Comparative Study of Exsolved and Impregnated Ni Nanoparticles Supported on Nanoporous Perovskites for Low-Temperature CO Oxidation

This study investigated the redox exsolution of Ni nanoparticles from a nanoporous La0.52Sr0.28Ti0.94Ni0.06O3 perovskite. The characteristics of exsolved Ni nanoparticles including their size, population, and surface concentration were deeply analyzed by environmental scanning electron microscopy (ESEM), transmission electron microscopy-energy dispersive X-ray spectroscopy (TEM-EDX) mapping, and hydrogen temperature-programmed reduction (H2-TPR). Ni exsolution was triggered in hydrogen as early as 400 °C, with the highest catalytic activity for low-temperature CO oxidation achieved after a reduction step at 500 °C, despite only a 10% fraction of Ni exsolved. The activity and stability of exsolved nanoparticles were compared with their impregnated counterparts on a perovskite material with a similar chemical composition (La0.65Sr0.35TiO3) and a comparable specific surface area and Ni loading. After an aging step at 800 °C, the catalytic activity of exsolved Ni nanoparticles at 300 °C was found to be 10 times higher than that of impregnated ones, emphasizing the thermal stability of Ni nanoparticles prepared by redox exsolution

Critical Role of the Semiconductor–Electrolyte Interface in Photocatalytic Performance for Water-Splitting Reactions Using Ta₃N₅ Particles

Distinct photocatalytic performance was observed when Ta₃N₅ was synthesized from commercially available Ta₂O₅ or from Ta₂O₅ prepared from TaCl₅ via the sol–gel route. With respect to photocatalytic O₂ evolution with Ag⁺ as a sacrificial reagent, the Ta₃N₅ produced from commercial Ta₂O₅ exhibited higher activity than the Ta₃N₅ produced via the sol–gel route. When the Ta₃N₅ photocatalysts were decorated with Pt nanoparticles in a similar manner, the Ta₃N₅ from the sol–gel route exhibited higher photocatalytic hydrogen evolution activity from a 10% aqueous methanol solution than Ta₃N₅ prepared from commercial Ta₂O₅ where no hydrogen can be detected. Detailed surface and bulk characterizations were conducted to obtain fundamental insight into the resulting photocatalytic activities. The characterization techniques, including XRD, elemental analysis, Raman spectroscopy, UV–vis spectroscopy, and surface-area measurements, revealed only negligible differences between these two photocatalysts. Our thorough characterization of the surface properties demonstrated that the very thin outermost layer of Ta₃N₅, with a thickness of a few nanometers, consists of either the reduced state of tantalum (TaN) or an amorphous phase. The extent of this surface layer was likely dependent on the nature of precursor oxide surfaces. DFT calculations based on partially oxidized Ta₃N_4.83O_0.17 and N deficient Ta₃N_4.83 consisting of reduced Ta species well described the optoelectrochemical properties obtained from the experiments. Electrochemical and Mott–Schottky analyses demonstrated that the surface layer drastically affects the energetic picture at the semiconductor–electrolyte interface, which can consequently affect the photocatalytic performance. Chemical etching of the surface of Ta₃N₅ particles to remove this surface layer unites the photocatalytic properties with the photocatalytic performance of these two materials. Mott–Schottky plots of these chemically etched Ta₃N₅ materials exhibited similar characteristics. This result suggests that the surface layer (1–2 nm) determines the electrochemical interface, which explains the different photocatalytic performances of these two materials