Water splitting in low-temperature ac plasmas at atmospheric pressure

Plasma-induced water splitting at atmospheric pressure has been studied with a novel fan-type Pt reactor and several tubular-type reactors: an all-quartz reactor, a glass reactor, and three metal reactors with Pt. Ni, and Fe as electrodes. Reaction products have been analyzed by using GC (gas chromatography) and Q-MS (quadrupole mass spectrometry). Optical emission spectroscopic studies of the process have been carried out by employing a CCD (charge-coupled device) detector. Water splitting with tubular quartz and glass reactors is probably non-catalytic. However, a heterogeneous catalytic function of surface of metal electrodes has been observed. The variation of hydrogen yield (Y_H) and energy efficiency (E_e) with operational parameters such as input voltages (U_in), flow rates of carrier gas (F_He), and concentrations of water (C_W) has been examined. Plasma-induced water splitting can be described with a kinetic equation of-dC_w/dt = kC_W ^0.2. The rate constants at 3.25 kV are 2.8 × 10⁻⁴, 3.5 × 10⁻³, and 3.4 × 10⁻² mol^0.8L^−0.8 min⁻¹ for tubular glass reactor, a tubular Pt reactor, and a fan-type Pt reactor, respectively. A CSTR (continuous-stirred tank reactor) and PFR (piston-flow reactor) model have been applied to a fan-type reactor and tubular reactor, respectively. A mechanism on the basis of optical emission spectroscopic data has been obtained comprising the energy transfer from excited carrier gas species to water molecules, which split via radicals of HO^·, O^·, and H^· to form H₂ and O₂. The fan-type Pt reactors exhibit highest activity and energy efficiency among the reactors tested. Higher yields of hydrogen are achieved at higher input voltages, low flow rates, and low concentrations of water (Y_H = 78 % at U_in of 3.75 kV, F_He of 20 mL/min, and C_W of 0.86 %). The energy efficiency exhibits an opposite trend (E_e = 6.1 % at U_in of 1.25 kV, F_He of 60 mL/min and C_W of 3.1 %).     

Stabilization of microwave arc plasmas of hydrocarbons at atmospheric pressure

Pure hydrocarbon plasmas have been generated at low pressures with good efficiency using methane as a reactant. Hydrocarbon plasma discharges containing high energy, free radical, and ionized intermediates were analyzed in situ using emission spectroscopy. Emission spectra were correlated with analytical data obtained from resultant product mixtures and literature assignments of emission bands in order to identify these intermediates. Stabilization of atmospheric methane plasmas using argon as a diluent has also been demonstrated in this study. Emission spectroscopy has also been used to identify reaction intermediates formed in plasmas at high pressures. Distinct differences in plasma discharges have been observed as a function of pressure, power, and methane concentrations at the molecular level using in situ spectroscopic techniques.     

Time-resolved optical emission spectrometry of Q-switched Nd:YAG laser-induced plasmas from copper targets in air at atmospheric pressure

Q-switched Nd:YAG laser induced plasma from a copper target was studied by time-gated optical multichannel analyser. The features of the spectra such as the plasma background, the type of lines of the solid sample (singlet-doublet, doublet-singlet, doublet-doublet, quadruplet-doublet, quadruplet-quadruplet), the buffer gas, the intensity-time dependence and the line broadening of the neutral and ionic lines are described. Nitrogen, oxygen ion and metal oxide emission, self-absorption, pressure and Stark broadening is predominant at the onset of plasma formation. A time delay is required to reach the best analytical performance.     

Critical analysis of viscosity data of thermal argon plasmas at atmospheric pressure

Reliable values of the viscosity in thermal argon plasmas are most important for our understanding of the momentum transfer and for realistic modeling of various plasma applications. Despite numerous attempts to determine reliable viscosity values over the last three decades, discrepancies still exist among the data reported by different authors. In this paper, a critical analysis is undertaken of calculated and experimental data of the argon viscosity based on recent publications. Our recalculation of viscosities in thermal argon plasmas are performed by using Lennard-Jones, Morse, Aziz, and exponential repulsive potentials for Ar-Ar atom interactions in different temperature ranges from 300 to 20,000 K. The contributions of elastic collisions of e-Ar, e-Ar⁺, and Ar⁺-Ar, as well as charge exchange of Ar⁺-Ar, to the viscosity become important with increasing temperature and degree of ionization in argon plasmas. Based on a critical analysis and recalculations, improved values of the argon viscosity are recommended, covering temperatures from 300 to 20,000 K. Polynomial expressions have been developed for calculating argon viscosities, which will be useful for numerical work and other applications of thermal argon plasmas at atmospheric pressure.     

Reactions of chloroethenes with atomic chlorine in air at atmospheric pressure

Relative rate method at the temperature of 298 K and pressure of 1013 hPa and GC-MS detection were used for the study of kinetics of the reactions of Cl atoms with H₂C=CCl₂, cis-ClHC=CHCl, trans-ClHC=CHCl, ClHC=CCl₂, and Cl₂C=CCl₂. The reaction products were identified by FTIR spectroscopy. A mechanism for the atmospheric degradation of chloro-ethenes has been suggested.     

The behavior of molecules in microwave-induced plasmas studied by optical emission spectroscopy. 1. Plasmas at atmospheric pressure

The behavior of molecules in different atmospheric microwave-induced plasmas (MIPs) has been studied by means of optical emission spectroscopy. This is in order to obtain more insight into molecular processes in plasmas and to investigate the feasibility of emission spectroscopy for the analysis of molecular compounds in gases, e.g. flue gases. Various molecular species (i.e. N₂, CO₂, H₂O, SF₆ and SO₂) have been introduced into discharges in argon or in molecular gases such as carbon dioxide or nitrogen. The plasmas were created and sustained by a guide-surfatron or a torch in the power range of 150 W to 2 kW. Only nitrogen sometimes yielded observable emission from the non-dissociated molecule (first and second positive system). Using other molecular gases, only dissociation and association products were observed (i.e. atomic species and diatomic molecules such as CN, C₂, CO, OH, NH and N₂⁺). The intensities of these products have been studied as a function of the concentration of introduced molecules, the position in the plasma and the composition of the plasma environment. Since in most cases the same diatomic association products are seen, observed associated molecules can only to some extend be related to the molecules originally present in the plasma gas. Therefore, it will be difficult to use atmospheric microwave discharges for the analysis of gas mixtures under the experimental conditions studied.     

Positive and negative gas-phase ion chemistry of chlorofluorocarbons in air at atmospheric pressure

This paper presents a report on the ionization/dissociation of some representative chlorofluorocarbons (CFCs) induced by corona discharges in air at atmospheric pressure. Both positive and negative ions formed from Freons 1,1,1-trichlorotrifluoroethane (CFC 113a), 1,1,2-trichlorotrifluoroethane (CFC 113), and 1,1,1,2-tetrachlorodifluoroethane (CFC 112a) were analyzed using an atmospheric pressure chemical ionization mass spectrometry (APCI-MS) instrument. Energy-resolved mass spectra were obtained by modulating the kinetic energy of the ions via adjustment of the sampling cone potential (V(cone)). Positive ion spectra of the CFCs (M) at low V(cone) show no signals due to either M(+)* or MH(+) but only those due to species [M - Cl](+) and CX(3)(+) (X = Cl, F), likely formed via C-Cl and C-C bond cleavages following ionization via charge exchange. Charge localization in the products of C-C bond cleavage in M(+)* is driven by the stability of the neutral fragment. At low V(cone) the hydrates [M - Cl](+)(H(2)O) are also observed. In the case of 1,1,2,-trichlorotrifluoroethane, [M - F](+) species also form as a result of ion-molecule reactions. As V(cone) is increased collision-induced dissociation of [M - Cl](+) and [M - F](+), i.e., the perhalogenated cations C(2)X(5)(+) (X = Cl, F), takes place via carbene elimination. In some cases such elimination is preceded or accompanied by rearrangements involving transfer of halogen from one carbon to the other. Evidence is also presented for the occurrence of a condensation reaction of C(2)Cl(3)F(2)(+) with water to form a C(2)Cl(2)F(2)HO(+) species via elimination of HCl. Negative ion spectra are dominated by Cl(-) and its ion-neutral complexes with M and with water. Additional components of the plasma include ion-neutral complexes O(3)(-)(M), the molecular anion M(-) (observed only with 1,1,2-trichlorotrifluoroethane), and an interesting species corresponding to [M - Cl + O](-). The origin and structure of these [M - Cl + O](-) species are discussed in terms of available thermochemical and reactivity data and current mechanistic views concerning reaction of O(2)(-) with halogenated compounds. The observation of both positive and negative ions containing oxygen is of special relevance to development of new processes for the treatment of volatile organic compounds (VOCs) based on oxidative decomposition induced by corona discharges in air at room temperature and pressure.     

A mass spectrometry study of alkanes in air plasma at atmospheric pressure

The positive APCI-mass spectra in air of linear (n-pentane, n-hexane, n-heptane, n-octane), branched [2,4-dimethylpentane, 2,2-dimethylpentane and 2,2,4-trimethylpentane (i-octane)], and cyclic (cyclohexane) alkanes were analyzed at different mixing ratios and temperatures. The effect of air humidity was also investigated. Complex ion chemistry is observed as a result of the interplay of several different reagent ions, including atmospheric ions O₂^+•, NO⁺, H₃O⁺, and their hydrates, but also alkyl fragment ions derived from the alkanes. Some of these reactions are known from previous selected ion/molecule reaction studies; others are so far unreported. The major ion formed from most alkanes (M) is the species [M − H]⁺, which is accompanied by M^+• only in the case of n-octane. Ionic fragments of C _n H_2n ^+1/+ composition are also observed, particularly with branched alkanes: the relative abundance of such fragments with respect to that of [M − H]⁺ decreases with increasing concentration of M, thus suggesting that they react with M via hydride abstraction. The branched C₇ and C₈ alkanes react with NO⁺ to form a C₄H₁₀NO⁺ ion product, which upon collisional activation dissociates via HNO elimination. The structure of t-Bu⁺(HNO) is proposed for such species, which is reasonably formed from the original NO⁺(M) ion/molecule complex via hydride transfer and olefin elimination. Finally, linear alkanes C₅–C₈ give a product ion corresponding to C₄H₇⁺(M), which we suggest is attributed to addition of [M − H]⁺ to C₄H₈ olefin formed in the charge-transfer-induced fragmentation of M. The results are relevant to applications of nonthermal plasma processes in the fields of air depuration and combustion enhancement.     

Influence of air plasma treatment at atmospheric pressure on wood extractives

The influence of an atmospheric pressure plasma treatment on wood extractives has been investigated by means of surface energy determination and XPS. Polar and disperse component of the surface energy show only marginal influence of plasma treatment, whereas XPS indicates plasma induced oxidation and degradation of the extractives.     

Practical Os/Cu-cocatalyzed air oxidation of allyl and benzyl alcohols at room temperature and atmospheric pressure

[reaction: see text] A new protocol for the oxidation of primary and secondary allyl and benzyl alcohols at room temperature and using 1 atm of air is described. The procedure uses low loadings of copper salts and osmium tetroxide, which is activated with quinuclidine and prereduced with an alkene. Chemoselectivity for allyl and benzyl alcohols is very high, no overoxidation is observed, and the reaction takes place under neutral conditions.     

An experimental study of the electrospraying of water in air at atmospheric pressure

Water solutions with electrical conductivities ranging from that of the deionized water up to 2 S/m have been electrosprayed in air through narrow silica tubes. Results show unambiguously that steady cone jets of water in air without the assistance of glow discharge can be formed for the range of electrical conductivities we have explored. The absence of corona discharge has been proven not only for the good agreement between the experimental results and the scaling laws given in the cone-jet literature but also for the independence of the spray current on the atmosphere (air or CO(2)) in which water was being electrosprayed. Other regimes such as the electric dripping and the assisted glow discharge cone-jet mode that appear in the electrospraying of water in air at room temperature have also been investigated.     

Evaporation of ionic liquids at atmospheric pressure: study by ion mobility spectrometry

A conventional ion mobility spectrometry (IMS) was used to study atmospheric pressure evaporation of seven pure imidazolium and pyrrolidinium ionic liquids (ILs) with [Tf₂N], [PF₆], [BF₄] and [fap] anions. The positive drift time spectra of the as-received samples measured at 220 °C exhibited close similarity; the peak at reduced mobility K₀ = 1.99 cm² V⁻¹ s⁻¹ was a dominant spectral pattern of imidazolium-based ILs. With an assumption that ILs vapor consists mainly of neutral ion pairs, which generate the parent cations in the reactant section of the detector, and using the reference data on the electrical mobility of ILs cations and clusters, this peak was attributed to the parent cation [emim]. Despite visible change in color of the majority of ILs after the heating at 220 °C for 5 h, essential distinctions between spectra of the as-received and heated samples were not observed. In negative mode, pronounced peaks were registered only for ILs with [fap] anion.     

Direct detection of benzene, toluene, and ethylbenzene at trace levels in ambient air by atmospheric pressure chemical ionization using a handheld mass spectrometer

The capabilities of a portable mass spectrometer for real-time monitoring of trace levels of benzene, toluene, and ethylbenzene in air are illustrated. An atmospheric pressure interface was built to implement atmospheric pressure chemical ionization for direct analysis of gas-phase samples on a previously described miniature mass spectrometer (Gao et al. Anal. Chem. 2006, 78, 5994–6002). Linear dynamic ranges, limits of detection and other analytical figures of merit were evaluated: for benzene, a limit of detection of 0.2 parts-per-billion was achieved for air samples without any sample preconcentration. The corresponding limits of detection for toluene and ethylbenzene were 0.5 parts-per-billion and 0.7 parts-per-billion, respectively. These detection limits are well below the compounds’ permissible exposure levels, even in the presence of added complex mixtures of organics at levels exceeding the parts-per-million level. The linear dynamic ranges of benzene, toluene, and ethylbenzene are limited to approximately two orders of magnitude by saturation of the detection electronics.     

Saturation effects in the laser ablation of stainless steel in air at atmospheric pressure

A pulsed Nd : YAG laser was used to generate a plasma from stainless steel targets in air at atmospheric pressure. Laser focusing was found to be an important factor in the ablation process. The influence of focal conditions on spatial profiles of plasma, emission intensity and averaged ablation rate (AAR, μm pulse^–1) of stainless steel samples as a function of laser energy are discussed. At high energies and depending on laser beam focusing, ablation efficiency tends to decrease compared to that at lower energies. This effect can be due to plasma shielding and air breakdown. The averaged ablation rate was found to be dependent on the thickness of the sample. This effect results in shielding of the incoming laser beam and redeposition of removed material in the crater. By focusing the beam inside the material free expansion of plasma is allowed, resulting in more efficient erosion of the sample at larger energies. For comparative purposes, data on ablated mass per pulse are presented. Received: 25 January 1999 / Revised: 7 April 1999 / Accepted: 30 April 1999     

Saturation effects in the laser ablation of stainless steel in air at atmospheric pressure

A pulsed Nd?:?YAG laser was used to generate a plasma from stainless steel targets in air at atmospheric pressure. Laser focusing was found to be an important factor in the ablation process. The influence of focal conditions on spatial profiles of plasma, emission intensity and averaged ablation rate (AAR, μm pulse^–1) of stainless steel samples as a function of laser energy are discussed. At high energies and depending on laser beam focusing, ablation efficiency tends to decrease compared to that at lower energies. This effect can be due to plasma shielding and air breakdown. The averaged ablation rate was found to be dependent on the thickness of the sample. This effect results in shielding of the incoming laser beam and redeposition of removed material in the crater. By focusing the beam inside the material free expansion of plasma is allowed, resulting in more efficient erosion of the sample at larger energies. For comparative purposes, data on ablated mass per pulse are presented.     

Radio-frequency induction plasmas at atmospheric pressure: Mixtures of hydrogen,nitrogen, and oxygen with argon

Numerical calculations are reported which simulate atmospheric-pressure radiofrequency induction plasmas consisting of either pure argon or mixtures of argon with hydrogen, nitrogen, or oxygen. These calculations are compared to observations of laboratory plasmas generated with the same geometry and run conditions. The major features of the laboratory plasmas are predicted well by the calculations: the pure argon plasma is the largest, with the argon-oxygen plasma slightly smaller. The argon-nitrogen plasma is considerably smaller and the argon-hydrogen plasma is the shortest, although somewhat fatter than the argon-nitrogen case. The calculations are not entirely successful in predicting the exact location of the plasmas relative to the coils. A likely explanation is that there is significant uncertainty regarding the actual power coupled to the laboratory plasmas.     

Transport of uranium free particles in a d.c. arc plasma burning in air at atmospheric pressure

The transport parameter ψ was calculated for the free particles of uranium in a d.c. arc plasma burning in air. Using the “wire method” the volatilization rate (Q_w), the total concentration of free particles (_nt), the axial velocity (_vj) of uranium particles and the plasma cross section (S) were measured. The transport parameter was calculated for cylindrical symmetry of the arc. The total particle concentration calculated by the wire method was compared to values obtained by absolute intensity measurements of ion and atom spectral lines of uranium. This led to an estimate of the molecular concentration of uranium in the d.c. arc plasma.     

Positive ion chemistry of esters of carboxylic acids in air plasma at atmospheric pressure

Ionization of esters of carboxylic acids RCOOR' (R = H, alkyl; R' = alkyl) within the air plasma of the Atmospheric Pressure Chemical Ionization (APCI) source occurs largely via H(+)-transfer and, to a minor extent, via NO(+) association. The protonated ester MH(+) is normally observed as M(2)H(+) and as higher aggregates (M(3)H(+), M(3)H(+)(H(2)O)) also at high source temperature. The behavior of M(2)H(+) upon collisional activation is consistent with the reported dissociation of proton-bound dimers to MH(+) species that, in turn, fragment according to the known paths of lowest energy. In addition, other important product ions form within the plasma, some in very high relative abundance, which are attributed to ion-molecule condensation reactions between neutral M and either MH(+) or M(2)H(+) resulting in the elimination of CO, R'OH, alkene from the alkoxy moiety of the ester and HCOOH. A general scheme is proposed to account for the experimental observations, which suggest that the encounter complex formed between MH(+) and M or between M(2)H(+) and M may either collisionally relax to the protonated dimer or trimer, respectively, or react via covalent bond forming and cleaving steps to eliminate stable neutral molecules. The proposed scheme is supported by both the observed concentration dependence and the temperature dependence of the products relative abundances within the plasma. Such reactions can be the dominant process, as in the case of formate esters. A second significant ionization route involves addition of NO(+) to form M(n)NO(+) (n = 1, 2, 3). An additional product corresponding to [M(2)NO(+) - CO(2)] is also observed with iso- and n-butyl formate esters.     

Chemical‐surface modification of polymers using atmospheric pressure nonequilibrium plasmas and comparisons with vacuum plasmas

We demonstrate that stable microwave‐coupled atmospheric pressure nonequilibrium plasmas (APNEPs) can be formed under a wide variety of gas and flow‐rate conditions. Furthermore, these plasmas can be effectively used to remove surface contamination and chemically modify polymer surfaces. These chemical changes, generally oxidation and crosslinking, enhance the surface properties of the materials such as surface energy. Comparisons between vacuum plasma and atmospheric plasma treatment strongly indicate that much of the vacuum‐plasma literature is pertinent to APNEP, thereby providing assistance with understanding the nature of APNEP‐induced reactions. © 2001 John Wiley & Sons, Inc. J Polym Sci Part A: Polym Chem 40: 95–109, 2002