Unraveling the Activity of Iron Carbide Clusters Embedded in Silica for Thermocatalytic Conversion of Methane

Isolated Fe-sites on 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 cluster embedded in silica. Using a combination of density-functional theoretic 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 conversion of methane to 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

formation of higher carbides can be inhibited by controlling the hydrogen partial pressure.