Synthesis, Characterization, and Application of Metal Nanoparticles Supported on Nitrogen-Doped Carbon: Catalysis beyond Electrochemistry

Ultra-small cobalt nanoparticles from molecularly-defined Co-salen complexes for catalytic synthesis of amines

We report the synthesis of in situ generated cobalt nanoparticles from molecularly defined complexes as efficient and selective catalysts for reductive amination reactions. In the presence of ammonia and hydrogen, cobalt–salen complexes such as cobalt(ii)–N,N′-bis(salicylidene)-1,2-phenylenediamine produce ultra-small (2–4 nm) cobalt-nanoparticles embedded in a carbon–nitrogen framework. The resulting materials constitute stable, reusable and magnetically separable catalysts, which enable the synthesis of linear and branched benzylic, heterocyclic and aliphatic primary amines from carbonyl compounds and ammonia. The isolated nanoparticles also represent excellent catalysts for the synthesis of primary, secondary as well as tertiary amines including biologically relevant N-methyl amines.

We report the synthesis of in situ generated cobalt nanoparticles from molecularly defined complexes as efficient and selective catalysts for reductive amination reactions.

Flexibility Induced Encapsulation of Ultrafine Palladium Nanoparticles into Organic Cages for Tsuji-Trost Allylation

A series of three positional isomers of organic cages namely o-OC, m-OC, and p-OC, have been self-assembled using dynamic covalent chemistry. Their room temperature controlled fabrication with palladium gives ultrafine diameter (1-2 nm) of palladium nanoparticles (Pd NPs). We observed that the shape-flexibility of cages have great impact on the formation of Pd NPs. Theoretical calculations reveals that theoretically obtainable size of Pd NPs for each cage which was complementary to the experimental results. Theoretical studies indicate that the driving forces for the specific orientational preference may be ascribed to subtle variations on the level of π-π interactions, which ultimately governs the growth of Pd NPs therein. It is the first example of shape-flexible synthesis of organic cages where flexibility governs the nanoparticle growth. Pd NPs have shown excellent catalysis of Tsuji-Trost allylation at room temperature and pressure in water.

Carbon-Supported Raney Nickel Catalyst for Acetone Hydrogenation with High Selectivity

Catalysts with high selectivity play key roles in green chemistry. In this work, a granular Raney Ni catalyst using carbon as support (Raney Ni/C) was developed by mixing phenolic resin with Ni-Al alloy, conducting carbonization at high temperature, and leaching with alkaline liquor. The as-prepared Raney Ni/C catalyst is suitable for use in fix-bed reactors. Moreover, it shows high activity and selectivity for catalytic acetone hydrogenation. For instance, at the reaction temperature of 120 °C, the conversion of acetone can reach up to 99.9% and the main byproduct methyl isobutylcarbinol (MIBC) content can be diminished to 0.02 wt%. The Raney Ni/C may represent a new type of shaped Raney metal catalysts, which are important fix-bed catalysts in chemical industry.

Oxidase-Inspired Selective 2e/4e Reduction of Oxygen on Electron-Deficient Cu

Development of low-cost and efficient (electro)catalysts with tunable 2e/4e oxygen reduction reaction (ORR) selectivity toward energy conversion, biomimetic catalysis, and biosensing has attracted growing interest. Herein, we reported that carbon nanohybrids with O- or N-coordinated Cu (Cu-OC or Cu-NC) showed superior activity for 2e and 4e electrocatalytic ORR with selectivities of 84.0% and 97.2%, respectively. Experimental evidence demonstrated that the strong electron-rich O-doped carbon in Cu-OC donated electrons to Cu²⁺, weakening the binding strength of H₂O₂ at Cu-O centers and facilitating the 2e ORR pathway for selective production of H₂O₂. However, the poor electron-donor ability of the N-doped carbon in Cu-NC made Cu-N sites more electron deficient due to the reduced electron transfer from N-doped carbon to Cu²⁺, promoting 4e ORR by enhancing adsorption of O₂ and the ORR intermediates. The high 4e ORR activity of Cu-NC rendered its potential for application in a Zn-air battery and oxidase-mimicking activity for 3,3',5,5'-tetramethylbenzidine (TMB) and ascorbic acid (AA) oxidation. The maximal velocity (V_max) of TMB and AA oxidation over Cu-NC was higher than some natural oxidases and noble-metal-based artificial enzymes. The lower activation energy for AA oxidation over Cu-NC resulted in a 263-fold higher oxidative rate than TMB, further prompting nonenzymatic sensing of AA by the competitive oxidation strategy.

Catalytically-active porous assembly with dynamic pulsating motion for efficient exchange of products and reagents

Despite recent advances in the use of porous materials as efficient heterogeneous catalysts which operate through effectively trapping reagents in a well-defined space, continuously uptaking reagents to substitute products in the cavity for efficient product turnover still remains challenging. Here, a porous catalyst is endowed with 'breathing' characteristics by thermal stimulus, which can enable the efficient exchange of reagents and products through reversible stacking from inflated aromatic hexamers to contracted trimeric macrocycles. The contracted super-hydrophobic tubular interior with pyridine environment exhibits catalytic activity towards a nucleophilic aromatic substitution reaction by promoting interactions between concentrated reagents and active sites. Subsequent expansion facilitates the exchange of products and reagents, which ensures the next reaction. The strategy of mesoporous modification with inflatable transition may provide a new insight for construction of dynamic catalysts.

Cross-Linked Cyclodextrins Bimetallic Nanocatalysts: Applications in Microwave-Assisted Reductive Aminations

The optimization of sustainable protocols for reductive amination has been a lingering challenge in green synthesis. In this context, a comparative study of different metal-loaded cross-linked cyclodextrins (CDs) were examined for the microwave (MW)-assisted reductive amination of aldehydes and ketones using either H₂ or formic acid as a hydrogen source. The Pd/Cu heterogeneous nanocatalyst based on Pd (II) and Cu (I) salts embedded in a β-CD network was the most efficient in terms of yield and selectivity attained. In addition, the polymeric cross-linking avoided metal leaching, thus enhancing the process sustainability; good yields were realized using benzylamine under H₂. These interesting findings were then applied to the MW-assisted one-pot synthesis of secondary amines via a tandem reductive amination of benzaldehyde with nitroaromatics under H₂ pressure. The formation of a Cu_xPd_y alloy under reaction conditions was discerned, and a synergic effect due to the cooperation between Cu and Pd has been hypothesized. During the reaction, the system worked as a bifunctional nanocatalyst wherein the Pd sites facilitate the reduction of nitro compounds, while the Cu species promote the subsequent imine hydrogenation affording structurally diverse secondary amines with high yields.

Ultrafine Rh nanoparticles confined by nitrogen-rich covalent organic frameworks for methanolysis of ammonia borane

An Rh/PC-COF was synthesized using a metal–nitrogen coordination reduction strategy and was applied as a highly efficient catalyst for methanolysis of ammonia borane.

Water-assisted one-pot synthesis of N-doped carbon supported Ru catalysts for heterogeneous catalysis

For the first time, a simple yet efficient water-assisted one-pot pyrolysis (WAOP) strategy was developed to in situ liberate the inaccessible Ru active sites confined inside N-doped carbon. The liberated Ru/CN catalysts exhibit a 9-fold improvement in catalytic activity for quinoline hydrogenation compared with catalysts obtained from the water-free pyrolysis process, and high tolerance for selective hydrogenation of various quinolines substituted with different functional groups. We anticipate that WAOP addresses a key issue that currently plagues carbon-based catalyst synthesis and should lead to improvements in fields as diverse as chemical production and environmental protection.

Catalytic reductive aminations using molecular hydrogen for synthesis of different kinds of amines

Catalytic reductive aminations using molecular hydrogen represent an essential and widely used methodology for the synthesis of different kinds of amines.

Room-Temperature Sustainable Synthesis of Selected Platinum Group Metal (PGM = Ir, Rh, and Ru) Nanocatalysts Well-Dispersed on Porous Carbon for Efficient Hydrogen Evolution and Oxidation

Electroless deposition via a spontaneous redox reaction between the metal precursor and support is believed to be a promising approach for the syntheses of supported metal nanoparticles (SMNPs). However, its widespread applications are significantly prohibited by the low reductivity and high cost of support. To overcome these shortcomings, a porous carbon (PC) is herein developed as a promising matrix for the electroless deposition of metal NPs. Benefiting from abundant oxygen-based surface functional groups, the PC shows stronger reducibility (low redox potential) than conventional carbon substrate such as carbon nanotubes or graphene oxide, enabling a facile electroless deposition of Ir, Rh, and Ru NPs on its surface. These SMNPs exhibit an impressive electrocatalytic activity for the hydrogen evolution reaction (HER) or hydrogen oxidation reaction (HOR). For example, the Rh NP/PC can deliver an HER current density of 10 mA cm^-2 with a small overpotential of 21 mV in 0.5 m H₂ SO₄ , while the Ru NP/PC exhibits excellent HOR activity in 0.1 m KOH in terms of high mass and surface specific exchange current density of 263 A g^-1 _Ru and 0.227 mA cm^-2 _Ru . The present strategy may open up opportunities for mass production of efficient supported NPs for diverse applications.

Integrated textile sensor patch for real-time and multiplex sweat analysis

Wearable sweat analysis devices for monitoring of multiple health-related biomarkers with high sensitivity are highly desired for noninvasive and real-time monitoring of human health. Here, we report a flexible sweat analysis patch based on a silk fabric-derived carbon textile for simultaneous detection of six health-related biomarkers. The intrinsically N-doped graphitic structure and the hierarchical woven, porous structure provided the carbon textile good electrical conductivity, rich active sites, and good water wettability for efficient electron transmission and abundant access to reactants, enabling it to serve as an excellent working electrode in electrochemical sensors. On the basis of the above, we fabricated a multiplex sweat analysis patch that is capable of simultaneous detection of glucose, lactate, ascorbic acid, uric acid, Na⁺, and K⁺. The integration of selective detectors with signal collection and transmission components in this device has enabled us to realize real-time analysis of sweat.

Amphiphilic Mesoporous Sandwich-Structured Catalysts for Selective Hydrogenation of 4-Nitrostyrene in Water

Selective catalytic hydrogenation of substituted nitro compounds (NCs) of hydrophobic nature in aqueous solution using transition-metal-based catalysts is highly desirable yet fairly challenging. Herein, we propose the idea of amphiphilic mesoporous catalysts for selective hydrogenation of hydrophobic NCs in aqueous solution. The amphiphilic catalyst Co@Co-N-C@SBA-15 with a sandwich-like structure is constructed by a one-step solvent-free melting coating method. The catalyst has an external hydrophilic silica support that facilitates catalyst dispersion in water. It has unique Co-N-C catalytic layers uniformly coated in the inner mesopore surfaces of the silica support, which enhance the selective adsorption and activation of hydrophobic NCs. It has a high surface area (448.2 m²/g) and a uniform mesopore size (∼7.0 nm) for fast mass transportation. It possesses ultrafine metallic Co nanoparticles uniformly anchored within the N-doped carbon (N-C) layers for easy magnetic separation. These features make the catalyst excellent for the selective hydrogenation of 4-nitrostyrene to form 4-aminostyrene, with a high conversion of 98.0% in 1.0 h, a superior selectivity of 98.8%, and a good stability under mild conditions. A comprehensive study confirms the excellence of the amphiphilic mesoporous catalysts compared with other control catalysts. The Co-N sites are the intrinsic active sites. They can selectively adsorb and activate the nitro groups other than the vinyl groups, leading to superior selectivity. Water as the solvent results in the best performance compared with typical organic solvents probably because of an enhanced water-mediated hydrogen spillover and transfer.

Eco-Friendly, Direct Deposition of Metal Nanoparticles on Graphite for Electrochemical Energy Conversion and Storage

Simple, green, and energy-efficient methods for preparing electroactive materials used to generate and store renewable energy are important for a sustainable future. In this study, we showed that noble and certain non-noble metal nanoparticles can be deposited on graphite without the aid of any reducing agent. This method of reducing metal ions to metal nanoparticles by graphite involves only one step (i.e., immersion into a solution) and one chemical (i.e., a metal salt). Hence, the method is exceedingly simple, green, and does not require any energy input. Large amounts of metal nanoparticles are generated both on the surface and deep into the bulk of graphite (∼100 μm). Despite the simplicity of this method, the metal deposited on graphite showed good electrocatalytic performance for ethanol oxidation and oxygen evolution reactions and also functioned as electrodes for supercapacitors. This method is thus ideal for preparing electrocatalytic materials and electrochemical energy storage devices due to its simplicity and environmental sustainability. The simplicity of the method is due to the inherent reducing potential of graphite (i.e., a material that is generally perceived as inert). Results from analyses showed that functionalization of the reactive edges in the regions of defects allowed the graphite to serve as a reducing agent. Increasing the amount of defects (e.g., via chemical or simple mechanical treatments) is shown to be the fundamental principle for increasing the reactivity of graphite.

0.2 V Electrolysis Voltage-Driven Alkaline Hydrogen Production with Nitrogen-Doped Carbon Nanobowl-Supported Ultrafine Rh Nanoparticles of 1.4 nm

The development of highly effective and low-cost electrocatalysts for energy-saving hydrogen production via water splitting is still a great challenge. Herein, porous nitrogen-doped carbon nanobowls (N-CBs) have been designed and used for the controlled growth of ultrafine rhodium (Rh) nanoparticles. With the aid of interfacial bonding of Rh and N, ultrafine Rh nanoparticles with an average size of 1.4 nm have been successfully immobilized on the N-CBs. This Rh/N-CB electrocatalyst shows superior activity and high stability for the hydrogen evolution reaction (HER) and the hydrazine oxidation reaction (HzOR). More importantly, the Rh/N-CBs exhibit high activity for hydrogen production from water electrolysis, marking with a cell voltage of 0.2 V to achieve a current density of 20 mA cm^-2 when they serve as cathodic electrocatalysts for the HER and anodic electrocatalysts for the HzOR in 1 M KOH with 0.5 M hydrazine. The density functional theory calculations demonstrate that a near-zero hydrogen adsorption free energy produced by the chemical bonding of Rh with the pyrrole-N doped in N-CBs is responsible for the excellent HER activity of Rh/N-CBs electrocatalysts.

Organosulfides: An Emerging Class of Cathode Materials for Rechargeable Lithium Batteries

Lithium-ion batteries have received significant attention over the last decades due to the wide application of portable electronics and increasing deployment of electric vehicles. In order to further increase the energy density of batteries and overcome the capacity limitations (<250 mAh g^-1) of inorganic cathode materials, it is imperative to explore new cathode materials for rechargeable lithium batteries. Organic compounds including organic carbonyl, radicals, and organosulfides are promising as they have advantages of high capacities, abundant resources, and tunable structures. In the 1980s, a few organosulfides, in particular organodisulfides, as cathode materials were studied to a certain extent in rechargeable lithium batteries. However, they showed low capacities and poor cycling performance, which made them unappealing then in comparison to transition metal oxide cathode materials. As a result, organosulfides have not been extensively studied like other cathode materials including organic carbonyls and radicals. In recent years, organosulfides with long sulfur chains (e.g., trisulfide, tetrasulfide, pentasulfide, etc.) in the structures have been receiving more attention in conjunction with the development of lithium-sulfur batteries. As a major class of sulfur derivatives, they have versatile structures and unique properties in comparison with elemental sulfur. In this Account, we first generalize the working principles of organosulfides in lithium batteries. We then discuss organosulfide molecules, which have precise lithiation sites and tunable capacities. The organic functional groups can provide additional benefits, such as discharge voltage and energy efficiency enhancement by phenyl groups and cycling stability improvement by N-heterocycles. Furthermore, replacing sulfur by selenium in these compounds can improve their electrochemical properties due to the high electronic conductivity and low bond energy associated with selenium. We list organosulfide polymers consisting of phenyl rings, N-heterocycles, or aliphatic segments. Organosulfides as electrolyte additives or components for forming a solid-electrolyte interphase layer on lithium metal anode are also presented. Carbon materials such as carbon nanotubes and reduced graphene oxide can enhance the battery performance of organosulfide cathodes. We discuss the synthesis methods for polysulfide molecules and polymers. Finally, we show the advantages of organosulfides over sulfur as cathode materials in lithium batteries. This Account provides a summary of recent development, in-depth analysis of structure-performance relationship, and guidance for future development of organosulfides as promising cathode materials for next generation rechargeable lithium batteries.

Fabrication of a Spherical Superstructure of Carbon Nanorods

Hierarchical superstructures in nano/microsize have attracted great attention owing to their wide potential applications. Herein, a self-templated strategy is presented for the synthesis of a spherical superstructure of carbon nanorods (SS-CNR) in micrometers through the morphology-preserved thermal transformation of a spherical superstructure of metal-organic framework nanorods (SS-MOFNR). The self-ordered SS-MOFNR with a chestnut-shell-like superstructure composed of 1D MOF nanorods on the shell is synthesized by a hydrothermal transformation process from crystalline MOF nanoparticles. After carbonization in argon, the hierarchical SS-MOFNR transforms into SS-CNR, which preserves the original chestnut-shell-like superstructure with 1D porous carbon nanorods on the shell. Taking the advantage of this functional superstructure, SS-CNR immobilized with ultrafine palladium (Pd) nanoparticles (Pd@SS-CNR) exhibits excellent catalytic activity for formic acid dehydrogenation. This synthetic strategy provides a facile method to synthesize uniform spherical superstructures constructed from 1D MOF nanorods or carbon nanorods for applications in catalysis and energy storage.

Switching Co/N/C Catalysts for Heterogeneous Catalysis and Electrocatalysis by Controllable Pyrolysis of Cobalt Porphyrin

Identifying the optimal synthetic and structural parameters in preparing pyrolyzed metal/nitrogen/carbon (M/N/C) materials is crucial for developing effective catalysts for many important catalytic processes. Here we report a group of mesoporous Co/N/C catalysts ranging from polymerized cobalt porphyrin to Co/N-doped carbons, which are prepared by pyrolysis of cobalt porphyrin using silica nanoparticles as templates at different temperatures, for boosting both heterogeneous catalysis and electrocatalysis. It is revealed that the polymerized cobalt porphyrin prepared at low temperature (500°C) is a polymer-like network with exclusive single-atom Co-N_x sites, and that the high-temperature-pyrolysis (>600°C) produces an electrically conductive Co/N-doped carbon, accompanied by part degradation of Co-N_x centers. We identify that the polymerized cobalt porphyrin with undecomposed Co-N_x centers is optimal for heterogeneous catalytic oxidation of ethylbenzene, whereas the electrically conductive Co/N-doped carbon is ideal for eletrocatalytic oxygen reduction. Our results provide new insights for rationally optimizing M/N/C catalysts for different reactions.

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A group of mesoporous Co/N/C catalysts have been successfully prepared

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HAADF-STEM and XAFS reveal structural and componential information of the catalysts

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The difference in optimizing Co/N/C catalysts for different reactions is identified

Alkene hydrosilylation catalyzed by easily assembled Ni(ii)-carboxylate MOFs

We report the first Ni MOF catalysts for anti-Markovnikov hydrosilylation of alkenes. These catalysts are bench-stable and easily-assembled from simple Ni salts and carboxylic acids. The best catalyst gives turnover numbers up to 9500 and is robust even after 10 recycling runs. The catalyst can be applied for the hydrosilylation of a wide range of alkenes, achieving good synthetic utility and functional group tolerance.

Nitrogen-Doped Carbon Nanotube Confined Co-Nx Sites for Selective Hydrogenation of Biomass-Derived Compounds

Biomass is the most abundant renewable resource on earth and developing high-performance nonprecious selective hydrogenation (SH) catalysts will enable the use of biomass to replace rapidly diminishing fossil resources. This work utilizes ZIF-67-derived nitrogen-doped carbon nanotubes to confine Co nanoparticles (NPs) with Co-N_x active sites as a high-performance SH catalyst. The confined Co NPs with Co-N_x exhibit excellent catalytic activity, selectivity, and stability toward a wide range of biomass-derived compounds. Such active sites can selectively hydrogenate aldehyde, ketone, carboxyl, and nitro groups of biomass-derived compounds into value-added fine chemicals with 100% selectivity. The reported approach could be adopted to create other forms of catalytically active sites from other nonprecious metals.