Tetravalent Ce in the Nitrate-Decorated Hexanuclear Cluster [Ce6(μ3-O)4(μ3-OH)4]12+: A Structural End Point for Ceria Nanoparticles

We describe the synthesis and characterization of three glycine-stabilized hexanuclear CeIV cluster compounds, each containing the [Ce6(μ3-O)4(μ3-OH)4]12+ core structure. Crystallized from aqueous nitrate solutions with pH < 0, the core cluster structures exhibit variable decoration by nitrate, glycine, and water ligands depending on solution conditions, where increased nitrate and glycine decoration of the cluster core was observed for crystals synthesized at high Ce and nitrate concentrations. No other crystalline products were observed using this synthetic route. In addition to confirming the tetravalent oxidation state of cerium in one of the reported clusters, cyclic voltammetry also indicates that CeIV is reduced at ∼+0.60 V vs Ag/AgCl (3 M NaCl), which is significantly less than the standard electrode potential. This large decrease in the CeIV/CeIII reduction potential suggests that CeIV is significantly stabilized relative to CeIII within the examined cluster. These compounds are discussed in terms of their importance as small, end member, ceric oxide nanoparticles. Single-crystal structural solutions, together with voltammetry and electrolysis data, permit the decoupling of CeIII defects and substoichiometry. In addition, Ce–Ce distances can be used to determine an "effective" CeO2–x lattice constant, providing a simple method for comparing literature descriptions. The results are discussed in terms of their potential implications for the mechanisms by which nanoparticle ceria serve as catalysts and oxygen-storage materials.