Enhanced resolution of ultra micropore size determination of biochars and activated carbons by dual gas analysis using N2 and CO2 with 2D-NLDFT adsorption models

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Carbide-Derived Carbons: Effect of Pore Size on Hydrogen Uptake and Heat of Adsorption

Cryoadsorption is a promising method of enhancing gravimetric and volumetric onboard H2 storage capacity for future transportation needs. Inexpensive carbide-derived carbons (CDCs), produced by chlorination of metal carbides, have up to 80 % open-pore volume with tunable pore size and specific surface area (SSA). Tuning the carbon structure and pore size with high sensitivity by using different starting carbides and chlorination temperatures allows rational design of carbon materials with enhanced C-H2 interaction and thus increased H2 storage capacity. A systematic experimental investigation of a large number of CDCs with controlled pore size distributions and SSAs shows how smaller pores increase both the heat of adsorption and the total volume of adsorbed H2. It has been demonstrated that increasing the average heat of H2 adsorption above 6.6 kJ mol-1 substantially enhances H2 uptake at 1 atm (1 atm = 101 325 Pa) and -196 °C. The heats of adsorption up to 11 kJ mol-1 exceed values reported for metal-organic framework compounds and carbon nanotubes

Confirmation of pore formation mechanisms in biochars and activated carbons by dual isotherm analysis

In this study biochars and activated carbons were synthesized either directly via the pyrolysis of sodium carboxymethyl cellulose (NC) or via hydrothermal carbonization of sawdust (SD) in an aqueous solution of KOH. The amount of porogen was varied by modulating the degree of sodium carboxymethyl substitution on NC or the amount of KOH mixed in solution with SD. Pore size distributions (PSDs) of these carbons were determined from the dual fit of kernels based on the two-dimensional version of the nonlocal density functional theory (2D-NLDFT) heterogeneous surface models to either N2 and H2 or O2 and H2 isotherms measured at 196 1C. By comparing PSDs of carbons from the same starting material at increasing degrees of activation, we show that those derived using O2 and H2 isotherms not only give more detail of variations in pore size but that the results also fit better with current understandings of porosity development in carbons derived through oxidative activation. This is likely a result of superior diffusion of O2 into ultramicropores at low pressure relative to N2

Carbide-derived carbons designed for efficient hydrogen storage

Carbide-derived carbons (CDCs) with specific surface area (SSA) ~ 2000 m2/g and open pore volume up to 80% are produced by chlorine etching of metal carbides. Tuning the pore size distribution by carbide precursor selection and etching temperature yields enhanced hydrogen storage capacity at both ambient and elevated pressure. Our goal is to establish the fundamental relation between capacity and SSA, pore size and pore volume

Disentangling the self-diffusional dynamics of H2 adsorbed in micro- and mesoporous carbide-derived carbon by wide temporal range quasi-elastic neutron scattering

Understanding the processes guiding the confinement of adsorbed H-2 in different porous structures is vital for the development of adsorbents for effective cryo-adsorptive H-2 storage systems. Quasi-elastic neutron scattering (QENS) is applied over a wide range of timescales (0.2 ps - 150 ps) to determine different self-diffusion mechanisms of H-2 adsorbed in a carbide (synthesized from TiC via the sol-gel method) derived carbon (sol-gel TiC-CDC) adsorbent with hierarchical porous structure. The bulk and porous structure is characterized by gas adsorption, Raman spectroscopy, and wide-angle X-ray scattering methods. Sol-gel TiC-CDC belongs to a series of CDCs that have been previously characterized and where the self-diffusion of adsorbed H-2 has been investigated with QENS. Sol-gel TiC-CDC is very mesoporous, has relatively high stacking (2.76 graphenic layers per stack), and small interlayer spacing of graphenic sheets (3.43 angstrom) in comparison to other CDCs in the series, thus, being a well-ordered highly porous CDC. Restricted rotational self-diffusion of adsorbed H-2 is determined in ultramicropores (pore width, w, < 7 angstrom) and translationally self-diffusing H-2 adsorbed in multilayers across multiple timescales are determined in micro- and mesopores (7 angstrom< w < 500 angstrom). The microporous and graphenic structure of the CDC does not remarkably affect the self-diffusion of H-2 at high surface coverages. The simultaneous determination of adsorbed H-2 motions across different timescales allows to analyze the influence of micro- and mesopores under H-2 loading conditions, which are close to the ones used in technical applications and are vital for adsorbent optimization

Evaluation of the textural properties of ultramicroporous carbons using experimental and theoretical methods

Spherical carbon molecular sieves (CMS) have selective adsorptive properties which are suitable for separation and purification of gas mixtures. Precise methods of characterization are needed to understand the performance of CMS in separation processes. To this end, the pore size distribution (PSD) of four carbon molecular sieves were evaluated experimentally using immersion calorimetry and complemented with gas adsorption measurements at cryogenic temperatures for N2, O2 and Ar, and at 273 K for CO2. Theoretical pore size distributions were estimated using two-dimensional non-local Density Functional Theory (2D-NLDFT) models. Calorimetry results showed that B and C samples had a narrow pore size distribution with pores below 0.7 nm. Meanwhile, the pore size distributions calculated from O2 and Ar adsorption isotherms, gave an apex in the 0.5–0.6 nm region for all the carbons together with a growing development of porosity at around 0.8 nm and above for carbons A and D. The agreement observed between experiments and theory confirmed the validity of the theoretical 2D-NLDFT models to anticipate the PSD. Carbon C with pores exclusively below 0.7 nm separated CO2 and CH4 while carbon D with pores in the supermicroporous region separated propane and propylene chromatographically.JSA would like to acknowledge financial support from the Ministerio de Economía y Competitividad (MINECO) (MAT2016-80285-p), Generalitat Valenciana (PROMETEOII/2014/004) and H2020 (MSCA-RISE-2016/NanoMed Project)

Alternative view of oxygen reduction on porous carbon electrocatalysts: the substance of complex oxygen-surface interactions

Electrochemical oxygen reduction reaction (ORR) is an important energy-related process requiring alternative catalysts to expensive platinum-based ones. Although recently some advancements in carbon catalysts have been reported, there is still a lack of understanding which surface features might enhance their efficiency for ORR. Through a detailed study of oxygen adsorption on carbon molecular sieves and using inelastic neutron scattering, we demonstrated here that the extent of oxygen adsorption/interactions with surface is an important parameter affecting ORR. It was found that both the strength of O2 physical adsorption in small pores and its specific interactions with surface ether functionalities in the proximity of pores positively influence the ORR efficiency. We have shown that ultramicropores and hydrophobic surface rich in ether-based groups and/or electrons enhance ORR on carbon electrocatalysts and the performance parameters are similar to those measured on Pt/C with the number of electron transfer equal to 4

Using DFT analysis of adsorption data of multiple gases including H2 for the comprehensive characterization of microporous carbons

6 pages, 6 figures.-- Available online Dec on 23, 2006.Hydrogen and nitrogen adsorption isotherms at cryogenic temperatures (77 and 87 K) were used to characterize the microporosity of a series of activated carbons, representing various pore size distributions (PSD). The PSD for each carbon was calculated by simultaneous fitting of the DFT model isotherms to their experimental counterparts. The resulting PSD represents robust characteristics of the carbon structure that is consistent with all the data used in the analysis. The range of pore size analysis in this method is extended to smaller pore sizes compared to the standard nitrogen adsorption analysis. In addition, it is shown that this approach allows to detect and exclude experimental points that are not fully equilibrated due to diffusion problems in narrow micropores. The results of the analysis of a series of carbons activated with systematically increasing burn-off show that the presented approach is a useful tool for a comprehensive characterization of microporous carbons, and for obtaining detailed and reliable carbon PSDs.Peer reviewe