Diego Cazorla-Amorós

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.

Advances in the study of methane storage in porous carbonaceous materials

This paper presents an overview of the results of our research group in methane storage, in which the behaviour of different carbon materials in methane storage has been studied. These materials include physically activated carbon fibres (ACFs), chemically activated carbons (ACs) and activated carbon monoliths (ACMs), all of them prepared in our laboratories. These results have been compared with those corresponding to commercial ACFs, commercial activated carbon cloths and felts, and a commercial activated carbon. An in depth analysis (different raw materials, activating agent and preparation variables) has been done in order to obtain the carbon material with the best methane adsorption capacity by unit volume of adsorbent. The important effect of the micropore volume, micropore size distribution (MPSD) and packing density of the carbon materials in the methane adsorption capacity and delivery has been analysed. After this study, activated carbons with volumetric methane uptake as high as 166 v/v and delivery of 145 v/v have been prepared. In addition, ACM with methane uptake of 140 v/v and a delivery of 126 v/v has also been obtained. Moreover, the results corresponding to preliminary in situ small angle neutron scattering (SANS) study of CD4 adsorption under pressure in different porous carbons and a zeolite are also included. These experiments have established SANS as a viable technique to investigate high-pressure methane adsorption. CD4 adsorption at supercritical conditions produces changes in the SANS curves. The changes observed are in agreement with theoretical speculations that the density of the adsorbed phase depends upon the pore size.

Structural characterization of N-containing activated carbon fibers prepared from a low softening point petroleum pitch and a melamine resin

The incorporation of heteroatoms like N in activated carbons is of interest to modify the surface chemistry of the materials and, then, to improve their behavior as catalyst or catalyst support. In this work, N-containing activated carbon fibers have been prepared using a petroleum pitch with a low softening point and an N-containing resin. The novelty of the preparation method is that it involves the steps used in the synthesis of activated carbon fibers, i.e. spinning, stabilization, carbonization and activation. The materials have been characterized with techniques such as XPS and UPS, which allows us to follow the changes in both the chemical state of N species and the valence band structure of the carbon samples during the preparation steps.

Enhanced capacitance of carbon nanotubes through chemical activation

Abstract Microporosity of pure multi-walled carbon nanotubes (MWNTs) has been highly developed using chemical KOH activation. Depending on the nanotubular material, the burn-off ranged from 20% to 45% after the activation process. At least twofold increase of surface area has been obtained with maximum values of ca. 1050 m 2 / g for KOH/C ratio of 4:1. The activated material still possesses a nanotubular morphology with many defects on the outer walls that give a significant increase of micropore volume, while keeping a noticeable mesoporosity. Such activated MWNTs have been used as electrode material for supercapacitors in alkaline, acidic and aprotic medium. Enhanced values of capacitance were always observed after activation: in some cases it increased almost seven times from 15 F/g (for non-activated nanotubes) to 90 F/g (after chemical activation).

Metal-support interaction in Pt/C catalysts. Influence of the support surface chemistry and the metal precursor

The influence of support surface chemistry and metal precursor species on the properties of Pt/C catalysts has been analyzed. The char of a phenolformaldehyde polymer is the carbon source for obtaining four different supports: three with different degree of surface oxidation and another one modified by ion-exchanged calcium. These supports have been impregnated with aqueous solutions of two platinum precursors with different ionic character: chloroplatinic acid and tetraaminplatinum chloride. The study of the system consists of a preliminary characterization of the supports (surface chemistry and textural properties), an EXAFS analysis of the fresh impregnated catalysts and the determination of platinum dispersion. The platinum precursor-support interaction, established after the impregnation step, and the platinum precursor distribution have been related to the surface chemistry of the supports and the platinum precursor used. The effect of these parameters in the final metal dispersion has also been investigated. The results obtained show that the degree of support surface oxidation has a strong influence on the distribution of the metal precursor on the support and, consequently, on the final platinum dispersion. The surface oxidation of the supports seems to have a negative effect on the platinum dispersion, independently of the platinum precursor used. Thus, the lower the number of surface oxygen complexes, the higher the metal dispersion. The reduction of precursor H2PtCl6 by interaction with the carbon has also been found to depend on the surface chemistry of the supports.

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).

The role of different nitrogen functional groups on the removal of SO2 from flue gases by N-doped activated carbon powders and fibres

Abstract SO 2 removal from flue gases by carbonaceous materials is determined by their behaviour as catalysts for SO 2 oxidation into SO 3 or H 2 SO 4 in the presence of O 2 or O 2 and steam, respectively. Previous studies have demonstrated that nitrogen (N) functional groups are active sites for the adsorption and oxidation of SO 2 , although the nature of the N groups with the higher activity had not been established yet. For this reason, in the present work a number of activated carbons (AC) and activated carbon fibres (ACF) doped with N atoms have been prepared using different methods. The number and nature of these N groups have been assessed by XPS. The materials prepared have a wide range of nitrogen content, which is distributed into different chemical species. In this way, we were able to determine the effect of the N content and the role of the different N-containing functional groups on the catalytic activity for SO 2 oxidation. The results confirm that, although the pore volume and the pore size distribution strongly influence the catalytic activity, the presence of N species at the surface increases the catalytic activity. They also demonstrate that, among the different N functional groups, pyridinic nitrogen is the most active for this reaction.

Factors controling the SO2 removal by porous carbons: relevance of the SO2 oxidation step

Activated carbons (AC) and activated carbon fibres (ACF) with different surface chemistry and porosity have been studied to analyse the SO2 retention in presence of O2 at room temperature. Samples surface chemistry was studied using temperature programmed desorption (TPD) and H2 temperature programmed reaction experiments. The porous texture was determined by CO2 and N2 adsorption isotherms at 273 K and 77 K, respectively. SO2 adsorption experiments were performed at 313 K on fresh and heat treated (N2 1173 K) samples using gas mixtures of 2000 ppm SO2 in N2 or 2000 ppm SO2/5% O2 in N2. Adsorption experiments on heat treated samples show that the presence of surface oxygen complexes impedes the SO2 adsorption and its oxidation to SO3. Additionally, no correlation has been found between the amount of SO2 adsorbed and the number of active sites created by the evolution of oxygen complexes during heat treatment. The results obtained have been explained using the fundamentals of gas adsorption for microporous solids. The SO2 uptake is analyzed considering the SO2 oxidation to SO3 as a new variable that is strongly affected by the pore size distribution. An optimum pore size exists (i.e. pore size of about 7A)in which the oxidation of SO2 to SO3 is favoured. A pore width enlargement decreases the conversion of SO2 to SO3 and, thus, the total amount of SO2 retained by the carbon sample.

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.