Formation Kinetics of Chemically Vapor-Deposited Carbon on Mesoporous Silica

Abstract

Formation kinetics of chemically vapor-deposited carbon on mesoporous silica was examined for its porous texture, activation energy and thermodynamic factors. The carbon was deposited by CVD using benzene, hexane and toluene. The amount of carbon in these hybrids increased with deposition in temperature and time. The increase of carbon resulted in decreases of their specific surface area and pore size. The density of carbon within mesopores in the hybrid prepared from benzene and toluene was around 0.6 and 1.9 g/cm³, respectively. For the hybrid modified with hexane, carbon was prone to enclose particles of the mesoporous silica. The activation energies of carbonization were estimated to be less than 100 kJ/mol for benzene and toluene; however, the energy from hexane was 279 kJ/mol. These energies imply that mesopores accelerate carbonization from benzene and toluene to form carbon within mesopores, and carbon was deposited on MPS particles for hexane. The estimated change of formation free energy (ΔG_f) of hexane was smaller than those of benzene and toluene over the temperature range of these experiments. From these estimated ΔG_f, hexane molecules tend to carbonize more quickly before introduction into mesopores than benzene and toluene. Eventually the lower ΔG_f of hexane presumably possibly gives a large amount of deposited carbon on the surface of the mesoporous silica particles.