Physical Experiment Report "Determination of Solid Specific Surface - BET Method"

First, the purpose of the experiment:
1. Learn how to determine the specific surface of activated carbon by BET method.
2. Understand the basic assumptions of BET multi-layer adsorption theory and the basic principle of BET method for determining solid specific surface area
3. Master the measurement method of the solid surface ratio of the BET method and master the working principle of the surface measuring instrument and the operation of the related measuring software.
Second, the experimental principle Gas chromatography is based on the BET multi-layer adsorption theory based on a determination of the specific surface of the porous material, the commonly used BET formula is:

)-1 + P / P0 VmC
The above formula expresses the relationship between the adsorption amount and the relative pressure of the adsorbate under constant temperature conditions.
Where V is the adsorption amount when the equilibrium pressure is P, P0 is the gas saturated vapor pressure at the experimental temperature, Vm is the adsorption amount when the first layer is full, and C is a constant. Therefore, the formula contains two constants of Vm and C, Also known as the BET two-constant equation. It associates the desired quantity Vm with the measurable parameters C, P.
The above formula is a general linear equation. If you obey this equation,
Then P/[V] plots P/P0 to obtain a straight line, and the slope/VmC of the straight line and the straight line on the vertical axis are 1/VmC to obtain Vm.
The specific surface area of ​​the sample to be tested is:
S= VmNAσA/
Where NA is the Avogadro constant; m is the mass of the sample; σm is the area occupied by each adsorbed molecule on the surface of the adsorbent, and the value of σm can be calculated from the close packing in the liquid state. The adsorbed molecules on the surface are arranged in a hexagonal close packed manner. For the entire adsorbed layer space, the repeating unit is a regular hexahedron, and the commonly used adsorbent N2 is calculated as σm=0.162 nm2.
Nowadays, the method of measuring the adsorption amount of nitrogen at liquid nitrogen temperature is the most common method, and the internationally recognized value of σm is 0.162 nm2.
In this experiment, the surface area of ​​the sample to be tested is measured by computer controlled chromatography.
Third, the experimental reagents and instruments than the surface analyzer, liquid nitrogen, high purity nitrogen, hydrogen. Soap membrane flowmeter, vacuum flask.
Four: experimental steps preparation
1. Turn the nitrogen pressure regulator and hydrogen pressure regulator on the surface analyzer panel counterclockwise to the relaxed position.
2. Turn the pressure reducing valve on the nitrogen cylinder to the relaxed position in the counterclockwise direction, open the main valve of the cylinder, and then slowly open the pressure reducing valve clockwise to the pressure of the decompression table is 0.2MPa, and open the hydrogen cylinder in the same way.
3. Slowly open the nitrogen pressure regulating valve and the hydrogen pressure regulating valve on the surface panel to the gas pressure in the clockwise direction to 0.1MPa.
4. Connect the soap membrane flowmeter to the instrument panel and vent it to the port. Pull out the No. 1 lever below the nitrogen resistance valve to measure the flow rate of nitrogen. Adjust the flow rate of nitrogen with a nitrogen resistance valve to 9 ml/min, then 1 The lever is pushed in.
5. Connect the soap membrane flowmeter to the instrument panel and vent the 2 port. Pull out the No. 2 lever below the hydrogen resistance valve to measure the hydrogen flow rate. Adjust the hydrogen flow rate to 36ml/min with the hydrogen resistance valve. The lever is pushed in.
6. Turn on the power switch on the panel of the surface analyzer, adjust the current adjustment knob to the bridge current of 120mA, turn on the computer, double-click the Pioneer icon on the desktop to start the software. Observe the baseline.
Measurement work
1. The liquid nitrogen is transferred from the liquid nitrogen cylinder into the thermos cup. After the sample tube is cooled, the sample tube is placed on the vacuum flask containing the liquid nitrogen, and the flask is fixed. Observe the baseline trend when the adsorption peak appears. Then, after recording the curve back to the baseline, hit the zero button and the measurement button, then remove the flask from the sample tube and observe the desorption curve. When the desktop pops up the report, select the standard parameters to be compared with it and record the result. Repeat this step and measure it three times in parallel, taking the average value as the specific surface area of ​​the sample. The article you are browsing is organized by the first 'Panwen.com', and the copyright belongs to the original author and the original source.
2. After the experiment is completed, close the measurement software in sequence, and the computer will adjust the current adjustment knob on the surface panel to 80mA. Turn off the power switch and turn off the main valve on the hydrogen cylinder and the nitrogen cylinder. shut down.
Operational precautions
1. Compared with the coarse adjustment on the surface of the surface analyzer, the fine adjustment and adjustment knob are fixed, do not move again;
2. When opening the cylinder, do not stand on the front of the meter to prevent the gas from rushing out the wound;
3. Be very careful when using liquid nitrogen. Do not violently shake the cup, and do not cover the cup lid tightly;
4. When the cup is placed in the sample tube or removed, the action should be slow to avoid the temperature change changes too quickly to cause the sample tube to burst;
5. When closing the cylinder main valve, do not close each pressure reducing valve;
Five: data recording and processing:
Sample No. Weight Surface Area Peak Area Standard Sample
70
200
1660630
Sample 1
70
199.241
1626622
Sample 2
70
198.646
1621763
Sample mean
70
198.944
1624192.5
The average surface area of ​​the sample is /2 = 198.944 m2 / g
The relative error is: /200.00=-0.0078)
Sixth, the error analysis has problems when zeroing, when the peak occurs, the baseline does not start from zero, and then improperly handled;
When the flask containing liquid nitrogen is taken out, the baseline has not yet started scanning.
At the time of desorption, the temperature is low and tailing occurs. It is generally believed that the hysteresis is caused by the porous structure, and in most cases the thermodynamic equilibrium of desorption is more complete.

Seven, matters needing attention
1. When the cylinder is opened, the front of the cylinder head is not allowed to stand, so as to prevent the dial from rushing out and hurting people;
2, the liquid nitrogen should be very careful, do not violently shake the cup and not close the lid of the cup;
2, pay attention to the switch valve, the direction of rotation of the knob;
3, when the cylinder main valve, be careful not to close the pressure reducing valve and the pressure regulating valve;
4. Pay attention to the computer operation during the measurement: do not click the measurement button during the adsorption, and then adjust the zero when the liquid nitrogen is ready for desorption after the adsorption is completed, and measure and enter the stage of measuring the adsorption amount;
5, close the instrument in strict accordance with the order.
6. The BET formula is only applicable to the specific pressure of between 0.05 and 0.35. This is because when deriving the formula, it is assumed that it is a physical adsorption of multiple layers. When the specific pressure is less than 0.05, the pressure is too small to establish multiple layers. Physical adsorption, even monolayer adsorption is not formed, the surface non-uniformity is prominent; when the specific pressure is greater than 0.35, the capillary condensation becomes significant, thus destroying the multi-layer physical adsorption equilibrium.