NICKEL-CATALYZED CONVERSION OF ACTIVATED CARBON EXTRUDATES INTO HIGH-SURFACE-AREA SILICON-CARBIDE BY REACTIVE CHEMICAL-VAPOR-DEPOSITION

A novel method for the synthesis of high surface area silicon carbide extrudates has been developed which consists of applying nickel onto a ctivated carbon extrudates followed by reaction with silicon tetrachlo ride and hydrogen, Utilization of nickel is shown to be essential in o rder to obtain a considerable conversion, Selective SiC formation has been obtained at 1380 K and 10 kPa. Thus, methane is formed at the int erior of the carbon via gasification: C(s) + 2H(2)(g) reversible arrow CH4(g), which subsequently reacts with silicon tetrachloride to silic on carbide: SiCl4(g) + CH4(g) reversible arrow SiC(s) + 4HCl(g). The t otal carbon conversion ranges from 20 to 55% for nickel contents of 2 and 8 wt%, respectively. Si-codeposition will occur when the gasificat ion reaction diminishes in time, due to deactivation of the nickel gas ification sites, Extensive whisker formation of SiC is encountered owi ng to the operative vapour-liquid-solid mechanism. Mass transport calc ulations show that methane is formed throughout the extrudate, whereas the front of SiC formation moves from the outside to the internal par t due to diffusion limitations of SiCl4 and nickel deactivation. The r esidual carbon can be removed after conversion by oxidation, resulting in high surface area SiC extrudates, The BET-surface areas after conv ersion 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 di stributions of the SiC supports show that the pore volume is evenly di stributed over the meso- and macro-pore region (diameter: 2 to 100 nm) which allows the following areas of application: (1) reactions at hig h temperatures and (2) liquid-phase reactions at demanding pH conditio ns. (C) 1997 Academic Press.