More than 95% (in volume) of all of today’s chemical products are manufactured through catalytic processes, making research into more efficient catalytic materials a thrilling and very dynamic rese...

Metal–Organic Frameworks in Heterogeneous Catalysis: Recent Progress, New Trends, and Future Perspectives

Metal-organic frameworks (MOFs) are a class of distinctive porous crystalline materials constructed by metal ions/clusters and organic linkers. Owing to their structural diversity, functional adjustability, and high surface area, different types of MOF-based single metal sites are well exploited, including coordinately unsaturated metal sites from metal nodes and metallolinkers, as well as active metal species immobilized to MOFs. Furthermore, controllable thermal transformation of MOFs can upgrade them to nanomaterials functionalized with active single-atom catalysts (SACs). These unique features of MOFs and their derivatives enable them to serve as a highly versatile platform for catalysis, which has actually been becoming a rapidly developing interdisciplinary research area. In this review, we overview the recent developments of catalysis at single metal sites in MOF-based materials with emphasis on their structures and applications for thermocatalysis, electrocatalysis, and photocatalysis. We also compare the results and summarize the major insights gained from the works in this review, providing the challenges and prospects in this emerging field.

Metal-Organic Framework-Based Catalysts with Single Metal Sites.

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.

Single-Atom Catalysts across the Periodic Table.

As emerging crystalline porous organic-inorganic hybrid materials, metal-organic frameworks (MOFs) have been widely used as sacrificial precursors for the synthesis of carbon materials, metal/metal compounds, and their composites with tunable and controllable nanostructures and chemical compositions for electrochemical energy applications. Herein, recent progress of MOF-derived nanomaterials for various electrochemical energy storage and conversion applications including Li-ion batteries, Li-S batteries, Na-ion batteries, supercapacitors, water splitting, and oxygen reduction reaction is reviewed. Structural and compositional design of MOF-derived nanomaterials is systematically summarized, which may hopefully offer inspirations and guidances for future development of MOF-derived nanomaterials for more efficient and more durable electrochemical energy applications.

Metal-organic framework-derived materials for electrochemical energy applications

Single‐Atom Catalysts for Electrocatalytic Applications

Electrochemical conversion of CO2 and H2O into syngas is an attractive route to utilize green electricity. A competitive system economy demands development of cost-effective electrocatalyst with dual active sites for CO2 reduction reaction (CO2RR) and hydrogen evolution reaction (HER). Here, a single atom electrocatalyst derived from a metal-organic framework is proposed, in which Co single atoms and N functional groups function as atomic CO2RR and HER active sites, respectively. The synthesis method is based on pyrolysis of ZnO@ZIF (zeolitic imidazolate framework). The excess in situ Zn evaporation effectively prevents Co single atoms (≈3.4 wt%) from aggregation and maintains appropriate Co/N ratio. The as-prepared electrocatalyst is featured with high graphitic degree of carbon support for rapid electron transport and sponge-like thin carbon shells with hierarchical pore system for facilitating active site exposure and mass transport. Therefore, the electrocatalyst exhibits a nearly 100% Faradic efficiency and a high formation rate of ≈425 mmol g-1 h-1 at 1.0 V with the gaseous product ratio (CO/H2) approximating ideal 1/2. With the assistance of an extensive material characterization and density functional theory (DFT) calculations, it is identified that Co single atoms are uniformly coordinated in the form of Co-C2N2 moieties, and act as the major catalytic sites for CO2 reduction.

https://onlinelibrary.wiley.com/doi/pdf/10.1002/advs.201800177

Bifunctional Nitrogen and Cobalt Codoped Hollow Carbon for Electrochemical Syngas Production

General Dimension-Controlled Synthesis of Hollow Carbon Embedded with Metal Singe Atoms or Core–Shell Nanoparticles for Energy Storage Applications

Graphitic carbon nitride (g-C3N4) nanosheets/titanium dioxide (TiO2) nanoparticles heterostructures have been in situ synthesized via a modified sol–gel method combined with a calcination process. The prepared g-C3N4/TiO2 heterostructured composites exhibited excellent photocatalytic hydrogen generation from water splitting under visible light irradiation. It was found that the TiO2 nanoparticles are well-dispersed on the g-C3N4 nanosheets. The as-obtained g-C3N4 coupled with TiO2 not only increased the surface area of g-C3N4, but also promoted the separation of photo-generated charge carriers. The developed composite exhibits an excellent hydrogen evolution rate of 40 μmol h−1, which is about 2.7 times higher than that of pure g-C3N4 nanosheets.

/pdf/in-situ-synthesis-of-g-c3n4-tio2-heterostructures-with-151d99hayy.pdf

In situ synthesis of g-C3N4/TiO2 heterostructures with enhanced photocatalytic hydrogen evolution under visible light

A hierarchically porous ternary Au/ZnO@ZIF-8 nanocomposite with flower-like morphology has been prepared using a new in situ encapsulation approach. In particular, Au nanoparticles (NPs) were spatially encapsulated from the ZnO surface into the ZIF-8 crystal matrix during its nucleation process. The ternary composite exhibited high catalytic activity for the reduction of p-nitrophenol.

Hierarchically porous ternary Au/ZnO@ZIF-8 nanocomposite: spatial in situ Au encapsulation and catalytic activity for the reduction of p-nitrophenol

The invention discloses a mono-dispersed nitrogen doped hollow carbon nano polyhedron and a preparation method thereof. The mono-dispersed nitrogen doped hollow carbon nano polyhedron takes a ZnO nano-sphere as a template and a Zn source and 2-methylimidazole as organic ligands to synthesize a core-shell-structure ZnO@ZIF-8 nano mono-crystal composite material; the core-shell-structure ZnO@ZIF-8 nano mono-crystal composite material is put into a high-temperature furnace and is calcined in an inert gas atmosphere for 2h to 4h, wherein the calcination temperature is 800 DEG C to 900 DEG C; after calcination, a carbon nano material with a hollow structure is directly obtained. A preparation process disclosed by the invention is simple, and a template removing process is not adopted. The carbon nano material with the hollow structure, which is prepared by the method disclosed by the invention, has a uniform polyhedron shape, a high specific surface area and high nitrogen content, and has a great application potential in the fields including electrochemical energy storage, catalysis, fuel batteries and the like.

Xin Cao

Papers

Bifunctional Nitrogen and Cobalt Codoped Hollow Carbon for Electrochemical Syngas Production

General Dimension-Controlled Synthesis of Hollow Carbon Embedded with Metal Singe Atoms or Core–Shell Nanoparticles for Energy Storage Applications

In situ synthesis of g-C3N4/TiO2 heterostructures with enhanced photocatalytic hydrogen evolution under visible light

Hierarchically porous ternary Au/ZnO@ZIF-8 nanocomposite: spatial in situ Au encapsulation and catalytic activity for the reduction of p-nitrophenol

Mono-dispersed nitrogen doped hollow carbon nano polyhedron and preparation method thereof