In Situ Aberration-Corrected Environmental TEM: Reduction of Model Co3O4in H2at the Atomic Level

Well-defined tricobalt tetraoxide's critical morphology effect on the structure-reactivity relationship

This review focuses on exploring the intricate relationship between the catalyst particle size and shape on a nanoscale level and how it affects the performance of reactions. Drawing from decades of research, valuable insights have been gained. Intentionally shaping catalyst particles makes exposing a more significant percentage of reactive facets possible, enabling the control of overactive sites. In this study, the effectiveness of Co3O4 nanoparticles (NPs) with nanometric size as a catalyst is examined, with a particular emphasis on the coordination patterns between oxygen and cobalt atoms on the surface of these NPs. Investigating the correlation between the structure and reactivity of the exposed NPs reveals that the form of Co3O4 with nanometric size can be modified to tune its catalytic capabilities finely. Morphology-dependent nanocatalysis is often attributed to the advantageous exposure of reactive crystal facets accumulating numerous active sites. However, experimental evidences highlight the importance of considering the reorganization of NPs throughout their actions and the potential synergistic effects between nearby reactive and less-active aspects. Despite the significant role played by the atomic structure of Co3O4 NPs with nanometric size, limited attention has been given to this aspect due to challenges in high-resolution characterizations. To bridge this gap, this review strongly advocates for a comprehensive understanding of the relationship between the structure and reactivity through real-time observation of individual NPs during the operation. Proposed techniques enable the assessment of dimensions, configuration, and interfacial arrangement, along with the monitoring of structural alterations caused by fluctuating temperature and gaseous conditions. Integrating this live data with spectroscopic methods commonly employed in studying inactive catalysts holds the potential for an enhanced understanding of the fundamental active sites and the dynamic behavior exhibited in catalytic settings.

Visualization of the structural transformation of NiO/YSZ/BZY nanocomposite particles using in situ gas environmental transmission electron microscopy

This study focused on investigating the dynamic structural transformations of spherical NiO/YSZ/BZY triple-phase nanocomposite particles, commonly employed for cermet anodes, during the hydrogen reduction reaction. We utilized both spherical aberration (Cs) corrected transmission electron microscopy (TEM) and high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) observation modes under a controlled gaseous environment. The environmental gas pressure was set to 1 atm (760 Torr), mirroring real-world conditions. To elucidate pre- and post-hydrogen reduction compositional alterations, we conducted elemental mapping using energy-dispersive X-ray spectroscopy (EDS). Our findings indicated that NiO nanoparticles underwent reduction to Ni particles upon heat treatments in an environment containing H2 gas. Significantly, this reduction of NiO led to the migration of Ni along the external surface of each composite particle, ultimately resulting in the agglomeration at the interparticle spaces among the three NiO/YSZ/BZY nanocomposite particles.

In Situ Transmission Electron Microscopy to Study the Location and Distribution Effect of Pt on the Reduction of Co3 O4 -SiO2

The addition of Pt generally promotes the reduction of Co3 O4 in supported catalysts, which further improves their activity and selectivity. However, due to the limited spatial resolution, how Pt and its location and distribution affect the reduction of Co3 O4 remains unclear. Using ex situ and in situ ambient pressure scanning transmission electron microscopy, combined with temperature-programmed reduction, the reduction of silica-supported Co3 O4 without Pt and with different location and distribution of Pt is studied. Shrinkage of Co3 O4 nanoparticles is directly observed during their reduction, and Pt greatly lowers the reduction temperature. For the first time, the initial reduction of Co3 O4 with and without Pt is studied at the nanoscale. The initial reduction of Co3 O4 changes from surface to interface between Co3 O4 and SiO2 . Small Pt nanoparticles located at the interface between Co3 O4 and SiO2 promote the reduction of Co3 O4 by the detachment of Co3 O4 /CoO from SiO2 . After reduction, the Pt and part of the Co form an alloy with Pt well dispersed. This study for the first time unravels the effects of Pt location and distribution on the reduction of Co3 O4 nanoparticles, and helps to design cobalt-based catalysts with efficient use of Pt as a reduction promoter.

Transformation of Co3O4 nanoparticles to CoO monitored by in situ TEM and predicted ferromagnetism at the Co3O4/CoO interface from first principles

Nanoparticles of Co₃O₄ and CoO are of paramount importance because of their chemical properties propelling their applications in catalysis and battery materials, and because of their intriguing magnetic properties. Here we elucidate the transformation of Co₃O₄ nanoparticles to CoO into nanoscale detail by in situ heating in the transmission electron microscope (TEM), and we decipher the energetics and magnetic properties of the Co₃O₄/CoO interface from first principles calculations. The transformation was found to start at a temperature of 350 °C, and full conversion of all particles was achieved after heating to 400 °C for 10 minutes. The transformation progressed from the surface to the center of the nanoparticles under the formation of dislocations, while the two phases maintained a cube-on-cube orientation relationship. Various possibilities for magnetic ordering were considered in the density functional theory (DFT) calculations and a favorable Co₃O₄/CoO {100}/{100} interface energy of 0.38 J m^-2 is predicted for the lowest-energy ordering. Remarkably, the DFT calculations revealed a substantial net ferromagnetic moment originating from the interface between the two antiferromagnetic compounds, amounting to approximately 13.9 μ _B nm^-2. The transformation was reproduced ex situ when heating at a temperature of 400 °C in a high vacuum chamber.

Visualizing single atom dynamics in heterogeneous catalysis using analytical in situ environmental scanning transmission electron microscopy

Progress is reported in analytical in situ environmental scanning transmission electron microscopy (ESTEM) for visualizing and analysing in real-time dynamic gas-solid catalyst reactions at the single-atom level under controlled reaction conditions of gas environment and temperature. The recent development of the ESTEM advances the capability of the established ETEM with the detection of fundamental single atoms, and the associated atomic structure of selected solid-state heterogeneous catalysts, in catalytic reactions in their working state. The new data provide improved understanding of dynamic atomic processes and reaction mechanisms, in activity and deactivation, at the fundamental level; and in the chemistry underpinning important technological processes. The benefits of atomic resolution-E(S)TEM to science and technology include new knowledge leading to improved technological processes, reductions in energy requirements and better management of environmental waste. This article is part of a discussion meeting issue 'Dynamic in situ microscopy relating structure and function'.

Well-Defined Materials for Heterogeneous Catalysis: From Nanoparticles to Isolated Single-Atom Sites

The use of well-defined materials in heterogeneous catalysis will open up numerous new opportunities for the development of advanced catalysts to address the global challenges in energy and the environment. This review surveys the roles of nanoparticles and isolated single atom sites in catalytic reactions. In the second section, the effects of size, shape, and metal-support interactions are discussed for nanostructured catalysts. Case studies are summarized to illustrate the dynamics of structure evolution of well-defined nanoparticles under certain reaction conditions. In the third section, we review the syntheses and catalytic applications of isolated single atomic sites anchored on different types of supports. In the final part, we conclude by highlighting the challenges and opportunities of well-defined materials for catalyst development and gaining a fundamental understanding of their active sites.

Confined small-sized cobalt catalysts stimulate carbon-chain growth reversely by modifying ASF law of Fischer-Tropsch synthesis

Fischer–Tropsch synthesis (FTS) is a promising technology to convert syngas derived from non-petroleum-based resources to valuable chemicals or fuels. Selectively producing target products will bring great economic benefits, but unfortunately it is theoretically limited by Anderson–Schulz–Flory (ASF) law. Herein, we synthesize size-uniformed cobalt nanocrystals embedded into mesoporous SiO₂ supports, which is likely the structure of water-melon seeds inside pulps. We successfully tune the selectivity of products from diesel-range hydrocarbons (66.2%) to gasoline-range hydrocarbons (62.4%) by controlling the crystallite sizes of confined cobalt from 7.2 to 11.4 nm, and modify the ASF law. Generally, larger Co crystallites increase carbon-chain growth, producing heavier hydrocarbons. But here, we interestingly observe a reverse phenomenon: the uniformly small-sized cobalt crystallites can strongly adsorb active C* species, and the confined structure will inhibit aggregation of cobalt crystallites and escape of reaction intermediates in FTS, inducing the higher selectivity towards heavier hydrocarbons.

Fischer–Tropsch synthesis (FTS) is theoretically limited by Anderson–Schulz–Flory (ASF) law. Here, the authors successfully tune the selectivity of products from diesel-range hydrocarbons to gasoline-range hydrocarbons in FTS by controlling the crystallite sizes of confined cobalt, and modify the ASF law.

Visualizing atomic-scale redox dynamics in vanadium oxide-based catalysts

Surface redox processes involving oxygen atom exchange are fundamental in catalytic reactions mediated by metal oxides. These processes are often difficult to uncover due to changes in the surface stoichiometry and atomic arrangement. Here we employ high-resolution transmission electron microscopy to study vanadium oxide supported on titanium dioxide, which is of relevance as a catalyst in, e.g., nitrogen oxide emission abatement for environmental protection. The observations reveal a reversible transformation of the vanadium oxide surface between an ordered and disordered state, concomitant with a reversible change in the vanadium oxidation state, when alternating between oxidizing and reducing conditions. The transformation depends on the anatase titanium dioxide surface termination and the vanadium oxide layer thickness, suggesting that the properties of vanadium oxide are sensitive to the supporting oxide. These atomic-resolution observations offer a basis for rationalizing previous reports on shape-sensitive catalytic properties.

Redox processes in metal oxide surfaces can exhibit structure sensitivities which are difficult to uncover. Here, the authors use atomic-resolution imaging to demonstrate facet dependent alterations in the surfaces of supported vanadium oxide upon reduction and oxidation.

Direct observations of dynamic PtCo interactions in fuel cell catalyst precursors at the atomic level using E(S)TEM

Reduction reactions in practical bimetallic platinum-cobalt electrode catalyst precursors containing platinum, cobalt and cobalt oxides in hydrogen at 200, 450 and 700 °C for 6 h have been studied in situ using an aberration corrected environmental (scanning) transmission electron microscope (AC E(S)TEM). Little difference was observed in reduction at 200 °C but during and after reduction at 450 °C, small nanoparticles less than 3 nm in diameter with tetragonal PtCo structures were observed and limited Pt₃ Co ordering could be seen on the surfaces of larger nanoparticles. During and after reduction at 700 °C, fully ordered Pt₃ Co and PtCo nanoparticles larger than 4 nm were produced and the average nanoparticle size almost trebled relative to the fresh precursor. After reduction at 450 and 700 °C, most nanoparticles were disordered platinum/cobalt alloys with fcc structure. After reduction at 700 °C many of the smallest nanoparticles disappeared suggesting Ostwald ripening had occurred. Mechanisms concerning the thermal transformation of mixed cobalt and platinum species are discussed, offering new insights into the creation of bimetallic platinum-cobalt nanoparticles in fuel cell catalysts.

Oxidation behavior of cobalt nanoparticles studied by in situ environmental transmission electron microscopy

The dynamics of oxidation of cobalt nanoparticles were directly revealed by in situ environmental transmission electron microscopy. Firstly, cobalt nanoparticles were oxidized to polycrystalline cobalt monoxide, then to polycrystalline tricobalt tetroxide, in the presence of oxygen with a low partial pressure. Numerous cavities (or voids) were formed during the oxidation, owing to the Kirkendall effect. Analysis of the oxides growth suggested that the oxidation of cobalt nanoparticles followed a parabolic rate law, which was consistent with diffusion-limited kinetics. In situ transmission electron microscopy allowed potential atomic oxidation pathways to be considered. The outward diffusion of cobalt atoms inside the oxide layer controlled the oxidation, and formed the hollow structure. Irradiation by the electron beam, which destroyed the sealing effect of graphite layer coated on the cobalt surface and resulted in fast oxidation rate, played an important role in activating and promoting the oxidations. These findings further our understanding on the microscopic kinetics of metal nanocrystal oxidation and knowledge of energetic electrons promoting oxidation reaction.

Oxidation and reduction processes of platinum nanoparticles observed at the atomic scale by environmental transmission electron microscopy

Oxidation and reduction of the surfaces of Pt nanoparticles were in situ examined in reactive gases (O2, CO and H2O vapor) by aberration-corrected environmental transmission electron microscopy. Atomic layers of Pt oxides were gradually formed on the surface of Pt nanoparticles at room temperature in O2. The surface Pt oxides were reduced to Pt promptly in both vacuum and gas including CO. We showed that H2O vapor suppressed the surface oxidation. The processes found in this study were induced by gases that were most likely activated by electron irradiation. The observation results provide atomistic insight into the oxidation and reduction process of the surface of Pt nanoparticles that is exposed to activated gases.