Client-run Instrument
Trainer: Beth Eves

Micromeritics SmartVac degasser, 3Flex analyzer, computer
SmartVac degasser, 3Flex analyzer, computer

WHAT INFORMATION THE CLIENT SHOULD KNOW BEFORE REQUESTING ANALYSIS

As a client-run instrument, some requirements should be recognized before asking for training in this particular technique or deciding that a compound should be analyzed on these instruments.

  • Only solid samples can be used.
  • What information are you wanting from the analysis of the sample?  Do you have any literature that you are wanting to compare this to that might have analysis parameters that could be replicated?
  • For physisorption:  Have an idea of what your theoretical surface area should be.  Try to be within an order of magnitude.  The amount of sample needed for physisorption is usually at least 100mg of sample in the tube, to help lesson any effect weighing errors may cause.  Another good amount of sample is enough for 200m2 of surface area in the analysis tube.  If you cannot provide these amounts, talk to the trainer before analyzing!
    • Have an idea of what gas(es) you want to use.
    • Does your compound hold onto water or solvent easily?  At what temperatures does your compound undergo phase changes or warping?
  • For chemisorption, an idea of what gas(es) you want to use is needed.  Also having at least 50mg of sample in the analysis tube.

Principles of Operation

Physisorption is a process in which the van der Waals forces of the surface of a cleaned and degassed compound can attract and hold onto low-temperature (liquefied) gases.  By comparing the amount of gas used, characteristics of the compound’s surface can then be calculated, like surface area, average pore size, total pore size, etc.  Physisorption can occur with any solid-fluid or solid-gas system.  A comparable real-world phenomenon would be dust settling all over the surface of an object.

Chemisorption is a process where there is a chemical reaction between the surface of the compound and the adsorbate gas, generating new chemical bonds.   With the use of an integral thermal conductivity detector, dynamic chemisorption analyses like temperature programmed reduction (TPR), oxidation (TPO), desorption (TPD), and reactions (TPRx) can be performed.  A real world example would be the rusting of iron.

The SmartVac handles the degassing of compounds before physisorption analysis.  It can degas six samples at a time, each with their own individual degassing program.  Starting to degas a new sample will not interfere with other samples already on the other SmartVac ports.

The 3Flex instrument can handle up to three samples at a time for physisorption, each with their own individual set-up.  It can handle one sample at a time for chemisorption.  It can also perform vapor adsorption or heat of adsorption analyses.  Accessories include a heating mantle for physisorption, a furnace capable of 1100°C for chemisorption, and abilities for both cold trap vapor and room-temperature vapor dosing.
 

Process of Analysis

Physisorption preparation:

Solid samples are weighed into a precisely-made glass tube which has a flattened bulb on the end.  A filler-rod and check-seal are then added, sealing the container.

A sample data file is then created, which contains the sample identification specifics, the inputted recorded weights, the degassing instructions used by the SmartVac, the analysis instructions used by the 3Flex and the reports calculated once the sample has been analyzed.  
The sample is then pumped down under vacuum on the SmartVac degasser according to the program file and heated to remove any remaining solvent or water still remaining in the pores of the compound.  If these are not removed completely, the data acquired will not be accurate or the analysis may not proceed at all until the sample is degassed for longer and/or with more heat.  The SmartVac can reach a temperature of 450°C.

The degassed sealed sample+container is then weighed again for a final weight.  An isothermal jacket is placed on the container and then the tube is attached to a port on the 3Flex analyzer.  (Three samples can be analyzed at the same time on the 3Flex for routine physisorption.)  A dewar of liquid nitrogen is prepared and installed onto the 3Flex elevator under the sample tube(s).  A Psat tube is also put into position for measuring the Po value during the analysis.  A shield is then placed to cover the analysis area.

The sample data file analysis instructions are then used to analyze the sample.  Once the sample analysis has completed, the previously selected reports are generated for data collection.  Reports that are available are adsorption and desorption isotherms, single and multipoint BET surface area, Langmuir method surface area, average pore size, total pore volume, t-Plot external surface area and micropore analysis, Alpha-S method, BJH adsorption and desorption, Dollimore-Heal adsorption and desorption, Horvath-Kawazoe, MP-method, DFT pore size and surface energy, Dubinin-Radushkevich and Dubinin-Astakhov reports.

An example of a typical physisorption run might be a 6-point BET analysis, followed by a total adsorption up to almost a full relative atmosphere, then a desorption down to a third of a relative atmosphere, then repeating the adsorption/desorption an additional two times.
 

Chemisorption preparation:

Solid samples are weighed into a precisely-made glass tube which fits into a heating furnace.  Quartz wool or filter disks can be used to prevent the compound from being pushed into the inner workings of the instrument.  

A sample data file is then created, which contains the sample identification specifics, the inputted recorded weights, the analysis instructions used by the 3Flex and the reports calculated once the sample has been finished.  It is possible to program the instrument to run a repeat of the chemisorption program on the same sample, after a programmed amount of time where the sample is cooled and any excess gas has been evacuated away, to observe any reversible sorption.

A filler rod is inserted into the sample tube and the sample and exhaust tubes are attached to the 3Flex.  A thermocouple is clipped onto the sample tube, for monitoring the temperature of the heating furnace (which can reach 1100°C).  The furnace is placed on the elevator and attached to the instrument for power and exhaust.  The elevator is raised, the furnace surrounds the sample tube and a ceramic top is placed around the top of the furnace around the tube.

The sample data file analysis instructions are then used to analyze the sample.  Once the analysis is finished, the sample is cooled and reweighed.  Once new weight is added to the file, the previously selected reports are generated for data collection.  Reports that are available are isotherms, Difference method, Sinfelt method, Langmuir surface area, Freundlich and Temkin reports.

Further Reading

BET Surface Area - Andy Connelly

Physical Adsorption Webinar - Micromeritics

Introduction to Chemical Adsorption Analytical Techniques and their Applications to Catalysis – Paul A. Webb