Nitrogen Matrix Research Articles

Method Validation of Gas Chromatography and Stability Study for Analysis of Standard Gas Mixtures of Carbon Dioxide in Nitrogen Matrix

Method Validation of Gas Chromatography and Stability Study for Analysis of Standard Gas Mixtures of Carbon Dioxide in Nitrogen Matrix

New emission band of solid nitrogen

New results on the study of radiation effects in solid nitrogen and N2-doped Ne matrix are presented, with a focus on the so-called γ-line origin. The irradiation was carried out in dc regime with an electron beam of subthreshold energy. The relaxation dynamics was monitored by emission spectroscopy: cathodoluminescence (CL) and nonstationary luminescence (NsL), along with current activation spectroscopy. Thermally stimulated luminescence (TSL) and exoelectron emission (TSEE) of pure nitrogen and N2 in the Ne matrix were measured in a correlated manner. Three emission bands were recorded in the NIR CL spectra of solid N2: 794, 802, and 810 nm. The band at 810 nm was detected for the first time. These three bands are characterized by similar behavior and form molecular series with spacing between adjacent vibrational energy levels of the ground state of 125 and 123 cm−1. These data cast doubt on the recently made assumption that the γ-line is attributed to the emission of the nitrogen anion N− [R. E. Boltnev, I. B. Bykhalo, I. N. Krushinskaya et al. Phys. Chem. Chem. Phys. 18, 16013 (2016)]. The processes of electron attachment and neutralization of positively charged species are discussed. It has been established that the γ-line in the TSL spectra of pure nitrogen and N2-doped Ne matrix correlates with TSEE currents and recombination emission of O+, N2+, and N4+ ions, which indicates its connection with the neutralization reaction. The measurement of NsL supported this conclusion. A new possible assignment of the γ-line and its satellites to the emission of tetranitrogen N4 is discussed.

Manganese-Doped Bimetallic (Co,Ni)2P Integrated CoP in N,S Co-Doped Carbon: Unveiling a Compatible Hybrid Electrocatalyst for Overall Water Splitting.

Rational design of highly efficient noble-metal-unbound electrodes for hydrogen and oxygen production at increased current density is crucial for robust water-splitting. A facile hydrothermal and room-temperature aging method is presented, followed by chemical vapor deposition (CVD), to create a self-sacrificed hybrid heterostructure electrocatalyst. This hybrid material, (Mn-(Co,Ni)2P/CoP/(N,S)-C), comprises manganese-doped cobalt nickel phosphide (Mn-(Co,Ni)2P) nanofeathers and cobalt phosphide (CoP) nanocubes embedded in a nitrogen and sulfur co-doped carbon matrix (N,S)-C on nickel foam. The catalyst exhibits excellent performance in both the hydrogen evolution reaction (HER; η10 = 61mV) and oxygen evolution reaction (OER; η10 = 213mV) due to abundant active sites, high porosity, and enhanced hetero-interface interaction between Mn-(Co2P-Ni2P) CoP, and (N,S)-C supported by significant synergistic effects observed among different phases through density functional theory (DFT) calculations. Impressively, (Mn-(Co,Ni)2P/CoP/(N,S)-C (+,-) shows an extra low cell voltage of 1.49 V@10mA cm-2. Moreover, the catalyst exhibits remarkable stability at 100 and 300mA cm-2 when operating as a single stack cell electrolyzer. The superior electrochemical activity is attributed to the enhanced electrode-electrolyte interface among the multiple phases of the hybrid structure.

Preparation and Spectroscopic Identification of the Cyclic CO2 Dimer 1,2-Dioxetanedione.

We report the preparation and infrared spectroscopic identification of 1,2-dioxetanedione, which is one of the two possible cyclic dimers of carbon dioxide. We prepared this hitherto experimentally incompletely characterized species in a solid nitrogen matrix at 3 K from the reaction of oxalyl dichloride with the urea·hydrogen peroxide complex. Surprisingly, irradiation at 254 nm does not lead to its dissociation into carbon dioxide but rather yields cyclic carbon trioxide. We further assert our spectroscopic assignments by 18O isotopic labeling and high-level N-electron valence state perturbation theory and coupled-cluster computations. The successful isolation of 1,2-dioxetanedione supports its viability as the postulated high-energy intermediate in the well-known and ubiquitously exploited "peroxyoxalate" chemiluminescent system.

Molybdenum Phosphide Quantum Dots Encapsulated by P/N-Doped Carbon for Hydrogen Evolution Reaction in Acid and Alkaline Electrolytes.

A facile and eco-friendly strategy is presented for synthesizing novel nanocomposites, with MoP quantum dots (QDs) as cores and graphitic carbon as shells, these nanoparticles are dispersed in a nitrogen and phosphorus-doped porous carbon and carbon nanotubes (CNTs) substrates (MoP@NPC/CNT). The synthesis involves self-assembling reactions to form single-source precursors (SSPs), followed by pyrolysis at 900 °C in an inert atmosphere to obtain MoP@NPC/CNT-900. The presence of carbon layers on the MoP QDs effectively prevents particle aggregation, enhancing the utilization of active MoP species. The optimized sample, MoP@NPC/CNT-900, exhibits remarkable electrocatalytic activity and durability for the hydrogen evolution reaction (HER). It demonstrates a low overpotential of 155 mV at 10 mA cm-2 , a small Tafel slope of 76 mV dec-1 , and sustained performance over 20 hours in 0.5 M H2 SO4 . Furthermore, the catalyst shows excellent activity in 1 M KOH, with a relatively low overpotential of 131 mV and long-term durability under constant current input. The exceptional HER activity can be attributed to several factors: the superior performance of MoP QDs, the large surface area and good conductivity of the carbon substrates, and the synergistic effect between MoP and carbon species.

Quartz‐Enhanced Photoacoustic Sensors for Detection of Eight Air Pollutants

A quartz‐enhanced photoacoustic spectroscopy sensor architecture capable of detecting eight different air pollutants (CH4, NO2, CO2, N2O, CO, NO, SO2, and NH3) is reported. Each analyte is targeted using the same sensor components (acoustic detection module, quartz‐tuning fork, etc.) and a dedicated laser source. Both interband cascade lasers and quantum cascade lasers are employed to target a well‐resolved and isolated absorption feature from each investigated gas, in a wavelength ranging from 3.35 to 9.06 μm. The sensor is calibrated with certified concentrations of each gas species, in a wet nitrogen matrix. For each analyte, the optimal pressure is determined. Minimum detection limits of 40, 13, 800, 230, 450, 78, 18, and 5.8 ppb are obtained for CH4, NO2, CO2, N2O, CO, NO, SO2, and NH3, respectively, at 100 ms of integration time.

Interface Density Engineering on Heterogeneous Molybdenum Dichalcogenides Enabling Highly Efficient Hydrogen Evolution Catalysis and Sodium Ion Storage.

Constructing active heterointerfaces is powerful to enhance the electrochemical performances of transition metal dichalcogenides, but the interface density regulation remains a huge challenge. Herein, MoO2 /MoS2 heterogeneous nanorods are encapsulated in nitrogen and sulfur co-doped carbon matrix (MoO2 /MoS2 @NSC) by controllable sulfidation. MoO2 and MoS2 are coupled intimately at atomic level, forming the MoO2 /MoS2 heterointerfaces with different distribution density. Strong electronic interactions are triggered at these MoO2 /MoS2 heterointerfaces for enhancing electron transfer. In alkaline media, the optimal material exhibits outstanding hydrogen evolution reaction (HER) performances that significantly surpass carbon-covered MoS2 nanorods counterpart (η10 : 156mV vs 232mV) and most of the MoS2 -based heterostructures reported recently. First-principles calculation deciphers that MoO2 /MoS2 heterointerfaces greatly promote water dissociation and hydrogen atom adsorption via the O-Mo-S electronic bridges during HER process. Moreover, benefited from the high pseudocapacitance contribution, abundant "ion reservoir"-like channels, and low Na+ diffusion barrier appended by high-density MoO2 /MoS2 heterointerfaces, the material delivers high specific capacity of 888 mAh g-1 , remarkable rate capability and cycling stability of 390 cycles at 0.1 A g-1 as the anode of sodium ion battery. This work will undoubtedly light the way of interface density engineering for high-performance electrochemical energy conversion and storage systems.

Infrared Spectra and Phototransformations of meta-Fluorophenol Isolated in Argon and Nitrogen Matrices

Monomers of meta-fluorophenol (mFP) were trapped from the gas phase into cryogenic argon and nitrogen matrices. The estimated relative energies of the two conformers are very close, and in the gas phase they have nearly equal populations. Due to the similarity of their structures (they only differ in the orientation of the OH group), the two conformers have also similar predicted vibrational signatures, which makes the vibrational characterization of the individual rotamers challenging. In the present work, it has been established that in an argon matrix only the most stable trans conformer of mFP exists (the OH group pointing away from the fluorine atom). On the other hand, the IR spectrum of mFP in a nitrogen matrix testifies to the simultaneous presence in this matrix of both the trans conformer and of the higher-energy cis conformer (the OH group pointing toward the fluorine atom), which is stabilized by interaction with the matrix gas host. We found that the exposition of the cryogenic N2 matrix to the Globar source of the infrared spectrometer affects the conformational populations. By collecting experimental spectra, either in the full mid-infrared range or only in the range below 2200 cm-1, we were able to reliably distinguish two sets of experimental bands originating from individual conformers. A comparison of the two sets of experimental bands with computed infrared spectra of the conformers allowed, for the first time, the unequivocal vibrational identification of each of them. The joint implementation of computational vibrational spectroscopy and matrix-isolation infrared spectroscopy proved to be a very accurate method of structural analysis. Some mechanistic insights into conformational isomerism (the quantum tunneling of hydrogen atom and vibrationally-induced conformational transformations) have been addressed. Finally, we also subjected matrix-isolated mFP to irradiations with UV light, and the phototransformations observed in these experiments are also described.

Simultaneous Tunneling Control in Conformer-Specific Reactions.

We present here a new example of chemical reactivity governed by quantum tunneling, which also highlights the limitations of the classical theories. The syn and anti conformers of a triplet 2-formylphenylnitrene, generated in a nitrogen matrix, were found to spontaneously rearrange to the corresponding 2,1-benzisoxazole and imino-ketene, respectively. The kinetics of both transformations were measured at 10 and 20 K and found to be temperature-independent, providing clear evidence of concomitant tunneling reactions (heavy-atom and H-atom). Computations confirm the existence of these tunneling reaction pathways. Although the energy barrier between the nitrene conformers is lower than any of the observed reactions, no conformational interconversion was observed. These results demonstrate an unprecedented case of simultaneous tunneling control in conformer-specific reactions of the same chemical species. The product outcome is impossible to be rationalized by the conventional kinetic or thermodynamic control.

Interaction of phonons with spherical nanoparticles embedded densely in simple crystalline matrix. Case of nitrogen-palladium nanocomposite

The results of experimental investigations of thermal conductivity of nanocomposites built of palladium spherical nanoparticles embedded in the structure of crystalline nitrogen are presented in this paper. The investigations were carried out on the samples containing the Pd nanospheres of diameters of 6, 8, 10, 12, 18, and 24 nm at the palladium fraction amounting to 15% of the nanocomposite volume. The measurements were performed with a steady-state heat flow method in the temperature interval of 2–35 K. For the analysis of the experimental results, the relaxation time approximation in the frame of the thermal conductivity Debye model was used. The analysis shows that the nitrogen matrix phonons can effectively interact with spherical nanoparticles in, at least, four different mechanisms. Two of those mechanisms, a diffuse scattering by the boundary of two media (which are the matrix and the material of the nanoparticle) and an oscillating resultant of interaction of nanoparticle phonons with the matrix phonons, result in a decrease of the nanocomposite thermal conductivity. The remaining two mechanisms would be considered nonthermal resistive scattering processes: The first of them is the specular scattering of the matrix phonons by the matrix and the nanoparticle material interface. The second one is a forward scattering, in which the matrix phonon penetrates the nanoparticle and afterward penetrates another one without any resistive interaction with phonons of the crystalline matrix. The analysis shows that the nonresistive processes are significantly more frequent in the investigated nanocomposites than the resistive ones. Such a conclusion is in perfect agreement with the results of the analysis of the phonon mean free path in the nitrogen crystal-palladium nanospheres composite.

Multi-gas quartz-enhanced photoacoustic sensor for environmental monitoring exploiting a Vernier effect-based quantum cascade laser

We report on a gas sensor based on quartz-enhanced photoacoustic spectroscopy (QEPAS) able to detect multiple gas species for environmental monitoring applications, by exploiting a Vernier effect-based quantum cascade laser as the excitation source. The device emission spectrum consists of ten separated emission clusters covering the range from 2100 up to 2250 cm−1. Four clusters were selected to detect the absorption features of carbon monoxide (CO), nitrous oxide (N2O), carbon dioxide (CO2), and water vapor (H2O), respectively. The sensor was calibrated with certified concentrations of CO, N2O and CO2 in a wet nitrogen matrix. The H2O absorption feature was used to monitor the water vapor within the gas line during the calibration. Minimum detection limits of 6 ppb, 7 ppb, and 70 ppm were achieved for CO, N2O and CO2, respectively, at 100 ms of integration time. As proof of concept, the QEPAS sensor was tested by continuously sampling indoor laboratory air and monitoring the analytes concentrations.

High loading of single atomic iron sites in Fe–NC oxygen reduction catalysts for proton exchange membrane fuel cells

Non-precious iron-based catalysts (Fe–NCs) require high active site density to meet the performance targets as cathode catalysts in proton exchange membrane fuel cells. Site density is generally limited to that achieved at a 1–3 wt%(Fe) loading due to the undesired formation of iron-containing nanoparticles at higher loadings. Here we show that by preforming a carbon–nitrogen matrix using a sacrificial metal (Zn) in the initial synthesis step and then exchanging iron into this preformed matrix we achieve 7 wt% iron coordinated solely as single-atom Fe–N4 sites, as identified by 57Fe cryogenic Mössbauer spectroscopy and X-ray absorption spectroscopy. Site density values measured by in situ nitrite stripping and ex situ CO chemisorption methods are 4.7 × 1019 and 7.8 × 1019 sites g−1, with a turnover frequency of 5.4 electrons sites−1 s−1 at 0.80 V in a 0.5 M H2SO4 electrolyte. The catalyst delivers an excellent proton exchange membrane fuel cell performance with current densities of 41.3 mA cm−2 at 0.90 ViR-free using H2–O2 and 145 mA cm−2 at 0.80 V (199 mA cm−2 at 0.80 ViR-free) using H2–air.

Warming promotes soil CO2 and CH4 emissions but decreasing moisture inhibits CH4 emissions in the permafrost peatland of the Great Xing'an Mountains

Permafrost peatlands, as large soil carbon pools, are sensitive to global warming. However, the effects of temperature, moisture, and their interactions on carbon emissions in the permafrost peatlands remain unclear, when considering the availability of soil matrixes. The permafrost peatland (0–50 cm soil) in the Great Xing'an Mountains was selected to explore the deficiency. The cumulative carbon dioxide (CO2) and methane (CH4) emissions from soil were measured under different temperatures (5 °C, 10 °C, and 15 °C) and moisture content (130%, 100%, and 70%) treatments by the indoor incubation. The results showed that the soil carbon and nitrogen matrix determined soil carbon emissions. Warming affected the availability of soil carbon and nitrogen substrates, thus stimulating microbial activity and increasing soil carbon emissions. With soil temperature increasing by 10 °C, soil CO2 and CH4 emission rates increased by 5.1–9.4 and 3.8–6.4 times respectively. Warming promoted soil carbon emissions, and the decrease of moisture content promoted CO2 emissions but inhibited CH4 emissions in the permafrost peatland. Soil moisture and the carbon and nitrogen matrix determined the intensity of CO2 and CH4 emissions. The results were important to assess soil carbon emissions from permafrost peatlands under the impact of future climate warming and to formulate carbon emission reduction policies.

Iron–Cobalt-Based Materials: An Efficient Bimetallic Catalyst for Ammonia Synthesis at Low Temperatures

Ammonia is one of the most commonly produced chemicals in the industry. As a result, ∼1–2% of the world’s electrical energy is used, and it produces ∼1.5–2% of global CO2 emissions. Therefore, developing efficient catalysts at milder conditions is highly desirable. Here, efficient iron/cobalt catalysts for ammonia synthesis are prepared with different Fe/Co ratios from phthalocyanine precursors, resulting in Fe–Co bimetallic nanoparticles embedded in a porous carbon–nitrogen matrix. The incorporation of Co to the Fe catalyst up to 20% wt of Co leads to ∼40% enhancement in the activity compared to the monometallic Fe-based catalyst. Interestingly, catalysts exhibit excellent activity even at low temperatures (350 °C). Ea for the most efficient 6K–FePc80CoPc20 catalyst is found to be 29 kJ·mol–1, suggesting facile activation of N2 at low temperatures. An in-depth kinetic study revealed that introducing Co in the Fe catalysts drastically tuned the surface of the catalyst by weakening the various NHx retarding species. Density functional theory calculations confirm the thermodynamic feasibility for introducing 25% of Co at surface Fe sites. This indicates that the presence of Co on the surface draws negative charges from neighboring Fe sites, making the exposed Fe sites neutral and more favorable for N2 activation on the bimetallic K2O/Co–Fe catalyst and displaying a more thermodynamically feasible energy profile as compared to the pristine K2O/Fe catalyst.

A matrix isolation infrared spectroscopic study of thermal isomerisation of 1-butanol

1-butanol has fourteen distinct conformers. Vibrational spectroscopy in O-H and C-O stretching frequency region is reported earlier. Conformer specific assignment is not yet carried out. Herein, 1-butanol is investigated in the nitrogen matrix at 8 K. Eight distinct bands have been observed in the O-H stretching region and nine distinct bands have been observed in the C-O stretching region. Conformer specific assignment of the observed O-H and C-O bands is carried out. Thermal cyclisation of the deposited matrix resulted isomerisation between the two most stable conformers along with others that has been monitored using O-H and C-O bands.

Confinement effects on C–H and C–F stretching vibrational frequencies of difluoromethane in cold inert gas matrixes: a combined infrared spectroscopy and electronic structure theory study

Mid-infrared spectra of difluoromethane (DFM) have been recorded by confining the molecule in the matrix cages of pure argon, nitrogen and in the mixed matrixes of argon and nitrogen obtained by mixing the two gases in definite proportions. The measured spectra depict distinct confinement effects in terms of the infrared spectral shifts of the C–H and C–F stretching vibrational fundamentals as compared to the free molecules in the gas-phase, and also some remarkable changes in band appearances and relative intensities. The signs of the spectral shifts are observed to be different for the $$\upnu _{\text {C--H}}$$ and $$\upnu _{\text {C--F}}$$ transitions. Pronounced blue shifts are observed for the two $$\upnu _{\text {C--H}}$$ modes in pure nitrogen matrix, which progressively reduce in magnitude in the mixed matrixes, and is least in the pure argon matrix. On the other hand, distinct red shifts of the $$\upnu _{\text {C--F}}$$ vibrations are evident in pure nitrogen matrix, which are reduced in magnitude in the mixed matrixes, and become even less pronounced in pure argon matrix. The effect of Fermi resonances in the $$\upnu _{\text {C--H}}$$ region, that has motivated several previous investigations of DFM including high resolution gas phase studies, becomes explicit upon comparison of the matrix isolation spectrum with the gas phase spectrum. Electronic structure calculations using several DFT and DFT-D methods, in conjunction with NBO analysis, have been carried out for various sizes of $$\hbox {DFM-N}_{2}$$ and DFM-Ar complexes in order to understand the underlying interactions responsible for the observed shifts. The calculations are found to satisfactorily reproduce the observed variations in magnitude and direction of the shifts of both the $$\upnu _{\text {C--H}}$$ and $$\upnu _{\text {C--F}}$$ transitions for the change of the medium. It is inferred that the inert gas matrix environment exerts an electronic re-organization within the DFM molecule, and the consequent rehybridization of the carbon-centric hybrid orbital of the C–H and C–F bonds is responsible for the observed spectral shifts.

Study on Electrochemical Oxygen Evolution Properties of Bimetallic Carbon Nan Sheets

Transition metal oxide doped two-dimensional materials, which combine the advantages of multiphase and homogeneous electrocatalysts, have attracted extensive attention in recent years. However, the electrocatalysts with single metal component can not further meet the requirements of catalytic performance. In this paper, a bimetallic organic complex, CuCo-MOF, was prepared from L-aspartic acid and copper ions and cobalt ions. The effects of temperature and metal ratio on the oxygen evolution properties of the prepared materials were investigated, and CN@CuO/Co2O3 nanosheets with excellent oxygen evolution electrochemical activity were optimized. The catalyst is a compound of CuO and Co2O3 nanoparticles doped into the carbon nitrogen matrix. A large number of homogeneous CuO and Co2O3 nanoparticles cooperate on the ultra-thin carbon nitrogen substrate with high conductivity, showing low initial oxygen evolution over potential and high catalytic current density. A current density of 10 mA cm-2 can be only achieved at 1.54 V vs. RHE.

Vibrationally Induced Conformational Isomerization and Tunneling in Pyrrole-2-Carboxylic Acid

The conformational behavior of carboxylic acids has attracted considerable attention, as it can be used as a gateway for the study of more complex phenomena. Here, we present an experimental and computational study of pyrrole-2-carboxylic acid (PCA) conformational space and the vibrational characterization of the compound by infrared spectroscopy. The possibility of promoting conformational transformations using selective vibrational excitation of the 2ν(OH) and 2ν(NH) stretching overtones is explored. Two conformers, exhibiting the cis configuration of the COOH group (O═C-O-H dihedral angle near 0°) and differing by the orientation of the carboxylic group with respect to the pyrrole ring (i.e., showing either a cis or a trans NCC═O arrangement), were found to coexist initially for the compound isolated in a cryogenic nitrogen matrix, in an 86:14 ratio, and were characterized by infrared spectroscopy. A third conformer, with the COOH group in the trans configuration, was produced, in situ, by narrowband near-infrared (NIR) excitation of the most stable PCA form (with a cis NCC═O moiety). The photogenerated PCA conformer was found to decay back to the most stable PCA form, by H-atom quantum mechanical tunneling, with a characteristic half-life time of ∼10 min in the nitrogen matrix at 10 K. Tunneling rates were theoretically estimated and compared for the observed isomerization of pyrrole-2-carboxylic acid and for the structurally similar furan-2-carboxylic acid. This comparison showcases the effect of small modifications in the potential energy surface and the implications of quantum tunneling for the stability of short-living species.

Full Implementation of Matrix Approach to Biogeochemistry Module of CLM5

AbstractEarth system models (ESMs) have been rapidly developed in recent decades to advance our understanding of climate change‐carbon cycle feedback. However, those models are massive in coding, require expensive computational resources, and have difficulty in diagnosing their performance. It is highly desirable to develop ESMs with modularity and effective diagnostics. Toward these goals, we implemented a matrix approach to the Community Land Model version 5 (CLM5) to represent carbon and nitrogen cycles. Specifically, we reorganized 18 balance equations each for carbon and nitrogen cycles among the 18 vegetation pools in the original CLM5 into two matrix equations. Similarly, 140 balance equations each for carbon and nitrogen cycles among the 140 soil pools were reorganized into two additional matrix equations. The vegetation carbon and nitrogen matrix equations are connected to soil matrix equations via litterfall. The matrix equations fully reproduce simulations of carbon and nitrogen dynamics by the original model. The computational cost for forwarding simulation of the CLM5 matrix model was 26% more expensive than the original model, largely due to calculation of additional diagnostic variables, but the spin‐up computational cost was significantly saved. We showed a case study on modeled soil carbon storage under two forcing data sets to illustrate the diagnostic capability that the matrix approach uniquely offers to understand simulation results of global carbon and nitrogen dynamics. The successful implementation of the matrix approach to CLM5, one of the most complex land models, demonstrates that most, if not all, the biogeochemical models can be reorganized into the matrix form to gain high modularity, effective diagnostics, and accelerated spin‐up.

Ethanol monomer revisited: Thermal isomerisation between anti and gauche conformers in Ar and N2 matrix

The anti-gauche conformational distribution of ethanol has been investigated using the O-H and symmetric C-C-O stretching infrared spectra in argon and nitrogen matrix. The dipole moment of the gauche conformer is higher compared to the anti conformer. The relative population of the anti and gauche conformer of ethanol depends on the rare gas mixture in the gas phase and also in the matrix after thermal cyclisation. In the gas phase, the anti conformer is more populated in nitrogen mixture whereas the gauche conformer is more populated in the argon mixture. After thermal cyclisation in the matrix, the anti conformer isomerise to the gauche conformer in nitrogen matrix and reverse happens in the argon matrix. Thermal cyclisation at 30 K in nitrogen matrix leads to the splitting of the gauche± states that is probed by monitoring symmetric C-C-O stretching frequency mode.