Thermoporometry of mesoporous carbons: Application of octamethylcyclotetrasiloxane as probe liquid for small mesopores

Abstract

The mesoporosity of carbons can be determined using traditional methods such as gas physisorption or mercury intrusion, as well as by an alternative technique, thermoporometry (TPM). TPM focuses on determining porosity in the wet state. If water is used as probe liquid, the mesopores larger than 10 nm are hard to evaluate due to the overlapping of pore and bulk melting peaks. To overcome this problem, octamethylcyclotetrasiloxane (OMCTS) is used. Due to the fact that OMCTS undergoes solid-solid phase change in the same temperature range as melting in small mesopores, its use for mesopores below 10 nm is problematic. Thus, the applicability of OMCTS as a probe liquid for TPM characterization of small mesopores was tested in this work. The separation of melting in small pores from solid-solid transition was elaborated by several approaches like changes of heating rate, implementation of prefreezing step, deconvolution of experimental DSC records with the use of basic kinetic equation as underlying functions). It was found that the prefreezing step and a measurement of pure OMCTS under the same conditions as expected for TPM measurement is useful since the solid-solid transition depends on the measurement conditions. The usefulness of the suggested measurement conditions and deconvolution method was successfully demonstrated on carbonaceous samples with exclusively small mesopores (5 nm), as well as carbons containing both small and large mesopores. The results indicate that this approach enables the investigation of the entire mesoporous range and makes OMCTS a more versatile probe liquid for TPM.