Raman evidence of the interaction between multiwalled carbon nanotubes and nanostructured TiO(2)

Nanocomposites of carbon nanotubes and titanium dioxide (TiO(2)) have attracted much attention due to their photocatalytic properties. Although many examples in the literature have visualized these nanocomposites by electron microscopic images, spectroscopic characterization is still lacking with regard to the interaction between the carbon nanotube and TiO(2). In this work, we show evidence of the attachment of nanostructured TiO(2) to multiwalled carbon nanotubes(MWNTs) by Raman spectroscopy. The nanostructured TiO(2) was characterized by both full-width at half-maximum (FWHM) and the Raman shift of the TiO(2) band at ca 144 cm(-1), whereas the average diameter of the crystallite was estimated as approximately 7 nm. Comparison of the Raman spectra of the MWNTs and MWNTs/TiO(2) shows a clear inversion of the relative intensities of the G and D bands, suggesting a substantial chemical modification of the outermost tubes due to the attachment of nanostructured TiO(2). To complement the nanocomposite characterization, scanning electronic microscopy and X-ray diffraction were performed. Copyright (C) 2011 John Wiley & Sons, Ltd.

Development and testing of an asymmetric capacitor with a nickel-carbon foam positive electrode

Electrochemical capacitors (ECs), also known as supercapacitors or ultracapacitors, are energy storage devices with properties between batteries and conventional capacitors. EC have evolved through several generations. The trend in EC is to combine a double-layer electrode with a battery-type electrode in an asymmetric capacitor configuration. The double-layer electrode is usually an activated carbon (AC) since it has high surface area, good conductivity, and relatively low cost. The battery-type electrode usually consists of PbO2 or Ni(OH)2. In this research, a graphitic carbon foam was impregnated with Co-substituted Ni(OH)2 using electrochemical deposition to serve as the positive electrode in the asymmetric capacitor. The purpose was to reduce the cost and weight of the ECs while maintaining or increasing capacitance and gravimetric energy storage density. The XRD result indicated that the nickel-carbon foam electrode was a typical α-Ni(OH)2. The specific capacitance of the nickel-carbon foam electrode was 2641 F/g at 5 mA/cm2, higher than the previously reported value of 2080 F/g for a 7.5% Al-substituted α-Ni(OH)2 electrode. Three different ACs (RP-20, YP-50F, and Ketjenblack EC-600JD) were evaluated through their morphology and electrochemical performance to determine their suitability for use in ECs. The study indicated that YP-50F demonstrated the better overall performance because of the combination of micropore and mesopore structures. Therefore, YP-50F was chosen to combine with the nickel-carbon foam electrode for further evaluation. Six cells with different mass ratios of negative to positive active mass were fabricated to study the electrochemical performance. Among the different mass ratios, the asymmetric capacitor with the mass ratio of 3.71 gave the highest specific energy and specific power, 24.5 W.h/kg and 498 W/kg, respectively.

Method for Aqueous Gold Thiosulfate Extraction Using Copper-Cyanide Pretreated Carbon Asdortion

A gold thiosulfate leaching process uses carbon to remove gold from the leach liquor. The activated carbon is pretreated with copper cyanide. A copper (on the carbon) to gold (in solution) ration of at least 1.5 optimizes gold recovery from solution. To recover the gold from the carbon, conventional elution technology works but is dependent on the copper to gold ratio on the carbon.

Activation of polymer blend carbon nanofibres by alkaline hydroxides and their hydrogen storage performances

In the present work we study the hydroxide activation (NaOH and KOH) of phenol-formaldehyde resin derived CNFs prepared by a polymer blend technique to prepare highly porous activated carbon nanofibres (ACNFs). Morphology and textural characteristics of these ACNFs were studied and their hydrogen storage capacities at 77 K (at 0.1 MPa and at high pressures up to 4 MPa) were assessed, and compared, with reported capacities of other porous carbon materials. Phenol-formaldehyde resin derived carbon fibres were successfully activated with these two alkaline hydroxides rendering highly microporous ACNFs with reasonable good activation process yields up to 47 wt.% compared to 7 wt.% yields from steam activation for similar surface areas of 1500 m2/g or higher. These nano-sized activated carbons present interesting H2 storage capacities at 77 K which are comparable, or even higher, to other high quality microporous carbon materials. This observation is due, in part, to their nano-sized diameters allowing to enhance their packing densities to 0.71 g/cm3 and hence their resulting hydrogen storage capacities.

Ammonia removal using activated carbons: effect of the surface chemistry in dry and moist conditions

The effect of surface chemistry (nature and amount of oxygen groups) in the removal of ammonia was studied using a modified resin-based activated carbon. NH3 breakthrough column experiments show that the modification of the original activated carbon with nitric acid, that is, the incorporation of oxygen surface groups, highly improves the adsorption behavior at room temperature. Apparently, there is a linear relationship between the total adsorption capacity and the amount of the more acidic and less stable oxygen surface groups. Similar experiments using moist air clearly show that the effect of humidity highly depends on the surface chemistry of the carbon used. Moisture highly improves the adsorption behavior for samples with a low concentration of oxygen functionalities, probably due to the preferential adsorption of ammonia via dissolution into water. On the contrary, moisture exhibits a small effect on samples with a rich surface chemistry due to the preferential adsorption pathway via Brønsted and Lewis acid centers from the carbon surface. FTIR analyses of the exhausted oxidized samples confirm both the formation of NH4+ species interacting with the Brønsted acid sites, together with the presence of NH3 species coordinated, through the lone pair electron, to Lewis acid sites on the graphene layers.

NaOH and KOH for preparing activated carbons used in energy and environmental applications

Alkaline hydroxides, especially sodium and potassium hydroxides, are multi-million-ton per annum commodities and strong chemical bases that have large scale applications. Some of them are related with their consequent ability to degrade most materials, depending on the temperature used. As an example, these chemicals are involved in the manufacture of pulp and paper, textiles, biodiesels, soaps and detergents, acid gases removal (e.g., SO2) and others, as well as in many organic synthesis processes. Sodium and potassium hydroxides are strong and corrosive bases, but they are also very stable chemicals that can melt without decomposition, NaOH at 318ºC, and KOH at 360ºC. Hence, they can react with most materials, even with relatively inert ones such as carbon materials. Thus, at temperatures higher than 360ºC these melted hydroxides easily react with most types of carbon-containing raw materials (coals, lignocellulosic materials, pitches, etc.), as well as with most pure carbon materials (carbon fibers, carbon nanofibers and carbon nanotubes). This reaction occurs via a solid-liquid redox reaction in which both hydroxides (NaOH or KOH) are converted to the following main products: hydrogen, alkaline metals and alkaline carbonates, as a result of the carbon precursor oxidation. By controlling this reaction, and after a suitable washing process, good quality activated carbons (ACs), a classical type of porous materials, can be prepared. Such carbon activation by hydroxides, known since long time ago, continues to be under research due to the unique properties of the resulting activated carbons. They have promising high porosity developments and interesting pore size distributions. These two properties are important for new applications such as gas storage (e.g., natural gas or hydrogen), capture, storage and transport of carbon dioxide, electricity storage demands (EDLC-supercapacitors-) or pollution control. Because these applications require new and superior quality activated carbons, there is no doubt that among the different existing activating processes, the one based on the chemical reaction between the carbon precursor and the alkaline hydroxide (NaOH or KOH) gives the best activation results. The present article covers different aspects of the activation by hydroxides, including the characteristics of the resulting activated carbons and their performance in some environment-related applications. The following topics are discussed: i) variables of the preparation method, such as the nature of the hydroxide, the type of carbon precursor, the hydroxide/carbon precursor ratio, the mixing procedure of carbon precursor and hydroxide (impregnation of the precursor with a hydroxide solution or mixing both, hydroxide and carbon precursor, as solids), or the temperature and time of the reaction are discussed, analyzing their effect on the resulting porosity; ii) analysis of the main reactions occurring during the activation process, iii) comparative analysis of the porosity development obtained from different activation processes (e.g., CO2, steam, phosphoric acid and hydroxides activation); and iv) performance of the prepared activated carbon materials on a few applications, such as VOC removal, electricity and gas storages.

Assessment of CO2 adsorption capacity on activated carbons by a combination of batch and dynamic tests

In this work, batch and dynamic adsorption tests are coupled for an accurate evaluation of CO2 adsorption performance for three different activated carbons obtained from olives stones by chemical activation followed by physical activation with CO2 at varying times, i.e. 20, 40 and 60 h. Kinetic and thermodynamic CO2 adsorption tests from simulated flue-gas at different temperature and CO2 pressure are carried out both in batch (a manometric equipment operating with pure CO2) and dynamic (a lab-scale fixed-bed column operating with CO2/N2 mixture) conditions. The textural characterization of the activated carbon samples shows a direct dependence of both micropore and ultramicropore volume on the activation time, hence AC60 has the higher contribution. The adsorption tests conducted at 273 and 293 K showed that, when CO2 pressure is lower than 0.3 bar, the lower the activation time the higher CO2 adsorption capacity and a ranking ωeq(AC20)>ωeq(AC40)>ωeq(AC60) can be exactly defined when T= 293 K. This result can be likely ascribed to a narrower pore size distribution of the AC20 sample, whose smaller pores are more effective for CO2 capture at higher temperature and lower CO2 pressure, the latter representing operating conditions of major interest for decarbonation of a flue-gas effluent. Moreover, the experimental results obtained from dynamic tests confirm the results derived from the batch tests in terms of CO2 adsorption capacity. It is important to highlight that the adsorption of N2 on the synthesized AC samples can be considered negligible. Finally, the importance of a proper analysis of characterization data and adsorption experimental results is highlighted for a correct assessment of CO2 removal performances of activated carbons at different CO2 pressure and operating temperature.

Activation of a spherical carbon for toluene adsorption at low concentration

This paper complements a previous one [1] about toluene adsorption on a commercial spherical activated carbon and on samples obtained from it by CO2 or steam activation. The present paper deals with the activation of a commercial spherical carbon (SC) having low porosity and high bed density (0.85 g/cm3) using the same procedure. Our results show that SC can be well activated with CO2 or steam. The increase in the burn-off percentage leads to an increase in the gravimetric adsorption capacity (more intensively for CO2) and a decrease in bed density (more intensively for CO2). However, for similar porosity developments similar bed densities are achieved for CO2 and steam. Especial attention is paid to differences between both activating agents, comparing samples having similar or different activation rates, showing that CO2 generates more narrow porosity and penetrates more inside the spherical particles than steam. Steam activates more from the outside to the interior of the spheres and hence produces larger spheres size reductions. With both activation agents and with a suitable combination of porosity development and bed density, quite high volumetric adsorption values of toluene (up to 236 g toluene/L) can be obtained even using a low toluene concentration (200 ppmv).

KOH activation of carbon materials obtained from the pyrolysis of ethylene tar at different temperatures

A complete study of the importance of the pyrolysis temperature (up to 1500 °C) of a petroleum residue (ethylene tar) in the activation with KOH of the resultant pyrolysis products (covering from the own ethylene tar to pitches and well developed cokes) has been carried out. The trend in the porosity found for activated carbons is as follows: the pore volume increases with the pyrolysis temperature reaching a maximum value (1.39 cm3/g) at about 460 °C, just at the transition temperature between a fluid pitch and a solid coke. It is the pitch with highest mesophase content that develops the maximum porosity when activated with KOH. The amount of H2, CO and CO2 produced during the reaction of the mesophase pitch and coke with KOH has been quantified, and a trend as described for the pore volume was found with the pyrolysis temperature. Therefore, there is a relationship between the reactivity of the precursor with KOH and the porosity developed by the activated carbon. Since the reactions that produce H2 initiate at temperatures as low as 300 °C, it seems that KOH is modifying the conditions under which the pyrolysis occurs, and this fact is critical in the development of porosity.

Preparation of high metal content nanoporous carbon

Activated carbons with high metal content have been prepared by the pyrolysis of ethylene tar with dissolved metal acetylacetonates (Ti, V, Fe, Co, Ni and Cu) and subsequent activation with KOH of the pitch obtained in pyrolysis. These metal compounds decompose during the pyrolysis of ethylene tar yielding metal nanoparticles formed by metal and/or oxide which are homogeneously distributed in the pitch and remain in the activated carbon, so that the concentration of metal is, in most cases, 4–5 times higher than in the pristine ethylene tar. Since KOH is an effective activating agent, all activated carbons combine a high porosity development with a high metal content. In some of the carbons, such as P2FeA (3.3% Fe, pore volume 1.84 cm3/g, BET surface area 3270 m2/g), there is even an increase in the pore volume when compared to the activated carbon prepared in the same way without metal, in spite of the fact that the metal increases the weight of carbon without contributing to the adsorptive capacity. It seems that iron, on the one hand modifies the pyrolysis to give a pitch with larger mesophase content and on the other hand it locally catalyzes carbon gasification with the CO2 produced along the synthesis of the carbon. In addition to its influence on activation, iron promotes the formation of graphitic carbon fibers.

High-Pressure Methane Storage in Porous Materials: Are Carbon Materials in the Pole Position?

Natural gas storage on porous materials (ANG) is a promising alternative to conventional on-board compressed (CNG) or liquefied natural gas (LNG). To date, Metal–organic framework (MOF) materials have apparently been the only system published in the literature that is able to reach the new Department of Energy (DOE) value of 263 cm3 (STP: 273.15 K, 1 atm)/cm3; however, this value was obtained by using the ideal single-crystal density to calculate the volumetric capacity. Here, we prove experimentally, and for the first time, that properly designed activated carbon materials can really achieve the new DOE value while avoiding the additional drawback usually associated with MOF materials (i.e., the low mechanical stability under pressure (conforming), which is required for any practical application).

Activated Carbons Impregnated with Na2S and H2SO4: Texture, Surface Chemistry and Application to Mercury Removal from Aqueous Solutions

The effects of treatment of an activated carbon with Sulphur precursors on its textural properties and on the ability of the complex synthesized for mercury removal in aqueous solutions are studied. To this end, a commercial activated carbon has been modified by treatments with aqueous solutions of Na2S and H2SO4 at two temperatures (25 and 140 °C) to introduce sulphur species on its surface. The prepared adsorbents have been characterized by N2 (-196 °C) and CO2 (0 °C) adsorption, thermogravimetric analysis, temperature-programmed decomposition and X-ray photoelectron spectroscopy, and their adsorption capacities to remove Hg(II) ions in aqueous solutions have been determined. It has been shown that the impregnation treatments slightly modified the textural properties of the samples, with a small increase in the textural parameters (BET surface area and mesopore volumes). By contrast, surface oxygen content was increased when impregnation was carried out with Na2S, but it decreased when H2SO4 was used. However, the main effect of the impregnation treatments was the formation of surface sulphur complexes of thiol type, which was only achieved when the impregnation treatments were carried out at low temperature (25 °C). The presence of surface sulphur enhances the adsorption behaviour of these samples in the removal of Hg(II) cations in aqueous solutions at pH 2. In fact, complete Hg(II) removal is only obtained with the sulphur-containing activated carbons.

Generation of nitrogen functionalities on activated carbons by amidation reactions and Hofmann rearrangement: Chemical and electrochemical characterization

Nitrogen functionalization of a highly microporous activated carbon (BET surface area higher than 3000 m2/g) has been achieved using the following sequence of treatments: (i) chemical oxidation using concentrated nitric acid, (ii) amidation by acyl chloride substitution with NH4NO3 and (iii) amination by Hoffman rearrangement. This reaction pathway yielded amide and amine functional groups, and a total nitrogen content higher than 3 at.%. It is achieved producing only a small decrease (20%) of the starting microporosity, being most of it related to the initial wet oxidation of the activated carbon. Remarkably, nitrogen aromatic rings were also formed as a consequence of secondary cyclation reactions. The controlled step-by-step modification of the surface chemistry allowed to assess the influence of individual nitrogen surface groups in the electrochemical performance in 1 M H2SO4 of the carbon materials. The largest gravimetric capacitance was registered for the pristine activated carbon due to its largest apparent surface area. The nitrogen-containing activated carbons showed the highest surface capacitances. Interestingly, the amidated activated carbon showed the superior capacitance retention due to the presence of functional groups (such as lactams, imides and pyrroles) that enhance electrical conductivity through their electron-donating properties, showing a capacitance of 83 F/g at 50 A/g.

Electrochemical behaviour of activated carbons obtained via hydrothermal carbonization

Activated carbons were prepared by chemical activation of hydrochars, obtained by hydrothermal carbonisation (HTC) using low cost and abundant precursors such as rye straw and cellulose, with KOH. Hydrochars derived from rye straw were chemically activated using different KOH/precursor ratios, in order to assess the effect of this parameter on their electrochemical behaviour. In the case of cellulose, the influence of the hydrothermal carbonisation temperature was studied by fixing the activating agent/cellulose ratio. Furthermore, N-doped activated carbons were synthesised by KOH activation of hydrochars prepared by HTC from a mixture of glucose with melamine or glucosamine. In this way, N-doped activated carbons were prepared in order to evaluate the influence of nitrogen groups on their electrochemical behaviour in acidic medium. The results showed that parameters such as chemical activation or carbonisation temperature clearly affect the capacitance, since these parameters play a key role in the textural properties of activated carbons. Finally, symmetric capacitors based on activated carbon and N-doped activated carbon were tested at 1.3 V in a two-electrode cell configuration and the results revealed that N-groups improved the capacitance at high current density.

Design of hybrid asymmetric capacitors in aqueous electrolyte using ZTC and ultraporous activated carbons

The use of two different materials as electrodes allows the construction of asymmetric and hybrid capacitors cells with enhanced energy and power density. This approach is especially well-suited for overcoming the limitations of pseudocapacitive materials that provide a huge capacitance boost, but in a limited potential window. In this work, we introduce the concepts and protocols that are required for a successful design of such systems, which is illustrated by the construction of an asymmetric hybrid cell where a zeolite-templated carbon and an ultraporous activated carbon have been combined.