Synthetic carbons activated with phosphoric acid: I. Surface chemistry and ion binding properties

Synthetic activated carbons were prepared by phosphoric acid activation of a styrene-divinylbenzene copolymer at various temperatures in the 400-1000 °C range. The resulting carbons were characterized by elemental analysis, cation-exchange capacity measurement, infrared spectroscopy, potentiometric titration with calculation of proton affinity spectra, and copper adsorption from solution. The results indicate that the synthetic carbons obtained possess acidic character and show cation-exchange properties similar to those of oxidized carbons. However, the acidic compounds arising from treatment with phosphoric acid are tightly bound to the carbon lattice and are chemically and thermally more stable than those introduced by oxidative treatments. The largest amount of cation-exchange surface groups is introduced after activation at 800 °C. Infrared investigations showed that phosphorus compounds may be polyphosphates bound to the carbon lattice. Proton affinity distribution curves calculated from potentiometric titration experiments showed four types of surface groups on synthetic phosphoric acid activated carbons. Among them phosphorus-containing groups are the most important for the adsorption of heavy metal ions (copper) from acid solutions. Thus, carbons activated with phosphoric acid may be regarded as prospective cation-exchangers for the removal of heavy metals from water solutions.     

Effects of carbon dioxide and acidic carbon compounds on the analysis of Boehm titration curves

In the characterization of the surface functionality of acid-oxidized carbon nanotubes (CNTs) using the Boehm titration method, dissolution of acidic carbon compounds (ACCs) fragmented from the acid-oxidized CNTs and atmospheric carbon dioxide (CO₂) in the reaction base results in a somewhat over-estimation of the surface functionality of the CNTs. We developed a modified Henderson–Hasselbalch equation to show that the influence of the carboxylic and phenolic groups of ACCs and atmospheric CO₂ on the resultant titration curve can be identified and quantitatively measured, which makes it possible to determine the true surface functionality of acid-oxidized CNTs and ACCs.     

Effect of oxidation debris on spectroscopic and macroscopic properties of graphene oxide

Oxidation debris (OD) and graphene oxide (GO) before and after OD removal were characterized by Fourier transform infrared (FTIR) spectroscopy, Raman spectroscopy, mass spectroscopy, X-ray diffraction, transmission electron microscopy and potentiometric titration, respectively. OD removal decreased GO absorption intensity in UV/Vis spectra, caused changes in peak position and absorption intensity in FTIR spectra, and resulted in the decrease of I_D/I_G in Raman spectra. OD was amorphous and had higher content of acidic groups than purified GO. OD contributed 10–25% of overall surface charge density to unpurified GO in spite of small amount (ca. 1% mass). OD removal decreased significantly GO dispersibility in aqueous solution, but increased obviously the electrical conductivity of reduced graphene oxide (rGO) and the apparent density of compacted rGO. The removal of OD was necessary because of its striking effects on both GO spectroscopic and macroscopic properties. Batch desorption in NaOH solution was recommended for OD removal from as-prepared graphite oxide because of slow OD desorption kinetics.     

Removal of nickel(II) from aqueous solution by carbon nanotubes

Single‐walled carbon nanotubes (SWCNTs) and multiwalled carbon nanotubes (MWCNTs) were oxidized by NaClO solutions and were employed as sorbents to study sorption characteristics of nickel(II) from aqueous solution. The surface properties of CNTs such as functional groups, total acidic sites and negatively charged carbons were greatly improved after oxidation, which made CNTs become more hydrophilic and resulted in sorption of more Ni²⁺. The amount of Ni²⁺ sorbed onto oxidized CNTs increased with a rise in agitation speed, initial Ni²⁺ concentration and solution pH in the range 1–8, but decreased with a rise in CNT mass and solution ionic strength. The sorption mechanisms are complicated and appear attributable to electrostatic forces and chemical interactions between the Ni²⁺ and the surface functional groups of the CNTs. The oxidized SWCNTs and MWCNTs have shorter equilibrium time and better Ni²⁺ sorption performance than the oxidized granular activated carbon, suggesting that both NaClO oxidized CNTs are efficient Ni²⁺ sorbents and that they possess good potential applications in water treatment. Copyright © 2006 Society of Chemical Industry     

Adsorption of cadmium(II) from aqueous solution by surface oxidized carbon nanotubes

Carbon nanotubes (CNTs) were oxidized with H₂O₂, KMnO₄ and HNO₃. Their physicochemical properties were investigated by BET N₂ adsorption, laser particle examination, Boehm’s titration, zeta potential measurement and cadmium(II) adsorption. The experimental results suggest that cadmium(II) adsorption capacities for three kinds of oxidized CNTs increase due to the functional groups introduced by oxidation compared with the as-grown CNTs. The cadmium(II) adsorption capacity of the as-grown CNTs is only 1.1 mg g⁻¹, while it reaches 2.6, 5.1 and 11.0 mg g⁻¹ for the H₂O₂, HNO₃ and KMnO₄ oxidized CNTs, respectively, at the cadmium(II) equilibrium concentration of 4 mg l⁻¹. Adsorption of cadmium(II) by CNTs was strongly pH-dependent and the increase of adsorption capacities for HNO₃ and KMnO₄ oxidized CNTs is more obvious than that of the as-grown and H₂O₂ oxidized CNTs at lower pH regions. The experiments of CNT dosage effect on the cadmium(II) adsorption show that the adsorption capacity for KMnO₄ oxidized CNTs has a sharper increase at the CNT dosage from 0.03 to 0.08 g per 100 ml than the as-grown, H₂O₂ and HNO₃ oxidized CNTs and its removal efficiency almost reaches 100% at CNT dosage of 0.08 g per 100 ml. Analysis revealed that the KMnO₄ oxidized CNTs hosted manganese residuals, and these surely contributed to cadmium sorption to a yet-undefined extent.     

Temperature dependence of the point of zero charge of oxidized and non-oxidized activated carbons

Three activated carbons in the form of grains, fibers and pellets were oxidized with ammonium peroxidisulfate. The oxidized and non-oxidized activated carbons were characterized by: N₂ adsorption at 77 K, temperature programmed desorption up to 1273 K, and potentiometric titrations in the temperature range between 288 and 318 K. The point of zero charge (PZC) of the oxidized activated carbons decreased with increasing temperature and this decrease was in the same direction as the change in 1/2pK_w, the neutral point of water. The non-oxidized activated carbons behaved in a similar manner, but the PZC decreased by more than the corresponding change in 1/2pK_w. Variation of surface charge of the three oxidized activated carbons was fitted to a single second-order function with respect to pH and temperature. However, a single equation could not be found for the three non-oxidized activated carbons. Standard thermodynamic functions at the PZC were obtained from potentiometric curves. Acidity constant distributions were obtained and showed an increase in the number of acidic groups when the temperature increased. The number of carboxylic and phenolic groups obtained from these distributions was compared with that obtained from the deconvolution of temperature programmed desorption spectra.     

Potentiometric titration of humic acids

The acid-base properties of humic acids from the brown coal of the Aleksandriiskoe deposit (Ukraine) were investigated. The possibility of determining COOH and OH groups in humic acid macromolecules by different experimental methods was analyzed. It was found that the results of potentiometric titration depend on the method of titration (homogeneous or heterogeneous), the concentration of humic substances in the system, and the time taken to establish an equilibrium value of pH in the titrate. It was shown that the amount of accessible functional groups (COOH and OH) in humic acid macromolecules, which can be determined by a potentiometric method, is greater in homogeneous titration and at low concentrations of humic substances in solution. It was established that the total amount of acid groups determined by a barium method is higher than that measured with the use of potentiometric titration. The concentration of carboxyl groups determined by an acetate method is much lower than the results of potentiometric titration and the barium method. It was shown that the potentiometric titration of homogeneous systems (sodium humate + HCl) is preferable in the analysis of the acid-base properties of humic acids.     

Determination of dissociable groups in natural and regenerated cellulose fibers by different titration methods

Different titration methods were applied with the purpose to determine the dissociation properties of a natural (cotton) and regenerated (viscose, modal and lyocell) cellulose fibers. Potentiometric and conductometric titration were used to determine the content of acidic groups. pK values were determined by potentiometric titration. Polyelectrolyte adsorption was used for surface and total charge determination, and to obtain information about charge location and accessibility of charged groups. It was found that the average content of acidic groups is higher in cotton fibers than in regenerated fibers. The fiber charge of cotton is due to the dissociation of two type of acidic groups, one with pK ≈3.5 and the other with pK ≈5.5. In regenerated fibers there is only one type of acidic groups (pK ≈3.5). The pK value of the stronger acid is typical for carboxyl group in uronic acids. The polyelectrolyte adsorption indicates that most of the carboxyl groups are located in an inner region of all cellulose samples (cotton and regenerated fibers). It is concluded that titration methods are powerful tools for monitoring the content, strength, and distribution of acidic groups, as well as the total charge of natural and regenerated cellulose fibers. The three methods give similar results on all analyzed samples and show good repeatability. The results of investigation make it quite clear that combination of all titrations yields relevant information about content and strength of acidic groups in both natural and regenerated cellulose fibers used in the manufacture of textiles. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 92: 3186–3195, 2004     

Rapid synthesis of water-soluble carbon nanotubes-supported PtRu nanoparticles for methanol electrooxidation

A one-pot, rapid microwave-assisted method is proposed to directly synthesize water-soluble surface-aminated carbon nanotubes (CNTs-NH₂) from pristine CNTs by covalently grafting ethylenediamine onto the surface of CNTs, requiring no cumbersome chemical oxidation of CNTs. The excellent water-solubility and abundant surface functional groups of the CNTs-NH₂ lead to the production of uniformly dispersed PtRu nanoparticles (NPs) with diameter ranging from 1.5 to 4.5 nm. Cyclic voltammetry and amperometric studies indicate that the resulting PtRu NPs/CNTs-NH₂ nanohybrids have higher electrochemical surface area, better electrocatalytic performance and higher stability towards methanol oxidation compared to PtRu NPs supported on the pristine CNTs. The high electrocatalytic performance is mainly contributed to the small particle size and high dispersion of PtRu NPs and good proton conductivity of CNTs-NH₂. This work may demonstrate a universal approach to fabricate superior metal nanocrystalline on CNTs for broad applications in energy systems and sensing devices.     

Role of microporosity and surface chemistry in adsorption of 4,6-dimethyldibenzothiophene on polymer-derived activated carbons

Two carbon samples derived from poly(4-styrenesulfonic acid-co-maleic acid) based polymer by carbonization between 700 and 800 °C were oxidized to two different levels of surface acidity. The surfaces of resulting adsorbents were characterized by potentiometric titration, adsorption of nitrogen, FTIR, SEM/EDAX and thermal analysis. The materials were used as adsorbents of 4,6-dimethyldibenzothiophene (4,6-DMDBT) from hexadecane with initial concentration of sulfur between 10-150 ppmw. Although it was found that pores with diameter less than 10 Å govern the amount of 4,6-DMDBT adsorbed, that amount is enhanced when acidic groups are present in the larger pores owing to the contributions of specific interactions. Surface chemistry plays an important role in reactive adsorption and deposition of the products of surface reactions in the pore system.     

Superabsorbent composite. X. Effects of saponification on properties of polyacrylamide/attapulgite

A novel superabsorbent composite from acrylamide (AM) and attapulgite (APT), polyacrylamide/attapulgite (PAM/APT), was prepared by polymerizing AM with the existence of APT in aqueous solution, and then saponified with NaOH solution. Considering the impacts of hydrophilic groups (? COONa, ? COOH, and ? CONH₂) on properties of the PAM/APT composite, the effects of saponification mode, molar ratio of NaOH to AM and saponification time on water absorbency, hydrophilic group content, and surface morphology were investigated systematically. The results indicate that the two‐step adding NaOH mode is superior to that of the one‐step mode. Among the superabsorbent composite incorporated with 30 wt% APT, the composite saponified at 95°C for 2 h with the molar ratio of 0.6 for NaOH to AM acquired the highest water absorbencies of 1715g g^?1 and 87.8g g^?1 in distilled water and in 0.9 wt% NaCl solution, respectively. The molar ratio of various hydrophilic groups at this time was 10:3:11 for ? COONa, ? COOH, and ? CONH₂ determined using linear potentiometric titration method. POLYM. ENG. SCI. 46:1762–1767, 2006. © 2006 Society of Plastics Engineers.     

Characterization of MCM-41 with Immobilized Bi-functional SH/SO3H Layer

A new approach for quantitative determination of –SH and –SO₃H functional groups on MCM-41 silica with covalently immobilized mercaptopropyl groups is proposed. A set of MCM-41 type samples containing 1 mmol g^?1 of mercaptopropyl groups was oxidized with proportional quantities of hydrogen peroxide and subsequently characterized with different ratios of grafted mercaptopropyl and propylsulfonic acid groups. Data calculated from conductometric titration were correlated with the corresponding X-ray photoelectron spectra and exhibit a linear relationship of results for both techniques. The described method was applied to thiol-containing MCM-41 samples to reveal ambient oxidation of organo-silicas with immobilized thiol groups.     

Surface chemistry of phosphorus-containing carbons of lignocellulosic origin

Activated carbon adsorbents were prepared by phosphoric acid activation of fruit stones in an argon atmosphere at various temperatures in the 400-1000 °C range and at different acid/precursor impregnation ratios (0.63-1.02). The surface chemistry of the carbons was investigated by elemental analysis, cation exchange capacity (CEC, measured by neutralization of NaOH with acidic surface groups), infrared spectroscopy and potentiometric titration. Porous structure was derived from adsorption isotherms (N₂ at −196 °C and CO₂ at 0 °C). It was demonstrated that all carbons show considerable cation exchange capacity, the maximum (CEC = 2.2 mmol g⁻¹) being attained at 800 °C, which coincides with the maximum contents of phosphorus and oxygen. The cation exchange properties of phosphoric acid activated carbons from fruit stones are chemically stable in very acidic and basic solutions. Proton affinity distributions of all carbons show the presence of three types of surface groups with pK at 2.0-3.3, 4.6-5.9 and 7.6-9.1. These pK ranges were ascribed primarily to: (a) phosphorus-containing and carboxylic groups; (b) lactonic groups, and (c) phenolic groups, respectively. Phosphoric acid activated carbons are microporous with a relatively small contribution of mesopores. A maximum BET surface area of 1740 m² g⁻¹ was attained at 400 °C.     

Hydrothermal Solubilization–Hydrolysis–Dehydration of Cellulose to Glucose and 5-Hydroxymethylfurfural Over Solid Acid Carbon Catalysts

Solid acid catalysts based on graphite-like mesoporous carbon material Sibunit were developed for the one-pot solubilization–hydrolysis–dehydration of cellulose into glucose and 5-hydroxymethylfurfural (5-HMF). The catalysts were produced by treating Sibunit surface with three different procedures to form acidic and sulfo groups on the catalyst surface. The techniques used were: (1) sulfonation by H₂SO₄ at 80–250 °C, (2) oxidation by wet air or 32 v/v% solution of HNO₃, and (3) oxidation-sulfonation what meant additional sulfonating all the oxidized carbons at 200 °C. All the catalysts were characterized by low-temperature N₂ adsorption, titration with NaOH, TEM, XPS. Sulfonation of Sibunit was shown to be accompanied by surface oxidation (formation of acidic groups) and the high amount of acidic groups prevented additional sulfonation of the surface. All the Sibunit treatment methods increased the surface acidity in 3–15 times up to 0.14–0.62 mmol g^?1 compared to pure carbon (0.042 mmol g^?1). The catalysts were tested in the depolymerization of mechanically activated microcrystalline cellulose at 180 °C in pure water. The main products 5-HMF and glucose were produced with the yields in the range of 8–22 wt% and 12–46 wt%, respectively. The maximal yield were achieved over Sibunit sulfonated at 200 °C. An essential difference in the composition of main products obtained with solid acid Sibunit carbon catalysts (glucose, 5-HMF) and soluble in water H₂SO₄ catalysts (formic and levulinic acids) as well as strong dependence of the reaction kinetics on the morphology of carbon catalysts argue for heterogenious mechanism of cellulose depolymerization over Sibunit.     

Characterization of the surface chemistry of carbon materials by potentiometric titrations and temperature-programmed desorption

Two methods currently used to characterize oxygen-containing functional groups on the surface of carbon materials are compared, namely potentiometric titration (PT) and temperature-programmed desorption (TPD). Two materials were used, activated carbon and multi-walled carbon nanotubes, which were subsequently modified by oxidative treatments in order to produce samples with different amounts of surface groups. The concentrations of carboxylic acid groups determined by both techniques are in good agreement, but the quantitative result obtained by TPD is closer to the analytical concentration of the species obtained by elemental analysis. Discrepancies between the quantitative results are more pronounced at higher pK_a values (weak acids), where the concentrations determined by PT are lower than those obtained by TPD. This directly reflects the effects of neglecting the electrostatic interaction parameter. The TPD method was particularly suited for the characterization of samples modified with ethylenediamine, which is anchored to specific oxygenated groups. PT results are useful to describe the material behaviour in aqueous solutions, where the activity of the surface groups depends not only on their concentrations, but also on their environment.     

Humidity switching properties of sensors based on multiwalled carbon nanotubes/polyvinyl alcohol composite films

Multiwalled carbon nanotubes (CNTs) were used as the conductive filler of composites for switching type humidity sensor. The CNTs were oxidized by mixed acids (H₂SO₄ : HNO₃) at a mild temperature to modify carboxylic acid (COOH) groups on the surface of the nanotubes. The dispersibility of acid treated CNTs (CNTs‐COOH) in water is much improved, which is beneficial for dispersing CNTs in the polyvinyl alcohol (PVA) matrix without external additives. The obtained CNTs‐COOH/PVA sensors show nonlinear response to relative humidity (RH), that is, switching properties. The resistances of the sensors remain constant before 80% relative humidity (RH) and then increase sharply with RH, indicating excellent switching characteristic of the sensors. The 10 wt % CNTs‐COOH/PVA sensor shows a sensitivity (ΔR/Ro) of 32.3 at 100% RH. The humidity switching properties of CNTs‐COOH/PVA are much better than that of pristine CNTs/PVA. The improvements are attributed to the improved balance between the dispersibility of CNTs‐COOH and electrical conductivity of the composite films. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 39726.     

Interaction between graphite oxide and Congo red in aqueous media

Interaction between graphite oxide and Congo red, a diazo dye containing NH₂ and SO₃ functional groups, was studied using UV–vis, FTIR, capillary electrophoresis and potentiometric methods. UV–vis experiments managed to elicit a colorful set of reactions, which were dependent on the sonication of solution. It was established that the molecules of Congo red after sonication are able to penetrate through the hydration shell and extract the protons attached to the functional groups on the surface of graphite oxide. Interaction between the graphite oxide particles and Congo red molecules is suggested to stabilize the aqueous suspensions of single-layer graphite oxide sheets. Potentiometric titration data show that the Congo red molecules adsorbed on the surface of graphite oxide are able to prevent the penetration of OH⁻ ions to reach the acidic functional groups inside the graphite oxide stacks. Electrostatic repulsion among these functional groups is quite high. Ionization of the graphite oxide colloidal particles with adsorbed Congo red molecules leads to the significant changes in their conformation.     

Radiation grafting of AMPSA and DMAEM onto membranes of acrylonitrile copolymer and polyamide for immobilization of glucose oxidase

Membranes of acrylonitrile copolymer and polyamide were modified by radiation grafting of 2-acrylamido-2-methylpropanesulphonic acid and 2-dimethylaminoethyl methacrylate. The effect of irradiation dose (from an electron beam accelerator) and method of introduction of ferrous ions on the grafting degree was studied. The sulpho- and quaternary amino groups introduced into the membranes were determined quantitatively by potentiometric titration. The grafting degree and hydrophilicity of the modified membranes were studied. Glucose oxidase was immobilized onto modified membranes, and its basic characteristics were determined: bound protein, relative activity, pH_opt, and T_opt, pH, and thermal stability, as well as storage of activity. © 1996 John Wiley & Sons, Inc.     

Carbon nanotube-Cu hybrids enhanced catalytic activity in aqueous media

In this work, we report the synthesis of a hybrid material, consisting of Cu nanoparticles (Cu-NPs) anchored on the surface of carbon nanotubes (CNTs), which is able to increase the catalytic behavior when compared to bare CuNPs. This enhanced activity was evaluated by the electrophoretic separation of three carbohydrates: sucrose, glucose and fructose. CNTs were first synthesized in the presence of minute amounts of oxygen, oxidized using H₂O₂-UV radiation and then decorated with Cu-NPs. The resulting hybrid structure was confirmed and characterized by X-ray photoelectron spectroscopy, transmission electron microscopy, high resolution transmission electron energy-dispersive X-ray spectroscopy, titration, thermogravimetric analysis, elemental analysis and Raman spectroscopy.     

Poly[N-Heterylmethyl(Propyl, Butyl)](Organo)Siloxanes — New Modifiers for Chemical Fibres and Cloth

A method was developed for production of chemically modified fibres containing covalently bound poly[N-heterylmethyl(propyl,butyl)](organo)siloxanes on the surface. These modifiers are of great interest as specific reagents and catalysts in organic and organometallic synthesis, sorbents for precious metals, and biologically active substances. The modified fibres are obtained with a high yield in the reaction of condensation of N-sodium derivatives of 3,5-dimethylpyrazole, 4-methyl-1-phenyl-3-pyrazolidone, 1, 2, 4-triazole, 2-methylindole, benzimidazole with (organo)siloxane chloromethyl(propyl,butyl)silyl groups covalently bound on the surface of the fibres. The concentration of bound groups was determined by elemental analysis. The new materials were characterized with the data from the IR spectra.