Pd/CeO 2 has attracted great attention owing to its unique activity for methane catalytic oxidation; however, the active sites for CH 4 catalytic oxidation still remain elusive, which affects the comprehensive understanding of the catalytic mechanism. In this work, the structures of PdO x nanoparticles (NPs) loaded on octahedrons, cubes, and rods of nanocrystal CeO 2 supports were systematically studied by Cs-corrected HRTEM/STEM, XPS, and Raman spectroscopy. Our results indicate that the Pd species on CeO 2 supports are morphology-dependent: PdO NPs (Pd 2+ ) on octahedrons, PdO x (x = 1−2) clusters (1−2 nm) on cubes, and dispersed Pd 4+ ions on the CeO 2 rods. Additionally, the chemical states of Pd can be tuned in oxidizing/reducing atmospheres via interactions between Pd and CeO 2 . Detailed studies reveal that the Pd 2+ species are the active centers for the catalytic oxidation of methane. The activity of Pd 0 could be ascribed to Pd 2+ produced through the gradual oxidation of Pd 0 during the CH 4 oxidation. Further, Pd 4+ in the CeO 2 lattice is inactive for CH 4 oxidation. In situ Fourier transform infrared spectroscopy results suggest that the mechanism of CH 4 oxidation reaction on PdO x /CeO 2 follows the Mars−van Krevelen mechanism, and adsorbed CO can be produced in CH 4 decomposition over Pd 2+ in the absence of gas-phase oxygen. As revealed by density functional theory calculations, the incomplete coordination of Pd 2+ ions and adjacent oxygen atoms has excellent activity in cracking the C−H bond of CH 4 , which leads to high methane oxidation ability.

Metal catalysts are of great importance in the modern chemical industry. It is well-known that the structures of metal catalysts determine their properties. However, recent studies suggested that the structures of metal catalysts change dynamically under reaction conditions, resulting in the deactivation or activation of metal catalysts. This Review summarizes the latest research progresses in the structural reconstruction of metal catalysts via controlled-atmosphere transmission electron microscopy. The stateof-the-art research technologies and crucial factors affecting the nanosized metal catalyst reconstruction are discussed. Various types of reconstruction phenomena are reviewed, including sintering and dispersion, reshaping, composition evolution, surface reconstruction of metal oxides, and strong metal−support interactions. Moreover, recent studies of the structure−property relationship of metal catalysts are also reviewed. Finally, we highlight current challenges and provide the perspectives for future research of this topic. We hope this Review provides insights for the rational design of highperformance metal catalysts.

Achieving metal nanocrystals with metastable phase draws much attention due to their anticipated fascinating properties, wheras it is still challenging because their polymorphism nature and phase transition mechanism remain elusive. Here, phase stability of face-centered cubic (fcc) Pd nanocrystals was studied via in situ spherical aberration (Cs)-corrected transmission electron microscopy (TEM). By constructing a well-defined Pd/C composite structure, Pd nanocrystals encapsulated by graphite, the dispersion process of fcc Pd was observed through a nucleation and growth process. Interestingly, Cs-corrected scanning TEM analysis demonstrated that the newly formed Pd nanocrystals could adopt a metastable hexagonal phase, which was considered challenging to obtain. Accordingly, formation mechanism of the hexagonal Pd nanocrystals was proposed, which involved the combined effect of two factors: (1) templating of graphite and (2) size effect. This work is expected to offer new insight into the polymorphism of Pd nanocrystals and pave the way for the future design of metastable metal nanomaterials.

Conspectus Heterogeneous catalysts are widely used in a variety of industrial fields, including environmental protection, energy conversion, and chemical production. Their performance during reactions is usually determined by a small fraction of sites at the catalyst surfaces/interfaces, namely, the active sites. Actually, since the concept of the “active sites” was proposed by Hugh Taylor in the 1920s, determining the active site at the atomic level and understanding the molecular processes that happened at the active site have become the top priority in catalysis research. Researchers tried different methods to acquire various information related to the surface/interface active sites, pursuing their rational design at the atomic level. Although great achievements have been made in catalyst surface study, in situ atomistic design of active sites remains challenging, due to the lack of direct information and effective manipulation means concerning the active sites. Specifically, many critical issues regarding the active sites under reaction conditions remain to be solved: (1) precisely identifying the active sites, which is the foundation for understanding the catalytical mechanism and rational design of the catalyst; (2) accurately manipulating the surface/interface active sites at the atomic scale, which is the basis for realizing the desired performance; (3) maintaining these designed active sites operating in a long-term with high efficiency without deactivation, which is extremely important to the practical applications of the catalysts. All these aspects rely on the fundamental understanding of the interactions between different surface/interface configurations of the catalyst and external environments (gas, pressure, temperature, etc.). In this Account, we present the recent progress in our group on the studies of surface/interface active sites via controlled atmosphere transmission electron microscopy (CATEM). We first briefly introduce the advances in CATEM technologies, including the window (closed) approach and aperture (open) approaches. Then, the challenge of identifying active sites is discussed, and our efforts in this target by determining surface atomic structures, tracking the active components, visualizing reacting molecules, and in situ evaluating catalytic performance are demonstrated. The next section focuses on the in situ manipulating active sites by controlling external environmental factors (e.g., gas, temperature, and pressure), including tailoring the catalyst shape, surface components, and interface with atomic precision. The fourth section discusses the strategies for the long-term stable operation of the active sites based on our in situ studies in understanding the deactivation mechanisms. In the end, we provide our perspectives on the future opportunities and some scientific and technical challenges in this booming area. This Account highlights the in situ atomic level design of the active sites based on the CATEM route, which provides an applicable strategy for deep understanding...

Understanding the oxidation mechanism of metal nanoparticles under ambient pressure is extremely important to make the best use of them in a variety of applications. Through ambient pressure transmission electron microscopy, we in situ investigated the dynamic oxidation processes of Ni nanoparticles at different temperatures under atmospheric pressure, and a temperature-dependent oxidation behavior was revealed. At a relatively low temperature (e.g., 600 °C), the oxidation of Ni nanoparticles underwent a classic Kirkendall process, accompanied by the formation of oxide shells. In contrast, at a higher temperature (e.g., 800 °C), the oxidation began with a single crystal nucleus at the metal surface and then proceeded along the metal/oxide interface without voids formed during the whole process. Through our experiments and density functional theory calculations, a temperature-dependent oxidation mechanism based on Ni nanoparticles was proposed, which was derived from the discrepancy of gas adsorption and diffusion rates under different temperatures.