Carbon aerogels and Cr-, Fe-, Co-, and Ni-containing carbon aerogels were obtained by pyrolysis, at temperatures between 500 and 1800 °C, of the corresponding aerogels prepared by the sol−gel method from polymerization of resorcinol with formaldehyde. All samples were characterized by mercury porosimetry, nitrogen adsorption, X-ray diffraction (XRD), high-resolution transmission electron microscopy (HRTEM), and Raman spectroscopy. Results obtained show that carbon aerogels are, essentially, macroporous materials that maintain large pore volumes even after pyrolysis at 1800 °C. For pyrolysis at temperatures higher than 1000 °C, the presence of the transition metals produced graphitized areas with three-dimensional stacking order, as shown by HRTEM, XRD, and Raman spectroscopy. HRTEM also showed that the metal−carbon containing aerogels were formed by polyhedral structures. Cr and Fe seem to be the best catalysts for graphitization of carbon aerogels.

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The mesoporosity and microporosity of carbon aerogels were controlled by the structure of the polymer precursor and by CO2 activation, respectively. Their adsorption isotherms of nitrogen at 77 K were measured, and the porosities were determined by the high-resolution αS-plot. The ratio of the micropore volume to the mesopore volume was 0.10 to 0.24. The adsorption isotherms of water on the carbon aerogels were also measured at 303 K. All water adsorption isotherms had an adsorption hysteresis near P/P 0 = 0.5. The hysteresis of the nonactivated sample is not marked, but activated samples showed a vertical and noticeable hysteresis. The saturated amounts of water adsorption were not close to the mesopore volume but the micropore volume. Hence, it was concluded that water molecules do not predominantly adsorb on carbon mesopores and that the adsorption hysteresis is not described by the Kelvin equation.

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The thermodynamic Saam-Cole approach including the molecule-surface interaction was applied to explain the critical pore width at which the adsorption hysteresis of N2 on regular mesoporous silica disappears. The difference of the critical thickness of physisorbed layers on the pore wall upon condensation and evaporation was associated with the pore wall roughness inherent to the atomic structures, showing that the critical pore width for the disappearance of the adsorption hysteresis is in the range of 3.6 to 3.8 nm.

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