area value should read 2.98

m2for 20% gas mixture after the three injection or DET should read 0.00 0.02.

Figure 7:Inject port for calibration of

the Flowsorb

Figure 6: Calibration and coarse and fine

adjustment knobs

Note that the calculated S value of 2.98 is only for the 20% mixture if a different gasmixture is used the value of S needs to be recalculated for that mixture using Eq. 8.

3. Surface area measurements

At this point you have to switch sample holder from DEGAS to TEST as fast as possible

to avoid air to accumulate in the column.

a) Adjust the gas flow rate if necessary.

b) Adjust the level of liquid nitrogen about the cold trap if needed.

c) Depress DET, pushbutton.

d) After air pulse passes, depress SURFACE AREA pushbutton. (X1 is fine for

this experiment)

e) Place the Dewar containing the liquid nitrogen (LN2) about the sample and

adjust its level.

f) Depress CLEAR SA DISPLAY pushbutton to clear display of adsorption

result when counting ceases.

g)

Counting will starts after a couple of minutes and could last for several

minutes (up to 15 min). The THRESHOLD light will indicate when the counting

starts and when it ceases (no flashing for 15 20 seconds). Record the value of

the sample surface area that is displayed. This is the adsorbed surface area of the

sample.

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h) Remove LN2 Dewar and promptly warm up the sample using a beaker

containing water at room temperature. For this measurement the Dewar support

has to be in the low position as shown in figures 1 and 3

i) Assure that the gas flow rate has returned to normal before counting begins.

j) After counting ceases (THRESHOLD light stops flashing for 15 20

seconds), record value displayed as the sample surface area.

You should repeat the measurements at least twice and if time permits use a different

gas mixture (30%) and repeat the measurements.

Calculations and Discussions

1. Record adsorbed (using LN2) and desorbed (water beaker at RT) surface area of

alumina. Compare these values (in m2/gram) with the surface area value given on

the bottle. Perform an error analysis.

2. Discuss some of the approximations that are made in order to determine this

value.

3. Describe what the isotherm would look like if the partial pressures of N2 used

approached one atmosphere pressure?

4. Would the experimental surface area be higher or lower if the conditions for BET

theory were not met? Explain.

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Single Surface Area Analysis