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    What is BET method?

    作者:管理員       發布日期: 2015-08-12      點擊: 2847

       BET method is based on the multi-layer molecular adsorption model proposed by Stephen Brunauer, Paul Hugh Emmett and Edward Teller, in which single layer adsorption volume, Vm is correlated with multi-layer adsorption volume, V. This is the famous BET equation. Since BET equation is based on multi-layer adsorption theory, which makes it more accurate to describe real adsorption process, measurement by BET method ensures better accuracy. Through measurement of 3-5 groups of multi-layer adsorption volume by sample at different partial nitrogen pressure, an adsorption isotherm can be plotted with P/V(P0 ? P) on the y-axis and P/P0 on the x-axis according to experimental results. Fit this curve to get a straight line, then slope and intercept can be obtained. Surface area of sample under testing can be calculated from known Vm. Based on theory and practice, BET equation fit well with real adsorption process when P/P0 is in the range of 0.35-0.05. Curve linearity is nice too in this range, so this range is preferred for practical measurement.

      BET equation is 

    where:      P:     nitrogen partial pressure 
               P0:  nitrogen saturation vapor pressure at liquid nitrogen temperature
               V:     real adsorption volume on sample surface
               Vm:  mono-layer nitrogen adsorption volume
               C:    constant determined by sample adsorption capability

     Operational procedures for BET are similar to reference comparison method, the difference is that BET method need to calibrate real nitrogen adsorption volume, also theoretic calculation is different for these two methods. BET method for surface area testing is applicable to numerous industries, and adopted popularly due to its accuracy and reliability, which is appealing to scientific research. When sample nitrogen adsorption is strong, single-point BET method can be adopted. This method is quick, similar to reference comparison method. Testing result using this single-point BET method has relatively small error comparing to multi-point BET method.

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