Chemical Composition,Physical Properties and Populating Mechanism of Some O(I) States for a DC Discharge in Oxygen with Water Cathode

This paper reports the results of the experimental study of parameters for a DC oxygen discharge with water cathode in the pressure range of 0.1–1 bar and the discharge current of 40 mA. The radius of positive column, the cathode voltage drop, the cathode current density and the electric field strength were measured. Rotational temperatures of N₂ (C³Π_u, V = 0) and OH (A²Σ, V = 0) and absolute line intensities of atomic oxygen with wave length of 845 and 777 nm were determined as well. Plasma composition modeling was carried out by the combined solution of the Boltzmann equation for electrons, the equations of vibrational kinetics for ground states of N₂, O₂, H₂O molecules, and the equations of chemical kinetics, and the plasma conductivity equation. Calculations were carried out taking into consideration the discharge radial heterogeneity and using experimental values of E/N and gas temperatures. The main particles being formed in plasma were shown to be ^·OH, H₂O₂, O(³P), O₂(a¹Δ_g), O₂(b¹Σ _g ⁺ ), H(¹S). On the basis of this calculation and experimental values of line intensities, the populating mechanism of (3p ³P) level of atomic oxygen was discussed. The comparison of some properties of discharges in O₂, N₂ and air was done.     

CARS Diagnostic and Modeling of a Dielectric Barrier Discharge

Dielectric barrier discharges (DBD) with planar- and knife-shaped electrodes are operated in N₂O₂NO mixtures under a pressure of 20 and 98 kPa. They are excited by means of consecutive unipolar or bipolar high-voltage pulse packages of 10 kV at a pulse repetition rate of 1 and 2 kHz. The rotational and vibrational excitation of N ₂ molecules and the reduction of nitric oxide (NO) in the discharge have been investigated using coherent anti-Stokes Raman scattering (CARS) technique. Rotational (gas) temperatures near the room temperature and vibrational temperatures of about 800 K at atmospheric pressure and 1400 K at a pressure of 20 kPa are observed. Therefore, chemical reactions of NO with vibrationally excited N ₂ are probably insignificant. One-dimensional kinetic models are developed that balance 35 chemical reactions between 10 species and deliver equations for the population density of excited vibrational levels of N ₂ together with a solution of the Boltzmann equation for the electrons. A good agreement between measured vibrational temperatures of N ₂ , the concentration of NO, and calculated data is achieved. Modeling of the plasma discharge verifies that a DBD operated with a N₂NO mixture reduces the NO content, the simultaneous presence of O ₂ , already 1%, is enough to prevent the NO reduction.     

Vibrational analysis of N2(B 3Πg and C 3Πu) and CO(X) excited in N2 discharge and post discharge

A spectroscopic characterization of a N₂ radiofrequency discharge and N₂CO post discharge has been performed. The relative vibrational distribution of the excited B ³Π_g and C ³Π_u states of nitrogen and their correlation with the ground state have been analyzed. The analysis confirms the importance of the metastable molecules. N₂(A ³Σ⁺_u), in affecting the vibrational distribution of nitrogen in its ground state in the discharge and post discharge. The vibrational analysis of the CO ground state, excited in the post discharge by vibrationally excited N₂ molecules, confirms the high degree of vibrational non-equilibrium in the ground state of nitrogen, in the presence of a low first-level vibrational temperature.     

Rotating Gliding Arc Assisted Water Splitting in Atmospheric Nitrogen

In this study, hydrogen production from water splitting in N₂ using an atmospheric pressure rotating gliding arc plasma was investigated. The effect of input H₂O concentration and total flow rate on the performance of the plasma water splitting process (e.g., H₂ and O₂ yield, H₂ production rate, and energy yield of H₂) was investigated. N₂ showed a pronouncedly facilitating effect on the H₂O splitting and H₂ production process due to the reactions of the excited N₂ species [e.g., electronically excited metastable N₂(A)] with the H₂O molecules. The maximum H₂ production rate reached up to 41.3 μmols^?1, which is much higher than that of other typical non-thermal plasmas (e.g., ~0.2 μmols^?1 for a dielectric barrier discharge). Optical emission diagnostics has shown that in addition to the NO, N₂, and N₂ ⁺ that were observed in the pure N₂ spectra, strong OH and NH emission lines also appeared in the H₂O/N₂ spectra. OH radical is considered as a key intermediate species that could contribute to the formation of H₂, O₂, and H₂O₂. The increase of the H₂O concentration could lead to a continuous enhancement of the OH intensity. The rotational temperature of N₂ ⁺ dropped drastically from 2875 ± 125 to 1725 ± 25 K with the addition of 1 % (mol/mol) H₂O into the N₂ plasma.     

Self-consistent field calculations using two-body density functionals for correlation energy component: II. Small molecules

In part I of this series, self-consistent calculations using two-body density functionals for correlation energy were done and applied to atomic systems, giving very good results. We now apply the same scheme to small molecules. The examples studied include diatomic (H₂, Li₂, B₂, C₂, N₂, O₂, F₂, HLi, HBe, HB, HF, and HCl) as well as polyatomic (H₂O, NH₃, H₂O₂, and O₃) molecules at their ground states. The values reported for equilibrium geometries, atomization energies, vibrational frequencies, and dipole moments are compared with experimental and other theoretical calculations, with good agreement in most cases. © 1998 John Wiley & Sons, Inc. J Comput Chem 19: 1899–1908, 1998     

Ionisation of small molecules by state-selected Ne* (3P0, 3P2) metastable atoms in the 0.06 < E < 6 eV energy range

The velocity dependence and absolute values of the total ionisation cross section for the molecules H₂, N₂, O₂, NO, CO, N₂O, CO₂, and CH₄ by metastable Ne* (³P₀) and Ne* (³P₂) atoms at collision energies ranging from 0.06 to 6.0 eV have been measured in a crossed beam experiment. State selection of the two metastable states of Ne* was obtained by optical pumping with a cw dye laser. We observe a strongly different velocity dependence at collision energies below about 1 eV for the ionisation cross section of the systems Ne*H₂, N₂, CO, and CH₄, and the systems Ne*O₂, NO, CO₂, and N₂O, respectively. The first group shows an increasing cross section in this energy range, similar to the Ne*Ar system, while the second group shows a very flat behaviour. This behaviour correlates with the difference in character (π or σ^b) of the orbital of the electron that is removed from the target molecule. For the molecules H₂, N₂, CO, and CH₄ an electron from a σ^b orbital is removed from the molecule, whereas for O₂, NO, N₂O, and CO₂ an outer π-ortibal electron is involved. For the systems Ne* (³P₀, ³P₂)H₂ we have derived the imaginary part of the optical potential by assuming a real potential similar to the theoretically calculated ground state NaH₂ potential of Botschwina et al. The resonance width Γ(r) as a function of the internuclear distance r shows a saturation at small r (r < 2.8 Å) for both the Ne*(³P₀)H₂ and the Ne*(³P₂)H₂ interaction. This supports previous conclusions of Verheijen et al. and Kroon et al. Reliable values for the absolute value of the total ionisation cross section have been obtained by performing a careful calibration of the density—length product of the supersonic secondary beam. The results are in good agreement with the values of West et al. for experiments without state selection. The total ionisation cross sections for molecules with π-type ionisation orbitals, with their larger spatial extent, in general are larger than those for molecules with σ^b-type ionisation orbitals.     

Transient Spark Discharge Generated in Various N2/O2 Gas Mixtures: Reactive Species in the Gas and Water and Their Antibacterial Effects

Reactive species generated in the gas and in water by cold air plasma of the transient spark discharge in various N₂/O₂ gas mixtures (including pure N₂ and pure O₂) have been examined. The discharge was operated without/with circulated water driven down the inclined grounded electrode. Without water, NO and NO₂ are typically produced with maximum concentrations at 50% O₂. N₂O was also present for low O₂ contents (up to 20%), while O₃ was generated only in pure O₂. With water, gaseous NO and NO₂ concentrations were lower, N₂O was completely suppressed and HNO₂ increased; and O₃ was lowered in O₂ gas. All species production decreased with the gas flow rate increasing from 0.5 to 2.2 L/min. Liquid phase species (H₂O₂, NO₂￣, NO₃￣, ^·OH) were detected in plasma treated water. H₂O₂ reached the highest concentrations in pure N₂ and O₂. On the other hand, nitrites NO₂￣ and nitrates NO₃￣ peaked between 20 and 80% O₂ and were associated with pH reduction. The concentrations of all species increased with the plasma treatment time. Aqueous ^·OH radicals were analyzed by terephthalic acid fluorescence and their concentration correlated with H₂O₂. The antibacterial efficacy of the transient spark on bacteria in water increased with water treatment time and was found the strongest in the air-like mixture thanks to the peroxynitrite formation. Yet, significant antibacterial effects were found even in pure N₂ and in pure O₂ most likely due to high ^·OH radical concentrations. Controlling the N₂/O₂ ratio in the gas mixture, gas flow rate, and water treatment time enables tuning the antibacterial efficacy.

     

An evaluation of the mechanism of nitrous acid formation in the urban atmosphere

Nitrous acid (HONO) has been observed to build in the atmosphere of cities during the nighttime hours and it is suspected that photolysis of HONO may be a significant source of HO radicals early in the day. The sources of HONO are poorly understood, making it difficult to account for nighttime HONO formation in photochemical modeling studies of urban atmospheres, such as modeling of urban O₃ formation. This paper reviews the available information on measurements of HONO in the atmosphere and suggest mechanisms of HONO formation. The most extensive atmospheric measurement databases are used to investigate the relations between HONO and potential precursors. Based on these analyses, the nighttime HONO concentrations are found to correlate best with the product of NO, NO₂ and H₂O concentrations, or possibly the NO, NO₂, H₂O, and aerosol concentrations. A new mechanism for nighttime HONO formation is proposed that is consistent with this precursor relationship, namely, reaction of N₂O₃ with moist aerosols (or other surfaces) to form two HONO molecules. Theoretical considerations of the equilibrium constant for N₂O₃ formation and the theory of gas-particle reactions show that the proposed reaction is a plausible candidate for HONO formation in urban atmospheres. For photochemical modeling purposes, a relation is derived in terms of gas phase species only (i.e., excluding the aerosol concentration): NO + NO₂ + H₂O → 2 HONO with a rate constant of 1.68 x 10^-17 e^6348/T (ppm^-2 min^-1). This rate constant is based on an analysis of ambient measurements of HONO, NO, NO₂ and H₂O, with a temperature dependence from the equilibrium constant for formation of N₂O₃. Photochemical grid modeling is used to investigate the effects of this relation on simulated HONO and O₃ concentrations in Los Angeles, and the results are compared to two alternative sources of nighttime HONO that have been used by modelers. Modeling results show that the proposed relation results in HONO concentrations consistent with ambient measurements. Furthermore, the relation represents a conservative modeling approach because HONO production is effectively confined to the model surface layers in the nighttime hours, the time and place for which ambient data exist to show that HONO formation occurs. The empirical relation derived here should provide a useful tool for modelers until such time as knowledge of the HONO forming mechanisms has improved and more quantitative relations can be derived.     

The Effect of Oxygen and Water Vapor on Nitric Oxide Conversion with a Dielectric Barrier Discharge Reactor

The effect of O₂ and H₂O vapor on the Nitric oxide (NO) removal rate, the NO₂ generation rate and the discharge characteristics were investigated using the dielectric barrier discharge (DBD) reactor at 1 atm pressure and at room temperature (20°). The results showed that the O₂ present in the flue gas always hampered the removal of NO and the generation of N₂O, but that the O₂ could enhance the generation of NO₂ in the NO/N₂/O₂ mixtures. Furthermore, with the increase of oxygen, the average discharge current gradually decreases in the reactor. The H₂O present in N₂/NO hindered the removal of NO and the generation of NO₂ but had no impact on the average discharge current in the reactor in the NO/N₂/H₂O mixtures in which the HNO₂ and HNO₃ was detected. The energy efficiency of the DBD used to remove the NO from the flue gas was also estimated.     

Variational calculation of vibrational energies of triatomic molecules using SCF optimized modes

A self-consistent field optimization of the vibrational coordinates for nonlinear triatomic molecules is presented. The optimal coordinates are obtained by making a three-dimensional rotational transformation of the normal modes and determining the rotation angles as those for which the SCF energy is stationary. The utility of the optimized coordinates in full variational calculations of vibrational energies is studied for the molecules of H₂O, O₃, H₂D⁺, H₂T⁺, and D₂T⁺. For H₂O and O₃, the optimization procedure leads to the local mode representation. It is shown that the use of the optimal coordinates in variational calculations allows a large reduction of the dimension of the Hamiltonian matrix to be diagonalized in order to reach convergence.     

The Mechanisms of Formation and Decay of Atoms in N2-H2 Plasmas

The formation rate and number densities of H(² S) and N(⁴ S) atoms, rate constants of their heterogeneous decay, and some electrophysical characteristics of plasma were measured in the positive column of a dc discharge in an H₂–N₂ gas mixture at a pressure of 266 Pa and a discharge current of 50 mA. From these data, by the combined solution of the Boltzmann equation, vibrational kinetics equations for ₂(X ¹_g) and N₂(X ¹_g), chemical kinetics equations, and plasma energy and heat balance equations, it was found that vibrationally excited nitrogen molecules must play a substantial role in the formation of both hydrogen and nitrogen atoms.     

Transport Coefficients in Water Plasma: Part I: Equilibrium Plasma

Knowledge of the transport coefficients of steam water plasma is important for modeling plasma flow processes and heat transfer. In this study, calculations of these properties were performed in a temperature range from 400 to 30,000 K and at pressures of 0.5, 1.0, 5.0 and 10 bar. Herein the composition of water plasma was determined at equilibrium. First, the most recent data on potential interactions and elastic differential cross sections for interacting particles were carefully examined in order to choose those most appropriate for determining the collision integrals. Second, we restricted the number of species to ten (e, H, O, H⁺, O⁺, O⁺⁺, H₂, O₂, OH and H₂O) and tested our collision integrals by comparing the thermal conductivity and viscosity to experimental data for water (at low temperatures). Finally, the total thermal conductivity, viscosity and electrical conductivity were calculated for different pressures.     

Infrared studies of the diatomic molecules O2, N2, NO and H2 adsorbed on Fe2O3

An infrared spectroscopic study of the diatomic molecules O₂, N₂, NO and H₂ adsorbed under different conditions on Fe₂O₃ has been performed.Complex patterns of absorption on both α-Fe₂O₃ and γ-Fe₂O₃ activated in O₂ at high temperature are assigned to vibrations of two different chemisorbed O₂ species.N₂ molecules do not interact with “oxygen rich” α-Fe₂O₃ surfaces, but give N₂O^? and N₂O₂^2? species when chemisorbed on evacuated surfaces.NO molecules give complex patterns of absorption, depending on the gas pressure. Three different types of nitrate structures can be identified, as well as NO, NO^? and cis-N₂O₂ chemisorbed species. Chemisorbed water molecules are formed by contact of H₂ with Fe₂O₃ surfaces even at room temperature.     

Ferromagnetism, paramagnetism, and thermally induced magnetism in photomagnetic CrIII/MnII and CrIII oxalates with the 7-methyl-3,3-diphenyl-3H-pyrano[3,2-f]quinolinium cation

Single crystals of the new cationic chromene, 7-methyl-3,3-diphenyl-3H-pyrano[3,2-f-quinolinium iodide (C₂₅H₂₀NO)I (1), were synthesized. The crystal structure of the new compound was studied, and quantum chemical calculation for the open and closed forms were carried out. The bifunctional compounds containing mono- and bimetallic 3d metal (tris)oxalates with the chromenium cation, (C₂₅H₂₀NO)₃[Cr(C₂O₄)₃] · 4H₂O (2) and (C₂₅H₂₀NO)[CrMn(C₂O₄)₃] · H₂O (3), were prepared. Compound 1 is paramagnetic due to low-lying thermally excited states of the chromene molecules. At low temperatures (∼2 K), the paramagnetic states are frozen, and the compound becomes diamagnetic. Compound 2 is paramagnetic and its magnetic properties are determined mainly by the Cr³⁺ ions and the thermally induced paramagnetic states of the chromene molecules. At high temperatures, the magnetic moment of compound 3 consists of the contributions of the paramagnetic Cr³⁺ and Mn²⁺ ions and the thermally induced paramagnetic states of chromenes. At low temperatures (2–3 K), the thermally induced magnetism of organic molecules is frozen, and the magnetically ordered (and, probably, spin-glass) state is observed in the two-dimensional network of metal oxalates (T _c = 3 K in the zero magnetic field). The UV irradiation leads to an increase in the magnetic moment of the compound in the paramagnetic region due to the generation of radiation defects.     

Conversion of NO in NO/N2, NO/O2/N2, NO/C2H4/N2 and NO/C2H4/O2/N2 Systems by Dielectric Barrier Discharge Plasmas

An experimental study on the conversion of NO in the NO/N₂, NO/O₂/N₂, NO/C₂H₄/N₂ and NO/C₂H₄/O₂/N₂ systems has been carried out using dielectric barrier discharge (DBD) plasmas at atmospheric pressure. In the NO/N₂ system, NO decomposition to N₂ and O₂ is the dominating reaction; NO conversion to NO₂ is less significant. O₂ produced from NO decomposition was detected by an on-line mass spectrometer. With the increase of NO initial concentration, the concentration of O₂ produced decreases at 298 K, but slightly increases at 523 K. In the NO/O₂/N₂ system, NO is mainly oxidized to NO₂, but NO conversion becomes very low at 523 K and over 1.6% of O₂. In the NO/C₂H₄/N₂ system, NO is reduced to N₂ with about the same NO conversion as that in the NO/N₂ system but without NO₂ formation. In the NO/C₂H₄/O₂/N₂ system, the oxidation of NO to NO₂ is dramatically promoted. At 523 K, with the increase of the energy density, NO conversion increases rapidly first, and then almost stabilizes at 93–91% of NO conversion with 61–55% of NO₂ selectivity in the energy density range of 317–550 J L⁻¹. It finally decreases gradually at high energy density. A negligible amount of N₂O is formed in the above four systems. Of the four systems studied, NO conversion and NO₂ selectivity of the NO/C₂H₄/O₂/N₂ system are the highest, and NO/O₂/C₂H₄/N₂ system has the lowest electrical energy consumption per NO molecule converted.     

Active Species Generated by a Pulsed Arc Electrohydraulic Discharge Plasma Channel in Contaminated Water Treatments

Pulsed arc electrohydraulic discharge (PAED) direct plasma technique was applied for various types of contaminated water treatment. The experimental system consists of a spark-gap switch type pulse power supply (0.5 kJ/pulse) and a 3 L stainless steel reactor with eccentrically configured rod-to-rod electrodes. The current and voltage waveforms are fundamentally different for different conductivity water. Double pulse current discharges were observed for pond water with relatively higher conductivity (637 mS/m) while a single pulse current discharge was observed for lake water with relatively lower conductivity (78 mS/m). From the optical emission spectrum and UV dosimeters, UV-A, UV-B, excited molecules and radicals including N₂*, O, H, OH, O₃ etc. were observed during the discharge period. Both optical emission and UV intensities in pond water are slightly lower than lake water. The decay time of the UV-A, N₂*, OH, H and O radicals were around 0.6 ms, where the discharge period ended around 0.4 ms. The results indicate that the radicals existed longer than the discharge period. The pH, dissolved oxygen and conductivity were changed during the course of PAED treatment. The ions and radicals such as H·, O·, H⁺, OH· etc. generated by PAED may cause alterations density decay time and the active species in water were present for a longer period. The reduction of total organic carbon (TOC) in pond water reached 80% after 5 min of PAED treatment. Based on local thermal plasma equilibrium (LTE) model, LTE thermal plasma chemical composition model for 1 mol water vapour was used to compare to present experiments. PAED discharge in contaminated pond water generated peak concentration of OH = 28% and O₂H = 0.012% mol in gas-phase and migrated to water-phase via gas–liquid interfaces generated by arc and micro-bubbles to form more stable O₃, H₂O₂, H₂ and O₂. The model suggested that the reduction of TOC occurred in gas/plasma phases or liquid-phase side of gas liquid interfaces.     

Characterisation by rotational spectroscopy of a hydrogen-bonded dimer formed between H2O and HCN

The rotational spectra in the vibrational ground states of (H₂O, HC¹⁴N) and (H₂O, HC¹⁵N) have been assigned in the frequency range 6–19 GHz. Values of rotational constants (B_O, C_O) and centrifugal distortion constants (Δ_J, Δ_JK) have been determined for both species, while the ¹⁴N-nuclear quadrupole coupling constants x_aa and x_bb have been established for the first. Observations concerning additional hyperfine structure arising from H,H nuclear spin-nuclear spin coupling in the H₂O subunit suggest that (H₂O,HCN) has a pair of equivalent protons and is effectively planar in the zero-point state. Observed spectroscopic constants are consistent only with the arrangement H₂O…HCN, with r(O…C) = 3.1387 Å.     

X-ray spectra and core hole energy curves of some diatomic molecules

Soft X-ray emission spectra of the molecules CO, N₂, NO and O₂ are examined for the purpose of deriving information on their core hole energy curves. Molecular force constant and equilibrium bond lengths are determined for the core hole species C*O and N₂*, and a qualitative analysis is made for CO*, N*O, NO* and O₂*. The results show that differences of equilibrium geometries between the core hole states and the ground states are very well reproduced (better than 1 pm) by SCF calculations within the Hartree-Fock formalism. Inclusion of anharmonicity in the Franck-Condon analysis gives a small but significant effect on the best fitted value for the core hole state bond lengths (about 0.5 pm). Oxygen is binding energies determined from the X-ray spectra are shown to agree with ESCA data, in most cases within a few tenths of an eV. Calculated ΔSCF transition energies reproduce the experimental data within a few eV.     

The lifetimes of gas phase CO2˙− and N2O˙− calculated from the transition probability of the autodetachment process A− → A + e−

A procedure to calculate the quantum mechanical transition probability of a unimolecular primary chemical process, A^? → A + e^? is investigated for the circumstance where A^? and A have different numbers of vibrational and rotational degrees of freedom (one is linear, the other not). A procedure is introduced to deal with the coupling between the vibrational and rotational motions. The proposed method was applied to calculating the lifetimes of CO₂^˙? and N₂O^˙? in the gas phase. The geometry optimizations and frequency calculations for CO₂, CO₂^˙?, N₂O, and N₂O^˙? are performed at HF, MP2, and QCISD(T) levels with 6-31G* or 6–31+G* basis sets, in order to obtain reliable geometric and spectroscopic information on these systems. Lifetimes are calculated for several of the lower vibrational–rotational states of the anions, as well as for the Boltzmann distribution of states at 298 K. The lifetime of the lowest vibrational–rotational state of CO₂^˙?, is 1.03 × 10^?4 s, and of the lowest vibrational state with rotational levels weighted by Boltzmann distribution at 298 K, 1.50 × 10^?4 s. These values are in good agreement with the experimental number, 9.0 ± 2.0 × 10^?5 s, and support the experimental evidence that CO₂^˙? was formed in its ground vibrational level by the techniques used. The lifetime of CO₂^˙? calculated with Boltzmann distribution over its vibrational and rotational levels at 298 K, is 1.51 × 10^?5 s. There are no direct measurements of the lifetime of N₂O^˙?, but it was estimated to be greater than 10^?4 s from experimental evidence. The predicted lifetimes of N₂O^˙?, at its lowest vibrational–rotational state (0 K) and lowest vibrational state with rotational levels weighted by the Boltzmann distribution at 298 K, are 238 and 19.1 s, respectively. The lifetime of N₂O^˙? at thermal equilibrium at 298 K is 6.66 × 10^?2 s, indicating that electron loss from the excited vibrational states of N₂O^˙? is significant. This study represents the first theoretical investigation of CO₂^˙? and N₂O^˙? lifetimes. © 1994 John Wiley & Sons, Inc.     

The structure of {[Co(pht)(bpy)(H2O)]·3H2O}n (pht is phthalate and bpy is 4,4′‐bipyridine) and the role of solvent water clusters in structure stability

The Co^II cation in poly[[aqua(μ‐benzene‐1,2‐dicarboxylato‐κ³O¹,O²:O¹)(μ‐4,4′‐bipyridine‐κ²N:N′)cobalt(II)] trihydrate], {[Co(C₈H₄O₄)(C₁₀H₈N₂)(H₂O)]·3H₂O}_n, is octahedrally coordinated by two N atoms of two 4,4′‐bipyridine ligands, three O atoms from phthalate anions and a fourth O atom from a coordinated water molecule. The packing consists of planes of coordination polymers linked by hydrogen bonds mediated by three solvent water molecules; the linkage is achieved by the water molecules forming intricate oligomeric clusters which also involve the O atoms of the phthalate ligands.