Effect of H<Subscript>2</Subscript>O<Subscript>2</Subscript> modification of H<Subscript>3</Subscript>PW<Subscript>12</Subscript>O<Subscript>40</Subscript>@carbon for m-xylene oxidation to isophthalic acid

The production of isophthalic acid (IPA) from the oxidation of m-xylene (MX) by air is catalyzed by H₃PW₁₂O₄₀ (HPW) loaded on carbon and cobalt. We used H₂O₂ solution to oxidize the carbon to improve the catalytic activity of HPW@C catalyst. Experiments reveal that the best carbon sample is obtained by calcining the carbon at 700 °C for 4 h after being impregnated in the 3.75% H₂O₂ solution at 40 °C for 7 h. The surface characterization displays that the H₂O₂ modification leads to an increase in the acidic groups and a reduction in the basic groups on the carbon surface. The catalytic capability of the HPW@C catalyst depends on its surface chemical characteristics and physical property. The acidic groups play a more important part than the physical property. The MX conversion after 180 min reaction acquired by the HPW@C catalysts prepared from the activated carbon modified in the best condition is 3.81% over that obtained by the HPW@C catalysts prepared from the original carbon. The IPA produced by the former is 46.2% over that produced by the latter.     

Purification of Wet‐Process Phosphoric Acid by Hydrogen Peroxide Oxidation,Activated Carbon Adsorption and Electrooxidation

In this work, three technologies are studied for the purification of phosphoric acid produced by the wet process: chemical oxidation with hydrogen peroxide, adsorption onto activated carbon, and electrochemical oxidation by boron‐doped diamond anodes. The treatment of wet‐process phosphoric acid by chemical oxidation with H₂O₂ as oxidizing agent can remove 75 % of the initial TOC as maximum, indicating that this wet‐process phosphoric acid contains an important amount of organics that cannot be oxidized by hydrogen peroxide under the operation conditions used. High temperatures and hydrogen peroxide/TOC ratios close to 150 g H₂O₂/g TOC allow obtaining the best chemical oxidation results. The adsorption onto activated carbon can remove between 40 and 60 % of the initial TOC as maximum. Adsorption times of 2 hours and activated carbon/WPA ratios close to 12 g AC/Kg WTP assure both steady state and maximum adsorption of organics. The electrochemical process is the only technique by which complete mineralization of WPA organics can be achieved. Operating at 60 mA cm^–2 and at room temperature, high current efficiencies are achieved which only seem to decrease by mass transport limitations.     

Automatic potentiometric titration of surface acidity of carbon black

Acid strength distribution on commercial carbon blacks and chemically oxidized carbon blacks was measured by automatic titration using sodium hydroxide solutions as titrants. The titration curves show that the weaker acids with pKa between 7.0 and 11.0 occur in the amount of 0.025–0.490 m eq/g, the stronger ones with pKa between 2.0 and 7.0 in the amount of 0–0.475 m eq/g and trace amount of pKa range below 2.0. The acid strength distribution obtained by this method coincides with that by the Boehm method. The observed amounts of total acid exceed active hydrogen contents obtained by the Zerewitinoff method in the carbon blacks. Three different oxidizing agents were used to determine their effect on carbon black acidity. Nitric acid produced acidic groups at the highest surface density; H₂O₂ produced a predominance of weaker acids; hot air produced stronger acids with an increase of specific surface area.     

Oxidation of activated carbon by dry and wet methods: Surface chemistry and textural modifications

The effects of dry and wet oxidation treatments of activated carbon (AC) on the surface chemistry and porous structure are studied. Using cherry stones (CS), AC was first prepared by carbonization at 900 °C for 2 h in N₂ and activation at 850 °C for 2 h in CO₂. Then, the resulting AC was oxidized in O₂(air) or O₃ atmosphere and with HNO₃ and H₂O₂ solutions. The acidic-basic surface sites were analyzed by FT-IR spectroscopy, Boehm method, and pH of the point of zero charge (pH_pzc) and the porous structure by N₂ adsorption and mercury porosimetry. It has been found that the oxidizing agent, under specific reaction conditions, rather than whether it was a gas or a solute in aqueous solution, is the main factor that controls the changes produced in the surface chemistry and porous structure of AC. O₃ and HNO₃ are the most effective oxidants to form acidic oxygen surface groups. However, the content of basic groups decreases for the four oxidants, the effect being much stronger for HNO₃. A microporosity reduction is also observed, which is more important for O₂(air) and especially for HNO₃ than for O₃ and H₂O₂. The percentage of microporosity loss is as high as 43.3 for HNO₃. Mesoporosity significantly develops, whereas macroporosity usually remains practically unchanged. Dry oxidation of AC at 100 °C in O₃ has proved to be the most promising method to increase the content of acidic oxygen surface groups in the material without greatly decreasing the content of basic sites and microporosity and with a significant mesoporosity development.     

Electrical conductivity of chemically modified multiwalled carbon nanotube/epoxy composites

The electrical conductivity of oxidized multiwalled carbon nanotubes (MWNT)/epoxy composites is investigated with respect to the chemical treatment of the MWNT. The oxidation is carried out by refluxing the as-received MWNT in concentrated HNO₃ and H₂O₂/NH₄OH solutions, respectively, under several different treatment conditions. The oxidized MWNT are negatively charged and functionalized with carboxylic groups by both solutions. The MWNT oxidized under severe conditions are well purified, but their crystalline structures are partially damaged. It is recognized that the damage to the MWNT has considerable influence on the electrical properties of the MWNT composites, causing the electrical conductivity to be lowered at a low content of MWNT and the percolation threshold to be raised. The MWNT oxidized by the mixture of H₂O₂ and NH₄OH solution provides epoxy composites with a higher conductivity than those produced with the MWNT oxidized by nitric acid over the whole range of MWNT, independently of the oxidation conditions.     

Carbon materials as catalyst supports for SO2 oxidation: catalytic activity of CuO-AC

Copper catalysts supported on acid treated activated carbon (AC) were prepared, characterized and tested in terms of their SO₂ oxidation activity. Reactions of CuO-AC in flow systems with sulfur dioxide, oxygen and nitrogen streams were investigated to determine the types of chemical interactions that occur on the sorbent surface. The effects of reaction temperature, acid treatment, metal loading, support particle size, SO₂ concentration and O₂ concentration on SO₂ oxidation activity were evaluated. It was found that carbon materials used as catalyst supports for copper oxide catalysts provided a high catalytic activity for adsorbing SO₂ from flue gas and oxidizing it. Acid pretreatment of the carbon supports increased the content of specific surface chemical groups to enhance the catalytic activity for SO₂ oxidation. Metal loading, as well as support particle size, have a significant influence on the SO₂ activity. The supported metals rather than surface oxygen functional groups on AC may be the active sites for adsorbing SO₂.     

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.     

Preparation of an activated carbon artifact: oxidative modification of coconut shell-based carbon to improve the strength

Coconut shell-based activated carbon was oxidized in aq. H₂SO₄, HNO₃ and H₂O₂ to induce surface oxygen functional groups on its surface and to increase the mechanical strength of the resultant activated carbon artifact with PVB as a binder. Although all oxidation was confirmed to significantly increase the strength, aq. H₂O₂ was found to be most effective, giving strength as high as 6000 kPa, which is believed to be sufficient for the electrode of an electric double layer capacitor (EDLC). The increase of CO₂ evolving groups induced on the surface of activated carbon appears to be responsible for the strength increase. There was an optimum extent of oxidation for the strength as well as the performance of the electrode. Too much oxidation reduces the electrical conductivity of the activated carbon. Facile oxidation by aq. H₂O₂ can be recommended as a practical modification of the surface since it takes place safely below 100°C without releasing any harmful gas.     

The effect of the gradual thermal decomposition of surface oxygen species on the chemical and catalytic properties of oxidized activated carbon

The physicochemical properties, surface chemical structure and some catalytic properties of a series of carbons prepared by nitric acid oxidation of an activated carbon and subsequent heat treatment under vacuum and mild temperature conditions (423-573 K) were studied. The porous structure characteristics of the partially evacuated samples were estimated from low-temperature nitrogen adsorption data. The thermal analysis and the quantitative determination of surface functional groups by selective neutralization of bases and pH-metric titration were carried out. The dehydration of 2-methylpropan-2-ol was used as a test reaction. While gradual annealing in vacuum alters the surface only slightly, it does differentiate strongly the number and the acidic strength of the surface groups. Progressive heating under mild conditions removes mainly those surface groups that are located in macropores or on the outer surface of the carbon. According to TPD results, the decomposed surface groups are single carboxylic groups, as expected. The decomposition of single, strong carboxylic groups is accompanied by rearrangements of other carboxylic groups with the simultaneous formation of additional cyclic structures like anhydrides, lactones or lactols. Catalytic tests support our previous findings that oxidized carbons have a high dehydration activity. This activity is controlled not only by the number and the strength of acidic groups, but also by their accessibility. There exists an optimum concentration of surface acidic groups above which the activity decreases due to steric restrictions.     

Decomposition of hydrogen peroxide in the presence of activated carbons with different characteristics

BACKGROUND: Catalytic ozonation promoted by activated carbon is a promising advanced oxidation process used in water treatment. Hydrogen peroxide generated as a by‐product from the reaction of ozone with some surface groups on the activated carbon or from the oxidation of some organic compounds present in the water being treated seems to play a key role in the catalytic ozonation process. Hydrogen peroxide decomposition promoted by two granular activated carbons (GAC) of different characteristics (Hydraffin P110 and Chemviron SSP‐4) has been studied in a batch reactor. The operating variables investigated were the stirring speed, temperature, pH and particle size. Also, the influence of metals on the GAC surface, that can catalyze hydrogen peroxide decomposition, was observed. RESULTS: Chemviron SSP‐4 showed a higher activity to decompose hydrogen peroxide than HydraffinP110 (70 and 50% of hydrogen peroxide removed after 2 h process, respectively). Regardless of the activated carbon used, hydrogen peroxide decomposition was clearly controlled by the mass transfer, although temperature and pH conditions exerted a remarkable influence on the process. Catalytic ozonation in the presence of activated carbon and hydrogen peroxide greatly improved the mineralization of oxalic acid (a very recalcitrant target compound). About 70% TOC (total organic carbon) depletion was observed after 1 h reaction in this combined system, much higher than the mineralization achieved by the single processes used. CONCLUSIONS: Of the two activated carbons studied, Chemviron SSP‐4 with an acidic nature presented a higher activity to decompose hydrogen peroxide. However the influence of the operating variables was quite similar in both cases. Experiments carried out in the presence of tert‐butanol confirmed the appearance of radical species. A kinetic study indicated that the process was controlled by the internal mass transfer and the chemical reaction on the surface of the activated carbon. The catalytic activity of hydrogen peroxide in oxalic acid ozonation promoted by activated carbon (O₃/AC/H₂O₂) was also studied. The results revealed the synergetic activity of the system O₃/AC/H₂O₂ to remove oxalic acid. Copyright © 2010 Society of Chemical Industry     

Combined XPS and TPD study of oxygen-functionalized carbon nanofibers grown on sintered metal fibers

A composite material consisting of carbon nanofibers (CNFs) grown on sintered metal fiber filters was modified by H₂O₂ or plasma-generated O₃. Coupling temperature programmed desorption (TPD) and X-ray photoelectron spectroscopy (XPS) techniques in the same UHV apparatus allowed the direct correlation of the nature of the created O-functional groups and their evolution as CO and CO₂ upon heating. The two oxidative treatments yielded different distributions of O-containing groups. The relative contribution of oxidized carbon was very low in the C1s region, hence the functional groups were more robustly analyzed through the O1s region. The comparison of the released oxygen by integration of the TPD CO, CO₂ and H₂O spectra with the intensity loss of the XPS O1s spectra showed good agreement. In order to fit the data adequately, the set of O1s spectra was deconvoluted in at least four peaks for the differently activated samples. Finally, it was shown that functional groups formed by H₂O₂-treatment (mostly non-phenolic OH groups) are more thermally stable than those formed by O₃-treatment. The latter treatment increases the concentration of carboxylic functionalities, which decompose at temperatures < 800 K; O₃-activated CNFs should therefore show a more pronounced acidic behavior.     

Modified activated carbons for catalytic wet air oxidation of phenol

This study aims at testing several activated carbons for the catalytic wet air oxidation (CWAO) of phenol solutions. Two commercial activated carbons were used both as received and modified by treatment with either HNO₃, (NH₄)₂S₂O₈, or H₂O₂ and by demineralisation with HCl. The activated carbons were characterised by measuring their surface area, distribution of surface functional groups and phenol adsorption capacity. The parent and treated activated carbons were then checked for CWAO using a trickle bed at 140 °C and 2 bar of oxygen partial pressure. The treatments increase the acidic sites, mostly creating lactones and carboxyls though some phenolic and carbonyl groups were also generated. Only (NH₄)₂S₂O₈ treatment yields a significant decrease in surface area. CWAO tests show that catalytic activity mainly depends on the origin of the activated carbon. The modifications generally had a low impact on phenol conversion, which correlates somewhat with the increase in the acidity of the carbons. Characterisation of the used activated carbon evidences that chemisorbed phenolic polymers formed through oxidative coupling and oxygen radicals play a major role in the CWAO over activated carbon.     

Surface properties of carbon micro-coils oxidized by a low concentration of oxygen gas

Vapor-grown carbon micro-coils were oxidized under a low O₂ flow-rate for introducing oxygen-containing functional groups on the surface. The surface characteristics were then examined. The O1s/C1s intensity ratios of the XPS spectra measured on the surface increased on using the oxidation treatment. The maximum O1s/C1s ratio of 11.4 at.% was obtained under the following conditions: (a) the flow-rate of the mixed gas of O₂+Ar (O₂/Ar=1/10) was 82.5 sccm; (b) the oxidation temperature was 600°C; and (c) the treatment time was 30 min. The maximum O1s/C1s ratio is about 3.5 times that of the source carbon coils. The specific surface area significantly increased due to the oxidation treatment and attained a maximum value of 1050 m² g⁻¹, which is about 10 times that of the source carbon coils. As the specific surface area increased, the surface morphology of the carbon coils became more complicated on a nanometer scale.     

Differentiation and quantification of surface acidities on MWCNTs by indirect potentiometric titration

Wet-chemically oxidized carbon nanotubes (CNTs) generally exhibit both covalently-bound acidic functional groups on the surface and surface-adsorbed acidic substances, i.e. carbonaceous CNT fragments from the oxidation procedure. Direct potentiometric titration of oxidizable high surface area materials with dynamically desorbing acidic fragments is slow and inaccurate. Adsorbed acidic fragments are deprotonated by sodium hydroxide and form anions in solution which is not the case for covalently bound acidic groups on the CNTs, so the following filtration after NaOH treatment separates desorbable acidic substances from non-desorbable or covalently bound groups. For a known concentration of NaOH, titration of the eluate with hydrochloric acid (HCl) allows determination of the concentrations of both types of acidities. However, contrary to reports in the literature, the NaOH consumed by non-desorbed acidic groups has to be accounted for and impedes distinction of desorbed acidic groups via their pK_a values. Results are presented of a study on the information content and the reliability of indirect potentiometric Boehm titration for different oxidized multi-walled CNTs.     

Characterization of activated carbon, graphitized carbon fibers and synthetic diamond powder using TPD and DRIFTS

A high surface area activated carbon, graphitized carbon fibers and synthetic diamond powder were characterized by X-ray diffraction, temperature-programmed desorption and diffuse reflectance infrared (IR) spectroscopy (DRIFTS). The activated carbon was analyzed as received as well as after either a nitric acid treatment to introduce oxygen functional groups on its surface or a high temperature treatment (HTT) in H₂ at 1223 K to remove surface groups. TPD evolution profiles of CO and CO₂ were combined with DRIFTS spectra of these carbon surfaces before and after pretreatments in H₂ at 723 and 1223 K to provide complementary information regarding the nature of these surface groups. Significant amounts of both low- and high-temperature CO₂ desorption occurred from the HNO₃-treated carbon, indicating that both strongly and weakly acidic groups were introduced on this surface and, in addition, comparable amounts of CO and CO₂ were desorbed. With the graphitized carbon fibers and diamond powder, larger amounts of CO were desorbed compared to CO₂, indicating the presence of predominantly weakly acidic or non-acidic groups on these surfaces. For the HNO₃-treated carbon, IR peaks associated with surface carboxylic acid groups initially present disappeared after treatment at 723 K, while bands attributable to anhydride, quinone, ester and phenol species remained. Small amounts of ether, furan and phenol groups were detected on the graphitized fiber surface, while ketonic carbonyl groups were dominant on diamond. Significant amounts of chemisorbed hydrogen were also detected, presumably occurring on edge atoms made available by the decomposition of CO-yielding complexes at temperatures >873 K.     

Characterisation of the surface of oxidised carbon adsorbents

The surface reactivity and functional group content of a series of oxidised active carbons has been assessed by elemental analysis, electrophoretic mobility measurements and potentiometric titrations. Oxidation of carbons with hot air resulted in a greater proportion of relatively weak acidic surface functional groups (i.e., phenolic), whereas nitric acid modification produced a greater amount of carboxylic groups. Electrophoretic mobility measurements suggest that the carbon surface is negatively charged within the range of the pH values studied. pH titration results indicate that the surface acidity of active carbons is stronger than that of a commercial polymeric carboxylic acid ion exchange resin. Possible mechanisms of carbon surface oxidation are discussed.     

Preparation and Study of Pd Catalysts Supported on Activated Carbon Cloth (ACC) for Direct Synthesis of H2O2 from H2 and O2

Activated carbon cloths (ACCs) were used as supports for Pd catalysts. The catalyst preparation was carried out by the impregnation method using acidic solution of palladium dichloride (PdCl₂) as metal precursor. The effects of the oxidation state of the loaded metal, heat treatment of the catalysts in different atmosphere (H₂, air) at different temperatures and surface chemistry of the support on the catalyst characterizations and the catalytic activities were investigated. Wet oxidation of ACC was done by nitric acid in order to induce oxygen-containing surface functional groups. Surface chemistry of the support and oxidation state of the metallic phase was investigated by means of XPS, TPD, SEM, DTA and TGA tests. Direct synthesis of hydrogen peroxide from H₂ and O₂ was performed batch wise in a stainless steel autoclave. The reactions were conducted under high pressure (38 bar) at 0 °C and methanol was used as reaction medium. The direct synthesis results showed that the oxygen-containing surface functional groups increase the selectivity of the catalysts by reducing the rate of water production. Existence of the oxidized state of Pd (PdO) also makes the catalyst more selective than the corresponding zerovalent state (Pd0). PdO affected on selectivity by increasing the rate of H₂O₂ production and reducing the amount of production of water, simultaneously.     

Effects of the oxidation temperature on the structure and properties of polyacrylonitrile‐based activated carbon hollow fiber

Polyacrylonitrile (PAN) hollow fibers were pretreated with ammonium dibasic phosphate, oxidized in air, carbonized in nitrogen, and activated with carbon dioxide. The effects of the oxidation temperature of the PAN hollow fiber precursor on the microstructure, specific surface, pore size distribution, and adsorption properties of PAN‐based activated carbon hollow fiber (PAN‐ACHF) were studied. When PAN hollow fibers were oxidized at 270°C, because of drastic oxidation, chain scission occurred, and the number of pores within and on the surface of the resultant PAN‐ACHF increased, but the pores were just in the thinner region of the skin of PAN‐ACHF. The surface area of PAN‐ACHF reached a maximum when the oxidation temperature was 270°C. The adsorption ratios to creatinine were all higher than 90% at all oxidation temperatures, and the adsorption ratio to VB₁₂ reached a maximum (97%) at 230°C. The dominant pore sizes of the mesopores in PAN‐ACHF ranged from 2 to 5 nm. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 98: 203–207, 2005     

Investigation of nitric acid treatment of activated carbon for enhanced aqueous mercury removal

In the present work, Hg(II) adsorption of a commercial activated carbon with and without nitric acid treatment was compared in a batch system. Iodine adsorption test and nitrogen adsorption and desorption experiments were carried out to investigate the changes in porous characteristics during acid treatment. Although the results for iodine adsorption of two samples were approximately similar, the increase in porous characteristics during acid treatment was determined by micropore volume and total pore volume of treated and untreated samples. To evaluate the effects of acid treatment on the surface functional groups, FTIR analysis for both types of activated carbons was performed, and showed oxidized surface for treated sample. Furthermore, composition of the gaseous by-product resulted from this treatment has been qualitatively analyzed using a FTIR device. Consequently, NO, NO₂, N₂O₄, N₂O, CO, and CO₂ were detected. Kinetic and equilibrium adsorption studies were performed considering effective parameters, including contact time, initial pH, and initial concentration. It can be seen that nitric acid treatment of activated carbon has enhanced Hg(II) adsorption capacity. Moreover, kinetic studies showed faster adsorption rate for treated activated carbon through changes in external surface rather than internal.     

Catalytic destruction of methyl tertiary butyl ether (MTBE) using oxidized carbon

Decomposition of methyl tert-butyl ether (MTBE) in the gas phase was studied using carbon catalysts with chemically modified surface. Carbon samples with different surface chemical properties were obtained from commercial activated carbon D43/1 (CarboTech, Essen, Germany) by oxidation in liquid phase with various oxidants as well as in air. The catalytic tests were performed in a flow reactor at a temperature range of 353–473 K. Isobutene and methanol are the only products of the MTBE decomposition. The generation of surface acidic oxides considerably enhances the catalytic activity of the carbons. However, the activity is controlled not only by the number and strength of acidic groups, but also by their accessibility. The most active carbon is that oxidized with air at 673 K, which contains pores wider than the pores of other oxidized carbons.