HIGH-SURFACE-AREA SILICON-CARBIDE AS CATALYST SUPPORT CHARACTERIZATION AND STABILITY

High surface area silicon carbide (SiC) of 30 m(2)/g has been synthesi zed by the catalytic conversion of activated carbon. The stability of this SIC in aqueous hydrogen fluoride and a boiling nitric acid soluti on is shown to be excellent. No corrosion is encountered by treatment with boiling HNO3, HF treatment causes the dissolution of the silica s urface layer present on the SiC while the SiC remains intact. Oxidatio n in air at elevated temperatures has been analyzed by thermal gravime tric analysis, diffuse reflectance infrared spectroscopy, nitrogen ads orption, and X-ray diffraction. The thermal stability in non-oxidizing environments is shown to be excellent; no significant sintering has b een 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 SiO2 and considerable sintering. Air oxidation at 1273 K of pure SiC, SiC load ed with 5 wt.% nickel, and HNO3 treated SiC is shown to cause substant ial 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 o xidation agrees with reports on the oxidation of non-porous Acheson Si C and SiC coatings formed by Chemical Vapour Deposition. It is conclud ed that at high surface area SIC cannot be used as a catalyst support in processes operating in oxidizing environments and temperatures abov e 1073 K. SiC based catalysts are very well suited for (1) high-temper ature gas-phase reactions operating in the absence of oxidizing consti tuents (O-2 or H2O) and (2) strong acidic liquid-phase processes. (C) 1998 Elsevier Science B.V.