Activated carbon fibre monoliths

Activated carbon fibre monoliths were prepared by physical activation of carbon fibre monoliths derived from two kinds of pitch-based carbon fibre (CF) (carbon fibres from a coal tar pitch and carbon fibres derived from a petroleum pitch). The monoliths were conformed using a coal tar pitch binder. The carbon fibre monoliths and the activated carbon fibre monoliths were studied by scanning electron microscopy (SEM) and gas adsorption (i.e. N2 at 77 K and CO2 at 273 K). The results obtained reveal that monoliths perform a good activation process that produce a quite high development of microporosity (BET surface areas around 2600 m2/g and N2 micropore volume of 1.23 cm3/g). On the other hand, it is remarkable that the activation process used allow to easily control the degree of activation and hence to select the adsorption capacities of the activated carbon fibre monoliths.

Metal and precursor effect during 1-heptyne selective hydrogenation using an activated carbon as support

Palladium, platinum, and ruthenium supported on activated carbon were used as catalysts for the selective hydrogenation of 1-heptyne, a terminal alkyne. All catalysts were characterized by temperature programmed reduction, X-ray diffraction, transmission electron microscopy, and X-ray photoelectron spectroscopy. TPR and XPS suggest that the metal in all catalysts is reduced after the pretreatment with H2 at 673 K. The TPR trace of the PdNRX catalyst shows that the support surface groups are greatly modified as a consequence of the use of HNO3 during the catalyst preparation. During the hydrogenation of 1-heptyne, both palladium catalysts were more active and selective than the platinum and ruthenium catalysts. The activity order of the catalysts is as follows: PdClRX > PdNRX > PtClRX ≫ RuClRX. This superior performance of PdClRX was attributed in part to the total occupancy of the d electronic levels of the Pd metal that is supposed to promote the rupture of the H2 bond during the hydrogenation reaction. The activity differences between PdClRX and PdNRX catalysts could be attributed to a better accessibility of the substrate to the active sites, as a consequence of steric and electronic effects of the superficial support groups. The order for the selectivity to 1-heptene is as follows: PdClRX = PdNRX > RuClRX > PtClRX, and it can be mainly attributed to thermodynamic effects.

Asymmetric hybrid capacitors based on activated carbon and activated carbon fibre–PANI electrodes

Composites consisting of polyaniline (PANI) coatings inside the microporosity of an activated carbon fibre (ACF) were prepared by electrochemical and chemical methods. Electrochemical characterization of both composites points out that the electrodes with polyaniline show a higher capacitance than the pristine porous carbon electrode. These materials have been used to develop an asymmetric capacitor based on activated carbon (AC) as negative electrode and an ACF–PANI composite as positive electrode in H2SO4 solution as electrolyte. The presence of a thin layer of polyaniline inside the porosity of the activated carbon fibres avoids the oxidation of the carbon material and the oxygen evolution reaction is produced at more positive potentials. This capacitor was tested in a maximum cell voltage of 1.6 V and exhibited high energy densities, calculated for the unpackaged active materials, with values of 20 W h kg−1 and power densities of 2.1 kW kg−1 with excellent cycle lifetime (90% during the first 1000 cycles) and high coulombic efficiency.

Flexible ruthenium oxide-activated carbon cloth composites prepared by simple electrodeposition methods

This work focuses on the preparation of flexible ruthenium oxide containing activated carbon cloth by electrodeposition. Different electrodeposition methods have been used, including chronoamperometry, chronopotentiometry and cyclic voltammetry. The electrochemical properties of the obtained materials have been measured. The results show that the potentiostatic method allows preparing composites with higher specific capacitance than the pristine activated carbon cloth. The capacitance values measured by cyclic voltammetry at 10 mV s−1 and 1 V of potential window were up to 160 and 180 F g−1. This means an improvement of 82% and 100% with respect to the capacitance of the pristine activated carbon cloth. This excellent capacitance enhancement is attributed to the small particle size (4–5 nm) and the three-dimensional nanoporous network of the ruthenium oxide film which allows reaching very high degree of oxide utilization without blocking the pore structure of the activated carbon cloth. In addition, the electrodes maintain the mechanical properties of the carbon cloth and can be useful for flexible devices.

Biodiesel production by acid catalysis with heteropolyacids supported on activated carbon fibers

Different catalysts, based on heteropolyacids supported on activated carbon fibers, have been prepared for palmitic acid esterification reaction. The influence of the catalyst (heteropolyacid) and the support on the catalytic activity have been analyzed. The results prove that an adequate combination of both is required to achieve the most suitable catalysts. Regarding to the heteropolyacid, phosphomolybdic acid seems to be the most suitable appropriate taking into account its lowest leaching. About the support, it must show an optimum microporosity, which must be wide enough to allow the entrance and exit of the reagents and products but not too wide in order to avoid the leaching of the catalyst. In addition, both decreasing of the catalytic activity and its recovery over several cycles have been analyzed.

Spherical activated carbon as an enhanced support for TiO2/AC photocatalysts

Titanium dioxide nanoparticles prepared in situ by sol–gel method were supported on a spherical activated carbon to prepare TiO2/AC hybrid photocatalysts for the oxidation of gaseous organic compounds. Additionally, a granular activated carbon was studied for comparison purposes. In both types of TiO2/AC composites the effect of different variables (i.e., the thermal treatment conditions used during the preparation of these materials) and the UV-light wavelength used during photocatalytic oxidation were analyzed. The prepared materials were deeply characterized (by gas adsorption, TGA, XRD, SEM and photocatalytic propene oxidation). The obtained results show that the carbon support has an important effect on the properties of the deposited TiO2 and, therefore, on the photocatalytic activity of the resulting TiO2/AC composites. Thus, the hybrid materials prepared over the spherical activated carbon show better results than those prepared over the granular one; a good TiO2 coverage with a high crystallinity of the deposited titanium dioxide, which just needs an air oxidation treatment at low-moderate temperature (350–375 °C) to present high photoactivity, without the need of additional inert atmosphere treatments. Additionally, these materials are more active at 365 nm than at 257.7 nm UV radiation, opening the possibility of using solar light for this application.

Electroadsorption of arsenic from natural water in granular activated carbon

The adsorption and electroadsorption of arsenic from a natural water has been studied in a filter-press electrochemical cell using a commercial granular activated carbon as adsorbent and Pt/Ti and graphite as electrodes. A significant reduction of the arsenic concentration is achieved when current is imposed between the electrodes, especially when the activated carbon was located in the vicinity of the anode. This enhancement can be explained in terms of the presence of electrostatic interactions between the polarized carbon surface and the arsenic ions, and changes in the distribution of most stable species of arsenic in solution due to As(III) to As(V) oxidation. In summary, electrochemical adsorption on a filter-press cell can be used for enhancement the arsenic remediation with activated carbon in the treatment of a real groundwater.

Tailoring the Surface Chemistry of Activated Carbon Cloth by Electrochemical Methods

This paper presents a systematic study of the effect of the electrochemical treatment (galvanostatic electrolysis in a filter-press electrochemical cell) on the surface chemistry and porous texture of commercial activated carbon cloth. The same treatments have been conducted over a granular activated carbon in order to clarify the effect of morphology. The influence of different electrochemical variables, such as the electrode polarity (anodic or cathodic), the applied current (between 0.2 and 1.0 A) and the type of electrolyte (HNO3 and NaCl) have also been analyzed. The anodic treatment of both activated carbons causes an increase in the amount of surface oxygen groups, whereas the cathodic treatment does not produce any relevant modification of the surface chemistry. The HNO3 electrolyte produced a lower generation of oxygen groups than the NaCl one, but differences in the achieved distribution of surface groups can be benefitial to selectively tune the surface chemistry. The porous texture seems to be unaltered after the electro-oxidation treatment. The validity of this method to introduce surface oxygen groups with a pseudocapacitive behavior has been corroborated by cyclic voltammetry. As a conclusion, the electrochemical treatment can be easily implemented to selectively and quantitatively modify the surface chemistry of activated carbons with different shapes and morphologies.

CeO2-promoted Ni/activated carbon catalysts for the water–gas shift (WGS) reaction

The low temperature water–gas shift (WGS) reaction has been studied over carbon-supported nickel catalysts promoted by ceria. To this end, cerium oxide has been dispersed (at different loadings: 10, 20, 30 and 40 wt.%) on the activated carbon surface with the aim of obtaining small ceria particles and a highly available surface area. Furthermore, carbon- and ceria-supported nickel catalysts have also been studied as references. A combination of N2 adsorption analysis, powder X-ray diffraction, temperature-programmed reduction with H2, X-ray photoelectron spectroscopy and TEM analysis were used to characterize the Ni–CeO2 interactions and the CeO2 dispersion over the activated carbon support. Catalysts were tested in the low temperature WGS reaction with two different feed gas mixtures: the idealized one (with only CO and H2O) and a slightly harder one (with CO, CO2, H2, and H2O). The obtained results show that there is a clear effect of the ceria loading on the catalytic activity. In both cases, catalysts with 20 and 10 wt.% CeO2 were the most active materials at low temperature. On the other hand, Ni/C shows a lower activity, this assessing the determinant role of ceria in this reaction. Methane, a product of side reactions, was observed in very low amounts, when CO2 and H2 were included in the WGS feed. Nevertheless, our data indicate that the methanation process is mainly due to CO2, and no CO consumption via methanation takes place at the relevant WGS temperatures. Finally, a stability test was carried out, obtaining CO conversions greater than 40% after 150 h of reaction.

Activated Carbon Fibre Monoliths for Hydrogen Storage

Porous adsorbents are currently investigated for hydrogen storage application. From a practical point of view, in addition to high porosity developments, high material densities are required, in order to confine as much material as possible in a tank device. In this study, we use different measured sample densities (tap, packing, compacted and monolith) for analyzing the hydrogen adsorption behavior of activated carbon fibres (ACFs) and activated carbon nanofibres (ACNFs) which were prepared by KOH and CO2 activations, respectively. Hydrogen adsorption isotherms are measured for all of the adsorbents at room temperature and under high pressures (up to 20 MPa). The obtained results confirm that (i) gravimetric H2 adsorption is directly related to the porosity of the adsorbent, (ii) volumetric H2 adsorption depends on the adsorbent porosity and importantly also on the material density, (iii) the density of the adsorbent can be improved by packing the original adsorbents under mechanical pressure or synthesizing monoliths from them, (iv) both ways (packing under pressure or preparing monoliths) considerably improve the storage capacity of the starting adsorbents, and (v) the preparation of monoliths, in addition to avoid engineering constrains of packing under mechanical pressure, has the advantage of providing high mechanical resistance and easy handling of the adsorbent.

Methane dry reforming over Ni supported on pine sawdust activated carbon: Effects of support surface properties and metal loading

The influence of metal loading and support surface functional groups (SFG) on methane dry reforming (MDR) over Ni catalysts supported on pine-sawdust derived activated carbon were studied. Using pine sawdust as the catalyst support precursor, the smallest variety and lowest concentration of SFG led to best Ni dispersion and highest catalytic activity, which increased with Ni loading up to 3 Ni atoms nm-2. At higher Ni loading, the formation of large metal aggregates was observed, consistent with a lower "apparen" surface area and a decrease in catalytic activity. The H2/CO ratio rose with increasing reaction temperature, indicating that increasingly important side reactions were taking place in addition to MDR.

Enhanced removal of 8-quinolinecarboxylic acid in an activated carbon cloth by electroadsorption in aqueous solution

The effect of the electrochemical treatment (potentiostatic treatment in a filter-press electrochemical cell) on the adsorption capacity of an activated carbon cloth (ACC) was analyzed in relation with the removal of 8-quinolinecarboxylic acid pollutant from water. The adsorption capacity of an ACC is quantitatively improved in the presence of an electric field (electroadsorption process) reaching values of 96% in comparison to 55% in absence of applied potential. In addition, the cathodic treatment results in higher removal efficiencies than the anodic treatment. The enhanced adsorption capacity has been proved to be irreversible, since the removed compound remains adsorbed after switching the applied potential. The kinetics of the adsorption processes is also improved by the presence of an applied potential.

Design of Activated Carbon/Activated Carbon Asymmetric Capacitors

Supercapacitors are energy storage devices that offer a high power density and a low energy density in comparison with batteries. Their limited energy density can be overcome by using asymmetric configuration in mass electrodes, where each electrode works within their maximum available potential window, rendering the maximum voltage output of the system. Such asymmetric capacitors are optimized using the capacitance and the potential stability limits of the electrodes, with the reliability of the design largely depending on the accuracy and the approach taken for the electrochemical characterization. Therefore, the performance could be lower than expected and even the system could break down, if a well thought out procedure is not followed. In this work, a procedure for the development of asymmetric supercapacitors based on activated carbons is detailed. Three activated carbon materials with different textural properties and surface chemistry have been systematically characterized in neutral aqueous electrolyte. The asymmetric configuration of the masses of both electrodes in the supercapacitor has allowed to cover a higher potential window, resulting in an increase of the energy density of the three devices studied when compared with the symmetric systems, and an improved cycle life.

Removal of BrO3 − from drinking water samples using newly developed agricultural waste-based activated carbon and its determination by ultra-performance liquid chromatography-mass spectrometry

Activated carbon was prepared from date pits via chemical activation with H3PO4. The effects of activating agent concentration and activation temperature on the yield and surface area were studied. The optimal activated carbon was prepared at 450 °C using 55 % H3PO4. The prepared activated carbon was characterized by Fourier transform infrared spectroscopy, scanning electron microscopy, thermogravimetric-differential thermal analysis, and Brunauer, Emmett, and Teller (BET) surface area. The prepared date pit-based activated carbon (DAC) was used for the removal of bromate (BrO3 −). The concentration of BrO3 − was determined by ultra-performance liquid chromatography-mass tandem spectrometry (UPLC-MS/MS). The experimental equilibrium data for BrO3 − adsorption onto DAC was well fitted to the Langmuir isotherm model and showed maximum monolayer adsorption capacity of 25.64 mg g−1. The adsorption kinetics of BrO3 − adsorption was very well represented by the pseudo-first-order equation. The analytical application of DAC for the analysis of real water samples was studied with very promising results.

Platinum supported on highly-dispersed ceria on activated carbon for the total oxidation of VOCs

Catalysts consisting in platinum supported on cerium oxide highly dispersed on activated carbon, with a Pt loading of 1 wt.% and ceria loadings of 5, 10 and 20 wt.% have been prepared by impregnation method and characterized by several techniques (N2 adsorption at 77 K, ICP, XRD, H2-TPR and XPS). Their catalytic behavior has been evaluated in the total oxidation of ethanol and toluene after reduction at 473 K. The obtained results show that the prepared catalysts have better performances than platinum supported on bulk CeO2. The best catalytic performance was obtained for 10 wt.% ceria loading, likely due to an optimum synergistic interaction between highly dispersed cerium oxide and platinum particles. Pt-10Ce/C achieves total conversion of ethanol and toluene to CO2 at 433 K and 453 K, respectively, and shows no deactivation during a test for 100 h. Under humid conditions (relative humidity, RH, of 40 and 80%), the activity was only slightly influenced due to the hydrophobic character of the activated carbon support, which prevents the adsorption of water.