High surface area silicon carbide as catalyst support characterization and stability
Introduction
Silicon carbide's physical bulk properties (high thermostability, high mechanical strength, and high heat conductivity) have been claimed to enable the use of this material as catalyst support at extreme process conditions, viz. processes operating at high temperatures and oxidizing environments [1]. The application of silicon carbide as catalyst support had mainly been limited by its low maximum attainable specific surface area (surface area for a commercially available silicon carbide is below 1 m2/g). The last decade, various other methods for the synthesis of high surface area SiC have been reported. The method applied by Vannice et al. [1]comprises the gas-phase decomposition of tetramethylsilane resulting in SiC of nearly 50 m2/g. A new synthesis procedure for the preparation of high surface area SiC has been developed in which activated carbon is catalytically converted into SiC by the following overall reaction 2, 3, 4, 5The mechanism of conversion comprises the gasification of carbon by hydrogen followed by the intra-particle deposition of SiC from SiCl4 and CH4 via the Vapour Liquid Solid (VLS) mechanism resulting in abundant whisker growth within the particle. Both reactions are catalyzed by nickel and the carbon acts as a source for CH4 formation as well as a template for SiC deposition. The synthesis of SiC with surface areas ranging from 30 to 80 m2/g has been achieved. Application of a Fluidized Bed Chemical Vapour Deposition (FBCVD) reactor allows a very reproducible and homogeneous conversion of several grams of activated carbon 3, 4which alleviates to a large extent the investigations for possible applications of SiC supports. Research regarding the use of high surface area SiC at severe process conditions is limited. Vannice et al. [1]investigated silicon carbide based Ni and Pt catalysts for the hydrogenation of carbon monoxide at 473 and 773 K. The well-dispersed Pt/SiC catalyst displayed similar catalytic behaviour as SiO2 and Al2O3 based catalysts. The Ni/SiC catalysts exhibited no H2 chemisorption and unusual activity for the methanation of CO. Ledoux and co-workers [6]explored the use of SiC based cobalt molybdenum catalysts in hydrodesulfurization reactions. These tests are of fundamental importance. They are, however, not to be regarded as future applications of high surface area SiC, because the expected production costs of this material are higher than that of conventional supports and these will not be justified by the marginal benefits gained by using SiC in that field. An area in which the use of SiC might be beneficial are high-temperature processes in which sintering of high surface area SiC is probably less than that of alumina and silica, e.g. reforming of methane and dehydrogenation of paraffins. Another area comprises for instance hydro-demetallization of heavy oil fractions, because of SiC's resistance against corrosion in strong acidic solutions during regeneration of the catalyst. Automotive exhaust catalysis based on SiC powder [7]and porous SiC [8]has been investigated as a possible application in the field of high-temperature processes. Here, the thermal stability of the silicon carbide itself and the limited reactions of the catalytically active components with the support are claimed to result in a superior operation compared to that of conventional alumina based catalysts. SiC has furthermore been suggested as a support for catalysts suitable for high-temperature combustion [9]. No extensive analysis and evaluation of high surface area SiC has been carried out to investigate the correctness of these assumptions. In this paper the characterization and stability testing of high surface area SiC synthesized by the FB-CVD method are reported. The results are compared with those obtained for other porous forms of SiC and non-porous SiC and evaluated for support applications at extreme process conditions. The investigated properties comprise the thermal stability in N2, N2/H2O mixtures, and air. Furthermore, the resistance against corrosion in aqueous hydrogen fluoride and a boiling solution of nitric acid has been investigated.
Section snippets
Materials
Nitric acid (65%) HNO3 (pro analysi) and Ni(NO3)2·6H2O (pro analysi) were obtained from Merck. SiO2 (Si-162-1, SBET=30 m2/g, Vpore=0.6 ml/g) has been obtained from Engelhard.
Synthesis of high surface area SiC
High surface area SiC has been synthesized by FB-CVD using washed Norit Elorit carbon granulates (300 to 425 μm) loaded with 5 wt.% nickel 3, 4. A gas composition of 45 mol% H2, 4.5 mol% SiCl4, and 50.5 mol% argon is reacted with the carbon at 1373 K and 100 kPa for 40 min. The residual carbon present after conversion has been
Thermal and hydrothermal stability in nitrogen and steam
The surface areas and pore volumes of SiC-5 after the thermal and hydrothermal stability tests are shown in Table 1.
The constant surface area after ageing in pure N2 at 1273 K displays the excellent thermal stability of SiC-5. The stability in air and steam environments is less pronounced, which is evidenced by the decrease in surface area after ageing at 1273 K and partial oxidation of the silicon carbide into SiO2.
Rate of oxidation
The results of the rate of oxidation are displayed in Fig. 1Fig. 2Fig. 3. The
SiC surface analysis
From the oxidation experiments (Fig. 1) it follows that the DRIFT spectra 5(c) and (d) correspond to silicon carbide converted for 20% and 55% into SiO2, respectively. Boutonnet-Kizling and co-workers obtained similar effects for oxidized Pt/α-SiC [5]. They correlated the intensity of Si–O vibrations with the degree of SiC oxidation [10]. A HF treatment of SiC-5 completely removes the silica layer present on the SiC as shown by the disappearance of the 1100 cm−1 absorption band. The SiC
Conclusion
The potential of high surface area SiC prepared by the catalytic conversion of activated carbon as catalyst support has been studied. The thermal stability in non-oxidizing environments is shown to be excellent. No significant sintering has been observed in nitrogen atmospheres at temperatures of 1273 K. Utilization of air or the presence of steam at 1273 K results in partial SiC oxidation into SiO2 and considerable sintering. Oxidation in air at elevated temperatures has been analyzed by thermal
List of symbols
a, b stoichiometric coefficients of the gas–solid reaction (mol mol−1) Cg concentration of O2 (mol m−3) Deff effective diffusion coefficient of O2 (m2 s−1) Ea activation energy (kJ mol−1) r radius of the non-converted part of the SiC (m) R initial radius (m) SBET BET surface area (m2 g−1) t time (h) T temperature (K) Vpore pore volume (ml g−1) ξ conversion (-) ρ, ρs density of SiC and SiO2, respectively (kg m−3) τ time necessary for complete oxidation (h)
Acknowledgements
This research was part of the Innovation-oriented Research Programme on Catalysis (project number 90017) and was financially supported by the Ministry of Economic Affairs of The Netherlands.
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