CuAs2O4: Design, Hydrothermal Synthesis, Crystal Structure, Photocatalytic Dye Degradation, Hydrogen Evolution Reaction, Knoevenagel Condensation Reaction, and Thermal Studies

CuAs2O4 has been explored as a heterogeneous catalyst in the fields of photocatalysis, electrocatalysis, and solvent-free organic transformation reactions. The homogeneity has been successfully attained for the first time by designing a pH-assisted hydrothermal synthesis technique. Single-crystal X-ray diffraction studies reveal that no phase transition has been observed by lowering the temperature up to 103 K with no existence of satellite reflections. The crystal structure exhibits tetragonal symmetry with space group P42/mbc and consists of [CuO6] octahedra and [AsO3E] tetrahedra (E represents the stereochemically active lone pair). Structural investigation shows a cylindrical void inside the structure, which could lead to interesting physical and chemical properties. The photocatalytic dye degradation efficiency with methylene blue (MB) showed ∼100% degradation, though the degradation efficiency increased by 2-fold with the addition of 6% H2O2. The reusability of the catalyst up to the 10th cycle with ∼35% MB dye degradation has been established. It can exhibit HER activity with a low overpotential of 165 mV with respect to RHE to attain the current density of j = 10 mA cm-2. SEM and TEM revealed rod-shaped particles, which supported the large number of catalytic active sites. The structural consistency of the catalyst after photodegradation and HER studies is confirmed by the PXRD pattern. XPS confirms the oxidation state of Cu and As in the compound. The catalytic activity toward the Knoevenagel condensation reaction at moderate temperature under solvent-free condition is also studied. TG-DTA shows an endothermic minimum (Tmin) at 436 °C due to the mass loss of As2O3.

Shining Light on Anion-Mixed Nanocatalysts for Efficient Water Electrolysis: Fundamentals, Progress, and Perspectives

This review introduces recent advances of various anion-mixed transition metal compounds (e.g., nitrides, halides, phosphides, chalcogenides, (oxy)hydroxides, and borides) for efficient water electrolysis applications in detail. The challenges and future perspectives are proposed and analyzed for the anion-mixed water dissociation catalysts, including polyanion-mixed and metal-free catalyst, progressive synthesis strategies, advanced in situ characterizations, and atomic level structure-activity relationship. Hydrogen with high energy density and zero carbon emission is widely acknowledged as the most promising candidate toward world's carbon neutrality and future sustainable eco-society. Water-splitting is a constructive technology for unpolluted and high-purity H₂ production, and a series of non-precious electrocatalysts have been developed over the past decade. To further improve the catalytic activities, metal doping is always adopted to modulate the 3d-electronic configuration and electron-donating/accepting (e-DA) properties, while for anion doping, the electronegativity variations among different non-metal elements would also bring some potential in the modulations of e-DA and metal valence for tuning the performances. In this review, we summarize the recent developments of the many different anion-mixed transition metal compounds (e.g., nitrides, halides, phosphides, chalcogenides, oxyhydroxides, and borides/borates) for efficient water electrolysis applications. First, we have introduced the general information of water-splitting and the description of anion-mixed electrocatalysts and highlighted their complementary functions of mixed anions. Furthermore, some latest advances of anion-mixed compounds are also categorized for hydrogen and oxygen evolution electrocatalysis. The rationales behind their enhanced electrochemical performances are discussed. Last but not least, the challenges and future perspectives are briefly proposed for the anion-mixed water dissociation catalysts.

Direct Synthesis of Two Inorganic Catalysts on Carbon Fibres for the Electrocatalytic Oxidation of Water

Two electrodes for anodic water oxidation made by direct synthesis of inorganic catalysts onto conductive carbon fibre sheets are evaluated. As catalysts two Co- and Sb-containing phases were tested, that is, Co₃ Sb₄ O₆ F₆ and the new compound CoSbO₄ . The compounds express large differences in their morphology: CoSbO₄ grows as thin needles whereas Co₃ Sb₄ O₆ F₆ grows as larger facetted crystals. Despite the smaller surface area the latter compound shows a better catalytic performance. When the compound Co₃ Sb₄ O₆ F₆ was used it gave a low increase of +0.028 mV h^-1 at an overpotential of η=472 mV after 10 h and a stability of +0.48 mV h^-1 at an overpotential of η=488 mV after 60 h. The leakages of Co and Sb were negligible and only <0.001 at % Co and approximately 0.02 at % Sb were detected in the electrolyte.