COATING OF ACTIVATED CARBON WITH SILICON-CARBIDE BY CHEMICAL-VAPOR-DEPOSITION

Coating of activated carbon with silicon carbide by chemical vapour de position (CVD) has been investigated to improve the oxidation resistan ce and the mechanical strength of activated carbon extrudates. The oxi dation resistance has been analyzed by thermal gravimetric analysis in air; the temperature at the maximum rate of oxidation (T-max) is used to compare the modified carbons. Selective deposition of SiC by react ing SiCl4 with the carbon surface cannot be achieved below 1400 K. Sil icon deposition has been encountered in all cases. Coating of activate d carbon using a CH4/SiCl4 mixture results in SiC deposition at 1376 K . The oxidation resistance of this modified activated carbon has been improved by 150 K (T-max = 1025 K), while the side crushing strength i mproved by a factor 1.7. The residual surface area was 176 m(2)/g. SiC coatings have also been obtained by decomposing CH3SiCl3 at temperatu res above 1200 K. The side crushing strength of the extrudates improve d by a factor of 1.4, while the resistance against oxidation remained similar to that of the original carbon. The residual surface areas and pore volumes averaged 530 m(2)/g and 0.33 ml/g, respectively. Both me thods of SiC deposition result in surface areas which are high enough for catalyst support applications. Evaluation of the infiltration perf ormance of this SiC-CVD process using CH3SiCl3 shows that 20-95% of th e SiC has been deposited inside the extrudates. The residual porosity of the extrudates is evaluated using a general mathematically develope d chemical vapour infiltration design chart, which correlates initial Thiele moduli with the porosity after deposition. Good agreement is ob tained between the experimental data and the design chart. Copyright ( C) 1996 Elsevier Science Ltd