Krisztina László

Effect of activation on the surface chemistry of carbons from polymer precursors

Abstract Carbonized chars prepared from polyacrylonitrile, polyethyleneterephthalate and cellulose were activated in a steam/nitrogen flow to ca. 50% burn-off (b.o.). The surface chemistry of these carbons has been characterized by X-ray photoelectron spectroscopy (XPS), adsorption from benzene–methanol binary liquid mixture, and potentiometric acid–base titration. The activation of the various polymer-based chars, in spite of the treatment similarity (temperature, burn-off) results in different physical and chemical surface properties. It has been confirmed by XPS analysis, that the elevated temperature of the activation not only promotes the relative extension of the graphitic regions by removal of noncarbon atoms and part of the amorphous carbon, but also helps on the further development of graphitic regions. The carbons contain both hydrophobic and hydrophilic sites on their surface already in their carbonized form, but the activated ones show a more hydrophobic character. The carbons exhibit an amphoteric surface, containing both acidic and basic surface groups. The titration curves show a hysteresis when performed upward and downward. Suspensions of the activated samples exhibit a basic pH. Both the increased hydrophobic property and the basicity can be attributed to extended graphitic regions. Part of the oxygen atoms forming basic functional groups may contribute to basicity, as well.

Surface characterization of polyethyleneterephthalate (PET) based activated carbon and the effect of pH on its adsorption capacity from aqueous phenol and 2,3,4-trichlorophenol solutions

A predominantly microporous activated carbon produced from polyethyleneterephthalate (PET) by physical activation 23 (burn-off 50%) has a BET surface area of 1170 m / g and a total pore volume of 0.625 cm / g. Its surface possesses an amphoteric nature due to the oxygen functionalities present, however, the majority of the groups are basic. The adsorption of phenol and 2,3,4-trichlorophenol has been studied from aqueous solution of different pH (pH53, 11 and unbuffered) on this carbon. The adsorption mechanism depends both on the pH of the solutions and the pK of the phenols, as the former a influences the surface chemistry of the carbon as well. In the case of the phenol, a competitive adsorption takes place, as the interactions are weak in all three media investigated. The triple chlorine substitution in the 2,3,4-trichlorophenol significantly increases the strength of the interactions in comparison with phenol. Due to its smaller p K value, in unbuffered medium a ionic interaction occurs, as has been concluded from the shape of the isotherm and the outstandingly high value of the adsorption equilibrium constant. Competitive adsorption takes place only in the basic media, where the adsorption of the 2,3,4-trichlorophenol is hindered by electrostatic repulsion.

Driving forces of conformational changes in single-layer graphene oxide

The extensive oxygen-group functionality of single-layer graphene oxide proffers useful anchor sites for chemical functionalization in the controlled formation of graphene architecture and composites. However, the physicochemical environment of graphene oxide and its single-atom thickness facilitate its ability to undergo conformational changes due to responses to its environment, whether pH, salinity, or temperature. Here, we report experimental and molecular simulations confirming the conformational changes of single-layer graphene oxide sheets from the wet or dry state. MD, PM6, and ab initio simulations of dry SLG and dry and wetted SLGO and electron microscopy imaging show marked differences in the properties of the materials that can explain variations in previously observed results for the pH dependent behavior of SLGO and electrical conductivity of chemically modified graphene-polymer composites. Understanding the physicochemical responses of graphene and graphene oxide architecture and performing selected chemistry will ultimately facilitate greater tunability of their performance.

Comparative adsorption study on carbons from polymer precursors

Abstract The effect of the precursor polymer on nitrogen adsorption properties, pore size distribution and the hydrophilic/hydrophobic character of both the pyrolyzed char and the activated carbon was studied in the case of three basically different polymers, i.e. polyacrylonitrile, polyethyleneterephthalate and cellulose. Pyrolyzed samples were produced from these polymers and activated by steam at 900°C. After a 50% burn-off, the pore volume and the specific surface area increase significantly and the pore size distribution is determined by that of the pyrolyzed char, i.e. the starting polymer. The activated sample derived from polyacrylonitrile is microporous, while the other two carbons contain both micro- and mesopores. Activation creates pores which are equally available for methanol and benzene and the heterogeneous surfaces become more benzophilic in all three polymers. The N-content has limited effect on the surface properties studied.

pH-driven physicochemical conformational changes of single-layer graphene oxide

Single-layer graphene oxides (SLGOs) undergo morphological changes depending on the pH of the system and may account for restricted chemical reactivity. Herein, SLGO may also capture nanoparticles through layering and enveloping when the pH is changed, demonstrating potential usefulness in drug delivery or waste material capture.

Redox- and pH-Responsive Cysteamine-Modified Poly(aspartic acid) Showing a Reversible Sol–Gel Transition

Synthesis and characterization of a pH- and redox-sensitive hydrogel of poly(aspartic acid) are reported. Reversible gelation and dissolution are achieved both in dimethylformamide and in aqueous medium via a thiol-disulphide interconversion in the side chain of the polymers. Structural changes are confirmed by Raman microscopy and rheological measurements. Injectable aqueous solutions of thiolated poly(aspartic acid) can be converted into mechanically stable gels by oxidation, which can be useful for drug encapsulation and targeted delivery. Reduction-facilitated release of an entrapped drug from disulphide cross-linked hydrogels is studied.

Water in contact with magnetite nanoparticles, as seen from experiments and computer simulations.

The adsorption of water vapor at the surface of magnetite nanoparticles has been investigated both by experimental and by computer simulation methods. The water vapor adsorption/desorption isotherm has been measured on freshly prepared magnetite nanocrystals of the size below 10 nm. The change of the isosteric heat of adsorption with the surface coverage has been determined from the temperature dependence of this isotherm using the isosteric method. The adsorption isotherm has also been determined by performing a set of grand canonical Monte Carlo simulations at 300 K. X-ray photoelectron spectroscopy results as well as the temperature and coverage dependence of the isosteric heat of adsorption clearly indicates that dissociative chemisorption of the water molecules in the first adsorption layer occurs at the bare magnetite surface, resulting in a high density of surface hydroxyl groups. This dissociative chemisorption is followed by a multilayer physisorption of water at higher pressures. Computer simulation results can reproduce excellently both the adsorption isotherm and the isosteric heat of adsorption beyond the first chemisorbed layer of water. Results of the computer simulations reveal that physisorbed water forms several well-distinguished molecular layers on the magnetite surface; however, these layers are not built up sequentially. Instead, the building up of several molecular layers occurs simultaneously. The adsorption of the water molecules in this range appears to be a nucleation-like process, resulting in a rather rough external surface of the adsorption layer.

Fractal approach of activated carbons from solid waste materials

Abstract Carbonaceous composite materials were produced from various pyrolyzed organic waste materials by steam activation. Fractal dimensions derived from SAXS and adsorption measurements were used to explain the changes in the matrix and the surface microstructure, due to the activation. The comparison of the fractal dimensions from SAXS and nitrogen adsorption data can be successfully applied to understand the development of the porous structure. In these materials, of heterogeneous matrix, no correlation was found between the B.E.T. specific surface area and the fractal dimensions derived from various methods. The adsorption of different substrates is disturbed by the chemical heterogeneity of the surface. In these cases, the applicability of aqueous solutions in the description of the surface geometry might be questioned.

Competitive adsorption of phenol and 3-chlorophenol on purified MWCNTs

A commercial multiwall carbon nanotube and its carboxylated derivate (CNTC and COOHC, respectively) was used after purification to study the competitive adsorption of phenol (P) and m-chlorophenol (CP) from 0.1 M aqueous NaCl solutions without external pH control. The adsorption takes place practically exclusively on the external surface of the nanotubes. The uptake of P is suppressed in comparison to its single solute behaviour on both nanotubes, independently of the initial pollutant concentration. The uptake of CP however is more sensitive to the concentration and the surface chemistry of the nanotube. The measured co-adsorption isotherms were compared to the isotherms calculated from the competitive Langmuir model (CLM). Preferential adsorption of CP was observed in about 95% of the relative concentration range. The total adsorption may exceed the corresponding single component sorption capacity.

Morphological and chemical features of nano and macroscale carbons affecting hydrogen peroxide decomposition in aqueous media

Chemical and structural factors of carbon materials affect their activity in adsorption and surface reactions in aqueous media. Decomposition of hydrogen peroxide studied is a probe reaction for exploring parameters of carbons that might be involved, such as specific surface area, nitrogen and oxygen doping and conformational changes. To date, a detailed comparison of the behavior of carbon nanoscale (Carbon Nanotubes, CNT, Single Layer Graphene Oxide, SLGO) with macroscale (Activated carbons, AC) materials in this reaction has not been forthcoming. Herein, we demonstrate that on their first cycle, ACs in doped and undoped forms outperform all nanoscale carbons tested in the H(2)O(2) decomposition. Among the nanocarbons, nitrogen-doped CNT exhibited the highest activity in this reaction. However, subsequent recycling of each carbon, without chemical regeneration between uses, reveals SLGO exhibits greater reaction rate stability over an extended number of cycles (n>8) than other carbons including nitrogen-doped CNT and ACs. The effects of pH, temperature and concentration on the reaction were analyzed. Quantum-chemical modeling and reaction kinetics analysis reveal key processes likely involved in hydrogen peroxide decomposition and show evidence that the reaction rate is linked to active sites with N-and O-containing functionalities.