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