Jacek Jagiello

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.

Characterization of the surfaces of activated carbons in terms of their acidity constant distributions

Abstract A method for characterization of carbon surfaces acidity using a continuous distribution of acidity constants is proposed. The method is based on potentiometric titration measurements. Titration curves are transformed into proton adsorption isotherms, and are analyzed to yield the distribution of acidity constants. Calculation of the distribution function is sensitive to experimental errors, and therefore a careful smoothing treatment must be applied to the experimental data. The method is tested by application to organic compounds and the calculated pK values are in excellent agreement with literature data. The method is then applied to study the evolution of acidic groups, in terms of their pK values, subsequent to modification processes such as oxidation and reduction of activated carbons. It is demonstrated that the method is sensitive to the changes in the number and character of surface acidic groups.

Carbon slit pore model incorporating surface energetical heterogeneity and geometrical corrugation

In our recent paper (Jagiello and Olivier, Carbon 55:70–80, 2013) we considered introducing energetical heterogeneity (EH) and geometrical corrugation (GC) to the pore walls of the standard carbon slit pore model. We treated these two effects independently and we found that each of them provides significant improvement to the carbon model. The present work is a continuation of the previous one, as we include both effects in one comprehensive model. The existing standard slit pore model widely used for the characterization of activated carbons assumes graphite-like energetically uniform pore walls. As a result of this assumption adsorption isotherms calculated by the non-local density functional theory (NLDFT) do not fit accurately the experimental N2 data measured for real activated carbons. Assuming a graphene-based structure for activated carbons and using a two-dimensional-NLDFT treatment of the fluid density in the pores we present energetically heterogeneous and geometrically corrugated (EH–GC) surface model for carbon pores. Some parameters of the model were obtained by fitting the model to the reference adsorption data for non-graphitized carbon black. For testing, we applied the new model to the pore size analysis of porous carbons that had given poor results when analyzed using the standard slit pore model. We obtained an excellent fit of the new model to the experimental data and we found that the typical artifacts of the standard model were eliminated.

Toward Understanding Reactive Adsorption of Ammonia on Cu-MOF/Graphite Oxide Nanocomposites

The adsorption of ammonia on HKUST-1 (a metal-organic framework, MOF) and HKUST-1/graphite oxide (GO) composites was investigated in two different experimental conditions. From the isotherms, the isosteric heats of adsorption were calculated from the Clausius-Clapeyron equation following the virial approach. The results on HKUST-1 were compared with those obtained using molecular simulation studies. All materials exhibit higher ammonia adsorption capacities than those reported in the literature. The ammonia adsorption on the composites is higher than that measured separately on the MOF component and on GO. The strong adsorption of ammonia caused by chemical interactions on different adsorption sites is evidenced by the trends in the isosteric heats of adsorption. The molecular simulations conducted on HKUST-1 support the trends observed experimentally. In particular, the strong chemisorption of ammonia on the metallic centers of HKUST-1 is confirmed. Nevertheless, higher adsorption capacities are predicted compared with the experimental results. This discrepancy is mainly assigned to the partial collapse of the MOF structure upon exposure to ammonia, which is not accounted for in the simulation study.

INVERSE GAS CHROMATOGRAPHY STUDY OF MODIFIED SMECTITE SURFACES

Inverse gas chromatography at infinite dilution, employing alkanes and alkenes as probes, has been used to characterize the surface properties of a series of smectites of varying chemical composition. The results of this study show that the acidic centers and the interlayer distances have a great influence on the specific interaction of the smectite surface with π-bonds of alkenes. High values of the specific interaction parameter, ɛπ, are caused by the existence of strong acidic centers that are connected with interlayer cations as well as with the chemical structure of the mineral sheets. On the other hand, alkanes, whose interaction with the smectites is predominantly dispersive, are unaffected by changes in the clays’ composition and/or structure.

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.

Surface energy and adsorption energy distribution measurements on some carbon blacks

Abstract The dispersive component of the surface energy (γsd) of various carbon blacks and of nitric acid or heat-treated samples has been measured by inverse gas chromatography (IGC). It is shown that IGC, at infinite dilution, applies poorly to carbon blacks and provides only apparent and excessively high values of γsd (a few hundreds of mJ/m2). IGC, at finite concentration, readily allows the obtention of n-alkanes, benzene and chloroform adsorption isotherms from which various thermodynamic values are computed (spreading pressure π, isosteric enthalpy of adsorption, surface energy characteristics). The γsd values calculated from πe are in the 80–120 mJ/m2 range (i.e. values which are in fair agreement with published data on graphite or graphitized carbons). Further, the specific interaction potential of polar probes is in relation, as expected, with the oxygen content of the carbons. Finally, from the adsorption isotherms, the adsorption energy distribution function is computed, supposing a patchwise distribution of adsorption sites. The results confirm the high surface heterogeneity of carbon blacks and the influence of surface treatments, but indicate also major differences between carbon blacks from different preparation processes.

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.