Synthesis of Porous Carbon Monoliths Using Hard Templates

The preparation of porous carbon monoliths with a defined shape via template-assisted routes is reported. Monoliths made from porous concrete and zeolite were each used as the template. The porous concrete-derived carbon monoliths exhibited high gravimetric specific surface areas up to 2000 m2·g−1. The pore system comprised macro-, meso-, and micropores. These pores were hierarchically arranged. The pore system was created by the complex interplay of the actions of both the template and the activating agent as well. On the other hand, zeolite-made template shapes allowed for the preparation of microporous carbon monoliths with a high volumetric specific surface area. This feature could be beneficial if carbon monoliths must be integrated into technical systems under space-limited conditions.

The carbon loss due to the activation step was determined by weighing. The crystalline structure of the materials was investigated by X-ray diffraction (XRD) with a Bruker D2 Phaser instrument (Madison, WI, USA). Cu Kα radiation (wavelength λ = 1.5418 Å ) was used as an X-ray source. The determination of graphitic and amorphous carbon structures was performed in the 2 θ angle range of 1-60° and with a step of 0.02° per second.

Comment: Theoretical value of the carbon BET surface area and the pore volume (according [27])
A direct template effect was defined as follows: the template pores and the carbon network have a common interface and so the internal surface area of the replica corresponds to those of the template; -the pore sizes of the replica are determined by the thickness of the template pore walls; -the pore volume stems by the skeleton volume of the template.
Assuming the direct template effect, the surface area of the carbon replica can be calculated from the template surface area and the mass of infiltrated carbon according template pore template skel carbon The symbols are: Stheo: calculated surface area of the carbon replica in m 2 per g carbon, Stemplate: surface area of the template in m 2 per g template, Vpore template: template pore volume in cm 3 per g template, ρskel carbon: skeleton density of nonporous carbon which is assumed to be 2.0 g carbon per cm 3 carbon i.e., somewhat lower than that of pure graphite.
From this, a ratio of the measured surface area and theoretical carbon surface area Smeasured/Stheo can be calculated. Any deviations from unity can be explained in terms of scenarios which differ from the direct template effect. If the ratio of Smeasured/Stheo is lower than unity, the carbon network, which has been formed in the template pores, was obviously not stable enough and collapsed (at least) partially. In case of ratios larger than unity, an inherent porosity of the carbon, mostly formed by gasification reactions, must be assumed.
In the same way, the theoretical value of the pore volume of the carbon replica can be calculated by Here the symbols are: Vpore theo: theoretical value of the pore volume given in cm 3 per g carbon, Vskel template: template skeleton volume in cm 3 per g template, Vpore template: template pore volume in cm 3 per g template, x: template pore filling degree expressed as the fraction of the carbon filled template pore volume in cm 3 carbon per cm 3 pore volume mcarbon infil: mass fraction of infiltrated carbon in g carbon per g template. From this, again, a ratio of the measured value and the theoretical value can be calculated. If the ratio of Vpore measured/Vpore theo is lower than unity, different scenarios must be considered: the carbon particle got broken (due to low carbon content in the template), -large unfilled template regions which would cause the formation of macropores in the replica; these pores cannot be detected by the applied adsorption method; -blocked pores in the replica which cannot be infiltrated by any probe molecule.