BET Surface Area Analysis

The BET Surface Area analysis evaluates total surface area of the catalyst before and after chemisorption. Loss of activity, which may occur due to the blocking of pores during the chemisorption reaction, as well as the occurrence of sintering of the support, can be studied.

After outgassing the sample, a mixture of 30% nitrogen and 70% helium is applied to the sample which is immersed in a liquid nitrogen (LN2) bath. The amount of nitrogen adsorbed (usually measured on desorption) at LN2 temperatures is used to calculate total surface area.

Because nitrogen uptake is a function of pore size, sample sintering can cause results to show reduced surface area. Therefore, it may be helpful to perform BET as the first and last experiments in the analysis in order to check for sintering of the support.

Preparation

Pretreatment Flow helium over the sample to remove impurities, usually at the maximum temperature tolerated by the sample (for example, the temperature at which the sample was calcined).
Analysis Flow 30% N2/He, using a dewar of liquid nitrogen, to measure the uptake of nitrogen. Then remove the LN2 dewar and replace it immediately with a dewar of water at ambient temperature. The amount of N2 desorbed is measured and the BET equation is used to calculate the active surface area.
Cold Trap Install the external long delay path. Do not use a cold trap dewar.
Pressure regulator For BET analysis, gas cylinders should be set to a level between 10 and 15 psig (69 and 103.5 kPag).
Furnace temperature During pretreatment, select a temperature high enough to remove contaminants or moisture, but not so high as to cause sintering or fusing of the sample. Hold the temperature long enough to remove contaminants or moisture. Maintain the target temperature for at least 30 minutes to ensure adequate degassing. Ensure the Done step is set to return the sample temperature to 0 ºC.

Calibration

BET experiments require a separate calibration consisting of three manual injections of nitrogen through the septum. This calibration can be performed either before or after the BET analysis. As with all TCD calibrations, use the Peak Editor to associate the calibration file with the analysis file. For this reason, it is necessary that the data on the manual injections are collected in a separate experiment from the BET experiment. The injections may be included in the same analysis as the BET experiment, or they may be performed as a completely separate analysis. Use the same recording options (zero, invert) for the Calibration experiment and the BET experiment.

The following method includes the calibration injections as a separate experiment which follows the BET experiment in the analysis.

Injection Size

Inject three volumes of gas with the goal of approximating the volume of gas taken up by the sample. Use one injection slightly larger than the expected volume, one injection slightly smaller than the expected volume, and one injection between these two volumes. The largest injection should be two to three times greater than the smallest injection.

Because the same syringe must be used for all three injections, select a syringe that can accommodate all three injection volumes. The volume of gas uptake can be estimated if the approximate surface area (SA) of the sample is known. Use the following formula to determine the approximate volume (Vm) to inject.

So:

For example, a sample of 0.1 g of 50 m2/g material has a surface area of 5 m2. Therefore, the volume of the gas which will be adsorbed is approximately 1.145 cm3 STP.

Procedure

  Before performing an analysis, ensure the sample and analyzer are adequately prepared.
  1. Obtain the sample weight, then install the loaded sample tube and thermocouple on the analyzer.
  2. Ensure the Delay Path is installed in the cold trap port.
  3. Create a sample file containing the appropriate analysis conditions and report options.
  1. Go to File > New Sample.
  2. Complete the sample Information window using appropriate values.
  3. Select the Analysis Conditions tab and insert the following experiment steps:
Insert Step Window Field Field Entry or Selection
Experiment New Experiment Experiment description Enter a description of the experiment
Type of analysis Physisorption Surface Area
Zones TCD Detector

Block zone

100

Filament

175
Valve Zones

Cold trap

110

Analysis

110

Loop

110

Back pressure

110
Gas Flows Prep Gas Helium
CarrierGas Helium
Loop Gas None
Rate 50 cm3/min
Cold trap valve Trap
Analysis valve Prepare
Loop valve Fill
Back pressure Prepare
Outputs Outputs Use default values
Peaks Peaks Use default values
Recording Options Invert the TCD signal Select this option
Temperature Ramp Temperature Ramp Ramp Type Sample ramp
End temperature 350
Ramp rate 50.0
Hold time 0.00
Enable KwikCool No
Temperature Ramp Temperature Ramp Ramp Type Return to ambient
Enable KwikCool Enable this selection
Change Gas Flows Gas Flows Prep Gas None
Carrier Gas Nitrogen-Helium
Loop Gas None
Cold trap valve Trap
Analysis valve Analyze
Loop valve Fill
Back pressure Prepare
Wait Wait Wait until baseline is stable Select this option
Wait Wait Wait for operator Click OK to proceed with physisorption measurement
Start Recording Start Recording One measurement every

0.1 seconds

Dose Dose Get physisorption point Select this option

The application automatically inserts a Stop Recording step when a Start Recording step is inserted. Ensure that a Dose step and a Temperature Ramp step is inserted within the Start/Stop Recording loop.

Termination Termination Return to ambient temperature Select this option
Leave detector enabled after analysis Option is not selected
Zones Not applicable
Gas Flows Not applicable

To include the calibration data collection (three manual injections through the septum) within this analysis but as a separate Experiment, also insert the following steps (before the Done step):

Insert Step Window Field Field Entry
Experiment New Experiment Experiment description Enter a description of the experiment
Type of analysis Other
Zones TCD Detector

Block zone

100

Filament

175
Valve Zones

Cold trap

110

Analysis

110

Loop

110

Back pressure

110
Gas Flows Prep Gas None
CarrierGas Nitrogen-Helium
Loop Gas None
Rate 50 cm3/min
Cold trap valve Trap
Analysis valve Analyze
Loop valve Fill
Back pressure Prepare
Outputs Outputs Use default values
Peaks Peaks Use default values
Wait Wait Wait until baseline is stable Select this option
Start Recording Start Recording One measurement every 0.1 seconds
Start Repeat Start Repeat Sequence Repeat 3 times
Wait Wait Wait for operator Prepare manual N2 injection. Click OK.
Dose Dose Manual injection Select this option
The application automatically inserts a Stop Repeat step when a Start Repeat step is inserted. Ensure that a Wait step is inserted within the Start/Stop Repeat loop.
The application automatically inserts a Stop Recording step when a Start Recording step is inserted. Ensure that the Dose step and the Temperature Ramp step is inserted within the Start/Stop Record loop.
  1. Select the Report Options tab and modify the values as needed.
  2. Click Save, then click Close.
  1. Start the analysis.
  1. Go to Unit [n] > Sample Analysis and select the sample file that was just created.
  2. Edit the sample file as needed, but for this example, editing is not required. Click Next.
  3. From the drop-down list, select the calibrations associated with each experiment in the sample file (if any). For this example, select None.
  4. Click Next.
  5. Read the cautionary window and make any necessary changes. Click Start to start the analysis. During the analysis, change the dewar when prompted.
  Change the dewar rapidly. It is recommended to hold the water dewar in one hand and remove the LN2 dewar with the other to minimize the time required for the change.
  1. When the analysis ends, remove the sample tube from the analyzer, place the caps on the tube, and dry off the outside of the tube with a paper towel.

 

Use the cotton gloves provided in the accessory kit to remove the sample tube before it has cooled. Rubber gloves may be used to handle the sample tube when it has cooled.

  1. Weigh the sample tube, sample, and caps. Subtract this weight from the weight obtained for the sample tube and caps. The resulting weight is the dry sample weight (the after analysis weight). This weight will be used when calculating the BET surface area in the following sections.

Generate the BET Report

The following tasks must be performed before generating the BET report:

  • Edit the sample file and the calibration file to ensure that peaks are properly marked
  • Create a TCD calibration file
  • Associate the TCD calibration file with the sample file

After completion of these tasks, go to Reports > Start Report. Verify that BET Surface Area is selected.

Edit the peaks in the sample file

  1. Open the Peak Editor.
  2. Select the Peak Editor experiment from the drop-down at the bottom of the window.
  3. Select Edit Peaks in the View Type group box.
  4. Click Find All Peaks.
  5. If needed, edit the edges of the desorption peak:
  The desorption peak is most often used to determine the surface area because the desorption process starts with the adsorbate equilibrated on the surface.
  1. Drag the cursor over the entire baseline of the peak to enlarge the editing area.
  2. Click on the baseline of the left side of the peak, right click and select Mark left edge.
  3. Repeat step b for the right side of the peak.
  1. As the peak edges are adjusted, the values associated with the peak (listed in the peak table) are also adjusted. When satisfied with the appearance of the peak, right click and select Save.

Edit the peaks in the calibration file

  1. Click Calibration.
  2. Select the calibration file used with the analysis. Click OK.
  3. Edit the peaks in the same manner as described for the sample file.

Create the TCD calibration file

  1. Go to Options > Signal Calibration > New.
  1. Select User defined and enable an Experiment. Click OK.

 

  1. Click Load Calibration Data.
  2. Select the sample file containing the calibration data. Click OK. The data in the file are inserted into the appropriate fields of the Signal Calibration window.
  3. The values in the Peak Quantity column are defaults. The correct values for the volume of each injection at STP must be determined using the formula:

Determine the volumes for each calibration peak listed in the table.

  1. Enter the converted values (cm3 STP) into the table.

Assess the Peak Area plot, Goodness of Fit, and Coefficients to decide if the calibration file is acceptable. Use your laboratory’s standards to determine what level of linearity is acceptable. As a general guideline, use a calibration file with a very low value for Goodness of Fit (less than 0.1), when 1st or 2nd Degree is specified.

To delete peaks that are outliers, highlight the peak data in the table and click Delete Peak. Evaluate the results when changing the degree (select a different degree from the drop-down list).

  1. Enter a new description into the Description field.
  2. When satisfied with all values in the file, click Save, then click Close

Associate the Calibration file with the Sample file

  1. Open the Peak Editor.
  2. Select the Peak Editor experiment from the drop-down at the bottom of the window.
  3. Select the experiment and click the Calibration button.
  4. Select the down-arrow to the right of the Calibration field. Select the calibration file just created. Click OK.
  5. Repeat steps 3 and 4 for each experiment.
  6. Click Save, then click Close.
 
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