D. Lozano-Castelló

Preparation of activated carbons from Spanish anthracite

Abstract In a previous work, the use of a Spanish anthracite for the preparation of activated carbons by chemical activation was analyzed. The results indicated that this raw material is promising for that purpose. In the present paper, that previous work is extended and the effect of different preparation variables on the final porous texture is discussed, such as KOH/anthracite ratio, heating rate, carbonization temperature and carbonization time. Among those different variables studied, the KOH/anthracite ratio seems to be the most important one. In addition, this study introduces an investigation of the nitrogen flow rate, showing that this variable has a very important effect on porosity development. The study confirms that the raw material used is appropriate for the preparation of activated carbons in a single stage pyrolysis process. The proper choice of the preparation conditions allows us to produce microporous activated carbons with a micropore volume up to 1.45 cm3/g and a BET surface area of 3290 m2/g. This work is extended in Part II with a detailed study using NaOH as activating agent and a different preparation method (physical mixing).

Influence of pore structure and surface chemistry on electric double layer capacitance in non-aqueous electrolyte

The performance as electric double layer capacitors (EDLC) in non-aqueous electrolyte of a series of alkaline agent-activated carbons with high surface area is presented in this work. The results obtained show that, in general, capacitance increases with surface area. However, the results obtained in this study confirm that capacitance not only depends on surface area, but also on two other parameters: pore size distribution and surface chemistry. It has been shown that capacitance is higher for a sample with wider micropore size distribution than for a sample with higher surface area but too narrow micropore size distribution. In addition, it has been observed that the sample with a very high amount of surface groups presents very high capacitance values. In the present study, a KOH-activated carbon with a capacitance as high as 220 F/g was prepared. Finally, the results obtained with a mesoporous sample have shown that the presence of mesopores in activated carbons with very high surface area (e.g. >2000 m2/g), do not seem to be effective for double layer capacitors.

Influence of pore size distribution on methane storage at relatively low pressure: preparation of activated carbon with optimum pore size

Activated carbons prepared by KOH activation of an anthracite were studied for methane storage applications. The effect of the different variables of the activation process (KOH/anthracite ratio, pyrolysis temperature and time) on methane storage and methane delivery was analyzed. Methane delivery was obtained in two different ways: calculated from the isotherm and measuring the volume of methane delivered from a carbon-filled vessel (5 cm3). Both methods give similar values. In addition to the well-known effect of the micropore volume and packing density, special attention was paid to the effect that the micropore size distribution has in methane storage performance. It was shown that this parameter is also a key parameter in the application of activated carbons for methane storage applications. Activated carbons prepared from a cheap raw material and using a single stage activation process have reached very high values of methane storage (155 V/V) and delivery (145 V/V).

Activated carbon monoliths for methane storage: influence of binder

Abstract In the present work, the agglomeration of a high adsorption capacity powdered activated carbon suitable for methane storage has been studied. Activated carbon monoliths have been prepared using the starting activated carbon and six different binders. Porous texture characterization of all the monoliths has been carried out by physical adsorption and helium density. Experimental methane adsorption capacity and delivery values have been obtained for all the samples. The results show that the adsorption capacities of the activated carbon monoliths are reduced with respect to the starting activated carbon. In addition to the adsorption capacity and delivery, the monolith density is also a crucial parameter for methane storage applications. This parameter has been obtained for all the samples. Moreover, the evaluation of the mechanical properties of the monoliths has been carried out with compression tests. According to our results, among all the binders studied, the one which produces monoliths with the best equilibrium between adsorption capacity and piece density has a methane delivery of 126 V/V. The important effect of the percentage of this binder in piece density and mechanical properties has been shown. Finally, a preliminary kinetic study of methane adsorption up to 4 MPa for the monoliths has shown that activated carbon monoliths do not present diffusional problems for adsorption of methane.

Characterization of pore distribution in activated carbon fibers by microbeam small angle X-ray scattering

Abstract In the present work, the results corresponding to the first experiments done with single activated carbon fibers (ACFs) at the microfocus beamline (ID13) in the ‘European Synchrotron Radiation Facility’ (Grenoble) are presented. The experiments done with CO2 and steam ACFs have demonstrated the suitability of this technique to characterize a single ACF. The experiments show that scattering intensity increases with the burn-off degree, which agrees with SAXS experiments carried out using bigger amounts of fibers. Moreover, the two-dimensional scattering patterns show that, in this type of ACFs, the porosity development during the activation process is isotropic. In addition, it has been demonstrated that the use of an X-ray microbeam of 2 μm diameter allows the characterization of different regions of the same fiber with microscopic position resolution. The scans across the fiber diameter are the first direct proof for the previous results obtained by our research group. Thus, in the case of CO2 ACFs, the scattering is high in different regions across the fiber diameter, confirming that CO2 activation takes place within the fibers, generating a quite homogeneous development of porosity. On the other hand, in the case of steam ACFs, the scattering is much higher in the external zones of the fibers than in the bulk, which means that steam focuses the activation in the outer parts of the fibers.

Investigation of Pd nanoparticles supported on zeolites for hydrogen production from formic acid dehydrogenation

Catalysts based on palladium nanoparticles supported on different zeolites (BETA, ZSM-5 and Y) were prepared and their catalytic performance in formic acid dehydrogenation was studied. The effects of the zeolite structure and porous texture on the catalytic activity were investigated by comparing the behaviour of these samples. The results revealed that the samples based on BETA zeolite are promising catalysts for this application.

Relationship between surface area and crystal size of pure and doped cerium oxides

Abstract Pure and Zr, La or Pr-doped cerium oxides were characterised by transmission electron microscopy (TEM), N 2 adsorption-desorption at −196 °C and X-ray diffraction (XRD). For crystal sizes calculation, the Scherrer and Williamson-Hall equations were compared, and the relationship between surface area and crystal size was critically discussed. It was demonstrated that the Williamson-Hall equation must be used instead of the Scherrer equation to calculate crystal sizes, since the latter equation underestimated the crystal size while the former one provided suitable values by taking into account the strain contribution to line broadening. The miscalculation of crystal size by using the Scherrer equation affected both pure and doped cerium oxides, but the size underestimation became more important for high dopant loading. Under the conditions of the BET surface area measurements, N 2 molecules were adsorbed on the surface of the single crystals identified by XRD.

Can highly activated carbons be prepared with a homogeneous micropore size distribution

From a practical point of view, microporous activated carbons with high surface area and a homogeneous micropore size distribution (MPSD) are very interesting materials. However, according to the results found in the literature, up to now, the preparation of activated carbons with high surface areas (BET>2000 m2/g) produces a widening of the microporosity in all the activation methods. In the present work, chemically activated carbons from several coal precursors have been prepared using KOH as activating agent. A deep study of the effect of the different preparation variables in the porous texture of the activated carbon has been carried out. After this analysis, the selection of the suitable coal precursor and preparation conditions has lead to obtain a highly activated carbon (BET>2000 m2/g) and a homogeneous MPSD.

Long-life vibration-free 4.5 K sorption cooler for space applications.

A breadboard 4.5 K helium sorption cooler for use in vibration-sensitive space missions was developed and successfully tested. This type of cooler has no moving parts and is, therefore, essentially vibration-free. The absence of moving parts also simplifies scaling down of the cooler to small sizes, and it contributes to achieving a very long lifetime. In addition, the cooler operates with limited dcs so that hardly any electromagnetic interference is generated. This cooler is a favorite option for future missions such as ESAs Darwin mission, a space interferometer in which the sensitive optics and detectors can hardly accept any vibration. The system design consists of a hydrogen stage cooling from 80 to 14.5 K and a helium stage establishing 5 mW at 4.5 K. Both stages use microporous activated carbon as the adsorption material. The two cooler stages need about 3.5 W of total input power and are heat sunk at two passive radiators at temperatures of about 50 and 80 K-radiators which are constructed at the cold side of the spacecraft. We developed, built, and tested a demonstrator of the helium cooler. This demonstrator has four sorption compressor cells in two compressor stages. Test experiments on this cooler showed that it performs within all specifications imposed by ESA. The cooler delivered 4.5 mW at 4.5 K with a long-term temperature stability of 1 mK and an input power of 1.96 W. So far, the cooler has operated continuously for a period of 2.5 months and has not shown any sign of performance degradation.

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.