Practical BET measurements

This is a very brief list of some considerations before running BET measurements. If you would like more detail about using your BELSORP for BET analysis, contact us.

Understand your instrument.

• How the AFSM function works
  • or maybe your instrument uses some other method of free space control
• Know the sensitivity and accuracy
• What is the recommended sample size
• Has the instrument been serviced ? Was a reference sample run recently ? Or other controls ?
• …

Know your sample

• Can it be heated ? Can it be exposed to vacuum ?
• Does it contain any solvent or water ?
  • The effect of heating may be far greater when solvents are present.
  • It may be necessary to degas in several stages, starting at low temperature, and progressively increasing.
• How long must it be degassed ?
• Does it contain evaporable components ?
  • Even metals, such as Zinc and Magnesium can evaporate when heated in vacuum.
• Is there a risk of damaging it ?
  • Heat can cause damage to polymers and other organic compounds
  • Heating zeolites which contain water can cause hydrothermal decomposition
  • Cements, clays, silicates, and many related materials
  • Residual solvents can decompose MOF-type materials
  • Natural biomaterials can crack and deform when dried
• Heat can also cause structural changes:
  • Recrystallisation after heating above the Glass transition temperature
  • Strain relief and annealing of disorder
• Could there be any pores ? What sizes ?
  • If you want to get knowledge about the pores, it is necessary to measure at suitable pressures
  • The presence of micropores normally increases the difficulty of outgassing.
• What form is it ?
  • If it is granular or monolithic :
    • Does it fit inside the cells ?
    • Is it necessary to grind or cut it ?
    • Is it necessary to use it whole ?
  • If it is a powder :
    • Is there a problem of elutriation - movement of the powder when vacuum is applied ?
    • Is there a problem of static adhesion on the sides of the cell?
• Do you already know what BET surface area you expect ?

Choose the analysis conditions

• Sorptive and temperature:
  • Nitrogen 77 K
  • Argon 77 K
  • Argon 87 K
  • Krypton 77 K
  • Water at 298 K
  • Other vapors
• Select an appropriate amount of sample
  • Typically, your manufacturer will indicate a minimum amount of surface area in the cell.
  • Remember there are other considerations in choosing sample size…
• cell and accessories
• degassing and sample preparation method
• measurement method
  • use AFSM
  • Use GDO ?

Verify your equipment

• Clean cell, frits, cell caps/ autoseal valves
  • Caution : In case of static problems, the cleaning method may be contributing, if the detergent leaves silicone or other residues.
• New seals or o-rings ?
• Recent verification of instruments ?
  • date of last periodic BELSORP system-check.
  • last measurement of a reference sample
  • last service of pumps - especially rotary pumps where a turbomolecular pump is not used
  • Verify the laboratory microbalance

Loading sample

• Prepare the empty cell - it must be clean and dry
  • It may be necessary to evacuate and backfill it with inert gas
  • Some samples absorb significant amounts of nitrogen, even at room temperature, so it may be necessary to use helium at every weighing stage
• Weigh the empty cell
• Load the sample
  • avoid sample in the stem of the cell
• Weigh the sample + cell, before any outgassing
• Evacuate or purge
• Verify there are no leaks
• Degas
  • External degasser? in-situ? both?
  • Set the correct Temperature
    • or temperature profile
  • Record the duration of degassing
  • Verify the apparent leak rate
  • Cooling
  • Backfill the cell with the same gas used for the empty weighing
• Reweigh the sample when the moisture has been removed

Configure the measurement

• Measurement of the value of \(P_0\) ( saturated vapour pressure )
• Measurement range of \(P\) or \(P/P_0\)
  • Must at least cover the BET range
  • It is very useful to include desorption points
    • If the adsorption interaction is truly physisorption, then the desorption curve should superpose the adsorption curve - it should be reversible. Otherwise, there may be a problem with the measurement.
  • Do you want to include a full isotherm to see details of the mesopores ?
• Dosing criteria
• Equilibration criteria
  • These are especially important for measurements of microporous materials at low pressures, where the equilibration can be very slow.

After the measurement :

• Verify the reversibility within the BET range (or understand why the isotherm is not reversible)
• Verify that the \(C\) constant is positive : \(C \gt 0\)
• Choose the region where the BET model is valid
  • Specifically, \(n_{ads}\,\left(1 - P/P_0\right)\) should be increasing continously with \(P/P_0\)
    • This is recommended by ISO9277 / IUPAC - it is commonly called the Rouquerol criterion
    • It is particularly important in the presence of micropores
• Verify that the relative pressure \(P/P_0\) corresponding to the monolayer coverage, \(n_m\), lies inside the range of \(P/P_0\) used for the fit.
• Choose A_m

Reporting the result

Explicit mentions to accompany the BET area:

• Gas species
• Temperature
• Outgassing conditions, duration, verification method
• Range of \(P/P_0\) used for fitting
  • and why that range was chosen
• Quality of fit, usually as an R^2 value
• The C value