Mykola Seredych

Revisiting the chemistry of graphite oxides and its effect on ammonia adsorption

Graphite oxide (GO) was synthesized using two different methods: one with sulfuric acid as part of the oxidizing mixture (Hummers–Offeman method) and another one without the sulfur-containing compound involved in the oxidation process (Brodie method). They were both tested for ammonia adsorption in dynamic conditions, at ambient temperature, and characterized before and after exposure to ammonia by X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy, potentiometric titration, energy-dispersive X-ray (EDX) spectroscopy, X-ray photoelectron spectroscopy (XPS) and elemental analysis. Analyses of the initial materials showed that besides epoxy, hydroxyl and carboxylic groups, a significant amount of sulfur is incorporated as sulfonic group for GO prepared by the Hummers–Offeman method. The process of ammonia adsorption seems to be strongly related to the type of GO. For GO prepared by the Brodie method, ammonia is mainly retained via intercalation in the interlayer space of GO and by reaction with the carboxylic groups present at the edges of the graphene layers. On the contrary, when GO prepared by the Hummers method is used, the ways of retention are different: not only is the intercalation of ammonia observed but its reaction with the epoxy, carboxylic and sulfonic groups present is also observed. In particular, during the ammonia adsorption process, sulfonic groups are converted to sulfates in the presence of superoxide anions O2−*. These sulfates can then react with ammonia to form ammonium sulfates. For both GOs, an incorporation of a significant part of the ammonia adsorbed as amines in their structure is observed as a result of reactive adsorption.

S-doped micro/mesoporous carbon–graphene composites as efficient supercapacitors in alkaline media

Composites of polymer (styrene-sulfonate sodium salt) derived carbon with highly oxidized graphite oxide were synthesized and tested as supercapacitors. The materials were characterized using adsorption of nitrogen, SEM/EDX, elemental analysis, thermal analysis-mass spectroscopy and potentiometric titration. The electrochemical performance was evaluated using cyclic voltammetry, galvanostatic charge–discharge techniques and impedance spectroscopy in 6 M KOH. Addition of the graphene phase increases DC conductivity, volume of small micropores, and sizes of mesopores. These changes enhance the capacitance. Sulfur species located in small micropores affect the charge of the carbon surface and decrease its affinity to adsorb water. This results in a specific electrosorption of electrolyte ions and thus in highly efficient space utilization. Sulfones and sulfoxides located in the larger pores (mesopores) contribute to a pseudocapacitive effect. On the materials tested ∼110 F g−1 was measured in spite of the small surface area (about 600 m2 g−1). This leads to a high volumetric capacitance of up to 65 F cm−3 (without a special densification).

Visible-light-enhanced interactions of hydrogen sulfide with composites of zinc (oxy)hydroxide with graphite oxide and graphene.

Composites of zinc(oxy)hydroxide-graphite oxide and of zinc(oxy)hydroxide-graphene were used as adsorbents of hydrogen sulfide under ambient conditions. The initial and exhausted samples were characterized by XRD, FTIR, potentiometric titration, EDX, thermal analysis, and nitrogen adsorption. An increase in the amount of H(2)S adsorbed/oxidized on their surfaces in comparison with that of pure Zn(OH)(2) is linked to the structure of the composite, the relative number of terminal hydroxyls, and the kind of graphene-based phase used. Although terminal groups are activated by a photochemical process, the graphite oxide component owing to the chemical bonds with the zinc(oxy)hydroxide phase and conductive properties helps in electron transfer, leading to more efficient oxygen activation via the formation of superoxide ions. Elemental sulfur, zinc sulfide, sulfite, and sulfate are formed on the surface. The formation of sulfur compounds on the surface of zinc(oxy)hydroxide during the course of the breakthrough experiments and thus Zn(OH)(2)-ZnS heterojunctions can also contribute to the increased surface activity of our materials. The results show the superiority of graphite oxide in the formation of composites owing to its active surface chemistry and the possibility of interface bond formation, leading to an increase in the number of electron-transfer reactions.

Superior performance of copper based MOF and aminated graphite oxide composites as CO2 adsorbents at room temperature.

New composites Cu-BTC MOF and graphite oxide modified with urea (GO-U) are developed and tested as CO2 adsorbents at room temperature. The composite containing GO-U with the highest nitrogen content exhibits an excellent CO2 uptake (4.23 mmol/g) at dynamic conditions. The incorporation of GO-U into MOF changes the chemistry and microstructure of the parent MOF and results in synergistic features beneficial for CO2 retention on the surface. To identify these features the initial and exhausted materials were extensively characterized from the points of view of their porosity and chemistry. Although the adsorption forces are relatively strong, the results indicate that CO2 is mainly physisorbed on the composites at dry dynamic conditions at ambient temperature and pressure. The primary adsorption sites include small micropores specific for the composites, open Cu sites, and cage window sites.

Removal of antibiotics from water using sewage sludge- and waste oil sludge-derived adsorbents

Sewage sludge- and waste oil sludge-derived materials were tested as adsorbents of pharmaceuticals from diluted water solutions. Simultaneous retention of eleven antibiotics plus two anticonvulsants was examined via batch adsorption experiments. Virgin and exhausted adsorbents were examined via thermal and FTIR analyses to elucidate adsorption mechanisms. Maximum adsorption capacities for the 6 materials tested ranged from 80 to 300 mg/g, comparable to the adsorption capacities of antibiotics on various activated carbons (200-400 mg/g) reported in the literature. The performance was linked to surface reactivity, polarity and porosity. A large volume of pores similar in size to the adsorbate molecules with hydrophobic carbon-based origin of pore walls was indicated as an important factor promoting the separation process. Moreover, the polar surface of an inorganic phase in the adsorbents attracted the functional groups of target molecules. The presence of reactive alkali metals promoted reaction with acidic groups, formation of salts and their precipitation in the pore system.

Enhancement in Dibenzothiophene Reactive Adsorption from Liquid Fuel via Incorporation of Sulfur Heteroatoms into the Nanoporous Carbon Matrix

Adsorption of dibenzothiophene (DBT) and 4,6-dimethyldibenzothiophene (DMDBT) from simulated diesel fuel was investigated with polymer-derived carbon matrices. Sulfur was incorporated to the carbon surface via a high-temperature hydrogen sulfide reduction of oxygen-containing groups. The resultant carbons were characterized by nitrogen adsorption, thermal analysis, potentiometric titration, and elemental analysis. The selectivities for DBT and DMDBT adsorption were calculated with reference to naphthalene. The carbon matrices studied had comparable structures, hence, the effects of the sulfur functionalities were evident in an increase in dibenzothiophenes selectivity and the breakthrough capacity; this was especially visible at a breakthrough point where small pores are expected to be active in the adsorption process. Incorporation of sulfur atoms into the aromatic rings of the carbon matrix increases the ability of the surface to attract dibenzothiophenes via dispersive interactions (sulfur-sulfur bridges). Sulfur and sulfur-oxygen groups present in larger pores enhance the amount of adsorbed dibenzothiophenes via specific acid-base and polar interactions. They also contribute to the reactive adsorption of DBT and DMDBT (oxidized) and their chemisorption on the carbon surface.

Interactions of 4,6-Dimethyldibenzothiophene with the Surface of Activated Carbons

Two carbon samples, commercial wood-based carbons and laboratory-derived polymer-based carbon, were oxidized to two different levels of surface acidity. The resulting adsorbents were characterized using adsorption of nitrogen, potentiometric titration, FTIR, SEM/EDAX, and elemental analysis. On the carbons obtained, the adsorption of 4,6-dimethyldibenzothiophene (4,6-DMDBT) from hexadecane was carried out in the range of the initial concentrations between 10 and 150 ppmw sulfur. The results indicate that pores with diameters less than 10 A are important for adsorption of 4,6-DMDBT. Chemical transformations, likely oxidation, occur in larger pores, and the extent of this process is governed by the availability of oxidants. Those oxidants might be either chemisorbed oxygen and/or iron oxides present in an inorganic matter. Surface acidic groups, when located in larger pores, attract 4,6-DMDBT via specific interactions, and this can increase the amount adsorbed. When the density of these groups is high, they create obstacles for the effective packing of the adsorbate in the pore space and, thus, the amount adsorbed decreases.

Enhanced reactive adsorption of hydrogen sulfide on the composites of graphene/graphite oxide with copper (hydr)oxychlorides.

Composites of copper (hydr)oxychlorides with graphite oxide or graphene were synthesized and used as adsorbents of hydrogen sulfide at dynamic conditions at ambient temperatures. The materials were extensively characterized before and after adsorption in order to link their performance to the surface features. X-ray diffraction, FTIR, thermal analysis, TEM, SEM/EDX, and adsorption of nitrogen were used. It was found that the composite with graphene has the most favorable surface features enhancing reactive adsorption of hydrogen sulfide. The presence of moisture in the H2S stream has a positive effect on the removal process owing to the dissociation process. H2S is retained on the surface via a direct replacement of OH groups and via acid-base reactions with the copper (hydr)oxide. Highly dispersed reduced copper species on the surface of the composite with graphene enhance activation of oxygen and cause formation of sulfites and sulfates. Higher conductivity of the graphene phase than that of graphite oxide helps in electron transfer in redox reactions.

Removal of dorzolamide from biomedical wastewaters with adsorption onto graphite oxide/poly(acrylic acid) grafted chitosan nanocomposite

A novel graphite oxide/poly(acrylic acid) grafted chitosan nanocomposite (GO/CSA) was prepared and used as biosorbent for the removal of pharmaceutical compound (dorzolamide) from biomedical synthetic wastewaters. The performance was evaluated taking into account pH, kinetics and thermodynamics of adsorption. GO/CSA presented higher adsorption capacity in comparison with the parent materials (graphite oxide and poly(acrylic acid) grafted chitosan). All adsorbents prepared were characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR), and potentiometric titration. The surface features were also evaluated after the dorzolamide adsorption in order to derive the adsorption mechanism. It was suggested that the reactive groups of GO and CSA can interact with the amino groups of dorzolamide and mainly the abundance of carboxyl groups of GO/CSA composite was the main reason for its enhanced adsorption capacity.

Zinc (hydr)oxide/graphite based-phase composites: effect of the carbonaceous phase on surface properties and enhancement in electrical conductivity

Composites of zinc hydroxide with graphite-oxide and graphite derived material are prepared using in situ precipitation of Zn(OH)2 in the presence of the dispersed graphite-based phases. The new materials are characterized by a range of methods, including SEM, infrared and Raman spectroscopy, thermal analysis, potentiometric titration, nitrogen adsorption, XPS, and electrical measurements. The results indicate that the final properties of the composites are determined by a complex interplay between the two components. When graphite-oxide is used, its oxygen containing functional groups are involved during synthesis with a zinc hydroxide precursor, leading to the formation of interface bonds between the inorganic and carbon-based phase. This new interface not only affects the chemistry of the materials but also determines texture and porosity. Another conversion of the inorganic phase to zinc oxide via thermal treatment further affects the properties of the composite, leading to the formation of additional chemical bonds between the zinc oxide and graphite-oxide phases. Even though the addition of graphite-oxide results in a highly porous material of heterogeneous nature, the thermally treated composite has a comparable electrical conductivity to the zinc hydroxide–graphite derived material composite, although having a lower density. This is attributed to the increased abundance of sp2 hybridization, the presence of Zn0, and improved local electrical connectivity between the two phases of the composite through the new interface bonds.