Metastable Nanoparticle Surfaces for Tunable Catalysis Systems
Controlling Oxide Properties through Cation Site Occupation
(Metastable Nanoparticle Surfaces for Tunable Catalysis Systems) The need for inexpensive high-activity materials in electrocatalytic applications such as water splitting has driven substantial research into the surface chemistry of non-noble metal catalysts, especially metal oxides. Great progress has been achieved in optimization of oxide electroactivity by the use of mixed metal oxides, which achieve the necessary physicochemical properties via controlled substitution of cation sites. Despite the obvious importance of the anionic partner in determining the catalyst electronic and surface structure, knowledge of the effects of anionic substitution in electrocatalysis is far less certain. In this talk I overview the creation of metastable phases in nanoparticles through anion exchange and show that, by controlling the relative proportions (e.g., oxide and sulfide), the activity of nanoparticle electrocatalysts can be substantially increased. For instance, in the cobalt oxide system we find that lightly doped CoOxS0.18 exhibits a metastable, S-substituted CoO structure and is 2-3 times more active in the hydrogen evolution reaction than either end-member of the oxide-sulfide series. We explain this result using density functional theory calculations, which reveal that S-substitution of the CoO surface increases the bonding strength of H to the surface, and reaches an optimum value at a low doping level, consistent with our observations. These results show that metastable nanoparticle catalysts are accessible and provide an important strategy for controlling the physicochemical properties of electrocatalyst surfaces for energy harvesting.
(Controlling Oxide Properties through Cation Site Occupation) Ternary spinel oxides are actively used and researched as catalysis for fuel cells and electrochemical energy storage. In spinels, the cation site occupation is an important determinant of materials properties. For ternary spinels, the two cation types can be found at multiple lattice sites in different oxidation states. This is especially true for non-stochiometeric compositions. Therefore, manipulation of cation configurational disorder can greatly affect properties that have a large impact on the spinels performance in energy harvesting devices. In this talk we investigate relationships between cation site occupation and the supercapacitor performance and show a strong correlation between structural properties and electronic properties in ternary spinels. Understanding the relationship between cation configurational disorder and spinel properties will lead to design principles for tailored properties in oxides.