ABSTRACT
A thorough characterization of the textural properties of hierarchical porous carbons (HPCs) is of utmost importance as it provides information that aids in the selection of a suitable material for a given application and in understanding the phenomena observed once the material becomes part of a system. Gas adsorption-desorption isotherms coupled with the application of density functional theory (DFT) models to these isotherms are common tools for the textural characterization of HPCs, for which pore shape is an essential factor for the determination of pore size distributions (PSDs). By analyzing the experimental adsorption data of a series of CO2-activated HPCs with a progressive development of porosity, it is shown that artifacts are found in the derived PSDs when a slit-cylinder pore shape boundary is fixed at 2 nm, which is the case for the original dual-shape nonlocal DFT (2D-NLDFT-HS) and hybrid quenched solid DFT (QSDFT) models. This study presents a new dual-shape 2D-NLDFT-HS (DS-HS) model that, combined with the 2D-NLDFT-HS model for CO2, provides the possibility of analyzing simultaneously N2 and CO2 adsorption-desorption isotherms and adjusting at the same time the limits for the assumed slit and cylindrical pore shapes. Using the DS-HS approach and adjusting the slit-cylinder boundary at 3 nm allowed eliminating PSDs artifacts. The interactive adjustment of the slit-cylindrical pore shape boundary of the DS-HS model represents a major advantage of this approach allowing for a comprehensive analysis of the adsorption data and a more accurate description of the textural properties of HPC materials.
ABSTRACT
A highly sensitive and selective silicon-based microanalytical prototype was used to identify a few ppb of volatile organic compounds (VOCs) in indoor air. Herein, a new nonactivated tannin-derived carbon synthesized by an environmentally friendly method, DM2C, a MIL-101(Cr) MOF, and a DaY zeolite were selected for the preconcentration of BTEX compounds (i.e., benzene, toluene, ethylbenzene, and xylenes). Integrating a small amount of these nanoporous solids inside a miniaturized preconcentration unit led to excellent preconcentration performance. By taking advantage of the high adsorption-desorption capacities of the DM2C adsorbent, concentrations as low as 23.5, 30.8, 16.7, 25, and 28.8 ppb of benzene, toluene, ethylbenzene, ortho- and para-xylene, respectively, were detected in a short analysis time (â¼10 min) even in the presence of 60% relative humidity at 25 °C. The DM2C showed excellent stability over a period of 4 months and more than 500 tests, as well as repeatability, which makes it a very reliable adsorbent for the detection of trace VOCs in indoor air under realistic conditions in the presence of humidity.
ABSTRACT
The electromagnetic properties of various carbon gels, produced with different bulk densities, were investigated in a wide frequency range (20 Hz-36 GHz). The values of dielectric permittivity and electrical conductivity at 129 Hz were found to be very high, i.e., more than 105 and close to 100 S/m, respectively. Both strongly decreased with frequency but remained high in the microwave frequency range (close to 10 and about 0.1 S/m, respectively, at 30 GHz). Moreover, the dielectric permittivity and the electrical conductivity strongly increased with the bulk density of the materials, according to power laws at low frequency. However, the maximum of microwave absorption was observed at lower densities. The DC conductivity slightly decreased on cooling, according to the Arrhenius law. The lower activation energies are typical of carbon gels presenting lower DC electrical conductivities, due to a higher number of defects. High and thermally stable electromagnetic properties of carbon gels, together with other unique properties of these materials, such as lightness and chemical inertness, open possibilities for producing new electromagnetic coatings.
