Automatic selection of data points and optimization of BET surface area

The selection of data used for fitting BET surface area is often an easy process using the MicroActive software.  A broad range of isotherm data is selected and then the BET fit may be further refined to yield the specific surface area of a material.  The process for data selection and optimization has been fully automated using the BET surface area rules advocated by Professor Jean Rouquerol, CNRS Marseille, FR.  Prof. Rouquerol recommends the following criteria:

  1. A linear fit to the (BET) transformed data should be obtained and this is a common practice for calculating specific surface area.
  2. The BET “C” constant must be positive ( C > 0 ).  This is also common practice for calculating specific surface area.
  3. The Rouquerol transform  n_{ads}( 1 - p/p^o) should be increasing with p/p^o for the data selected to calculate the BET parameters.
  4. The monolayer capacity – n_m should be within the limits of data that were used to fit the BET parameters.
  5. The value of 1/(\sqrt{C} + 1) \approx p/p^o at the monolayer capacity.

Rules 3, 4, and 5 have been contributed by Prof. Rouquerol and these rules allow for the automatic selection of points to calculate the BET surface area.  Using rule three, n_{ads}( 1 - p/p^o) is monotonically increasing with p/p^o.  This provides a clear basis for determining the maximum values of p/p^o and n_{ads} as this plot (the Rouquerol Transform) has a peak maximum corresponding to the maximum values to be used in the BET transform.

Rule four is used to confirm that the calculated value of the monolayer capacity n_m falls within the range of data used to calculate the BET parameters.  This confirms that the monolayer capacity has not been extrapolated.

Rule five 1/(\sqrt{C} + 1) \approx p/p^o provides a check for consistency and may be used to further optimize the selection of points used to calculate the BET fit.  The value of 1/(\sqrt{C} + 1) should approach the relative pressure calculated from the BET transform at the monolayer capacity.

This new report is available for use with MicroActive 2.0 and the MicroActive included with: Gemini, TriStar II Plus, ASAP 2460, and 3Flex.   Please contact your local Micromeritics representative to obtain this new report or send a request to techsupport.micromeritics.com.

The automatic selection of points for BET surface area has been tested using a broad range of materials.  Below is an example of the automated selection and optimization for a Fluid Cracking Catalyst (FCC).  It is often difficult to select the proper range for the BET calculation since it is a combination of zeolite (faujasite) and clay binder and the microporous component (zeolite) may often cause the BET “C” value to be negative.  This new report validates the optimized range against the previously given rules and gives both the summary data and the results of the rule checking.

Automatic selection of data used for calculating the BET surface area of a fluid cracking catalyst.

Automatic selection of data used for calculating the BET surface area of a fluid cracking catalyst.

Calculating the BET surface area of metal organic frameworks (MOFs) may be equally challenging.  Below is an example of the automatically optimized BET fit for Basolite C300.

BET surface area for Basolite C300 with data automatic selected using the five rules propsed by Prof. Jean Rouqueol

BET surface area for Basolite C300 with data automatic selected using the five rules propsed by Prof. Jean Rouquerol

The automatic selection is also effective for non-microporous materials such as silica and titania (see below).

BET surface area for meso-porous silica.

BET surface area for meso-porous silica.

Automatically selected surface area points for titania.

Automatically selected surface area points for titania.

These five rules are equally applicable to microporous carbons and an example is given below.

Automatic selection and optimization of data used for calculating BET surface area of an activated carbon using the Rouquerol criteria.

Automatic selection and optimization of data used for calculating BET surface area of an activated carbon using the Rouquerol criteria.

The instruments used to obtain the isotherm data may be viewed at www.micromeritics.com.

Technical support for Micromeritics users is freely available through techsupport.micromeritics.com.

Accredited training and education for Micromeritics instruments users are available through www.micro.edu.