Catalytically active ceria-supported cobalt-manganese oxide nanocatalysts for oxidation of carbon monoxide

A low-concentration cobalt (∼6 at%) and manganese (∼3 at%) bimetallic oxide catalyst supported on ceria nanorods (CoMnO_x/CeO₂), as well as its related single metal oxide counterparts (CoO_x/CeO₂ and MnO_x/CeO₂) was synthesized via a deposition-precipitation approach. The fresh samples after air-calcination at 400 °C were tested under the reaction conditions of CO oxidation, and showed the following order of reactivity: CoMnO_x/CeO₂ > CoO_x/CeO₂ > MnO_x/CeO₂. X-ray diffraction (XRD) and transmission electron microscopy (TEM) data identified that the structure of the CeO₂ support was maintained during deposition of metal (Co, Mn) ions while the corresponding vis-Raman spectra verified that more oxygen vacancies were created after deposition-precipitation than those in pure ceria nanorods. Aberration-corrected, high-angle, annular dark-field scanning transmission electron microscopy (HAADF-STEM) images with the help of electron energy loss spectroscopy (EELS) analyses determined two types of cobalt species, i.e. ultra-fine clusters (<2 nm) and smaller nanocrystals (up to 5 nm) in CoO_x/CeO₂ while only bigger nanostructures (∼10 nm) of cobalt-manganese oxides in CoMnO_x/CeO₂. X-ray absorption fine structure (XAFS) measurements demonstrated the presence of a cubic Co₃O₄ phase in all the cobalt-based catalysts. The fitting results of the extended X-ray absorption fine structure (EXAFS) indicated that the introduction of the secondary metal (Mn) oxide significantly enhanced the two-dimensional growth of cobalt oxide nanostructures on the surface of CeO₂. Therefore, the enhanced activity of CO oxidation reaction over the bimetallic cobalt-manganese oxide nanocatalyst can be attributed to the higher crystallinity of the Co₃O₄ phase in this work.