Heterogeneous Recombination Of Oxygen Atoms Research Articles

Application of the catalytic probe method for measuring the concentration of oxygen atoms in Ar/O2 plasma of a low-pressure arc

The monitoring of the degree of oxygen dissociation in the discharge plasma is critical for various plasma applications associated with the etching and oxidation of surfaces or the reactive deposition of oxide coatings. The use of existing measurement techniques is limited owing to their complexity, significant error rate, or application conditions. This study deals with the development of a catalytic probe method for measuring the degree of oxygen dissociation in dense arc discharge plasma. A method for measuring and processing the experimental results is presented, which allows the determination of the thermal contribution of the heterogeneous recombination of oxygen atoms at a high total heating power of the catalytic probe by particle streams and plasma radiation. The atomic oxygen concentration was measured in low-pressure arc plasma with a self-heating hollow cathode in an Ar/O2 mixture with changes in the discharge current and oxygen partial pressure over a wide range of 30–70 A and 0.2–0.6 Pa, respectively. It has been demonstrated that the maximum degree of oxygen dissociation (up to 25% of the O2 content) is achieved at the maximum discharge current and is practically independent of the oxygen flow, whereas the highest concentration of atomic oxygen is achieved when the maximum current and O2 flow values are combined. This conclusion is important for technologies based on plasma-chemical processes in high-current discharges.

Heterogeneous recombination of oxygen atoms on an aluminum foil surface under low-temperature plasma conditions

According to direct kinetic measurements under conditions of a positive column of a low-pressure glow discharge, the probabilities of recombination of oxygen atoms (γ) on an aluminum foil surface have been determined and the effective activation energies of the recombination process have been calculated. It has been found that the γ value depends on the gas pressure in the system at a discharge current above 30 mA. The fluxes of the main active components of oxygen plasma onto the plasma-bounding surface have been calculated. It has been shown that an increase in the heterogeneous recombination probability is attributed to an increase in the flux of metastable oxygen molecules О2(b1∑g+ onto the surface.

Kinetic characteristics of the process of heterogeneous recombination of O(3P) atoms in O2-Ar plasma

The probabilities of the heterogeneous recombination of oxygen atoms have been determined using the method of electron paramagnetic resonance (EPR) in a positive glow gap of O2-Ar (0–90%) mixtures in a wide range of temperatures of the reactor wall. The apparent activation energies of the process of heterogeneous recombination have been calculated.

Analysis of heterogeneous recombination of oxygen atoms on aluminum oxide by methods of quantum mechanics and classical dynamics

To analyze the catalytic properties of heat shield materials of space vehicles, the cluster models of the adsorption of oxygen atoms on aluminum oxide are constructed and the corresponding potential energy surface is calculated on the basis of the density functional theory. Quantum-mechanical calculations showed that it is necessary to take into account the relaxation of the surface monolayers. Using this surface in molecular dynamics, calculations made it possible to obtain the probabilities of the heterogeneous recombination of oxygen atoms on the α-Al2O3 surface, which are in good agreement with experimental data. The calculations performed substantially decrease the amount of experimental investigations necessary reliably to describe the heterogeneous catalysis on promising reusable heat shield coatings for analyzing heat transfer during spacecraft entry into the atmosphere.

Simulation of oxygen atom adsorption on an Al2O3 surface by the density functional method

Several cluster models of oxygen atom adsorption on an Al2O3 surface are constructed on the basis of the density functional method. The performed quantum mechanical computations allow one to reveal a number of important features of the potential energy surface to describe the heterogeneous catalytic processes with the use of molecular dynamics methods. The heterogeneous recombination of oxygen atoms is simulated according to the Eley-Rideal mechanism. It is shown that the potential energy surface should be used with consideration of the internal relaxation of surface monolayers to correctly describe the process under study.

Analysis of the heterogeneous recombination of oxygen atoms on aluminum oxide by methods of quantum mechanics and classical dynamics

On the basis of the density-functional theory, cluster models of the adsorption of oxygen atoms on aluminum oxide are constructed and the corresponding potential-energy surface is calculated. Quantum-mechanical calculations showed that it is necessary to take into account the angular dependence of the potential-energy surface and the relaxation of the surface monolayers. Using this surface in molecular dynamics calculations made it possible to obtain the probabilities of the heterogeneous recombination of oxygen atoms on the α-Al2O3 surface, which are in good agreement with experimental data. The calculations performed substantially decrease the amount of experimental investigations necessary reliably to describe the heterogeneous catalysis on promising reusable heat shield coatings for analyzing heat transfer during spacecraft entry into the atmosphere.

Luminescence of solids excited by adsorbed oxygen atoms

The luminescence excited upon the discharge of molecular gases O2, N2O, and CO onto a Zn2SiO4-Mn crystalline phosphor surface prefilled with oxygen atoms is studied. An interpretation of this phenomenon in terms of the model of acceleration of the surface heterogeneous recombination of oxygen atoms in the appearing adsorption layer of oxygen-containing molecules O2, N2O, and CO by the exchange-association mechanism is given.

Probabilities of the heterogeneous recombination of oxygen atoms in O2-Ar plasma

The probabilities of the heterogeneous recombination of oxygen atoms in the positive glow gap and afterglow for O2-Ar (0–90%) mixtures were determined for the pressure range of 50–300 Pa and discharge currents of 10–80 mA. A molybdenum glass was used as the recombination surface in the first case, while a quartz glass surface was used in the second case.

The kinetics of production and loss of O(3P) oxygen atoms in air plasma

The method of electron paramagnetic resonance is used to measure the production rate and concentration of O(3P) oxygen atoms at pressures from 50 to 320 Pa and discharge currents from 50 to 100 mA in the positive column of a dc glow discharge in air. The probability of heterogeneous recombination of oxygen atoms proceeding by the first kinetic order with respect to their concentration is determined in the region of flux afterglow and in the zone of air plasma.

Recombination of preadsorbed oxygen atoms at the surface of solids

Relaxation measurements are used to establish that for a concentration of gas-phase oxygen atoms n=1014 cm−3 heterogeneous recombination of oxygen atoms at the surface of copper and zinc sulfide takes place by a mechanism involving recombination of preadsorbed (precursor state) atoms. A hypothesis is put forward for the existence of a universal mechanism for the catalytic acceleration of heterogeneous chemical reactions.