High Surface Area Silicon Carbide Research Articles

A general method for rapid synthesis of refractory carbides by low-pressure carbothermal shock reduction

Refractory carbides are attractive candidates for support materials in heterogeneous catalysis because of their high thermal, chemical, and mechanical stability. However, the industrial applications of refractory carbides, especially silicon carbide (SiC), are greatly hampered by their low surface area and harsh synthetic conditions, typically have a very limited surface area (<200 m2 g-1), and are prepared in a high-temperature environment (>1,400 °C) that lasts for several or even tens of hours. Based on Le Chatelier's principle, we theoretically proposed and experimentally verified that a low-pressure carbothermal reduction (CR) strategy was capable of synthesizing high-surface area SiC (569.9 m2 g-1) at a lower temperature and a faster rate (∼1,300 °C, 50 Pa, 30 s). Such high-surface area SiC possesses excellent thermal stability and antioxidant capacity since it maintained stability under a water-saturated airflow at 650 °C for 100 h. Furthermore, we demonstrated the feasibility of our strategy for scale-up production of high-surface area SiC (460.6 m2 g-1), with a yield larger than 12 g in one experiment, by virtue of an industrial viable vacuum sintering furnace. Importantly, our strategy is also applicable to the rapid synthesis of refractory metal carbides (NbC, Mo2C, TaC, WC) and even their emerging high-entropy carbides (VNbMoTaWC5, TiVNbTaWC5). Therefore, our low-pressure CR method provides an alternative strategy, not merely limited to temperature and time items, to regulate the synthesis and facilitate the upcoming industrial applications of carbide-based advanced functional materials.

Exploring Different Synthesis Parameters for the Preparation of Metal-Nitrogen-Carbon Type Oxygen Reduction Catalysts

The influence of various synthesis conditions of a metal-nitrogen-carbon (M-N/C) catalyst material on oxygen reduction reaction (ORR) kinetics is discussed. Seven M-N/C catalysts based on cobalt are obtained by changing various synthesis conditions, such as the mixing environment, pyrolysis gas, and post-treatment. The ORR activity and stability measurements are performed using the classical three-electrode configuration in a 0.1 M HClO4 solution. The most active and stable ORR catalyst proves to be the material obtained by mixing a cobalt salt, 2,2’-bipyridine, and a high surface area silicon carbide derived carbon together in water and pyrolyzing the mixture in argon. In a fuel cell test, however, a maximum power density value of 135 mW cm−2 is achieved with the catalyst mixed together in a planetary ball-mill at a low catalyst loading of 1.0 ± 0.1 mg cm−2 and at a test cell temperature of 60 oC despite of the fact that preparing the catalyst via dry ball-milling reduces the surface area of the material roughly 40% more than in the case of using a solution-based method. Consequently, mixing the catalyst precursors together without any additional chemicals in a planetary ball-mill instead of in a solution appears to be the most promising choice.

The Synthesis of High-Surface Area Silicon Carbide by Conversion Method Using Carbosilane Polymer

Since mesoporous silica such as MCM-41 and SBA-15 was developed, the study of the properties of high-surface area materials was accelerated. Moreover, the mesoporous silica is used as a template to produce high-surface materials by nanocasting technology. The purpose of this paper is the synthesis of a high surface silicon carbide sphere by the nanocasting technology. In this study, KCC-1 silica sphere was used as a template, and polycarbosilane and poly (phenyl carbosilane) were selected for precursor of silicon carbide. Carbosilane polymer gives advantage of synthesis silicon carbide under low temperature, and hollow spheres were produced. In this study, the polycarbosilane was more effective for the synthesis of SiC hollow spheres by inversion of template structure showing a fibrous morphology on the sphere wall. And it was confirmed that the sphere was composed of nanosized SiC crystals, and has high surface area using TEM, XRD and BET analysis.

ChemInform Abstract: Synthesis, Stability, and Catalytic Properties of High Surface Area Silicon Oxynitride and Silicon Carbide

Abstract The aim of this work is to extend the range of inorganic materials which find application in heterogeneous catalysis, as catalysts or catalyst supports, through use of progress in the field of advanced ceramics. Developments in synthetic routes to high surface area non-oxides (nitrides, carbides, borides) are briefly reviewed. Work on the synthesis, stability and catalytic properties of high surface area silicon oxynitride and silicon carbide is described. For the former compound, three synthetic routes were investigated: the reaction of an amorphous silica with ammonia proved to be the preferred route, giving a silicon oxynitride of ca. 150 m 2 g −1 . The surface composition of the material obtained by the latter method showed good stability, as indicated by retention of surface nitrogen under reducing or oxidising conditions at about 700°C. The oxynitride surface was unreactive towards butene-1, causing no double-bond isomerisation at 400°C. However, the material was also found to be capable of functioning as a solid basic catalyst, catalysing Knoevenagel condensations in solution at 50°C. The use of nitrides as solid basic catalysts appears to be novel, and this result suggests new applications for such materials. In the case of silicon carbide, a mesoporous form was obtained by polymer pyrolysis. The texture shows improved thermal (but not hydrothermal) stability compared to silica. The surface is less reactive than that of the oxynitride, catalysing neither the double-bond isomerisation of butene-1 nor the Knoevenagel condensation. This compound, and the oxynitride, may be useful as inert support materials for reactions involving alkenes.

Preparations and Catalytic Applications of High Surface Area Silicon Carbide

We discuss recent developments in the preparation methods of high surface area silicon carbide,such as templated synthesis,sol-gel processing,and polycarbosilane pyrolysis.We provide an overview and an outlook for the applications of high surface area silicon carbide as catalyst support materials.

Synthesis and characterization of monolithic, high surface area SiO2/C and SiC/C composites

In this study, the synthesis and characterization of high surface area carbon-supported silica and silicon carbide aerogels are presented. An activated carbon aerogel with surface area greater than 3000 m2 g−1 was used as a support for the sol–gel deposition of silica. The resulting silica-coated carbon aerogel retained a surface area greater than 2000 m2 g−1 and showed improved thermal stability in air. The carbon-supported silicon carbide aerogel was made by the carbothermal reduction of the silica-coated carbon aerogel under flowing Ar at 1500 °C. The resulting monolith maintained a surface area greater than 2000 m2 g−1 and was stable to temperatures approaching 600 °C, 100 °C higher than that of the pristine carbon aerogel.

Synthesis and characterization of high surface area silicon carbide by dynamic vacuum carbothermal reduction

Silicon carbide (SiC) precursor was obtained by sol–gel used tetraethoxysilane as silicon source and saccharose as carbon source, and then the precursor was used to prepare SiC by carbothermal reduction under dynamic vacuum condition. The samples were characterized by X-ray diffraction, scanning electron microscope, and low-temperature nitrogen adsorption–desorption measurement. The results showed that the carbothermal temperature for synthesizing SiC needed to be at 1,100 °C under dynamic vacuum. At this temperature, the obtained sample is composed of agglomerated regular grains with size ranging from 20 to 40 nm and has a high surface area of 167 m2/g and the main pore size center at 5.3 nm.

Synthesis of High Surface Area Silicon Carbide and Its Application in CO Oxidation Reaction

A sol-gel process was employed to prepare SiC precursors, using tetraethyl orthosilicate (TEOS) and sucrose as raw materials. And oxalic acid, ferric nitrate, and nickel nitrate were added respectively as catalyst for hydrolysis and polymeration reactions. The effects of catalyst, reaction temperature, and reaction time on the gel formation were studied. The results showed that ferric nitrate was optimal for gel formation. SiC of 133 m2·g-1 was obtained by further carbonthermal reduction of the xerogel from the sol-gel system involved ferric nitrate, at 1350 ℃ in an Ar flow for 10 h. Such SiC was then used as catalyst support for CO oxidation. Wet impregnation method was adopted to obtain Pt/SiC catalyst. The as-prepared Pt/SiC catalyst showed good activity and stability for CO oxidation. It was also found that its performance could be improved significantly with the synergistic action of Ni promoter.

Thermal stability of high surface area silicon carbide materials

The synthesis of mesoporous silicon carbide by chemical vapor infiltration of dimethyl dichlorosilane into mesoporous silica SBA-15 and subsequent dissolution of the silica matrix with HF was investigated. The influence of the synthesis parameters of the composite material (SiC/SBA-15) on the final product (mesoporous SiC) was determined. Depending on the preparation conditions, materials with specific surface areas from 410 to 830 m 2 g −1 and pore sizes between 2 and 10 nm with high mesopore volume (0.31–0.96 cm 3 g −1) were prepared. Additionally, the thermal stability of mesoporous silicon carbide at 1573 K in an inert atmosphere (argon) was investigated, and compared to that of SBA-15 and ordered mesoporous carbon (CMK-1). Mesoporous SiC has a much higher thermal textural stability as compared to SBA-15, but a lower stability than ordered mesoporous carbon CMK-1.

Novel Synthesis of High Surface Area Silicon Carbide by RAPET (Reactions under Autogenic Pressure at Elevated Temperature) of Organosilanes

We report on a very simple, efficient, and economical synthetic technique that produces SiC of a high surface area, and under different conditions of a silicon carbide−carbon nanocomposite (SCCN) a...

High Surface Area Silicon Carbide Whiskers and Nanotubes Nanocast Using Mesoporous Silica

We report here the formation of high surface area silicon carbide materials comprising whiskers and nanotubes via a synthesis route, which utilizes mesoporous silica as sacrificial solid template. The silicon carbide materials are obtained via carbothermal reduction of mesoporous silica/carbon (i.e., SBA-15/sucrose) composites. Varying the carbothermal reduction conditions (i.e., temperature or duration) readily modifies the morphology of the silicon carbide so as to obtain whiskers or nanotubes. The whiskers, which grow in the [111] direction, are achieved by subjecting the mesoporous silica/carbon composites to carbothermal reduction at high temperature (1250 or 1300 °C) for reduction periods of up to 14 h. For reduction periods of up to 14 h, using the higher temperature (1300 °C) optimizes whisker formation. The diameter of the whiskers is 50−90 nm and their length is typically greater than 20 μm. The surface area of the whisker containing SiC materials varies between 120 and 145 m2/g, while their por...

Silicon Carbide: A Novel Catalyst Support for Heterogeneous Catalysis

Progress in developing a new class of support materials based on silicon carbide (SiC)is reviewed. Silicon carbide has superior mechanical and thermal properties which, coupled to chemical inertness,avoids several of the problems inherent in the use of commercial oxide and carbon based supports and catalysts. High surface area SiC can now be prepared easily in a commercially viable shape,with good mechanical properties,and at reasonable cost.I t can be shaped directly into monolith or honeycomb forms including some catalytically active material, rendering fabrication simple and cost effective. Furthermore, it can be modified for specific catalytic applications through the addition of metals. In many respects, it combines the best properties of oxide and carbon based supports without suffering many of their disadvantages.

Synthesis and thermal stability of Ni, Cu, Co, and Mo catalysts based on high surface area silicon carbide

The potential of high surface area silicon carbide as catalyst support has been evaluated regarding the metal–support interaction (MSI), metal–support stability (MSS), and affinity for ion-adsorption. Nickel, cobalt, copper, and molybdenum catalysts have been prepared by incipient wetness impregnation. These SiC based catalysts all show after calcination at 773 K, an MSI lower than that of their silica and alumina based counterparts. Reaction of the metal with SiC at elevated temperatures may cause the formation of metal silicides and limits the maximum temperature of application. The MSS of the incipient wetness Ni/SiC catalyst is high. An easily reducible NiO species is retained on the SiC surface after calcination at 1273 K, whereas substantial deactivation of the Ni/Al 2O 3 and Ni/SiO 2 catalysts occurs. These results suggest a high potential of Ni/SiC catalysts in high-temperature processes. Highly dispersed Ni/SiC catalysts (the diameter of the nickel particles equals 4 nm) have been prepared by adsorption of Ni(NH 3) 4(H 2O) 2 2+ on SiC. The nickel is thus grafted on SiC and SiO 2 as nickel silicate (antigorite). The formed amount of antigorite per unit surface area is on SiC three times higher than that on silica, which points to the presence of a very reactive oxidic layer on the SiC. Calcination at 1273 K causes substantial SiC conversion and nickel sintering, which points to a low MSS, in contrast to the Ni/SiC catalyst prepared by incipient wetness. This probably originates from the intimate contact of the nickel phase with the SiC surface and the resulting catalyzed oxidation of the SiC.

High surface area silicon carbide doped with zirconium for use as catalyst support. Preparation, characterization and catalytic application

Zirconium doped SiC with a surface area from 88 to 200 m 2 g −1 was synthesized using the shape memory concept method followed by calcination in air at a temperature of ≤480°C. The material obtained was composed of β-SiC and small ZrO 2 particles dispersed throughout the material matrix and a significant amount of an amorphous phase containing Si, Zr and O. Molybdenum oxycarbide, the active isomerization phase, supported on such a material displayed a similar behavior to that obtained on pure SiC for the n-heptane isomerization reaction. A comparison made with the molybdenum oxycarbide catalyst supported on pure ZrO 2 showed that the Zr doped SiC was not simply made of silicon carbide coated with a layer of ZrO 2 on the surface but probably an amorphous phase containing Si, Zr and O which displays a similar behavior as pure SiC.

High surface area silicon carbide as catalyst support characterization and stability

High surface area silicon carbide (SiC) of 30 m 2/g has been synthesized by the catalytic conversion of activated carbon. The stability of this SiC in aqueous hydrogen fluoride and a boiling nitric acid solution is shown to be excellent. No corrosion is encountered by treatment with boiling HNO 3, HF treatment causes the dissolution of the silica surface layer present on the SiC while the SiC remains intact. Oxidation in air at elevated temperatures has been analyzed by thermal gravimetric analysis, diffuse reflectance infrared spectroscopy, nitrogen adsorption, and X-ray diffraction. The thermal stability in non-oxidizing environments is shown to be excellent; no significant sintering has been observed after ageing in nitrogen for 4 h at 1273 K. The presence of 2 v% steam at 1273 K results in partial SiC oxidation into SiO 2 and considerable sintering. Air oxidation at 1273 K of pure SiC, SiC loaded with 5 wt.% nickel, and HNO 3 treated SiC is shown to cause substantial SiC conversion, viz. 60% to 70% after 10 h. Air oxidation at 1080 K will result in complete conversion in about 100 days. This rate of oxidation agrees with reports on the oxidation of non-porous Acheson SiC and SiC coatings formed by Chemical Vapour Deposition. It is concluded that at high surface area SiC cannot be used as a catalyst support in processes operating in oxidizing environments and temperatures above 1073 K. SiC based catalysts are very well suited for (1) high-temperature gas-phase reactions operating in the absence of oxidizing constituents (O 2 or H 2O) and (2) strong acidic liquid-phase processes.

Synthesis of high surface area silicon carbide by fluidized bed chemical vapour deposition

Activated carbon granulates loaded with small amounts of nickel have successfully been converted into high surface area silicon carbide granulates by reaction with hydrogen and silicon tetrachloride at 100 kPa and 1380 K (C+SiCl 4+2H 2→SiC+4HCl). A cone shaped Fluidized Bed Chemical Vapour Deposition reactor is demonstrated to achieve reproducible and homogeneous conversions of considerable amounts of activated carbon. High surface area silicon carbide has thus been synthesized with surface areas ranging between 25 and 80 m 2 g −1. The shape memory concept is applicable, because the shape of the activated carbon determines the morphology of the silicon carbide. The conversion process is shown to be limited by the gasification of carbon into methane. The formed methane is then totally converted into SiC via the Vapour Liquid Solid mechanism and conventional Chemical Vapour Deposition. Carbon conversions range from 10% to 45%. The high surface area SiC obtained after conversion and removal of the residual carbon has high potential as catalyst support for liquid-phase application at demanding pH conditions and processes operating at high temperatures.

Nickel-Catalyzed Conversion of Activated Carbon Extrudates into High Surface Area Silicon Carbide by Reactive Chemical Vapour Deposition

A novel method for the synthesis of high surface area silicon carbide extrudates has been developed which consists of applying nickel onto activated carbon extrudates followed by reaction with silicon tetrachloride and hydrogen. Utilization of nickel is shown to be essential in order to obtain a considerable conversion. Selective SiC formation has been obtained at 1380 K and 10 kPa. Thus, methane is formed at the interior of the carbon via gasification: C(s) + 2H 2 (g) ⇄ CH 4 (g), which subsequently reacts with silicon tetrachloride to silicon carbide: SiCl 4 (g) + CH 4 (g) ⇄ SiC(s) + 4HCl(g). The total carbon conversion ranges from 20 to 55% for nickel contents of 2 and 8 wt%, respectively. Si-codeposition will occur when the gasification reaction diminishes in time, due to deactivation of the nickel gasification sites. Extensive whisker formation of SiC is encountered owing to the operative vapour-liquid-solid mechanism. Mass transport calculations show that methane is formed throughout the extrudate, whereas the front of SiC formation moves from the outside to the internal part due to diffusion limitations of SiCl 4 and nickel deactivation. The residual carbon can be removed after conversion by oxidation, resulting in high surface area SiC extrudates. The BET-surface areas after conversion vary from 359 to 154 m 2 /g; BET-surface areas after removal of the residual carbon are in the range of 57 to 32 m 2 /g. Pore size distributions of the SiC supports show that the pore volume is evenly distributed over the meso- and macro-pore region (diameter: 2 to 100 nm) which allows the following areas of application: (1) reactions at high temperatures and (2) liquid-phase reactions at demanding pH conditions.

High Surface Area Silicon Carbide Doped with Zirconium for use as Heterogeneous Catalyst Support

ABSTRACTHigh surface area (&gt; 100 m2 · g−1) SiC doped with zirconium was prepared by the gas-solid reaction. The material was made up of three phases: β-SiC, covered by ZrO2 and an amorphous phase composed of Si, Zr and O. The characterization of the sample was performed by means of powder X-ray diffraction (XRD), surface area and porosity measurements by the BET method, scanning electron microscopy (SEM) and high-resolution transmission electron microscopy (HRTEM). Preliminary catalytic tests, the standard n-C7 isomerization on supported MoOxCy showed that this new support was at least as effective as pure SiC.

High surface area silicon carbide from rice husk: A support material for catalysts

Ultrafine β-SiC with high surface area (150 m2 g−1) has been synthesized by inflight processing of charred rice husk in a r.f. plasma reactor operating at atmospheric pressure. The plasma-synthesized particles were doped with platinum (1%) and tested as a catalytic support material. The catalyst (1% Pt doped β-SiC) showed 100% conversion of CO to CO2 at a temperature as low as 175°C.

Synthesis, stability, and catalytic properties of high surface area silicon oxynitride and silicon carbide

The aim of this work is to extend the range of inorganic materials which find application in heterogeneous catalysis, as catalysts or catalyst supports, through use of progress in the field of advanced ceramics. Developments in synthetic routes to high surface area non-oxides (nitrides, carbides, borides) are briefly reviewed. Work on the synthesis, stability and catalytic properties of high surface area silicon oxynitride and silicon carbide is described. For the former compound, three synthetic routes were investigated: the reaction of an amorphous silica with ammonia proved to be the preferred route, giving a silicon oxynitride of ca. 150 m 2 g −1. The surface composition of the material obtained by the latter method showed good stability, as indicated by retention of surface nitrogen under reducing or oxidising conditions at about 700°C. The oxynitride surface was unreactive towards butene-1, causing no double-bond isomerisation at 400°C. However, the material was also found to be capable of functioning as a solid basic catalyst, catalysing Knoevenagel condensations in solution at 50°C. The use of nitrides as solid basic catalysts appears to be novel, and this result suggests new applications for such materials. In the case of silicon carbide, a mesoporous form was obtained by polymer pyrolysis. The texture shows improved thermal (but not hydrothermal) stability compared to silica. The surface is less reactive than that of the oxynitride, catalysing neither the double-bond isomerisation of butene-1 nor the Knoevenagel condensation. This compound, and the oxynitride, may be useful as inert support materials for reactions involving alkenes.