Universal strategy for homogeneously doping noble metals into cyano-bridged coordination polymers

Advanced Strategies for Stabilizing Single-Atom Catalysts for Energy Storage and Conversion

Well-defined atomically dispersed metal catalysts (or single-atom catalysts) have been widely studied to fundamentally understand their catalytic mechanisms, improve the catalytic efficiency, increase the abundance of active components, enhance the catalyst utilization, and develop cost-effective catalysts to effectively reduce the usage of noble metals. Such single-atom catalysts have relatively higher selectivity and catalytic activity with maximum atom utilization due to their unique characteristics of high metal dispersion and a low-coordination environment. However, freestanding single atoms are thermodynamically unstable, such that during synthesis and catalytic reactions, they inevitably tend to agglomerate to reduce the system energy associated with their large surface areas. Therefore, developing innovative strategies to stabilize single-atom catalysts, including mass-separated soft landing, one-pot pyrolysis, co-precipitation, impregnation, atomic layer deposition, and organometallic complexation, is critically needed. Many types of supporting materials, including polymers, have been commonly used to stabilize single atoms in these fabrication techniques. Herein, we review the stabilization strategies of single-atom catalyst, including different synthesis methods, specific metals and carriers, specific catalytic reactions, and their advantages and disadvantages. In particular, this review focuses on the application of polymers in the synthesis and stabilization of single-atom catalysts, including their functions as carriers for metal single atoms, synthetic templates, encapsulation agents, and protection agents during the fabrication process. The technical challenges that are currently faced by single-atom catalysts are summarized, and perspectives related to future research directions including catalytic mechanisms, enhancement of the catalyst loading content, and large-scale implementation are proposed to realize their practical applications.

Graphical Abstract

Single-atom catalysts are characterized by high metal dispersibility, weak coordination environments, high catalytic activity and selectivity, and the highest atom utilization. However, due to the free energy of the large surface area, individual atoms are usually unstable and are prone to agglomeration during synthesis and catalytic reactions. Therefore, researchers have developed innovative strategies, such as soft sedimentation, one-pot pyrolysis, coprecipitation, impregnation, step reduction, atomic layer precipitation, and organometallic complexation, to stabilize single-atom catalysts in practical applications. This article summarizes the stabilization strategies for single-atom catalysts from the aspects of their synthesis methods, metal and support types, catalytic reaction types, and its advantages and disadvantages. The focus is on the application of polymers in the preparation and stabilization of single-atom catalysts, including metal single-atom carriers, synthetic templates, encapsulation agents, and the role of polymers as protection agents in the manufacturing process. The main feature of polymers and polymer-derived materials is that they usually contain abundant heteroatoms, such as N, that possess lone-pair electrons. These lone-pair electrons can anchor the single metal atom through strong coordination interactions. The coordination environment of the lone-pair electrons can facilitate the formation of single-atom catalysts because they can enlarge the average distance of a single precursor adsorbed on the polymer matrix. Polymers with nitrogen groups are favorable candidates for dispersing active single atoms by weakening the tendency of metal aggregation and redistributing the charge densities around single atoms to enhance the catalytic performance. This review provides a summary and analysis of the current technical challenges faced by single-atom catalysts and future research directions, such as the catalytic mechanism of single-atom catalysts, sufficiently high loading, and large-scale implementation.

Highly dispersed Ag nanoparticles in situ creating rich cyano defects in carbon nitride for efficient photocatalytic H2 production

In the obtained Ag–C–CN photocatalyst, the cyano defects act as charge transfer channels to promote electron transfer, making Ag nanoparticles more active as a HER cocatalyst.

Synthesis of hierarchical porous Prussian blue analogues in partially miscible ionic liquid/ethanol solution near the phase boundary

Hierarchical porous PBAs with excellent catalytic performance were controllably synthesized in a partially miscible IL/ethanol system.

Self-assembled single-atom nanozyme for enhanced photodynamic therapy treatment of tumor

Hypoxia of solid tumor compromises the therapeutic outcome of photodynamic therapy (PDT) that relies on localized O₂ molecules to produce highly cytotoxic singlet oxygen (¹O₂) species. Herein, we present a safe and versatile self-assembled PDT nanoagent, i.e., OxgeMCC-r single-atom enzyme (SAE), consisting of single-atom ruthenium as the active catalytic site anchored in a metal-organic framework Mn₃[Co(CN)₆]₂ with encapsulated chlorin e6 (Ce6), which serves as a catalase-like nanozyme for oxygen generation. Coordination-driven self-assembly of organic linkers and metal ions in the presence of a biocompatible polymer generates a nanoscale network that adaptively encapsulates Ce6. The resulted OxgeMCC-r SAE possesses well-defined morphology, uniform size distribution and high loading capacity. When conducting the in situ O₂ generation through the reaction between endogenous H₂O₂ and single-atom Ru species of OxgeMCC-r SAE, the hypoxia in tumor microenvironment is relieved. Our study demonstrates a promising self-assembled nanozyme with highly efficient single-atom catalytic sites for cancer treatment.

Halloysite nanotubes based heterogeneous solid acid catalysts

Taking into account the excellent catalytic performance of halloysite nanotubes, the main focus of this review article is to unveil the research on halloysite nanotubes for the preparation of solid acids and their applications in acid catalysis.

Ir-Doped Ni-based metal–organic framework ultrathin nanosheets on Ni foam for enhanced urea electro-oxidation

NiIr-based metal–organic frameworks grown on a nickel foam substrate (NiIr-MOF/NF) are synthesized by a solvothermal method and directly used for urea electro-oxidation.

Gold nanoparticles conjugated to bimetallic manganese(II) and iron(II) Prussian Blue analogues for aptamer-based impedimetric determination of the human epidermal growth factor receptor-2 and living MCF-7 cells

An aptamer-based assay is described for the determination of trace levels of the cancer marker human epidermal growth factor receptor-2 (HER2) and living MCF-7 cells. The method is based on the use of a bimetallic MnFe Prussian blue analogue coupled to gold nanoparticles (MnFePBA@AuNP). Compared to pristine MnFe PBA nanocubes, the series of MnFePBA@AuNP exhibits a core-shell spherical nanostructure, and the shell thickness decreases from 99.9 nm down to 49.3 nm on increasing the fraction of AuNPs. The composite was placed on a gold electrode and incubated with the aptamer solution through electrostatic interaction. Then the modified electrode was employed to detect HER2 and MCF-7 cells using [Fe(CN)₆]^3-/4- as redox probe and displays good responses to both of them. Electrochemical impedance spectroscopy data show that the signal variation between each step during the whole procedure for the HER2 and MCF-7 cells detection can be embodied as the resistance value change between the [Fe(CN)₆]^3-/4- and electrode surface. The assay has a very low detection limit (0.247 pg∙mL^-1) and works in the 0.001-1.0 ng∙mL^-1 HER2 concentration range. It was also used to sense HER2 in MCF-7 cells, and this results in an assay that works within the 500-5 × 10⁴ cell∙mL^-1 cell concentration range and a 36 cell∙mL^-1 detection limit. Furthermore, the aptamer-based assay is selective, acceptably reproducible, stable, and well feasible for the detection of HER2 and living MCF-7 cells in human serum. Graphical abstract Schematic of an electrochemical aptasensor based on the bimetallic MnFe Prussian blue analogue (MnFe PBA) coupling with gold nanoparticles (represented by MnFePBA@AuNPs). It was employed as the aptasensor for human epidermal growth factor receptor-2 (HER2), and living MCF-7 cells.

Assembling Ultrasmall Copper-Doped Ruthenium Oxide Nanocrystals into Hollow Porous Polyhedra: Highly Robust Electrocatalysts for Oxygen Evolution in Acidic Media

Here, a facile and novel strategy for the preparation of Cu-doped RuO2 hollow porous polyhedra composed of ultrasmall nanocrystals through one-step annealing of a Ru-exchanged Cu-BTC derivative is reported. Owing to the optimized surface configuration and altered electronic structure, the prepared catalyst displays a remarkable oxygen evolution reaction (OER) performance with low overpotential of 188 mV at 10 mA cm-2 in acidic electrolyte, an ultralow Tafel slope of 43.96 mV dec-1 , and excellent stability in durability testing for 10 000 cycles, and continuous testing of 8 h at a current density of 10 mA cm-2 . Density functional theory calculations reveal that the highly unsaturated Ru sites on the high-index facets can be oxidized gradually and reduce the energy barrier of rate-determining steps. On the other hand, the Cu dopants can alter the electronic structures so as to further improve the intrinsic OER activity.

Tuning the Activity of Carbon for Electrocatalytic Hydrogen Evolution via an Iridium-Cobalt Alloy Core Encapsulated in Nitrogen-Doped Carbon Cages

Graphene, a 2D material consisting of a single layer of sp² -hybridized carbon, exhibits inert activity as an electrocatalyst, while the incorporation of heteroatoms (such as N) into the framework can tune its electronic properties. Because of the different electronegativity between N and C atoms, electrons will transfer from C to N in N-doped graphene nanosheets, changing inert C atoms adjacent to the N-dopants into active sites. Notwithstanding the achieved progress, its intrinsic activity in acidic media is still far from Pt/C. Here, a facile annealing strategy is adopted for Ir-doped metal-organic frameworks to synthesize IrCo nanoalloys encapsulated in N-doped graphene layers. The highly active electrocatalyst, with remarkably reduced Ir loading (1.56 wt%), achieves an ultralow Tafel slope of 23 mV dec^-1 and an overpotential of only 24 mV at a current density of 10 mA cm^-2 in 0.5 m sulfuric acid solution. Such superior performance is even superior to the noble-metal catalyst Pt. Surface structural and computational studies reveal that the superior behavior originates from the decreased ΔG_H* for HER induced by the electrons transferred from the alloy core to the graphene layers, which is beneficial for enhancing CH binding.

Design of 3d–4f molecular squares through the [Fe{(HB(pz)3)}(CN)3]− metalloligand

A new series of cyanido-bridged {FeIII2LnIII2} heterobimetallic molecular squares [Ln = La (1), Gd (2), Tb (3) and Dy (4)] has been prepared and magneto-structurally investigated.

Porous Nanobimetallic Fe-Mn Cubes with High Valent Mn and Highly Efficient Removal of Arsenic(III)

Iron (Fe) oxides are the most commonly used adsorbent materials for the aqueous removal of Arsenic (As), but they have deficiencies, including low uptake and poor removal of the relatively higher toxicity As(III). Introduction of transition metals into Fe-containing adsorbents, an inexpensive method of altering Fe chemical states, is likewise of interest for removing As(III) from water by means of adsorption. Porous cubic Fe-Mn structures with BET surface area of 450 m² g^-1 were herein prepared via chemical etching of Mn-substituted Prussian Blue analogues (PBAs). Cyclic voltammetry showed a "protective" role of polyvinylpyrrolidone (PVP) during the preparation process, so that Mn with high valence state was readily preserved in this binuclear corner-sharing structure. The calculated reaction Gibbs free energy, which was the most negative of the studied adsorbents, indicated that the adsorption was promoted in the presence of high valence Mn. The structures were capable of directly capturing As(III) oxyanions to below the acceptable limits in drinking water standards, and the saturation uptake capacity of 460 mg g^-1 was substantially higher than comparable materials under consideration. The work therefore presents a new benchmark for As-adsorbent materials and demonstrates the promise of the porous Fe-Mn structure for rapid removal of other metal ions from contaminated water for environmental remediation.

Ruthenium-cobalt nanoalloys encapsulated in nitrogen-doped graphene as active electrocatalysts for producing hydrogen in alkaline media

The scalable production of hydrogen could conveniently be realized by alkaline water electrolysis. Currently, the major challenge confronting hydrogen evolution reaction (HER) is lacking inexpensive alternatives to platinum-based electrocatalysts. Here we report a high-efficient and stable electrocatalyst composed of ruthenium and cobalt bimetallic nanoalloy encapsulated in nitrogen-doped graphene layers. The catalysts display remarkable performance with low overpotentials of only 28 and 218 mV at 10 and 100 mA cm⁻², respectively, and excellent stability of 10,000 cycles. Ruthenium is the cheapest platinum-group metal and its amount in the catalyst is only 3.58 wt.%, showing the catalyst high activity at a very competitive price. Density functional theory calculations reveal that the introduction of ruthenium atoms into cobalt core can improve the efficiency of electron transfer from alloy core to graphene shell, beneficial for enhancing carbon–hydrogen bond, thereby lowing ΔG_H* of HER.

Ruthenium is the cheapest platinum-group metal, yet active hydrogen evolution catalysts with low amounts of ruthenium have yet to be designed. Here, the authors report the preparation of a ruthenium–cobalt nanoalloy and demonstrate its potential as an effective hydrogen evolution catalyst in basic media.

Enhanced Activity for Hydrogen Evolution Reaction over CoFe Catalysts by Alloying with Small Amount of Pt

The hydrogen evolution reaction highly relied on Pt electrocatalysts, with high activity and stability. In the past few years, a host of efforts have been made in the development of novel platinum nanostructures with a low amount of Pt because the scarcity and high price of Pt hinder its practical applications. Here, we report the preparation of PtCoFe@CN electrocatalysts with a remarkably reduced Pt loading amount of 4.60% by annealing Pt-doped metal-organic frameworks (MOFs). The electrocatalyst demonstrated an outstanding performance with only 45 mV overpotential to achieve the 10 mA cm^-2 current density, which is quite close to that of the commercial 20% Pt/C catalyst. The enhanced catalytic capability is originated from the modification of the electronic structures of CoFe by alloying with Pt. The results indicate that robust and superstable alloy electrocatalysts which contain a very small amount of noble metal could be prepared by annealing noble metal-doped MOFs.

Photoluminescence distinction of lung adenocarcinoma cells A549 and squamous cells H520 using metallothionein expression in response to Cd-doped Mn3[Co(CN)6]2 nanocubes

Luminescent Mn₃[Co(CN)₆]₂ nanocubes doped with Cd²⁺ can induce human lung adenocarcinoma cells A549 generating more metallothioneins (MTs) than squamous cells H520.