Structure, Bonding, and Catalytic Activity of Monodisperse, Transition-Metal-Substituted CeO₂ Nanoparticles

We present a simple and generalizable synthetic route toward phase-pure, monodisperse transition-metal-substituted ceria nanoparticles (M_0.1Ce_0.9O_2–x, M = Mn, Fe, Co, Ni, Cu). The solution-based pyrolysis of a series of heterobimetallic Schiff base complexes ensures a rigorous control of the size, morphology and composition of 3 nm M_0.1Ce_0.9O_2–x crystallites for CO oxidation catalysis and other applications. X-ray absorption spectroscopy confirms the dispersion of aliovalent (M³⁺ and M²⁺) transition metal ions into the ceria matrix without the formation of any bulk transition metal oxide phases, while steady-state CO oxidation catalysis reveals an order of magnitude increase in catalytic activity with copper substitution. Density functional calculations of model slabs of these compounds confirm the stabilization of M³⁺ and M²⁺ in the lattice of CeO₂. These results highlight the role of the host CeO₂ lattice in stabilizing high oxidation states of aliovalent transition metal dopants that ordinarily would be intractable, such as Cu³⁺, as well as demonstrating a rational approach to catalyst design. The current work demonstrates, for the first time, a generalizable approach for the preparation of transition-metal-substituted CeO₂ for a broad range of transition metals with unparalleled synthetic control and illustrates that Cu³⁺ is implicated in the mechanism for CO oxidation on CuO-CeO₂ catalysts.