Teresa J. Bandosz

Surface functionality and porosity of activated carbons obtained from chemical activation of wood

a `´ ´ ´ ´ Abstract Mixtures of wood with either phosphoric acid or diammonium hydrogen phosphate ((NH ) HPO ) were heated under 42 4 nitrogen or steam / nitrogen flows at temperatures between 300 and 500 8C. As a result of these processes, activated carbons with various pore size distribution and surface properties were obtained. The samples were characterised using potentiometric titration and sorption of nitrogen at its boiling point. The results showed the significant influence of the nature of the activating agent and the atmosphere on the final properties of the materials. It was demonstrated that steam inhibits the incorporation of heteroatoms into the carbon matrix. Activation of wood in the presence of phosphoric acid together with 2 steam gives carbon of high surface area (|1800 m / g) with well-developed mesoporosity and an almost neutral surface.

Role of surface chemistry in adsorption of phenol on activated carbons

Two samples of activated carbon of wood origin were oxidized using ammonium persulfate. The structural properties and surface chemistry of the samples and their oxidized counterparts were characterized using sorption of nitrogen and Boehm titration, respectively. Phenol adsorption from solution (at trace concentrations) was studied at temperatures close to ambient without maintaining a specific pH of the solution. The results showed, as expected, that the phenol uptake is dependent on both the porosity and surface chemistry of the carbons. Furthermore, phenol adsorption showed a strong dependence on the number of carboxylic groups due to two factors: (1) phenol reacts with carboxylic groups on the carbon surface, forming an ester bond, and (2) carboxylic groups on the carbon surface remove the pi-electron from the activated carbon aromatic ring matrix, causing a decrease in the strength of interactions between the benzene ring of phenol and the carbons basal planes, which decreases the uptake of phenol.

Study of carbon-smectite composites and carbons obtained by in situ carbonization of polyfurfuryl alcohol

Abstract A sodium form of Wyoming smectite was intercalated with hydroxyaluminum cations and saturated with furfuryl alcohol; polymerization and carbonization were done to obtain carbon-smectite composites. Carbons were obtained from the smectite inorganic matrices by washing them with HCl and HF. The synthesized materials were studied by X-ray diffraction, inverse gas chromatography, DTA, and adsorption of nitrogen and methane. Analyses of data indicate the inorganic matrix acts as a limiting pore size former in the carbons washed from smectite precursors.

Inverse gas chromatographic study of activated carbons : the effect of controlled oxidation on microstructure and surface chemical functionality

Abstract Activated carbons derived from four different source materials were subjected to controlled oxidation by nitric acid, and their resulting microstructure and surface chemical groups were studied. Classical titration procedures indicated the acid/base character of the carbons and inverse gas chromatography was used to study other properties. We find that carboxylic groups are the primary acidic functionality added by oxidation and that these groups can be removed by high temperature treatment without altering the microstructural changes induced by oxidation. Nitric acid treatment of these carbons produces highly acidic carbonaceous materials, and in some cases modifies microstructure. We conclude that inverse gas chromatography at infinite dilution, along with the procedures outlined, provides a convenient way to study the effects of surface modification procedures on the structure and surface chemical properties of porous adsorbents.

Application of inverse gas chromatography at infinite dilution to study the effects of oxidation of activated carbons

Abstract Effects of the oxidation with nitric acid of active carbons from different origins have been studied by inverse gas chromatography. It is shown that the oxidation of activated carbons with nitric acid increases their acidity considerably, and at the same time decreases their adsorption affinity for n-alkanes, which is manifested by the decrease of the dispersive component of surface energy, γsD, and adsorption enthalpies of alkanes. A correlation has been established between the acidity of carbons measured by base titration and the energy of the specific interactions with π bonds of unsaturated hydrocarbons.

Sorption and desorption of lithium ions from activated carbons

Abstract The influence of surface functional groups on activated carbons on the adsorption and desorption of lithium ions, present in trace amounts in aqueous solution, has been studied in the absence and presence of an applied electric field. Application of an electric current during the experiments dramatically increased the sorption of the ions. Moreover, lithium could be released into the solution by reversing the current, showing the reversibility of this process. In its absence, a significant enhancement in the lithium sorption uptake was found when phosphoric acid, trapped in the carbon pores was eliminated. To understand the differences induced by the chemical and electrochemical treatments of the materials, pore volumes and surface acidity were characterized.

Application of inverse gas chromatography to the study of the surface properties of modified layered minerals

Abstract Inverse gas chromatography at infinite dilution has been used to study the effects of modifications of cationic (montmorillonite) and anionic (hydrotalcite) clays. By using alkanes and alkenes as probe molecules both the dispersive component of the surface energy and the surface acidic sites were characterized. The introduction of polymers into the structure of these minerals significantly changed their chemical character. Carbonization of samples whose interlayer spaces were saturated with polymers produced new carbon-modified materials. These composite materials became microporous while retaining their initial acid/base surface properties.

Effect of template constraints on adsorption properties of synthetic carbons prepared within the gallery of layered double hydroxides

Abstract Mg-Al layered double hydroxides have been synthesized and 1,5-naphthalene disulfonate dianions have been intercalated; the organic molecules were carbonized within the layered framework. For comparison, a reference carbon was prepared from the sodium salt of the 1,5-naphthalene disulfonic acid precursor using conventional carbonization under the same thermal and post-treatment conditions. The surface properties of the carbonized product extracted from the carbon-mixed oxide nanocomposite have been studied by adsorption techniques and the results compared to the adsorption characteristics of the reference carbon. The Sorption of nitrogen at 77 K and carbon dioxide at 298 K demonstrate that, although the two carbons have almost the same specific surface area and total pore volumes, there is a significant difference in their micropore structure. Micropore size distributions calculated from adsorption isotherms of methane and sulfur hexafluoride at near ambient temperatures reveal a more heterogeneous micropore structure for the template derived carbon compared to the reference. The factors controlling micropore structure development during two-dimensional carbonization are discussed based on DTA, results of sorption of carbon dioxide and FTIR spectroscopic measurements.

Hydrotalcite-like structures as molecular containers for preparation of microporous carbons

Abstract Microporous carbons have been prepared by thermal treatment of poly (4-styrenesulfonate) derivative of MgAl hydrotalcites and subsequently the carbons extracted from the matrix. The interlayer carbonization process and the characteristics of the carbonaceous materials were studied by X-ray diffraction, inverse gas chromatography at infinite dilution (IGC), diffuse reflectance infrared spectroscopy (DRIFT), differential thermal analysis (DTA) and nitrogen and methane adsorption measurements. The structural and surface properties of the template carbons were compared to carbon prepared from the same organic precursor using a conventional carbonization method. It was found that carbonization between the layers of hydrotalcite leads to a formation of highly microporous carbons without additional activation processes.

Study of carbon microstructure by using inverse gas chromatography

Abstract Carbons with different pore structures were studied by means of inverse gas chromatography at infinite dilution and finite concentration. Thermodynamic quantities, such as the enthalpy or the free energy of adsorption, are dependent on the carbon micropore structure. Normal alkanes in conjunction with their branched isomers having different critical diameters were used to assess the molecular sieving properties of the investigated carbons. Chromatographic measurements at finite concentration yielded adsorption isotherms, and from these the isosteric heat of adsorption as a function of amount adsorbed for the parent n-alkane and its isomers were calculated. Both isotherms and isosteric heats of adsorption are consistent with the carbon pore structure.