Surface Area Silicon Research Articles

Low temperature use of SiC-supported NiS 2-based catalysts for selective H 2S oxidation : Role of SiC surface heterogeneity and nature of the active phase

High activity for the direct oxidation of H 2S into elemental sulfur at low reaction temperature (40–60 °C) on medium surface area silicon carbide-supported nickel sulfide (NiS 2/SiC) catalyst was attributed to the formation of a highly active superficial nickel oxysulfide. The hypothesis of the superficial formation of either nickel oxide or nickel sulfate was rejected. The superiority of the SiC support in terms of performance as compared to silica, high surface area alumina and activated charcoal was evidenced. The high stability of the SiC-supported catalyst as a function of time and solid sulfur loading in the presence of water was explained by a peculiar mode of sulfur deposition, involving the role of water and the hydrophilic/hydrophobic duality of the SiC support surface. Water acts as a conveyor belt, continuously cleaning the active phase particles, located on the hydrophilic oxygen-containing surfaces of the support. Hydrophobic pure SiC, located outside the mesoporosity and exempt of active phase, remains as an available surface for the storage of high amounts of solid sulfur.

Porous Silicon Nitride as a Superbase Catalyst

The synthesis of a new mesoporous support based on high surface area silicon nitride and its use as a solid superbase is reported. The 2,3-dimethylbut-1-ene isomerization is used as a test reaction to compare the activity of individual catalysts depending on the synthesis conditions of the support. Various impregnation methods and different metal loadings are studied. In particular, potassium catalysts, obtained by impregnation from ammoniacal solution, show high catalytic activity for alkene isomerization. Heat treatments of the support at 1273 K and potassium loadings up to 30% are beneficial for high conversion at low temperature.

Non-Oxide Sol-Gel Chemistry: Preparation from Tris(dialkylamino)silazanes of a Carbon-Free, Porous, Silicon Diimide Gel.

The acid-catalyzed ammonolysis of the hitherto unknown compound tris(dimethylamino)silylamine (1), which is prepared from SiCl4 in high yield and purity, results in the preparation of a silicon diimide gel. The probable first step in this process is the acid-catalyzed self-condensation of 1 to the cyclic trimer [{(CH3)2N}2SiNH]3. This ammonolysis under mild conditions in THF provides a semirigid translucent gel. On drying under mild conditions in an ammonia atmosphere this non-oxide gel yields a high surface area silicon diimide xerogel that is the first example of a porous non-oxide silicate gel.

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 Imidonitrides: A New Class of Microporous Solid Base

Shape- or size-selective micro- and mesoporous solids find applications as catalysts, catalyst supports, and membrane materials. Here the synthesis of a high surface area microporous base is reported. This material, prepared by ammonolysis of oligomeric methylsilazanes, is a nitride analogue of amorphous silica-based mixed metal oxides. The sharply defined microporosity of silicon imidonitride materials and their observed activity in the base-catalyzed Knoevenagel reaction, together with their large number of N–H groups, raises the possibility that these materials will function as size-selective solid base catalysts.

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.

Pore Creation in Silicon Oxycarbides by Rinsing in Dilute Hydrofluoric Acid

We have prepared silicon oxycarbides by the pyrolysis of siloxane polymers at a maximum temperature of 1000 °C. The resulting silicon oxycarbides are networked glasses which we believe also contain some graphene sheets. After washing in a dilute solution of hydrofluoric acid for times between 2 min and 24 h, these materials lost, at most, 40% of their mass. Powder X-ray diffraction (XRD), small-angle scattering (SAX), BET surface area measurements, elemental analysis, and silicon K-edge X-ray absorption spectroscopy (XAS) were used to determine the physical structure of the bulk material and the local chemical environments of the Si atoms. It was found that a microscopic pore network was created in the material upon HF washing. The number of pores, but not their size, increased with HF washing time. The HF etching revealed a passivating layer, which we believe consists mainly of silicon and carbon, that prevented further etching of the material. The electrochemical behavior of these materials in Li batteries was not affected by the radical changes in surface chemistry.

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 (> 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.

Nickel Catalyzed Conversion of Activated Carbon into Porous Silicon Carbide

ABSTRACTHigh surface area silicon carbide (SiC) of 31 m2/g has been synthesized by the catalytic conversion of activated carbon. The thermal stability in non-oxidizing environments is shown to be excellent; no significant sintering has been observed after ageing in nitrogen for 4 hours at 1273 K. The presence of 2v% steam or the use of air results in SiC oxidation into SiO2 and considerable sintering at 1273 K. Air oxidation of SiC is shown to cause substantial SiC conversion, viz. 60 % after 10 hours at 1273 K. Complete conversion is achieved at 1080 K in about 100 days. This rate of oxidation agrees with reports on the oxidation of non-porous Acheson SiC and Chemical Vapour Deposited SiC coatings. The use of SiC based catalysts is, therefore, limited to (1) high temperature gas phase reactions operating in the absence of oxidizing constituents (O2 or H2O) and (2) liquid phase processes at demanding pH. Syntheses of highly dispersed and highly loaded Ni/SiC catalysts are feasible by applying an ion-exchange technique, resulting in supported nickel particles of 4 nm.

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.

Characterization of High Surface Area Silicon Oxynitrides

ABSTRACTHigh surface area silicon oxynitrides have been prepared by nitrida- tion of silica with ammonia. Characterization by Fourier-transform infrared spectroscopy has allowed quantitative determination of hydroxyl, amido and imido groups. Data obtained by X-ray photoelectron spectroscopy show that the nitrogen is well distributed in the surface of the materials.

The use of a high surface area silicon oxynitride as a solid, basic catalyst

High surface area silicon oxynitride is shown to be a basic catalyst through its ability to catalyse Knoevenagel condensations when suspended in toluene at 50 °C; the activity is due to the presence of surface nitrogen.

Buried Layer Tungsten Deposits In Porous Silicon: Metal Penetration Depth and Film Purity Determinants

AbstractInfiltration of anodically prepared porous silicon with tungsten hexafluoride gas has been investigated as a function of silicon porosity, source gas pressure and carrier gas type and flow rate. The depth of tungsten metallization in the silicon has been shown to depend most sensitively on the WF, partial pressure, and less on the flow rate and carrier gas type. Penetration depths of 30 pirn have been attained. Structural integrity of the tungsten layer is dependent on the porosity of the starting material and the degree of internal oxidation of the porous silicon surface area.

The preparation and use of high surface area silicon carbide catalyst supports

The preparation and use of high surface area silicon carbide catalyst supports

The preparation and use of high surface area silicon carbide catalyst supports

High surface area SiC has been prepared in a tubular reactor by the high-temperature, vapor phase decomposition of Si-containing compounds such as tetramethylsilane. Surface areas near 50 m 2 g −1 for β-phase SiC were obtained in these initial experiments. These powders did not sinter during heating to 1173 K, even in the presence of oxygen, and surface areas actually increased somewhat due to burnoff of residual elemental carbon. Using these SiC powders as a support resulted in well-dispersed Pt/SiC catalysts which possessed normal kinetic behavior for methanation. However, Ni/SiC catalysts exhibited unusual chemisorption behavior, a range of activity for CO hydrogenation, and selectivity only to methane.