  Chemical looping methane partial oxidation provides an energy and cost
effective route for methane utilization. However, there is considerable CO2
co-production in state-of-the-art chemical looping systems, rendering a
decreased productivity in value-added fuels or chemicals. In this work, we show
that the co-production of CO2 can be dramatically suppressed in methane partial
oxidation reactions using iron oxide nanoparticles, with a size of 2~8 nm, as
the oxygen carrier. To stabilize these nanoparticles at high temperatures, they
are embedded in an ordered, gas-permeable mesoporous silica matrix. We
experimentally obtained near 100% CO selectivity in a cyclic redox system at
750{\deg}C to 935{\deg}C, which is a significantly lower temperature range than
in conventional oxygen carrier systems. Density functional theory calculations
elucidate the origins for such selectivity and reveal that CH4 adsorption
energies decrease with increasing nanoparticle size. These calculations also
show that low-coordinated lattice oxygen atoms on the surface of nanoparticles
significantly promote Fe-O bond cleavage and CO formation. We envision that
embedded nanostructured oxygen carriers have the potential to serve as a
general materials platform for achieving 100% selectivity in redox reactions at
high temperatures.
