Influence of microporosity of activated carbons as a support of polyoxometalates

H₃[PMo₁₂O₄₀] (PMo) was supported on four activated carbons (AC) in order to analyze the influence that the porous texture of AC exercises in the adsorption of PMo, trying to study where they are adsorbed and how their adsorption modifies the porous texture of AC. The amount of PMo adsorbed on AC follows a clear correlation with the porous texture, microporosity being the best evidence for this. Such a correlation indicates that PMo is only adsorbed in supermicroporosity as a consequence of the size of PMo, which limits the entrance on the smallest micropores, about 0.8 nm. XRD results indicate that the PMo is adsorbed on the micropore wall, that it is highly dispersed and that it does not develop any crystalline phase.     

On the determination of surface areas in activated carbons

The paper examines the validity of two approaches frequently used to determine surface areas in activated carbons, namely the BET method and the use of immersion calorimetry. The study is based on 21 well characterized carbons, whose external and microporous surface areas, S_e and S_mi, have been determined by a variety of independent techniques. It appears clearly that S_BET and the real surface area S_mi + S_e are in agreement only for carbons with average pore widths L_o around 0.8-1.1 nm. Beyond, S_BET increases rapidly and S_BET − S_e is practically the monolayer equivalent of the micropore volume W_o. This confirms that a characterization of surface properties based on S_BET is, a priori, not reliable. The study of the enthalpy of immersion of the carbons into benzene at 293 K, based on Dubinin’s theory, shows that Δ_iH consists of three contributions, namely from the interactions with the micropore walls (−0.136 J m⁻²), the external surface (−0.114 J m⁻²), and from the volume of liquid found between the surface layers in the micropores (−141 J cm⁻³). It appears that for carbons where L_o > 1 nm, the real surface area cannot be determined in a reliable way from the enthalpy of immersion and a specific heat of wetting alone.     

Role of microporosity and surface chemistry in adsorption of 4,6-dimethyldibenzothiophene on polymer-derived activated carbons

Two carbon samples derived from poly(4-styrenesulfonic acid-co-maleic acid) based polymer by carbonization between 700 and 800 °C were oxidized to two different levels of surface acidity. The surfaces of resulting adsorbents were characterized by potentiometric titration, adsorption of nitrogen, FTIR, SEM/EDAX and thermal analysis. The materials were used as adsorbents of 4,6-dimethyldibenzothiophene (4,6-DMDBT) from hexadecane with initial concentration of sulfur between 10-150 ppmw. Although it was found that pores with diameter less than 10 Å govern the amount of 4,6-DMDBT adsorbed, that amount is enhanced when acidic groups are present in the larger pores owing to the contributions of specific interactions. Surface chemistry plays an important role in reactive adsorption and deposition of the products of surface reactions in the pore system.     

Removal of organic impurities with activated carbons for ultra-pure hydrogen peroxide preparation

The aim of this research was to select suitable activated carbon (AC) for effective removal of organic impurities from industrial aqueous hydrogen peroxide solution to produce ultra-pure hydrogen peroxide. The textural parameters and surface chemistry of four kinds of AC samples were measured and analyzed. Static and dynamic equilibrium adsorption experiments were carried out to compare the effect of AC on organic impurities adsorption and hydrogen peroxide decomposition. The effects of AC pore structure and surface chemistry on the adsorption of organic impurities were investigated. The fitting operation conditions, i.e., operating temperature and AC dosage, were also examined. The results showed that AC adsorption capacity on organic impurities from industrial aqueous hydrogen peroxide solution was mainly influenced by micropore structure of AC, as well as decomposition of hydrogen peroxide. The pore size of 1–3 nm is most effective for adsorption of organic impurities. It was found that the organic impurities in industrial hydrogen peroxide solution could be reduced effectively to meet the standard of ultra-pure hydrogen peroxide of SEMI-C8 level with the proposed AC and adsorption techniques.     

Hydrogen production by catalytic decomposition of methane over activated carbons: Deactivation study

The amount of deposited carbon on activated carbons was adjusted by varying the space time and the time on stream. Carbon nuclei formation appeared to occur initially but was terminated soon and then the carbon crystallite growth became dominant. The methane decomposition rate over activated carbons had a nearly linear relationship with the amount of carbon deposited. This suggests that the carbon deposition occurs uniformly and the activity decreases due to pore blocking, resulting in loss of accessibility to the active sites. The rate was also nearly proportional to the surface area for the same kind of activated carbon, which is contrasted with the previous finding that no discernible trend was observed between the initial activity and the surface area among different kinds of the activated carbons.     

高比表面积活性炭吸附存储天然气性能研究

天然气中少量乙烷和丙烷的存在会直接影响活性炭对天然气的吸附存储容量。为此,体积法测定了高比表面积活性炭对甲烷、乙烷和丙烷的吸附等温线,吸附温度分别为283,293,303和313K;采用Langmuir-Freundlich(LF)方程拟合吸附等温线,得到各气体的方程参数,进而采用LRC关联式预测多组分吸附平衡数据,并计算活性炭对模拟天然气的存储能力。结果表明:活性炭对3种气体的吸附等温线都属于I型等温线,采用L-F方程可以很好地描述各气体的吸附等温线;高比表面积活性炭对模拟天然气的存储量随吸附温度的升高而显著降低,在吸附存储压力为3.5 MPa,吸附温度从283 K上升到313 K,相应的存储量(体积比)由139降低为103;与纯甲烷的吸附存储相比,模拟天然气的吸附储量(体积比)提高约20。     

Adsorption from binary and ternary mixtures on activated carbons containing different amounts of chemically bonded oxygen

The adsorption process from the gas and liquid phase on activated carbons was investigated. Unmodified and chemically modified activated carbon Norit RKD-3 with different contents of chemisorbed oxygen were used. The surfaces were characterized by their content of surface functional groups, and the pore structure was characterized on the basis of adsorption-desorption isotherms of benzene vapor. Surface excess isotherms from binary and ternary mixtures of dioxane, n-heptane, and benzene were also determined. The influence of the chemical composition of the carbon surface on the adsorption from the gaseous and liquid phase is discussed.     

Application of different equations to adsorption isotherms of phenolic compounds on activated carbons prepared from cork

Activated carbons were prepared from solid cork wastes by physical activation with carbon dioxide or steam, and chemical activation by impregnation with phosphoric acid. In this work we show the possibility of using these activated carbons for the adsorption of phenolic compounds from the aqueous phase. The materials present a different response to the adsorptives used (p-nitrophenol, p-chlorophenol, p-cresol and phenol), depending on the type of activation and the parameters (burn-off, absolute concentration) used in each case. All the samples were capable of retaining the contaminants, with the best result being reached by the sample with higher burn-off and the worst with the carbonised, while intermediate values were reached with the remaining samples. The experimental isotherms were analysed with two and three parameters equations (Freundlich, Langmuir, Dubinin-Radushkevich-Kaganer and Redlich-Peterson). The results obtained from the application of the equations are similar in some aspects, but the degree of confidence is quite different. The best fit was achieved with the Redlich-Peterson equation, which can be explained by the fact that this has three adjustable parameters. However, overall the Freundlich and DRK equations appear to be more useful and provide parameters which can be correlated with the structural characteristics of the solids obtained from N₂ adsorption measurements.     

Production of microporous palm shell based activated carbon for methane adsorption: Modeling and optimization using response surface methodology

In this study, the optimization of the palm shell based activated carbon production using combination of chemical and physical activation for methane adsorption is investigated. response surface methodology (RSM) in combination with central composite design (CCD) was used to optimize the operating parameters of the production process. Physical activation temperature, chemical impregnation ratio and physical activation time were chosen as the main process variables and the amount of methane adsorption was selected as the investigated response. Phosphoric acid and carbon dioxide were used as chemical and physical agents, respectively. The optimum reaction conditions were found to be a physical activation temperature of 855 °C, H₃PO₄ impregnation ratio of 9.42 g of phosphorous per gram palm shell and physical activation time of 135 min. The results exhibited significant increase in methane adsorption after physio-chemical activation.     

Formulation of activated carbons and evaluation of methane storage by compression and adsorption

Activated carbons were produced from coconut shells by chemical activation using H₃PO₄ (CAQ1) and ZnCl₂ (CAQ2), and used as adsorbents for natural gas storage. CAQ1 presented a wider pore size distribution than CAQ2. Storage evaluations were made in a semi‐pilot unit (0.5–4.0 MPa and 298–313 K). According to the used reactant, at 313 K and 3.0 MPa, the adsorptive capacities were 0.060 g/g (CAQ2) and 0.092 g/g (CAQ1). The shortest storage time to charge the tank to 4.0 MPa was obtained by CAQ2 (3.1 min), compared to CAQ1 (3.4 min), due to its higher specific heat capacity. © 2011 Canadian Society for Chemical Engineering     

Controlling carbon microporosity: the structure of carbons obtained from different phenolic resin precursors

Carbons were prepared from resins synthesised using the phenolic precursors phenol, para methylphenol, para ethylphenol, para n-propylphenol, para isopropylphenol and 3,5-dimethylphenol. Loss of phenolic OH from these materials was followed using solid-state nuclear magnetic resonance. The surface areas of the carbons were determined using N₂ and CO₂ adsorption. No significant differences in the loss of phenolic OH were observed. Under the same carbonisation conditions, the para alkyl phenols gave carbons with wide micropores, while the phenol and 3,5-dimethylphenol gave carbons with narrow micropores. Grinding the cured resins prior to carbonisation was found to significantly increase the surface area of the carbons obtained, with the microporous surface area increasing rapidly with a fall in particle size, without a significant increase in burn-off. Higher carbonisation temperatures widened the micropore size distribution, as shown by fitting the CO₂ adsorption isotherm with the Dubinin-Astakhov equation. The ability to change the carbon micropore structure obtained from a simple, well defined precursor, has many potential applications in carbon molecular sieves, catalyst supports and the investigation of adsorption processes.     

Role of activated carbon structural properties and surface chemistry in mercury adsorption

In this work the performance of activated carbons prepared from raw and demineralised lignite for gas-phase Hg° removal was evaluated. A two-stage activation procedure was used for the production of the activated samples. In order to study the effect of mineral matter on pore structure development and surface functionality of the activated carbons, a demineralisation procedure involving a three-stage acid treatment of coals, was used, prior to activation. Hg° adsorption tests were realized in laboratory-scale unit consisted of a fixed-bed reactor charged with the tested activated samples. The examined adsorbent properties that may affect removal capacity were the pore structure, the surface chemistry and the presence of sulphur on the surface of activated carbons. The obtained results revealed that activated carbons produced from demineralised lignite posses a high-developed micropore structure with increased total pore volume and BET surface area. These samples exhibit enhanced Hg° adsorptive capacity. In all cases, mercury removal efficiency increased by sulphur addition. Finally, the starting material properties and activation conditions affect the concentration and the type of the oxygen groups on activated carbon surface, that have been determined with TPD-MS experiments.     

Effect of adsorption of nitroaromatic compounds on the characteristics of bound water layers in aqueous suspensions of activated carbons

The impact of chemical treatment (oxidation, reduction) of microporous carbon prepared from plum stones on the structural parameters was analysed using the nitrogen adsorption method. The effects of pre-adsorption of 2,4,6-trinitrotoluene (TNT) and 1-chloro-2,4-dinitrobenzene (CDNB) on the characteristics of the interfacial water layers in the aqueous suspensions of these materials were explored using ¹H NMR spectroscopy with freezing out of the bulk water between 210 and 273 K. Substantial variations in the Gibbs free energy of the interfacial water in the aqueous suspension of treated carbons were observed, especially for a reduced sample with pre-adsorbed nitroaromatics, due to changes in the hydrophilic-hydrophobic properties of the systems.     

Liquid-phase adsorption of dyes and phenols using pinewood-based activated carbons

Physical properties of activated carbons prepared from pinewood at different activation times (0.5, 1.5, 2.7, and 4.0 h) in steam at 900 °C were studied. The adsorption equilibria and kinetics of three dyes and three phenols (phenol, 3-chlorophenol, and o-cresol) from aqueous solutions on such carbons were then examined at 30 °C. The adsorption isotherms of phenols could be well fitted by the Freundlich equation, and those of dyes were adequately described by the Langmuir-Freundlich equation. The effect of microporosity of the carbons on adsorption capacity was explored. Four simplified kinetic models including pseudo-first-order equation, pseudo-second-order equation, intraparticle diffusion model, and the Elovich equation were selected to follow the adsorption processes. The adsorption of all six adsorbates could be best described by the Elovich equation. The kinetic parameters of this best-fit model were calculated and discussed.     

Micropore structure of activated carbons and predictions of adsorption equilibrium

A method for calculating micropore size distributions based on a molecular model of adsorption and analytical solution for the adsorption isotherm is presented in this study. Micropore volume filling is considered to be an evolution of two-dimensional condensation, which occurs on micropore walls at the critical condensation pressure. The treatment of adsorption isotherms of propane, propylene, acetylene, ethylene, cyclopentane and benzene shows that the method offers a quantitative approach to determining a reliable carbon texture, which is independent of adsorbate employed and adsorption temperature. The invariant parameters of porous structure coupled with molecular constants of adsorbate provide a prediction of the adsorption equilibrium in a wide range of pressures. Good agreement between experimental and calculated data is demonstrated for adsorption of both gases and vapors.     

Effect of activated carbons derived from different precursors on the hydrogen sorption properties of magnesium

The absorption-desorption characteristics towards hydrogen of obtained by ball milling under argon atmosphere magnesium based composites (containing respectively 95 wt% Mg and 5 wt% activated carbon (AC) derived from bean pods (BP), apricot stones (AS) and mixture of coal tar pitch and furfural (CTPF)), were investigated. Actually there is no substantial difference between the rate of hydriding and the absorption capacity for magnesium composite with AC derived from apricot stones, and for magnesium composite with AC derived from a mixture of coal tar pitch and furfural — the capacity is 6.13 wt% for the former and 5.98 wt% for the latter. More significant difference between the investigated composites was observed for the dehydriding process. The magnesium composite with synthetic carbon (from furfural and coal tar pitch) desorbs at 623 K and 0.15 MPa and the absorbed hydrogen about twice faster than magnesium composite with AC from bean pods. The activated carbon derived from a mixture of coal tar pitch and furfural has the highest specific surface area, and obviously this characteristic has important influence especially on the dehydriding kinetics of magnesium.     

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.     

On the methane adsorption capacity of activated carbons: in search of a correlation with adsorbent properties

BACKGROUND: There exists now a widely held view that the methane storage capacity on an activated carbon is not related to any of the routinely determined properties of the adsorbent, such as surface area or micropore volume. This has been confirmed and a correlation pursued with other physical and/or chemical properties of both commercially available carbons and those prepared in the laboratory. Textural characteristics (from nitrogen adsorption isotherms at 77 K) considered were BET‐equivalent specific surface area, DR micropore volume and Horvath–Kawazoe micropore size distribution. Chemical properties were evaluated using Fourier transform infrared (FTIR) spectroscopy, thermal programmed decomposition (TPD) and Boehm titrations. Both kinetic and equilibrium methane adsorption experiments were performed at 273 and 298 K and up to 3.5 MPa. RESULTS: Using phosphoric acid to activate peach stones together with additional thermal treatment enabled the production of activated carbons with 137 v/v methane adsorption capacity at 298 K. CONCLUSIONS: The presence of acidic surface functional groups has a detrimental influence on methane uptake, due to the chemical inertness of the adsorbate and/or to pore blockage of the adsorbent. Basic surface functional groups (pyrone), together with a desirable pore size distribution centered at ca 0.8 nm, are thought to be responsible for improved methane adsorption capacity on such activated carbons. Copyright © 2009 Society of Chemical Industry     

Water adsorption by activated carbons in relation to their microporous structure

The present paper examines the adsorption of water by microporous carbons in the absence of specific interactions. The modelling of water adsorption for 293 and 310 K, using variable pore size distributions (PSD), shows that the type V isotherms follow the Dubinin-Astakhov (DA) equation and fulfill the requirement for temperature invariance. Furthermore, the parameters of the DA equation can be related in a simple way to structural properties of the model carbons. For a number of well-characterized carbons, the type V isotherms generated by combining model isotherms with the corresponding PSDs are in good agreement with the limiting isotherms at 293 and 310 K derived on the basis of a recent development of Dubinin’s theory. This approach will provide the basis for further studies including specific interactions.