Influence of synthesis conditions and heat treatment on the structure of Ti₃SiC₂-derived carbons

Abstract

We investigate the characterization and adsorption modeling of Ti₃SiC₂-derived carbons (Ti₃SiC₂-DCs) prepared by chlorination at three temperatures (600, 800, and 1000 °C) as well as samples heat-treated at 1100 °C for one and three day periods. The modeling exploits our recent finite wall thickness model-based density functional theory approach (Nguyen, T. X.; Bhatia, S. K. Langmuir 2004, 20, 3532), utilizing experimental adsorption isotherms of Ar at 87 K for the characterization. In general, characterization results show that the carbon wall structure of carbide-derived carbons (CDCs) is precursor-inherited, and Ti₃SiC₂-DCs have predominantly graphitic walls whose helium density is close to the true density of graphite (2.27 g/cm³), while SiC-derived carbons (SiC-DCs) have predominantly diamond-like walls whose helium density approaches the true density of diamond (3.52 g/cm³). This precursor-inherited character is also seen to give rise to two distinct evolutions of the pore structure of Ti₃SiC₂-DCs and SiC-DCs under chlorination temperature or post-heat-treatment conditions. In particular, both pore enlargement and opening are observed for Ti₃SiC₂-DCs, but only pore opening, especially in the region of smallest pores (<5 Å), is found for SiC-DCs. For adsorption modeling results, the ordering of pore network accessibility, Ar at 87 K < CH₄ at (313K and 333K) < CO₂ at or above 273 K, has been specifically found for the Ti₃SiC₂-DC samples prepared by chlorination at 800°C, providing high CO₂/CH₄ selectivity by the carbon. Heat treatment of this sample at 1100 °C for a 1 day period gives rise to significant improvement of pore network accessibility but subtle change in pore bodies. Finally, shrinkage is found for all investigated CDCs and the 1 day heat-treated sample, but swelling is found for the 3 day heat-treated carbon sample, under high-pressure CO₂ adsorption conditions.