Nickel Carbide Nanoparticle Catalyst for Selective Hydrogenation of Nitriles to Primary Amines.

Despite its unique physicochemical properties, the catalytic application of nickel carbide (Ni3 C) in organic synthesis is rare. In this study, we report well-defined nanocrystalline Ni3 C (nano-Ni3 C) as a highly active catalyst for the selective hydrogenation of nitriles to primary amines. The activity of the aluminum-oxide-supported nano-Ni3 C (nano-Ni3 C/Al2 O3 ) catalyst surpasses that of Ni nanoparticles. Various aromatic and aliphatic nitriles and dinitriles were successfully converted to the corresponding primary amines under mild conditions (1 bar H2 pressure). Furthermore, the nano-Ni3 C/Al2 O3 catalyst was reusable and applicable to gram-scale experiments. Density functional theory calculations suggest the formation of polar hydrogen species on the nano-Ni3 C surface, which were attributed to the high activity of nano-Ni3 C towards nitrile hydrogenation. This study demonstrates the utility of metal carbides as a new class of catalysts for liquid-phase organic reactions.

Synergistic catalytic enhancement of metal-organic framework derived nanoarchitectures decorated on graphene as a high-efficiency bifunctional electrocatalyst for methanol oxidation and oxygen reduction.

Designing highly efficient, long-lasting, and cost-effective cathodic and anodic functional materials as a bifunctional electrocatalyst is essential for overcoming the bottleneck in fuel cell development. Herein, a novel two-step synthesis strategy is developed to synthesize metal-organic framework (MOF) derived nitrogen-doped carbon (NC) with improved spatial isolation and a higher loading amount of cobalt (Co) and nickel carbide (Ni3C) nanocrystal decorated on graphene (denoted as Co@NC-Ni3C/G). Benefiting from multiple active sites of high N-doping level, uniform dispersion of Co and Ni3C nanocrystals, and a large active area of graphene, the Co@NC-Ni3C/G hybrids exhibit excellent methanol oxidation reaction (MOR) and oxygen reduction reaction (ORR) efficiency in an alkaline environment. For MOR, the optimized Co@NC-Ni3C/G-350 catalyst achieved a current density of 44.8 mA cm-2 at an applied potential of 1.47 V (V vs. RHE), which is significantly higher than Co@NC-Ni3C (42.07 mA cm-2) and Co@NC (24.1 mA cm-2) in 0.5 M methanol + 1.0 M KOH solutions. In addition, during the CO retention test, the Co@NC-Ni3C/G-350 catalyst exhibits excellent CO tolerance capacity. Excitingly, the as-prepared Co@NC-Ni3C/G-350 hybrid exhibits significantly improved ORR catalytic efficiency in terms of positive onset and half-wave potential (Eonset = 0.90 V, E1/2 = 0.830 V vs. RHE), small Tafel slope (34 mV dec-1) and excellent durability (only reduced 0.016 V after 5000 s test). This work provides new insights into MOF-derived functional nanomaterials for anode and cathode co-catalysts for methanol fuel cells.

Simple Synthesis and Characterization of Shell-Thickness-Controlled Ni/Ni3C Core-Shell Nanoparticles.

Ni/Ni3C core-shell nanoparticles with an average diameter of approximately 120 nm were carburized via a chemical solution method using triethylene glycol. It was found that over time, the nanoparticles were covered with a thin Ni3C shell measuring approximately 1-4 nm, and each Ni core was composed of poly grains. The saturation magnetization of the core-shell nanopowders decreased in proportion to the amount of Ni3C. The synthesis mechanism of the Ni/Ni3C core-shell nanoparticles was proposed through X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and transmission electron microscopy (TEM) analyses.

A Self-Reconstructed Bifunctional Electrocatalyst of Pseudo-Amorphous Nickel Carbide @ Iron Oxide Network for Seawater Splitting.

Here, a sol-gel method is used to prepare a Prussian blue analogue (NiFe-PBA) precursor with a 2D network, which is further annealed to an Fe3 O4 /NiCx composite (NiFe-PBA-gel-cal), inheriting the ultrahigh specific surface area of the parent structure. When the composite is used as both anode and cathode catalyst for overall water splitting, it requires low voltages of 1.57 and 1.66 V to provide a current density of 100 mA cm-2 in alkaline freshwater and simulated seawater, respectively, exhibiting no obvious attenuation over a 50 h test. Operando Raman spectroscopy and X-ray photoelectron spectroscopy indicate that NiOOH2-x active species containing high-valence Ni3+ /Ni4+ are in situ generated from NiCx during the water oxidation. Density functional theory calculations combined with ligand field theory reveal that the role of high valence states of Ni is to trigger the production of localized O 2p electron holes, acting as electrophilic centers for the activation of redox reactions for oxygen evolution reaction. After hydrogen evolution reaction, a series of ex situ and in situ investigations indicate the reduction from Fe3+ to Fe2+ and the evolution of Ni(OH)2 are the origin of the high activity.

Simultaneous determination of dihydroxybenzene isomers in cosmetics by synthesis of nitrogen-doped nickel carbide spheres and construction of ultrasensitive electrochemical sensor.

N-doped nickel carbide spheres (N-NiCSs) were synthesised for the first time by controlling the type of surfactant, surfactant-to-Ni molar ratio, reaction temperature, and reaction time. The morphology, composition, and electrochemical behaviour of the synthesised spheres revealed that the spheres presented a large specific surface area, abundant pores, and good conductivity, with excellent electrocatalytic performance. A glassy carbon electrode-modified with N-NiCSs was used for the simultaneous identification of hydroquinone (HQ), catechol (CC), and resorcinol (RS) utilising differential pulse voltammetry. The oxidation peaks of HQ, CC, and RS were observed at 9.8, 119, and 470 mV, respectively (vs. SCE). Under optimal conditions, the oxidation peak currents of HQ, CC, and RS were linear in the concentration ranges of 0.005-100 µM, 0.05-200 µM, and 5-500 µM, respectively. The detection limits of HQ, CC, and RS were 0.00152 µM, 0.015 µM, and 0.24 µM (S/N = 3), respectively. The sensitivities of HQ, CC, and RS were 4.635, 2.069, and 0.985 µA µM-1 cm-2 (S/N = 3), respectively. The fabricated sensor was successfully used to detect HQ, CC, and RS in hair dye, whitening cream, and local tap water samples. Moreover, the sensor presented a good repeatability, reproducibility, and stability during cosmetic testing and a relatively wide linear range, an ultralow detection limit, and an ultrahigh sensitivity.

Self-standing heterostructured NiC x -NiFe-NC/biochar as a highly efficient cathode for lithium-oxygen batteries.

Lithium-oxygen batteries have attracted research attention due to their low cost and high theoretical capacity. Developing inexpensive and highly efficient cathode materials without using noble metal-based catalysts is highly desirable for practical applications in lithium-oxygen batteries. Herein, a heterostructure of NiFe and NiC x inside of N-doped carbon (NiC x -NiFe-NC) derived from bimetallic Prussian blue supported on biochar was developed as a novel self-standing cathode for lithium-oxygen batteries. The specific discharge capacity of the best sample was 27.14 mAh·cm-2 at a stable discharge voltage of 2.75 V. The hybridization between the d-orbital of Ni and s and p-orbitals of carbon in NiC x , formed at 900 °C, enhanced the electrocatalytic performance due to the synergistic effect between these components. The structure of NiC x -NiFe-NC efficiently improved the electron and ion transfer between the cathode and the electrolyte during the electrochemical processes, resulting in superior electrocatalytic properties in lithium-oxygen batteries. This study indicates that nickel carbide supported on N-doped carbon is a promising cathode material for lithium-oxygen batteries.

Selective Methanol-to-Formate Electrocatalytic Conversion on Branched Nickel Carbide.

A methanol economy will be favored by the availability of low-cost catalysts able to selectively oxidize methanol to formate. This selective oxidation would allow extraction of the largest part of the fuel energy while concurrently producing a chemical with even higher commercial value than the fuel itself. Herein, we present a highly active methanol electrooxidation catalyst based on abundant elements and with an optimized structure to simultaneously maximize interaction with the electrolyte and mobility of charge carriers. In situ infrared spectroscopy combined with nuclear magnetic resonance spectroscopy showed that branched nickel carbide particles are the first catalyst determined to have nearly 100 % electrochemical conversion of methanol to formate without generating detectable CO2 as a byproduct. Electrochemical kinetics analysis revealed the optimized reaction conditions and the electrode delivered excellent activities. This work provides a straightforward and cost-efficient way for the conversion of organic small molecules and the first direct evidence of a selective formate reaction pathway.

Synergistic Coupling of Ni Nanoparticles with Ni3 C Nanosheets for Highly Efficient Overall Water Splitting.

Exploring earth-abundant bifunctional electrocatalysts with high efficiency for water electrolysis is extremely demanding and challenging. Herein, density functional theory (DFT) predictions reveal that coupling Ni with Ni3 C can not only facilitate the oxygen evolution reaction (OER) kinetics, but also optimize the hydrogen adsorption and water adsorption energies. Experimentally, a facile strategy is designed to in situ fabricate Ni3 C nanosheets on carbon cloth (CC), and simultaneously couple with Ni nanoparticles, resulting in the formation of an integrated heterostructure catalyst (Ni-Ni3 C/CC). Benefiting from the superior intrinsic activity as well as the abundant active sites, the Ni-Ni3 C/CC electrode demonstrates excellent bifunctional electrocatalytic activities toward the OER and hydrogen evolution reaction (HER), which are superior to all the documented Ni3 C-based electrocatalysts in alkaline electrolytes. Specifically, the Ni-Ni3 C/CC catalyst exhibits the low overpotentials of only 299 mV at the current density of 20 mA cm-2 for the OER and 98 mV at 10 mA cm-2 for the HER in 1 m KOH. Furthermore, the bifunctional Ni-Ni3 C/CC catalyst can propel water electrolysis with excellent activity and nearly 100% faradic efficiency. This work highlights an easy approach for designing and constructing advanced nickel carbide-based catalysts with high activity based on the theoretical predictions.

Atomic Layer Deposition of Nickel Carbide from a Nickel Amidinate Precursor and Hydrogen Plasma.

A new atomic layer deposition (ALD) process for depositing nickel carbide (Ni3C x) thin films is reported, using bis( N, N'-di- tert-butylacetamidinato)nickel(II) and H2 plasma. The process shows a good layer-by-layer film growth behavior with a saturated film growth rate of 0.039 nm/cycle for a fairly wide process temperature window from 75 to 250 °C. Comprehensive material characterizations are performed on the Ni3C x films deposited at 95 °C with various H2 plasma pulse lengths from 5 to 12 s, and no appreciable difference is found with the change of the plasma pulse length. The deposited Ni3C x films are fairly pure, smooth, and conductive, and the x in the nominal formula of Ni3C x is approximately 0.7. The ALD Ni3C x films are polycrystalline with a rhombohedral Ni3C crystal structure, and the films are free of nanocrystalline graphite or amorphous carbon. Last, we demonstrate that, by using this ALD process, highly uniform Ni3C x films can be conformally deposited into deep narrow trenches with an aspect ratio as high as 20:1, which thereby highlights the broad and promising applicability of this process for conformal Ni3C x film coatings on complex high-aspect-ratio 3D architectures in general.

Controlled Growth of a Hierarchical Nickel Carbide "Dandelion" Nanostructure.

We present a new type of highly hierarchical but nonporous nanostructure with a unique "dandelion" morphology. Based on the time evolution of these Ni3 C nanostructures, we suggest a mechanism for their formation. This type of hierarchical nanocrystal, with high accessible specific surface area in a relatively large (ca. 750 nm overall diameter) stable structure, can be valuable in catalysis and related applications.