Modified surface chemistry of activated carbons

In this study, immersion calorimetry was used to characterise different samples of commercial granular activated carbon (GAC) which undergo oxidation with HNO₃ (GACOxN) and thermal treatments to modify its superficial group contents, as well as to determine the textural characteristics of the materials through nitrogen adsorption at 77 K and its superficial chemistry by Boehm titration and zero point of charge. Correlations between the immersion enthalpies and the results of the other techniques of characterisation were established. The immersion enthalpies in dichloromethane obtained were greater, which were found to be between ?88.36 and ?155.6 J g^?1, in contrast to those in carbon tetrachloride, which were found to be between ?50.21 and ?94.29 J g^?1. The dependence of the immersion enthalpies in water on the contents of total acidity and basicity surface groups was also established, and a good correlation between the accessible surface area determined by calorimetric technique and the BET area was found.     

In situ electrochemical dilatometry of carbide-derived carbons

The long life durability and extraordinary stability of supercapacitors are ascribed to the common concept that the charge storage is purely based on double-layer charging. Therefore the ideal supercapacitor electrode should be free of charge induced microscopic structural changes. However, recent in-situ investigations on different carbon materials for supercapacitor electrodes have shown that the charge and discharge is accompanied by dimensional changes of the electrode up to several percent. This work studies the influence of the pore size on the expansion behavior of carbon electrodes derived from titanium carbide-derived carbons with an average pore size between 5 and 8 Å. Using tetraethylammonium tetrafluoroborate in acetonitrile, the swelling of the electrodes was measured by in situ dilatometry. The experiments revealed an increased expansion on the negatively charged electrode for pores below 6 Å, which could be described with pore swelling.     

Nitrogen modified carbide-derived carbons as adsorbents of hydrogen sulfide

Carbide-derived carbons produced from titanium carbide at temperatures from 600 degrees C to 1000 degrees C and exhibiting different porosities were treated with urea in order to introduce nitrogen containing species to their surface. Adsorption of hydrogen sulfide in the dynamic conditions in the presence of moisture was studied on initial and modified samples. The samples, before and after exposure to hydrogen sulfide, were characterized using adsorption of nitrogen, potentiometric titration, elemental analysis, and thermal analysis. The results showed that the introduction of nitrogen significantly enhances the performance of carbons in the process of hydrogen sulfide removal. The amount adsorbed and the degree of oxidation depended on the porosity. On the samples with very small pores, the adsorption was limited, probably owing to the sterical hindrances. With an increase in the size and volume of micropores, in which water and hydrogen sulfide can be accommodated, the efficiency of H(2)S removal by CDC increased.     

Role of surface chemistry in adsorption of phenol on activated carbons

Two samples of activated carbon of wood origin were oxidized using ammonium persulfate. The structural properties and surface chemistry of the samples and their oxidized counterparts were characterized using sorption of nitrogen and Boehm titration, respectively. Phenol adsorption from solution (at trace concentrations) was studied at temperatures close to ambient without maintaining a specific pH of the solution. The results showed, as expected, that the phenol uptake is dependent on both the porosity and surface chemistry of the carbons. Furthermore, phenol adsorption showed a strong dependence on the number of carboxylic groups due to two factors: (1) phenol reacts with carboxylic groups on the carbon surface, forming an ester bond, and (2) carboxylic groups on the carbon surface remove the pi-electron from the activated carbon aromatic ring matrix, causing a decrease in the strength of interactions between the benzene ring of phenol and the carbon's basal planes, which decreases the uptake of phenol.     

Tailoring of nanoscale porosity in carbide-derived carbons for hydrogen storage

The poor performance of hydrogen storage materials continues to hinder development of fuel cell-powered automobiles. Nanoscale carbons, in particular (activated carbon, exfoliated graphite, fullerenes, nanotubes, nanofibers, and nanohorns), have not fulfilled their initial promise. Here we show that carbon materials can be rationally designed for H2 storage. Carbide-derived carbons (CDC), a largely unknown class of porous carbons, are produced by high-temperature chlorination of carbides. Metals and metalloids are removed as chlorides, leaving behind a collapsed noncrystalline carbon with up to 80% open pore volume. The detailed nature of the porosity-average size and size distribution, shape, and total specific surface area (SSA)-can be tuned with high sensitivity by selection of precursor carbide (composition, lattice type) and chlorination temperature. The optimum temperature is bounded from below by thermodynamics and kinetics of chlorination reactions and from above by graphitization, which decreases SSA and introduces H2-sorbing surfaces with binding energies too low to be useful. Intuitively, pores of different size and shape should not contribute equally to hydrogen storage. By correlating pore properties with 77 K H2 isotherms from a wide variety of CDCs, we experimentally confirm that gravimetric hydrogen storage capacity normalized to total pore volume is optimized in materials with primarily micropores ( approximately 1 nm) rather than mesopores. Thus, in agreement with theoretical predictions, a narrow size distribution of small pores is desirable for storing hydrogen, while large pores merely degrade the volumetric storage capacity.     

Supercapacitors based on carbide-derived carbons synthesised using HCl and Cl2 as reactants

Micro- and mesoporous carbide-derived carbons (CDCs) were synthesised from TiC powder via a gas-phase reaction using HCl and Cl₂ within the temperature range of 700–1,100 °C. Analysis of X-ray diffraction results show that TiC-CDCs consist mainly of graphitic crystallites. The first-order Raman spectra showed the graphite-like absorption peaks at ~1,577 cm^?1 and the disorder-induced peaks at ~1,338 cm^?1. The low-temperature N₂ sorption experiments were performed, and specific surface areas up to 1,214 and 1,544 m²?g^?1 were obtained for TiC-CDC (HCl) synthesised at T?=?800 °C and TiC-CDC (Cl₂) synthesised at T?=?900 °C, respectively. For the TiC-CDC powders synthesised, a bimodal pore size distribution has been established with the first maximum in the region up to 1.5 nm and the second maximum from 2 to 4 nm. The energy-related properties of supercapacitors based on 1 M (C₂H₅)₃CH₃NBF₄ in acetonitrile and TiC-CDC (Cl₂) and TiC-CDC (HCl) as electrode materials were also investigated by cyclic voltammetry, impedance spectroscopy, galvanostatic charge/discharge and constant power methods. The specific energy, calculated at U?=?3.0 V, are maximal for TiC-CDC (Cl₂ 800 °C) and TiC-CDC (HCl 900 °C), which are 43.1 and 31.1 W?h?kg^?1, respectively. The specific power, calculated at cell potential U?=?3.0 V, are maximal for TiC-CDC (Cl₂ 1,000 °C) and TiC-CDC (HCl 1,000 °C), which are 805.2 and 847.5 kW?kg^?1, respectively. The Ragone plots for CDCs prepared by using Cl₂ or HCl are quite similar, and at high power loads, the TiC-CDC material synthesised using Cl₂ at 900 °C, i.e. the material with optimal pore structure, delivers the highest power at constant energy.     

Influence of surface chemistry on the SAXS response of polymer-based activated carbons

Small-angle X-ray scattering (SAXS) measurements using contrast variation are reported for activated carbons prepared from poly(ethyleneterephthalate). The carbon surfaces are functionalized to different degrees by cold and hot nitric acid treatment. The latter treatment reduces the surface area by 75%, but the pore size distribution in the micropore range is hardly affected. Seven liquids, n-hexane, i-octane, i-propanol, cyclohexane, toluene, alpha-pinene, and nitrobenzene, in addition to water vapor, were used as contrast modifiers. Although the values of the specific surface area S(X) deduced from these measurements are relatively insensitive to the solvent, the detailed SAXS spectra reveal interactions occurring on different spatial scales that depend on the surface chemistry of the carbon and on the physicochemical properties of the solvent. In the most heavily oxidized sample, the amphiphilic molecule i-propanol stabilizes the surface structure, whereas nonpolar molecules make the rough surface smoother. In the untreated and the lightly functionalized carbons, water vapor at 50% relative humidity condenses only partially in the micropores at room temperature, whereas in the heavily treated sample condensation in the micropores is practically complete.     

Influence of surface chemistry on the adsorption of oxygenated hydrocarbons on activated carbons

The objective of this work was to study the adsorption of different oxygenated hydrocarbons (methanol, ethanol, 1 and 2-butanol, methyl acetate) on activated carbons from organic mixtures with cyclohexane. Three activated carbons prepared by thermal and chemical treatments of a commercial carbon were employed for this purpose. Their textural properties were found to be similar, whereas their surface chemistries were modified, as shown by temperature-programmed desorption coupled to mass spectrometry (TPD-MS) and X-ray photoelectron spectroscopy (XPS). The adsorption isotherms were obtained by depletion method, and the analysis of adsorbed species was evaluated by TPD-MS to obtain new insight into the interactions between the different hydrocarbons and the carbon surface. Ethanol leads to a high-energy interaction between its hydroxyl function and the oxygenated surface groups and also to a lower energy interaction between the aliphatic part of the molecule and the carbon material. The desorption activation energy for this hydrophilic interaction is high (50 to 105 kJ/mol), and it is related to the nature of the carbon surface groups. The relative importance of these two interactions depend on the size of the alcohol/methanol is similar to ethanol, whereas butanols lead to more dispersive interactions. Methyl-acetate cannot undergo this kind of strong interaction and behaves like cyclohexane, having desorption activation energies ranging between 25 and 45 kJ/mol no matter the molecule and the carbon surface chemistry.     

Adsorption of dyes on carbon xerogels and templated carbons: influence of surface chemistry

The removal of textile dyes by adsorption onto carbon materials with extended mesoporosity is addressed in the present work. Two types of high surface area carbon adsorbents were prepared, namely a carbon xerogel and a templated carbon. Both materials were subsequently subjected to appropriate treatments in order to modify their surface chemistries, while keeping their textural properties relatively unchanged. The carbon adsorbents were extensively characterized by different techniques in order to correlate their adsorption performances with the corresponding surface properties. The behavior of the different materials was evaluated by determining equilibrium adsorption isotherms of two anionic dyes (Reactive Red 241 and Acid Blue 113) at different pH values. The results are compared with data previously obtained with commercial activated carbons subjected to the same treatments, and discussed in terms of the carbon surface chemistry and the interaction between the dye molecules and the adsorbent surface (dispersive and electrostatic interactions).     

Water vapor adsorption and microporous structure of carbon adsorbents. 18. Effect of surface chemistry of activated carbons on water vapor adsorption

This investigation has been devoted to a study of the chemical composition of the surfaces of activated carbons. A study has been made of the way in which changes in the surface chemistry of a series of carbons, as a result of heat treatment, affects the nature of their adsorption of water vapor. A differentiation has been made between oxygen-containing groups found on the surface of activated carbons before and after their heat treatment. It has been established that the original adsorption centers, which play a determining role in water vapor adsorption by activated carbons, comprise functional groups like strongly acidic free hydrogen ions, carboxylic and phenolic groups, situated on on the pore surface of the activated carbons. The number of these functional groups on the pore surface of the activated carbons has been correlated with the parametera ₀ (the number of original adsorption centers) in the isotherm equation for water vapor adsorption. The relative pressure corresponding to the formation of an adsorption layer on the surface of the activated carbons has been shown to depend on the number of original adsorption centers, the acidic functional groups.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 1, pp. 35–40, January, 1991.     

Enthalpic characterization of activated carbons with different surface chemistry with organic solvents and water

Journal of Thermal Analysis and Calorimetry - Immersion enthalpies of activated carbons modified on their chemical surface were determined in benzene, cyclohexane, hexane, ethanol and water. Three...     

Adsorption of carbon dioxide-methane mixtures in porous carbons: effect of surface chemistry

A combined experimental and molecular simulation study of the coadsorption of CO₂ and CH₄ in porous carbons is reported. We address the effect of surface chemistry by considering a numerical model of disordered porous carbons which has been modified to include heterochemistry (with a chemical composition consistent with that of the experimental sample). We discuss how realistic the numerical sample is by comparing its pore size distribution (PSD), specific surface area, porous volume, and porosity with those for the experimental sample. We also discuss the different criteria used to estimate the latter properties from a geometrical analysis. We demonstrate the ability of the MP method to estimate PSD of porous carbons from nitrogen adsorption isotherms. Both the experimental and simulated coadsorption isotherms resemble those obtained for pure gases (type I in the IUPAC classification). On the other hand, only the porous carbon including the heterogroups allows simulating quantitatively the selectivity of the experimental adsorbent for different carbon dioxide/methane mixtures. This result shows that taking into account the heterochemistry present in porous carbons is crucial to represent correctly adsorption selectivities in such hydrophobic samples. We also show that the adsorbed solution theory describes quantitatively the simulated and experimental coadsorption isotherms without any parameter adjustment.     

Influence of surface treatments on micropore structure and hydrogen adsorption behavior of nanoporous carbons

The scope of this work was to control the pore sizes of porous carbons by various surface treatments and to investigate the relation between pore structures and hydrogen adsorption capacity. The effects of various surface treatments (i.e., gas-phase ozone, anodic oxidation, fluorination, and oxygen plasma) on the micropore structures of porous carbons were investigated by N(2)/77 K isothermal adsorption. The hydrogen adsorption capacity was measured by H(2) isothermal adsorption at 77 K. In the result, the specific surface area and micropore volume of all of the treated samples were slightly decreased due to the micropore filling or pore collapsing behaviors. It was also found that in F(2)-treated carbons the center of the pore size distribution was shifted to left side, meaning that the average size of the micropores decreased. The F(2)- and plasma-treated samples showed higher hydrogen storage capacities than did the other samples, the F(2)-treated one being the best, indicating that the micropore size of the porous carbons played a key role in the hydrogen adsorption at 77 K.     

Effects of structural properties of silicon carbide-derived carbons on their electrochemical double-layer capacitance in aqueous and organic electrolytes

High surface area silicon carbide-derived carbons (Si-CDCs) synthesized by chlorination of beta silicon carbide (βSiC) with two different particle sizes (6 μm and 50 nm) show different porosities with graphitic structure. Transmission electron microscopy, Raman spectroscopy and argon (Ar) and carbon dioxide (CO₂) sorption analyses are used to examine the textural properties of the Si-CDCs. The results show that the particle size of the precursor affects the surface area and porosity of carbons. Furthermore, an additional heat treatment of the Si-CDC with 50-nm particle size for 24 h at 1,000 °C results in a collapse of the pore structure and reduces the surface area. The capacitive behaviours are investigated in H₂SO₄ and in tetraethyl ammonium tetrafluoroborate (TEABF₄)/acetonitrile (AN). The electrochemical performance of the Si-CDCs is influenced by the particle size, surface area, pore volume and pore size distribution. The Si-CDCs exhibit capacitances in 1 M H₂SO₄ of up to 179 F g^?1 and very stable charge–discharge performance over 5,000 cycles. This study shows the crucial importance of ultramicropores less than 1 nm combined with nanosized particles for achieving high capacitance in aqueous electrolyte. Moreover, the graphitic degree at the surface of the Si-CDCs enhances considerably the rate capability and stability in both electrolytes.     

Soft chemistry synthesis, crystal structure and 3D-AFM-micro-structural surface features of calcium-barium selenate molecule with AB2X5 structure

Acetate-citrate precursors were applied to synthesize amorphous gelatinous calcium barium selenate with molecular formula (Ca_0.5Ba_0.5Se₂O₅). The gel-product was sintering at 800°C for 2 hrs then well characterized by X-ray diffraction, SEM and EDX elemental analyses and proved that the product Ca_0.5Ba_0.5Se₂O₅ is mainly belong to AB2X5-type structure with average grain size ranged in between 1.2?C3.8 ??m. A visualized study was constructed depending upon SEM and 3D-AFM experimental data to confirm the the microstructure morphological features of calcium-barium selenate surface.     

A study of crystal structure and surface chemistry of silicalites

Crystal structure and sorption properties of silicalites, a new microporous crystalline silica, have been studied. The low temperature phase transition of silicalite into α-cristobalite is detected as being promoted by alkali cations. Removal of alkali cations by acid treatment results in higher thermo-stability of the crystals (to over 1150°C) without any change in maximum sorption capacity for n-hexane. Hydroxyl modes are found to be similar between silicalite and isostructural zeolite and were spectroscopically identified (the band at 3680 cm⁻¹) as hydrolyzed sodium-silicate bonds forming on acid treatment and washing the precursor crystals with water.     

The interdependence of defects, electronic structure and surface chemistry

In this article we present three diverse applications of first-principles simulations to problems of materials chemistry and chemical physics. Their common characteristic is that they are essentially problems of the relationships among atomic structures and the properties they promote in real materials and real applications. The studies are on transition-metal oxide surface chemistry, the reactivity and electronic structure of sp(2)-bonded carbon systems, and defects and electrochromic properties in WO(3). In these demanding applications we must have concern for how realistic our model systems are and how well current implementations of DFT perform, and we comment on both.     

Effects of surface curvature and surface chemistry on the structure and activity of proteins adsorbed in nanopores

The interactions of proteins with the surface of cylindrical nanopores are systematically investigated to elucidate how surface curvature and surface chemistry affect the conformation and activity of confined proteins in an aqueous, buffered environment. Two globular proteins, lysozyme and myoglobin, with different catalytic functions, were used as model proteins to analyze structural changes in proteins after adsorption on ordered mesoporous silica SBA-15 and propyl-functionalized SBA-15 (C(3)SBA-15) with carefully controlled pore size. Liquid phase ATR-FTIR spectroscopy was used to study the amide I and II bands of the adsorbed proteins. The amide I bands showed that the secondary structures of free and adsorbed protein molecules differ, and that the secondary structure of the adsorbed protein is influenced by the local geometry as well as by the surface chemistry of the nanopores. The conformation of the adsorbed proteins inside the nanopores of SBA-15 and C(3)SBA-15 is strongly correlated with the local geometry and the surface properties of the nanoporous materials, which results in different catalytic activities. Adsorption by electrostatic interaction of proteins in nanopores of an optimal size provides a favorably confining and protecting environment, which may lead to considerably enhanced structural stability and catalytic activity.     

Self-catalyzed synthesis of mesoporous carbons with tunable pore size and structure by soft-templating method

A series of mesoporous carbons (MCs) have been obtained through organic–organic self-assembly method by using phloroglucinol–formaldehyde as carbon precursor and a reverse amphiphilic triblock copolymer as a template. Because of its acidity, the phloroglucinol was used as a catalyst itself. Results show that the pore size and structure of MCs were tailored by simply tuning the weight content of formaldehyde while keeping other reactants constant. A cylindrical mesostructure was obtained when the weight content was 1.0, 1.2 and 1.4. Further increasing the weight content to 1.6 or 2.0, a three-dimensional cage-like mesostructure was obtained. Specific surface area and pore volume up to 485 m²/g and 0.78 cm³/g can be reached, respectively. In addition, the pore size can be tuned in the range of 4.9–14.8 nm by changing the content of formaldehyde.