The Merits of Static and Dynamic High-Resolution Electron Microscopy (HREM) for the Study of Solid Catalysts

Single-Atom Catalysts on Covalent Triazine Frameworks: at the Crossroad between Homogeneous and Heterogeneous Catalysis

Heterogeneous single-site and single-atom catalysts potentially enable combining the high catalytic activity and selectivity of molecular catalysts with the easy continuous operation and recycling of solid catalysts. In recent years, covalent triazine frameworks (CTFs) found increasing attention as support materials for particulate and isolated metal species. Bearing a high fraction of nitrogen sites, they allow coordinating molecular metal species and stabilizing particulate metal species, respectively. Dependent on synthesis method and pretreatment of CTFs, materials resembling well-defined highly crosslinked polymers or materials comparable to structurally ill-defined nitrogen-containing carbons result. Accordingly, CTFs serve as model systems elucidating the interaction of single-site, single-atom and particulate metal species with such supports. Factors influencing the transition between molecular and particulate systems are discussed to allow deriving tailored catalyst systems.

Recent advances in co-reaction accelerators for sensitive electrochemiluminescence analysis

In electrochemiluminescence sensing platforms, co-reaction accelerators are specific materials used to catalyze the dissociation of co-reactants into active radicals, which can significantly boost the ECL emission of luminophores.

Isolating Clusters of Light Elements in Molecular Sieves with Atom Probe Tomography

Understanding the 3-D distribution and nature of active sites in heterogeneous catalysts is critical to developing structure–function relationships. However, this is difficult to achieve in microporous materials as there is little relative z-contrast between active and inactive framework elements (e.g., Al, O, P, and Si), making them difficult to differentiate with electron microscopies. We have applied atom probe tomography (APT), currently the only nanometer-scale 3-D microscopy to offer routine light element contrast, to the methanol-to-hydrocarbons (MTH) catalyst SAPO-34, with Si as the active site, which may be present in the framework as either isolated Si species or clusters (islands) of Si atoms. ²⁹Si solid-state NMR data on isotopically enriched and natural abundance materials are consistent with the presence of Si islands, and the APT results have been complemented with simulations to show the smallest detectable cluster size as a function of instrument spatial resolution and detector efficiency. We have identified significant Si–Si affinity in the materials, as well as clustering of coke deposited by the MTH reaction (¹³CH₃OH used) and an affinity between Brønsted acid sites and coke. A comparison with simulations shows that the ultimate spatial resolution that can be attained by APT applied to molecular sieves is 0.5–1 nm. Finally, the observed ¹³C clusters are consistent with hydrocarbon pool mechanism intermediates that are preferentially located in regions of increased Brønsted acidity.

Metal Catalysts for Heterogeneous Catalysis: From Single Atoms to Nanoclusters and Nanoparticles

Metal species with different size (single atoms, nanoclusters, and nanoparticles) show different catalytic behavior for various heterogeneous catalytic reactions. It has been shown in the literature that many factors including the particle size, shape, chemical composition, metal-support interaction, and metal-reactant/solvent interaction can have significant influences on the catalytic properties of metal catalysts. The recent developments of well-controlled synthesis methodologies and advanced characterization tools allow one to correlate the relationships at the molecular level. In this Review, the electronic and geometric structures of single atoms, nanoclusters, and nanoparticles will be discussed. Furthermore, we will summarize the catalytic applications of single atoms, nanoclusters, and nanoparticles for different types of reactions, including CO oxidation, selective oxidation, selective hydrogenation, organic reactions, electrocatalytic, and photocatalytic reactions. We will compare the results obtained from different systems and try to give a picture on how different types of metal species work in different reactions and give perspectives on the future directions toward better understanding of the catalytic behavior of different metal entities (single atoms, nanoclusters, and nanoparticles) in a unifying manner.

Insights into the physical chemistry of materials from advances in HAADF-STEM

HAADF-STEM provides atomic-resolution real space imaging. Here an image of a single Si dopant atom in a graphene lattice is shown adjacent to a schematic of the instrument. Simultaneous EELS on electrons scattered to low angles can provide chemical identification of the species preset. Differences between the Si L-edge spectra reveal differences in atomic bonding and hybridization for different configurations of Si atoms in graphene.

Nanocrystalline materials: recent advances in crystallographic characterization techniques

Most properties of nanocrystalline materials are shape-dependent, providing their exquisite tunability in optical, mechanical, electronic and catalytic properties. An example of the former is localized surface plasmon resonance (LSPR), the coherent oscillation of conduction electrons in metals that can be excited by the electric field of light; this resonance frequency is highly dependent on both the size and shape of a nanocrystal. An example of the latter is the marked difference in catalytic activity observed for different Pd nanoparticles. Such examples highlight the importance of particle shape in nanocrystalline materials and their practical applications. However, one may ask 'how are nanoshapes created?', 'how does the shape relate to the atomic packing and crystallography of the material?', 'how can we control and characterize the external shape and crystal structure of such small nanocrystals?'. This feature article aims to give the reader an overview of important techniques, concepts and recent advances related to these questions. Nucleation, growth and how seed crystallography influences the final synthesis product are discussed, followed by shape prediction models based on seed crystallography and thermodynamic or kinetic parameters. The crystallographic implications of epitaxy and orientation in multilayered, core-shell nanoparticles are overviewed, and, finally, the development and implications of novel, spatially resolved analysis tools are discussed.

Transmission electron microscopy and the science of carbon nanomaterials

Carbon is a unique and versatile element that is capable of forming different architectures at nanoscale. The element has become a key component in nanoscience and nanotechnology. Transmission electron microscopy (TEM) acts as "our eyes" enabling us not only to reveal the morphology, but also to provide structural, chemical and electronic information of nanocarbon on the atomic level. In fact, except for fullerene, nearly all types of carbon nanomaterials were discovered by TEM, such as carbon nanotubes, carbon nanocones, and graphene-like nanocarbon. It cannot be imagined what nanoscience and nanotechnology would be without the contributions of TEM. Herein, the "interaction" between TEM and the science of carbon nanomaterials is reviewed and it is demonstrated for some selected examples that TEM provides a dramatic driving force for the development of nanocarbon science.

A new approach to the investigation of nanoparticles: electron tomography with compressed sensing

The principal purpose of this contribution is to illustrate the potential of compressed sensing electron tomography for the characterisation of nanoparticulate materials that are vulnerable to electron beam damage. Not only is there growing interest in nanoparticles of organic materials in medical and allied contexts, there is also the need to investigate nanoparticles and nanoclusters of metals supported on biological macromolecular entities in the context of drug delivery. A qualitative account of the principles of electron tomography is outlined with illustrations from the field of heterogeneous catalysis, where electron beam damage is less of an issue, and an appendix deals with more quantitative aspects of how compressed sensing promises to expand the range of samples that have hitherto been accessible to investigation.

Single-site photocatalysts with a porous structure

A photocatalytic reaction involves charge separation and transfer under photo-irradiation, and the photogenerated charge carriers (holes and electrons) are responsible for the photocatalytic activity of the catalyst. The active centres in a single-site photocatalyst are the isolated and spatially separated sites that may interact with reactants after photo-irradiation. Generally, single-site photocatalysts perform better than other types of photocatalysts owing to the presence of the efficient active centres. A porous structure can provide more single sites and special passages for charge transport. Thus, the introduction of a porous structure into a photocatalyst may result in markedly enhanced photocatalytic reactivity, providing a promising strategy for the design and fabrication of novel photocatalysts with high performances. In this review, we summarize the developments in single-site photocatalysts, particularly those with a porous structure, such as metal-incorporated zeolites, metal–organic frameworks and porous semiconductor photocatalysts. The synthesis, structures and catalytic performances of these single-site photocatalysts have been described, and characterization and reaction mechanisms for single-site photocatalysts have also been detailed. Finally, we point out the significance of study on single-site photocatalysts with a porous structure.