Evaluation of carbon nanopores using large molecular probes in grand canonical Monte Carlo simulations and experiments

Nanopores (pores between 1 and 5 nm) have been the object of a great deal of attention because they can selectively adsorb relatively large molecules such as macromolecules and polymer molecules. Conventional methods for analyzing porous structures—such as N₂ adsorption measurements at 77 K—can be used to investigate microporous and mesoporous structures, but there is a lack of investigation of nanopores or the boundary between micropores (<2 nm) and mesopores (2–50 nm). Here, we propose the evaluation method of nanopores using a large probe molecule, SF₆. Grand canonical Monte Carlo simulations for N₂ and SF₆ suggested that SF₆ was adsorbed in 1.5–5 nm nanopores, while there was N₂ adsorption for the wide range of pore sizes. The SF₆ adsorption could therefore be used to confirm existence of the nanopores. To test this, we used single-walled carbon nanohorns as porous carbons with widely distributed pore size. SF₆ was well adsorbed only in the nanopores at 195 K, whereas N₂ adsorption was observed in all micropores and mesopores. This structural analysis of nanopores using a large-molecule probing method complements structural analyses using N₂ adsorption, as well as other techniques.     

Graphitization of highly porous carbons derived from poly(p-phenylene benzobisoxazole)

We studied the development of graphitization in carbonaceous materials derived from poly(p-phenylene benzobisoxazole) (PBO) pre-activated with carbon dioxide to burn-off degrees of 15 and 51 wt.%. The activated materials were subsequently heat-treated at various temperatures comprised between 1600 and 2700 °C. The changes in porous texture, structure and nanostructure were characterized by gas adsorption analysis, X-ray diffraction, Raman spectroscopy and high-resolution transmission electron microscopy. The two sets of materials pre-activated to different degrees of burn-off exhibited similar changes as a function of the heat treatment temperature. Quite unexpectedly, the presence of porosity generated by physical activation of PBO chars does not affect their graphitizability. The materials seem to retain a sufficiently large degree of the order acquired during the spinning of the PBO fibres to allow the remaining bi-periodic structural units to re-arrange, allowing subsequent graphitization.     

Extrusion of honeycomb monoliths employed with activated carbon-LDPE hybrid materials

A facile preparation method of activated carbon (AC) honeycomb monoliths is suggested in this work. A composite consisting of activated carbon, organic polymer binder (LDPE), and organic lubricant was effectively extruded above the melting temperature of the organic polymer in the shape of a honeycomb monoliths (dimensions: 256 cells in 4 cm × 4 cm). Three types of AC powders are investigated to elucidate the relationships between the processibility of the honeycomb monoliths and the size of their AC particles. The surface area of activated carbon filters increases as the size of the AC particle increases. This may be due to pore blocking by organic polymer binder on the surface of the AC particle. The mechanical properties of the honeycomb monolith are enhanced drastically when extrusion molding is used as compared to the use of a conventional press-molding process.     

A method to increase the energy density of supercapacitor cells by the addition of multiwall carbon nanotubes into activated carbon electrodes

The performance of supercapacitor cells with activated carbon (AC) electrodes was improved by adding a small amount of multiwall carbon nanotubes (MWCNTs). The electrode structure investigated comprised AC, four different types of MWCNTs and two polymer binders, polyvinylidene fluoride or polyvinyl alcohol. All fabricated devices were of the electrochemical double layer capacitor type. The organic electrolyte used was tetraethyl ammonium tetrafluoroborate (TEABF₄) in two different solvents: propylene carbonate or acetonitrile (AN). The electrodes were characterised with scanning electron microscopy and tested for their specific surface area and pore size distribution. The electrode fabrication process was fine-tuned by investigating the effect of the coating thickness on the supercapacitor cell performance. It was established that an AC/MWCNT-based supercapacitor with 30 μm thick roll-coated, composite electrodes of just 0.15%w/w MWCNT content provided superior tested power and energy densities of 38 kW/kg and 28 W h/kg, respectively, compared to 18 kW/kg and 17 W h/kg for AC only–based cells in a 1.5 TEABF₄/AN electrolyte. The increased energy density was attributed to a fine lace of MWCNTs covering the AC microparticles with visible 20–30 nm lace pores and to the high specific area of micropores.     

Restricted dynamics of molecular hydrogen confined in activated carbon nanopores

Quasi-elastic neutron scattering was used for characterization of dynamics of molecular hydrogen confined in narrow nanopores of two activated carbon materials: a carbon derived from polyfurfuryl alcohol and an ultramicroporous carbon. Fast, but incomplete ortho–para conversion was observed at 10 K, suggesting that scattering originates from the fraction of unconverted ortho isomer which is rotation-hindered because of confinement in nanopores. Hydrogen molecules entrapped in narrow nanopores (<7 Å) were immobile below 22–25 K. Mobility increased rapidly with temperature above this threshold, which is higher than the melting point of bulk hydrogen (13.9 K). Diffusion obeyed fixed-jump length mechanism, indistinguishable between 2D and 3D processes. Thermal activation of diffusion was characterized between ～22 and 37 K, and structure-dependent differences were found between the two carbons. Activation energy of diffusion was higher than that of bulk solid hydrogen. Classical notions of liquid and solid do not longer apply for H₂ confined in narrow nanopores.     

Binderfree synthesis of high-surface-area carbon electrodes via CO2 activation of resorcinol–formaldehyde carbon xerogel disks: Analysis of activation process

High-surface-area carbon xerogels were prepared in the form of disks via carbonization of precursor resorcinol–formaldehyde (RF) polymer disks and subsequent activation of the resultant RF carbon xerogels by CO₂. RF carbon xerogels allow the preparation of a set of pre-activated carbon disks having different mesopore volumes. Analysis of the relationship between the mesopore volume of the samples and their CO₂ activation efficiency showed that the presence of mesopores is crucial for obtaining a high-surface-area carbon with minimal burn-off of carbon atoms. Activation of an RF carbon disk with a mesopore volume of 1.0 cm³ g⁻¹ up to a burn-off of 81% yielded an activated carbon disk with a high BET surface area of ∼3000 m² g⁻¹. Such disks could be readily used as electrode materials for an electric double layer capacitor without filler or binder addition and exhibited competitive EDLC performance against other electrode materials previously reported.     

Evaluation of active carbon fibers used in cell biocompatibility and rat cystitis treatment

Escherichia coli is the predominant pathogen in catheter-associated urinary tract infection (CAUTI) and cystitis. CAUTI treated with prophylactic antibiotics induces antibiotic resistance in hospitals and nursing homes. Therefore, we developed a physical method to treat cystitis by using activated carbon (AC) fibers to remove lipopolysaccharide (LPS) from the bladder. An in vitro assay showed that AC had remarkable LPS adsorption capability (20.24 EU per mg AC for 60 min) and good biocompatibility, as revealed by morphological observation through field-emission scanning electron microscopy and MTS assay in SV-HUC-1 cell line. In an in vivo assay in rats, 3 groups were formed: control (pyrogen-free saline), LPS, and AC (n = 10, each group). The LPS and AC groups were pretreated with protamine sulfate (10 mg/mL), and E. coli LPS (5 mg/kg) was administered to induce cystitis. Subsequently, the AC group received 0.4 mg/kg AC suspension. After 24 h, histologic analysis revealed increased urinary bladder weight (edema) and marked increase in vascular congestion and hemorrhage in the LPS group. In contrast, the AC group showed significant reduction in degree of edema, inflammatory cell infiltration, and hemorrhage (p < 0.05). Thus, AC has the potential to act as an auxiliary agent for cystitis amelioration.     

Effect of pre-oxidation on the porosity development in a heavy oil fly ash by CO2 activation

The effect of pre-oxidation on the porosity evolution in heavy-oil fly ash subjected to activation with CO₂ has been investigated. After preliminary acid leaching, used to reduce the mineral matter content, the leached fly ash has been oxidised in air at 250 °C for 36 h. Pyrolysis was conducted on the unoxidised and oxidised leached fly ash at 900 °C for 2 h and the resultant chars were activated with CO₂ at 900 °C for different times. The activated samples have been characterised as regards the surface area and the pore volume.The pre-oxidation enhances the porosity development mainly in terms of mesoporosity leading to obtain activated products with higher surface area (about 270 m²/g at a 40% burn-off).     

Annealing-induced structural changes of carbon onions: High-resolution transmission electron microscopy and Raman studies

The first- and second-order Raman spectra of carbon nano-onions (CNOs), produced via annealing of detonation nanodiamonds with a mean grain size of ∼5 nm in the argon ambience at the maximal temperature of annealing process (T_MAX) varying from 1500 to 2150 °C, are analyzed together with the high-resolution transmission electron microscopy (HRTEM) images. The combined analysis provides a deep insight into the annealing-induced atomic-scale structural modifications of the CNO nanoparticles. The Raman and HRTEM data unambiguously demonstrate the reduction in the number of defects in the CNO structure, as well as indicate the conversion from the diamond sp³-bonded carbon phase to the sp²-bonded carbon phase with increasing T_MAX and its almost full completion for T_MAX = 1600 °C.     

Activation of graphene aerogel with phosphoric acid for enhanced electrocapacitive performance

We present a facile and efficient route to introduce in-plane nanopores on the graphene sheets by activation of graphene aerogel (GA) with phosphoric acid (H₃PO₄). Results from N₂ adsorption and TEM images showed that H₃PO₄ activation created mesopores with pore size of 2–8 nm on the graphene sheets. With such nanopores on graphene sheets, the activated GA exhibits a specific capacitance of 204 F g⁻¹, enhanced rate capability (69% capacitance retention from 0.2 to 30 A g⁻¹), reduced equivalent series resistance (3.8 mΩ) and shortened time constant (0.73 s) when comparing with the hydrothermally-derived pristine GA and thermally annealed GA in the absent of H₃PO₄. The excellent capacitive properties demonstrate that introduction of nanopores on GA by H₃PO₄ activation not only provides large ion-accessible surface area for efficient charge storage, but also promotes the kinetics of electrolyte across the graphene two-dimensional planes.     

Microporous carbons finely-tuned by cyclic high-pressure low-temperature oxidation and their use in electrochemical capacitors

Microporous carbons with a finely controlled porosity have been prepared from non-porous chars by cyclic oxidation/thermal desorption and further used in supercapacitor electrodes working in organic medium. The described activation method is shown to be effective for at least two types of non-porous carbons derived from sucrose and cellulose. The low temperature oxidation is realized by H₂O₂ at 200 °C and followed by thermal desorption of the surface functional groups at 900 °C under nitrogen flow. The porosity-forming procedure involves 4–5 oxidation/decomposition cycles, thus allowing a gradual adjustment of average pore size to that of ions making up the standard organic electrolyte ?1 mol L^?1 TEA⁺ BF₄^? in acetonitrile. The build-up of pore volume during the initial cycles proceeds essentially through the opening/formation and deepening of narrow micropores (L₀ ≈ 0.8 nm), whereas a slight pore widening appears to be the main outcome of further cycles. Due to the low burn-off of the overall process, the carbons are shown to form much denser coatings (0.71 g cm^?3) than a steam-activated carbon used in industrial supercapacitors (0.52 g cm^?3).     

Effect of pre-oxidation on the porosity development in a heavy oil fly ash by CO2 activation

The effect of pre-oxidation on the porosity evolution in heavy oil fly ash subjected to activation with CO₂ has been investigated. After preliminary acid leaching, used to reduce the mineral matter content, the leached fly ash has been oxidised in air at 250 °C for 36 h. Pyrolysis was conducted on the unoxidised and oxidised leached fly ash at 900 °C for 2 h and the resultant chars were activated with CO₂ at 900 °C for different times. The activated samples have been characterised as regards the surface area and the pore volume.The pre-oxidation enhances the porosity development mainly in terms of mesoporosity leading to obtain activated products with higher surface area (about 270 m²/g at a 40% burn-off).     

Synergetic removal of aqueous phenol by ozone and activated carbon within three-phase fluidized-bed reactor

Synergetic removal of aqueous phenol by decomposition with ozone and adsorption on activated carbon was experimentally investigated. To enhance phenol removal performance, two activated carbons (AC1 and AC2) with BET surface areas of 1106 and 1150 m² g^?1 and average pore diameters of 2.3 and 1.7 nm, respectively, were employed. While the slowest initial removal of phenol was achieved with introduction of ozone only, the much better removal of phenol was obtained with utilization of activated carbon with ozone. Some intermediate products, which were detected as total organic carbon (TOC), were found to remain even after phenol was completely decomposed. Regarding to higher mesopore fraction, AC1 could better remove intermediates than AC2. With the synergetic performance of AC1 and ozone it was found that the highest removal of phenol and TOC was up to 100% and 89%, respectively.     

Random forest model for removal of bromophenol blue using activated carbon obtained from Astragalus bisulcatus tree

In this research, activated carbon (AC) simply was prepared from a local, abundant tree in south of Iran. The AC with low cost and toxicity is a good candidate for bromophenol blue (BPB) removal from aqueous media. The AC with nano scale pore diameter is applicable for this dye removal following optimization of the influence of various parameters including contact time, pH, initial dye concentration and amount of adsorbent. Subsequently, experimental data was analyzed by four kinetic models including pseudo first and second-order, Elovich and the intraparticle diffusion equations and subsequently their respective parameters such as rate constants, equilibrium adsorption capacities and correlation coefficients was investigated and based on well known criterion their applicability was judged. The result shows that adsorption of BPB onto proposed adsorbent at all conditions such as versatile adsorbent dosages and initial BPB concentrations sufficiently described by the combination of the pseudo second-order equation and interparticle diffusion model. It was found that equilibrium rate of the BPB adsorption at various adsorbent dosage well fitted by Langmuir. Investigation of experimental result by two approaches (multiple linear regressions (MLR) and random forest (RF)) models show that RF is a powerful tool for prediction of BPB adsorption by activated carbon obtained from Astragalus bisulcatus tree. The optimal tuning parameters for RF model are obtained based on the n_tree = 100, m_try = 2. For the training data set, the MSE values of 0.0006 and the coefficient of determination (R²) values of 0.9895 for RF model and the MSE value of 0.0104 and the R² value of 0.823 for MLR model are obtained.     

Advanced ozone treatment of heavy oil refining wastewater by activated carbon supported iron oxide

The catalytic ozonation of heavy oil refining wastewater (HORW) was investigated over activated carbon supported iron oxides (FAC) catalysts using activated carbon (AC) as the reference. The catalyst was characterized by chemical analysis, XRD, N₂ adsorption–desorption and SEM. A significant increase in COD removal efficiency was observed in FAC + ozone compared with AC + ozone due to more hydroxyl radicals, identified by tert-butyl alcohol (TBA). The composition analysis of organic pollutant in HORW by FT-ICR MS discovered organic pollutants chain scission and oxidation process during the treatment. A great improvement of biodegradability for treated HORW had been obtained. The investigation uncovered the catalytic potential of FAC catalysts for ozonation of HORW.     

Spherical potassium intercalated activated carbon beads for pulverised fuel CO2 post-combustion capture

Spherical carbon beads with a uniform diameter of ca. 0.6–0.8 mm and high mechanical strength can be prepared by hydrothermal synthesis. To optimise the performance of these adsorbents for pulverised fuel post-combustion capture, the efficacy of potassium intercalation via a KOH treatment has been investigated, deliberately using nitrogen-free phenolic resin derived activated carbon (AC) beads so that the enhanced CO₂ adsorption achieved by potassium intercalation could be delineated from any other effects. At 25 °C and CO₂ partial pressure of 0.15 bar, the adsorption capacity of K-intercalated ACs nearly doubled from 0.79 mmol/g for the untreated carbons to 1.51 mmol/g whilst the effect on the morphology and mechanical strength is relatively small. It was found that only slightly more than ca. 1 wt.% of K is required to give the maximum benefit from intercalation that increases the surface polarity and the affinity towards CO₂. The notably increased CO₂ uptake of the K-AC beads as a result of modest increase in adsorption heat (32–40 kJ/mol compared to 27 kJ/mol for the original AC), coupled with the fast adsorption kinetics, suggest that the overall energy penalty is potentially superior to strongly basic polyethyleneimine and other amine-based solid adsorbent systems for carbon capture.     

Characterization of accessible and inaccessible pores in microporous carbons by a combination of adsorption and small angle neutron scattering

We determine the pore size distribution for five activated carbons (comprising carbide derived as well as commercial activated carbon samples) by the interpretation of experimental small angle neutron scattering (SANS) intensity profiles, based on the primary assumption of an infinitely dilute solution of hollow spherical particles. The interpretation yields the pore size distribution of the carbon samples that have predominantly micropore populations (size <20 Å), but not for carbons which have significant mesopore populations of sizes up to 48 Å and high mass fractal degrees. The pore size distribution (PSD) results based on SANS data reveal significant populations of micropores of size <6.1 Å, and mesopores of size >20 Å, which are not present in the PSD results based on adsorption isotherms of either Ar at 87 K or CO₂ 273 K. This inaccessible porosity becomes accessible to CO₂ and Ar on heat treatment, leading to increase in the adsorption based pore volume. However, the surface area does not commensurately increase, indicating the inaccessible microporosity to predominantly comprise surface defects and roughness that are removed on heat treatment or activation. This finding sheds the light onto the evolution of porosity of activated carbons during gasification or post synthesis-treatment.     

The composite material based on Dawson-type polyoxometalate and activated carbon as the supercapacitor electrode

A supercapacitor electrode assembled from activated carbon (AC) and (NH₄)₆[P₂Mo₁₈O₆₂]·14.2H₂O (P₂Mo₁₈) was fabricated for the first time, and showed remarkable electrochemical performance ascribed to the synergy of the double layer capacitance of AC and the pseudocapacitance of P₂Mo₁₈. The investigations indicate that the AC/P₂Mo₁₈ electrode exhibits a specific capacitance of 275 F g^− 1 at a high current density of 6 A g^− 1, which is substantially larger than the 182 F g^− 1 of the AC electrode. In addition, the AC/P₂Mo₁₈ electrode possesses a remarkable rate capability (89%) when the current density is increased from 2 to 6 A g^− 1.     

Interaction between single walled carbon nanotube and 1D crystal in CuX@SWCNT (X = Cl,Br, I) nanostructures

CuX@SWCNT (X = Cl, Br, I) nanostructures were prepared by capillary filling of 1.4–1.6 nm single-walled carbon nanotubes (SWCNT) with copper halides. The structure of CuX@SWCNT (X = Cl, Br, I) represents a distorted two-layer hcp of halogen atoms arranged along the SWCNT. The EXAFS and the high angle angular dark field (HAADF) HRTEM data indicate that Cu is partially coordinated by C. According to the optical absorption, valence band photoemission spectroscopy and work function measurements, a Fermi level (FL) downshift as compared with the initial value for the nanotubes and a corresponding charge transfer from the nanotubes to the 1D crystals is observed for CuX@SWCNT nanostructures. The FL shift increases in the sequence CuI < CuBr < CuCl due to an increase of the electron affinity for the halogen atoms. The XPS data confirm the acceptor effect of copper halides and indicate that metallic and semiconducting nanotubes behave differently. Raman spectroscopy performed under electrochemical charging allowed estimation of the value of charge transfer between the nanotube walls and the intercalated 1D crystal. The X-ray absorption and emission spectra for carbon and copper thresholds revealed a new energy level composed of the carbon 2р_z and copper 3d-orbitals. This indicates the Cu–C bonding, which in line with the structural HAADF HRTEM and EXAFS data.