Carbon Whiskers Research Articles

Investigation of nanomechanical oscillators based on amorphous carbon whiskers in vacuum and at ambient pressure

Nanomechanical oscillators (NMO) on the base of amorphous C nanowhiskers, localized on the tops of W needles have been created and studied. Trajectories of resonant oscillations were visualized using a scanning electron microscope and a confocal laser scanning microscope. Resonant frequencies and the quality factor of NMO were determined at low pressure and in air. Reduction of the nanomechanical oscillators quality factor after the transition from vacuum condition to ambient pressure was not observed.

Q‐factor study of nanomechanical system “metal tip – carbon nanowhisker” at low and ambient pressure

Nanomechanical oscillators based on amorphous carbon whiskers, localized on the top of tungsten tip were fabricated and investigated. The whiskers were grown in the scanning electron microscope chamber using focused electron beam technique. Oscillation trajectories and amplitude‐frequency characteristic of the oscillator were visualized at low and ambient pressure using a scanning electron microscope and a confocal laser‐scanning microscope, respectively. We experimentally show that at ambient pressure, the resonant frequency decreases significantly but the Q‐factor of the oscillator unexpectedly increases with respect to experimental data acquired at low pressure. The explanation was provided taking into account the role of thin water layer absorbed on the whisker from atmosphere. The model of the coupled oscillators is considered.

Equilibrium potassium coverage and its effect on a Ni tar reforming catalyst in alkali- and sulfur-laden biomass gasification gases

Biomass conversion to syngas via gasification produces certain levels of gaseous by-products, such as tar and inorganic impurities (sulfur, potassium, phosphorus etc.). Nickel, a commonly used catalyst for hydrocarbon steam reforming, suffers reduced reforming activity by small amounts of sulfur (S) or potassium (K), while resistance against deleterious carbon whisker formation increases. Nevertheless, the combined effect of biomass derived gas phase alkali at varying concentrations together with sulfur on tar reforming catalyst performance under realistic steady-state conditions is largely unknown. Prior to this study, a methodology to monitor these effects by precise K dosing as well as K co-dosing with S was successfully developed. A setup consisting of a 5kW biomass fed atmospheric bubbling fluidized bed gasifier, a high temperature hot gas ceramic filter, and a catalytic reactor operating at 800°C were used in the experiments. Within the current study, two test periods were conducted, including 30h with 1ppmv potassium chloride (KCl) dosing followed by 6h without KCl dosing. Besides an essentially carbon-free operation, it can be concluded that although K, above a certain threshold surface concentration, is known to block active Ni sites and decrease activity in traditional steam reforming, it appears to lower the surface S coverage (θs) at active Ni sites. This reduction in θs increases the conversion of methane and aromatics in tar reforming application, which is most likely related to K-induced softening of the SNi bond. The K-modified support surface may also contribute to the significant increase in reactivity towards tar molecules. In addition, previously unknown relevant concentrations of K during realistic operating conditions on typical Ni-based reforming catalysts are extrapolated to lie below 100μgK/m2, a conclusion based on the 10–40μgK/m2 equilibrium coverages observed for the Ni/MgAl2O4 catalyst in the present study.

Fabrication of SiC Composites with Synergistic Toughening of Carbon Whisker andIn Situ3C-SiC Nanowire

The SiC composites with synergistic toughening of carbon whisker andin situ3C-SiC nanowire have been fabricated by hot press sinter technology and annealed treatment technology. Effect of annealed time on the morphology of SiC nanowires and mechanical properties of theCw/SiC composites was surveyed in detail. The appropriate annealed time improved mechanical properties of theCw/SiC composites. The synergistic effect of carbon whisker and SiC nanowire can improve the fracture toughness forCw/SiC composites. The vapor-liquid-solid growth (VLS) mechanism was proposed. TEM photo showed that 3C-SiC nanowire can be obtained with preferential growth plane ({111}), which corresponded to interplanar spacing about 0.25 nm.

Exsolution of Fe and SrO Nanorods and Nanoparticles from Lanthanum Strontium Ferrite La0.6Sr0.4FeO3-δ Materials by Hydrogen Reduction.

Formation of uniform Fe and SrO rods as well as nanoparticles following controlled reduction of La0.6Sr0.4FeO3−δ (LSF) and Ni-LSF samples in dry and moist hydrogen is studied by aberration-corrected electron microscopy. Metallic Fe and SrO precipitate from the perovskite lattice as rods of several tenths of nm and thicknesses up to 20 nm. Based on a model of Fe whisker growth following reduction of pure iron oxides, Fe rod exsolution from LSF proceeds via rate-limiting lattice oxygen removal. This favors the formation of single iron metal nuclei at the perovskite surface, subsequently growing as isolated rods. The latter is only possible upon efficient removal of reduction-induced water and, subsequently, reduction of Fe +III/+IV to Fe(0). If water remains in the system, no reduction or rod formation occurs. In contrast, formation of SrO rods following reduction in dry hydrogen is a catalytic process aided by Ni particles. It bears significant resemblance to surface diffusion-controlled carbon whisker growth on Ni, leading to similar extrusion rods and filaments. In addition to SrO rod growth, the exsolution of Fe nanoparticles and, subsequently, Ni–Fe alloy particles is observed. The latter have also been observed under static hydrogen reduction. Under strict control of the experimental parameters, the presented data therefore open an attractive chemically driven pathway to metal nanoarchitectures beyond the formation of “simple” nanoparticles.

FDI Oriented Simulation of an MSR Reactor for Application in SOFC Systems

The feasibility of direct internal reforming in SOFCs has been widely investigated, but at present it is not a state-of-the-art option for SOFC systems, which, instead, embed a Methane Steam Reforming (MSR) reactor prior to the SOFC stack itself [1]. Despite these reactors implement an established technology, they still present some issues. In particular, carbon can deposit under the form of whiskers and coat the active sites of the catalyst, reducing its activity and limiting gas diffusion through the catalytic bed. Even if it is well known that coke deposition is inhibited by a high steam to carbon (S/C) ratio, generally in the range 2-6, nevertheless, the problem of carbon deposition is still not solved, as widely discussed in the literature [2]. In case coke deposition occurs in the MSR reactor, the reformate fuel contains high levels of methane. This could force the SOFC stack to perform direct internal reforming, which could then lead to dangerous SOFC stack operation [1], with high solid temperature gradients associated to mechanical stresses, fractures and eventually break-down. Early detection, isolation, identification and possibly correction of a coke deposition fault in the MSR reactor is essential in view of reducing the maintenance cost of SOFC plants, and to increase of their lifetime. To this end, Fault Detection and Isolation (FDI) tools, under development for SOFC systems, can be suitably tailored to be applied specifically to the MSR reactor. A number of FDI methods have been proposed and applied to a variety of systems. A survey can be found in [3], where they are classified as: (i) quantitative model-based ; (ii) qualitative model-based, and (iii) process history-based methods. In practice, the classification regards the approach used to produce the fault data for the FDI: in the first two cases, the data are obtained from a model developed for the system under analysis, while the latter approach is based on historical experimental data. In the present work, we propose a first-principle model based on a quantitative and detailed simulation developed for the MSR reactor, which is intended to provide the basis for the development of an FDI tool based on pattern recognition techniques, which will be developed in a subsequent phase. Modelling of industrial MSR reactors has received wide attention so far [4-5], mainly aimed at optimizing operating conditions and identifying innovative geometries, particularly with reference to the arrangement of the burners within the combustion chamber where the MSR reactor tubes are situated. These studies include the kinetic equations of the MSR reactions, but to date there are no literature papers in which the kinetics of the coke deposition reaction is embedded into the MSR reactor model, and this is a distinctive feature of the present work. Our MSR reactor model is based on microscopic balances of mass, energy and momentum, written under transient form. The evaluation of heat and mass transport phenomena within the catalyst pellets is included as well, together with, as already mentioned, chemical kinetics. The equations of the model form a PDAE (Partial Differential and Algebraic Equation) system, with appropriate boundary conditions, which is integrated numerically using a finite element method, implemented through COMSOL Multiphysics. Then the result is a 3-D and time-dependent distribution of all the chemical-physical variables within the MSR reactor, in particular composition, pressure and temperature of the reactant gas. An example of simulated temperature distribution in a full size industrial MSR reactor tube is reported in Fig. 1. In a subsequent development of this work, the MSR reactor diagnostics will be addressed through the classical model-based philosophy, based on the comparison between the chemical and physical variables measured in the reactor, and the same variables calculated by the model, under the hypothesis of unfaulty reactor operation. [1] Sorce A., Greco A., Magistri L., Costamagna P., “FDI oriented Modelling of an Experimental SOFC System, Model Validation and Simulation of Faulty States,” Applied Energy, vol. 136, pp. 894-908, 2014. [2] S. Helveg, J. Sehested, J.R. Rostrup-Nielsen, “Whisker carbon in perspective,” Catal. Today, vol. 178, pp. 42-46, 2011. [3] Venkatasubramanian V, Rengaswamy R, Yin K, Kaviri SN. “A review of process fault detection and diagnosis. Part I: Quantitative model-based methods,” Comput. Chem. Eng., vol. 27, pp. 293-311, 2003. [4] J. Xu, G.F. Froment, “Methane steam reforming: II. Diffusional limitations and reactor simulation,” AIChE J., vol. 35, pp. 97-103, 1989. [5] M. Behnam, A.G. Dixon, P.M. Wright, M. Nijemeisland, E.H. Stitt, “Comparison of CFD simulations to experiment under methane steam reforming reacting conditions,” Chem. Eng. J., vol. 207-208, pp. 690-700, 2012. Figure 1

The effect of CH4 decomposition temperature on the property of deposited carbon over Ni/SiO2 catalyst

The activity and stability of Ni/SiO2 catalyst for the decomposition of methane at different temperature were investigated and the types of deposited carbon were also monitored. The catalyst was prepared by impregnation method and characterized by XRD, TG, SEM, Raman, TPH and TPO techniques. The dependence of the morphology, structure, and amount of deposited carbon over the used catalyst on the reaction temperature was studied. The results suggested that the Ni/SiO2 catalyst showed the best initial catalytic activity at 700 °C for the catalytic decomposition of methane, where the carbon whisker was the finest and densest, and the proportion of the well-ordered graphic was enhanced with increased reaction temperature. The largest amount of carbon deposition was observed at 550 °C used catalyst, and at this reaction temperature the Ni/SiO2 catalyst showed relatively the highest stability.

Examining Carbon Nanofibers: Properties, growth, and applications

One-dimensional carbon ?nanostructures have been known and fabricated for more than a hundred years and were originally referred to as filamentous carbon,carbon filaments, or carbon whiskers. However, it was only during the 1990s and 2000s with the demonstration of carbon nanotubes (CNTs)and the introduction of catalytic plasma-enhanced chemical vapor deposition (PECVD) that one-dimensional carbon nanostructures emerged as a promising solution for a large variety of applications. In this article, carbon nanofibers (CNFs), a particular type of one-dimensional carbon with unique properties, are described, their growth is discussed, and their main applications are presented.

Macroscopically Aligned Carbon and Graphite Whiskers Prepared from Poly(m-phenylene) Derivatives with Helicene-like Helical Structures

We report novel macroscopically aligned carbon and graphite whiskers prepared from self-assembled helicene-like helical poly(m-phenylene) (PMP) derivatives with whisker morphologies. The PMP derivatives bearing carboxylic acid moieties were synthesized through esterolysis of the alkyl side chains and used as carbonization precursors. Polarizing optical microscopy and scanning electron microscopy observations showed that the randomly aligned whisker morphologies of the PMP derivatives were preserved during the carbonization, even at 800 °C. Furthermore, graphite whiskers were prepared with heat treatment at 2600 °C for the carbon whiskers. Next, two types of oriented substrates were prepared through polytetrafluoroethylene rubbing and stretching methods. The PMP films with macroscopically aligned whisker morphologies were prepared by annealing polymers on the oriented substrates. The carbon and graphite whiskers with macroscopically aligned morphologies were then prepared through heat treatment. The aligned carbon whiskers showed an electrical anisotropy, where the conductivity parallel to the alignment direction of whisker was higher than that perpendicular to it.

Preparation and properties of epoxy/BN highly thermal conductive composites reinforced with SiC whisker

A simple method is reported to increase the thermal conductivity and improve the poor mechanical properties caused by high filler loadings of epoxy composites, simultaneously. Epoxy composites were prepared with micro-boron nitride (BN) and silicon carbon whisker (SiCw) chemically treated by 3-aminopropyltriethoxysilane (KH550) and 3-glycidyloxypropyltrimethoxysilane (KH560), respectively. Effects of surface modification of BN particles on the thermal conductivity and flexural strength of epoxy/BN composites were investigated. About 3% SiCw particles grafted with KH560 were incorporated into composites with BN grafted with KH550, which led to about 13.8%–17.8% increase of the flexural strength as well as a marginal improvement of the thermal conductivity of composites, and they possessed good dielectric properties. In addition, dynamic mechanical analysis results showed that the storage modulus of composites increased significantly with the addition of fillers, while the glass transition temperature exhibited a slight decrease. POLYM. COMPOS., 2015. © 2015 Society of Plastics Engineers

High Stability of Ni-Co/SBA-15 Catalysts for CH4 Reforming with CO2

A series of SBA-15 supported Ni-Co/SBA-15 bimetallic catalysts (CN-x, Ni = 13 wt%, Co wt% = x, x = 0, 2, 5, 8, 100) are prepared using a co-impregnation method. Catalyst screening with the reforming reaction shows that Ni-Co bimetallic catalysts have superior catalytic performance. In addition, the Ni-Co bimetallic catalyst with 2% Co content shows the best stability and the lowest carbon deposition rate (1.79 mgc.gcat−1.h−1) in 800-h time on stream. The results indicated that carbon deposition is still the main reason for deactivation. The addition of Co increases the basicity of catalysts and enhances the CO2 adsorption, which effectively reduce the carbon deposition rate, while the addition of Co excess will produce a large number of O species, leading to Co0 oxidation, accelerated catalysts deactivation. In addition, the Co species on Ni-Co/SBA-15 catalysts effectively changed the morphology of carbon deposition. Carbon deposit on Ni monometallic catalysts is mostly in the form of shell-like carbon making catalyst rapid deactivation, whereas the kind of whisker carbon species mainly exhibited on the surface of Ni-Co bimetallic catalysts possessed little toxicity.

Enhanced dry reforming of methane on Ni and Ni-Pt catalysts synthesized by atomic layer deposition

Atomic layer deposition (ALD) was used to deposit Ni and Pt on alumina supports to form monometallic and bimetallic catalysts with initial particle sizes of 1–2.4nm. The ALD catalysts were more active (per mass of metal) than catalysts prepared by incipient wetness (IW) for dry reforming of methane (DRM), and they did not form carbon whiskers during reaction due to their sufficiently small size. Catalysts modified by Pt ALD had higher rates of reaction per mass of metal and inhibited coking, whereas NiPt catalysts synthesized by IW still formed carbon whiskers. Temperature-programmed reduction of Ni catalysts modified by Pt ALD indicated the presence of bimetallic interaction. Density functional theory calculations suggested that under reaction conditions, the NiPt surfaces form Ni-terminated surfaces that are associated with higher DRM rates (due to their C and O adsorption energies, as well as the CO formation and CH4 dissociation energies).

A study of the methanation of carbon dioxide on Ni/Al2O3 catalysts at atmospheric pressure

Abstract The hydrogenation of carbon dioxide producing methane and CO has been investigated over Ni/Al2O3 catalysts. The as prepared catalysts have been characterized by XRD and Temperature Programmed Reduction. Spent catalysts have been characterized by XRD and Field Emission SEM. Catalytic activity needs the presence of Ni metal particles which may form in situ if the Ni loading is higher than that needed to cover the alumina surface with a complete monolayer. If Ni content is lower, pre-reduction is needed. Catalysts containing very small Ni particles obtained by reducing moderate loading materials are very selective to methane without CO formation. The larger the Ni particles, due to higher Ni loadings, the higher the CO production. Cubic Ni metal particles are found in the spent catalysts mostly without carbon whiskers. The data suggest that fast methanation occurs at the expense of CO intermediate on the corners of nanoparticles interacting with alumina, likely with a “via oxygenate” mechanism.

Modifying perovskite-type oxide catalyst LaNiO3 with Ce for carbon dioxide reforming of methane

Perovskite-type oxide catalysts LaNiO3 and La1−xCexNiO3 (x ≤ 0.5) were prepared by the Pechini method and used as catalysts for carbon dioxide reforming of methane to form synthesis gas (H2 + CO). The gaseous reactants consisted of CO2 and CH4 in a molar ratio of 1:1. At a GHSV of 10,000 hr−1, CH4 conversion over LaNiO3 catalyst increased from 66% at 600 °C to 94% at 800 °C, while CO2 conversion increased from 51% to 92%. The achieved selectivities of CO and H2 were 33% and 57%, respectively, at 600 °C. To prevent the deposition of carbon and the sintering nickel species, some of the Ni in perovskite-type oxide catalyst was substituted by Ce. Ce provided lattice oxygen vacancies, which activated C–H bonds, and increased the selectivity of H2 to 61% at 600 °C. XRD analysis indicates that the catalyst exhibited a typical perovskite spinel structure and formed La2O2CO3 phases after CO2 reforming. The FE-SEM results reveal carbon whisker of the LaNiO3 catalyst and the BET analysis indicates that the specific surface area increases after the reforming reaction. The H2-TPR results confirm that Ce metals can store and provide oxygen.

The Properties of Individual Carbon Residuals and Their Influence on The Deactivation of Ni–CaO–ZrO2 Catalysts in CH4 Dry Reforming

AbstractNi–CaO–ZrO2 catalysts with different properties were prepared and tested for CO2 reforming of methane. The catalysts were characterized by means of transmission electron microscopy, thermogravimetric analysis, Raman spectroscopy, X‐ray photoelectron spectroscopy, and X‐ray diffraction to reveal their distinct properties and carbon deposition behaviors in the reforming reaction. It was found that the catalyst prepared by a coprecipitation method and ageing by heating to reflux exhibited a nanocrystalline structure with strong metal–support interaction, which was responsible for both high activity and stability, but it also exhibited the highest carbon formation rate among the tested catalysts. This result suggests that catalyst deactivation might not necessarily correlate with the amount of formed carbon, and the individual properties of carbon residuals could play a more decisive role. Carbon residuals on different catalysts were identified as amorphous carbon, encapsulating carbon, whisker carbon, and graphite, which had different influence on the deactivation. On the surface of the most active and stable catalyst, the carbon species mainly consisted of amorphous and whisker carbon, suggesting that the formation of such carbon species does not necessarily lead to catalyst deactivation. In contrast, the deactivation was found to be closely related to the formation of encapsulating carbon and graphite, which could coat the catalyst surface. The accumulation of different carbon residuals was proven to follow a formation–diffusion/elimination scenario, which was significantly influenced by the Ni particle size and Ni–ZrO2 interactions.

Catalyst deactivation by carbon formation during CO hydrogenation to hydrocarbons on mesoporous Co3O4

Mesoporous cobalt oxide (Co3O4) was prepared through the template-replicating method by using the templating material of KIT-6, and its catalytic activity for CO hydrogenation to hydrocarbons by the Fischer–Tropsch (FT) synthesis was investigated and compared with that of precipitated Co3O4. The mesoporous Co3O4 showed an enhanced catalytic activity at initial reaction stage by an increased mass-transfer rate of heavier hydrocarbons in large regular pore structures above 6nm in size and high metallic surface area of cobalt. Different deactivation phenomena were observed during the FT reaction due to collapse of the pore structure and carbon deposition such as formation of graphitic whisker carbon on mesoporous Co3O4 and encapsulation of amorphous carbon on precipitated Co3O4. The different catalytic performance and deactivation behavior of the two Co3O4 catalysts were mainly due to the variation of surface morphologies and the types of coke formed.

A novel nickel-based catalyst for methane dry reforming: A metal honeycomb-type catalyst prepared by sol–gel method and electroless plating

A honeycomb-type nickel-based catalyst was prepared by a combination technique consisting of a sol–gel method and an electroless plating method on a stainless steel substrate. The catalyst layer formed onto the fin substrate consisted of an alumina layer and a nickel component. The alumina layer was porous and was about 7μm in thickness, and the nickel component was deposited not only on the alumina surface but also inside the pores. The nickel particles formed on the surface were 70–150nm in diameter. The prepared nickel-based honeycomb catalyst demonstrated a high performance for methane dry reforming, indicating that the catalyst degradation was quite small even under severe CO2/CH4 conditions. The amount of the deposited carbon onto the nickel-based catalyst was much smaller than that on the commercial catalyst, which was thought to be one of the factors in the reduced deterioration of this honeycomb catalyst. Furthermore, the H2/CO ratio as a synthesis gas over this nickel-based catalyst was a smaller value than that of the commercial catalyst, which is a convenient reforming property for alkane manufacturing. The deposited carbon on the nickel-based honeycomb catalyst was a whisker carbon with non-condensed state.

Synthetic natural gas from wood: Reactions of ethylene in fluidised bed methanation

The synthesis step in the production of synthetic natural gas from wood, i.e. the methanation, was investigated by systematic experiments with commercial nickel catalyst in a micro-fluidised bed reactor. Ethylene in the feed is always converted completely; dominantly serial reactions of ethylene to ethane and further to methane under isothermal fluidised bed methanation conditions could be shown. Lower temperatures favour the production of the intermediate ethane while high temperatures cause the formation of carbon depositions and carbon whiskers. Applying optimal operation conditions, the hydrogenation of the unsaturated olefin not only avoids the deposition of carbon or coke, but also leads to an increase of the higher heating value (HHV) of the produced (raw) SNG.

Diffusion in quasi-one-dimensional nano- and microobjects

A new approach to investigations of the volume and surface diffusion in quasi-one-dimensional nano- and microobjects is described in application to the diffusion of tungsten from the substrate into a carbon whisker crystal. The proposed experimental technique employs special positioning marks formed on the whisker surface. The interrelated processes of volume and surface diffusion have been simulated using a combination of the integrated field sources and finite difference methods. The coefficients of volume and surface diffusion of tungsten atoms in carbon whiskers have been estimated.

Alumina Supported Ni-Mg-La Tri-Metallic Catalysts for Toluene Steam Reforming as a Biomass Gasification Tar Model Compound

The steam reforming of toluene as a model compound of biomass gasification tar was carried out over Mg and La oxide-promoted Ni-metal oxide/Al2O3 catalysts. Catalysts were prepared by two different methods, co- and sequential impregnation. The findings indicate that conversion of gas products was improved with the use of prepared catalysts especially on syn-gas (H2 and CO) species and the highest conversions were obtained at the reaction temperature of 800°C. LHV’s of product gas when using catalysts at 800°C were over 4 MJ/m3 and ratios of H2 to CO were between 2.49-2.77. For long term test, Carbon and hydrogen conversion to CO and H2 of the catalysts with respect to time on stream in the steam toluene reforming for 480 min were studied. La2O3+MgO+Ni/Al2O3 catalyst revealed the highest and stable conversion rate of closely 50% and 70% for CO and H2, respectively. Whisker carbon species and encapsulating carbon were found on used catalysts after reaction. The La2O3+MgO+Ni/Al2O3 catalyst showed lesser amount of whisker carbon and encapsulating carbon.