Dissociation Of Nitrogen Molecules Research Articles

Formation of HfCxNy by carbothermal reduction nitridation and their laser ablation behavior

Hafnium carbonitride (HfCxNy) powders have been successfully prepared by heating a mixture of fine-grained HfO2 and nano-carbon powders during 1873 K-2273 K in nitrogen. The carbothermal reduction nitrdation (CTRN) process is composed of two steps: (1) the formation of microporous HfC and (2) catalytic nitridation of microporous HfC. The results demonstrate that the as-prepared HfCxNy powder consists of a single phase and various nitrogen content through different sintering parameters. The mechanism consists of three stages: (1) Formation of cubic mixed crystal HfCxOy, (2) Transformation into microporous hafnium carbide, and (3) Crucially, microporous hafnium carbide catalyzes the activation and dissociation of nitrogen molecules, leading to the formation of HfCxNy. Additionally, we introduce a laser etching method to confirm the ultra-high-temperature melting behavior of HfCxNy. This study enhances our understanding of HfCxNy powders synthesis and their laser ablation performance, offering valuable insights for future material applications.

Electron transfer from yttrium hydride to Mo-carbonitride boosts low-temperature ammonia synthesis

Ammonia is pivotal to various chemical industries, which is produced using the Haber-Bosch process. This process requires harsh conditions (e.g., 20.0 MPa and (500–600)°C) to generate ammonia and thus, is an energy intensive process. The growing consensus is to mild the conditions for ammonia generation. Herein, we demonstrate the use of hydride to increase the activity of molybdenum carbonitride (MCN). YH@MCN (95:5, w/w) generates ammonia at a rate of 448μmolg−1h−1 at 255 °C and 0.1 MPa. To address the stability issue of hydride, we incorporate iron. The experiment demonstrates two-fold rates for such catalyst, comparing MCN alone. The activation energy (19.73kJmol-1) for YFeH@MCN is considerably low, owing to the electron transfer from YH to MCN that facilitates nitrogen dissociation. Hence, the use of distinct sites for easy dissociation of nitrogen molecules, and the synergy between them, results in an activity that exceeds of conventional molybdenum and iron-based catalysts, and is comparable to ruthenium-based catalysts. Our results illustrate the potential of using synergistic multiple active sites in catalysts, and introduce a design concept for ammonia synthesis catalysts, using readily available elements.

A DFT study on nitrogen enhancing mechanism of SRF niobium cavity performance

High-temperature nitriding has been shown to improve the quality factor (Q) of the superconducting radio frequency (SRF) cavity recently, while its inherent promoting mechanism is still unclear. Therefore, based on first-principles calculations, the adsorption and dissociation processes of nitrogen molecules on the Nb(110) surface, and the adsorption, diffusion processes of nitrogen atoms on the Nb(110) surface, were studied in this paper to explain the principle of its performance improvement. To establish the reaction mechanism, adsorption preference sites were determined. All possible reaction paths were investigated for each step including the energy barrier based on the climbing image nudged elastic band (CI-NEB) method. We establish multiple reaction paths and the only optimal one was found. It is found that the Nb(110) surface can facilitate the dissociation of nitrogen molecules (with only a 1.14 eV energy barrier), and nitrogen atomic energy can readilyreach the subsurface (only a 1.73 eV energy barrier). Still, the high energy barrier (up to 6.64 eV) prevents it from diffusing inward. This revelation provides the nitriding mechanism of pure niobium and the rationale for enhancing niobium cavity performance after nitrogen mixing.

The role of lanthanum hydride species in La2O3 supported Ru cluster catalyst for ammonia synthesis

To exploit the ammonia as a carrier for sustainable energy, development of efficient catalysts is highly desired. Oxide supported Ru catalysts have been widely studied in ammonia synthesis for their superior catalytic performances under mild conditions. Here, sub-nanometer Ru clusters on La2O3 (Ru clusters/La2O3) catalyst has been synthesized and exhibits superior catalytic performance in ammonia synthesis under mild conditions. Multiple characterizations results reveal that surface La-H species, in situ generated on the surface of catalyst, can significantly boost the activity of Ru clusters/La2O3 catalyst in ammonia synthesis. Detailed mechanistic studies demonstrate that the active La-H species on the La2O3 can not only promote activation and dissociation of nitrogen molecules but also directly involve in ammonia synthesis reaction process via a La-H species-mediated reaction pathway. This work promotes molecular level understanding the active sites and reaction mechanism under real reaction conditions, and will also offer opportunity for guiding the design of efficient Ru-based catalysts for ammonia synthesis.

Mechanism and experimental study on preparation of high-quality vanadium nitride by one-step vacuum carbothermal reduction nitridation method

With high hardness, high electrical conductivity, and good catalytic activity, VN is widely used in alloy materials, catalysis and electrode materials, and other fields. However, the conventional carbothermal reduction nitridation (CRN) methods are often limited by high temperatures and long reaction times. In this paper, the reaction process of vanadium trioxide one-step vacuum carbothermal reduction nitridation (VCRN) synthesis of vanadium nitride was studied by combining theoretical calculation and experimental research. The experimental results show that reducing the pressure is favorable for the V2O3-CRN reaction and high-quality VN was successfully synthesized under the conditions of 1473 K (300 Pa, 3 h) and the carbon mass ratio (mC/mV2O3) is 0.25. In addition, the reaction mechanism of V2O3-VCRN was elucidated by ab initio molecular dynamics (AIMD) simulations. The results show that carbon plays important role in the VCRN reaction process of vanadium trioxide. First, Carbon catalyzes the dissociation of nitrogen molecules on the surface of V2O3–C, which promotes the formation of the V–N bond, second is to react with O in the V2O3 lattice and reduces V2O3.

Boosting Photocatalytic Nitrogen Fixation via In Situ Constructing Bi Metal Active Sites over BiOBr/BiOI Heterojunction

The main obstacles to the photocatalytic reduction of nitrogen are the low separation efficiency of photogenerated charges and the few activation sites for nitrogen. It is highly desirable to explore new strategies for improving the nitrogen fixation performance of catalysts. Herein, the Bi metal active sites are constructed on the surface of BiOBr/BiOI heterojunction by in situ reaction, which promote the absorption, activation, and dissociation of nitrogen molecules. Moreover, the existence of Bi metal and BiOBr/BiOI heterojunction enhances the light absorption ability and facilitates the separation and transfer of photogenerated charges. The theoretical calculation also demonstrates that the BiOBr/BiOI/Bi composite has excellent electron structure and electron transfer efficiency. So, the ternary BiOBr/BiOI/Bi catalyst shows excellent performance of photocatalytic reduction of nitrogen to ammonia. The nitrogen reduction rate is 221.9 μmol g−1 h−1, which is 7.6 and 5 times higher than that of pure BiOBr and BiOBr/BiOI. The mechanism of photocatalytic nitrogen fixation of the BiOBr/BiOI/Bi is proposed based on the experimental and theoretical results. This study provides a novel method for improving the photocatalytic nitrogen reduction performance of catalysts.

Synergistic Effects of Crystal Phase and Strain for N2 Dissociation on Ru(0001) Surfaces with Multilayered Hexagonal Close-Packed Structures.

The synergistic effects of strain and crystal phase on the reaction activity of nitrogen molecule dissociation have been studied using density functional theory calculations on Ru(0001) surfaces with multilayered hexagonal close-packed structures. The phase transformation from hexagonal close-packed phase (2H) to face-centered cubic (3C) phase or unconventional phases (4H, DHCP, 6H1, and 6H2) would occur under the uniaxial tensile strain loaded along the c axis. The close-packed surfaces of unconventional crystal phases show an enhanced chemical reactivity for N adsorption due to the upshifted d-band center of Ru. However, the N2 adsorption energy is almost independent of the applied strain and crystal phase. The optimized catalytic activity of Ru(0001) surfaces with the unconventional phases is found for the N2 dissociation through breaking the scaling relationships between the reaction barrier and reaction energy. Our results indicate that the strain-induced phase transformation is an effective method to improve the catalytic activity of noble metal catalysts toward the N2 dissociation reaction.

Влияние процентного содержания аргона в смеси газов Ar-N2 на относительное количество частиц Ar+, N2+, N и N+ в плазме несамостоятельного тлеющего разряда низкого давления с полым катодом

The paper presents the results of studies of argon-nitrogen plasma of a non-self-sustaining low-pressure glow discharge with a hollow cathode by optical emission spectrometry. The plasma was generated in a mixture of Ar-N2 gases with an argon percentage from 0 to 100% at a total pressure of 1 Pa. The discharge current and voltage were maintained constant and amounted to 18 A and 165 V, respectively. A significant increase in the amount of Ar+ (up to 30%) at a nitrogen content of 10-25% was shown, which is associated with a shift in the reaction of nitrogen recharging with argon towards the generation of argon ions. It was found that at a low partial pressure of nitrogen (≈ 10%), a sharp increase in the content of atomic nitrogen is observed, which is probably due to the dissociation of nitrogen molecules upon collisions with an excited argon atom.

The effects of the driving frequencies on micro atmospheric pressure He/N2 plasma jets driven by tailored voltage waveforms

Capacitively coupled micro atmospheric pressure plasma jets are important tools for the generation of radicals at room temperature for various applications. Voltage waveform tailoring (VWT), which is based on the simultaneous use of a set of excitation frequencies, has been demonstrated to provide an efficient control of the electron energy probability function (EEPF) in such plasmas and, thus, allows optimizing the electron impact driven excitation and dissociation processes as compared to the classical single-frequency operation mode. In this work, the effects of changing the driving frequencies on the spatio-temporally resolved electron power absorption dynamics, the generation of helium metastables and the dissociation of nitrogen molecules are investigated in He/N2 plasmas based on experiments and simulations. We find that under a single-frequency excitation, the plasma and helium metastable densities are enhanced as a function of the driving frequency at a fixed voltage. When using valleys-type driving voltage waveforms synthesized based on consecutive harmonics of the fundamental driving frequency, the spatial symmetry of the electron power absorption dynamics and of the metastable density profile is broken. Increasing the fundamental frequency at a constant voltage is found to drastically enhance the plasma and metastable densities, which is a consequence of the change of the EEPF. Finally, we compare the energy efficiency of the formation of radicals under single-frequency and VWT operation at different driving frequencies. For a given power dissipated in the plasma, VWT yields a higher helium metastable as well as electron density and a higher dissociation rate of N2.

Characterization of low-pressure high-current cascaded arc plasma in large volumes

A comprehensive study of behavior and parameters of the low-pressure (1–20 mTorr) large-area plane–symmetric direct current arc discharge plasma in pure Ar and Ar-N2 mixtures was carried out. A low-pressure arc discharge was ignited in a large chamber between a planar vacuum arc cathode with magnetic steering of arc spots and a surrounding grounded primary anode while a long-length remote arc discharge was extended toward a remote linear anode parallel to the cathode plate. The arc column moved up and down along the cylindrical cathode target positioned perpendicular to the cathode-to-remote anode direction, following the motion of the cathodic arc spots. Current-voltage characteristics of the low-pressure direct current arc discharge, electron density, electron temperature, electron energy distribution function (EEDF), ion energy distribution function (IEDF) and dissociation of nitrogen molecules in the discharge were studied using electro-technical methods, electrostatic probes, a microwave resonant probe, an ion energy analyzer and optical emission spectroscopy (OES) methods. A number of OES methods were used to determine temperature and density of electrons in the discharge: the Te was measured using 1) the IArI,415/IArI,306 intensity ratio, 2) an intensity ratio of the (0–0) vibrational bands of 1st negative and 2nd positive systems of N2; the ne was obtained by 1) the IArII,487/IArI,451 intensity ratio and 2) ratio of N2(C3Πu,v = 1) and N2(C3Πu,v = 0) populations. In addition, the nitrogen dissociation degree was monitored using INI,868.03/IArI,852.14 and INI,862.92/IArI,852.14 intensity ratios. Spectral and probe measurements of Te and ne in the large-area arc discharge show reasonable agreement. The electron population analysis based on the second derivative of the Langmuir probe current-voltage characteristics allowed accurate measurements of electron population parameters in the discharge including EEDF, electron drift velocity along with plasma potential, electron density and energy. Modeling of the low-pressure large-volume arc discharge using the axially symmetric drift-diffusion approximation exhibited good qualitative and acceptable quantitative agreement between calculated plasma parameters and experimental data.

Influence of Cobalt Crystal Structures on Activation of Nitrogen Molecule: A First-Principles Study

Identification of the structure sensitivity of nitrogen molecule (N2) activation and ammonia synthesis on metal surfaces is important for the mechanistic understanding and rational design of more efficient catalysts. In the present work, density functional theory calculations together with microkinetic simulations were performed to study the influence of cobalt crystal structures including hexagonal close-packed (HCP) and face-centered cubic (FCC) on nitrogen molecule dissociation and ammonia synthesis. Molecular and dissociative adsorption energies of N2 as well as dissociation barriers are calculated for a total of ten cobalt surfaces. It is found that molecular adsorption energies on Co surfaces vary modestly on the order of 0.25 eV, whereas dissociative adsorption energies and the corresponding barriers vary considerably in magnitude by about 0.80 eV. First-principles microkinetic simulations show that HCP Co displays higher activity than FCC cobalt for nitrogen molecule dissociation and ammonia synth...

Construction of a coarse-grain quasi-classical trajectory method. II. Comparison against the direct molecular simulation method.

This work presents the analysis of non-equilibrium energy transfer and dissociation of nitrogen molecules (N2(Σg+1)) using two different approaches: the direct molecular simulation (DMS) method and the coarse-grain quasi-classical trajectory (CG-QCT) method. The two methods are used to study thermochemical relaxation in a zero-dimensional isochoric and isothermal reactor in which the nitrogen molecules are heated to several thousand degrees Kelvin, forcing the system into strong non-equilibrium. The analysis considers thermochemical relaxation for temperatures ranging from 10 000 to 25 000 K. Both methods make use of the same potential energy surface for the N2(Σg+1)-N2(Σg+1) system taken from the NASA Ames quantum chemistry database. Within the CG-QCT method, the rovibrational energy levels of the electronic ground state of the nitrogen molecule are lumped into a reduced number of bins. Two different grouping strategies are used: the more conventional vibrational-based grouping, widely used in the literature, and energy-based grouping. The analysis of both the internal state populations and concentration profiles show excellent agreement between the energy-based grouping and the DMS solutions. During the energy transfer process, discrepancies arise between the energy-based grouping and DMS solution due to the increased importance of mode separation for low energy states. By contrast, the vibrational grouping, traditionally considered state-of-the-art, captures well the behavior of the energy relaxation but fails to consistently predict the dissociation process. The deficiency of the vibrational grouping model is due to the assumption of strict mode separation and equilibrium of rotational energy states. These assumptions result in errors predicting the energy contribution to dissociation from the rotational and vibrational modes, with rotational energy actually contributing 30%-40% of the energy required to dissociate a molecule. This work confirms the findings discussed in Paper I [R. L. Macdonald et al., J. Chem. Phys. 148, 054309 (2018)], which underlines the importance of rotational energy to the dissociation process, and demonstrates that an accurate non-equilibrium chemistry model must accurately predict the deviation of rovibrational distribution from equilibrium.

First principles calculations on nitrogen reactivity on tungsten surfaces

Using the spin-polarized gradient-corrected density functional theory we investigate the adsorption and dissociation of nitrogen molecule and hydrogen nitride radicals on two model surfaces of tungsten, W{1 0 0} and W{1 1 0}. The goal of the investigations is to predict the number and nature of the nitrogenized moieties that could be found on a tungsten surface after nitrogen-seeded hydrogen (deuterium) plasma discharge. The surfaces are considered clean or saturated in hydrogen or nitrogen atoms as they are expected to be after nitrogen seeded hydrogen plasma irradiation. We find that the radicals NH, NH2 and NH3 are dissociated by the catalytic action of the metallic surfaces if they are animated with an initial kinetic energy of less than 2.5 eV. The products of the reaction are a nitrogen atom incorporated into the metal surface and an adsorbed proton. The general conclusion is that nitrogen reduces hydrogen retention on the surface and more generally that the nitrided surfaces should be less reactive than the clean tungsten even if the two surfaces we consider behave quite differently.

Angular distributions of multiply charged fragments from dissociation of Nitrogen molecules by Xe9+ impact

SynopsisThe angular distribution of the correlated molecular fragments with respect to the incoming beam direction for several of dissociation channels of multiply charged N2 molecule on collision with Xe9+ was studied.

Determination of vibrational and rotational temperatures in highly constricted nitrogen plasmas by fitting the second positive system of N2 molecules

Highly constricted plasmas are an active research area because of their ability to generate high activity of plasma beams, which exhibit potential in applications of material processing and film deposition. In this study, optical emission spectroscopy was used to study the highly constricted nitrogen plasma created at low pressure. The vibrational and rotational temperatures of molecules were determined by fitting the second positive system of nitrogen molecule. Under the conditions of the power densities as high as 7 ∼ 85 W/cm3 and the pressures of 2 ∼ 200 Pa, the determined rotational temperature was found to be relatively low, increasing from 350 to 700 K and the vibrational temperature keeping at ∼ 5000 K. The analysis of dissipated power revealed that ∼ 80 % of input power is dissipated for the nitrogen molecule dissociation and the creation/loss of ions at the tube wall, producing an as high as 1012 ∼ 1013 cm−3 plasma with the nitrogen dissociation degrees of 2%∼15%. With the increase in the discharge pressure, more input power was found to be dissipated in the dissociation of nitrogen molecules instead of creation of ions, resulting in a higher density of radicals.

Optimal control of dissociation of nitrogen molecule with intense ultra-short laser pulse shaping

The quantum optimal control theory in conjunction with time dependent density functional theory is used to optimize the laser pulse shape for dissociation of nitrogen molecule. For several initial peak intensities and frequency ranges, the optimum shapes are produced and compared to determine the most efficient pulse. Ehrenfest molecular dynamics model is also used to test the dissociation process. The corresponding snapshots of density and time dependent electron localization function are presented. It is noticed that when the frequency ranges of laser pulses are doubled, it leads to 60% faster dissociation of N2 molecule.

Dissociation of nitrogen in a pulse-periodic dielectric barrier discharge at atmospheric pressure

Nitrogen molecule dissociation in a pulse-periodic atmospheric-pressure dielectric barrier discharge is numerically analyzed. It is shown that the quenching rate of predissociation states at atmospheric pressure is relatively low and the production of nitrogen atoms in this case can be adequately described using the cross section for electron-impact dissociation of N2 molecules taken from the paper by P.C. Cosby [J. Chem. Phys. 98, 9544 (1993)].

Charge-asymmetric dissociation of nitrogen molecules colliding with highly charged ions

Ion pair fragmentation of nitrogen molecules following Ar8+ ion collision was observed by using a position-sensitive time-of-flight method. Kinetic energy release (KER) distributions were derived from the double hit data. Higher-asymmetric fragmentation channels have peak positions of the KER higher than the Columbic value.

Space-resolved characterization of high frequency atmospheric-pressure plasma in nitrogen, applying optical emission spectroscopy and numerical simulation

Averaged plasma parameters such as electron distribution function and electron density are determined by characterization of high frequency (2.4 GHz) nitrogen plasma using both experimental methods, namely optical emission spectroscopy (OES) and microphotography, and numerical simulation. Both direct and step-wise electron-impact excitation of nitrogen emissions are considered. The determination of space-resolved electron distribution function, electron density, rate constant for electron-impact dissociation of nitrogen molecule and the production of nitrogen atoms, applying the same methods, is discussed. Spatial distribution of intensities of neutral nitrogen molecule and nitrogen molecular ion from the microplasma is imaged by a CCD camera. The CCD images are calibrated using the corresponding emissions measured by absolutely calibrated OES, and are then subjected to inverse Abel transformation to determine space-resolved intensities and other parameters. The space-resolved parameters are compared, respectively, with the averaged parameters, and an agreement between them is established.

Modeling and experimental study of molecular nitrogen dissociation in an Ar–N2 ICP discharge

The dissociation of nitrogen molecules in an Ar–N2 inductively coupled plasma (ICP) discharge is studied both experimentally and theoretically. To measure the absolute N atom density and emission intensity of Ar and N2 excited levels, two-photon absorption laser-induced fluorescence (TALIF) spectroscopy and optical emission spectroscopy are used. We observe an increase in N atom density with increasing pressure whereas the N atom density decreases for pressures higher than 100 mTorr in a pure nitrogen discharge. On adding argon to the mixture, we observe that the dissociation rate is enhanced when going from a pure nitrogen discharge to an argon mixed discharge. To calculate the plasma parameters, a global (volume-averaged) model is developed. The variation of the electron temperature and the particle densities are calculated by solving the particle and energy balance equations. The model calculations are compared with the measurement results and the production and loss rates of each species are described under each discharge condition. From the model calculation, the dissociation of N2 molecules in the Ar–N2 mixed discharge occurs mainly by electron impact dissociation at low pressures, while at high pressures the dissociative recombination is enhanced by charge transfer between Ar+ and N2(X) as well as metastable–metastable pooling dissociation due to the high density. In addition, the surface sticking coefficient of nitrogen atoms in a planar ICP discharge (including glass and stainless steel walls) is deduced from TALIF measurements and is estimated to be 0.02 under our set-up conditions.