Inhibition of catalytic oxidation of carbon/carbon composites by phosphorus

The inhibition effectiveness of thermally deposited phosphorus (P) compounds on the carbon oxidation catalyzed by potassium or calcium acetate has been investigated. The P deposit was formed by impregnating carbon/carbon composite samples with methanol solution of methyl-phosphoric acid or phosphorus oxychloride and heating at ca. 600 °C. An amorphous layer formed by a relatively large amount of P deposit functioned as a barrier for the access of the catalyst to the carbon surface even though it had almost no barrier effect for O₂ access. The catalytic effect of calcium was almost completely suppressed by such deposit, but the effect of potassium was only partially suppressed due to the superior wetting ability and mobility of potassium species. Small amounts of P deposit showed similar inhibition effects on non-catalyzed oxidation, while their effects on catalytic oxidation were not as good. Characterization of P-deposited carbon samples by XPS, XRD, SEM and TPD, as well as ab initio MO calculations, suggested that the inhibition effect mainly resulted from the formation of oxygen-containing P groups which may include metaphosphates, C-O-PO₃ groups and C-PO₃ groups. Those groups are suggested to act as a physical barrier against carbon/catalyst interfacial contact as well as to block the active carbon sites. The presence of bridge oxygen bonded to a carbon site and a P group appears to be a critical factor for maintaining the inhibition effect. Indeed, the loss of such oxygen or connecting bond seems to result in loss of inhibition.     

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.     

Carbon infiltration of carbon-fiber preforms by catalytic CVI

Experimental studies were conducted to assess catalytic chemical vapor infiltration processing for preparing carbon/carbon composites as a potential improvement to conventional one. The catalyst was introduced into the carbon fiber preforms by wet impregnation. Using C₃H₆/Ar/H₂ as the original gas, catalytic carbon was formed at 500-1000 °C for 1-3 h. It was found that carbon filaments were formed as the preparing temperatures were 500-700 °C, and carbon particles could be obtained at 800-1000 °C. The increasing rate of density was up to 0.916 g/ml/h when the sample was formed at 600 °C for 1 h with the catalytic of 0.7 wt.% Ni, and the carbon yield arrived to 90 wt.% . According to the micrographs of catalytic carbon, the forming mechanism of carbon filaments agreed with that of carbon filaments due to a metal catalyst. The weighted average interlayer spacing of C/C composites with catalytic carbon decreased to 0.341.     

PtCo supported on ordered mesoporous carbon as an electrode catalyst for methanol oxidation

A catalyst for methanol oxidation, PtCo supported on graphitized mesoporous carbon, has been synthesized and its electrochemical activity for methanol oxidation has been investigated. The graphitized mesoporous carbon support with ordered pore structure and high surface area of 585 m² g⁻¹ was prepared by one-step melt casting method using Al doped hexagonal mesoporous silica as hard templates and mineral pitches as carbon precursors followed by carbonization at 800 °C. The materials were characterized by X-ray diffraction, Raman spectra, field emission scanning electron microscopy, transmission electron microscopy and nitrogen sorption techniques. Cyclic voltammetry and amperometric i-t tests were adopted to characterize the electro-catalytic activities of the materials for methanol oxidation. The results show that the graphitized mesoporous carbon exhibits large electrochemical capacitance and good electric property. After supported with 20 wt%Pt or 20 wt%PtCo nanoparticles, the resultant mesostructured composites show 26-97% higher electrochemical catalytic activity for methanol oxidation than commercial catalyst 20 wt%Pt/C in mass activity (mA mg Pt⁻¹).     

Effect of heat treatment of activated carbon supports on the loading and activity of Pt catalyst

The study has been done on the effect of heat treatment of activated carbon at 1573-1773 K on its structural and electronic properties as a catalyst support. The X-ray diffraction result indicated that a partly graphitized structure was formed when the activated carbon was heated to a high temperature (1673 K). From the X-ray photoelectron spectroscopy result, it was found that Pt⁰ concentration was increased, but PtO and PtO₂ concentrations were decreased with an increase in the heat treatment temperature. From the van Dam’s model applied to this result, it might be concluded that more “π-sites” are created as the heat treatment temperature becomes higher. From the CO-chemisorption result, the highest loading was observed in case of Pt/AC1673 sample. This improved loading ability could be explained by the special interaction of the graphitic planes (π-sites) with the metal particles. Based on the catalytic activity, CO-chemisorption and XPS results, it is concluded that the well-loaded Pt⁰ species mainly contribute to the catalytic activity. Moreover, it was found that different degrees of graphitization of heat treated activated carbon could cause different surface Pt⁰ and improve the resistance of carbon support against gasification under air oxidation.     

Diesel fuel desulfurization with hydrogen peroxide promoted by formic acid and catalyzed by activated carbon

Desulfurization of diesel fuels with hydrogen peroxide was studied using activated carbons as the catalysts. Adsorption and catalytic properties of activated carbons for dibenzothiophene (DBT) were investigated. The higher the adsorption capacity of the carbons is, the better the catalytic performance in the oxidation of DBT is. The effect of aqueous pH on the catalytic activities of the activated carbons was also investigated. Oxidation of DBT is enhanced when the aqueous pH is less than 2, and addition of formic acid can promote the oxidation. The effect of carbon surface chemistry on DBT adsorption and catalytic activity was also investigated. Adsorption of DBT shows a strong dependence on carboxylic group content. The oxidative removal of DBT increases as the surface carbonyl group content increases. Oxidative desulfurization of a commercial diesel fuel (sulfur content, 800 wt. ppm) with hydrogen peroxide was investigated in the presence of activated carbon and formic acid. Much lower residual sulfur content (142 wt. ppm) was found in the oxidized oil after the oxidation by using the hydrogen peroxide-activated carbon-formic acid system, compared with a hydrogen peroxide-formic acid system. The resulting oil contained 16 wt. ppm of sulfur after activated carbon adsorption without any negative effects in the fuel quality, and 98% of sulfur could be removed from the diesel oil with 96.5% of oil recovery. Activated carbon has high catalytic activity and can be repeatedly used following simple water washing, with little change in catalytic performance after three regeneration cycles.     

Carbon nanofibres and activated carbon nanofibres as electrodes in supercapacitors

Carbon nanofibres have been prepared by a floating catalyst procedure at industrial scale in a metallic furnace. The nanofibres (50-500 nm diameter and 5-200 μm length) are grown from the Fe particles used as catalyst. Soot appears together with the carbon nanofibres. The sample has been chemically activated using KOH as activating agent. Scanning electron microscopy has shown a smooth surface for the as-prepared carbon nanofibres but a rough surface for the activated ones. The specific surface area increases from 13 to 212 m²/g due to the activation. The volume of the micropores (in the 1-2 nm range) and the mesopores (2-5 nm range), as deduced by density functional theory methods, also increases after the activation. Electrochemical behaviour of the as-prepared and activated carbon nanofibres has been tested in a supercapacitor at laboratory scale using 6 M KOH aqueous solution as electrolyte. The specific capacitance, which is less than 1 F/g for the as-prepared sample, increase up to ≈60 F/g for the activated sample. Only a slight decrease in capacitance has been observed as the current density increases. Specific power of ≈100 W/kg at specific energy of 1 Wh/kg has been found in some particular cases. We have compared the electrochemical parameters of our activated carbon nanofibres with those of activated carbon nanofibres coming from a commercial sample; the latter was activated by the same way as our sample.     

Effect of carbon fiber surface functional groups on the mechanical properties of carbon-carbon composites with HTT

The present investigation describes the quantitative measurement of surface functional groups present on commercially available different PAN based carbon fibers, their effect on the development of interface with resol-type phenol formaldehyde resin matrix and its effect on the physico-mechanical properties of carbon-carbon composites at various stages of heat treatment. An ESCA study of the carbon fibers has revealed that high strength (ST-3) carbon fibers possess almost 10% reactive functional groups as compared to 5.5 and 4.5% in case of intermediate modulus (IM-500) and high modulus (HM-45) carbon fibers, respectively. As a result, ST-3 carbon fibers are in a position to make strong interactions with phenolic resin matrix and HM-45 carbon fibers make weak interactions, while IM-500 carbon fibers make intermediate interactions. This observation is also confirmed from the pyrolysis data (volume shrinkage) of the composites. Bulk density and kerosene density more or less increase in all the composites with heat treatment up to 2600 °C. It is further observed that bulk density is minimum and kerosene density is maximum upon heat treatment at 2600 °C in case of ST-3 based composites compared to HM-45 and IM-500 composites. It has been found for the first time that the deflection temperature (temperature at which the properties of the material start to decrease or increase) of flexural strength as well as interlaminar shear strength is different for the three composites (A, B and C) and is determined by the severity of interactions established at the polymer stage. Above this temperature, flexural strength and interlaminar shear strength increase in all the composites up to 2600 °C. The maximum value of flexural strength at 2600 °C is obtained for HM-45 composites and that of ILSS for ST-3 composites.     

Microstructures and mechanical properties of carbon/carbon composites reinforced with carbon nanofibers/nanotubes produced in situ

Using ferrocene as catalyst and toluene as the liquid precursor, carbon/carbon (C/C) composites were prepared by chemical liquid-vapor infiltration at 850-1100 °C. The microstructures and properties of C/C composites obtained with different ferrocene contents were studied. The results show smooth laminar and isotropic pyrocarbon are obtained after adding ferrocene to the precursor. Carbon nanofibers can be formed as the catalyst content is 0.3-1 wt.%. When the ferrocene content is 2 wt.%, multi-walled carbon nanotubes with the diameter about 20-90 nm are obtained together with carbon-encapsulated iron nanoparticles. After adding ferrocene to the precursor, the fracture modes of the composites change from brittle facture to tough fracture. The flexural strength of the composites is a maximum for 0.3 wt.% ferrocene in the precursor, higher than for ferrocene contents of 0, 0.5, 1 and 2 wt.%. The flexural modulus of the composites decreases after adding ferrocene to the precursor.     

Oxidation resistance of a gradient self-healing coating for carbon/carbon composites

A gradient self-healing coating consisting of three layers, SiC-B₄C/SiC/SiO₂, was examined as a multilayer protection for carbon/carbon composites. The inner layer was made of B₄C and β-SiC, the middle layer was a SiC based layer, and the outer layer was SiO₂ as an airproof layer. Both inner and middle layers were produced to be diphase structure by a pack cementation technique, and the outer airproof layer was prepared by hydrolyzing tetraethylorthosilicate. SEM and EDS investigations showed that the coating had a compositional gradient between B₄C and SiC. The coating showed great self-healing properties from 500 °C to 1500 °C. The weight loss rate of the coated composites was less than 1.3% after 50 h at 1500 °C, and coating represented excellent thermal shock resistance at 1500 °C. The oxidation kinetics of coated carbon/carbon composites showed that the Arrhenius curve consisted of three parts with two broken points at about 700 °C and 1100 °C, and the three parts corresponded to three different self-healing mechanisms in different temperature regions.     

An STM study of phosphoric acid inhibition of the oxidation of HOPG and carbon catalyzed by alkali salts

The role of phosphoric acid as an inhibitor in the oxidation of HOPG and as a neutralizer of alkali salt catalysts is examined using scanning tunneling microscopy, supported by thermogravimetric analysis of carbon powder samples. HOPG samples were oxidized in air primarily at 700 °C, with a few samples oxidized at 800 °C. Reaction time was 20 min. Powder samples were oxidized for 5 min at temperatures ranging from 500 °C to 900 °C and rates of oxidation were determined. STM images of impurity deposits and oxidized samples are presented and analyzed. Two alkali salts are examined, sodium hydroxide and potassium acetate, and both catalyze oxidation at 700 °C. Phosphoric acid proves to be an inhibitor at 700 °C but begins to lose its inhibiting effect at 800 °C. It also demonstrates neutralization of potassium acetate at 700 °C but results for NaOH/phosphoric acid mixtures are less conclusive.     

A multilayer coating of dense SiC alternated with porous Si-Mo for the oxidation protection of carbon/carbon silicon carbide composites

In order to improve the oxidation behavior of carbon/carbon silicide carbide composites prepared by liquid silicon infiltration of carbon/carbon porous preforms, a multilayer coating of dense SiC alternated with porous Si-Mo was prepared by chemical vapor deposition combined with slurry painting. Oxidation test showed that weight loss of the coated sample was only 0.25% after 150 h oxidation in air at 1673 K. And the coated sample gained weight in the course of 46 cycles of thermal shock test between 1673 K and 373 K. The coating remained intact during the two kinds of tests and no obvious failure was found. The excellent oxidation protective ability and thermal shock resistance of the SiC/Si-Mo coating can be attributed to the alternated structure.     

Microstructure of carbon/carbon composites reinforced with pitch-based ribbon-shape carbon fibers

Carbon/carbon composites were prepared with ribbon-shape pitch-based carbon fibers serving as reinforcement and thermosetting PFA resin and thermoplastic pitch as matrix precursors. The composites were heat treated to 1000, 1600 and 2700 °C. Microstructural transformations taking place in the reinforcement, carbon matrix, and the interface were studied using polarized optical and scanning electron microscopy. The fiber/matrix bond and ordering of the carbon matrix in heat-treated composites was found to vary depending on the heat treatment temperature of the fibers. Stabilized fiber cleaved during carbonization of resin-derived composites. In contrast, fibers retain their shape during carbonization of pitch matrix composites. Optical activity was observed in composites made with carbonized fibers; the extent decreases with increased heat treatment of the fibers. Studies at various heat treatment temperatures indicate that ribbon-shape fibers developed ordered structure at 1600 °C when co-carbonized with thermosetting resin or thermoplastic pitches.     

Potassium hydroxide catalyst supported on palm shell activated carbon for transesterification of palm oil

In this study, potassium hydroxide catalyst supported on palm shell activated carbon was developed for transesterification of palm oil. The Central Composite Design (CCD) of the Response Surface Methodology (RSM) was employed to investigate the effects of reaction temperature, catalyst loading and methanol to oil molar ratio on the production of biodiesel using activated carbon supported catalyst. The highest yield was obtained at 64.1 °C reaction temperature, 30.3 wt.% catalyst loading and 24:1 methanol to oil molar ratio. The physical and chemical properties of the produced biodiesel met the standard specifications. This study proves that activated carbon supported potassium hydroxide is an effective catalyst for transesterification of palm oil.     

Borate treatment of carbon fibers and carbon/carbon composites for improved oxidation resistance

Carbon fibers and carbon/carbon composites have been treated with borate additives and then cured at 500–600°C to produce a continuous film of boron oxide on all exposed surfaces.This treatment has been found to be highly effective in retarding oxidation of the carbonaceous substrate for extended periods in flowing air at temperatures up to 1000°C. At higher temperatures, and in the presence of water vapor, borate species were appreciably volatile and the oxidation protection provided by the coatings was less effective.     

Temperature and catalyst effects on the production of amorphous carbon nanotubes by a modified arc discharge

Large amounts of amorphous carbon nanotubes (ACNTs) were prepared with Co-Ni alloy powders as catalyst in hydrogen gas atmosphere by a modified arc discharging furnace which can control temperature during the electric arcing process. The experimental results indicate that the cooperative function of temperature and catalyst plays an important role in the soot production rate and the relative ACNT purity. When temperature increases from 25 °C to 700 °C, the soot production rate increases from around 1 g/h to 8 g/h, the best relative ACNT purity at 600 °C can reach up to 99% compared to the room temperature sample. Without catalyst, only plate graphite is formed at 25 °C and very few carbon nanotubes are found when temperature increases to 600 °C. TEM, SEM, HRTEM and XRD analysis showed that the as-prepared carbon nanotubes are almost amorphous. The soot production rate is 8 g/h and diameter range of amorphous carbon nanotubes is about 7-20 nm, respectively.     

Preparation of carbon nanofibers/carbon foam monolithic composite from coal liquefaction residue

Carbon nanofibers/carbon foam composites that are made by growing carbon nanofibers (CNFs) on the surface of a carbon foam (CF) have been prepared from coal liquefaction residues (CLR) by a procedure involving supercritical foaming, oxidization, carbonization, and catalytic chemical vapour deposition (CCVD) treatment. These new carbon/carbon composites were examined using SEM, TEM and XRD. The results show that the as-made CF has a structure with cell sizes of 300-600 μm. X-ray diffraction studies show that iron-containing contaminates are present in the CLR. However, these species may act as a catalyst in the CCVD process as established in the literature. After the CCVD treatment, the cell walls of CF are covered by highly compacted CNFs that have external diameters of about 100 nm and lengths of several tens of micrometers. This work may open a new way for direct and effective utilization of the CLR.     

Two structural types of carbon bi-filaments

The structure of carbon bi-filaments synthesized on nickel wire by a hot filament-assisted CVD (HF-CVD) technique has been investigated by TEM, HRTEM and SAED. Two main types of bi-filament have been found. The first type consists of elongated dense regions (“rods”) formed by bundles of graphene layers. The “rods” are inclined at an angle to the filament axis and appear to be arranged around the filament axis with pseudo-rotational symmetry. Along these bi-filaments (diameter 140-260 nm) lens-like cavities of different sizes are observed. The Ni catalytic particles have lenticular shape with pseudo-rotational symmetry and possess either a fcc or a hcp lattice. The top and bottom parts of a catalyst particle are usually terminated by {1 1 1} planes, (or {0 0 0 1} for hexagonal lattice), while its inclined part is formed by stepped terraces parallel to (1 1 1). The structural organization of bi-filaments as well as their defects are determined by time dependent surface structure of the catalyst particles and by an oscillatory process of carbon concentration on nickel (1 1 1) or (0 0 0 1) facets. The second type of bi-filament consist of two subfilaments, semi-circular in cross section, connected together by flat sides with pseudo-mirror symmetry. The diameter of the complete filaments is 65-75 nm. Nanometre-sized catalyst particles are distorted pyramids and usually have a hcp lattice. The basic structure of these bi-filaments consists of elongated regions (“rods”) formed by bundles of graphene layers. In this case the “rods” form a layer structure.     

Microstructure and anti-oxidation properties of multi-composition ceramic coatings for carbon/carbon composites

To protect carbon/carbon (C/C) composites from oxidation at elevated temperature, an effective WSi₂-CrSi₂-Si ceramic coating was deposited on the surface of SiC coated C/C composites by a simple and low-cost slurry method. The microstructures of the double-layer coatings were characterized by X-ray diffraction, scanning electron microscopy and energy dispersive spectroscopy analyses. The coating exhibited excellent oxidation resistance and thermal shock resistance. It could protect C/C composites from oxidation in air at 1773 K for 300 h with only 0.1 wt.% mass gain and endure the thermal shock for 30 cycles between 1773 K and room temperature. The excellent anti-oxidation ability of the double-layer WSi₂-CrSi₂-Si/SiC coating is mainly attributed to the dense structure of the coating and the formation of stable vitreous composition including SiO₂ and Cr₂O₃ produced during oxidation.     

Control of catalyst particle crystallographic orientation in vertically aligned carbon nanofiber synthesis

Vertically aligned carbon nanofibers (VACNF) have been synthesized where the crystallographic orientation of the initial catalyst film was preserved in the nanoparticle that remained at the nanofiber tip after growth. A substantial percentage of catalyst particles (75%), amounting to approximately 200 million nanofibers over a 100 mm Si wafer substrate, exhibited a sixfold symmetry attributed to a cubic Ni(1 1 1)∥Si(0 0 1) orientation relationship which was verified by X-ray diffraction studies. The Ni catalyst films were prepared by rf-magnetron sputtering under substrate bias conditions to yield a single (1 1 1) film texture. The total energy of the Ni thin film was estimated by calculating the sum of the surface free energy and strain energy. The total film energy was minimized by the evolution of the plane of lowest surface free energy, the (1 1 1) texture. This result was in agreement with X-ray diffraction measurements. The preferred orientation present in the Ni catalyst film prior to nanofiber growth was preserved in the Ni catalyst particles throughout the VACNF growth process. The Ni catalyst particles at the nanofiber tips were not pure single crystals but rather consisted of a mosaic structure of Ni nanocrystallites embedded within Ni catalyst nanoparticles (200-400 nm). The tip-located nanoparticles exhibited a faceted, crystal morphology with the faceting transferred to the underlying carbon nanofiber during the growth process. The possibility of precisely and accurately controlling VACNF growth velocity over macroscopic wafer dimensions with uniformly aligned catalyst particles is discussed.