For characterization of porous sorbents like activated carbons or zeolites the determination of adsorption isotherms of test gases like (He, H2, Ar, C4H10 etc) and subsequent data evaluation using different theories or models like the BET-, BJH- or other methods, is a very common and useful approach. The experimental devices needed for these purpose are commercially available and in most cases computerized (so called “one button systems”). Thus the specific sorbent parameters like surface area, pore size distribution and pore volume for example are easily available. Though these parameters are valuable and in most cases sufficient for surface characterisation, additional measurements of the heat of adsorption may be helpful for energetic characterization and examination of the surface heterogeneity of the porous material. Enthalpy measurements also can serve as consistency tests for model equations of adsorption isotherms used to correlate experimental data.

In this paper we present results for simultaneous volumetric measurements of gas uptake and adsorption enthalpy to study the surface loading and energetic heterogeneity of two different activated carbons (BAX 1100 , BAX 1500). The adsorptive used for this comparison was butane (n-C4H10). Additionally, the results for adsorption of i-butane and propane on BAX 1500 are displayed. The affect of wall adsorption in volumetric measurements for both – gas uptake and heats of adsorption - is discussed to a certain extent. Measurements were taken using a (fairly) novel sensor gas calorimeter (SGC) [1],allowing to simultaneously perform combined volumetric and calorimetric measurements from vacuum up to 1 MPa in the temperature range 77 K - 373 K.

References:

1.W. Zimmermann, J. U. Keller, A New Calorimeter for Simultaneous Measurement of Isotherms and Heats of Adsorption, Thermochimica Acta 405 (2003), 31-41