Microporous Active Carbons Research Articles

Effect of Pore Structure of Activated Carbon on its Electrochemical Performance in Non-Aqueous Electrolyte

The surface area and pore distribution of activated carbons (ACs) were characterized by nitrogen adsorption isotherms at 77 K, and double layer capacitors were assembled to demonstrate the electrochemical performance. With 1 mol·L-1 tetraethylammonium tetrafluoroborate dissolved in acetonitrile (Et4NBF4/AN) as electrolyte, electrochemical methods including cyclic voltammetry (CV) and galvanostatic charge-discharge tests were used to evaluate the capacitors. Results show that the surface area has a limited effect on the specific capacitance of ACs and the increaing of mesoporous content can decrease the capacitance decay rate at large discharge current. When current increases from 0.15A·g-1 to 9.6A·g-1, the minimum specific capacitance decay rate of mesoporous ACs is 10.93%, while that of the microporous ACs is 15.21%;the contributions of mesoporous surface area to the specific capacitance drop from 10.10 to 9.95μF·cm-2, while that of microporous surface area drop from 5.68 to 4.21μF·cm-2.

Dehydrochlorination of activated carbon-bound 1,1,2,2-tetrachloroethane: Implications for carbonaceous material-based soil/sediment remediation

Carbonaceous materials such as activated carbon (AC) are effective adsorbents for a number of organic contaminants, and are often used in soil and sediment remediation to sequester organic contaminants. However, little is known about the abiotic reactivity (and thus, the long-term fate) of the carbonaceous material-bound contaminants. We studied the base-promoted dehydrochlorination kinetics of low-level (∼1.25 mg/g) 1,1,2,2-tetrachloroethane (TeCA) adsorbed to 17 different ACs varying in origin and physicochemical properties. Activated carbon-bound TeCA exhibited significant reactivity—at a given pH the apparent reaction kinetic constants of AC-bound TeCA are 13–70% of the respective kinetic constants of dissolved TeCA. Binding to ACs likely have affected the reactivity of TeCA via two important mechanisms. First, the deprotonated surface O-functional groups (e.g., carboxyl and phenolic groups) catalyzed the base-promoted dehydrochlorination reaction by serving as the conjugate bases. Second, at low contaminant loadings TeCA molecules likely resided primarily in the micropores of ACs (particularly, micropores with diameters of 0.67–1.6 nm), wherein mass transfer of OH− (the nucleophile that reacts with TeCA) was inhibited. An important environmental implication is that carbonaceous materials not only sequester contaminants, but also allow contaminants to be slowly degraded in situ.

Mechanistic Study on Adsorption and Oxidation of NO over Activated Carbon

Themechanism of NO adsorption and oxidation over activated carbon at lowtemperature has been studied by NO adsorption and oxidation, NO2adsorption and Fourier transform infrared spectroscopy. The results show thatwhen oxygen is present, activated carbon catalyses NO oxidation into NO2and a stable NO conversion rate is achieved. NO is adsorbed in the form of (NO)2due to the confinement effects of the activated carbon micropore. (NO)2is oxidized into NO2 by oxygen. The disproportionation of NO2gives NO and NO3, that causes the peak of the maximum NOconcentration. When the active sites are saturated by NO3, NO2adsorption and disproportionation gradually diminish, that results in NO2breakthrough and increases the concentration of NO2 to thestationary one. At the same time, the concentration of NO gradually decreasesto the stationary one after the maximum.

Low-cost metal oxide activated carbon prepared and modified by microwave heating method for hydrogen storage

Novel microporous activated carbon (MAC) with high surface area and pore volume has been synthesized by microwave heating. Iron oxide nanoparticles were loaded into MAC by using Fe(NO3)3·9H2O followed by microwave irradiation for up to five minutes. The surface modified microporous activated carbon was characterized by BET, XRD, SEM and thermogravimetric examinations. Adsorption data of H2 on the unmodified and modified MACs were collected with PCT method for a pressure range up to 120 bar at 303 K. Greater hydrogen adsorption was observed on the carbon adsorbents doped with 1.45 wt% of iron oxide nanoparticle loaded due to the joint properties of hydrogen adsorption on the carbon surface and the spill-over of hydrogen molecules into carbon structures.

Influence of the electrolyte distribution near the micropores of the activated carbon (AC) electrode on high rate performance of high voltage capacitors

This paper presents a method to enhance the rate performance of high voltage capacitors using an electrolyte additive, 1,3,5-trifluorobenzene (TFB). With increasing discharge rate, the capacity of the activated carbon (AC)/lithium (Li) cell decreases with increasing the slope of the discharge curve and its potential drop at 4.6V. By adding TFB, the discharge slope improves to increase the rate performance of the cell, and EIS showed that the charge transfer resistance (Rc) of the AC cell decreases. These results suggest that TFB affects the distribution of the electrolyte components near the microporous AC and improves the rate performance of the AC cell.

Regeneration methods to restore carbon adsorptive capacity of dibenzothiophene and neutral nitrogen heteroaromatic compounds

Activated carbon (AC) spent by model diesel fuel was regenerated using thermal, ultrasound and solvent approaches. The variations of physical and chemical properties of AC in the cycles of adsorption and desorption were studied. The results suggest that micropores play a more significant role in the carbon regeneration than mesopores do. The loss of micropores results in a lower capacity of carbon to adsorb S and N heteroaromatic compounds. Toluene extraction retains more porosity and oxygen functional groups of carbon, leading to better recovery than those by the other approaches. The toluene extracted AC can achieve 70% of the fresh capacities after nine cycles’ regeneration. Thermal regeneration results in the loss of pores in carbon as well as the decomposition of oxygen functional groups. N,N-dimethylformamide (DMF) residues on regenerated AC samples, which helps with the adsorption of DBT to some extent even though this DMF lowers the carbon porosity. The difficulty in the diffusion of adsorbed sulfur- and nitrogen-containing compounds from the micropores of AC hinders the regeneration process.

Electrical double-layer capacitance of micro- and mesoporous activated carbon prepared from rice husk and beet sugar

To produce micro- and mesoporous RH-derived activated carbons (ACs), a mixture of rice husk (RH) and beet sugar (BS) was carbonized to produce a precursor, which was subjected to the silica-leaching process and then CO2 activation. The silica, intrinsically present in natural RH, was used as a template for mesopores, and BS was used for additional development of micropores. The RH-derived ACs were prepared with the intention of application as the electrode materials of electrical double-layer capacitors. The RH-derived AC, exhibiting BET specific surface area of 1357m2/g, total pore volume of 0.99mL/g, and mesopore volume fraction of 44.4%, showed superior capacitive performance in both aqueous (1M H2SO4) and organic (1M TEMA·BF4/PC) electrolytes, in comparison with commercial microporous and mesoporous ACs with similar BET specific surface area (1200–1600m2/g). The specific capacitance, evaluated by means of cyclic voltammetry using a three-electrode cell system at a scan rate of 5mV/s, was 106F/g for the aqueous electrolyte, and 114F/g for the organic electrolyte. Higher specific capacitance in the organic electrolyte was obtained from the RH-derived AC, and was much higher than those of commercial ACs. The good compatibility with both the aqueous and organic electrolytes was attributed to the well-balanced micro- and mesoporosity. The role of the microporosity and mesoporosity on the capacitive performance of ACs in both electrolytes is also discussed.

Dust removal and purification of calcium carbide furnace off-gas

A large amount of CO contained in calcium carbide furnace off-gas can be recycled and used as an important raw material for C1 (one carbon) industrial chemical manufacturing. The key to the recovery is the removal of dust and gaseous impurities from the off-gas. For this purpose, an industrial device has been used for removing the dust. This consists of a laboratory-scale fixed-bed reactor with two kinds of adsorbents, prepared in our laboratory, for removing HCN and COS. The results show that the dust content after the dedusting process was less than 20mg/m3. The dust particles are composed of microspheres, with an obvious tendency to agglomerate, and with an average diameter of about 6.71μm. After adsorption, the outlet concentrations of both HCN and COS were less than 1mg/m3. In order to remove HCN, activated carbon (AC) was used, impregnated with 0.15mol/L Cu(CH3COO)2 solution. The optimum calcination temperature was 350°C. The AC, modified with Cu(CH3COO)2, was examined by means of N2-BET and X-ray photoelectron spectroscopy (XPS) before and after off-gas treatment. The results indicate that the chemisorption products were mainly present in the AC micropores and mesopores, with radius 0.81–1.51nm. After dust removal and purification, the calcium carbide furnace off-gas is fit for use as a raw material in the C1 chemical manufacturing industry.

Thermodynamics of adsorption of krypton, xenon, nitrogen, and oxygen on microporous active carbon at temperatures above critical values

It was shown earlier that adsorption isotherms on microporous adsorbents at temperatures above the critical values can be described using the Dubinin-Radushkevich and Tolmachev-Aranovich equations when saturated adsorptive vapor pressures used as standard pressures at temperatures below the critical values are replaced by standard pressures P*(T) found by linear extrapolation of dependences of the logarithm of the saturated adsorptive vapor pressure on the inverse temperature. This was confirmed using the example of description of experimental adsorption isotherms of krypton, xenon, oxygen, and nitrogen on microporous active carbons AUK at the temperatures above the critical value.

Tailoring the porosity of chemically activated hydrothermal carbons: Influence of the precursor and hydrothermal carbonization temperature

Advanced porous materials with tailored porosity (extremely high development of microporosity together with a narrow micropore size distribution (MPSD)) are required in energy and environmental related applications. Lignocellulosic biomass derived HTC carbons are good precursors for the synthesis of activated carbons (ACs) via KOH chemical activation. However, more research is needed in order to tailor the microporosity for those specific applications. In the present work, the influence of the precursor and HTC temperature on the porous properties of the resulting ACs is analyzed, remarking that, regardless of the precursor, highly microporous ACs could be generated. The HTC temperature was found to be an extremely influential parameter affecting the porosity development and the MPSD of the ACs. Tuning of the MPSD of the ACs was achieved by modification of the HTC temperature. Promising preliminary results in gas storage (i.e. CO2 capture and high pressure CH4 storage) were obtained with these materials, showing the effectiveness of this synthesis strategy in converting a low value lignocellulosic biomass into a functional carbon material with high performance in gas storage applications.

Anchoring a halogenated amine on the surface of a microporous activated carbon for carbon dioxide capture

Activated carbon (AC) surface may be modified by grafting amine-containing compounds to improve the CO2 adsorption capacity. Two solid sorbents were prepared by anchoring a halogenated amine, i.e., 2-chloroethylamine hydrochloric acid (CEA) on the surface of a microporous AC using a two-stage modification. At the first stage, the samples of AC were oxidized by nitric acid to increase the amount of oxygen surface groups and at the second stage, the oxidized samples were modified by anchoring CEA on the surface to produce a superior CO2 adsorbent. The oxidized samples were compared with the aid of proximate and ultimate analysis, nitrogen adsorption–desorption at −196°C (77K) and temperature programmed desorption (TPD) to decide on the best oxidation conditions. The amine-modified samples were analyzed in terms of texture, surface chemistry and CO2 adsorption. The latter was studied using isothermal CO2 capture, temperature-programmed (TP) CO2 adsorption and cyclic operation. The modified samples had a lower surface area than the parent sample. The best modified sample presented an increase of 45% in CO2 capture capacity at 100°C. Based on unit surface area, the modified samples showed great CO2 capture capacities, compared to the virgin sample. Moreover, the modified samples presented a less dependency of CO2 capacity on temperature. This indicates that the adsorption mechanism shifts from physisorption to chemisorption by increasing temperature.

Molecular nanostructures of alcohols adsorbed in micropores of active carbons

Quantitative topological analysis of molecular nanostructures of alcohols adsorbed in a model pore of active carbon depending on the micropore width and temperature is carried out. The analysis is performed using a method (MDTG) that we suggested earlier. It is based on a combination of molecular-dynamic calculations and graph theory and allows detecting and recording all molecular nanostructures observed in each shot of the molecular-dynamic path; averaging the data for any amount of such shots thus obtaining “average” concentrations of associates (dimers, trimers, etc.); and then determining concentrations and characteristics of isomers (e.g., chains, branched chains, cycles, etc.), bond widths, angles, etc., in each group of associates.

Characterization of activated carbons derived from short rotation hardwood pyrolysis char

Chars from fast and flash pyrolysis of different short rotation hardwoods are activated by steam or KOH, resulting in a variety of activated carbons (ACs). Analysis of nitrogen adsorption isotherm shows that microporous ACs are obtained. Density functional theory calculation is applied to obtain information concerning the micropore size distribution. Additionally, methylene blue (MB) and iodine adsorption experiments are performed to evaluate the AC adsorptive properties. The textural and adsorptive properties are compared with a commercial AC (Norit). KOH activation yields ACs with very high surface area, micropore volume and adsorption capacity, while steam activation promotes formation of narrower micropores but yields less pore volume. The MB adsorption kinetics can be described by the pseudo-second-order model, and most ACs adsorb at least 70% of their maximum MB adsorption capacity within 15min.

Preparation of a new adsorbent from activated carbon and carbon nanofiber (AC/CNF) for manufacturing organic-vacbpour respirator cartridge

In this study a composite of activated carbon and carbon nanofiber (AC/CNF) was prepared to improve the performance of activated carbon (AC) for adsorption of volatile organic compounds (VOCs) and its utilization for respirator cartridges. Activated carbon was impregnated with a nickel nitrate catalyst precursor and carbon nanofibers (CNF) were deposited directly on the AC surface using catalytic chemical vapor deposition. Deposited CNFs on catalyst particles in AC micropores, were activated by CO2 to recover the surface area and micropores. Surface and textural characterizations of the prepared composites were investigated using Brunauer, Emmett and Teller’s (BET) technique and electron microscopy respectively. Prepared composite adsorbent was tested for benzene, toluene and xylene (BTX) adsorption and then employed in an organic respirator cartridge in granular form. Adsorption studies were conducted by passing air samples through the adsorbents in a glass column at an adjustable flow rate. Finally, any adsorbed species not retained by the adsorbents in the column were trapped in a charcoal sorbent tube and analyzed by gas chromatography. CNFs with a very thin diameter of about 10-20 nm were formed uniformly on the AC/CNF. The breakthrough time for cartridges prepared with CO2 activated AC/CNF was 117 minutes which are significantly longer than for those cartridges prepared with walnut shell- based activated carbon with the same weight of adsorbents. This study showed that a granular form CO2 activated AC/CNF composite could be a very effective alternate adsorbent for respirator cartridges due to its larger adsorption capacities and lower weight.

Investigation on capacitive behaviors of porous Ni electrodes for electric double layer capacitors

The present study aims at assessing the applicability of porous metals to the electrodes of electric double layer capacitors (EDLC). Porous Ni materials for test electrodes were prepared from commercially available Ni–Al alloys through an alkali-leaching process and the subsequent dry and surface-treatment processes. The surface area and pore structure (mesoporous structure) have been found to depend on the class of the alloys. Furthermore, the porous Ni materials prepared via the dry processes with heating and with exposure to air have shown high electrochemical stability but quite low capacitance. In contrast, the dry process without heating and in no contact with air has made the surface and bulk sate electrochemically more active, which induces pseudocapacitance as well as double-layer capacitance. The pseudocapacitive reaction is responsible for the gradual modification of the active electrode to a high-resistance material with charge/discharge cycles. The volumetric total capacitance has reached as high as 31.9F/cm3 (in a three-electrode system) irrespective of not so high specific surface areas (SSA) of 43m2/g. At present, the capacitance is lower than the values, i.e., 40.3–78.0F/cm3, of the commercial microporous active carbons with the high SSAs of 1508–2164m2/g. However, the capacitance normalized by the surface areas has reached 10.2μF/cm2, which is beyond 3.6–6.6μF/cm2 of the active carbons. In addition, the mesoporous Ni electrodes have shown more favorable rate performance than the microporous active carbons due to the low ion-transfer resistance.

Study of carbon aerogel-activated carbon composite electrodes for capacitive deionization application

Electrodes synthesized using mesoporous carbon aerogel (CA), microporous-activated carbon (AC), and different combinations of the two were evaluated for capacitive deionization application. Composite electrodes with CA and AC in the ratio of 75:25 provided the highest specific capacitance value (90 F/g at scan rate of 2 mV/s). Further, it was observed that on increasing the scan rates to up to 10 mV/s, the reduction in specific capacitance values for the electrodes with 75:25 CA/AC composition was only 5%, whereas the reduction for pure CA and pure AC electrodes were 23 and 52%, respectively. The test cell made using 75:25 CA/AC composite electrodes (size 10 cm × 10 cm with material loading of 15 mg/cm2) showed fast adsorption and desorption cycle of ∼15 min and salt removal efficiency of 51% was obtained.

A priori calculation of adsorption equilibria on microporous active carbons

A method is proposed for a priori calculating adsorption isotherms based on the combination of the molecular dynamics method and Dubinin-Radushkevich and Tolmachev-Aranovich equations. The isotherms of adsorption of individual gases and vapors, as well as components of gaseous and vaporous mixtures and liquid solutions on microporous active carbons, are calculated using this method.

Degradation of paracetamol by catalytic wet air oxidation and sequential adsorption – Catalytic wet air oxidation on activated carbons

The concern about the fate of pharmaceutical products has raised owing to the increasing contamination of rivers, lakes and groundwater. The aim of this paper is to evaluate two different processes for paracetamol removal. The catalytic wet air oxidation (CWAO) of paracetamol on activated carbon was investigated both as a water treatment technique using an autoclave reactor and as a regenerative treatment of the carbon after adsorption in a sequential fixed bed process. Three activated carbons (ACs) from different source materials were used as catalysts: two microporous basic ACs (S23 and C1) and a meso- and micro-porous acidic one (L27). During the first CWAO experiment the adsorption capacity and catalytic performance of fresh S23 and C1 were higher than those of fresh L27 despite its higher surface area. This situation changed after AC reuse, as finally L27 gave the best results after five CWAO cycles. Respirometry tests with activated sludge revealed that in the studied conditions the use of CWAO enhanced the aerobic biodegradability of the effluent. In the ADOX process L27 also showed better oxidation performances and regeneration efficiency. This different ageing was examined through AC physico-chemical properties.

Micropore Size Analysis of Activated Carbons Using Nitrogen, Carbon Dioxide and Methane Adsorption Isotherms: Experimental and Theoretical Studies

Nitrogen (N2, at 77 K and sub-atmospheric pressure), carbon dioxide (CO2, at 273 K, sub-atmospheric pressure and high pressure) and methane (CH4, at 298 K and high pressure) adsorption isotherms have been obtained for three microporous-activated carbons in order to characterize their micropore size distributions (MPSDs). A comparison of the results has shown that the shape of the isotherms was different, which was due in part to differences in the overall micropore volume and the MPSDs for the carbon samples studied in this work. According to the results, despite the different characteristics of CO2 and CH4, a quite good consistency between the MPSDs from these two gases was obtained for the three carbon samples. Finally, it was shown that the adsorption isotherms of CO2 and CH4 at the higher temperatures should be considered as a complement to N2 adsorption at 77 K for the characterization of porous texture (micropore volume and MPSDs) of activated carbon samples.

Developing criteria for the recovery of ionic liquids from aqueous phase by adsorption with activated carbon

Adsorption with activated carbons (ACs) has been recently reported as an affordable environmental application to remove and recover ionic liquids (ILs) from water streams. In this work, an extensive analysis of the separation of imidazolium-based ILs from aqueous solution, based on adsorption equilibrium experiments at 308K, has been performed. The study includes five commercial and four modified ACs, with a wide variety of structural and chemical properties, a carbon black and also two inorganic adsorbents. The materials tested were characterized by several techniques in order to establish the role of the porous structure and chemical surface on their adsorption capacities. We found that the microporous ACs with low concentration of polar groups in their surface are the best candidates for adsorption of hydrophobic ILs, reaching uptakes of up to 1g of IL per gram of adsorbent. The influence of both cation and anion on the adsorption process was analyzed from the adsorption isotherms of 27 different ILs. The dependence of the adsorption uptake with the chemical structure of the ILs was explained by mean of the quantum-chemical COSMO-RS model, which provides information of the intermolecular interactions between the IL adsorbate and the AC adsorbent. As a result, some insight into the adsorbent modifications needed for improving of the adsorption of specific ILs were achieved and successfully applied to the adsorption of hydrophilic ILs by oxidized ACs.In addition, a procedure for regenerating the exhausted adsorbent and recovering the IL by acetone extraction was validated upon successive adsorption–desorption cycles with two commercial ACs. Finally, the acetone was separated by atmospheric distillation, being the IL recovered without structural modification.