Role of oxygen surface groups in catalysis of hydrogasification of carbon black by potassium carbonate

The reaction rates of a nonporous graphitic carbon lampblack with hydrogen gas at 865°C and 500 psi were measured following treatments such as degassing and/or oxidation with and without impregnation of K₂CO₃. The results lead to the conclusion that oxygen surface groups enhance the reactivity of the carbon by generating “nascent” active sites on decomposition into CO and CO₂. In the catalyzed case the basic and C=O carbonyl groups form active and stable surface complexes. The acidic groups, on the other hand, could not be well characterized and may form stable and inactive or unstable and active complexes depending on their nature.     

Measuring oxygen permeabilities of polymer films by a new singlet oxygen technique

A new sensitive method is described for measuring the permeation and diffusion of oxygen through polymer films. For these measurements an ethyl cellulose film containing 9,10-dimethylanthracene (DMA) and erythrosin may be either tightly sealed between two pieces of sample film in a simple cell or sealed in a pouch made from the test material, and is used to absorb oxygen which has passed through the polymer film sample from the air. On illumination this oxygen is converted by the erythrosin to singlet oxygen which then reacts with the DMA. After an initial scavenging period, the rate of permeation of oxygen under the atmospheric pressure gradient across the film is monitored by determining the rate of disappearance of DMA spectrophotometrically.     

Self-assembling of multi-walled carbon nanotubes on a porous carbon surface by catalyst-free chemical vapor deposition

Synthesis of carbon nanotubes (CNTs) by catalyst-free chemical vapor deposition (CFCVD) is one of the most important challenges in nanotube science. Self-assembling multi-walled CNTs (MWCNTs) were produced on a porous carbon surface using carbon black (CB) as a substrate, at 800 °C by the decomposition of diluted ethylene. MWCNTs with an outer-diameter distribution of 20–80 nm, examined by scanning and transmission electron microscopy, could be self-assembled on pore structures of CB surface by CFCVD.     

Combustion of porous carbon particles in oxygen

A model is developed for the combustion of porous carbon particles in oxygen in a high-temperature furnace. Conditions for the existence of solutions to the equations of the model were found. A collation between the found region of existence of the solutions and experimental data shows that only some portions of the experimental curve fall within the region of existence of the solutions. These portions correspond to four different modes of particle combustion, and the realization of one or another mode depends primarily on the internal surface area of the particle. The mechanisms and some features of the modes of combustion of porous carbon particles in oxygen were considered.     

Polyaniline/porous carbon electrodes by chemical polymerisation: Effect of carbon surface chemistry

Polyaniline/porous carbon composite electrodes were prepared by chemical polymerisation and characterized in terms of porosity and performance as electrochemical capacitors.To obtain the composite electrodes two methods were used. The first method consisted of mixing, directly, the activated carbon with chemically polymerised polyaniline. The second one consisted of mixing the activated carbon with aniline and subsequent chemical polymerisation. Additionally, the second process was carried out with the porous carbon previously thermally treated in N₂ up to 900 °C in order to remove surface oxygen groups.Changes in porosity with the polyaniline addition were analysed. It has been proved that the method used strongly affects the porous structure. Dealing with the electrochemical performance, polyaniline and carbon mechanically mixed seem to work independently, being the composite behaviour a combination of the corresponding performance of both materials separately. The composites prepared by the second method (polymerisation over carbon) reveal the key role of surface chemistry in polyaniline coating. Aniline reacts with the oxygen complexes and their positive effect in capacitance is not observed.The second method (polymerisation over carbon) using a thermally treated carbon seems to be the best one since a more porous (or thinner) polyaniline film is produced.     

Kinetics of heterogeneous reactions of carbon and oxygen during combustion of porous carbon particles in oxygen

A model of combustion of a high-porosity carbon particle in oxygen is considered, which takes into account heterogeneous and homogeneous chemical reactions inside the particles and radiative heat transfer. The boundaries of the domain where the burning rate depends on the particle temperature are determined. The possibility of two combustion regimes is demonstrated: regime with a high burning rate, where the carbon-oxygen reaction proceeds in a layer adjacent to the particle surface, and regime with a low burning rate, where the reaction proceeds in the entire particle volume. In the regime with a high burning rate, the main product of the reaction between carbon and oxygen is carbon monoxide, whereas both carbon monoxide and carbon dioxide can be formed in the regime with a low burning rate. The kinetic equations of heterogeneous reactions C + O₂ = CO₂ and 2C + O₂ = 2CO are determined, which reveal the retarding effect of carbon monoxide and dioxide on the rates of these reactions. __________ Translated from Fizika Goreniya i Vzryva, Vol. 42, No. 3, pp. 11–22, May–June, 2006.     

XPS studies by use of the digital difference spectrum technique of functional groups on the surface of carbon fiber

The digital difference spectrum technique in X-ray photoelectron spectroscopy has proved to be effective for the analysis of the functional groups introduced by surface oxidation treatment of carbon fibers. The major functional groups introduced to the carbon fiber surface by surface oxidation are hydroxyl and carboxyl groups. Disordering of the crystal lattice is also observed together with generation of carboxyl groups in the case of surface oxidation of graphite fibers.     

Enhanced electro-oxidation resistance of carbon electrodes induced by phosphorus surface groups

The electro-oxidation of carbon materials enormously degrades their performance and limits their wider utilization in multiple electrochemical applications. In this work, the positive influence of phosphorus functionalities on the overall electrochemical stability of carbon materials has been demonstrated under different conditions. We show that the extent and selectivity of electroxidation in P-containing carbons are completely different to those observed in conventional carbons without P. The electro-oxidation of P-containing carbons involves the active participation of phosphorus surface groups, which are gradually transformed at high potentials from less-to more-oxidized species to slow down the introduction of oxygen groups on the carbon surface (oxidation) and the subsequent generation of (C*OOH)-like unstable promoters of electro-gasification. The highest-oxidized P groups (–C–O–P-like species) seem to distribute the gained oxygen to neighboring carbon sites, which finally suffer oxidation and/or gasification. So it is thought that P-groups could act as mediators of carbon oxidation although including various steps and intermediates compared to electroxidation in P-free materials.     

对用于活性炭表面含氧官能团分析的Boehm滴定法的几点讨论

不同的研究者在采用Boehm滴定法测定活性炭表面含氧官能团的含量时在操作方法上存在差异.本文探讨了Boehm滴定法中CO2的去除,直接滴定与返滴定,振荡时间,溶液的浓度对滴定过程的影响,提出适宜的操作建议.     

煤燃烧过程中碳、氧官能团演化行为

采用X射线光电子能谱（XPS）系统研究燃烧过程中准格尔褐煤及其煤焦颗粒表面中碳、氧的存在形态及演化过程.研究结果表明，准格尔褐煤颗粒表面含碳官能团中石墨化碳含量非常低，但同时具有一定含量的双键碳;含氧官能团中无机氧含量较低，有机氧含量相对较高.随着燃烧的进行，颗粒表面碳、氧官能团呈现不同的变化趋势.     

Modification of the adsorption properties of high surface area graphites by oxygen functional groups

The role of the concentration of oxygen functional groups on the surface of commercial high surface area graphite has been studied in this work. For this purpose, two samples of the parent HSAG have been both oxidized with aqueous HNO₃ solution and pyrolysed at 900 °C under an He flow, in order to remove surface oxygen groups. TPD results, in agreement with XPS experiments, reveal the existence of oxygen groups on the surface of the parent HSAG, whose concentration increases substantially after the oxidative treatment, whereas they disappear after the thermal treatment. The adsorption of different alkanes, aromatics and chlorohydrocarbons on the three samples were compared. Adsorption capacities were derived from the adsorption isotherms, whereas thermodynamic properties have been determined from chromatographic retention data. Both the capacity and the strength of adsorption decrease after the oxidative treatment of the graphites. For n-alkanes and cyclic compounds, it was demonstrated that the presence of oxygen surface groups affects their interaction in lower extension. In the adsorption of aromatics and double-bonded compounds, the influence of surface functionalization is more pronounced, since the removal of electrons from the π-electron system of the basal planes, weaken these forces.     

Continuous production of fullerenes by pyrolysis of acetylene at a glassy carbon surface

A reactor is described for generating fullerenes from hydrocarbon gas or vapor. Distinguishing features are a thermal system in which the fullerene product is not exposed to destructive energetic photons (as in an arc) and ease of control. Acetylene was chosen as the first feedstock to reduce the hydrogen content. Argon is the carrier gas. The reactor is a radio-frequency induction heated cylinder of glassy polymeric carbon having multiple holes through which the gas mixture passes. Operation as high as 2500 K is possible, but fullerene production is seen only below 1500 K. Preliminary tests give <1% yield, but improvement is expected by manipulation of the temperature, dwell time at temperature, feedstock species and partial pressure. A hydrogen scavenger may also be useful.     

Coal gasification at pressure by mixtures of carbon dioxide and oxygen

This Paper discusses results obtained from experiments on coal gasification by mixtures of carbon dioxide and oxygen under pressure. This research was carried out at the INIEX pilot station as part of the Belgian-German programme for the development of underground gasification. Tests were carried out to study the influence of the ratio of carbon dioxide to injected oxygen on the composition and the heating value of the gas obtained, the gasification efficiency and the consumption of the gasification medium. The experimental results can be satisfactorily explained by means of a model based on thermodynamic equilibria, assuming that gasification is brought about by an internal gasification agent formed by the volatile components of the coal and by an external gasification agent consisting of injected carbon dioxide and oxygen.     

Quantitative study on cross-linking reactions of oxygen groups during liquefaction of lignite by a new model system

Cross-linking reactions (CLR) of oxygen groups during liquefaction of lignite were quantitatively studied by a new model system. Chinese Yitai lignite (YT) was first oxidized by nitric acid at 70 °C and about 98% of the oxidized sample could be dissolved in tetrahydrofuran (THF) at room temperature. Then benzyl alcohol, PhCH₂OH (BA), as a model compound was added into the oxidized coal, also acted as solvent in the subsequent liquefaction. Temperature-programmed reactions (TPR) at liquefaction conditions under hydrogen atmosphere were performed to evaluate the CLR by quantitative analysis of THF-insoluble solid products (THFI) after reaction. Extensive CLR were observed even under high pressure of H₂ at 200-400 °C, and more than 51.7% and 81.2% of the THFS fraction was converted into the THFI at 300 °C with tetralin (TET) and BA as solvent, respectively. The THFI fraction was almost solely caused by the CLR, which makes it possible to quantitatively study the CLR by analyzing the amount of the cross-linked solid products (CSP). The pyrolysis behaviours of CSP and oxidized coal were examined by TG. Other model compounds containing oxygen-functional groups (alcohol, phenol, carboxyl, carbonyl and ether groups) can also be used in this model system to study CLR of oxygen groups in low-rank coals.     

Wear properties of new porous carbon materials: Woodceramics

The purpose of this investigation was to analyze the fundamental wear properties of new porous carbon materials Woodceramics. Wear tests were carried out by sliding an alumina ball (R=1.5, 4.0 mm) or hemispherical diamond pin (R=0.075 mm) against a Woodceramics plate (MDF-800), under three lubrication conditions: unlubricated in air, impregnated with base oil, and in water. The specific wear rate was calculated from the profile of the worn surfaces, and the relationship between the specific wear rate and the contact pressure parameter (W/R ²)^1/3 was obtained. Worn surfaces were observed with a scanning electron microscope (SEM) to clarify the microscopic wear mechanisms. The following principal results were obtained: (1) The specific wear rate of Woodceramics increases rapidly with increasing contact pressure parameter (W/R ²)^1/3, under all lubricated conditions; (2) When the contact pressure parameter (W/R ²)^1/3 is less than a certain critical value, the specific wear rate of Woodceramics is less than 10^–8 [mm²/N], which is low enough for practical use; (3) The wear mode of Woodceramics can be classified into the following three modes: large-scale brittle fracture-induced wear (flake formation), small-scale brittle fracture-induced wear (powder formation), and ultra mild wear (ploughing).     

Friction properties of new porous carbon materials: Woodceramics

Woodceramics are new porous carbon materials obtained from wood or woody materials impregnated with phenol resin, and carbonized in a vacuum furnace at high temperature. Woodceramics have several superior characteristics from the viewpoints of engineering materials and ecological materials: they are hard and strong, have porous structure and low density, are made from natural resources, do not cause environmental pollution, and are cheap to manufacture. This paper describes the fundamental friction properties of Woodceramics in sliding contact with several materials. Woodceramics made of medium density fiberboard (MDF) and beech impregnated with phenol resin and carbonized in a vacuum furnace at 800°C and 2000°C were rubbed against alumina, silicon nitride, bearing steel and diamond by using a reciprocating friction apparatus. Experiments were carried out unlubricated in air, impregnated with base oil and in water, at several normal loads and sliding velocities. The following principal results were obtained: (1) The friction coefficient is around 0.15, under all three lubrication conditions; (2) The friction coefficient slightly decreases and then stays constant with increasing normal load; (3) The friction coefficient is not affected by sliding velocity; (4) Woodceramics have a good self-lubricity.     

Thermal desorption analysis of oxygen surface complexes on carbon

The energetics of oxygen surface complex groups bound to a carbon surface were investigated using linear-temperature programmed thermal desorption. The rate of CO production from the decomposition of surface complex formed by the low temperature oxygen chemisorption on an activated Graphon surface indicated that at least two types of functional groups and/or sites must exist. In addition the data indicate that the desorption energies of these groups and/or sites is a function of their coverage. Mathematical modeling has also shown that those workers using the step-temperature programmed thermal desorption technique would not have energy resolution sufficient to detect surface heterogeneity.     

IR spectroscopy studies of oxygen surface compounds on carbon

Spectral studies of oxidation process were carried out on carbon films prepared by carbonization of polyfurfuryl alcohol and of cellulose. Investigation of the oxygen role in the initial stages of carbonization shows that oxygen surface compounds formed during such process have different chemical structure from those produced during oxidation of carbonic film previously desorbed at 600°C. Chemisorption of oxygen at room temperature on the carbonic film (desorbed previously at 600°C) occurs with the participation of π electrons of condensed aromatic systems. Iono-radical structures formed during that process show absorption bands in the range of 1590cm⁻¹. Oxidation of the carbon films carbonized at 800°C causes a decrease in the absorption coefficient, and oxygen surface compounds formed show absorption bands at about 1760, 1600 and 1260cm⁻¹. The carbon film as a model substance gives possibilities for broader application of IR spectroscopy in studies of carbon, carbonization and activation processes, sorption effects and catalytic mechanisms.