They work by reaction of gaseous Cl2 with basic H2O2 solution producing O2(a. 1â) with yield 60-90% at partial pressures 1-100 Torr at room temperature.
MODELING OF SINGLET OXYGEN PRODUCTION IN NONEQUILIBRIUM O2 GAS DISCHARGE PLASMA V.V.NAUMOV1, S.A.ZHDANOK2, A.M.STARIK3, A.CENIAN4, A.P.CHERNUKHO2 1
Institute of Fundamental Problems for High Technology, Ukrainian Academy of Sciences Prospect Nauki 45, Kiev 03028 Ukraine 2 A.V.Luikov Heat-Mass Transfer Institute, Belarus Academy of Sciences P.Brovki St. 15, Minsk 220072 Belarus 3 P.I.Baranov Central Institute of Aviation Motors, SCR “Raduga” Aviamotornaya St. 2, Moscow 111116 Russia 4 Institute of Fluid-Flow Machinery, Polish Academy of Sciences Ul.Fiszera 14, Gdansk 80-952 Poland Abstract
The theoretical modeling of singlet oxygen production in electrical gas discharges based on detailed state-to-state plasma-chemical kinetics is presented. The effects of electric field and gas-discharge parameters on the electron excitation, energy distribution function and concentration of excited singlet metastables, specifically O2(a1∆) and O2(b1Σ), are studied. Introduction Singlet oxygen is one of the most remarkable molecular species in kinetics of atmospheric gases. After discovery in oxygen discharges (Noxon, 1961, Ogryzlo, 1964) singlet molecular oxygen excites an enormous practical interest since it plays an important role in nonequilibrium gas-plasma systems relevant to astrophysics, aerodynamics, plasma- photo- biochemistry, materials processing [1-2]. Physically, singlet oxygen can exist in two electronically excited states (ES), singlet sigma O2(b1Σ) and singlet delta O2(a1∆). The potential energy diagram for the low lying ES of O2 is in Fig.1 (Krupenie, 1972). The electronic transitions O2 b1Σ→X3Σg and a1∆→X3Σg are known as atmospheric bands (Herzberg, 1950). Among all ES, singlet delta state O2(a1∆) has a super long lifetime (τR ≈ 4x103 s in a gas phase, 3 ms in a liquid phase) and serves a reservoir of electronic energy (carrying excitation 0.98 eV). So far it was hard to image how it can be possible to produce and accumulate such metastables in a large amount. But there is a good example in the field of gas lasers where a high 62
efficient chemically driven oxygen-iodine laser (COIL) of multi-kW power (IR radiation at 1.315 µm) has been developed [McDermott, 1978]. In this system O2(a1∆) is a precursor energy donor molecule which gives a near resonant E-E pumping of the lasing atom I*(2P1/2). For use in the COIL, O2(a1∆) is produced in the singlet oxygen generators (SOG). One can build a SOG of high productive efficiency (yield more 50 %) and get a high density product metastables (up to 1018 cm-3). Due to extraordinary metastabilty against collisional quenching, singlet oxygen can be transported to relatively long distances. In practice, there are known four main types of chemical SOGs: bubbler, film, jet and spray. They work by reaction of gaseous Cl2 with basic H2O2 solution producing O2(a1∆) with yield 60-90% at partial pressures 1-100 Torr at room temperature. It would be interesting to produce O2(a1∆) without liquid chemicals. People believe that sufficient O2(a1∆) production may be achieved purely by electrical means [3]. All-gas-phase discharge technique has been demonstrated in glow, ebeam and HF discharges [Benard, 1978, Fournier, 1980, Velikhov, 1983] with O2(a1∆) yield of 5-10%. Last years the progress is grown up to 17% O2(a1∆) in RF discharge [Fujii, 1994], 21% in an improved MW discharge [Itami, 1999], and 32% in a hybrid RF jet discharge [Schmiedberger, 2000]. It is hoped that ElectriSOG concept [Carroll, 2000] will give a O2(a1∆) yield number of 33% or higher. Although it seems to be not so easy to realize this idea at high pressures [Ivanov, 1999]. This paper presents our studies on the prospects of singlet oxygen production in nonequilibrium discharges based on detailed kinetic plasma-chemical modeling. Here, we outline effects of the electric field and gas-discharge parameters on the electron excitation, energy distribution function and concentration of singlet metastables O2(a1∆) and O2(b1Σ). Methodology For the modeling of plasma-chemistry kinetics under discharge conditions, we applied our PLASKIN model. This self-consistent model is based on the electron Boltzmann equation (in 2TSH approximation) coupled to the kinetic rate balance equations for reactive atoms, molecules and ions, combined with the continuity and transport equations for charged species, which are solved simultaneously in order to take into account the coupling between electron and heavy particle kinetics, including effects of the vibrationally excited molecules and electronically excited metastables. Oxygen model involves 9 neutral active species: O2(X3Σg), O2(a1∆), O2(b1Σ), O2(A3Σu+,C3∆u,c1Σu-), O2(B3Σu-), O3(1A1), O(3P), O(1D), O(1S), 6 ion: O+, O2+, O4+, O-, O2-, O3- and electrons e, which react following 82 selected reactions. Electrical discharge model corresponds to classic theory of a positive column in electronegative gases [2], using electron transport and collisional data that are in agreement with swarm parameters. Cross-sections for electron scattering are taken (with some corrections) from Phelps [1985]. It includes (Fig.2): 1 – momentum transfer; 2 – vibrational excitation; 3 – excitation of O2(a1∆); 4 – excitation of O2(b1Σ); 5– attachment; 6 – excitation of Herzberg’s group ES O2(A3Σu+,C3∆u,c1Σu-) with threshold 4.5 eV; 7 – dissociation into 63
O(3P)+O(3P); 8 – dissociation into O(3P)+O(1D); 9 – ionization; 10 – ionization with dissociation; 11 – rotational excitation. Reactive plasma kinetics model involves all the processes controlling charge particles dynamics and kinetics of electron-ion-molecular-atom reactions under the selected reaction scheme. It includes elastic and superelastic collisions, ionization, detachment, attachment, ion conversion, dissociation, recombination, vibrational excitation, de-excitation and other elementary processes. The electron rate coefficients ke(E/N) and electron temperature Te are determined on the basis of the EEDF (electron energy distribution function) solution from the kinetic Boltzmann code. Electrons are assumed to have a quasi-Maxwellian EEDF in the considered range of excitation. The heavy particle rate constants kr(T) and temperature dependences are collected from the expert estimations [3-5]. Vibrational kinetics of O2 is considered in multilevel approximation taking into account the molecular anharmonicity, V-V, V-T energy exchange in O3 is described in Landau-Teller multimode approach similar to those in CO2 [4-5]. Heterogeneous deactivation of excited states and recombination of atoms at the wall is accounted by the surface sticking coefficients. The discharge is assumed homogenous, and the flow is modeled by 1D gasdynamic equations. Results and discussions The grounds for the use of an oxygen discharge for the production of singlet oxygen can be provided without going into details of discharge operation by he electron energy balance in Fig.3. This is a numerical calculation from the Boltzmann equation (EEDF) that tracks the fraction of the discharge power utilized for each electron energy loss process as a function of the reduced electric field E/N. It is seen that both the singlet O2(a1∆) and O2(b1Σ) states of molecular oxygen may be readily populated via electron pumping. The basic production channel for singlet oxygen is an electron impact on the ground state O2(X3Σ). In both the pure O2 and mixture of He (Ar) with O2, near 45% of the electrical power goes to produce 1 1 O2(a ∆), and 20% goes for the production of O2(b Σ) at E/N ≈ 11 Td (1 Td = 10-17 V⋅cm2) for pure O2 and a lower E/N ≈ 8 Td for O2 :He mixture (electron energy corresponds to Te ~2 eV). Evidently, the discharge efficiency in producing O2(a1∆) is critically dependent on the electric field-to-gas number density ratio, E/N, and there is no guarantee that oxygen discharge will work at this optimum. This situation is almost unique in oxygen. At low E/N the rovibrational excitation dominates, at high E/N the dissociation and ionization predominates. Nevertheless, experiments is in existence, and from the typical V-I characteristics of the low-pressure preionization discharge, we suggest an effective reduced filed E/N ≈ 12 Td and electron density n ~1011 cm-3 is a reasonable estimate. For this E/N, ~40% of the total power is used to excite O2(a1∆) state (0.98 eV/state) and ~15% is used to excite O2(b1Σ) state (1.63 eV/state). A typical mass flow rate of ~10 mmol/s (corresponding to N ~1.3x1017 cm-3) yields a flow 64
velocity of ~20 m/s, and thus a transport time in the flow discharge is τg ≈5 ms. If one uses the fractional power deposition mentioned above, one can obtain a first order estimate of a density of O2(a1∆) ~2x1016 cm-3 (yield ≈15%). There is a simple reason to predict an exit ratio of [O2(b1Σ)]/[O2(a1∆)] ≈ 1/4, since the cross-section for the electron impact production of O2(a1∆) is at least 4 times more than for production of O2(b1Σ). In real discharge kinetics, singlet molecular oxygen takes part in energy pooling reaction O2(a1∆)+O2(a1∆)→O2(b1Σ)+ O2(X3Σg,υ) and in quenching collisions with atoms O and ozone O3 (whose densities are proportional to a gas pressure). Here, concentration of O3 is one of the most important factor affecting the efficiency of O2(a1∆) production. Ozone is formed from oxygen atoms in a three-body recombination O(3P)+O2+M→O3+M. Atomic oxygen, mainly in O(3P) state, is created from oxygen molecules by the fast electron impact dissociation e+O2→O(3P)+O(3P)+e and by dissociative attachment e+O2→O(3P)+O-. Besides, metastable atoms O(1D) is created by dissociative electron excitation e+O2→O(3P)+O(1D)+e and by direct electron impact e+O(3P)→O(1D)+e. The formation time of oxygen atoms is much smaller than that of ozone and, on the other hand, ozone formation is much faster than diffusion time in the discharge. Therefore, the lifetime of O2(a1∆) may be finite, and gas phase mass transport may influence the O2(a1∆) yield. Regarding to ozone, it can disappear in the fast destruction reactions with metastables O(1D)+O3→2O2, O2(a1∆)+O3→2O2+O if discharge will be well organized. The ground-state oxygen O2(X3Σg) is a dominant neutral in discharge, but singlet oxygen O2(a1∆) should be a major excited product in principle. In self-sustained discharge in pure oxygen, due to non-optimal E/N (high electric field), concentration of dissociated atoms O(3P) towards the end of discharge phase is practically the same as O2(a1∆). It is not very well from the energy and kinetic point of view. For larger atom concentration, the SOG efficiency drops drastically. Moreover, due to the fast O2 V-T relaxation at collisions with O atoms, the VDF (vibrational distribution function) is nearly equilibrium (Tv is not much more than T), thus a kinetic effect of vibrationally excited oxygen is negligible. The situation should be improved by the proper choice of the mixture (diluent He or Ar) and addition agents (NO2 or other catalytic oxides) which are able to bind oxygen atoms O and remove undesirable ozone O3. In this way, one can reach a higher degree of nonequilibrium output. In contrast with a liquid-chemical SOG, in a gas-discharge SOG the vibrational excitation may play more significant role, making more efficiency. Since oxygen discharge presents weakly ionized electronegative plasma, and its electronegativity increases with increased pressure, the negative ions are expected to contribute significantly to overall charge balance. Among negative charged particles, the O- has to be a dominant negative ion. It is of the same density as electrons, whereas the density of the O2- is smaller and the fraction of the O3- is negligible. Dissociative electron attachment to O2(X3Σg) and O2(a1∆) is a main channel for the O- creation: e+ O2→O(3P)+O- process dominates at range of pressures. Detachment by O- collisions on O(3P) via O-+O(3P)→e+O2(X3Σg) is the main channel for the O- ion destruction. Singlet metastables contribute to this loss by collisional detachments in O-+O2(a1∆)→O2-+O(3P) and O-+O2(a1∆)→O3+e. Another negative ion O2- is produced by dissociative attachment of ozone e+O3→O(3P)+O2- in the case than ozone is almost entirely created through detachment by collisions of O2(a1∆) with O-. The creation of O2- through the detachment O-+O2(a1∆)→ O(3P)+O2- and charge transfer O3+O(P)→O2(X3Σg)+O2- is also important. The main loss channels for O2- are charge transfer O2-+O(3P)→O2(X3Σg)+O-, O2-+O3→O2(X3Σg)+O3-. The detachment by collisions of O2- with O2(a1∆) takes place also O2-+O2(a1∆)→2O2+e. Evidently, kinetics of negative ions in oxygen discharge at high pressures is strongly determined by detachment processes with metastable 65
singlet oxygen, in that way O2(a1∆) is a strong influence on discharge electrodynamics and structure. As to positive charged particles, the O2+ is the major positive ion that dominates over the O+ ion all the time practically independent of pressure. The O2+ is created by electron impact ionization of the O2(X3Σg) and O2(a1∆), while the O+ is created by electron impact ionization of O(3P) and O(1D). In the post-discharge phase, all positive and negative ions have recombined and neutralized very soon. Afterwards, atoms O are assumed lost due to surface recombination at the wall (with probability γ∆ ≈10-1). Following calculations, we found the density of singlet oxygen in discharge is sufficiently large despite of all destruction processes in oxygen plasma. The amount of singlet oxygen destroyed during the first phase is replenished during the next phase of discharge. Of course, O2(a1∆) relax itself traveling in afterglow, but not too much due to its extra metastabilty. In the absence of O and O3, deactivation of O2(a1∆) in the flow will be mainly due to heterogeneous quenching at the wall (with probability γ∆ ≤10-3). To minimize the singlet oxygen loss, the tract should have a small surface-to-volume ratio and should be as short as possible. To prevent the overheating due to energy release, it is worth to take measures for the fast flow cooling. These requirements result in the optimal design of the discharge SOG. Conclusion Obviously, the formation of singlet molecular oxygen in electrical discharges cannot be explained on the basis of ‘normal’ or ground-state chemical kinetics because even a simple oxygen discharge produces quite a few excited species. We expand for that our PLASKIN model based on the basic principles of gas-discharges physics combined the detailed plasmachemical kinetics including excited states and multiple interactions under extensive reaction mechanism. Due to comprehensible simplified assumptions, including volume-averaged spatial uniformity, quasi-stationary Maxwellian EEDF, some uncertainties of cross-sections and rate coefficients in oxygen discharge, this model does not meant to calculate exact values of plasma parameters. However, it allows to understand and to determine which reactions in the set determine the discharge properties and which reactions among the species are important in the production and destruction of electrically produced singlet oxygen. Thus it can predict well the major kinetic regularities of the singlet oxygen synthesis in nonequilibrium plasma: theoretical limits of the efficiency of generation of O2(a1∆) and its attainable concentration (yield) depending on the initial gas-discharge parameters: electrical power, discharge geometry, gas pressure, temperature and mixture composition.
Acknowledgments This work was partially supported by the INTAS Grants No. 00-00556, 99-00464 and the RFBR. References 1. H.H.Wasserman, R.W.Murray, Singlet Oxygen, Academic Press, N.Y. (1979) 2. M.Capitelli, C.M.Ferreira, B.F.Gordiets, A.I.Osipov, Plasma Kinetics in Atmospheric Gases, Springer, Berlin (2000); V.D.Rusanov, A.A.Fridman, Physics of Chemically Active Plasma, Nauka, Moscow (1984); Yu.Raizer, GasDischarge Physics, Nauka, Moscow (1987) 3. D.L.Carrol, W.Solomon, SPIE 4184 40 (2001); D.L.Carrol, D.King, AIAA P-2277 (2002) A.P.Napartovich, A.A.Deryugin, I.V.Kochetov, J.Phys.D: Appl.Phys. 32 1827 (2001); 4. A.Cenian, A.Chernukho, J.Tech.Phys. 40 231 (1999); J.Phys.D:Appl.Phys. 30 1503 (1997) 5. A.M.Starik, N.S.Titova, Chem.Phys. 19(9) 61 (2000); 20(5) 17 (2001) 66
