The catalytic hydrogenation of CO₂ into value-added chemicals and fuels represents a key pathway toward establishing a circular carbon economy. In this context, exsolution has emerged as a powerful materials design strategy for the development of advanced thermocatalysts, enabling the controlled formation of socketed metal and alloy nanoparticles that are strongly anchored to oxide supports. Beyond their exceptional resistance to sintering and deactivation, growing evidence suggests that exsolved systems function as dynamic catalytic interfaces rather than static nanoparticle assemblies, with catalytic performance governed by the interplay between exsolved nanoparticles, oxygen defect chemistry, and active support participation under reaction conditions.

In this presentation, we will explore the design and application of exsolved systems for the thermocatalytic conversion of CO₂ into value-added chemicals and synthetic fuels. We will explore the pathways leading to methanol synthesis, as well as the production of paraffinic and olefinic hydrocarbon fractions. We will examine how catalyst composition can be precisely tuned through multi-metal exsolution, alloy or heterostructures formation, enabling control over adsorption energetics and reaction pathways toward targeted products on demand. We will also look into different host materials, including perovskites, spinels and fluorite-type structures, where composition, defect chemistry, and redox behavior dictate nanoparticle emergence, catalyst stability, and catalytic functionality. By connecting advances in exsolution chemistry with thermocatalytic CO₂ conversion, this work highlights how exsolution can evolve beyond a synthetic methodology into a versatile platform for the rational design of robust catalytic systems for sustainable fuel and chemical production.

The development of stable and efficient catalysts for CO₂ hydrogenation to methanol is essential for advancing carbon recycling technologies. In this work, catalysts were synthesized via an exsolution approach from CuZnAl₂O₄ spinel precursors and benchmarked against a commercial Cu/ZnO/Al₂O₃ catalyst. The materials were prepared by modified co-precipitation method and subjected to controlled reduction (350–550 °C) for 10 hours to induce nanoparticle exsolution.
Characterization techniques including XRD, TEM, CO₂-TPD, and XPS confirmed the formation of well-dispersed, socketed metallic nanoparticles. Reduction conditions influenced nanoparticle distribution and size. Higher temperatures favored larger monometallic Cu nanoparticles, while intermediate temperatures promoted CuZn alloy formation. These structural features enabled improved metal–support interaction and resistance to sintering and deactivation.
Catalytic testing (200–300 °C, 23 barg, H₂/CO₂ = 3) showed CO₂ conversion up to ~13% and methanol selectivity up to ~80% at lower temperatures. Dimethyl ether formation indicated tandem reaction pathways. Compared to the commercial catalyst, exsolved catalysts exhibited higher copper-normalized activity and superior structural stability, with no significant sintering after ~50 h on stream.
These results demonstrate that exsolution-derived Cu-based catalysts are promising candidates for stable and efficient CO₂ to methanol conversion.

Exsolution has traditionally been viewed as a strategy for producing exceptionally stable catalytic nanoparticles with outstanding resistance to sintering and deactivation. In this presentation, I discuss that exsolution should instead be viewed as a platform for engineering multifunctional materials that integrate CO₂ capture, activation, and catalytic conversion within a single material architecture. Rather than functioning solely as stable catalytic sites, exsolved nanoparticles cooperate with oxygen-deficient perovskite supports and carbonate-forming sorbents to regulate the dynamic capture, activation, and conversion of CO₂.

Using Ni/CaTiO₃/CaO as a model platform, reductive exsolution simultaneously generates highly dispersed socketed Ni nanoparticles and oxygen vacancies within the perovskite support, while CaO provides reversible carbonate storage that dynamically buffers CO₂ during reaction [1]. Together, these coupled functionalities establish cooperative reaction pathways that enhance CO formation during the reverse water-gas shift reaction, suppress methanation through dynamic carbonate buffering, and enable stable cyclic performance during integrated CO₂ capture and dry reforming of methane. In situ spectroscopy and cyclic reaction studies demonstrate that catalytic performance emerges from the cooperative interplay among exsolved nanoparticles, oxygen vacancies, and reversible carbonate chemistry rather than from any individual component alone. The presentation will also highlight recent efforts to translate these multifunctional materials into structured monolithic reactors for intensified carbon capture and conversion.

Collectively, these studies position exsolution not simply as a catalyst stabilization strategy, but as a design paradigm for multifunctional materials that couple sorption, activation, and catalytic conversion within a single material platform. This framework expands the role of exsolution beyond catalyst design, opening new opportunities for dynamic reactor architectures capable of intensified carbon capture and carbon-neutral chemical manufacturing.

While most exsolution research has focused on high-temperature applications, this study demonstrates how exsolution can be used as a catalyst design strategy in alkaline water electrolysis for the oxygen evolution reaction (OER). Dense polished pellets of the A-site deficient perovskite Sr_0.98Ti_0.7Fe_0.25Ni_0.05O_3−δ (STFNO) were employed as a model system to study the intrinsic oxygen evolution reaction activity and stability. [1]

Reductive treatment induces the exsolution of finely dispersed Ni–Fe-based nanoparticles, resulting in enhanced catalytic activity and enabling overpotentials as low as 199 mV at 10 mA cm⁻² under industrially relevant conditions (75 °C, 50 bar, 10 M KOH). The use of dense model electrodes provides unique insights into catalytic activity and stability under harsh operating conditions, revealing that the stability window of exsolved nanoparticles is strongly linked to temperature. While nanoparticles remain visible after electrolysis at 75 °C, they are no longer observed after operation at 100 °C, despite no significant loss in catalytic activity. [1]

Beyond performance enhancement, environmental transmission electron microscopy reveals that exsolution temperature can be used to tailor nanoparticle structure. Distinct exsolution pathways lead either to compositionally homogeneous nanoparticles or to core–shell structures, depending on the reduction temperature. [1] These findings demonstrate how exsolution parameters can be used to engineer catalyst structure. These insights highlight the potential of exsolution-derived catalysts for other low-temperature electrochemical technologies, including proton exchange membrane water electrolysis (PEMEC).

Electrode durability for alkaline water electrolysis (AWE) systems still remains a major challenge that needs to be addressed [1]. Transition metal phosphides (TMPs), are considered as one of most promising electrocatalysts due to their high electrocatalytic performance that competes with several noble-based metals catalysts [2]. However, the instability under AWE conditions due to TMPs degradation limits their applicability for large-scale operations. On the other hand, exsolution materials due to their strong metal-host interactions exhibit high electrochemical stability even under demanding conditions but lack in terms of performance [3-4]. Herein, we report a novel synthetic method of devising a hybrid exsolved-TMP catalyst that exhibits high electrocatalytic performance for both HER and OER, with ηHER10: 109 mV, ηHER100: 247 mv and ηOER10: 130 mV, ηΟER100: 440 mv, respectively. While the performance of the hybrid catalyst is noteworthy, the electrochemical stability is of immense interest. By performing HER and OER stability experiments at 0.1 A cm-2 for 100 hours, our exsolved hybrid catalyst superior electrochemical stability than traditional TMP catalyst. This approach provides a new synthetic route of devising durable, high-performing TMPs catalysts via exsolution approach, for electrochemical-based energy conversion applications.

[1] Harun Tüysüz,Acc. Chem. Res. 2024, 57, 4, 558–567

[2] Z.Pu, T.Liu, I. S.Amiinu, R.Cheng, P.Wang, C.Zhang, P.Ji, W.Hu, J.Liu, S.Mu, Transition-Metal Phosphides: Activity Origin, Energy-Related Electrocatalysis Applications, and Synthetic Strategies. Adv. Funct. Mater.2020, 30, 2004009. https://doi.org/10.1002/adfm.202004009

[3] Dragos Neagu et al 2023 J. Phys. Energy 5 031501

[4] Tao, S., Irvine, J. A redox-stable efficient anode for solid-oxide fuel cells. Nature Mater 2, 320–323 (2003). https://doi.org/10.1038/nmat871

The defect chemistry and resulting ionic structure is imperative for the properties of complex oxides, allowing for controlling and tailoring their functionality by thermodynamic means. This is particularly the case for exsolution-active oxides, in which reduction leads to the controlled decomposition of the host under the formation of metallic nanoparticles and compensating defect structures in the surrounding oxide matrix. In this talk, we attempt to provide a generalized perspective on defect-mediated phenomena in exsolution-active perovskites.

As we discuss, the exsolution response as well as the thermal stability of the resulting nanoparticles can be tuned by controlling the surface chemistry of the exsolution-active oxide, and thus controlling defect concentrations, space charge, and bonding between nanoparticle and host. Further, due to the dynamic loss of reducible transition metal species from the oxide lattice during exsolution, the characteristic defect compensation mechanisms vary as the exsolution process propagates.
For the co-doped exsolution model system, STNNi (Nb- and Ni-co-doped SrTiO₃), we observe a continuous transition from donor-type defect chemistry towards acceptor-type defect chemistry, indicative of an increasing counter-compensation of Nd-donors via B-site vacancies as the exsolution process proceeds. As a result, electronic connectivity of the nanocomposite and specifically the electrical contact resistances of the nanoparticles to their surrounding host oxide may strongly dependent on the intrinsic defect structure established after depletion of transition metals from the B-site. The results highlight the importance of both surface- and bulk-defect chemistry in establishing functional exsolution oxides.
 

1. Introduction

Exsolution is a beneficial process that promotes the uniform growth of highly dispersed catalytically active nanoparticles partially embedded in surface sockets of the host oxide, enhancing their stability. Perovskite oxides (ABO₃) are widely favored as host materials due to their remarkable structural flexibility, allowing for various element substitutions and controlled crystal defect chemistry, which are crucial for manipulating the exsolution process. Their stability under redox conditions and high temperatures makes them highly suitable for energy conversion applications such as solid oxide fuel cells (SOFCs) and solid oxide electrolysis cells (SOECs). Nickel is one of the most effective exsolvable species owing to its high reducibility, favorable incorporation as a B-site dopant, and excellent catalytic activity. The in-situ formation of Ni nanoparticles enhances the electrochemical activity of perovskite-based electrodes while providing a cost-effective alternative to noble metals. [1,2]

2. Materials and Methods

In this study, we investigate Ni exsolution from La(Mn,Cr)O₃-based perovskites with controlled A-site doping and defect chemistry through Sr substitution. Ni-doped La(Mn,Cr)O₃ powders were synthesized by Solution Combustion Synthesis. The obtained materials were characterized by X-ray diffraction (XRD) combined with Rietveld refinement, scanning electron microscopy (SEM), thermogravimetric analysis (TGA), and High-Energy Resolution Fluorescence Detected X-ray Absorption Near Edge Structure (HERFD-XANES) spectroscopy at the Ni K-edge. The powders were reduced under 5% H₂-containing atmosphere to promote exsolution.

3. Results and Discussion

Thermogravimetric analysis under reducing conditions revealed significant differences in mass loss behavior among the investigated compositions, which can be attributed to variations in oxygen vacancy formation and the onset of Ni exsolution. These results indicate a strong dependence of the reducibility of the perovskite lattice on dopant concentration and A-site defectivity.

HERFD-XANES measurements provided direct insight into the evolution of the Ni oxidation state during reduction. The spectra clearly show the progressive reduction of Ni species and the formation of metallic Ni. The extent of metallic Ni formation was found to depend on composition and defect chemistry, highlighting the role of Sr doping in controlling the exsolution process. SEM analysis of the reduced samples confirmed the formation of highly dispersed Ni nanoparticles exsolved at the perovskite surface. The particle size and surface density were closely related to the A-site chemistry, the overall defect structure of the host lattice, and the reduction conditions. These findings demonstrate a clear correlation between compositional tuning, defect chemistry, and exsolution dynamics, which ultimately govern the microstructural evolution of the materials. The combined structural, spectroscopic, and microstructural characterization provides a comprehensive understanding of the mechanisms driving Ni exsolution in this family of perovskites and supports their suitability as durable and efficient fuel electrodes for SOFC and SOEC applications. [3]

4. Conclusions

The final goal of this investigation is to establish structure–property relationships across a twelve-composition series by varying dopant concentrations and defect chemistry to control the exsolution process and microstructural evolution of La(Mn,Cr)O₃-based perovskites. The results elucidate the interplay between crystal chemistry, defect chemistry, and Ni exsolution behavior. HERFD-XANES demonstrates the formation of metallic Ni upon reduction, while SEM confirms the development of well-dispersed exsolved nanoparticles. By correlating A-site chemistry and Ni content with nanoparticle formation and stability, this work contributes to the rational design of exsolution-based fuel electrode materials with enhanced performance and long-term durability for SOFC and SOEC technologies.

Exsolution involves redox precipitation of metal nanoparticles from metal oxides, resulting in materials that exhibit excellent electrocatalytic properties and hold great promise for novel types of nanoscale heterostructures. Exsolution is described by three point defect equations involving the reduction of the host oxide, reduction and exsolution of the transition metal, and annihilation of host unit cells. Guided by their predictions, significantly enhanced exsolution in terms of molar amount, rate, and nucleation density is achieved by acceptor substitution to the system La_0.2Ca_0.7Ti_0.95Cu_0.05O_3−δ through atomic scale imaging and in situ X-ray diffraction and spectroscopy [1]. Moreover, multiple stages of exsolution are deconvoluted with increasing thermal activation: 1) minor exsolution of anchored surface nanoparticles, 2) exsolution of endoparticles within the bulk, and 3) diffusion and coalescing in the bulk and at the surfaces [2]. Strain in both the host oxide and the exsolved metal can constitute an additional thermodynamic barrier for exsolution beyond the availability of the required point defects.

Although sessile drop experiments provide important information about solid-liquid interactions in processing environments, the energy of solid-solid interfaces and their influence on adhesion and charge transfer are of central importance in many materials applications. These relationships are often complicated by capillarity-driven deviations from ideal flat-facet geometries, as well as by changes in interfacial energy caused by the adsorption (Gibbsian segregation) of dopants and/or impurities. In parallel, measurement of free surface energy as a function of dopant adsorption is critical for understanding the equilibrium morphology of metal particles supported on ceramic substrates.

This presentation will explore an experimental approach based on solid-state equilibration of thin films on defined substrates (so called solid-state dewetting) coupled with aberration-corrected microscopy to measure interfacial and surface energies while simultaneously resolving the atomistic structure and local chemistry of catalyst systems. The combination of these methods enables a direct connection between equilibrium particle shape, interface structure, and composition, providing a pathway to understand how capillarity, segregation, and adsorption collectively control adhesion and charge-transfer phenomena. Two approaches to measure the equilibrium crystal shape, and thus the relative surface energy of facets, will be explored.  Examples from catalyst-support systems will be discussed to illustrate how nanoscale interfacial energetics can be quantified and related to functional materials behavior

Socketing refers to the embedment of a metal particle on an oxide surface, and it is known to be correlated with decreased particle coarsening. The phenomenon is the result of a thermodynamic balance of interface and surface energies and is not directly linked with the mechanism of particle formation. Examples are discussed for nickel in two oxide systems, yttria stabilized zirconia and yttrium doped barium zirconate (BZY). For BZY, socketing is shown to occur for exsolved particles and for those formed by dewetting a film of nickel. However, exsolved particles form with a highly specific interface orientation relationship, compared with particles formed from dewetting, likely imparting superior coarsening resistance.

There are several purported benefits of exsolution catalysts, such as thermal stability and regenerability (through reoxidation). Currently, the mechanisms for these advantages are under discussion, and there remains some disagreement in the literature on how exsolved nanoparticles behave at the nanoscale. In situ experiments within the transmission electron microscope (TEM) provide a unique ability to investigate the behavior of exsolved nanoparticles with high spatial resolution. In this work, we utilize a variety of in situ TEM experiments to probe the behavior of exsolution-active materials during and after the exsolution reaction in several different systems.

First, we track the exsolution process from a compositionally complex spinel oxide through in situ gas experiments with simultaneous heating. Through a combination of heating while under a pressure of 1 bar of pure hydrogen up to 700 °C, we can observe the formation of metallic nanoparticles along with a restructuring of the oxide support. Furthermore, we show a progressive exsolution process occurs as different elements with differences in nobility exsolve from the support through electron energy loss spectroscopy at pressure and elevated temperature. By changing the gas atmosphere to oxygen while remaining at 1 bar pressure, we investigate the reversibility of the exsolution reaction in this system. The in situ results are compared with ex situ analyses on the same material, highlighting the dynamic and spatially resolved information gained from the use of in situ TEM techniques. We then contrast results from spinel oxides with in situ exsolution results from doped perovskite oxide thin films. Through the utilization of atomic resolution secondary electron imaging we demonstrate the formation of nanoparticles near and away from defects within the films, and that the perovskite structure remains stable. Additionally, we observe that defects not only act as nucleation sites, but also stabilize particles and prevent migration on the surface of the oxide support. Finally, an outlook on the in situ investigation of exsolution behaviors in the presence of external stimuli (i.e. strain, bias) is discussed.

Metal exsolution from oxide hosts is a powerful route to generate socketed, stable, and regenerable catalytic nanoparticles. Yet, exsolution is often treated mainly as a thermal reduction process, while its practical control depends on a broader set of coupled factors: the amount of metal that effectively emerges from the host lattice, the pathways available for cation migration, the nucleation landscape at the surface, and the ability of the parent oxide to withstand repeated redox operation.

In this talk, I will discuss exsolution as an architecture-dependent redox transformation in functional oxides. We demonstrate that vertically aligned perovskite-fluorite heterostructures can guide Ni migration through dense arrays of vertical interphases, promoting surface nanoparticle formation while preserving the structural integrity of the oxide scaffold after reduction. We also show that small fractions of exsolved Ni can be quantified by exploiting the magnetic contrast between ferromagnetic metallic nanoparticles and a paramagnetic oxide matrix, providing a sensitive whole-sample route to evaluate exsolution beyond the limits of conventional X-ray diffraction.

This architectural control is not limited to catalyst formation. Related redox-stable vertical heterostructures also act as functional electrochemical interfaces, where the nanoscale coexistence of mixed-conducting perovskite and ion-conducting fluorite phases increases reaction-site density and supports operation in both oxidizing and reducing atmospheres. These results suggest that oxide exsolution should be understood not only in terms of composition and temperature, but also in terms of interface density, strain, cation mobility, and oxygen chemical potential. I will conclude by briefly discussing ongoing efforts to extend these concepts toward electrochemically driven exsolution, where applied bias may offer an additional handle for forming, regenerating, and possibly reversing catalytic states during operation.

The exsolution of metallic nanoparticles from perovskite oxides is a promising route to obtain highly dispersed and stable catalysts via annealing in H₂-containing atmospheres. These nanoparticles remain anchored to the oxide surface, preventing sintering and enhancing resistance to carbon deposition, which leads to superior long-term catalytic performance compared to conventional noble metal-supported systems. Moreover, compositional tuning of the host oxide enables the controlled exsolution of alloy nanoparticles with unique electrocatalytic properties. However, dealing with the exsolution of multiple cations requires careful control of processing conditions and fine tuning of B-site composition. This is mainly due to the different reducibility that each element presents (following Ellingham trends) that, afterward,highly influence the concentration of each constituent in the exsolved nanoparticle.

Our group has devoted the last years to the understanding of multicomponent exsolution using as platform Sr₂Fe_1.5Mo_0.5O_6-δ based perovskites. First, because these materials are excellent electrodes for Solid Oxide Electrochemical cells. Secondly, this perovskite class can easily allocate multiple cations substituting Fe in the B-site, facilitating the exsolution of multicomponent nanoparticles. We initially investigate the exsolution of Ni-Co-Fe multicomponent alloys. By optimizing the microstructure and fine-tuning the exsolution treatment parameters, we achieved functionalization with ternary alloy nanoparticles (~10 nm) that previously showed excellent performance in CO₂ electrolysis, with high Faradaic efficiency and low polarization resistance.¹ Here, we demonstrate that adjusting gas atmosphere, temperature, and pressure allows further control of nanoparticle composition. Notably, high-pressure exsolution (up to 100 bar) revealed a volcano-like dependence of both exsolution extent and alloy composition on pressure.² We also show that redox cycling under atmospheric pressure modifies surface chemistry, leading to Fe enrichment.³ Lastly, we explore temperature effects on Cu-Co-Fe-Ni exsolution, identifying conditions that favor the formation of Janus-type nanoparticles, mainly affected by the low miscibility of Cu and metallic Fe. Interestingly, these phase-separated Janus-type nanoparticles greatly alter the reversibility property of the exsolution in this system, leading to the formation of pyramidal NiO nanoparticles instead of total redissolution.⁴

Finally, we reveal how the compositional tuning of these multielemental exsolved nanoparticles (via phase separation or Fe-enrichment) affects the catalytic properties of these materials, directing reactions to targeted products. These results highlight the potential and versatility of multicomponent nanoparticle exsolution in (electro)catalytic processes for renewable fuel production.

In heterogeneous catalytic systems, active metallic species are often supported on metal oxides. The deposited nanoparticles (NPs), however, often suffer from sintering, which can lead to deactivation of the catalyst. An alternative fabrication route for catalysts involves direct exsolution of metallic NPs from doped metal oxide hosts under thermal reductive treatments.[1] This approach can produce highly active and stable catalysts, where the exsolved NPs can remain strongly anchored to the host oxide.[1]

In our recent work, we demonstrate tunable exsolution of multimetallic NPs from a series of compositionally complex first-row transition metal spinel oxides.[2] We track the electronic, atomic structural, and microstructural evolution of the host oxide and exsolved NPs during thermal reductive treatment in diluted H₂ by in situ X-ray diffraction and total scattering followed by Rietveld and pair distribution function analysis. The results are complemented by electron microscopy and near-ambient pressure X-ray photoelectron spectroscopy. We observe exsolution of Cu-rich NPs from ca. 360 °C, while Cu@NiCo core-shell NPs form at 500 °C, which subsequently alloy into trimetallic CuNiCo NPs at 700 °C. Furthermore, we demonstrate that compositional tuning of the host oxide significantly affects the exsolution mechanisms, allowing tuning of the overall process. Our findings identify compositionally complex solid solutions as promising platforms for the rational design of innovative exsolved catalysts.

Metal nanoparticles supported on solid surfaces constitute the active component of many catalysts used in energy conversion, electrocatalysis, and environmental applications. Their catalytic performance is governed not only by particle size and composition but also by the electronic interaction with the supporting material, which determines nanoparticle stability, strain, and surface reactivity. This presentation summarizes a series of density functional theory (DFT) investigations aimed at revealing the atomic-scale mechanisms governing the electronic properties and stability of supported nanoparticles across carbon, oxide, and mixed metal-oxide supports. Systematic DFT calculations demonstrate that the binding strength of Pd and Au nanoparticles can be tuned through the surface curvature. Highly curved surfaces induce stronger orbital overlap, significantly increasing adsorption energies and suppressing nanoparticle mobility. First-principles molecular dynamics simulations further reveal reduced nanoparticle migration on curved supports, providing a microscopic explanation for the experimentally observed enhancement in catalyst durability. Platinum nanoparticles supported on carbon and SnO₂ demonstrate that strong metal-support interactions with oxide surfaces induce pronounced lattice distortions extending throughout the nanoparticle. Atomic-scale defects on the oxide support generate localized strain fields that modify the Pt electronic structure, providing an atomistic explanation for the experimentally measured strain distributions and their correlation with catalytic activity. Finally, we addresses self-exsolved Cu/Fe₃O₄ heterostructured nanoparticles formed during CO₂/H₂O co-electrolysis on CuFe₂O₄ spinel electrodes. First-principles calculations reveal that the metal-oxide interface creates electronically distinct active sites unavailable on the individual phases. Charge redistribution across the Cu/Fe₃O₄ interface promotes activation of CO₂ and H₂O while simultaneously stabilizing the exsolved nanoparticles through strong interfacial bonding. The calculations explain the exceptional activity and long-term stability observed experimentally and identify the heterostructured cermet interface as the primary catalytic active site.Together, these studies demonstrate how first-principles simulations can establish direct relationships between support geometry, electronic structure, lattice strain, and nanoparticle stability. By combining electronic structure theory with advanced experimental characterization, they provide fundamental design principles for engineering robust nanocatalysts with enhanced activity and durability for electrochemical energy conversion and heterogeneous catalysis.