Evaluation of slitlike porosity of carbon adsorbents

Deviations of a pore shape from a slitlike one were analyzed for a variety of carbon adsorbents on the basis of calculations of pore size distributions (PSDs) with respect to the pore volume (f_V(x)) used for estimation of the PSDs related to the specific surface area (f_S(x)) applying several models of pores: individual slitlike pores and mixtures of slitlike and cylindrical pores and gaps between spherical particles. The use of complex pore models allows us to diminish the difference between S_BET and the specific surface area (S_sum) calculated from f_S(x).     

Modelling of water adsorption by activated carbons: effects of microporous structure and oxygen content

The present paper examines the adsorption of water by microporous carbons containing various amounts of surface oxygen and a smaller proportion of basic centres. The modelling of water adsorption for 293 and 310 K, using variable pore size distributions (PSD), confirms that the overall type IV isotherm is the sum of a type I isotherm associated with the specific interactions, and a type V isotherm reflecting the non-specific interactions. The principle of temperature invariance is followed by these isotherms, which indicates that modelling leads to the Dubinin-Astakhov equation.The present approach allows the prediction of water adsorption near room temperature, on the basis of the PSD and the density of oxygen present on the surface area of the micropores. It is assumed, to a first and good approximation, that the pores are slit-shaped and the oxygen distribution is random.     

Radial distribution of porosity in spherical activated carbon particles

The radial distribution of porosity in spherical activated carbon particles and its relationship to both activation temperature and conversion of the reaction C-CO₂ are discussed. Char spheres, obtained by pelletizing Quercus agrifolia powdered char, were activated using CO₂ as activating agent at 820 and 860 °C. For both temperatures, the gasification reaction was run until the desired conversion (30, 50 and 70%) was reached. The activation process was performed using TGA apparatus. The porosity of the samples was measured by physical adsorption of nitrogen at −196 °C. BET and Dubinin-Raduskhevich equations were used to analyse the results. From each activated carbon sphere several samples were prepared, corresponding to different layers of the sphere; an outer layer, a middle layer and the core of the particle. From these results, the radial porosity evolution as a function of activation temperature and reaction conversion was obtained and a mathematical expression that correlates pore volume evolution and reaction conversion is proposed.     

Overcoming calcium catalysis during the thermal reactivation of granular activated carbon: Part II. Variation of steam-curing reactivation parameters

In order to overcome the deleterious effects of calcium catalysis during thermal reactivation, the authors have developed a methodology for first steam-curing spent GAC at 548-748 K, and then ramping the furnace temperature to 1023-1223 K while exposing the GAC to flowing N₂. In this article, the authors evaluated the influence of an array of parameters on the steam-curing plus ramped-temperature protocol that included curing time, curing temperature, ramped-to temperature and steam flow rate. Pore size distribution (PSD) measurements employed the density functional theory (DFT), and these revealed that the steam-curing time had the greatest influence on pore size distribution: increasing the steam-curing time from 15 to 60 min increased the <500 Å cumulative pore volume by ca. 10% and the 5.4 to 32 Å pore volume by ca. 12%. Several of the other process parameters exhibited only a slight effect on PSD. Furthermore, a spent GAC that first experienced the steam-curing and ramped-temperature protocol and then experienced acid washing had identical micropore volume as a spent GAC that first experienced acid washing and then experienced conventional reactivation. This confirmed that the steam-curing protocol overcame calcium catalysis and its destruction of microporosity.     

Co-adsorption of n-butane/water vapour mixtures on activated carbon fibre-based monoliths

Water-n-butane co-adsorption experiments on Activated Carbon Fibre based Monoliths (ACFMs) under equilibrium and dynamic conditions were performed at 30 °C. ACFMs proved to be better adsorbents than active carbon particles for the dry adsorption of n-butane at low concentration levels. The presence of water vapour in the gas stream has a negligible influence on n-butane adsorption for values of relative humidity (RH) equal to or below 25%. Higher water pressures cause a significant loss of adsorption capacity in ACFMs. Equilibrium water adsorption on ACFMs can be described by the Do and Do model [Carbon 38 (2000) 767] modified in order to account for both the variable size of the water aggregates on acid centres and the variable size of the clusters detached from the aggregates and inserted into the micropores. The model results show close agreement between the amounts of active sites for adsorption calculated by the model and the number expected from subjecting the ACFMs to different pre-treatments.The equilibrium of co-adsorption of water and n-butane can be described by a pore filling concept according to which the volume occupied by both gases adsorbed together from the gas mixture is approximately equal to the volume occupied by the pure component adsorbed separately to a greater extent.Pre-moistening of the monolith has little effect on its effective n-butane adsorption capacity (breakthrough time) during dynamic adsorption experiments. For values of relative humidity equal to or below 25%, the effective n-butane adsorption capacity of the monoliths was better than that of active carbon powder. Moreover, the monoliths produced a much lower pressure drop in the system. At these conditions, adsorption efficiency for the ACFMs never showed values below 85%.     

Comparison of molecular sieve properties in microporous chars from low-rank bituminous coal activated by steam and carbon dioxide

A Polish high volatile bituminous coal was subjected to air oxidation, carbonization and gaseous activation. The activation with steam and carbon dioxide was performed to low levels of burn-off: 5-25%. Sorption measurements of CO₂, as well as of organic vapours with increasing molecular sizes (CH₂Cl₂, C₆H₆, C₆H₁₂, CCl₄) were applied to evaluate the porous structure of the activated chars. Steam and carbon dioxide develop the microporous system according to the same mechanism—opening (burn-off 5-10%) and then widening of the narrow micropores. For char from the oxidized coal mainly a widening of the narrow micropores takes place. Comparing both activating agents, it was stated that for steam greater micropore volumes were obtained. This was confirmed by other authors for chars from brown coal and coking coal, but was in disagreement with the results for olive stones and carbon fibres. This would indicate the importance of the carbon precursor in the formation of the porous structure of carbon materials by different activating agents. In the region of studied burn-offs, among the micropore sizes useful for separation of gases and vapours with small molecules, micropore volumes with widths close to 0.4-0.5 nm are dominating. At very low burn-offs (5-10%), steam activation renders greater micropore volumes within these sizes, than does activation with carbon dioxide. But with increasing burn-off (15-25%), this phenomenon becomes reversed. This effect is still more accentuated for the preoxidized coal.     

Adsorption of volatile organic compounds by means of activated carbon fibre-based monoliths

Activated carbon fibre monoliths (ACFMs) were prepared from the rejects of polymeric fibres (Nomex™). These were carbonised, agglomerated with a phenolic resin and steam activated at burnoff degrees between 0 and 40%. Adsorption experiments with n-butane at 30 °C show that, at high adsorbate concentrations, the amount adsorbed is a function of pore volume, but at low concentrations this mainly depends on pore size distribution. The porosity of Nomex-based ACFMs is formed by narrow micropores, which permit higher amounts of vapour to be adsorbed in low concentrations compared to monoliths prepared from different commercial activated fibres and a commercial granular activated carbon, which exhibits wider pores. The agglomeration of Nomex-fibres to form ACFMs does not cause any loss in adsorption properties with respect to non-agglomerated activated fibres. From the adsorption experiments of different vapours on a Nomex-based ACFM (40% burnoff) it was found that at high concentrations (p/p_o=1) the adsorbed volume was independent of the nature of the adsorbate and depended only on pore volume. However, at low vapor concentrations (p/p_o=0.004), the amount adsorbed depended on the adsorbate being well correlated to the molecular parachor and the polarizability of the adsorbates     

Kinetics of 1,3,6-naphthalenetrisulphonic acid ozonation in presence of activated carbon

Processes based on the simultaneous use of ozone and activated carbon have proven very effective for removing contaminants of high toxicity and low biodegradability. The present study is aimed to determine the kinetic constants involved in this purification process and their relationship with the surface chemistry of the activated carbon. For this purpose, the ozonation of 1,3,6-naphthalenetrisulphonic acid (NTS), selected as model compound, was carried out in the presence of different activated carbons. Determination of the Weisz-Prater parameter (C_WP) revealed that intraparticular diffusion limitations exist in the system for particles >500 μm. The degradation kinetics of NTS in the presence of activated carbon depends on the concentrations of both, the contaminant and the dissolved ozone, with a global reaction order of 2. The heterogeneous reaction constants were determined using a model that allowed quantification of the capacity of the activated carbon to increase the NTS degradation rate and of the chemical surface properties responsible for this increase. The basicity of the activated carbon is mainly responsible for the catalytic activity of the carbon in NTS ozonation, even though, mineral matter contributes positively to the catalytic activity.     

Catalytic oxidation of Fe(II) by activated carbon in the presence of oxygen.: Effect of the surface oxidation degree on the catalytic activity

The catalytic oxidation of Fe(II) species in aqueous solution by activated carbons with different degrees of surface oxidation is described. The parent activated carbon was oxidized with aqueous solutions of nitric acid or hydrogen peroxide, and submitted to thermal treatment at 373, 523 and 773 K. The activated carbons prepared were characterized by N₂ adsorption and temperature-programmed desorption, and their catalytic behavior was determined by measuring the oxidation rate of Fe(II) to Fe(III) and the generation of hydrogen peroxide. Catalytic activity is a function of the nature of oxygen surface groups generated by oxidation.     

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).     

Catalytic oxidation of Fe(II) by activated carbon in the presence of oxygen.: Effect of the surface oxidation degree on the catalytic activity

The catalytic oxidation of Fe(II) species in aqueous solution by activated carbons with different degrees of surface oxidation is described. The parent activated carbon was oxidized with aqueous solutions of nitric acid or hydrogen peroxide, and submitted to thermal treatment at 373, 523 and 773 K. The activated carbons prepared were characterized by N₂ adsorption and temperature-programmed desorption, and their catalytic behavior was determined by measuring the oxidation rate of Fe(II) to Fe(III) and the generation of hydrogen peroxide. Catalytic activity is a function of the nature of oxygen surface groups generated by oxidation.     

Adsorption of p-nitrophenol on an activated carbon with different oxidations

An activated carbon prepared from olive stones has been modified through oxidation by nitric acid or sodium hypochlorite. These treatments introduced large amounts of oxygen groups, which were characterized by mass-spectrometry, temperature-programmed desorption (DTP-MS). Both CO₂- and CO-evolving groups were created by these oxidation treatments. A part of these oxidized samples was then outgassed under vacuum up to 823 K in order to remove most of the CO₂-evolving groups from their surface. Oxidized samples have a smaller surface area than the original sample. The subsequent partial outgassing increases the surface area which, however, does not reach the value it had before oxidation. p-Nitrophenol (PNP) adsorption isotherms from aqueous solutions were determined at 298 K for the original, oxidized, and partly outgassed samples. The results confirm the presence of an intermediate plateau at low equilibrium PNP concentration (at about 10 mg/l). The relative effects of textural versus surface chemistry on PNP uptakes are then discussed. The presence of CO-evolving groups showed no influence on PNP uptakes. The conclusion is that models in which carbonylic groups are basic adsorption sites for substituted phenols can be ruled out for the entire isotherm of PNP obtained with the original carbon. These models are also unlikely for PNP adsorption on oxidized and partly outgassed samples.     

Influence of oxygen treatment on electric double-layer capacitance of activated carbon fabrics

Oxygen treatment at 250°C on polyacrylonitrile-based activated carbon fabric was conducted to explore the influence of carbon-oxygen complexes on the performance of capacitors fabricated with the carbon fabric. Surface analysis showed that most of the oxygen functional groups created from the oxygen treatment were the carbonyl or quinone type. The performance of the capacitors was tested in 1 M H₂SO₄, using potential sweep cyclic voltammetry and constant current charge-discharge cycling. It was found that the Faradaic current, the contributor of pseudocapacitance, increased significantly with the extent of oxygen treatment, while the increase in the double-layer capacitance was minor. Due to the treatment the overall specific capacitance showed an increase up to 25% (e.g., from 120 to 150 F g⁻¹ at a current density of 0.5 mA cm⁻²). However, the distributed capacitance effect, the inner resistance and the leakage current were found to increase with the extent of oxidation. It is suggested that due to the local changes of charge density and the increase in redox activity the presence of the carbonyl- or quinone-type functional groups may induce double-layer formation, Faradaic current, surface polarity, and electrolyte decomposition.     

What kind of pore size distribution is assumed in the Dubinin–Astakhov adsorption isotherm equation?

Two most sophisticated methods of carbon porosity characterization (high resolution α_s-plot and the procedure proposed by Nguyen and Do, (ND)) were utilized for the assessment of porosity from the series of numerically generated adsorption isotherms. Basing on the Dubinin–Astakhov (DA) adsorption isotherm equation, two series of adsorption isotherms of nitrogen (T=77.5 K) were generated for constant E₀ and different n values, and for constant n and different E₀. They were described by the both above-mentioned methods. The types of obtained α_s-plots as well as the pore size distribution curves (PSD) lead to suggestions about the basic features of the DA and the meaning of the parameters of this adsorption isotherm equation.     

Simultaneous pentachlorophenol decomposition and granular activated carbon regeneration assisted by microwave irradiation

This paper describes an integrated granular activated carbon (GAC) adsorption/microwave (MW) irradiation process used for the treatment of high concentration pentachlorophenol (PCP) wastewater. Firstly, PCP in water was adsorbed onto GAC, then the PCP was decomposed and GAC regenerated by MW irradiation. The liquid and gas produced during irradiation were collected through condensing, and absorbed by a 0.1 mol l⁻¹ NaOH solution, respectively. The determination of the PCP concentrations in the distillate, absorption solution and GAC was accomplished by a HPLC. The results suggested that when irradiated with 850 W MW for 10 min, most of the PCP adsorbed by GAC and whatever fragments formed were decomposed to CO₂, H₂O and HCl or tightly bound to GAC, and less than 2% of PCP was transformed into intermediates in the distillate. Some dechlorination and dehydroxylation products were identified by the GC/MS analysis, and the degradation mechanism was proposed. It was also confirmed that GAC could be reused after several adsorption/MW regeneration cycles and its adsorption capacity could maintain a relatively high level, even higher than that of virgin GAC, as indicated by BET results, iodine values and PCP adsorption isotherms.     

Effect of the ozone-carbon reaction on the catalytic activity of activated carbon during the degradation of 1,3,6-naphthalenetrisulphonic acid with ozone

The present work investigated the evolution of catalytic activity of activated carbon in 1,3,6-naphthalenetrisulphonic acid (NTS) ozonation in aqueous phase. Activated carbons pre-treated with ozone showed a reduction in NTS oxidation rate and efficacy of TOC removal that increased with ozone exposure time. The ozone treatment increased the number of surface oxygenated (electron-withdrawing) groups on the carbon, therefore reducing its basic character and its reductive properties. This effect reduced the reactivity of the activated carbon to ozone and, therefore, the extent of the ozone decomposition into highly oxidative species.     

Relationship between the highest occupied molecular orbital (HOMO) electron density of adsorption sites on carbon and the Freundlich exponent

This paper discusses the relationship between the electron density for various groups of phenolic compounds and their adsorption capacity on activated carbon. Chloro- and nitrophenols were used as test adsorbates, and as-received or HNO₃-modified granular activated carbon (GAC) samples were used as adsorbents. The isotherms for these systems were determined by the batch bottle technique. The highest occupied molecular orbital (HOMO) electron density at the adsorption site was estimated using the CAChE program, and a relationship between the Freundlich exponent 1/n and the HOMO density of the adsorbent was found by combining the experimental and computational results for the modified GAC. The Freundlich equation is described as: n_ads=k(C)^1/n, where n_ads, k, C, 1/n are the weight of adsorbate per weight of adsorbent (g g⁻¹), the Freundlich coefficient, the concentration of adsorbate in bulk solution (g l⁻¹), and the Freundlich exponent, respectively. The value of the Freundlich exponent 1/n for the systems investigated decreased linearly with an increase in ‘adsorbent’ HOMO density. The HOMO electron density of both the adsorbate and the adsorbent were the major factors determining the value of the Freundlich exponent, 1/n, for phenolic adsorbate-carbonaceous adsorbent systems.