Trends and Prospects of Bimetallic Exsolution

Metal exsolution from perovskite-based anodes in solid oxide fuel cells

Solid oxide fuel cells (SOFCs) are highly efficient and environmentally friendly devices for converting fuel into electrical energy. In this regard, metal nanoparticles (NPs) loaded onto the anode oxide play a crucial role due to their exceptional catalytic activity. NPs synthesized through exsolution exhibit excellent dispersion and stability, garnering significant attention for comprehending the exsolution process mechanism and consequently improving synthesis effectiveness. This review presents recent advancements in the exsolution process, focusing on the influence of oxygen vacancies, A-site defects, lattice strain, and phase transformation on the variation of the octahedral crystal field in perovskites. Moreover, we offer insights into future research directions to further enhance our understanding of the mechanism and achieve significant exsolution of NPs on perovskites.

Real-time insight into the multistage mechanism of nanoparticle exsolution from a perovskite host surface

In exsolution, nanoparticles form by emerging from oxide hosts by application of redox driving forces, leading to transformative advances in stability, activity, and efficiency over deposition techniques, and resulting in a wide range of new opportunities for catalytic, energy and net-zero-related technologies. However, the mechanism of exsolved nanoparticle nucleation and perovskite structural evolution, has, to date, remained unclear. Herein, we shed light on this elusive process by following in real time Ir nanoparticle emergence from a SrTiO₃ host oxide lattice, using in situ high-resolution electron microscopy in combination with computational simulations and machine learning analytics. We show that nucleation occurs via atom clustering, in tandem with host evolution, revealing the participation of surface defects and host lattice restructuring in trapping Ir atoms to initiate nanoparticle formation and growth. These insights provide a theoretical platform and practical recommendations to further the development of highly functional and broadly applicable exsolvable materials.

Dynamic Surface Evolution of Metal Oxides for Autonomous Adaptation to Catalytic Reaction Environments

Metal oxides possessing distinctive physical/chemical properties due to different crystal structures and stoichiometries play a pivotal role in numerous current technologies, especially heterogeneous catalysis for production/conversion of high-valued chemicals and energy. To date, many researchers have investigated the effect of the structure and composition of these materials on their reactivity to various chemical and electrochemical reactions. However, metal oxide surfaces evolve from their initial form under dynamic reaction conditions due to the autonomous behaviors of the constituent atoms to adapt to the surrounding environment. Such nanoscale surface phenomena complicate reaction mechanisms and material properties, interrupting the clarification of the origin of functionality variations in reaction environments. In this review, the current findings on the spontaneous surface reorganization of metal oxides during reactions are categorized into three types: 1) the appearance of nano-sized second phase from oxides, 2) the (partial) encapsulation of oxide atoms toward supported metal surfaces, and 3) the oxide surface reconstruction with selective cation leaching in aqueous solution. Then their effects on each reaction are summarized in terms of activity and stability, providing novel insight for those who design metal-oxide-based catalytic materials.

Low-Temperature Exsolution of Ni-Ru Bimetallic Nanoparticles from A-Site Deficient Double Perovskites

Exsolution of stable metallic nanoparticles for use as efficient electrocatalysts has been of increasing interest for a range of energy technologies. Typically, exsolved nanoparticles show higher thermal and coarsening stability compared to conventionally deposited catalysts. Here, A-site deficient double perovskite oxides, La_{2- x} NiRuO_{6- δ} (x = 0.1 and 0.15), are designed and subjected to low-temperature reduction leading to exsolution. The reduced double perovskite materials are shown to exsolve nanoparticles of 2-6 nm diameter during the reduction in the low-temperature range of 350-450 °C. The nanoparticle sizes are found to increase after reduction at the higher temperature (450 °C), suggesting diffusion-limited particle growth. Interestingly, both nickel and ruthenium are co-exsolved during the reduction process. The formation of bimetallic nanoparticles at such low temperatures is rare. From the in situ impedance spectroscopy measurements of the double perovskite electrode layers, the onset of the exsolution process is found to be within the first few minutes of the reduction reaction. In addition, the area-specific resistance of the electrode layers is found to decrease by 90% from 291 to 29 Ω cm² , suggesting encouraging prospects for these low-temperature rapidly exsolved Ni/Ru alloy nanoparticles in a range of catalytic applications.

Bimetallic Exsolved Heterostructures of Controlled Composition with Tunable Catalytic Properties

In this paper, we show how the composition of bimetallic Fe-Ni exsolution can be controlled by the nature and concentration of oxygen vacancies in the parental matrix and how this is used to modify the performance of CO₂-assisted ethane conversion. Mesoporous A-site-deficient La_0.4Sr_0.6-αTi_0.6Fe_0.35Ni_0.05O_3±δ (0 ≤ α ≤ 0.2) perovskites with substantial specific surface area (>40 m²/g) enabled fast exsolution kinetics (T < 500 °C, t < 1 h) of bimetallic Fe-Ni nanoparticles of increasing size (3-10 nm). Through the application of a multitechnique approach we found that the A-site deficiency determined the concentration of oxygen vacancies associated with iron, which controlled the Fe reduction. Instead of homogeneous bimetallic nanoparticles, the increasing Fe fraction from 37 to 57% led to the emergence of bimodal Fe/Ni₃Fe systems. Catalytic tests showed superior stability of our catalysts with respect to commercial Ni/Al₂O₃. Ethane reforming was found to be the favored pathway, but an increase in selectivity toward ethane dehydrogenation occurred for the systems with a low metallic Fe fraction. The chance to control the reduction and growth processes of bimetallic exsolution offers interesting prospects for the design of advanced catalysts based on bimodal nanoparticle heterostructures.

Trends and Prospects of Bimetallic Exsolution

Supported bimetallic nanoparticles used for various chemical transformations appear to be more appealing than their monometallic counterparts, because of their unique properties mainly originating from the synergistic effects between the two different metals. Exsolution, a relatively new preparation method for supported nanoparticles, has earned increasing attention for bimetallic systems in the past decade, not only due to the high stability of the resulting nanoparticles but also for the potential to control key particle properties (size, composition, structure, morphology, etc.). In this review, we summarize the trends and advances on exsolution of bimetallic systems and provide prospects for future studies in this field.