Behaviour of activated carbons with different pore size distributions and surface oxygen groups for benzene and toluene adsorption at low concentrations

This paper deals with the study of the effect that the porosity and the surface chemistry of the activated carbons have on the adsorption of two VOC (benzene and toluene) at low concentration (200 ppmv). In this sense, activated carbons with very different porosities and contents in oxygen surface groups have been tested. Our results regarding the effect of the porosity show that the volume of narrow micropores (size <0.7 nm) seems to govern the adsorption of VOC at low concentration, specially for benzene adsorption. Regarding the surface chemistry, AC with low content in oxygen surface groups have the best adsorption capacities. Among the AC tested, those prepared by chemical activation with hydroxides exhibit the higher adsorption capacities for VOC. The adsorption capacities achieved are higher than those previously shown in the literature for these conditions, specially for toluene. Adsorption capacities as high as 34 g benzene/100 g AC or 64 g toluene/100 g AC have been achieved.     

Role of surface chemistry on electric double layer capacitance of carbon materials

A large number of porous carbon materials with different properties in terms of porosity, surface chemistry and electrical conductivity, were prepared and systematically studied as electric double layer capacitors in aqueous medium with H₂SO₄ as electrolyte. The precursors used are an anthracite, general purpose carbon fibres and high performance carbon fibres, which were activated by KOH, NaOH, CO₂ and steam at different conditions. Among all of them, an activated anthracite with a BET surface area close to 1500 m²/g, presents the best performance, reaching a value of 320 F/g, using a three-electrode system. The results obtained for all the samples, agree with the well-known relationship between capacitance and porosity, and show that the CO-type oxygen groups have a positive contribution to the capacitance. A very good correlation between the specific capacitance and this type of oxygen groups has been found.     

The influence of various factors on aqueous ozone decomposition by granular activated carbons and the development of a mechanistic approach

The decomposition of aqueous ozone in the presence of various granular activated carbons (GAC) was studied. The variables investigated were GAC dose, presence of tert-butyl alcohol (TBA), aqueous pH as well as textural and chemistry surface properties of GAC. All the GAC tested enhanced the rate of ozone decomposition to some extent. From the analysis of experimental results it was deduced that ozone transformation into HO radicals mainly occurred in the liquid bulk through a radical chain reaction initiated by OH⁻ and ions. Hydroperoxide ions arise from the formation of H₂O₂ on surface active sites of GAC and its further dissociation. No direct relationship between textural properties of GAC and the rate of ozone decomposition was found. However, a multiple regression analysis of data revealed that basic and hydroxyl surface oxygen groups (SOG) of GAC favor the kinetics of the ozone decomposition process. It is thought that these groups are the active sites for ozone transformation into H₂O₂. Repeated used of GAC in ozonation experiments resulted in loss of basic and hydroxyl SOG with formation of carboxyl, carbonyl and lactone-type groups. Then, pre-ozonation of GAC reduces its ability to enhance the aqueous ozone transformation into hydroxyl radicals.     

Effect of pore size distribution of coal-based activated carbons on double layer capacitance

A series of coal-based activated carbons representing a wide range of mesopore content, from 16.7 to 86.9%, were investigated as an electrode in electric double layer capacitors (EDLCs) in 1 mol l⁻¹ H₂SO₄ and 6 mol l⁻¹ KOH electrolytic solutions. The activated carbons (ACs) used in this study were produced from chemically modified lignite, subbituminous and bituminous coals by carbonization and subsequent activation with steam. The BET surface area of ACs studied ranged from 340 to 1270 m² g⁻¹. The performance of ACs as EDLC electrodes was characterized using voltammetry, galvanostatic charge/discharge and impedance spectroscopy measurements. For the carbons with surface area up to 1000 m² g⁻¹, the higher BET surface area the higher specific capacitance (F g⁻¹) for both electrolytes. The surface capacitance (μF cm⁻²) increases also with the mesopore content. The optimum range of mesopore content in terms of the use of ACs studied for EDLCs was found to be between 20 and 50%. A maximum capacitance exceeding 160 F g⁻¹ and a relatively high surface capacitance about 16 μF cm⁻² measured in H₂SO₄ solution were achieved for the AC prepared from a sulfonated subbituminous coal. This study shows that the ACs produced from coals exhibit a better performance as an electrode material of EDLC in H₂SO₄ than in KOH electrolytic solutions. For KOH, the capacitance per unit mesopore surface is slightly lower than that referred to unit micropore surface (9.1 versus 10.1 μF cm⁻²). However, in the case of H₂SO₄ the former capacitance is double and even higher compared with the latter (23.1 versus 9.8 μF cm⁻²). On the other hand, the capacitance per micropore surface area is the same in both electrolytes used, about 10.0 μF cm⁻².     

Surface-related capacitance of microporous carbons in aqueous and organic electrolytes

This paper examines the surface-related capacitance C/S of carbons with average micropore widths between 0.73 and 1.80 nm, using 1–2 M H₂SO₄, 6 M KOH and 1 M (C₂H₅)₄NBF₄ in acetonitrile as electrolytes. Following corrections for pseudocapacitance effects for the aqueous electrolytes and the use of an average surface area suggested by independent techniques, different from the BET area, it appears that C/S is practically independent of the micropore width. The analysis of the data with the help of recent models suggests that the dielectric constants ?_r of the different electrolytes may decrease with the pore size. It is surprising that the coexistence of two sets of values for the surface area of microporous carbons and its consequence on C/S have not received more attention in the past.     

Synthetic carbons activated with phosphoric acid III. Carbons prepared in air

Synthetic activated carbons were prepared by phosphoric acid activation of a styrene-divinylbenzene copolymer in an air atmosphere at various temperatures in the 400-900 °C interval. The carbons were characterized by elemental analysis, cation-exchange capacity measurement, infrared spectroscopy, potentiometric titration, copper adsorption from solution and physical adsorption of N₂ at −196 °C and CO₂ at 0 °C. It was shown that, similarly to synthetic phosphoric acid activated carbons obtained in argon, the synthetic carbons activated with phosphoric acid in air possess an acidic character and show considerable cation-exchange properties. The contribution of oxygen-containing surface groups along with phosphorus-containing groups to CEC is higher for carbons obtained in air. Three types of surface groups were identified on carbons prepared at temperatures up to 600 °C, and four types on carbons prepared at higher temperatures. These groups were assigned to ‘super-acidic’ (pK<0), phosphorus-containing (pK=1.1-1.2), carboxylic (pK=4.7-6.0) and phenolic (pK=8.1-9.4) groups. The cation-exchange capacity was at a maximum for the carbon prepared at 800 °C. Copper adsorption by synthetic phosphoric acid activated carbons obtained in air at temperatures lower than 800 °C is higher than for similar carbons obtained in argon. The increase is due to additional formation of oxygen-containing surface groups. Calculated copper binding constants revealed the importance of phosphorus-containing and carboxylic groups for adsorption of copper from aqueous solution. All carbons show a multimodal pore size distribution including simultaneously micropores and mesopores, but the porous texture is not a prime factor in determining the cation-exchange capacities of these carbons. Synthetic phosphoric acid activated carbons show a greater development of porosity when obtained in air as compared to carbons carbonized in argon.     

Influence of carbon-oxygen surface complexes on the surface acidity of tungsten oxide catalysts supported on activated carbons

Tungsten oxide catalysts supported on activated carbons were prepared by using tungsten hexacarbonyl, ammonium tungstate, and tungsten pentaethoxide as precursors. An activated carbon was obtained from olive stone by physical activation. A portion of this activated carbon was oxidized with ammonium peroxydisulfate in order to introduce different oxygen surface complexes. Subsequently, different portions of this oxidized activated carbon were heat treated in nitrogen flow at various temperatures to partially remove the oxygen surface complexes. In this way, activated carbons with different amounts of oxygen surface complexes were obtained, which were then used as supports for the tungsten oxide catalysts. Both the supports and the supported catalysts were pre-treated either in He, dry air or wet air flow at 623 K for 6 h. They were then characterized by X-ray photoelectron spectroscopy, X-ray diffraction, measurements of the pH of the point of zero charge, and activity in the decomposition reaction of isopropanol. Turnover frequencies for the formation of propene were obtained. According to these results, the oxygen surface complexes of the support have a major influence on the total acidity of the tungsten oxide supported catalysts. In some supported catalysts, W(VI) was reduced to W(V) during the decomposition reaction of isopropanol as a consequence of the hydrogen evolution. The results indicate that oxygen surface complexes of the support may play an important role in this reduction process, which was inhibited when the support had high surface oxygen content.     

The influence of the texture and surface properties of carbon adsorbents obtained from biomass products on the adsorption of manganese ions from aqueous solution

Studies on the adsorption of manganese ions from aqueous solution on carbon obtained from a mixture of biomass products indicate the importance of acidic surface oxides for manganese ion adsorption that is predominantly site specific. The results show that oxygen remaining from the raw material participates in the formation of surface oxides and indicates the possibility of controlling the content of acidic surface sites of the carbon surface by appropriate selection of the precursor composition and surface properties modification. The surface functionalities of oxidized carbon from a mixture of biomass products resembles the behavior of an ion-exchange resin. Oxidized carbon obtained from a 50:50 mixture of tar from steam pyrolysis of apricot stones and furfural contains a balance of surface area and high surface concentration of functional groups favorable for adsorption of positively charged manganese ions.     

The performance of electric double layer capacitors using particulate porous carbons derived from PAN fiber and phenol-formaldehyde resin

Activated carbon fibers are known to contain pores with a small resistance for electrolyte migration while possessing a large electrical resistance between the fibers. A carbon powder derived from pulverization of PAN-based carbon fibers was examined as an electrode for electric double layer capacitors using H₂SO₄ as the electrolyte solution. The performance of conventional-type activated carbon powders derived from phenol-formaldehyde resin char was also measured for comparison. The fiber-derived carbon exhibited an electrical resistance comparable to that of the conventional carbons while showed a larger specific capacitance as well as a lesser extent of capacitance decrease at high currents due to a smaller pore resistance. An ultimate capacitance as high as 290 F g⁻¹ can be reached for this fiber-derived carbon powder (with a BET surface area of ≈1300 m² g⁻¹). This large capacitance value was suggested to be associated with the high activity feature of the pore wall.     

Removal efficiency of radioactive methyl iodide on TEDA-impregnated activated carbons

Activated carbons were prepared by different series of carbon dioxide and steam activation from walnut shells for their optimal use as radioactive methyl iodide adsorbents in Nuclear Plants. The knowledge of the most favourable textural characteristics of the activated carbons was possible by the previous study of the commercial activated carbon currently used for this purpose. In order to increase their methyl iodide affinity, the effect of triethylenediamine impregnation was studied at 5 and 10 wt.%. The results obtained indicated that in both cases the adsorption efficiency is markedly improved by the addition of impregnant, which allows the adsorbate uptake to occur not only by physical adsorption, via non-specific interactions (as in non-impregnated carbons) but also by the specific interaction of triethylenediamine with radioactive methyl iodide. Methyl iodide retention efficiencies up to 98.1% were achieved.     

Surface chemistry and contrast-modified SAXS in polymer-based activated carbons

The adsorption of non-polar and polar molecules, n-hexane and water, on activated carbons, functionalized by oxidation with concentrated nitric acid, is investigated by small angle X-ray scattering. A relative mass density function p(q) is introduced in order to trace the filling characteristics of these probe molecules in the pore structure. Inspection of p(q) shows that while the pores affected by the applied relative pressure are completely filled by n-hexane, pore filling by water is only partial, even in the most oxidized carbon. The differences between the solid-liquid contact areas of these two molecules adsorbed from the vapour phase on the various carbons illustrate the influence of surface chemistry and molecular polarity on the contrast-modified SAXS response.     

Production of fibrous activated carbons from natural cellulose (jute, coconut) fibers for water treatment applications

Different fibrous activated carbons were prepared from natural precursors (jute and coconut fibers) by physical and chemical activation. Physical activation consisted of the thermal treatment of raw fibers at 950 °C in an inert atmosphere followed by an activation step with CO₂ at the same temperature. In chemical activation, the raw fibers were impregnated in a solution of phosphoric acid and heated at 900 °C in an inert atmosphere. The characteristics of the fibrous activated carbons were determined in the following terms: elemental analysis, pore characteristics, SEM observation of the porous surface, and surface chemistry. As the objective of this study was the reuse of waste for industrial wastewater treatment, the adsorption properties of the activated carbons were tested towards pollutants representative of industrial effluents: phenol, the dye Acid Red 27 and Cu²⁺ ions. Chemical activation by phosphoric acid seems the most suitable process to produce fibrous activated carbon from cellulose fiber. This method leads to an interesting porosity (S_BET up to 1500 m² g⁻¹), which enables a high adsorption capacity for micropollutants like phenol (reaching 181 mg g⁻¹). Moreover, it produces numerous acidic surface groups, which are involved in the adsorption mechanisms of dyes and metal ions.     

Adsorption of phenol onto activated carbons having different textural and surface properties

Adsorption of phenol in aqueous phase onto activated carbons (ACs) having different textural and surface properties has been considered. Six types of ACs were used: three were commercial, and three were obtained from Kraft lignin chemically activated with sodium hydroxide, potassium hydroxide or ortho-phosphoric acid. The apparent surface areas of the commercial ACs varied from 620 to 1320 m²/g, while ACs made from lignin presented surface areas as high as 1300 m²/g and 2900 m²/g when prepared with H₃PO₄ and alkaline hydroxides, respectively; moreover, the highest proportion of microporosity was found for ACs derived from lignin. A kinetic study was carried out, showing that the phenol adsorption data may be correctly adjusted, for all the ACs tested, by an equation corresponding to a pseudo second-order chemical reaction. Freundlich, Langmuir and Tempkin equations were tested for modelling the adsorption isotherms at equilibrium, and it was concluded that Langmuir model fitted adequately the experimental data. However, Tempkin model fitted even better the adsorption data obtained with ACs derived from lignin activated with alkaline hydroxides, which are characterized by the highest number of surface groups. Remarkably high phenol adsorption capacities were found for the ACs prepared by activation of Kraft lignin with NaOH and KOH: 238 and 213 mg/g of AC, respectively. Finally, the adsorption of phenol was found to depend not only on the micropore volume, but also on the total amount of carbonyl and basic groups and on the ratio of acid to basic groups.     

Diesel fuel desulfurization with hydrogen peroxide promoted by formic acid and catalyzed by activated carbon

Desulfurization of diesel fuels with hydrogen peroxide was studied using activated carbons as the catalysts. Adsorption and catalytic properties of activated carbons for dibenzothiophene (DBT) were investigated. The higher the adsorption capacity of the carbons is, the better the catalytic performance in the oxidation of DBT is. The effect of aqueous pH on the catalytic activities of the activated carbons was also investigated. Oxidation of DBT is enhanced when the aqueous pH is less than 2, and addition of formic acid can promote the oxidation. The effect of carbon surface chemistry on DBT adsorption and catalytic activity was also investigated. Adsorption of DBT shows a strong dependence on carboxylic group content. The oxidative removal of DBT increases as the surface carbonyl group content increases. Oxidative desulfurization of a commercial diesel fuel (sulfur content, 800 wt. ppm) with hydrogen peroxide was investigated in the presence of activated carbon and formic acid. Much lower residual sulfur content (142 wt. ppm) was found in the oxidized oil after the oxidation by using the hydrogen peroxide-activated carbon-formic acid system, compared with a hydrogen peroxide-formic acid system. The resulting oil contained 16 wt. ppm of sulfur after activated carbon adsorption without any negative effects in the fuel quality, and 98% of sulfur could be removed from the diesel oil with 96.5% of oil recovery. Activated carbon has high catalytic activity and can be repeatedly used following simple water washing, with little change in catalytic performance after three regeneration cycles.     

Chemical and electrochemical characterization of porous carbon materials

Chemical and electrochemical techniques have been used in order to asses surface functionalities of porous carbon materials. An anthracite has been chemically activated using both KOH and NaOH as activating agents. As a result, activated carbons with high micropore volume (higher than 1 cm³/g) have been obtained. These samples were oxidized with HNO₃ and thermally treated in N₂ flow at different temperatures in order to obtain porous carbon materials with different amounts of surface oxygen complexes. Thermal treatment in H₂ was also carried out. The sample treated with H₂ was subsequently treated in air flow at 450 °C. Thus, materials with very similar porous texture and widely different surface chemistry have been compared. The surface chemistry of the resulting materials was systematically characterized by TPD experiments and XPS measurements. Galvanostatic and voltammetric techniques were used to deepen into the characterization of the surface oxygen complexes. The combination of both, chemical and electrochemical methods provide unique information, regarding the key role of surface chemistry in improving carbon wettability in aqueous solution and the redox processes undergone by the surface oxygen groups. Both contributions are of relevance to understand the use of porous carbons as electrochemical capacitors.