CO oxidation catalysis with substituted ceria nanoparticles

Abstract

The low-temperature and cost-effective oxidation of carbon monoxide to carbon dioxide remains a fundamental challenge in heterogeneous catalysis that would enable a diverse range of technologies for electrochemical storage and respiratory health. The development of new catalysts is often driven by high-throughput screening and many of the resulting compounds are mixed-phase, which obscures a rigorous identification of active sites and mechanisms at play for catalysis. In this thesis, the preparation of substituted ceria nanoparticles is described to bring about a fundamental understanding of the structure of the active sites, mechanism and design descriptors for CO oxidation on ceria-based catalysts. Monodisperse, single-phase nanoparticles of late first-row transition-metal-substituted ceria (MyCe₁.yO₂-x, M = Mn, Fe, Co, Ni and Cu) are prepared from the controlled pyrolysis of heterobimetallic precursors in amine surfactant solutions. By means of kinetic analyses, X-ray absorption spectroscopy (XAS) and transmission electron microscopy (TEM), the active site for CO oxidation catalysis is identified as atomically-dispersed, square-planar M³+ and M²+ moieties substituted into the surface of the ceria lattice. The introduction of CuO does not contribute to the catalytic activity of CuyCe₁.yO₂-x, lending support to the hypothesis that the substituted ceria itself is responsible for the catalytic rate enhancement in mixed-phased catalysts like CuO/CeO₂ Under oxygen-rich conditions, the kinetic parameters for CO oxidation are consistent with lattice oxygen from the dispersed copper sites contributing directly to the oxidation of CO in the rate-determining step. In-situ X-ray photoelectron spectroscopy (XPS) and FTIR studies indicate that adsorbed CO can be directly oxidized to CO₂ in the absence of gaseous O₂, while in-situ XAS confirms that electron transfer is localized to the copper sites. XAS studies demonstrate that the reversible reducibility of dispersed copper ions is a contributing factor for the special catalytic activity of CuO/CeO₂ catalysts. The oxygen-ion vacancy formation energy is introduced as an activity descriptor to rationalize trends in the catalytic activities measured for MyCe₁-yO₂-x nanoparticles that span over three orders of magnitude. As such, the DFT-calculated vacancy formation energy serves to guide in the rational design of catalysts through computational, rather than experimental, screening of candidate compounds for CO oxidation catalysis.