Dopant‐Driven Positive Reinforcement in Ex‐Solution Process: New Strategy to Develop Highly Capable and Durable Catalytic Materials

Abstract

Supported catalysts, in which metal nanoparticles (NPs) are distributed on oxide supports, are widely used in a variety of industrial processes, including in catalysis, [1] sensing, [2] and renewable energy. [3] To improve the reactivity, costeffectiveness, and durability of catalysts, decreasing their particle size to ensure uniform dispersion on the support and to suppress their aggregation during operation, particularly at high temperatures (>500 °C) is of utmost importance. Thus far, not only conventional impregnation and coprecipitation approaches but also numerous new synthesis and dispersion techniques have been actively studied. [4] Recently, the spontaneous growth of metal NPs on the surface of a host oxide, called "ex-solution," has attracted much attention as a new route to obtain supported catalysts. [5] It is based on the phenomenon that certain cations in a complex oxide lattice selectively precipitate on the surface when the oxide is partially reduced at high temperatures. [6] In particular, when catalytically active and highly reducible transition metals are used as dopants, metal NPs can be uniformly dispersed on the oxide surface with only a single heat treatment. [7] Furthermore, the metal NPs produced by ex-solution are strongly bound to the host oxide and thus have excellent thermal and chemical stability. [8,9] Accordingly, thanks to these advantages, the ex-solution process is now being applied in high-temperature chemical and electrochemical applications, such as solid oxide fuel cells, [10] electrolyzers, [11] and oxidation catalysts. [12] However, the high-temperature heat treatment required to obtain ex-solved particles is a substantial barrier that significantly limits the use of this process in a wider range of applications. To achieve high catalytic reactivity, not only the metal particles but also the oxide support should have a large specific surface area, whereas at high temperatures, sintering between the oxide supports often occurs, deactivating the catalysts. Therefore, it is necessary to develop a strategy for activating the ex-solution process at a temperature low enough that the nanoscale oxide supports do not aggregate. In this regard, Irvine et al. proposed facilitating the extrusion of B-site cations The ex-solution phenomenon, a central platform for growing metal nanoparticles on the surface of host oxides in real time with high durability and a fine distribution, has recently been applied to various scientific and industrial fields, such as catalysis, sensing, and renewable energy. However, the high-temperature processing required for ex-solutions (>700 °C) limits the applicable material compositions and has hindered advances in this technique. Here, an unprecedented approach is reported for lowtemperature particle ex-solution on important nanoscale binary oxides. WO 3 with a nanosheet structure is selected as the parent oxide, and Ir serves as the active metal species that produces the ex-solved metallic particles. Importantly, Ir doping facilitates a phase transition ...

Emergence and Future of Exsolved Materials

“…Small published by Wiley-VCH GmbH convert CH 4 using CO 2 [155] and the other two on CO oxidation. [156,157] In all cases, it is demonstrated that the improved catalytic activity of the material was induced after the emergence of the Ir nanoparticles on the surface. In the former case this was shown to be due to the particles' ability to catalyze the cleavage of the CH bond in CH 4 with high coking resistance.…”

“…At the same time exsolution offers exceptional control over particle characteristics (size, population, and distribution), which allows for fine tuning of the activity and the selectivity of processes that employ exsolved materials. [5,16] Due to the combination of activity, durability, as well as compositional, structural, and functional tunability, the application of exsolved materials has proven to be [5,[8][9][10][11]15,16,22,23,26,40,125,140,157] In the exsolution method, illustrated schematically in Figure 1a, the active (i.e., exsolvable) elements are substituted in a host lattice under oxidizing conditions, forming an oxide solid solution, and released as metallic particles upon exposure to reducing conditions, leaving behind the host lattice as support. There are thus a number of key processes fundamental to exsolution, as schematically illustrated in Figure 1b: formation of a solid solution phase, exposure to reducing conditions to provide the driving force for the phase segregation, nucleation and growth of the exsolved phase, diffusion of ions, and oxide ions, and electrons across the host lattice to fuel the nucleation and growth process (generally shuttling between the bulk and the surface, but also locally if this occurs within the bulk or near-surface).…”

“…Timeline of key developments of the exsolution concept. Adapted with permission [5,[8][9][10][11]15,16,22,23,26,40,125,140,157]. Top row from left to right: Copyright 2006, Wiley-VCH, Copyright 2013, Springer Nature, Copyright 2016, Springer Nature, Copyright 2017, Springer Nature, Copyright 2010, Elsevier.…”

Oxide/ZIF‐8 Hybrid Nanofiber Yarns: Heightened Surface Activity for Exceptional Chemiresistive Sensing

“…[7,8] In addition, thermal degradation of the sensing layer and the agglomeration of metal catalysts could both degrade the sensor performance, compromising its reliability in long-term operations. [9,10] Moreover, the brittleness of oxides limits the scope of utilization from flexible and wearable devices, which are key aspects for consumer applications in the immediate future. To this end, it is necessary to develop an effective design strategy which integrates nanostructured oxide layers with a flexible polymer substrate as an underlying layer [11,12] and a molecular sieving layer for selective gas filtration.…”

Nanoparticle Ex-solution for Supported Catalysts: Materials Design, Mechanism and Future Perspectives

“…On the basis of this discovery, Jang et al recently demonstrated a highly stable chemiresistive sensor which employs Ir as the exsolution catalyst on two-dimensional (2D) WO 3 nanosheets (NSs). 152 Specifically, the substitution of W with Ir led to an improved reducibility of the WO 3 host oxide, enabling the exsolution at significantly low temperatures (300 °C) as compared to those in other approaches. As shown in Figure 21a, the extraction of Ir nanoparticles from the host WO 3 at a low temperature was accelerated due to the phase transformation of the host WO 3 from monoclinic to tetragonal WO 3 and WO 2.9 phases during the reduction process.…”