Graphite Atomizer Research Articles

How Nitrogen Doping Affects Hydrogen Spillover on Carbon-Supported Pd Nanoparticles: New Insights from DFT

The process of hydrogen spillover on metal nanoparticle decorated carbon surfaces has been discussed for many years due to its importance for heterogeneous catalysis and hydrogen storage. The present density functional theory (DFT) study supports recent experimental observations of the hydrogen spillover process. By use of model systems of carbon-supported palladium nanoparticles, the details of hydrogen spillover are investigated and the effects of nitrogen doping on such processes are elucidated. It is found that graphitic nitrogen atoms and water molecules significantly facilitate the hydrogen spillover reaction and serve as anchoring sites for the spillover hydrogen atoms.

CO2 and H2 adsorption on 3D nitrogen-doped porous graphene: Experimental and theoretical studies

Carbon dioxide (CO2) and hydrogen (H2) adsorptions were investigated on 3D nitrogen-doped porous graphene (GO-PAA) produced by chemical activation of graphene oxide impregnated with polyallylamine (PAA). GO-PAA characterizations by Raman and X-ray photoelectron spectroscopies, thermogravimetric analysis, N2 adsorption-desorption isotherms, scanning electron microscopy and energy-dispersive X-ray spectroscopy revealed that GO-PAA shows excellent thermal stability, decomposing at temperatures higher than 450 °C, specific surface area of 1155 m2 g−1, with pyrrolic, pyridinic and graphitic nitrogen atoms homogeneously dispersed throughout its 3D porous structure. The mesoporous nature of GO-PAA and the nitrogen doping level of 7.5 wt% resulted in remarkable H2 (1.3 wt%) and CO2 (20 mmol g−1) adsorption capacities at room temperature (RT, 25 °C) under high-pressure regime (40 bar). This H2 adsorption capacity at RT is among the highest values reported in the open literature. The role of the nitrogen atoms on the gas adsorption properties was investigated by combining experimental data, such as isosteric heat and Diffuse Reflectance Infrared Fourier Transform spectroscopy, with theoretical calculations using Density-Functional Theory. Inclusion of nitrogen atoms in the graphene structure reduces the energies of the Frontier Molecular Orbitals, facilitating polarization and increasing the interaction energy.

Exploiting encapsulated FeCo alloy decorated N-doped hierarchically porous carbon electrocatalysts in rechargeable Zn-air batteries

Abstract Exploiting highly active non-noble metal catalyst with designed nanoarchitecture is essential and formidable for the rechargeable Zn-air batteries (ZABs). To this end, hierarchically porous N-doped carbon hybrids with encapsulated FeCo alloy (FeCo@NC) is designed by template-assisted method. The optimized sample pyrolyzed at 800 °C (FeCo@NC800) possesses a large specific surface area of 1195 m2 g−1, plentiful micro-/meso-/macro-pores and is rich in defects and graphitic N-doped carbon atoms. These preeminent characteristics endow the catalyst with superior bifunctional oxygen electrocatalytic activities (ΔE = E10-E1/2 =852 mV) in alkaline electrolyte, as well as outstanding stability. As air-electrode catalyst in ZAB, FeCo@NC800 reveals high power density of 169 mW cm−2, excellent rechargeable property and cycling stability. Moreover, the as-obtained catalyst displays promising application potentiality in solid-state ZAB.

Constructing FeN4/graphitic nitrogen atomic interface for high-efficiency electrochemical CO2 reduction over a broad potential window

Summary Atomically dispersed Fe–N–C catalyst has shown great performance in the CO2-to-CO conversion, yet the high CO selectivity is achieved only within a rather narrow potential range, which cannot well meet the requirements for the following CO dimerization or hydrogenation. Here, we developed a hydrogen-pyrolysis etching strategy to precisely manipulate the uncoordinated N dopants in the Fe–N–C catalyst. This strategy could preferentially eliminate pyridinic and pyrrolic N atoms while leaving graphitic N. The resulting catalyst gave a CO faradaic efficiency (FECO) above 90% over a broad window from −0.3 to −0.8 V (versus RHE) (in particular, FECO > 97% at −0.6 V). In situ ATR-SEIRAS and first-principle calculations further revealed that this strategy can not only suppress the parasitic H2 generation but also promote CO2 activation and protonation with the assistance of co-adsorbed H2O on the “atomic interface” of “FeN4/graphitic N.”

Catalytic role of graphitic nitrogen atoms in the CO oxidation reaction over N-containing graphene: a first-principles mechanistic evaluation

The catalytic role of graphitic nitrogen atoms of a series of nitrogen-doped graphene surfaces is explored for low-temperature oxidation of CO using periodic DFT calculations.

Polymer-Assisted Electrophoretic Synthesis of N-Doped Graphene-Polypyrrole Demonstrating Oxygen Reduction with Excellent Methanol Crossover Impact and Durability.

The design and synthesis of metal-free catalysts with superior electrocatalytic activity, high durability, low cost, and under mild conditions is extremely desirable but remains challenging. To address this problem, a polymer-assisted electrochemical exfoliation technique of graphite in the presence of an aqueous acidic medium is reported. This simple, cost-effective, and mass-scale production approach could open the possibility for the synthesis of high-quality nitrogen-doped graphene-polypyrrole (NG-PPy). The NG-PPy catalyst displays an improved half wave potential (E1/2 =0.77 V) in alkaline medium compared with G-PPy (E1/2 =0.66 V). Most importantly, this catalyst demonstrates excellent stability with high methanol tolerance, and it outperforms the commercial Pt/C catalyst and other previously reported metal-free catalysts. The content of graphitic nitrogen atoms is the key factor for the enhancement of electrocatalytic activity towards oxygen reduction reactions (ORR). Interestingly, the NG-PPy catalyst can be used as a cathode material in a zinc-air battery, which demonstrates a higher peak power density (59 mW cm-2 ) than G-PPy (36.6 mW cm-2 ), highlighting the importance of the low-cost material synthesis approach towards the development of metal-free efficient ORR catalysts for fuel cell and metal-air battery applications. Remarkably, the polymer-assisted electrophoretic exfoliation of graphite with a high yield (≈88 wt %) of few-layer graphene flakes could pave the way towards the mass production of high-quality graphene for a variety of applications.

Defect topology and annihilation by cooperative movement of atoms in neutron-irradiated graphite

Graphite has been used as neutron moderator or reflector in many nuclear reactors. The irradiation of graphite in a nuclear reactor results in a complex population of defects. Heating of the irradiated graphite at high temperatures results in annihilation of the defects with release of an unusually large energy, called the Wigner energy. From various experiments on highly irradiated graphite samples from CIRUS reactor at Trombay and ab-initio simulations, we have for the first time identified various 2-, 3- and 4-coordinated topological structures in defected graphite, and provided microscopic mechanism of defect annihilation on heating and release of the Wigner energy. The annihilation process involves cascading cooperative movement of atoms in two steps involving an intermediate structure. Our work provides new insights in understanding of the defect topologies and annihilation in graphite which is of considerable importance to wider areas of graphitic materials including graphene and carbon nanotubes.

Determination of arsenic in edible oils by direct graphite furnace atomic absorption spectrometry

A direct analytical method of arsenic content in edible oil with graphite furnace atomic absorption spectrophotometer was studied. Edible oil was diluted with n-heptane followed by the determination of arsenic with Zeeman graphite furnace atomic absorption spectrophotometer and transverse heated graphite atomizer (THGA). The arsenic organic AAS standard solution was used as calibration standard, and bis (benzonitrile) dichloro palladium solution was used as a matrix modifier. The results showed that the optimal ashing and atomizing temperatures were 1200°C and 2300°C, respectively. The detection limit was estimated to be 0.010 ppm, and the percentages of recovery for the spike of 0.05, 0.10 and 0.20 ppm arsenic to salad oil, fish oil, palm oil, and lard were 91.2-94.3%, 94.7-95.6%, 94.2-96.7% and 93.3-94.1%, respectively. With the use of this method, thirty edible oils were analyzed and the arsenic contents were found to be below the detection limits.

Infrared Spectroscopy of Ultraviolet-Irradiated Carbon Nanotubes

The possibility of using the UV irradiation for a functionalization of carbon nanotubes with different degrees of structural perfection is considered. In investigations, the method of infrared spectroscopy is used. A change in the number of functional groups under the short-term UV irradiation of specimens with multiwall carbon nanotubes is estimated by a change in the relative intensity of the IR spectral bands corresponding to vibrations of the functional groups in comparison with the relative intensity of the band corresponding to vibrations of the carbon atoms in graphite.

Noble metal nanocomposites as tools for fast and reliable speciation analysis of mercury in water samples

ABSTRACT A novel non-chromatographic approach for speciation analysis of mercury was developed based on sequential application of silver and gold nanocomposites as sorbents for solid phase extraction. The nanocomposite materials consisted by AgNPs or AuNPs grafted on submicronsized silica spheres (SiO2-NH2@AgNPs and SiO2-NH2@AuNPs) were prepared by multistep synthesis procedures and characterised in details by different techniques – Transmission Electron Microscopy (TEM), Scanning Electron Microscopy (SEM), Atomic Force Microscopy (AFM), electrokinetic measurements, UV–Vis spectroscopy and X-ray diffraction. Excellent separation and enrichment of Hg(II) and CH3Hg+ was achieved due to the specific behaviour and affinity of Ag and Au nanoparticles (NPs) towards particular Hg chemical forms. In the first extraction step, Hg(II) was strongly retained by highly selective amalgamation on the surface of Ag nanocomposite (SiO2-NH2@AgNPs). In the second extraction step, CH3Hg+ remained in the supernatant solution was sorbed on the surface of Au nanocomposite (SiO2-NH2@AuNPs). Both Hg species trapped in the sorbents precipitates were quantified by electrothermal Atomic Absorption Spectroscopy (AAS) after direct injection in the graphite atomiser under optimal instrumental parameters (Ag and Au act as modifiers). The optimal solid-phase extraction conditions were examined and defined by batch method. Recoveries achieved by the developed analytical procedure were in the range 97–102% for both Hg(II) and methylmercury with limits of detection and quantification of 0.03 and 0.1 µg L−1, respectively. The proposed analytical procedure was applied for speciation analysis of mercury in water samples.

Graphenes and CNTs: Adatoms, islands, nanocrystals, and intercalants as interacting multipoles

The functionalization (adsorption) of graphene and carbon nanotubes (CNT) is investigated in the case of charge transfer between a functionalizing molecule (adatom) and a substrate (graphenes or CNT), and the first principles charge transfer calculations are briefly reviewed. It is shown that electrostatic dipoles caused by charge transfer describe the interaction between the adsorbed atoms or islands (clusters) at low concentration, that is, at the initial and intermediate stages of functionalization. It is shown that intercalated atoms in graphite, bi-, and tri-graphene can be described by the electrostatic quadrupoles, their magnitudes are found. The quadrupoles’ axes are perpendicular to the layers. On the surface of the CNT, the adsorbed nanocrystals (clusters) are described as electrostatic quadrupoles, their magnitudes are found. The quadrupoles’ axes are directed along the CNT. At long distances, the interaction energies and repulsion forces are calculated for the clusters. The results explain the experimentally found homogeneous distribution of the adsorbed particles and clusters.

Diffusion & concentration effect of Li/Li+ to the efficiency of LIBs

In this work the concentration of Li/Li+ has applied for increasing the efficiency of Lithium ion batteries. Various numbers of lithium and lithium cations have been simulated as diffused atoms in graphite as anode materials. We have found the structure of (G// (h-BN) //G) can be to improve the voltage and electrical transport in anodic sheets-based LIBs. This system could also be assembled into free-standing electrodes without any binder or current collector, which will lead to increased specific energy density for the overall battery design. Therefore, the above modification of BN-G sheet and designing of this kind structure provide strategies for improving the performance of material based anodes in LIBs.

ZnCl2 Enabled Synthesis of Highly Crystalline and Emissive Carbon Dots with Exceptional Capability to Generate O2⋅–

Summary Highly crystalline and emissive carbon dots (CDs) are firstly synthesized via an ionothermal method in organic solvents. In contrast to well-established approaches, ZnCl2 is utilized as the pyrolysis-promoting agent to prepare emissive CDs with tunable wavelengths, directly by carbonization from different O and N precursors under a mild reaction condition at 210°C. More than 50% synthetic yield and 39% photoluminescent quantum yield (blue CD) are obtained. XAFS measurements, in conjunction with TEM, HAADF-STEM, UPS, and ESR analyses, allow us to obtain energy diagram configuration and confirm the luminescent nature of these hard-core structured CDs. Importantly, the resulting CDs exhibit excellent capability to photogenerate superoxide radical anion (O2⋅–), which can be exemplified by both photodynamic therapy and photooxidative cyclization synthesis of tetrahydroquinolines. Representative pharmaceutical mifepristone can be derivatized in 90% yield within 10 min of irradiation of the CDs in an elegant flow reactor.

CVD grown nitrogen doped graphene is an exceptional visible-light driven photocatalyst for surface catalytic reactions

The photocatalytic potential of large area CVD grown nitrogen doped graphene (NGr) has been explored though the chemical transformation of 4-nitrobenzene thiol into p,p′-dimercaptoazobenzene. Decoration of NGr with Ag nanocubes with rounded edges to form NGr/Ag nanohybrids resulted in a slight increase in the work-function and a decrease in the n-type character of NGr due to ground state transfer of negative charge from NGr to Ag. The Ag nanocubes exhibited a localized surface plasmon resonance (LSPR) at ~425 nm. When the NGr/Ag nanohybrids were illuminated with visible light of wavelength close to the LSPR peak, Kelvin probe force microscopy (KPFM) indicated a dramatic change in surface potential of −225 mV and Raman spectra detected electron accumulation in NGr, which are attributed to a high local field enhancement-mediated hot electron injection into NGr and the formation of long-lived charge separated states. Pristine nitrogen doped graphene and its coupled system with plasmonic Ag nanoparticles showed superior photocatalytic performance compared to bare plasmonic Ag catalyst. While standalone Ag NPs were unable to complete the transformation of 4-NBT into DMAB even at a laser power of 10 mW, NGr/Ag nanohybrids completed this transformation at a laser power of 1 mW, pointing to the high photoreduction strength of NGr/Ag. Density functional theory (DFT) based computational modeling was used to examine the electronic structure of graphene doped with graphitic, pyridinic and pyrrolic nitrogen dopant atoms. DFT results indicated an enhanced chemical reactivity of NGr due to stronger localization of charge at the dopant sites and a pronounced difference in the projected density of states (PDOS) for carbon atoms in proximity to, and distant from, the nitrogen dopant sites.

Preconcentration of Pb with Aminosilanized Fe3O4 Nanopowders in Environmental Water Followed by Electrothermal Atomic Absorption Spectrometric Determination

A new preconcentration method to determine lead in environmental waters using the aminosilanized magnetite Fe3O4 powder sorbent has been developed. The preconcentration method was combined with electrothermal atomization atomic absorption spectrometry (ETAAS) and a graphite atomizer. Trace amount of sorbent (3 mg) could be applied into the preconcentration of Pb. According the preconcentration, the detection limits were 14 and 19 pg·mL−1 with bare and aminosilanized Fe3O4, respectively. The effect of interferent elements such as Al, Ca, Co, Fe, K, Mg, Na, Ni, and Zn (1000 ng·mL−1 versus Pb 1 ng·mL−1) on the preconcentration of Pb using bare magnetite was evaluated, and some elements (Al, Ni, and Zn) were found to interfere with the Pb preconcentration. The aminosilanized Fe3O4 sorbent was found to be effective in eliminating the severe interferences. The enrichment factors were 34 for the preconcentration with aminosilanized Fe3O4. The recovery of spiked Pb in the case of the sorbent with aminosilanized Fe3O4 was in the range of 75 to 110%. From the analytical data, the preconcentration technique was found to be useful for the determination of trace lead in environmental waters.

Comparative study of density functionals for the description of lithium-graphite intercalation compounds.

The lack of description of van der Waals interactions in layered materials such as graphite and binary graphite intercalation compounds remains a main drawback of conventional density functional theory. Two fundamentally different approaches to overcome this issue are the employment of semiempirical dispersion correction scheme such as Grimme dispersion correction or nonlocal density functionals. We carefully compare these two approaches for the description of the geometric structure and the thermodynamic stability of pure graphite and Li-GICs at different lithium concentrations and stages. Based on the computed formation energies, we also evaluate the lithium-graphite intercalation potential. We find that PBE-D3(BJ) accurately reproduces the lattice parameters and the interlayer binding energy of graphite, although it underestimates the thermodynamic stability of stage-II Li-GICs mainly due to overbinding of carbon atoms in pure graphite. The nonlocal van der Waals functionals optB88-vdW, optB86b-vdW, and revPBE-vdW show a good agreement with experiments concerning stability of Li-GICs of different stages, although they overestimate the van der Waals interactions in graphite. The experimentally determined decreasing step-function behavior of Li-graphite intercalation potential can be qualitatively reproduced only by employing the revPBE van der Waals functional, whereas the other density functionals fail in the description. © 2019 Wiley Periodicals, Inc.

Aluminium release by coated and uncoated fluid-warming devices.

The use of fluid-warming systems is recommended for infusion rates >500ml.h-1 to avoid peri-operative hypothermia. Some fluid-warming devices use disposable aluminium-heated plates for heat transfer, but there is no protective coating to separate the fluid from the heated aluminium surface. It is unknown if this could promote release of aluminium into infusion fluids. We investigated a coated (Fluido compact) and an uncoated (enFlow) fluid-warming device using normal saline or balanced electrolyte solution as infusion fluids, pumped through the heated disposables at flow rates of 2, 4 and 8ml.min-1 for 60min each. Aluminium concentrations in the fluid samples were analysed using graphite furnace atomic absorption spectrometry. With saline the coated and uncoated devices yielded aluminium concentrations below the level of quantification (<128μg.l-1 ). Similarly, balanced electrolyte solution in the coated device yielded aluminium concentrations <128μg.l-1 . However, balanced electrolyte solution in the uncoated device yielded aluminium concentrations of up to 6794 (3465-8002 [1868-7421]) μg.l-1 . Repeating this last study at a flow rate of 2ml.min-1 resulted in quite high aluminium concentrations when the uncoated device was not heated (~1000μg.l-1 ) and higher concentrations after the device was heated. We conclude that using uncoated aluminium plates in fluid-warming systems can lead to a risk of administering potentially harmful concentrations of aluminium when balanced crystalloid solutions are used. The mechanism is unclear, but heat is in part involved. Coating for aluminium within medical devices in direct contact with infusion fluids should be recommended.

Preparation of Mo2C–carbon nanomaterials for hydrogen evolution reaction

Highly active, stable and low-cost noble metal-free electrocatalysts are essential for production of hydrogen. However, preparation of such catalysts is still highly challenging so far. In this work, the Mo2C–carbon nanomaterials have been prepared by controlled thermal technique. By controlling concentration of the reactants in the experimental condition, the Mo2C–carbon nanomaterials have been fabricated, which leads to decreases in contact resistance b/w Mo2C–carbon nanomaterials and graphitic carbon atoms. As a result, the Mo2C–carbon nanomaterial electrode shows remarkable activity for hydrogen evolution reactions with a small onset overpotential of 95 mV, a Tafel slope of 62 mV dec−1, an high exchange current density of 0.32 mA cm−2, good stability during long-term 1000 cycles and exhibits long-term durability for several days. This study opens a new method for the preparation of highly active non-noble electrode for production of hydrogen from water splitting.

Model of Forming Isotropic and Anisotropic Graphite Under High Temperatures and Fluences Neutron Irradiation

A model is proposed for describing the shape change of isotropic and anisotropic graphite under the influence of high temperatures and high neutron radiation fluences. The model is based on a new approach, which uses the following provisions: description of the near-pore neighborhood in graphite as a solid solution using the theory of phase transformations of the first kind; consideration of a new phase as a spheroidal pore of small eccentricity, flattened along the direction of greatest stress; taking into account the clustering of carbon atoms at fluences of more than . The graphite non-isotropy is characterized by different pore sizes, different diffusion coefficients, the lengths of the paths of graphite atoms along and across volume of the sample, which in turn depend on the temperature of the sample. It is proposed that the initial element on the basis of which a new phase will be formed is the spheroidal pore of small eccentricity, flattened along the direction of the highest tension. A kinetic equation that describes the diffusion of pores under the influence of high temperatures and intense neutron fluxes is obtained. Initially, the pores are in a field of predetermined stresses oriented along and across the sample. The contribution of diffusion processes is due to the term proportional to the pore distribution function in the sample, and the effect of the neutron flux is described by an additional term in the kinetic equation. The obtained kinetic equation for anisotropic graphite can be transformed for isotropic graphite. For isotropic and anisotropic graphite, model solutions have been obtained that characterize the change in its volume with time under the influence of a neutron flux and high temperature. It is shown that the magnitude of the change in the relative volume of reactor graphite for isotropic graphite exceeds a similar value for anisotropic graphite. Theoretical confirmation of the laws governing the swelling of anisotropic graphites under the influence of large neutron fluences and in the high-temperature field, previously obtained by other authors, is obtained: longitudinal compression of anisotropic graphite samples corresponds to a change in the linear dimensions of isotropic graphites; the transverse compression of anisotropic graphite samples is less than the change in the longitudinal linear dimensions of isotropic graphites.

Selection of a photodetector for atomic absorption spectrometers with a two-stage probe atomizer

A two-stage graphite atomizer with an independently heated tungsten probe has been investigated by shadow spectral visualization, which is a promising method for analyzing the direct atomic absorption of complex substances. Fall-offs in optical density of the formed Ag, Fe, and Al vapors and smoke are measured. The dependences of the shape and size of the analytical pulses of absorption on the size of the photometric zone in a graphite furnace, and on the photodetector type (multipixel photodetectors with spatial resolution and a simple photomultiplier), are established. It is shown that the multipixel photodetectors recommended earlier for one-stage atomization exhibit some advantages. However, for analytical purposes, this probe atomizer can be used successfully with a photomultiplier or photodiode.