Enhancing Electrochemical Water-Splitting Kinetics by Polarization-Driven Formation of Near-Surface Iron(0): An In Situ XPS Study on Perovskite-Type Electrodes

Progress of Exsolved Metal Nanoparticles on Oxides as High Performance (Electro)Catalysts for the Conversion of Small Molecules

Utilizing electricity and heat from renewable energy to convert small molecules into value-added chemicals through electro/thermal catalytic processes has enormous socioeconomic and environmental benefits. However, the lack of catalysts with high activity, good long-term stability, and low cost strongly inhibits the practical implementation of these processes. Oxides with exsolved metal nanoparticles have recently been emerging as promising catalysts with outstanding activity and stability for the conversion of small molecules, which provides new possibilities for application of the processes. In this review, it starts with an introduction on the mechanism of exsolution, discussing representative exsolution materials, the impacts of intrinsic material properties and external environmental conditions on the exsolution behavior, and the driving forces for exsolution. The performances of exsolution materials in various reactions, such as alkane reforming reaction, carbon monoxide oxidation, carbon dioxide utilization, high temperature steam electrolysis, and low temperature electrocatalysis, are then summarized. Finally, the challenges and future perspectives for the development of exsolution materials as high-performance catalysts are discussed.

Crystallographic and electronic evolution of lanthanum strontium ferrite (La0.6Sr0.4FeO3−δ) thin film and bulk model systems during iron exsolution

We study the changes in the crystallographic phases and in the chemical states during the iron exsolution process of lanthanum strontium ferrite (LSF, La_0.6Sr_0.4FeO_3−δ).

Structural investigations of La0.6Sr0.4FeO3−δ under reducing conditions: kinetic and thermodynamic limitations for phase transformations and iron exsolution phenomena

The crystal structure changes and iron exsolution behavior of a series of oxygen-deficient lanthanum strontium ferrite (La_0.6Sr_0.4FeO_3−δ, LSF) samples under various inert and reducing conditions up to a maximum temperature of 873 K have been investigated to understand the role of oxygen and iron deficiencies in both processes. Iron exsolution occurs in reductive environments at higher temperatures, leading to the formation of Fe rods or particles at the surface. Utilizing multiple ex situ and in situ methods (in situ X-ray diffraction (XRD), in situ thermogravimetric analysis (TGA), and scanning X-ray absorption near-edge spectroscopy (XANES)), the thermodynamic and kinetic limitations are accordingly assessed. Prior to the iron exsolution, the perovskite undergoes a nonlinear shift of the diffraction peaks to smaller 2θ angles, which can be attributed to a rhombohedral-to-cubic (R3̄c to Pm3̄m) structural transition. In reducing atmospheres, the cubic structure is stabilized upon cooling to room temperature, whereas the transition is suppressed under oxidizing conditions. This suggests that an accumulation of oxygen vacancies in the lattice stabilize the cubic phase. The exsolution itself is shown to exhibit a diffusion-limited Avrami-like behavior, where the transport of iron to the Fe-depleted surface-near region is the rate-limiting step.

A dependence of structural transformation and iron exsolution on chemical environment and reducing conditions is proven for the perovskite La_0.6Sr_0.4FeO_3−δ.

Exsolution of Fe and SrO Nanorods and Nanoparticles from Lanthanum Strontium Ferrite La0.6Sr0.4FeO3-δ Materials by Hydrogen Reduction

Formation of uniform Fe and SrO rods as well as nanoparticles following controlled reduction of La_0.6Sr_0.4FeO_3-δ (LSF) and Ni-LSF samples in dry and moist hydrogen is studied by aberration-corrected electron microscopy. Metallic Fe and SrO precipitate from the perovskite lattice as rods of several tenths of nm and thicknesses up to 20 nm. Based on a model of Fe whisker growth following reduction of pure iron oxides, Fe rod exsolution from LSF proceeds via rate-limiting lattice oxygen removal. This favors the formation of single iron metal nuclei at the perovskite surface, subsequently growing as isolated rods. The latter is only possible upon efficient removal of reduction-induced water and, subsequently, reduction of Fe +III/+IV to Fe(0). If water remains in the system, no reduction or rod formation occurs. In contrast, formation of SrO rods following reduction in dry hydrogen is a catalytic process aided by Ni particles. It bears significant resemblance to surface diffusion-controlled carbon whisker growth on Ni, leading to similar extrusion rods and filaments. In addition to SrO rod growth, the exsolution of Fe nanoparticles and, subsequently, Ni-Fe alloy particles is observed. The latter have also been observed under static hydrogen reduction. Under strict control of the experimental parameters, the presented data therefore open an attractive chemically driven pathway to metal nanoarchitectures beyond the formation of "simple" nanoparticles.