Facile Synthesis and Characterization of Nanostructured Transition Metal/Ceria Solid Solutions (TM_xCe_1-xO_2-δ, TM = Mn, Ni, Co, or Fe) for CO Oxidation

We developed a general synthetic route for preparing nanoporous transition metal/ceria solid solutions with nanocrystalline frameworks (TM_xCe_1-xO_2-δ, TM = Mn, Ni, Co, or Fe). Their structural properties were characterized using transmission electron microscopy (TEM), X-ray powder diffraction (XRPD), and N₂ sorption. Through thermolysis of bimetallic coordination polymers, hierarchically nanoporous frameworks composed of 3-4 nm TM_xCe_1-xO_2-δ solid solution nanocrystals in which the transition metal ions are well-dispersed in the ceria lattice as evidenced by the Rietveld refinement of the XRPD patterns were synthesized. The electronic properties of the Mn_xCe_1-xO_2-δ solid solutions at up to 20 mol % were examined by Raman spectroscopy and X-ray photoelectron spectroscopy analysis, and H₂-temperature-programmed reduction results demonstrated the altered physicochemical properties, e.g., hydrogen reduction behaviors, due to the doping. CO oxidation studies of Mn_xCe_1-xO_2-δ reveal that the Mn species are responsible for increasing the catalytic activity by an order of magnitude compared to that of pure ceria, by creating nanostructures with accessible pores and active sites on the inner surface. This facile synthetic approach can create nanoporous solid solutions with nanocrystalline frameworks and devise structures and compositions. Therefore, our approach opens new avenues for developing multimetallic catalyst systems.