Structure and conductivity of carbon materials produced from coal pitch with carbon additives

Attention focuses on the structure and electrical conductivity of carbon materials obtained by the carbonization of coal pitch in the presence of additives (nanotubes, graphite foam, and graphite), at temperatures up to 900°C. In some cases, ultrasonic mixing is used on introducing the additives to the pitch. Ultrasonic mixing is found to change the properties of the pitch and affect the properties of the carbon material produced. In particular, the proportion of carbon with an ordered structure is increased; the electrical conductivity at temperatures below 40 K is increased; and the energy barrier E _g between individual crystallites is reduced almost fourfold. At higher temperatures, the electrical conductivity is practically unchanged. Adding nanotubes to the pitch reduces the content of ordered carbon structures in the carbon material produced and lowers its electrical conductivity. Adding graphite foam and graphite to the pitch increases the order and electrical conductivity of the carbon material produced and lowers the energy barrier E _g between individual crystallites in the samples. The electrical conductivity of all the carbon materials below 16 K is described by the characteristic formula for fluctuation-induced tunneling conduction. This indicates that contacts between individual crystallites are mainly responsible for the electrical conductivity.     

Catalytic graphitization of non-graphitizable carbon by chromium and manganese oxides

Carbon prepared from the benzene-insoluble fraction of a solvent refined coal (non-fusible), an active carbon, a charcoal and PAN carbon fibres have been heat-treated with oxides of chromium, manganese, molybdenum and vanadium to investigate catalytic graphitization in the temperature range 1673–2773 K. Resultant materials were examined by X-ray diffraction, SEM and phase contrast high resolution electron microscopy. The chromia is an effective catalyst at 2273 K at concentration of chromium of 30%, no changes being observed at higher temperatures. With other oxides the extent of catalytic graphitization (using L_c) increased with HTT to 2773 K, values of C₀ being less sensitive to HTT. Soak times are important, equilibration taking 2 hr at 2073 K (SRC-BI: Cr₂O₃) and 10 hr at 1673 K (SRC-BI; MnO₂). Large concentrations of additives (up to 30% of metal) are required. The microscopy reveals the development of flaky graphite (SRC-BI) and the layered stacking arrangements of graphite planes in the SRC-BI graphites.     

The effects of aluminum on structural development of a carbon derived from phenolic resin

The catalytic graphitization of a phenolic resin carbon by aluminum powder was carried out under an N₂ atmosphere. The following catalytic mechanism is proposed: In the heating process up to 2000°C, AlN is formed at first by the reaction of the aluminum with the N₂ gas. At approximately 2000°C, the AlN begins to react with the carbon matrix to form an intermediate, A₁₄C₃. The Al₄C₃ penetrates into the surrounding carbon matrix through an Al₄C₃ formation-decomposition sequence, leaving a graphite crystal behind. As the penetration proceeds, smaller graphite crystals are formed because of the division of the Al₄C₃ particles. When a most finely divided Al₄C₃ is produced by the further penetration, instead of the graphite, only turbostratic carbon is formed by this catalytic process.     

Effects of boron doping in low- and high-surface-area carbon powders

Two distinctive carbon materials (Saran char and SP-1 graphite) were doped with B at different loading to clarify the intrinsic effect of substitutional B on carbon reactivity. The carbon precursors would be affected in different style by substitutional B due to different important properties (crystallinity and surface area). The B retentivity depended on the nature of B dopant and carbon substrate; a less ordered carbon has higher B loading than its counterpart. Graphitization was enhanced by substitutional B, as expected. Furthermore, the B incorporation was still beneficial for SP-1 although it already had high crystallinity. An interesting behavior was noticed; the increase in L_a was greater than L_c. The intrinsic effect of substitutional B in carbon oxidation was proved to be a catalytic one. Unlike highly ordered SP-1 graphite, Saran char showed both a catalytic effect at low B loading and low conversion, and an inhibiting effect at high B loading and high conversion. The inductive effect was proposed to explain this catalytic effect on different crystallite size. Different sizes of carbon clusters were calculated by Gaussian 98W; the extent of the effect of substitutional B did get smaller to the carbon in bigger size of carbon cluster.     

Oxygen transfer and carbon gasification in the reaction of different carbons with CO2

The oxidation of carbon in CO₂-CO mixtures can be discussed assuming the mechanism CO₂ = CO + O (adsorbed) (1) O (adsorbed) + C → CO (2) This treatment is supported and supplemented by measurements of both partial reactions at different carbons, electrode graphite, charcoal, natural graphite and iron doped graphite. From the kinetics it can be seen that the reactions (1) and (2) predominantly take place at different sites. Nevertheless the kinetics of both reactions are correlated, since a uniform oxygen activity is established at the carbon surface by the interplay of the oxygen surface diffusion and of both reactions. The rate of reaction (2) and accordingly the rate of the overall reaction is proportional to the stationary oxygen activity which can be calculated from the steady state condition for oxygen adsorption. Several rate equations are derived for the overall reaction at special reaction conditions and for different carbons and it is shown that no general explicit rate equation can be given for the oxidation of carbon in CO₂.     

Fe/N/C non-precious catalysts for PEM fuel cells: Influence of the structural parameters of pristine commercial carbon blacks on their activity for oxygen reduction

Fifteen commercial SRCC furnace carbon blacks of various grades, ranging from N1 to N9, were used as carbon supports in the preparation of Fe/N/C type electrocatalysts for the oxygen reduction reaction (ORR) in PEM fuel cell conditions. All catalysts were prepared by loading the various carbon grades with 0.2 wt.% Fe as iron acetate and heat-treating the resulting material at 950 °C in pure NH₃. This reaction provides the nitrogen content and the microporosity necessary to synthesize and host the Fe/N/C catalytic sites that perform ORR. The maximum catalytic activity (V_pr max) for each carbon grade was determined by optimizing pyrolysis time. The aim of this study is to determine which structural characteristics of the pristine carbon black are important for maximizing catalytic activity. Three structural parameters that influenced the catalytic site density on the carbon support were identified. They are: (i) the average particle diameter of the pristine carbon black, d_particle, available from BET area measurements; (ii) the amount of disordered phase which is proportional to W_D, the width at half maximum of the D peak in the Raman spectrum of the pristine carbon; and (iii) the mean size of the graphene layers characterizing the graphitic crystallites in the carbon black, L_a. The latter is available by Rietveld analysis of the XRD spectra of the pristine carbons. The best catalytic activities are obtained for the smallest d_particle, the largest W_D, and the largest L_a. Optimizing these three parameters maximizes the fraction of the pristine carbon black that becomes microporous upon reaction with NH₃ and, therefore, enables the formation of Fe/N/C catalytic sites. A FeN₂₊₂/C structure bridging two adjacent graphitic crystallites is proposed as a potential model for most of the catalytic sites present in such Fe/N/C type catalysts.     

Controlled atmosphere electron microscopy studies of graphite gasification—The catalytic influence of vanadium and vanadium pentoxide

CAEM has been used to examine the behaviour of V and V₂O₅ as catalysts in the graphite-oxygen reaction. The similarities in both qualitative and quantitative effects indicates that the same reactive species is operative in both systems. The pronounced activity of these catalysts is attributed to their ability to wet and spread over the graphite surface at moderately low temperatures. It is also shown that V₂O₅ has the ability to penetrate into graphite and that catalytic attack occurred mainly by a channelling mode, although it is possible that the intercalated material may exert a catalytic influence on the reaction.     

Comparison between carbonization of wood charcoal with Al-triisopropoxide and alumina

A comparison was made between the catalytic carbonization of biomass carbon suspended in Al-triisopropoxide and in biomass carbon mixed with 40 μm sized Al₂O₃ particles. Both types of samples were plasma sintered during 5 min under an argon pressure of 50 MPa at temperatures up to 2200 °C. Plate-like catalytic graphitization develops by formation and dissociation of plate-like Al₄C₃. Plasma sintering under the proper CO partial pressure and heat treatment temperature is instrumental in forcing the Al₂O₃ to react with the carbon, forming first Al₄C₃ and subsequently graphite. The difference between Al-triisopropoxide and Al₂O₃ is a matter of intensity of the graphite reaction versus the size of the graphite patches.     

Entropy change in lithium ion cells on charge and discharge

Open circuit voltage (OCV) was measured as a function of temperature and state of charge (SOC) for six kinds of lithium ion cells. The following cells were used: four kinds of commercial cell using a LiCoO₂ cathode and a graphite or hard carbon anode; a trial manufacture cell using a Li–Ni–Co complex oxide cathode and a graphite-coke hybrid carbon anode; and a trial manufacture cell using a LiMn₂O₄ cathode and a graphite anode. The entropy change in the cell reaction was determined by calculating the derivative of the OCV with temperature. Results were compared and discussed to determine the influence of the phase transition in the electrode materials due to cell reaction. It was clarified that the entropy change in cells using a LiCoO₂ cathode is negative except for the part of the SOC region where Li _x CoO₂ phase transition occurred. An endothermic reaction then occurs during discharge and an exothermic reaction during charge. In cells using LiCoO₂ cathodes, there was a fluctuation in the entropy change originating from the Li _x CoO₂ phase transition in the SOC range between 70% and 90%. This fluctuation was influenced by temperature and by additives or excess lithium in the cathode material. The entropy change in both cells using a Li–Ni–Co complex oxide cathode or a LiMn₂O₄ cathode was comparatively small.     

The reaction of hydroxyl radicals with carbon at 298 K

The reaction of free OH radicals with graphite was studied in a flow system by mass spectrometry, the OH being produced by the reaction H + NO₂ → OH + NO. The OH radicals react rapidly at 298 K to produce approximately equal amounts of CO and CO₂. The collision efficiency (γ) for gasification of the carbon is>5 × 10^?3. OH radicals are much more reactive than free oxygen atoms towards graphite at 298 K. Carbon is an efficient heterogeneous catalyst for the reaction H + OH → H₂O, and when free hydrogen atoms are present, this reaction is several times faster than the gasification of the carbon by OH. Carbon is also an efficient catalyst for the recombination of H atoms: 2H → H₂.     

The graphitization of carbon nanofibers produced by catalytic decomposition of methane: Synergetic effect of the inherent Ni and Si

The catalytic effect of the inherent Ni and Si on the graphitization of carbon nanofibers produced by catalytic decomposition of methane is reported. The participation of the inherent Ni and Si metals as co-catalysts in the graphitization of the carbon nanofibers through the formation of Ni₂Si and SiC was inferred. Taking advantage of this catalytic effect, graphite materials showing structural characteristics comparable to oil-derived graphites which are employed in several industrial applications have been prepared from the carbon nanofibers. Unlike SiC which is further descomposed to graphite, the role of Ni₂Si remains unclear. At the CNFs heat treatment temperatures employed, Ni₂Si is in a liquid state where the carbon can be dissolved to form a supersaturated solution from which the SiC can be produced by segregation, thus being an intermediate stage in the catalytic graphitization of the carbon nanofibers. Further work are currently in progress to go insight this issue.     

Graphite oxidation in molten sodium carbonate

The oxidation of spectroscopic grade graphite using air or oxygen in molten sodium carbonate was investigated at 900, 1000 and 10502C. The oxidation rate increased with increasing temperature, increasing oxygen concentration, and increasing graphite surface area but decreased slightly as the reaction air was diluted with increasing carbon dioxide concentrations. At high-graphite loadings, the reaction rate was 0.45 order in oxygen, 0.45 order in graphite surface area with an apparent activation energy (E_a) of 35 kcal/mole and appeared to tend toward a rate limit imposed by the available oxidant in the melt. At low-graphite loadings, the rate was 0.42 order in oxygen, 0.78 order in graphite surface area with E = 32 kcal/mole and appeared to tend toward a rate limit imposed by the available graphite surface area. Virtually no carbon monoxide was observed under the conditions of the experiments. A sequence of reactions is proposed in which sodium peroxide, formed by the reaction of oxygen with sodium carbonate is the active oxidizing species.     

Facile synthesis of spherical carbon composite particles via a dry granulation process

We synthesized spherical carbon composite particles from oil-based raw coke using a dry granulation process. Metal and metal oxide nanoparticles (NPs) such as TiO₂, SiO₂, and Si were used as additives in the carbon composites. Core–shell and internally mixed composites with microporous structures were formed depending on the type of NPs added and synthesis conditions. In the composites containing silicon species, SiO₂ was reduced to SiO_x by carbon and volatile constituents in the raw coke during heat treatment. The electrochemical performance of the carbon composites as negative electrodes in lithium-ion batteries was investigated. Interestingly, the carbon composites prepared from raw coke and Si showed better performance as negative electrodes than pure graphite.     

Catalytic effects of alkaline earth carbonates in the carbon-carbon dioxide reaction

The results of a thermogravimetric study of the catalytic effects of alkaline earth carbonates in the CCO₂ reaction are described. Using graphite powder as the carbonaceous phase the rates of the reaction between 700 and 1100 °C were found to be substantially increased on addition of small amounts of BaCO₃ and SrCO₃ to the graphite. CaCO₃ was a much less active catalyst whereas MgCO₃ had only a slight effect on the kinetics. The catalytic process is interpreted in terms of a solid-state reaction between the salt and the graphite to yield the metal oxide which subsequently regenerates carbonate by reaction with gaseous CO₂.     

On porosity-overvoltage correlation for carbon anodes in cryolite-alumina melts

The anodic overvoltage in aluminium electrolysis was studied by steady-state current/voltage measurements with carbon anodes of different porosities: vitreous carbon, which is a non-porous material, spectral graphite, which contains micropores and industrial baked carbon of 8–24% porosity (P₁) due to pores larger than 7.5μm. Replacement of vitreous carbon with baked carbon was accompanied by a drastic decrease in overvoltage. For a given overvoltage, baked carbon yields a linear increase of cd with increasing P₁. It was established that the larger the P₁ parameter, the narrower is the range of Tafel behaviour, the lower are the Tafel slopes and the higher is the exchange current. These findings were interpreted in terms of reaction and diffusion overvoltage; their contribution depends directly on the porosity P₁. The higher P₁ for baked carbon, the larger is the diffusion overvoltage due to the diffusion into the anode pores.     

Effects of calcium,strontium, and barium as catalysts and sulphur scavengers in the steam gasification of coal chars

Effects of alkaline earths on the steam gasification of two coal chars, one of low and one of high intrinsic reactivity, were evaluated gravimetrically. The chars were derived from coal powders which had been impregnated with Ca, Sr, or Ba, pelletized, and pyrolysed in nitrogen. The additives increased the gasification rates in the order Ca < Sr < Ba. It follows from reaction kinetics that catalysis is caused by a large increase in the density of reaction sites, not by a lowering of the true activation energy. As shown by electron micrographs and elemental maps obtained by X-ray analysis, the strong catalytic effect is closely associated with the ability of the alkaline-earth species to spread over the char and to preserve contact with a freshly formed carbon surface as the gasification proceeds. The alkaline-earth catalysts are severely poisoned by hydrogen sulphide or sulphur dioxide from an external source.     

Catalytic activity of graphite-based nanofillers on cure reaction of epoxy resins

The strong influence of graphite oxide (GO) nanofiller on the glass transition temperature (T_g) of epoxy resins, generally attributed to restricted molecular mobility of the epoxy matrix by the nanofiller or to the crosslinking of GO layers via the epoxy chains, is investigated. The study confirms that large increases of the glass transition temperature of the nanocomposite can be observed in presence of GO. However, similar T_g increases are observed, when the filler is a high-surface-area graphite (HSAG), lacking oxidized groups. Moreover, these T_g differences tend to disappear as a consequence of aging or thermal annealing. These results suggest that the observed T_g increases are mainly due to a catalytic activity of graphitic layers on the crosslinking reaction between the epoxy resin components (epoxide oligomer and di-amine), rather than to reaction of the epoxide groups with functional groups of GO. This hypothesis is supported by investigating the catalytic activity of graphite-based materials on reactions between analogous monofunctional epoxide and amine compounds.     

A thermogravimetric analysis/mass spectroscopy study of the thermal and chemical stability of carbon in the Pt/C catalytic system

Vulcan XC72 carbon powder and Pt/Vulcan XC72 catalytic powder were characterized by transmission electron microscopy (TEM) and their reactivity under controlled atmospheres was studied as a function of the temperature. Under air atmosphere, production of water was detected by thermogravimetric analysis coupled with mass spectroscopy (TGA-MS) measurement at m/z 18, which evidenced that hydrogenated surface functions were present on the carbon substrate. Under argon atmosphere, the comparison of TGA-MS measurements performed at m/z 18 and m/z 44 with TEM and XRD results, together with XPS measurements, indicated that platinum surface oxides are rather Pt(OH)₂ than PtO or PtO₂ species. Such reactive surface species is involved in the degradation mechanism of carbon support under air and inert atmospheres. Under H₂(3%)/Ar atmosphere, hydrocarbon production coming from “reforming” reactions of the carbon support started at very low temperatures (below 373 K). TEM images of the same catalytic powder region before and after thermal treatment at 423 K under reducing atmosphere clearly displayed consumption of the carbon substrate. The reaction products may not only affect the intrinsic properties of the support but also the catalytic properties of platinum particles: reaction products could poison the anodic catalyst.     

Catalytic oxidation of carbon/carbon composite materials in the presence of potassium and calcium acetates

The catalytic effects of potassium acetate (KAC) and calcium acetate (CaAC) on the oxidation of carbon/carbon composites (C/C composites) used in aircraft brake system have been characterized. Potassium exhibited a very strong catalytic effect on the oxidation of the selected carbon samples, including C/C composite blocks impregnated with aqueous KAC solution and graphite powder physically mixed with KAC powder. The initial amount of catalyst loading and the pre-treatment in inert gas were found to affect its catalytic effectiveness. Impregnated calcium was also a good catalyst for the oxidation of C/C composites, but its effectiveness is much lower than that of potassium and is much less sensitive to catalyst loading amount and pre-treatment. Calcium acetate physically mixed with graphite powder only showed a slight catalytic effect. The experimental results suggested that the interfacial contact between catalyst and carbon is the key factor determining catalytic effectiveness, in agreement with previous studies using porous carbon materials. Due to its unique wetting ability and mobility on the carbon surface, potassium can form and maintain such contact with carbon and is, therefore, more effective in the C-O₂ reaction than calcium. The formation and development of such contact, which can also be affected by catalyst loading and pre-treatment process, can explain well the influence of these experimental conditions on the catalytic effect of potassium. The decreasing trend of reactivity with increasing burn-off in calcium-catalyzed oxidation is a result of interfacial contact loss because calcium does not have the necessary mobility to maintain such contact during reaction.     

Comparative study of support effects in ruthenium catalysts applied for wet air oxidation of aromatic compounds

Catalytic wet air oxidation (CWAO) reactions of aniline and phenol were conducted over supported ruthenium catalysts. Three support materials were employed: ZrO₂ and graphite, which exhibit medium adsorption capacities for pollutants and present mesopores in their texture, and an activated carbon. This latter has higher adsorption capacity for pollutants because of the large capability of the micropores for contaminant retention from water. The Ru catalysts supported on the activated carbon material showed the higher values of conversion in the oxidation of aniline and of conversion and mineralization in the reaction of phenol. Under our experimental conditions the role of micropores present on the support material seems to be relevant for improving catalytic performances. The incorporation of Ru nanoparticles from different precursors has been also evaluated. Even if the final Ru particle size is a key parameter for the catalytic mineralization, a cooperative effect with the activated carbon support has been established.