Thermochemical splitting of carbon dioxide by lanthanum manganites - understanding the mechanistic effects of doping

This review investigates the effect of different dopants on the oxygen evolution and carbon dioxide splitting abilities of the lanthanum manganites. Particular focus was placed on the lanthanide, alkaline earth metals, the redox-active transition metal, and non-redox active Group 3 metals. The review suggests that a small ionic radius lanthanide on the A-site can increase the size discrepancy, leading to Mn-O6 octahedra tilting and more facile Mn-O bond breaking. Doping the A-site with a divalent alkaline earth element can increase the valance of the transition metal, leading to greater reduction capabilities. A transition metal with one electron in the eg orbital is the most effective for reduction while for oxidation, zero electrons in the high-energy eg orbitals is optimal. Finally, doping of the B-site with metals such as gallium or aluminium aids in sintering resistance and allows reactivity to remain constant over multiple cycles. Higher reduction temperatures and moderate re-oxidation temperatures also promote higher fuel yields as does the active reduction of the perovskite under hydrogen, although the total energy consumption implications of this are unknown. Far more is known about the mechanism of the reduction reaction than the oxidation reaction, therefore more research in this area is required.