Microwave-induced Plasma Afterglow Research Articles

Kinetic study in a microwave-induced plasma afterglow of the Fe (a 5D4) reaction with NO2 from 303 to 814 K

A kinetic study of the reaction of ground state Fe atoms (a 5D4) with NO2 has been performed in a fast-flow reactor. A microwave-induced plasma afterglow system with H atoms reacting with the gaseous FexCl3x(g) oligomers was used as Fe atom source. The Fe atoms were monitored by atomic absorption spectroscopy (AAS) at 248.3 nm. The rate constant k1 of the reaction Fe + NO2 → products has been determined at temperatures between 303 and 814 K and was found to be equal to k1 = (4.8+0.7−0.6) × 10−10exp(−(4.5 ± 0.4)/RT kJ mol−1) cm3 molecule−1 s−1. The mechanistic implications in terms of the “electron-jump” mechanism are also discussed and it was concluded that the concept of “harpooning” is not applicable to reaction (1). Instead, the value of k1 can remarkably well be simulated in terms of the simple gas kinetic collision model.

Kinetic Study in a Microwave-Induced Plasma Afterglow of the Cu (2S) Atom Reaction with CH3Br at Temperatures between 300 and 804 K and with CH3I between 300 and 696 K

A kinetic study of the reactions Cu + CH3Br → CuBr + CH3 (k1) and Cu + CH3I → CuI + CH3 (k2) has been carried out in a fast-flow reactor. The gas-phase copper atoms were generated using the microwave-induced plasma (MIP) afterglow technique. Atomic absorption spectroscopy at 327.4 nm was used as the detection technique. The influence of experimental parameters such as the hydrogen content, sublimation temperature of the CuCl pellet, and reactor pressure on k1 and k2 has been verified. The rate constant k1 was measured at temperatures between 300 and 804 K, which resulted in the Arrhenius expression k1 = (1.7 ± 0.2) × 10-11 exp[(−8.2 ± 0.5 kJ mol-1)/RT] cm3 molecule-1 s-1. Since the Arrhenius plot shows a slight curvature, the values of k1 were also fitted to the modified Arrhenius equation k(T) = A Tn exp(−E/RT). Meaningful kinetic parameters can only be derived when n is fixed. The measured values of k1 can be best described as a function of temperature over the 300−804 K range by the expression log k1(T) = +223.6669 − 264.2058(log T) + 97.7693(log T)2 − 11.9244(log T)3. The Arrhenius expression for rate constant k2 is k2 = (7.9 ± 0.5) × 10-11 exp[(−0.8 ± 0.2 kJ mol-1)/RT] cm3 molecule-1 s-1 between 300 and 696 K. The obtained results will be discussed in terms of the electron-jump model, and some reactivity/structure relations will be presented.

Kinetic Study in a Microwave-Induced Plasma Afterglow of the Cu(2S) Atom Reaction with CH3Cl in the Temperature Range 389−853 K

A kinetic study of the reaction Cu + CH3Cl CuCl + CH3 has been carried out in a fast-flow reactor. The gas phase copper atoms were generated using the microwave-induced plasma (MIP) afterglow technique. Atomic absorption spectroscopy at 327.4 nm was used as the detection technique. The influence of the experimental parameters such as the hydrogen content, sublimation temperature of CuCl pellet, and reactor pressure on k1 has been verified. The rate constant k1 was measured at temperatures between 389 and 853 K, which resulted in the Arrhenius expression k1 = (1.73 ± 0.45) × 10-11 exp(−34.7 ± 1.2 kJ mol-1/RT) cm3 molecule-1 s-1. Since the Arrhenius plot shows a slight curvature, the values of k1 were also fitted to the modified Arrhenius equation k(T) = ATn exp(−E/RT). Meaningful kinetic parameters can only be derived when n is fixed. A value of n = 2.15 was obtained using transition state theory combined with ab initio molecular orbital calculations. In that case the activation barrier of the reaction is lowered to 24.8 kJ mol-1. The measured values of k1 as a function of temperature can best be calculated over the 389−853 K range by the expression log k1(T) = −21.22 − 10.99(log T) + 3.30(log T)2.

Kinetic study of the Cu(2S) atom reaction with chlorine (1Σg+) at 305–809 K

A kinetic study of the reaction of Cu(4 2S)+ Cl2 [graphic omitted] CuCl(X 1Σ)+ Cl(2P) has been performed in the temperature range 305–809 K. The measurements are carried out in a fast-flow reactor in which the ground-state copper atoms are generated by the microwave-induced plasma afterglow atomisation technique. Atomic absorption spectroscopy at 324.7 nm is used as the detection technique. From the temperature dependence of k2 the Arrhenius expression k2=(3.3 ± 1.1)× 10–10 exp(–2.2 ± 0.4 kJ mol–1/RT) can be derived. This result is discussed in the light of the electron-jump or harpooning mechanism.

Kinetic study in a microwave-induced plasma afterglow of the Cu(4 2S) atom reaction with N2O from 458 to 980 K and with NO2 from 303 to 762 K

The rate constants for the reaction of ground-state copper atoms (4 2S) with N2O and NO2 have been derived in a fast-flow reactor. The microwave-induced plasma (MIP) afterglow technique was used for the generation of copper atoms in the gas phase. The rate constant for the reaction Cu(4 2S)+ N2O → CuO + N2 at temperatures between 458 and 980 K is found to be (2.4 ± 0.6)× 10–10 exp –(48.6 ± 1.4 kJ mol–1/RT) cm3 molecule–1 s–1. The reaction Cu(4 2S)+ NO2→ products, was followed in the temperature range from 303 to 762 K and has a rate constant (1.3 ± 0.6)× 10–11 exp –(0.2 ± 1.3 kJ mol–1/RT) cm3 molecule–1 s–1. From the copper atom decays in argon at 528 K, a diffusion coefficient, DCu, Ar= 543.2 ± 60.2 cm2 Torr s–1, could be derived.

Kinetic study of the Na + O2+ He reaction in the temperature range 392–777 K

A kinetic study of the reaction Na + O2+ He [graphic omitted] NaO2+ He has been carried out in a fast-flow reactor in the temperature and pressure ranges of, respectively, 392–777 K and 4–14 Torr. Sodium atoms are generated either by thermal evaporation of sodium metal or by the technique of microwave-induced plasma afterglow atomization. Atomic absorption spectroscopy is used as the detection technique for atomic sodium. A fit of k2 to the expression A(T/300)n yields k2=(8.6 ± 0.7)× 10–31(T/300)–1.04 ± 0.16 cm6 molecule–2 s–1. This rate expression will be compared with kinetic data derived for the above reaction on the basis of time-resolved flash photolysis and excimer laser photolysis measurements. Using the Troe formalism, values of k2 are calculated in the temperature range 200–2000 K and the importance of the reaction in the burnt-gas region of oxygen-rich flames is discussed.

Mechanism of CuCl and CuO chemiluminescence in microwave-induced plasma afterglows at 300 K

The sublimation of the copper halides CuCl or CuCl2 in microwave-induced plasma afterglows results in the chemiluminescence of CuCl at 300 K. In O2–He afterglows the A, B, C, D, E and F transitions up from v′= 3 are observed, while H2–He afterglows are much less effective, showing only A and B transitions. In addition, a weak chemiluminescence of CuO is detected in O2–He afterglows. Various chemiluminescence mechanisms are discussed, from which it appears that the termolecular recombination of hydrogen or oxygen atoms, with CuCl or CuO acting as a third body, is the most plausible chemiexcitation mechanism.

Mechanistic aspects of atomization processes in microwave-induced plasma afterglows

Some mechanistic aspects of the atomization processes in the afterglow of microwave-induced plasmas are reconsidered. Besides lattice atomization, where only heterogeneous interactions between radicals and the solid occur, a pure gas-phase atomization process also has to be taken into account. The latter mechanism becomes dominant with the chlorides or organic salts of transition metals. The role of charged species in the overall atomization process is found to be negligible, indicating that an electrochemical mechanism is not important in most cases.

Generation of sodium and copper atoms in the gas phase by microwave-induced plasma afterglows

A fast-flow reactor technique is described by which metal atoms in the gas phase can be produced in the afterglow of microwave-induced plasmas in hydrogen–helium or –argon mixtures. A metal compound MX brought into the gas phase by thermal sublimation can be converted rapidly in the afterglow to form metal atoms M which are detected by atomic absorption spectroscopy. Various parameters which influence the observed metal absorbance have been investigated such as the afterglow composition, the temperature and the axial distance in the flow reactor. Conditions are established for generating the relatively volatile sodium (3 2S) and the non-volatile copper (4 2S) atoms in the temperature range 300–800 K. For copper atoms a diffusion coefficient at 300 K and 10 Torr of 47.1 and 23.6 cm2 s–1 is derived in helium and argon, respectively. This system could become an interesting new source of metal atoms for kinetic studies of metal atom–gas reactions in the temperature range 300–800 K.

A kinetic study of the reaction of copper atoms in the gas phase with molecular oxygen at 300 K in microwave-induced plasma afterglows

The reaction between gas-phase copper atoms Cug(42S) and molecular oxygen has been investigated in a fast-flow reactor in the pressure range 5.5–12 Torr. As a source of Cug the new technique of plasma afterglow atomization of CuCl and Cu(CH3CO2)2 in H2/Ar microwave induced plasmas was used. The copper atoms were detected by atomic absorption spectroscopy. The rate constant for the reaction Cug+O2+Ar → CuO2+Ar was found to be (3.8±0.2)× 10–31 cm6 molecule–2s–1 at 300 K. For the diffusion coefficient of Cug in argon the value of DCu,Ar= 219.9±9.2 cm2 Torr s–1 was derived at 300 K.

Study of the lattice atomization of metal oxides in hydrogen atom containing plasma afterglows

The process of lattice atomization through free radical interactions was investigated for a number of alkaline earth and transition metal oxides reacting with hydrogen atoms. The latter were produced in a hydrogen-argon microwave-induced plasma afterglow and were brought into contact with Co 3O 4, NiO, ZnO, CdO, Fe 2O 3, Cr 2O 3, Cu 2O, CuO, PbO, PbO 2, SnO 2, MoO 3, MgO, CaO, SrO, BaO, BeO and Al 2O 3 in a fast flow reactor in the temperature range 400–800 K and at a pressure of 10 mbar. The occurrence of lattice atomization was observed for PbO 2, ZnO, CdO, MgO, SrO, CaO and BaO by detection of metal atoms in the gas phase by atomic-absorption spectrometry. Simultaneously, a number of modifications at the solid surface occur, such as colour changes and the formation of Ni, Co, CoO, Cd, Fe 3O 4, Cu, Cu 2O and Pb, as shown by X-ray diffraction. A mechanism of lattice atomization is proposed that shows that the vaporization of the solid metal into the gas phase is probably the rate-determining step.