Quenched solid density functional theory method for characterization of mesoporous carbons by nitrogen adsorption

Quenched solid density functional theory (QSDFT) model for characterization of mesoporous carbons using nitrogen adsorption is extended to cylindrical and spherical pore geometries. The kernels of theoretical isotherms in the range from 0.4 to 50 nm are constructed accounting for different possible variations of the pore shapes in micropore and mesopore regions. The results of QSDFT method are illustrated with experimental data on adsorption on novel CMK-3 and 3DOm carbons. The proposed method is recommended for pore size distribution calculations for micro–mesoporous carbons obtained through various templating mechanisms.     

基于密度泛函理论的活性炭孔径分布改进算法

为提高非定域密度泛函理论（NLDFT）预测活性炭孔径分布（PSD）的精度,考虑了活性炭孔壁面晶体粗糙度对结果的影响。在传统NLDFT基础上,结合吸附壁面碳原子的密度分布,推导出改进NLDFT,预测了氩在光滑及具粗糙碳晶体表面的吸附平衡,并根据87.3 K、氩在活性炭上的吸附平衡数据,在由两种NLDFT确定了不同孔径的理论等温线核后,由寻优函数确定活性炭在0.35~12 nm区间的PSD。结果表明,以改进的NLDFT预测活性炭的PSD时,确定的活性炭孔径呈连续分布,预测平衡数据的相对误差小于10%;传统NLDFT确定的孔径在1 nm处出现断点,最大的预测相对误差范围达45%。改进NLDFT能较准确预测氩在具有粗糙晶体碳表面活性炭的PSD。     

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

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.     

Enhanced hydrogen storage capacity in carbon aerogels treated with KOH

Organic aerogels are prepared by the sol-gel polymerization of resorcinol with furfural catalysed by potassium hydroxide solution using ambient pressure drying. These aerogels are further carbonised in nitrogen in order to obtain their corresponding carbon aerogels. Nitrogen adsorption at 77 K allowed the determination of surface areas and pore volumes, further analysed by the Dubinin-Radushkevich model and density functional theory model. In particular, a KOH catalysed carbon aerogel exhibits a hydrogen uptake of ∼5.2 wt.% at 77 K and 3.5 MPa. The correlation of maximum hydrogen uptake with surface area and micropore volume was investigated.     

Comparison of the Dubinin-Radushkevich and Quenched Solid Density Functional Theory approaches for the characterisation of narrow microporosity in activated carbons obtained by chemical activation with KOH or NaOH of Kraft and hydrolytic lignins

The classical DR method and the Quenched Solid Density Functional Theory (QSDFT) approach have been used to analyse N₂ at 77 K isotherms determined on activated carbons prepared by alkaline chemical activation of different lignins. The QSDFT pore size distributions are bimodal with a narrow peak below 1 nm and a broad peak from 1 to 2.5-3.5 nm. Deconvolution allows estimation of the volumes and widths of the narrow micropores. These are lower than estimated by the DR analysis as this does not separate micropore and non-micropore adsorption. On the basis of the QSDFT analysis, the optimum conditions for obtaining materials with a high volume of narrow micropores were activation temperatures of 550-650 °C, hydroxide/lignin ratio of 1 and dwell time at the maximum activation temperature of 30 min. KOH was preferable to NaOH as it requires lower temperatures and results in materials with higher narrow micropore volumes. The “best” material obtained, prepared with KOH at 550 °C, had mean micropore width of 0.7 nm and micropore volume of 0.37 cm³ g⁻¹ which compares very favourably with molecular sieve carbons prepared from synthetic polymers. Furthermore, this material was obtained with an activation yield of 32.9%, which is quite high for alkaline chemical activation.     

Electron microscopy and nitrogen adsorption studies of film-type carbon replicas with large pore volume synthesized by using colloidal silica and SBA-15 as templates

Mesoporous carbons synthesized by the film-type replication of colloidal silica and SBA-15 templates are studied by electron microscopy and nitrogen adsorption. This synthesis strategy involves the formation of thin carbon film on the pore walls of these templates using resorcinol-crotonaldehyde polymer as carbon precursor. For the silica templates consisting of 20-80 nm colloids this synthesis affords carbons with extremely large pore volumes (5-9 cm³/g) and uniform spherical pores reproducing the size of the colloids used.     

Microporosity of carbon deposits collected in the Tore Supra tokamak probed by nitrogen and carbon dioxide adsorption

Nitrogen and carbon dioxide adsorption experiments have been used to investigate the porosity of carbon deposits formed in the Tore Supra tokamak as a consequence of the erosion of the plasma-facing components. We compare BET, α_s-, and Dubinin-Raduskevich methods to distinguish between micropore volume (∼0.04 cm³ g⁻¹) and external surface (∼90 m² g⁻¹). Consistent results have been obtained for nitrogen and carbon dioxide, and the smallest pores are shown to be reversibly closed and opened under air exposure and outgassing at 600 °C, respectively, probably due to blocking of pore entrances by surface oxides. Pore size distribution is calculated using the non-local density functional theory: a novel and straightforward method is used to fit the experimental isotherms by Lorentzian distributions of pores centered in some relevant pore size regions. We thus show that the tokamak sample micropores are mainly ultra-micropores (∼75%) whose widths are centered at 0.6 nm. This latter result is in good qualitative agreement with the outgassing effect and in good quantitative agreement with what is deduced from α_s-plot.     

Characterization of heat-treated porous carbons using argon adsorption

The microstructural variation of Norit R1 Extra activated carbon, progressively heated at 1373 K, was explored in terms of pore size and pore wall thickness distributions, for various periods of heating time, determined by argon adsorption at 87 K, both using an infinite as well as and finite wall thickness model. The latter approach has recently been developed in our laboratory and has been applied to several virgin carbons. The current results show significant variations in small pore size regions (<7 Å) in association with strong growth of thick walls having at least three carbon sheets, as a result of heat treatment. In particular, shrinkage of the smallest pores due to strong interaction between their opposite walls as well as smoothening of carbon wall surfaces due to an increase in graphitization degree under thermal treatment have been found. Further, the results of pore wall thickness distribution are well corroborated by X-ray diffraction. The results of pore size and pore wall thickness distributions are also shown to be consistent with transmission electron microscopy analyses.     

Analysis of the evolution of the pore size distribution and the pore network connectivity of a porous carbon during activation

The development of the microporosity, pore size distribution and pore network connectivity has been studied in the production of activated carbons from lignite char. Grand Canonical Monte Carlo simulation of adsorption was applied to the characterisation of a set of activated carbons produced at a sequence of times. The pore size distributions obtained from nitrogen at 77 K and ethane at 264 K were used as inputs to a method based on percolation theory to study the changing connectivity of the system. The incorporation of percolation concepts in the study of the porosity development gives an insight into the processes involved. The analysis is applied to a particular environmental application, the adsorption of phenanthrene.     

Carbon aerogels from acetic acid catalysed resorcinol-furfural using supercritical drying for hydrogen storage

Organic aerogels were derived from acetic acid catalysed resorcinol and furfural and then dried directly in supercritical carbon dioxide without the use of a solvent exchange process. These aerogels were further carbonised in nitrogen and activated in CO₂ in order to obtain their corresponding carbon aerogels. The carbon aerogels prepared by this method had a greater proportion of micropores in addition to a much shorter preparation time (on the order of days) than those prepared by other studies. The effect of different drying techniques on the microstructure of the wet gels was investigated by nitrogen adsorption at cryogenic liquid nitrogen temperature. Nitrogen adsorption at 77 K allowed the determination of surface areas and pore volumes, further analysed by the Dubinin-Radushkevich model and density functional theory model. The surface area and micropore volume of carbon aerogels prepared by this method increased by 19% and 12%, and accordingly, hydrogen uptake capacity was increased by 10% from 4.9 ± 0.2 wt.% to 5.4 ± 0.3 wt.% at 4.6 MPa and 77 K.     

The effect of Mo2C derived carbon pore size on the electrical double-layer characteristics in propylene carbonate-based electrolyte

Carbide-derived carbon (CDC) was synthesised from molybdenum carbide by extracting Mo atoms in a high-temperature chlorine atmosphere. A systematic study of the influence of pore size on the electrical double layer (EDL) performance was carried out with carbons synthesised in the temperature interval of 500-900 °C. Strong effect of chlorination conditions on the pore-size distribution was noticed that gives wide possibilities to vary the pore structure of Mo₂C derived carbons. An average pore size of carbons varied between 1 nm and 2 nm depending on chlorination temperature. The relationships were established between the pore-size distribution and the electrochemical performance of micro/mesoporous carbons. The EDL characteristics of carbon materials in a propylene carbonate solution of triethylmethylammonium tetrafluoroborate were obtained using the cyclic voltammetry at ΔE of 3.8 V and the constant current methods in a 3-electrode test cell. A novel test method was developed to demonstrate the power characteristics of the electrode materials. The results of this study affirmed the great potential of Mo₂C derived carbons, whose EDL capacitance reaches ∼65 F cm⁻³ and 132 F g⁻¹ and the 20-s discharge power density is 2.1 W cm⁻³.     

Comparative study of nanopores in activated carbons by HRTEM and adsorption methods

Detailed analysis of nanopores (IUPAC micropores at pore half-width x < 1 nm) of carbonised porous phenolformaldehyde resin microbeads used as a precursor of activated carbon (AC) and CO₂ activated carbon (at 50% burn-off) has been performed on the basis of high-resolution transmission electron microscopy (HRTEM) image analysis and nitrogen adsorption data analysed using several density functional theory (DFT) methods. The results of quenched solid DFT (QSDFT) and nonlocal (NLDFT) are in agreement with the pore size distributions of nanopores based on the HRTEM image analysis. Development of porosity with progressive activation degree in a series of ACs leads to enhancement of the deviation of the pore shape from the used pore models. The TG/DTA data and Raman spectra show nonlinear but weak changes in the AC characteristics with increasing burn-off degree.     

Influence of oxidation process on the adsorption capacity of activated carbons from lignocellulosic precursors

A set of activated carbon materials non-oxidised and oxidised, were successfully prepared from two different lignocellulosic precursors, almond shell and vine shoot, by physical activation with carbon dioxide and posterior oxidation with nitric acid. All samples were characterised in relation to their structural properties and chemical composition, by different techniques, namely nitrogen adsorption at 77 K, elemental analysis (C, H, N, O and S), point of zero charge (PZC) and FTIR. A judicious choice was made to obtain carbon materials with similar structural properties (apparent BET surface area ∼ 850-950 m²g⁻¹, micropore volume ∼ 0.4 cm³g⁻¹, mean pore width ∼ 1.2 nm and external surface area ∼ 14-26 m²g⁻¹). After their characterisation, these microporous activated carbons were also tested for the adsorption of phenolic compounds (p-nitrophenol and phenol) in the liquid phase at room temperature. The performance in liquid phase was correlated with their structural and chemical properties. The oxidation had a major impact at a chemical level but only a moderate modification of the porous structure of the samples. The Langmuir and Freundlich equations were applied to the experimental adsorption isotherms of phenolic compounds with good agreement for the different estimated parameters.     

Importance of activated carbon's oxygen surface functional groups on elemental mercury adsorption

The effect of varying physical and chemical properties of activated carbons on adsorption of elemental mercury (Hg⁰) was studied by treating two activated carbons to modify their surface functional groups and pore structures. Heat treatment (1200 K) in nitrogen (N₂), air oxidation (693 K), and nitric acid (6N HNO₃) treatment of two activated carbons (BPL, WPL) were conducted to vary their surface oxygen functional groups. Adsorption experiments of Hg⁰ by the activated carbons were conducted using a fixed-bed reactor at a temperature of 398 K and under N₂ atmosphere. The pore structures of the samples were characterized by N₂ and carbon dioxide (CO₂) adsorption. Temperature-programmed desorption (TPD) and base-acid titration experiments were conducted to determine the chemical characteristics of the carbon samples. Characterization of the physical and chemical properties of activated carbons in relation to their Hg⁰ adsorption capacity provides important mechanistic information on Hg⁰ adsorption. Results suggest that oxygen surface complexes, possibly lactone and carbonyl groups, are the active sites for Hg⁰ capture. The carbons that have a lower carbon monoxide (CO)/CO₂ ratio and a low phenol group concentration tend to have a higher Hg⁰ adsorption capacity, suggesting that phenol groups may inhibit Hg⁰ adsorption. The high Hg⁰ adsorption capacity of a carbon sample is also found to be associated with a low ratio of the phenol/carbonyl groups. A possible Hg⁰ adsorption mechanism, which is likely to involve an electron transfer process during Hg⁰ adsorption in which the carbon surfaces may act as an electrode for Hg⁰ oxidation, is also discussed.     

Sulfur removal from model diesel fuel using granular activated carbon from dates’ stones activated by ZnCl2

Samples of granular activated carbon (GAC) were produced from dates’ stones by chemical activation using ZnCl₂ as an activator. Textural characteristics of GAC were determined by nitrogen adsorption at 77 K along with application of BET equation (Brunauer, Emmett and Teller) for determination of surface area. Pore size distribution and pore volumes were computed from N₂ adsorption data by applying the nonlinear density function theory (NLDFT). FT-IR spectra of GAC samples were also obtained to determine the functional groups present on the surface. GAC samples were used in desulfurization of a model diesel fuel composed of n-C₁₀H₃₄ and dibenzothiophene (DBT) as sulfur containing compound. More than 86% of DBT is adsorbed in the first 3 h which gradually increases to 92.6% in 48 h and no more sulfur is removed thereafter. The adsorption data were fitted to both Freundlich and Langmuir equations to estimate the adsorption parameters. The optimum operating conditions for GAC preparation based on high adsorption capacity are T_carb = 700 °C, θ_carb = 3.0 h and R = 0.5. Moreover, the efficiency of sulfur removal by GAC is reduced when applied to commercial diesel fuel. Finally, linear regression of experimental data was able to predict the critical pore diameter for DBT adsorption (0.8 nm) and validating the reported impact of average pore diameter of activated carbon on the adsorption capacity.     

Adsorption prediction for three binary supercritical gas mixtures on activated carbon based on a NDFT/PSD approach

A non-local density functional theory (NDFT) in combination with the pore size distribution (PSD) analysis has been applied to make a comprehensively theoretical study for correlating and predicting the adsorption equilibria of pure supercritical gases and the corresponding binary supercritical gas mixtures on activated carbon. In this approach, the required PSDs were determined from an input of experimental adsorption data of pure components. By comparing with the experimental data of three different binary systems, CH₄/N₂, CH₄/CO₂, and CO₂/N₂, at high pressure up to 13.6 MPa and temperature 318.2 K with various concentration ranges, the predictive performance of the theoretical approach was evaluated. The adsorption of the mixtures has also been predicted by applying the ideal adsorbed solution (IAS) theory. It was shown that the NDFT/PSD method could be used to predict the mixture adsorption behaviors under high pressure. The developed method has greater superiority over the IAS theory in the prediction of the mixture adsorption.     

Modeling the selectivity of activated carbons for efficient separation of hydrogen and carbon dioxide

Grand canonical Monte Carlo simulations (GCMC) are carried out to investigate the separation of hydrogen and carbon dioxide via adsorption in activated carbons. In the simulations, both hydrogen and carbon dioxide molecules are modeled as Lennard-Jones spheres, and the activated carbons are represented by a slit-pore model. At elevated temperatures (T = 505 and 923 K), the activated carbons exhibit essentially no preference over the two gases and the selectivity of carbon dioxide relative to hydrogen falls monotonically as the pore size increases. At room temperature, however, the selectivity of carbon dioxide relative to hydrogen reaches up to 90, indicating that hydrogen and carbon dioxide can be efficiently separated. Furthermore, the optimized pore sizes, of width H = 1.48 nm for the bulk mole fraction ratio of x_CO2/x_H2=1:2 and H = 1.18 nm for x_CO2/x_H2=1:8, are identified in which the activated carbons show the highest selectivity for the separation of hydrogen and carbon dioxide.