(446d) Speciation and Reactivity of Small Fe, Cu, or Co Oxide Clusters Supported on Ceria and Silica

The redox activity of supported metal oxides determines their utility in emissions abatement and their role in activation of H₂ or O₂ for chemicals synthesis. This talk describes two routes to controlling the redox activity of dispersed metal oxides. In the first part, we support Fe(III), Co(II), and Cu(II) oxides by incipient wetness impregnation of aqueous EDTA complexes of Fe, Cu, and Co on neat and alkali-modified CeO₂, followed by calcination to give small clusters, following prior work on SiO₂. [1] The catalysts are characterized principally by TPR, in situ XANES, and diffuse reflectance UV-visible spectroscopy (DRS), and it is seen that a larger fraction of the oxide is redox active in reversible 1-electron cycles than compared to impregnation with more typical precursors. This improvement in activity is hypothesized to be due to the presence of more uniform interactions between CeO₂ and the oxide afforded by the bulky, anionic precursor. These materials are active in crotonaldehyde hydrogenation and H₂-SCR of NO, with rates of the latter quantitatively correlated to the fraction of redox active sites.

In a second example, we deposit pre-formed clusters of Cu(I) oxides onto SiO₂. The L_xCu_y clusters were previously synthesized as potential models for active sites in Cu-exchanged zeolites and in oxidase enzymes, where L are bridging siloxane ligands. [2] The Cu(I) oxidation state and some degree of the cluster structure is stabilized by incipient wetness impregnation onto SiO₂, such that they are resistant to oxidative decomposition into bulk CuO up to ~400°C. These Cu(I) materials activate O₂ at lower temperatures, as we demonstrate through their ability to carry out oxidative dehydrogenation of cyclohexane.

[1] Prieto-Centurion, D.; Boston, A. M.; Notestein, J. M., Structural and electronic promotion with alkali cations of silica-supported Fe(III) sites for alkane oxidation. J. Catal. 2012, 296, 77-85.

[2] Schax, F.; Limberg, C.; Mugge, C., Copper(I) Siloxides - Aggregated Solid-State Structures, Cu-Cu Interactions and Dynamic Solution Behavior. Eur. J. Inorg. Chem. 2012, 4661-4668.