Coating of activated carbon with silicon carbide by chemical vapour deposition

Abstract

Coating of activated carbon with silicon carbide by chemical vapour deposition (CVD) has been investigated to improve the oxidation resistance and the mechanical strength of activated carbon extrudates. The oxidation 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 reacting SiCl 4 with the carbon surface cannot be achieved below 1400 K. Silicon deposition has been encountered in all cases. Coating of activated carbon using a CH 4 SiCl 4 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 improved by a factor 1.7. The residual surface area was 176 m 2/g. SiC coatings have also been obtained by decomposing CH 3SiCl 3 at temperatures above 1200 K. The side crushing strength of the extrudates improved 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 methods of SiC deposition result in surface areas which are high enough for catalyst support applications. Evaluation of the infiltration performance of this SiC-CVD process using CH 3SiCl 3 shows that 20–95% of the SiC has been deposited inside the extrudates. The residual porosity of the extrudates is evaluated using a general mathematically developed chemical vapour infiltration design chart, which correlates initial Thiele moduli with the porosity after deposition. Good agreement is obtained between the experimental data and the design chart.