Direct Visualization of Independent Ta Centers Supported on Two-Dimensional TiO2 Nanosheets

Computational and Experimental Characterization of the Ligand Environment of a Ni-Oxo Catalyst Supported in the Metal-Organic Framework NU-1000

Heterogeneous catalysts exhibit significant changes in composition due to the influence of operating conditions, and these compositional changes can have dramatic effects on catalytic performance. For traditional bulk metal heterogeneous catalysts, relationships between composition and catalytic operating conditions are well documented. However, the influence of operating conditions on the compositions of single-site heterogeneous catalysts remains largely unresolved. To address this, we report a combined computational and experimental characterization of a Ni oxo catalyst under catalytic hydrogenation conditions. Specifically, pair distribution function (PDF) analysis is combined with ab initio thermodynamic modeling to investigate ligand environments present on a Ni oxo cluster supported in the metal-organic framework NU-1000. Comparisons of the experimentally observed and simulated Ni-O coordination numbers and Ni-O, Ni···Ni, and Ni···Zr distances provide insight into the Ni ligand environment under H₂ (g). These comparisons suggest significant OH and H₂O content and, further, that different Ni ions within the cluster and/or NU-1000 structure may comprise subtly different numbers of these ligands. Further, the observation of significant H₂O content under H₂ (g) suggests that the NU-1000 support supplies H₂O to the cluster. Examples of ligand environments that could lead to the observed PDFs are provided. The combination of simulations and experiments provides new insights into the ligand environment for Ni-NU-1000 catalysts that will be useful for understanding the ligand environments of other single-site Ni catalysts as well.

Controlled Synthesis of Perforated Oxide Nanosheets with High Density Nanopores Showing Superior Water Purification Performance

A method for creating genuine nanopores in high area density on monolayer two-dimensional (2D) metallic oxides has been developed. By use of the strong reduction capability of hydroiodic acid, active metal ions, such as Fe^III and Co^III, in 2D oxide nanosheets can be reduced to a divalent charge state (2+). The selective removal of FeO₂ and CoO₂ metal oxide units from the framework can be tuned to produce pores in a range of 1-4 nm. By monitoring of the redox reaction kinetics, the pore area density can be also tuned from ∼0.9 × 10⁴ to ∼3.3 × 10⁵ μm^-2. The universality of this method to produce much smaller pores and higher area density than the previously reported ones has been proven in different oxide nanosheets. To demonstrate their potential applications, ultrasmall metal organic framework particles were grown inside the pores of perforated titania oxide nanosheets. The optimized hybrid film showed ∼100% rejection of methylene blue (MB) from the water. Its water permeance reached 4260 L m^-2 h^-1 bar^-1, which is 1-3 orders of that for reported 2D membranes with good MB rejections.

Techniques for the characterization of single atom catalysts

Single atom catalysts (SACs) are a hot research area recently. Over most of the SACs, the singly dispersed atoms are the active sites, which contribute to the catalytic activities significantly compared with a catalyst with continuously packed active sites. It is essential to determine whether SACs have been successfully synthesized. Several techniques have been applied for the characterization of the dispersion states of the active sites over SACs, such as Energy Dispersive X-ray spectroscopy (EDX), Electron Energy Loss Spectroscopy (EELS), etc. In this review, the techniques for the identification of the singly dispersed sites over SACs are introduced, the advantages and limitations of each technique are pointed out, and the future research directions have been discussed. It is hoped that this review will be helpful for a more comprehensive understanding of the characterization and detection methods involved in SACs, and stimulate and promote the further development of this emerging research field.

Visualization of Transition Metal Decoration on h-BN Surface

Functional h-BN (hexagonal boron nitride) has been prepared via the incorporation of transition metal (TM) impurities like nanoparticles and single atoms. Herein, scanning transmission electron microscopy (STEM) combined with density functional theory (DFT) was employed to study Ta-, Co-, Ni-, and Ir-decorated h-BN monolayers to provide an overview of their preferential site occupancies and morphological evolutions on h-BN. Ta, Ni, Ir, and Co single atoms are all positioned on the nitrogen of h-BN; however DFT predicts the occupancy site can vary with their spin state. In terms of microstructural evolution, Co, Ni, and Ir atoms form 3D nanoclusters while Ta atoms are well dispersed and thus the single Ta atom can be decorated on h-BN. This study highlights on TM/h-BN interaction dynamics and presents an avenue for designing nanostructures for electrocatalytic application.

Single-Atom Catalysts across the Periodic Table

Isolated atoms featuring unique reactivity are at the heart of enzymatic and homogeneous catalysts. In contrast, although the concept has long existed, single-atom heterogeneous catalysts (SACs) have only recently gained prominence. Host materials have similar functions to ligands in homogeneous catalysts, determining the stability, local environment, and electronic properties of isolated atoms and thus providing a platform for tailoring heterogeneous catalysts for targeted applications. Within just a decade, we have witnessed many examples of SACs both disrupting diverse fields of heterogeneous catalysis with their distinctive reactivity and substantially enriching our understanding of molecular processes on surfaces. To date, the term SAC mostly refers to late transition metal-based systems, but numerous examples exist in which isolated atoms of other elements play key catalytic roles. This review provides a compositional encyclopedia of SACs, celebrating the 10th anniversary of the introduction of this term. By defining single-atom catalysis in the broadest sense, we explore the full elemental diversity, joining different areas across the whole periodic table, and discussing historical milestones and recent developments. In particular, we examine the coordination structures and associated properties accessed through distinct single-atom-host combinations and relate them to their main applications in thermo-, electro-, and photocatalysis, revealing trends in element-specific evolution, host design, and uses. Finally, we highlight frontiers in the field, including multimetallic SACs, atom proximity control, and possible applications for multistep and cascade reactions, identifying challenges, and propose directions for future development in this flourishing field.