Unraveling the activity of iron carbide clusters embedded in silica for thermocatalytic conversion of methane

Abstract

Isolated Fe-sites on a silica substrate have recently been reported for direct and non-oxidative conversion of gaseous methane with high selectivity. The activated catalyst was proposed to be FeC₂ clusters embedded in silica. Using a combination of density functional theory methods and micro-kinetic modeling, we show that under the same reaction conditions (1223 K, 1 atm), FeC₂ sites convert to FeC₃ and the latter is instead responsible for the observed activity. We investigate the detailed mechanism of the conversion of methane to a methyl radical and hydrogen on FeC₃@SiO₂ under different conditions of methane partial pressure. We find that methyl radical evolution is the rate-determining step for the overall conversion. Our calculations also indicate that the conversion of embedded FeC₃ to FeC₄ competes with methyl radical evolution from the active catalyst. However, due to the higher stability of FeC₃ sites, we anticipate that the formation of higher carbides can be inhibited by controlling the hydrogen partial pressure.