The characterization of activated carbons with oxygen and nitrogen surface groups

The physicochemical properties and the surface chemical structure of the carbon materials obtained by the modification of the commercial activated carbon D43/1 (Carbo-Tech, Essen, Germany) were studied. The previously de-ashed activated carbon was subjected to the following modification procedures: high-temperature treatment (1000 K) under vacuum; oxidation with conc, nitric acid; and ammonia-treatment of annealed and oxidised carbons at high temperatures. The porous structure and the surface area of the five different carbon samples obtained were estimated by means of mercury porosimetry and from low-temperature nitrogen adsorption data. The thermogravimetric analysis and the quantitative determination of surface functional groups by selective neutralisation of bases and pH-metric titration were carried out. FTIR spectra (transmission) and X-ray photoelectron spectra (Cls, Ols and Nls) were obtained for all the carbon samples and compared with one another. The changes in the porosity and the chemical properties of the carbon surface caused by the modification were analysed. Some possible surface functional species, their structure and surface state are discussed.     

Effect of surface phosphorus functionalities of activated carbons containing oxygen and nitrogen on electrochemical capacitance

Micro/mesoporous activated carbons containing oxygen and phosphorus heteroatoms were modified by incorporation of nitrogen using melamine and urea precursors. The surface chemistry was analyzed by the means of elemental analysis, XPS, and ³¹P MAS NMR. The results indicate that upon the incorporation of nitrogen at high temperatures not only new species involving carbon/nitrogen/oxygen are formed but also the phosphorous environment is significantly altered. Both urea and melamine precursors have similar effects on formation of P-N and P-C bonds. These compounds, although present in small but measurable quantities seem to affect the performance of carbons in electrochemical capacitors. With an increase in the heterogeneity of phosphorus containing species and with a decrease in the content pyrophosphates the capacitance increases and the retention ratio of the capacitor is improved.     

Well-defined mesoporosity on lignocellulosic-derived activated carbons

Activated carbons with a highly developed mesoscale cavitation-linked structure have been prepared from natural products (e.g. peach stones) by combining chemical and physical activation processes. Characterization results show that these materials exhibit a large “apparent” surface area (～1500 m²/g) together with a well-defined mesoporous structure, i.e. large cavities connected to the external surface through narrower mesoporous necks (cavitation effects).     

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.     

Adsorption from benzene-ethanol binary solutions on activated carbons with different contents of oxygen surface complexes

Adsorption from benzene-ethanol binary solutions on preparations of modified activated carbon CWZ-3 containing different amounts of oxygen bound to the surface was investigated. The modification was carried out in such a way that the porous structure of the carbons was but slightly affected. However, significant differences were observed in the total contents of oxygen as well as in the particular types of oxygen surface functional groups (especially of strongly acidic character). It was found that the higher the contents of those groups the stronger is the preference to adsorb the more polar component of the solution. The most significant role should be ascribed here to carboxylic and phenolic groups which can give hydrogen bonds with alcohol molecules.     

Amination and ammoxidation of activated carbons

Four commercially available carbons were subjected to ammonia and ammonia/oxygen gas mixtures at temperatures between 200 and 420°C. The pore structure of the carbons, as determined by nitrogen porosimetry, was virtually unchanged after the treatments. The surface chemistry of aminated and ammoxidized carbons was found to differ from that of carbons treated with ammonia at high temperatures (600 to 900°C). Probably imide-, amide-, and lactam-type surface species are formed as indicated by FTIR, TPD-MS, acid/base-titration, and elemental analysis. The amide-lactam-imide ratio depends on the modification technique; these surface groups can be converted to aromatic nitrogen species by heat treatment above 600°C. Nitrogen contents and surface fractions are obtainable of 4.4 wt% and 0.14, respectively, for amination and of 3.2 wt% and 0.11 for ammoxidation. A high nitrogen incorporation efficiency on applied ammonia can be reached for ammoxidation (58%). The lactam, imide, and amide groups enhance the absorption of the graphitic plane at 1600 cm⁻¹ in FTIR spectroscopy by induction. The intensity of this absorption is generally related to the amount of oxygen.     

Pentachlorophenol retention on to lignocellulosic activated carbons

The rate and equilibrium retention of pentachlorophenol, in an aqueous medium, by activated carbons obtained from peach stones and Eucalyptus globulus chips are studied and related to the chemical and textural properties of the adsorbents. E globulus‐activated carbons exhibited a higher pentachlorophenol initial retention rate, due to the higher contribution of the macropores to the porosity of these materials. However, the pentachlorophenol retention rate in activated carbons from peach stones increases to a greater extent than in E globulus with the burn‐off growth, due to the greater development of mesopores in the former. Under the experimental conditions used, the initial retention rate of pentachlorophenol in both adsorbents showed a mixed chemical and diffusional dependence, and the most important variables are the particle size of the adsorbents and the temperature of the medium. Pentachlorophenol equilibrium retention capacity depends on the apparent specific surface area, the presence of carbonylic‐like surface functional groups and the accessibility of the adsorbate to the adsorption sites. The dependence of pentachlorophenol retention capacity with the surface concentration of carbonylic functional groups suggests some chemical interactions between the aromatic pentachlorophenol ring and the carbonylic‐like surface functional groups present in the activated carbons. © 1999 Society of Chemical Industry     

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.     

Iodine adsorption and structure of activated carbons

Studies were conducted on the adsorption of iodine from saturated aqueous solutions and from saturated vapor by eight activated carbons of greatly diverse pore structures. Water adsorption data were used to determine the pore size distribution curves which provided both the distributions of the pore constriction (desorption) and cavity (adsorption) diameters. Adsorption from aqueous phase formed a unimolecular layer on the carbon surface while adsorption from saturated vapor produced pore-filling of micropores (pores less than 30 Å diameter) and surface coverage of the macropores. A great deal of steric interference was present because of the small difference in the diameter of iodine molecule (4.94 Å) relative to the 10–30 Å diameter pores. Good correlation was attained between adsorption and pore structure when corrections were made for the steric effect and the mean diameter distributions of the constrictions and cavities were used. The model for the iodine-on-carbon adsorption resembles packing of spheres into cylinders.     

Catalytic oxidation of hydrogen sulphide on activated carbons

Activated carbon is a suitable adsorbent for removal of hydrogen sulphide from natural, synthesis or other product gases. The process depends predominantly on physical adsorption, though catalytic oxidation is also involved. During catalytic oxidation the H₂S is converted in the presence of oxygen to elemental sulphur, which is adsorbed onto the internal surface of the activated carbon, thus leading to a sulphur load of up to 120% by weight. The oxidation rate depends on the partial pressure of both reactants, H₂S and O₂ and is largely controlled by the characteristics of the activated carbon. The activity of the catalyst can be improved by impregnating the activated carbon with promoters such as iron and iodine. The regeneration of spent carbon is currently carried out using hot gas desorption methods at temperatures around 450 °C.     

Adsorption equilibrium of solvent vapors on activated carbons

Adsorption amounts of solvent vapors such as n-hexane, toluene, and MEK on two commercial activated carbons (SLG-2PS and X-7000) were measured experimentally using a quartz spring balance connected with a high vacuum system. Single-species equilibrium data show typical type II isotherms, which are most frequently encountered in separation processes. Equilibrium data obtained at relative vapor pressures less than 0.3 were fitted to four well-known isotherms such as Langmuir, Freundlich, Sips, and Dubinin-Astakov (D-A) equations. All isotherm equations were found to describe experimental equilibrium data satisfactorily. Only the Langmuir equation was not proper to fit the equilibrium data of solvent vapors on SLG-2PS. In order to evaluate the adsorption energy distribution of the solvent vapors on activated carbons, a simple model based on the condensation approximation (CA) approach was applied. According to the result, it was proven that two carbons have energetically heterogeneous surfaces.     

Adsorption of methylene chloride vapor on activated carbons

A laboratory investigation on the adsorption of hazardous methylene chloride (METH) vapor on the commercial activated carbons BPL and PCB, which were made from bituminous coal and coconut shell, respectively, was conducted at 283, 293, 303, and 313 K. The physical properties and surface functional groups of the two activated carbons were also measured and compared with each other. The experimental results indicate that the adsorption capacity of carbon PCB is slightly higher than that of carbon BPL. It was found that the Langmuir, Freundlich, and Dubinin–Radushkevich adsorption equations were well fitted by the measured adsorption data. The values of the parameters of the adsorption equations were determined for the two adsorbents. The physical properties (e.g. micropore volume) of the adsorbents are consistent with the parameters obtained from the adsorption results.     

A new low temperature approach to developing mesoporosity in metal-doped carbons for adsorption and catalysis

Key factors in achieving effective adsorption are the size of the pores relative to those of the adsorbate molecules and often the presence of small metal particles which can confer catalytic activity. While microporous carbons are excellent adsorbents for small molecules they are not as effective for larger species. A new low temperature approach to activation using an oxygen gas pulsing technique to achieve a carbon with controllable meso/micropore structure is described which also minimises metal sintering. The porosity of the samples was analysed by nitrogen adsorption at 77 K. Microporous metal-doped ASC carbon showed significant increases in the level of mesoporosity, its mesopore volume increasing from 0.06 to 0.24 cm³ g⁻¹. However, undoped BPL carbon treated under the same conditions remained unchanged. The catalytic effect of the metals in the ASC carbon is thought to account for the observed mesopore development. At the temperatures used, a continuous oxidative activation yielded no pore widening in either carbon, suggesting that the gas pulsing method is far more effective in increasing mesopore sizes.     

Liquid-phase adsorption of phenol by activated carbons prepared from bituminous coals with different oxygen contents

Activated carbons prepared from two bituminous coals were used to adsorb phenol in aqueous solutions. The major difference between the coal precursors is the oxygen content. The carbon preparation consisted of carbonization of the coals followed by activation in CO ₂ to various extents of burn-off. Experimental results show that the amount of phenol adsorbed generally increases with the BET surface area of the carbon. The carbons prepared from the coal with a higher oxygen content have larger surface areas, and, therefore, exhibit higher capacities for phenol. The surface area of the carbon increases with the extent of carbon burn-off, whereas the increase in the adsorptive capacity due to the increasing burn-off level does not show a linear relationship with the increase in area; the ratio of the capacity to BET surface area is not constant and decreases with the burn-off level. This has been attributed to the accessibility of phenol to the surface being affected by the length of diffusion path, which is an increasing function of the burn-off level or the particle size. The amount of phenol adsorbed decreases with the temperature for these carbons. It was found, according to the Langmuir model, that the adsorption process was significantly affected by the oxygen content in the coal precursors. © 1999 Society of Chemical Industry     

The influence of porosity and surface oxygen groups of peat-based activated carbons on benzene adsorption from dry and humid air

Activated carbons (ACs) prepared from peat were used for benzene adsorption (5 ppmv) from dry and humid (relative humidity (RH) 70%) air streams. Benzene uptake by the ACs was lower in the presence of water vapor due to competition between benzene and water molecules for the adsorption sites. Adsorption of benzene from dry and humid air on the ACs with low content of surface oxygen groups was attributed to the presence of narrow micropores (size <0.7 nm). A linear correlation between the amount of adsorbed benzene and micropore volume calculated from CO₂ adsorption isotherms was found. The coefficients of determination R² were 0.87 for benzene adsorption in the absence of water vapor and 0.83 for adsorption at relative humidity 70%. It was shown that the presence of surface groups in the ACs reduces benzene uptake more profoundly in the presence of moisture than in the dry conditions.     

Study of thermal regeneration of spent activated carbons: Thermogravimetric measurement of various single component organics loaded on activated carbons

Thermogravity analysis of the activated carbons loaded with 32 different single component organics showed that TGA curves could be classified into three distinct groups with regard to their shapes. The organics that belong to Group (I) are rather volatile and TGA curves can be explained by equdibrium desorption model. Group (II) organics are relatively easy to decompose and TGA curves were interpreted in terms of first-order cracking kinetics. The parameters included in these models were obtained from the measured TGA curves by utilizing half desorbed temperature T_{$\text{1}\text{2}$} and reciprocal slope of TGA at T_{$\text{1}\text{2}$}, ΔT. Group (III) consists of phenol, β-naphtol, lignin etc. and gave high residuals on activated carbons after heating up to 800°C. This suggests that these organics are the ones that are critical to the ordinary thermal regeneration method.A rough classification of organics into these groups was done by using the boiling point and the aromatic carbon content.