Formation Of Co Nanoparticles Research Articles

Carbon layer derived carrier transport in Co/g-C3N4 nanosheet junctions for efficient H2O2 production and NO removal

Co nanoparticles are embedded in N-doped carbon nanotubes (Co-N-carbon nanotubes) using combinations of mechano-chemical pre-treatment and thermal polymerization process (at 760 °C). A further thermal polymerization process done on these Co-N-carbon nanotubes and ultrathin graphitic carbon nitrate (g-C3N4) leads to the formation of Co nanoparticles (coated with carbon layers) embedded g-C3N4 nanosheets composite, along with the disappearance of carbon nanotube morphology on the material. The Co/g-C3N4 junctions are constructed with a Schottky barrier formed between the carbon layer coated Co nanoparticles and the g-C3N4 nanosheets. The presence of carbon layers in the composite system facilitates transport of photogenerated electrons at the Fermi level of Co, which results in enhanced photocatalytic H2O2 generation and NO oxidation performances of the composite material in visible light irradiation condition with excellent cyclic stability. The Schottky junction composites constructed using optimized preparation conditions in case of no co-catalyst incorporation shows a photocatalytic H2O2 generation efficiency of 3618 μmolL−1h−1 which is about 1000 times of that of pristine g-C3N4 nanosheets. A NO removal rate of 62 % (higher than that of the commercial P25 (TiO2)) is also observed. These results provide possible approaches on constructing advanced photocatalysts for energy and environmental applications.

Cobalt doping amount determines dominant reactive species in peroxymonosulfate activation via porous carbon catalysts co-doped by cobalt and nitrogen

Switching the reaction routes in peroxymonosulfate (PMS)-based advanced oxidation processes have attracted much attention but remain challenging. Herein, a series of Co-N/C catalysts with different compositions and structures were prepared by using bimetallic zeolitic imidazolate frameworks based on ZIF-8 and ZIF-67 (xZn/Co-ZIFs). Results show that Co doping amount could mediate the transformation of the activation pathway of PMS over Co-N/C. When Co doping amount was less than 10%, the constructed xCo-N/C/PMS system (x ≤ 10%) was singlet oxygen-dominated reaction; however further increasing Co doping amount would lead to the generation and coexistence of sulfate radicals and high-valent cobalt, besides singlet oxygen. Furthermore, the nitrogen-coordinated Co (Co-NX) sites could serve as main catalytically active sites to generate singlet oxygen. While excess Co doping amount caused the formation of Co nanoparticles from which leached Co ions were responsible for the generation of sulfate radicals and high-valent cobalt. Compared to undoped N/C, Co doping could significantly enhance the catalytic performance. The 0.5% Co-N/C could achieve the optimum degradation (0.488 min−1) and mineralization abilities (78.4%) of sulfamethoxazole among the investigated Co-N/C catalysts, which was superior to most of previously reported catalysts. In addition, the application prospects of the two systems in different environmental scenarios (pH, inorganic anions and natural organic matter) were assessed and showed different degradation behaviors. This study provides a strategy to regulate the reactive species in PMS-based advanced oxidation process.

Large-scale synthesis of high loading Co single-atom catalyst with efficient oxidase-like activity for the colorimetric detection of acid phosphatase

Single-atom catalysts (SACs) have shown broad application prospects in different area due to their maximum atomic utilization and excellent catalytic properties. However, how to realize large-scale synthesis of high loading SACs is still a problem for various application requirements. Herein, we report a novel strategy to synthesize Co single-atom catalyst (Co-SAC) by controllable releasing of Co atoms at high temperature, which can be effective to avoid the formation of Co nanoparticles. Particularly, the synthesized Co-SAC with Co content of 7.04 wt% contained exclusive single Co atoms coordinated Co-N4 active sites, demonstrated by inductively coupled plasma optical emission spectroscopy (ICP-OES) and X-ray absorption spectroscopy (XAS). The obtained Co-SAC exhibited excellent intrinsic oxidase-like activity by catalyzing the dissolved oxygen to superoxide radicals. Based on the antioxidant of ascorbic acid (AA) and the enzymatic property of acid phosphatase (ACP), a colorimetric assay for AA and ACP was established by utilizing the oxidase-like activity of Co-SAC. The results indicated that the limit of detection (LOD) of AA and ACP was 1.31 μM and 0.08 U/L, respectively. Considering the large-scale synthetic method and its excellent catalytic property, this kind of Co-SAC would have a good prospect in biosensor fields.

Electrocatalytic reduction of nitrate by in situ generated cobalt nanoparticles.

The cobalt pyridinophane complex [Co(HN4)Cl2]+ (HN4 = 3,7-diaza-1,5(2,6)-dipyridinacyclooctaphane) is converted under catalytic conditions to an electrode-adsorbed species. Aqueous Co2+ solutions similarly deposit a species under these conditions. Surface characterization reveals the formation of Co nanoparticles. These nanoparticles are active in the electrocatalytic redution of aqueous nitrate.

Engineering Active Sites in Three‐Dimensional Hierarchically Porous Graphene‐Like Carbon with Co and N‐Doped Carbon for High‐Performance Zinc‐Air Battery

AbstractThe design of active sites plays an important role in developing highly active oxygen electrocatalysts in Zn‐air batteries (ZnABs). Here, we report the formation of cobalt (Co) nanoparticles and thin graphitic N‐doped carbon (NC) supported on three‐dimensional hierarchically porous graphene‐like carbon (Co‐NC/3DHPGC) to maximize the accessibility of Co‐NC active sites for oxygen reduction/evolution reactions (ORR/OER). The produced Co‐NC/3DHPGC exhibits a broad size distribution (5–30 nm) of Co nanoparticles dispersed on the external surface of 3DHPGC and coated with NC to a thickness of ∼2 nm. We attributed the formation of Co nanoparticles with broad size distribution to the hierarchical porosity of 3DHPGC, which served as a cage to stabilize the Co nanoparticles and increase the metal dispersion; the produced Co nanoparticles catalyze the formation of graphitic NC. Compared with commercial Pt/C and RuO2 catalysts, the resultant Co‐NC/3DHPGC exhibits excellent bifunctional ORR/OER electrocatalytic activity and high durability. The high electrocatalytic performance is ascribed to the accessibility of highly active Co‐NC sites through mesopores of 3DHPGC. The ZnAB assembled with Co‐NC/3DHPGC exhibits high energy density and efficiency. This systematic engineering and rational synthesis strategy may provide new insight into the development of high‐performance oxygen electrocatalysts for metal‐air batteries and fuel cell technology.

Co@Pd core-shell nanoparticles embedded in nitrogen-doped porous carbon as dual functional electrocatalysts for both oxygen reduction and hydrogen evolution reactions

Developing efficient bi-functional electrocatalysts for both oxygen reduction reaction (ORR) and hydrogen evolution reaction (HER) is crucial for producing hydrogen and utilizing hydrogen effectively to promote electrochemical energy storage in proton membrane exchange fuel cells (PEMFCs). Herein, we report Co@Pd core-shell nanoparticles encapsulated in porous carbon derived from zeolitic imidazolate framework 67 (ZIF-67) for both ORR and HER. The controlled pyrolysis of ZIF-67 can lead to the formation of Co nanoparticles encapsulated in nitrogen-doped porous carbon (Co NC), which subsequently underwent galvanic replacement with Na2PdCl4 to form Co@Pd core-shell nanoparticles embedded in nitrogen-doped porous carbon (Co@Pd NC). The Co@Pd NC exhibited outperformance in ORR and HER than commercial Pd/C, as manifested by more positive onset potential and larger diffusion-limited current density in ORR tests, as well as a small overpotential to drive a current density of 10 mA cm−2, and much lower Tafel slope in HER tests. It also demonstrated more robust long-term stability than commercial Pd/C for both ORR and HER. Multiple techniques inter-confirmed that the Pd loading in the sample was very low. The findings can pave a path for fabricating a core-shell structured nanocomposite with ultralow noble metal usage as a bifunctional catalyst for electrochemical energy storage and conversion with high-efficiency and remarkable longevity.

Cobalt Molybdenum Oxide Derived High-Performance Electrocatalyst for the Hydrogen Evolution Reaction

The design and synthesis of high-performance hydrogen evolution reaction (HER) catalysts requires an overall consideration of intrinsic activity and number of active sites as well as electric conductivity. We herein report a facile synthesis of a cost-effective catalyst that can simultaneously address these key issues. A cobalt molybdenum oxide hydrate (CoMoO4·nH2O) with a 3D hierarchical nanostructure can be readily grown on nickel foam using a hydrothermal method. Calcination treatment of this precursor material under a reductive atmosphere resulted in the formation of Co nanoparticles on the Co2Mo3O8 surface, which worked in concert to act as active sites for the HER. In addition, the resulting Co2Mo3O8 from the dehydration and reduction reactions of CoMoO4·nH2O showed remarkable increases in both active surface area and electrical conductivity. As a consequence of these favorable attributes, the catalyst exhibited electrocatalytic performance comparable to that of the commercial Pt/C catalyst for the ...

Interfaces between Model Co-W-C Alloys with Various Carbon Contents and Tungsten Carbide.

Interfaces between alloys simulating binders in WC-Co cemented carbides and tungsten carbide were examined on the micro-, nano-, and atomic-scale. The precipitation of fine WC grains and η-phase occurs at the interface of the alloy with the low carbon content. The precipitation of such grains almost does not occur in the alloy with the medium-low carbon content and does not take place in the alloy with the high carbon content. The formation of Co nanoparticles in the binder alloy with the medium-low carbon content was established. Interfaces in the alloy with the medium-low carbon content characterized by complete wetting with respect to WC and with the high carbon content characterized by incomplete wetting were examined at an atomic scale. The absence of any additional phases or carbon segregations at both of the interfaces was established. Thus, the phenomenon of incomplete wetting of WC by liquid binders with high carbon contents is presumably related to special features of the Co-based binder alloys oversaturated with carbon at sintering temperatures.

Pt Hollow Nanospheres As an Electrocatalyst for the Oxygen Reduction Reaction

H2-fed proton exchange membrane fuel cells (PEMFCs) represent the most advanced fuel cell technology and have a great deal of potential applications in low/zero-emission electric vehicles, distributed home power generators, and power sources for small and portable electronics. However, the commercialization of PEMFC technology has been greatly hindered by some challenges, mainly sluggish kinetics of the oxygen reduction reaction (ORR) at the cathode and the high cost of the noble metal Pt (1, 2). Alloying platinum with non-noble metals such as Co is an effective approach to improve the catalytic performance and reduce the usage of Pt. In addition, through transferring Pt nanoparticles (NPs) into a hollow nanostructure, electrocatalytic performance can be improved greatly due to its relatively lower density and higher surface area-to-volume ratio than its solid counterpart (i.e., NPs). In this work, Pt hollow nanospheres (HNSs) were fabricated through a replacement reaction of Co atoms by PtCl6 2- ions to reduce Pt usage and improve the catalytic activity towards the ORR. Carbon black Vulcan XC-72R (VC) was introduced into a solution prior to the addition of Co(II) and the formation of Co NPs and the replacement of Co by PtCl6 2-ions for a uniform dispersion of Co NPs and the Pt HNSs on the carbon support. Some Co atoms have alloyed Pt in the synthesis and exist in the Pt HNSs (3). The hollow mesoporous core in the Pt HNSs can be utilized as an electrolyte solution buffering reservoir to minimize the diffusion distance to the interior surface of the porous shell of Pt crystallites while the porous nanochannels (i.e., the micropores between the Pt crystallites) in the shell open to the mesoporous hollow core form fast mass transport networks providing more accessible sites for oxygen transfer. In addition, triple phase boundaries (i.e., gas-electrolyte-Pt NPs) can be developed more easily in the Pt HNS enabling individual Pt crystallites in the shell to be accessible to electrolyte ions and oxygen, and thus catalytically active. Furthermore, alloying Pt with Co might result in a significant lattice shrinking because of the change in Pt-Pt bond distance which also contributes to the improved electrocatalytic activity. In contrast, for the state-of-the-art Pt NPs/VC catalyst, the Pt NPs can agglomerate more easily to form larger particles, resulting in reduced active sites. Besides, the interior voids between Pt NPs may not be accessible to electrolyte ions and do not contribute to the ORR activity due to the lack of triple phase boundaries. As a result, the as-developed PtCo (20 wt%) HNS/VC catalyst outperforms significantly the state-of-the-art Pt(20 wt%)NP/VC catalyst.

Co@Pt Core@Shell nanoparticles encapsulated in porous carbon derived from zeolitic imidazolate framework 67 for oxygen electroreduction in alkaline media

Abstract Nanocomposites based on Co@Pt core@shell nanoparticles encapsulated in nitrogen-doped porous carbons were prepared as a new type of high-performance electrocatalysts for oxygen reduction reaction (ORR). Controlled pyrolysis of zeolitic imidazolate framework 67 (ZIF-67) led to the formation of Co nanoparticles encapsulated in nitrogen-doped porous carbon (Co-NC), which underwent galvanic replacement reactions with K2PtCl4 forming Co@Pt core@shell nanoparticles. The surface microstructure and composition of the resulting Co@Pt-NC nanocomposite were examined by electron microscopic as well as X-ray photoelectron spectroscopic (XPS) measurements. With the Co@Pt particles encapsulated in nitrogen-doped porous carbon, the hybrids exhibited a high specific surface area and abundant catalytically active sites for ORR. Electrochemically, the specific activity and mass activity of the Co@Pt-NC composite at +0.85 V (0.145 mA cm−2 and 71.9 A g−1) were superior to those of commercial Pt/C (0.123 mA cm−2 and 38.4 A g−1). Furthermore, the Co@Pt-NC composite also exhibited remarkably higher durability and more robust tolerance against methanol crossover than commercial Pt/C.

Pt Hollow Nanospheres As an Electrocatalyst for the Oxygen Reduction Reaction

H2-fed proton exchange membrane fuel cells (PEMFCs) represent the most advanced fuel cell technology and have a great deal of potential applications in low/zero-emission electric vehicles, distributed home power generators, and power sources for small and portable electronics. However, the commercialization of PEMFC technology has been greatly hindered by some challenges, mainly sluggish kinetics of the oxygen reduction reaction (ORR) at the cathode and the high cost of the noble metal Pt (1, 2). Alloying platinum with non-noble metals such as Co is an effective approach to improve the catalytic performance and reduce the usage of Pt. In addition, through transferring Pt nanoparticles (NPs) into a hollow nanostructure, electrocatalytic performance can be improved greatly due to its relatively lower density and higher surface area-to-volume ratio than its solid counterpart (i.e., NPs). In this work, Pt hollow nanospheres (HNSs) were fabricated through a replacement reaction of Co atoms by PtCl6 2- ions to reduce Pt usage and improve the catalytic activity towards the ORR. Carbon black Vulcan XC-72R (VC) was introduced into a solution prior to the addition of Co(II) and the formation of Co NPs and the replacement of Co by PtCl6 2-ions for a uniform dispersion of Co NPs and the Pt HNSs on the carbon support. Some Co atoms have alloyed Pt in the synthesis and exist in the Pt HNSs (3). The hollow mesoporous core in the Pt HNSs can be utilized as an electrolyte solution buffering reservoir to minimize the diffusion distance to the interior surface of the porous shell of Pt crystallites while the porous nanochannels (i.e., the micropores between the Pt crystallites) in the shell open to the mesoporous hollow core form fast mass transport networks providing more accessible sites for oxygen transfer. In addition, triple phase boundaries (i.e., gas-electrolyte-Pt NPs) can be developed more easily in the Pt HNS enabling individual Pt crystallites in the shell to be accessible to electrolyte ions and oxygen, and thus catalytically active. Furthermore, alloying Pt with Co might result in a significant lattice shrinking because of the change in Pt-Pt bond distance which also contributes to the improved electrocatalytic activity. In contrast, for the state-of-the-art Pt NPs/VC catalyst, the Pt NPs can agglomerate more easily to form larger particles, resulting in reduced active sites. Besides, the interior voids between Pt NPs may not be accessible to electrolyte ions and do not contribute to the ORR activity due to the lack of triple phase boundaries. As a result, the as-developed Pt(20 wt%) HNS/VC catalyst outperforms significantly the state-of-the-art Pt(20 wt%)NP/VC catalyst.

INVESTIGATION OF PLATINUM-CATALYZED FORMATION OF COBALT NANOPARTICLES BY MIXED POTENTIAL

Cobalt (Co) nanoparticles with diameters of 20 to 60 nm were successfully formed by seed-mediated electroless deposition in aqueous solution at room temperature. The formation of Co nanoparticles was investigated by in situ monitoring of mixed potential and deposition rate. Co nanoparticles was produced when the mixed potential was below the oxidation-reduction potential of Co(II)/Co, which coincided with a sharp increase in deposited mass on a gold-sputtered quartz crystal substrate. The mixed potential was positive and Co nanoparticles were not produced without platinum (Pt) addition. Increasing concentration of Pt addition resulted in a decrease in mixed potential and Co nanoparticle diameter. Thus, Pt effectively enhanced Co deposition rate and served as nucleation seeds for Co nanoparticles.

Metal-polymer nanocomposites based on polydiphenylamine and cobalt nanoparticles

It is shown for the first time that the infrared heating of polydiphenylamine in the presence of cobalt (II) acetate Co(CH3CO2)2 · 4H2O in an inert atmosphere at a sample temperature of T = 250−600°C leads to the formation of a metal-polymer nanocomposite in which Co nanoparticles are dispersed in a polymer matrix at the expense of condensation of diphenylamine crystalline oligomers with hydrogen release, promoting the reduction of Co2+ to Co0. The formation of Co nanoparticles is confirmed by X-ray phase analysis. According to data from transmission electron microscopy, Co nanoparticles have sizes of 2 < d < 8 nm. The investigation into the magnetic and thermal properties of Co/polydiphenylamine nanocomposite has shown that the nanocomposite obtained is superparamegnetic and thermostable.

PREPARATION OF PURE COBALT NANOPARTICLES BY ELECTRIC ARC DISCHARGE METHOD IN ETHYLENE GLYCOL

Synthesis of Cobalt nanoparticles often entails toxic and expensive physical-chemistry methods. Fabrication of pure cobalt nanoparticles (NPs) using a simple and low-cost electric arc discharge method in ethylene glycol (EG) is suggested for the first time. The effect of different arc discharge currents (10, 20 and 30 A) on the size and optical absorption of the NPs are studied. Dynamic light scattering (DLS) and UV-visible spectroscopy result indicate that at an arc current of 10 A NPs of about 92.95 nm are produced and increasing the arc current leads to larger NPs. UV-visible spectroscopy data shows that the solvent gets more and more transparent with time. Sonication proves that this effect is related to agglomeration of the NPs. Formation of the pure Co NPs are evidenced by means of X-ray diffraction (XRD) measurements which gives an average size of about 21 nm using Scherrer's relation. Magnetization measurements of the samples are carried out by Alternating Gradient Force Magnetometer (AGFM). The results demonstrate the ability of the arc discharge method for direct formation of Co NPs in EG medium.

Quick-XAS and Raman operando characterisation of a cobalt alumina-supported catalyst under realistic Fischer–Tropsch reaction conditions

The work reported herein describes the structure and behaviour of alumina-supported nano-sized metallic cobalt particles under realistic activation and working Fischer–Tropsch (FT) synthesis conditions (Ptot=20bar). The combination of Quick-XAS for monitoring the change of the local order around Co with time resolved Raman spectroscopy for monitoring hydrocarbons produced by FT synthesis provides a better comprehension of the behaviour of FT catalysts in operation. This is of prime importance to improve cycle length, catalytic activity and selectivity towards heavy paraffins of FT cobalt-based catalysts. Under the activation process used in this work, we evidenced the formation of Co nanoparticles which undergo a reversible oxidation/reduction process depending on the water partial pressure in the cell controlled by the reaction temperature and CO conversion.

Formation of Co nanoparticles on a pyrolytic graphite surface in ultrahigh vacuum

Isolated cobalt nanoparticles have been obtained by annealing Co films produced by the electron-beam evaporation of a solid target onto a highly oriented pyrolytic graphite substrate in ultrahigh vacuum. Preliminary ion bombardment of the substrate reduces the average size of the particles formed during coalescence, but the shape of large particles is distorted. It is established that the Co nanoparticles retain their shape when exposed to air. Magnetic force images show that the particles of 20–40 nm in height are single-domain.

Preparation and magnetic properties of anisotropic (Sm,Pr)Co5/Co composite particles

Anisotropic (Sm,Pr)Co5/Co nanocomposite particles have been fabricated by chemical coating the 2h ball milled (Sm,Pr)Co5 flakes with Co nanoparticles. The Co nanoparticles were synthesized with mean particle sizes in the range of 20–50nm. The nanocomposite particles present [001] out-of-plane texture and improved magnetic properties, e.g., an enhanced remanent magnetization of 72emu/g for (Sm,Pr)Co5/Co and 66emu/g for (Sm,Pr)Co5. In addition, by using the 8h ball milled powders (much smaller than the 2h ball milled powders) as the starting materials, Co nanoparticles can also be successfully coated on the surface of the flakes. A plausible mechanism for the formation of Co nanoparticles on the surface of (Sm,Pr)Co5 flakes is discussed.

Synthesis of cobalt nanoparticles in supercritical methanol

Cobalt nanoparticles with a face-centered cubic (fcc) structure were synthesized by the reduction of Co(NO 3) 2 in supercritical methanol. The reduction of Co(NO 3) 2 to fcc Co was completed within 15 min at 400 °C and 300 bar. The reduction mechanism was investigated by varying the reaction temperature (200–400 °C) and the reaction time (0.5–15 min). The results suggested that cobalt methoxynitrate formed at an initial stage was converted to CoO, and then reduced to Co. In addition, an introduction of oleic acid as a stabilizer achieved the formation of Co nanoparticles (∼10 nm).

Surface phenomena involved in the formation of Co nanoparticles on amorphous carbon and SiO2 deposited by magnetron sputtering

High-pressure DC magnetron sputtering was used for the deposition of cobalt on amorphous carbon (a-C) and SiO2. Deposition conditions, substrate surface morphology and annealing parameters are investigated in order to promote the synthesis of large arrays of nanoparticles, with regular size and shape. Uniformly distributed Co nanoparticles a few nanometers in size were formed under annealing at 700°C in H2. Particle nucleation and growth are discussed based on X-ray photoelectron spectroscopy, transmission and scanning electron microscopy and kinetic Monte Carlo modeling (KMC).

Electroless Deposition of Ferromagnetic Cobalt Nanoparticles in Propylene Glycol

Ferromagnetic Co nanoparticles with diameters of about 40–400 nm are synthesized by electroless deposition in boiling propylene glycol. The Co particle size is decreased to a certain degree by varying the concentration of starting materials and by adding nucleating agents. The electrochemical behavior of propylene glycol is investigated by in situ measurements of mixed potential to understand the formation of Co nanoparticles in polyol systems. The mixed potential decreases with an increase in temperature and in the presence of NaOH, which suggests the faster decomposition of propylene glycol. It also shifts abruptly to a more negative value when nucleating agents are added. This indicates that nucleating agents catalyze both the oxidation reaction of propylene glycol and the reduction reaction of Co(II) species, as well as aid in the formation of Co nanoparticles as heterogeneous nucleation sites.