High surface area silicon carbide as catalyst support characterization and stability

Abstract

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.