Electrochemical surface oxidation of carbon nanofibers

Carbon nanofiber (CNF) surfaces were functionalized with oxygen-bearing groups through electrochemical oxidation. The electrode was prepared without a binder, allowing easy separation of the functionalized CNFs for subsequent applications. The relationships between the applied potential and the CNF structure with the resulting O/C atomic ratio and the distribution of oxygen functional groups were investigated. Surface groups were identified and characterized by elemental analyses, X-ray photoelectron spectroscopy, micro-attenuated total reflectance FTIR, and cyclic voltammetry. The oxidation of herringbone CNFs was initiated at a relatively low potential at both the anodic and cathodic electrodes, while the O/C atomic ratio remained relatively constant within the range of potentials investigated. The relative concentration of carbonyl and hydroxyl groups increased with increasing potential while the amount of carboxylic groups decreased. The structure of the CNF was important in determining the O/C atomic ratio, which was especially dependent on the spatial arrangement of graphene layers. Tubular CNFs exhibited low O/C atomic ratios while herringbone CNFs, which have a higher surface area, exhibited the largest ratios. The dispersion of the CNFs in water was much more homogeneous following electrochemical oxidation.     

Nitric acid oxidation of vapor grown carbon nanofibers

Vapor grown carbon nanofibers (Pyrograf III™) with 100–300 nm diameters and ∼10–100 μm lengths were oxidized in 69–71 wt.% nitric acid (115 °C) for various times (10 min to 24 h). These fibers were remarkably oxidation-resistant. XPS (O_1s) showed that the surface atomic oxygen percent increased from 6.3 to 18.3–22.5% for 10–90 min oxidations followed by a drop to 14–15% after 10–24 h oxidations. No damage was observed by TEM. Little change in surface area was observed by N₂ BET but CO₂-DR measurements exhibited an increase from 20–25 m²/g to 41–73 m²/g after 10–90 min of oxidation followed by a decrease to 35–22 m²/g after 10 h, consistent with the XPS findings. Shallow ultramicropore formation could account for the surface area increase. NaOH titrations showed ∼3-fold increase in surface acidic functions (∼27 to 76 μmol/g) occurring after 10 min of oxidation. Then this level remained constant through 24 h of oxidation. XPS (C_1s), O_1s) confirmed that carboxyl groups were removed and ester, anhydride, quinoid and phenolic hydroxyls appeared upon HNO₃ oxidation. Oxidized fibers dispersed when shaken in water, demonstrating wettability had increased. A model for this oxidation behavior is proposed.     

Regeneration of spent activated carbon by wet air oxidation

The regeneration of activated carbon was studied using the wet air oxidation process in the temperature range of 150–240°C and oxygen partial pressure range of 0.2 to 1.0 MPa. Phenol was used as substrate. The overall mechanism of regeneration has been analysed and the different steps taking place during the regeneration process were individually investigated. Kinetics of oxidation of phenol and oxygen mass transfer coefficients have been estimated in the ranges of temperature and pressure studied. Oxidative degeneration characteristics of activated carbon were also studied. The conditions of temperature and pressure have been found at which the extent of regeneration is favourable.     

Influence of pH on the wet oxidation of phenol with copper catalyst

This work reports the influence of pH on the catalytic wet oxidation (CWO) of phenol performed with a commercial copper-based catalyst. The results obtained show that pH is a critical parameter able to modify the chemical stability of the catalyst, the significance of the oxidation reaction in the liquid phase, the reaction mechanism and, consequently, the oxidation route of phenol. Experiments have been carried out to study the mentioned aspects. Stirred basket and fixed bed reactors (FBRs) have been employed, at 140 °C and at 16 bar of oxygen pressure. Three initial pH values have been used: 6 (the pH of the phenol solution), 3.5 (adjusted by H₂SO₄) and 8 (by addition of Na₂CO₃). Furthermore, some phenol oxidation runs without solid catalyst but with different concentrations of copper in solution have been accomplish at pHo=3.5. At acid pH, important leaching of copper from the catalyst to the solution was achieved, finding this negligible at pH 8. It was found that the major contribution to the phenol conversion reached at acid pH by using the solid catalyst was due to the catalytic activity of the leached copper. Both oxidation mechanisms at acid and basic conditions have been elucidated to explain the differences in the type and distribution of the intermediates obtained. The catalytic phenol oxidation route found at pH=8 comprises intermediates less toxic than phenol while at acid pH the cyclic intermediates formed as first oxidation intermediates are far more toxic than phenol.     

The efficiency of the oxidation of carbon nanofibers with various oxidizing agents

The oxidation of carbon nanofibers (CNFs) with various oxidizing agents; 6 M HNO₃, KMnO₄, RuO₄, and a mixture of concentrated H₂SO₄/HNO₃ is studied to determine the reaction conditions that optimizes yield and chemical functionalization. The oxidized nanofibers were examined by scanning electron microscopy, and Raman spectroscopy to characterize the defects caused by oxidation. The amount of acidic sites generated during oxidative treatment was quantified by reacting them with amine terminated octadecane and the subsequent quantification of the products of this grafting process by Fourier transform infrared spectroscopy. The concentrated H₂SO₄/HNO₃ mixture created the maximum number of carboxylic acid groups and the highest amount of defect sites, but had very low yield (∼12%). Six molar of HNO₃ generated fewer defect sites and a lower number of -COOH groups, but did exhibit a 70% yield. RuO₄ and KMnO₄ resulted in similar amounts of overall defect sites, however the yield and amount of -COOH groups for KMnO₄ treated CNF were significantly lower than the RuO₄ treatment. Overall, oxidation of these CNF with RuO₄ provides the best balance of yield and carboxylation.     

Production of acidified active carbon by wet oxidation and its carbon structure

Active carbon was oxidised in several oxidising solutions and measured for its surface acidity, specific surface area, X-ray diffraction patterns, ESR spectra and gas adsorption isotherms. The favourable oxidising conditions for acidifying active carbon were speculated. Three kinds of steam activated coconut-shell carbon were treated by solutions of nitric acid, alkaline permanganate, acidic permanganate with nitrate, acidic dichromate and of chlorate in fuming nitric acid. Graphite was similarly treated as a control. When the carbon was oxidised by all the above mentioned reagents except the acidic dichromate, its surface acidity increased unavoidably accompanied by the surface area reduction in almost a linear relation. However, the surface area reduction was extensive in the carbons oxidised by acidic dichromate. Neither the crystalline nor the amorphous carbon fraction was selectively oxidised by any of the oxidants to a significant extent when observed by X-ray diffraction, probably because of the lower levels of crystallisation in the micelles. On the other hand, the change of free radicals was observed by most of the oxidations which indicated the conformational change of the carbon at molecular level. The improved adsorption of ammonia was observed on the oxidised carbon up to 20 times and that of sulphur dioxide and water vapour to a few times. The adsorption of n-hexane was negatively affected by the oxidations.     

Influence of the inherent metal species on the graphitization of methane-based carbon nanofibers

Carbon nanofibers (CNFs) containing different proportions of Ni and Si were produced from methane decomposition in a fluidized bed reactor with a nickel–copper based catalyst. They were subjected to heat treatment in the temperature interval 1800–2800 °C for the purpose of studying the influence of the inherent metal species on their ability to graphitize. The participation of Ni and Si species on the graphitization of the methane-based CNFs through the formation of a nickel silicide phase as an intermediate state which further promotes the production of silicon carbide was inferred. Moreover, since silicon carbide was observed by X-ray diffraction after the heat treatment of the CNFs at temperatures ?2400 °C, the formation of graphite at the expense of the carbide decomposition seems to be a plausible mechanism to explain the catalytic graphitization of these CNFs. Because of this effect, carbon materials with crystalline parameters in the range of synthetic graphites which are currently employed in energy applications were prepared in this work. A progressive improvement of the degree of the structural order of the materials prepared with increasing Si/Ni weight ratio in the CNFs was observed.     

Robust growth of herringbone carbon nanofibers on layered double hydroxide derived catalysts and their applications as anodes for Li-ion batteries

Herringbone carbon nanofibers (CNFs) were efficiently produced by chemical vapor deposition on Ni nanoparticles derived from layered double hydroxide (LDH) precursors. The as-obtained CNFs with a diameter ranging from 40 to 60 nm demonstrated herringbone morphologies when they grew on Ni/Al LDH derived catalysts both in the fixed-bed and fluidized-bed reactor. The Ni/Mg/Al, Ni/Cu/Al, as well as Ni/Mo/Mg/Al catalysts were also effective to grow herringbone CNFs. The diameter and specific surface area of the as-obtained CNFs highly depended on the catalyst composition and the growth temperature. When CNFs were grown at 550 °C on Ni/Al catalyst, the as-obtained products had an outer diameter of ca. 50 nm and a specific surface area of 242 m² g⁻¹, possessed a discharge capacity of 330 mAh g⁻¹ as the electrode in a two-electrode coin-type cell. With the increase of the surface area, the discharge capacity increased at a rate of 0.90 mAh cm⁻², while the initial coulombic efficiency decreased gradually on nanocarbon anodes. This is attributed to the fact that CNFs with higher surface area afford smaller sp² carbon layer that facilitated more Li ions to extract from the anodes.     

The effect of graphitization temperature on the structure of helical-ribbon carbon nanofibers

Structural rearrangement of helical-ribbon carbon nanofibers (CNFs) was studied as a function of graphitization temperature. The as-produced nanofibers are composed of a helical ribbon of graphene spiralled about and angled to the fiber axis. The discrete layers of graphene ribbon overlap each other forming the helical-ribbon in contrast to the discontinuous cones of the more common stacked-cup CNF morphology. After heat treatment to 2400 °C and above, the CNFs were completely free of residual metal catalyst inclusions, principally nickel used in their synthesis, and other functionalities. The formation of loops at the graphene edges was also observed. Heat treatment through the temperature range 1500-2800 °C resulted in a relatively minor contraction in interlayer spacing d₀₀₂ from 0.3381 to 0.3363 nm. This was attributed to the highly graphitic character of the as-produced CNFs. However, there were significant increases in the crystallite thickness L_c through this temperature range. In addition, heat treatment above 2400 °C induced a marked change of the nanofiber morphology from circular to faceted polygonal cross-section resulting from the re-ordering of the turbostratic, curved graphene layers to regions of planar graphene layers with 3-dimensional graphitic structure (AB stacking).     

Temperature effect on the formation of catalysts for growth of carbon nanofibers

Turbostratic carbon nanofibers (CNFs), platelet graphite nanofibers (PGNFs) and tubular graphite nanofibers (TGNFs, also called multi-walled carbon nanotubes) were synthesized using thermal decomposition from a mixture of poly(ethylene glycol) and NiCl₂. A detailed study found that the synthesis temperature dramatically affected the morphology and topography of the catalysts, which play an important role in the synthesis of the various CNFs. At the temperature of 600 °C, irregular shape nanocatalysts with very rough surfaces were formed for the synthesis of turbostratic CNFs. Cubic-like nanocatalysts were formed at 750 °C for PGNFs and truncated cone-like nanocatalysts were formed at 850 °C for TGNFs. The surface roughness and the shape of the catalysts determined the stacking order of the graphene layers so that different types of CNF were formed. The growth direction of the graphene layers was from the Ni(1 1 1) plane for PGNFs and from the Ni(1 1 0) plane for TGNFs. Characterizations and field emission properties of these materials were also studied and compared.     

The effect of the functionalization of carbon nanofibers on their electronic conductivity

The effects of the functionalization of carbon nanofibers (CNFs) on their electronic conductivity, in addition to their physico-chemical properties have been studied. Oxygen surface groups have been created on the surface of three CNFs with different properties, following three oxidation treatments with diverse severity. The oxygen content increased from two to six times the original content, depending on the CNF texture, from 1.5–2.6 wt.% up to 15.1 wt.%. Whereas some important properties are not significantly modified after functionalization (texture, crystalline structure, etc.), other properties like the electronic conductivity are affected depending on the extent of the process. The electronic conductivity of CNFs decreases from 200–350 S m⁻¹ up to 20–100 S m⁻¹ (the precise value depends on carbon crystallinity and compaction degree) when surface oxygen content increases from 1.5 wt.% to 5 wt.%. A further oxidation degree leads to a 90% decrease in conductivity, and in the end can even destroy the original fibrous structure. As a first approach, oxidizing at room temperature with rather strong acid solutions is a better strategy to create functional groups and maintain the electronic conductivity than increasing the process temperature with less severe oxidizing agents.     

Kinetics of carbon oxide evolution in temperature-programmed oxidation of carbonaceous laydown deposited on wet oxidation catalysts

The combustion kinetics of coke laydown on wet oxidation catalysts was studied by means of temperature-programmed oxidation and mass spectrometry within the temperature range (30–600°C). The coke deposits were formed over three different catalysts 1 wt.% Pt/Al₂O₃, MnO₂/CeO₂ and 1 wt.% Pt–MnO₂/CeO₂ during phenol deep oxidation in a three-phase slurry reactor at various reaction conditions (exposure time, temperature, oxygen pressure, catalyst loading). The carbon oxides, oxygen and water fluxes arising from the combustion of the carbonaceous deposits in a 5% O₂/He mixture, were continuously monitored. In all cases, unimodal quasi-Gaussian distributions were obtained for CO₂ while no CO was detected. These evolutions were successfully described by a modified “fractal power-law” grain model. The coke-dependence of the carbon dioxide profiles was related to the fractal dimension of the catalyst surface and to the oxygen partial order during coke burn-off. The corresponding change in O₂ partial order was ascribed to competition between three steps in the combustion mechanism: non-dissociative O₂ chemisorption, interaction of oxygen with undissociated dioxygen bearing surface species, physical desorption of the complex oxide as carbon dioxide.     

Airside performance of herringbone fin-and-tube heat exchangers in wet conditions

The airside performance for herringbone fin-and-tube heat exchangers under dehumidifying conditions are reported in this study. A total of eighteen samples were tested and compared. The effects of the number of tube rows, fin pitch, tube size, and wave height were examined in this study. A unique characteristic of the herringbone fin pattern is that the friction factors are strongly related to the number of tube rows. The airside performance of the herringbone fin pattern is strongly related to condensate retention. Based on the present test results, heat transfer and friction correlations, in terms of the Colburn j and Fanning friction factors, were proposed to describe the airside performance of the herringbone fin geometry. The correlation of the Colburn j factor gives a mean deviation of 5.76%, while the Fanning f factor shows a mean deviation of 8.27%.     

The influence of oxidation on the texture and the number of oxygen-containing surface groups of carbon nanofibers

The effect of liquid-phase oxidation on the texture and surface properties of carbon nanofibers has been studied using XRD, TEM, SEM, N₂-physisorption, TGA-MS, XPS and acid-base titrations. Oxidation was performed by refluxing the nanofibers in HNO₃ and mixtures of HNO₃/H₂SO₄ for different times. The graphite-like structure of the treated fibers remained intact, however, the specific surface area and the pore volume increased with the severity of oxidation treatment. For the first time it is shown that the most predominant effect that gives rise to these textural modifications is the opening of the inner tubes of the fibers. Moreover, it is demonstrated that both the total oxygen content (O/C=0.02-0.07 at/at) as well as the number of acidic groups (1-3 nm⁻²) are a function of the type of oxidizing agent used and the treatment time. The total oxygen content of the oxidized samples turns out to be substantially higher than can be accommodated in the form of oxygen-containing groups at the exterior surface.     

Nanofibrous carbon with herringbone structure as an effective catalyst of the H2S selective oxidation

Granular nanofibrous carbons (NFCs) with herringbone structure were synthesised by the decomposition of natural gas over Ni/Al₂O₃ catalysts, and their performance in the selective oxidation of hydrogen sulphide was studied. Samples obtained over pure Ni catalysts are the strongest mechanically and easiest to produce. However, they show low selectivity for sulphur and are unstable during operation. Boiling in nitric acid followed by annealing led to improvements in catalytic stability and a significant increase in the selectivity for sulphur in the direct oxidation of hydrogen sulphide. The addition of large amounts of water vapour to the reaction mixture dramatically improved the selectivity and stability of the NFCs.     

Activated carbon as catalyst in wet oxidation of phenol: Effect of the oxidation reaction on the catalyst properties and stability

Catalytic wet oxidation (CWO) of phenol has been carried out in a continuous three-phase reactor by using a commercial activated carbon (AC) as catalyst, feeding oxygen as gas phase and an aqueous solution 1000 ppm in phenol to the reactor. A stable catalyst under operation conditions is one of the main difficulties to pass up in the catalytic wet oxidation process, so the stability of the activated carbon with the time on stream (TOS) was investigated. To do this the phenol conversion change was analyzed with TOS and results were contrasted to the change of the physicochemical properties of the AC with the TOS. Gas adsorption/desorption, TPD, XPS and SEM measurements were applied to the AC taken from the reactor after several TOS values. A significant reduction of the micro-pore volume and BET surface area of the catalyst was observed with TOS. However, as reaction proceeded the external surface area and the total amount of oxygen surface group increased. Moreover, regeneration of the initial catalyst properties was done by washing with water saturated in oxygen, at the reaction conditions or by heating in N₂ atmosphere at 450, 700 and 900 °C. The total micro-pore volume and internal surface area of the catalyst were not recovered by the regeneration process, probably due to blockage of the narrow micropores by pyrolytic carbon produced during the first step of the wet oxidation process.     

Ru和Cu协同催化湿式氧化处理氨氮废水

采用化学还原法制备了RuCu/TiO₂双金属催化剂,并探究了Ru和Cu的协同作用对催化湿式氧化（CWAO）无害化处理氨氮废水催化性能的影响。研究结果表明,Cu的添加可有效改善Ru/TiO₂催化剂的N₂选择性,而Ru的存在可有效提高Cu/TiO₂催化剂的催化活性。反应条件为0.5 MPa、150℃、[NH₃]₀=1000 mg·L^-1、pH=12、模拟废水处理量为33 L·（kg cat）^-1·h^-1时,1Ru2Cu/TiO₂能使废水的氨氮转化率和N₂选择性分别高达87.7%和85.9%。表征结果表明：Ru和Cu的协同在催化氧化氨氮废水过程中起了关键作用,主要体现在：Ru和Cu的强相互作用导致1Ru2Cu/TiO₂催化剂具有良好的抗流失性能,进而使得催化剂具有良好的稳定性;Ru和Cu的电子转移使得1Ru2Cu/TiO₂具有适中的亲氧性能,有效提高了催化剂的催化活性。     

Efficient catalytic wet oxidation of phenol using iron acetylacetonate complexes anchored on carbon nanofibres

Iron acetylacetonate complexes anchored on oxidized carbon nanofibres (CNFs) were prepared by a three steps procedure: (i) oxidation of commercial CNFs by treatment with HNO₃, (ii) synthesis of acetylacetonate (acac) functional groups on the oxidized CNFs surfaces (acac/CNFs) and (iii) iron ions complexation on the acac sites of the CNFs (Fe-acac/CNFs). The surface groups exposed on the functionalized CNFs were characterized by using attenuated total reflectance Fourier transform infrared spectroscopy, temperature programmed desorption, thermogravimetric analysis and X-ray photoelectron spectroscopy. The functionalized CNFs and the iron complexes anchored on the CNFs were tested as heterogeneous catalysts for the wet oxidation of phenol with pure oxygen. Complete phenol conversion and high mineralization values were achieved with fresh and reused Fe-acac/CNFs catalysts, which demonstrate the improved stability of the catalysts under the phenol degradation reaction conditions. Furthermore these conditions are comparatively mild, typically 413 K of reaction temperature and 2.0 MPa of oxygen pressure.     

Preparation of graphene/carbon hybrid nanofibers and their performance for NO oxidation

A series of novel microdomain-graphitized polyacrylonitrile (PAN)-based nanofibers were prepared by adding varied amounts of graphene oxide into the precursor via the electrospinning method. These hybrid electrospun nanofibers with were stabilized in ambient atmosphere, carbonized in nitrogen atmosphere and treated in NH₃ atmosphere for NO oxidation with low concentration (50 ppm) at room temperature. The samples were characterized by scanning electron microscopy, X-ray diffraction, Fourier transform infrared spectroscopy, Raman spectroscopy, and nitrogen adsorption at 77 K. Oxidation of NO into NO₂ at room temperature was investigated in a fiber fixed-bed. The results demonstrated that the reduced graphene oxide sheets provide catalytic active sites embedded in the PAN-based nanofibers. In addition it was determined that nitrogen-containing functional groups played important roles in the enhancement of the catalytic oxidation of NO to NO₂. The samples with 5 wt.% GO exhibit the most catalytic oxidation of NO into NO₂.