Ca–Mn Composite Oxides as Effective Alternatives to Metal Nanoparticle Catalysts in CO Oxidation

Composite Ca–Mn oxide catalysts with distinct crystal structures—orthorhombic perovskite CaMnO3, delafossite-related layered Ca2Mn3O8, and marokite-type CaMn2O4—were synthesized via a coprecipitation method and evaluated for CO oxidation. Among these oxides, Ca2Mn3O8 exhibited the highest low-temperature activity, achieving significant CO conversion above 120°C, comparable to that of Pd/Al2O3. Repeated reaction tests confirmed the structural robustness of Ca2Mn3O8. CO-TPD measurements revealed that CO oxidation on Ca–Mn oxides proceed predominantly via the Mars–van Krevelen (MvK) mechanism. It is considered that the structural feature of Ca2Mn3O8, layered framework containing ordered Mn vacancies, enhances lattice-oxygen mobility, thereby accelerating the MvK-type CO oxidation pathway. These results demonstrate that Ca2Mn3O8 is a structurally stable, non-noble-metal catalyst capable of efficient low-temperature CO oxidation, offering a promising alternative to conventional metal nanoparticle catalysts.