Confrontation of various adsorption models for assessing the porous structure of activated carbons

International audienceHerein, a comprehensive analysis of DFT methods as a tool for evaluating the impact of the nature of the activating agent on the porous structure of activated carbons derived from hazelnut shells is given. The study was based on the use of NLDFT, QSDFT, and 2D-NLDFT methods applied to nitrogen adsorption isotherms, and the results were compared with those formerly obtained by using DR, BET, and LBET methods. Analyses conducted with NLDFT, QSDFT, and 2D-NLDFT revealed a very strong dependence of the results on assumptions about the specific pore model, which calls into question the reliability and credibility of these methods. However, if one takes into account the measurement errors that may during the determination of the adsorption isotherms, as well as the difficulty of selecting a representative sample in a batch of materials (most often non-homogeneous) to be analysed, some imperfections of the DFT methods become acceptable. The analyses in question revealed some limitations of the LBET method which became obvious when the analysis concerned bimodal porous materials with a considerable proportion of mesopores. In such cases, the LBET method, which was formerly designed for analysing microporous materials, may become less reliable

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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Application of the modified Dubinin-Astakhov equation for a better understanding of high-pressure hydrogen adsorption on activated carbons

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Application of the modified Dubinin-Astakhov equation for a better understanding of high-pressure hydrogen adsorption on activated carbons

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Assessment of hydrogen storage in activated carbons produced from hydrothermally treated organic materials

12 activated carbons (ACs) were prepared by KOH activation using hydrochars as precursors. These hydrochars were prepared by hydrothermal carbonisation (HTC) of sucrose solutions at concentrations ranging from 0.2 to 1.6 mol L-1. The KOH to hydrochar weight ratio (W) was varied from 1 to 5, and the activation temperature was set to 1023K. ACs texture was assessed by nitrogen and carbon dioxide adsorption at 77 and 273 K, respectively; pore size distribution was calculated by using both isotherms and the SAIEUS© software. ACs with surface areas between 790 and 2240 m2 g-1 were obtained. Hydrogen excess adsorption was determined at 298K and pressures up to 10 MPa in a volumetric device, and the isosteric heat of adsorption (Qst) was calculated for four ACs, using hydrogen isotherms obtained at 278, 298 and 308K. Potassium intercalation between graphitic planes was assumed to account for the high Qst values, 7-8 kJ mol-1. Hydrogen uptake at 2 MPa was compared with hydrogen adsorption data of 38 other ACs reported in the open literature. Hydrogen adsorption fundamentally depends on micropore volume and preliminary HTC did not enhance hydrogen storage although it could be a good strategy for doping carbon with heteroelements.The authors gratefully acknowledge the financial support of the CPER 2007-2013 “Structuration du Pôle de Compétitivité Fibres Grand’Est” (Competitiveness Fibre Cluster), through local (Conseil Général des Vosges), regional (Région Lorraine), national (DRRT and FNADT) and European (FEDER) funds. Part of this work was supported by CHEERS project (FEDER funds) and the COST Action MP1103 “Nanostructured materials for solid-state hydrogen storage”.Peer reviewe

Biochar as a catalyst for hydrogen production from methane conversion

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Hydrothermal pre-treatment, an efficient tool to improve activated carbon performances

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Short-length carbon nanotubes as building blocks for high dielectric constant materials in the terahertz range

Due to the high polarizability of finite-length carbon nanotubes (CNTs) in the quasi-static regime, they can be considered as building blocks for the fabrication of high dielectric constant material. Our theoretical estimations, based on an effective medium approach and solutions of a boundary value problem for individual CNT, predict that composite materials comprising short-length CNTs can have very high dielectric constants (up to 300) and low dielectric loss tangents (below 0.03) in the terahertz range. In order to prove this, 500–1000 nm thick films comprising single- and multi-walled CNTs of both long (0.5–2 μm) and short (0.1–0.4 μm) lengths have been fabricated. The analysis, based on the time-domain terahertz spectroscopy in the range 0.2–1.0 THz, demonstrated a decrease in the dielectric loss tangents of the CNT-based materials with a reduction in CNT length. In the terahertz range, the films comprising short-length CNTs had a relative effective permittivity with a large real part (25–136) and dielectric loss tangent (0.35–0.60)

Biomass-derived carbons physically activated in one or two steps for CH4/CO2 separation

The present study aims at evaluating the suitability of producing activated carbons (ACs) derived from wheat straw by a one-step synthesis approach, as an alternative to more conventional two steps production processes (i.e., pyrolysis and subsequent activation). The performance of the produced ACs, in one or two steps, as sustainable and selective CO2 adsorbents for CH4/CO2 separation is compared. In addition, the influence of pyrolysis conditions on the properties of the resulting two-step ACs is carefully analyzed. We show that the biochar-based precursors of ACs presenting the best textural properties were obtained under mild conditions of maximum temperature and absolute pressure during pyrolysis. The one-step ACs were fully comparable —in terms of textural properties as well as CO2 uptake and selectivity— to those produced by the more conventional two-step synthesis process. In addition, results obtained from breakthrough curve simulations highlight that the best AC in terms of CH4 recovery under dynamic conditions was produced by a one-step activation. Therefore, the one-step process appears to be as an attractive route for the production of engineered carbon materials, which can lead to significant cost savings in large-scale production systems

Towards a feasible and scalable production of bio-xerogels

© 2015 Elsevier Inc. Hypothesis: The synthesis process of carbon xerogels is limited, mainly due to two drawbacks that prevent their introduction onto the market: (i) the long time required for producing the material and (ii) the reagents used for the synthesis, which are costly and harmful to the environment. Microwave radiation is expected to produce a reduction in time of more than 90%, while the use of tannin instead of resorcinol will probably result in a cost-effective carbonaceous material. Experiments: Resorcinol-tannin-formaldehyde xerogels containing different amounts of tannin, either with or without a surfactant (sodium dodecyl sulphate), were synthesized by means of two different heating methods: conventional and microwave heating. The effects of the surfactant, the heating method and the addition of tannin upon the porous structure and the chemical composition of the final materials were evaluated. Findings: It was found that the addition of surfactant is essential for obtaining highly porous xerogels when using tannins. The heating method also plays an important role, as conventionally synthesized samples display a greater volume of large pores. However, tannins are less sensitive to microwave radiation and their use results in tannin-formaldehyde xerogels that have a porous structure and chemical composition similar to those of resorcinol-formaldehyde xerogels.Financial support from the Ministerio de Economía y Competitividad of Spain MINECO (under Projects MAT2011-23733, IPT-2012-0689-420000 and CTQ2013-49433-EXP) is greatly acknowledged. NRR is also grateful to MINECO for her predoctoral research grant. The French authors also gratefully acknowledge the financial support of the CPER 2007–2013 “Structuration du Pôle de Compétitivité Fibres Grand’Est” (Competitiveness Fibre Cluster), through local (Conseil Général des Vosges), regional (Région Lorraine), national (DRRT and FNADT) and European (FEDER) funds.Peer Reviewe