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

Abstract

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.