Zur Kristallchemie der Oxoiridate

Hydrothermal Synthesis of Iridium-Substituted NaTaO3 Perovskites

Iridium-containing NaTaO₃ is produced using a one-step hydrothermal crystallisation from Ta₂O₅ and IrCl₃ in an aqueous solution of 10 M NaOH in 40 vol% H₂O₂ heated at 240 °C. Although a nominal replacement of 50% of Ta by Ir was attempted, the amount of Ir included in the perovskite oxide was only up to 15 mol%. The materials are formed as crystalline powders comprising cube-shaped crystallites around 100 nm in edge length, as seen by scanning transmission electron microscopy. Energy dispersive X-ray mapping shows an even dispersion of Ir through the crystallites. Profile fitting of powder X-ray diffraction (XRD) shows expanded unit cell volumes (orthorhombic space group Pbnm) compared to the parent NaTaO₃, while XANES spectroscopy at the Ir L_III-edge reveals that the highest Ir-content materials contain Ir⁴⁺. The inclusion of Ir⁴⁺ into the perovskite by replacement of Ta⁵⁺ implies the presence of charge-balancing defects and upon heat treatment the iridium is extruded from the perovskite at around 600 °C in air, with the presence of metallic iridium seen by in situ powder XRD. The highest Ir-content material was loaded with Pt and examined for photocatalytic evolution of H₂ from aqueous methanol. Compared to the parent NaTaO₃, the Ir-substituted material shows a more than ten-fold enhancement of hydrogen yield with a significant proportion ascribed to visible light absorption.

Controlling the crystallisation of oxide materials by solvothermal chemistry: tuning composition, substitution and morphology of functional solids

Three aspects in the synthesis of oxides under solvothermal conditions are reviewed: materials discovery, substitutional chemistry and crystal habit control.

Hydrogeniridate(VI) anion and the geometries of tetrahedral oxo-anions

The compound KHIrO4, potassium hydrogentetraoxidoiridate(VI)(1-), crystallizes in a Scheelite-type structure containing discrete, slightly flattened, [Ir(O3OH)](-) tetrahedra--the first observation of a group 9 element in the 6+ oxidation state as an oxoanion.

Chemistry of precious metal oxides relevant to heterogeneous catalysis

The platinum group metals (PGMs) are widely employed as catalysts, especially for the mitigation of automotive exhaust pollutants. The low natural abundance of PGMs and increasing demand from the expanding automotive sector necessitates strategies to improve the efficiency of PGM use. Conventional catalysts typically consist of PGM nanoparticles dispersed on high surface area oxide supports. However, high PGM loadings must be used to counter sintering, ablation, and deactivation of the catalyst such that sufficient activity is maintained over the operating lifetime. An appealing strategy for reducing metal loading is the substitution of PGM ions into oxide hosts: the use of single atoms (ions) as catalytic active sites represents a highly atom-efficient alternative to the use of nanoparticles. This review addresses the crystal chemistry and reactivity of oxide compounds of precious metals that are, or could be relevant to developing an understanding of the role of precious metal ions in heterogeneous catalysis. We review the chemical conditions that facilitate stabilization of the notoriously oxophobic precious metals in oxide environments, and survey complex oxide hosts that have proven to be amenable to reversible redox cycling of PGMs.