Gas Adsorption

In this experiment you will learn about gas adsorption to a solid surface.

Experimental Procedure

Absorption from the gas phase can be measured by either gravimetric or volumetric techniques. In this experiment you will use the volumetric technique: You will quantify the amount of adsorption that occurs by measuring the pressure before and after the adsorption takes place.

Measurement of an Unknown Volume

  1. Pump down the system. What is the minimum pressure that we can achieve? Consult your instructor.
  2. Close the valve between the known and unknown volumes. Record the pressure of the known volume, Pknown.
  3. Let N2 gas flow into the unknown volume. Record the pressure, Punknown.
  4. Open the valve and record the equilibrium pressure of the two volumes, Ptotal.
  5. Repeat steps 2 through 4 five more times. How consistent are your results?
  6. Assuming that the gases are ideal and that material (moles) is conserved, calculate the unknown pressure from the following equations:
    7. $$P_f V_f=P_{unknown} V_{unknown} + P_{known} V_{known}\\ V_f=V_{unknown} +V_{known}\\ (P_{known}-P_f)V_{known}=V_{unknown} (P_f -P_{unknown})$$

    where Pf is the equilibrium pressure you recorded in step 4. Plot a graph of (Pknown-Pf)Vknown versus (Pf-Punknown) and calculate Vunknown from the slope.

  7. Calculate the unknown volume based on its dimensions and compare with your results from previous steps.
  8. The value of known volume will not be used for the rest of the experiment. Close the valve between this volume and rest of the setup.

Measurement of the Surface of Activated Carbon

    Preparation of the system

  1. At the experiment station you will find a cleaning pole, a ruler and a hollow tube. Use the cleaning pole to clean the tube. Weigh 150 mg of activated carbon and gently fill the tube. Use the ruler to measure the height and mark the height on the tube.
  2. In the experimental setup, open valve 2 and take out the disk from its bottom.
  3. Attach the activated carbon rod to valve 2. Make sure to place a filter between the valve and the activated carbon!
  4. Open valves 1 and 2. Turn on the vacuum pump and heat the activated carbon using a blow drier until the pressure reaches ~10-3 Torr. Fill the trap with liquid nitrogen and observe the pressure changes over a 10 minutes period.
    5. Safety instructions:
    1. Start heating only after opening valves 1 and 2!
    2. Make sure that the plastic tubes don't melt!
    3. Filling the trap with liquid nitrogen must be done while wearing proper gloves and safety goggles.

  5. Once the pressure stabilizes close valve 1. Make sure that valve 1 is closed during all your measurments (unless you need to adjust the pressure in the tube). Close valve 2 and record the pressure, Ptube. Fill a dewar with liquid nitrogen, and slowly insert the tube into it. Only the lower third of the tube needs to be cooled with liquid nitrogen!

    6. The adsorption isotherm:

  6. Let N2 gas flow into the system and measure the line pressure, Pline.
  7. Open valve 2 between the volumes and measure the pressure, Ptotal. Note: In the first measurment record the pressure change in 5-second intervals, for a total of 300 seconds (this measurement will allow you to study the process kinetics).
  8. Repeat steps 6 and 7, and make sure you sample at least 6-7 points in the range of 10-3-600 Torr.

    9. The desorption isotherm:

  9. Once pressure stabilizes, close valve 2 and measure Ptube.
  10. Gently open valve 1. Measure the line pressure, Pline.
  11. Open valve 2 and measure the total pressure once it stabilizes, Ptotal.

    12. Shutting down:

    Must be done under supervision!

  12. Open both valves so that the activated carbon rod is under vacuum.
  13. Separate the nitrogen dewar from the system. As a result of the rapid desorption of gas from the activated carbon rod pressure will rise to high values (that is why all the valves must be open).

Data Analysis:

  1. The change in the number of adsorbed and desorbed moles can be calculated using the following equation:
    2. $$\Delta n= {P_{line} V_1\over RT_{room}} + {P_{tube} V_2\over RT_{room}}+ {P_{tube} V_3\over RT_{77K}} - ({P_f V_1\over RT_{room}} + {P_f V_2\over RT_{room}} + {P_f V_3\over RT_{77K}})$$
  2. Analysis of the Langmuir isotherm.

    3. Use equations 4-11 to analyze the results.

    1. Plot Pf/n as function of Pf, and extract ns and K.
    2. Calculate the change in free energy, ΔG0, from the following relation: K=e-ΔG0/RT .
    3. Calculate the maximum adsorption surface area using equation 15.
    4. Plot the fractional coverage θ of adsorption and desorption on the same graph.

  3. Analysis of the BET isotherm.

    4. Use equations 12-15 to analyze the results.

    1. Plot Z/[n(1-Z)] as function of Z, and extract ns and c.
    2. Find the known value of ΔHvap in the literature, and calculate ΔHdes.
    3. Calculate the maximum adsorption surface area using equation 15.
    4. Plot the fractional coverage θ of adsorption and desorption on the same graph.