Reaction Kinetics and Chemical Quasi‐Equilibria of the Ozone Synthesis in Oxygen DC Discharges

Abstract

The spatial distribution of ozone and of oxygen atoms was studied along the active and the passive zone of a dc discharge (positive column, pressure: p = ( 4 . . .lo) . 10' Pa, current: I = 2 . . .50 mA, flow rate: F = 5 . . .lo0 sccm) in flowing oxygen. The composition of the final output 02/03-mixture is controlled by relaxation processes in the passive reactor zone. It is affected sensitively by the total number density and the gas temperature in the afterglow. Steady states meaning reversible chemical qua+-equilibria were observed and analysed extensively. Within a detailed kinetic model the formation of these equilibria can be explained quantitatively. The synthesis to ozone is controlled above all by the metastable O2 (a'd,) species, which modify drastically the results for the basic mechanism, considering the Oatoms in the 3 P and 'D states.

Isotope Evidence for Ozone Formation on Surfaces

“…This is due to the fact that ozone is formed and destroyed effectively by species, such as O and O 2 (a 1 Δ g ), that survive the chemical climate of a discharge on the time scale of milliseconds, unlike ionic and highly excited species that exist on the microsecond time scale only. Quasi equilibrium concentrations of O 3 are thus almost independent of the discharge conditions, which may vary from a very low degree to complete oxygen dissociation . In agreement with these findings of Jacobs et al, we will thus adopt the picture according to which the gas is first excited in the active zone of the discharge reactor and ozone and then finally forms in the afterglow region, when O atoms and O 2 (a 1 Δ g ) fade away (see Figure 12 of ref ).…”

“…Quasi equilibrium concentrations of O 3 are thus almost independent of the discharge conditions, which may vary from a very low degree to complete oxygen dissociation . In agreement with these findings of Jacobs et al, we will thus adopt the picture according to which the gas is first excited in the active zone of the discharge reactor and ozone and then finally forms in the afterglow region, when O atoms and O 2 (a 1 Δ g ) fade away (see Figure 12 of ref ). Ozone produced in an electric discharge can thus be expected to acquire, at least to a large extent, its isotopic composition from the formation processes according to eq or eq , which has led us to adopt the isotope approach depicted in eq .…”

“…Ignoring ozone sources, such as the wall process (eq ), however, leads to a systematic overestimation of the importance of eq . Assuming significant amounts of O and O 2 (a 1 Δ g ) in the postdischarge, ozone once formed may react with atomic oxygen O + O 3 → 2O 2 leading to the catalytic conversion of odd (O, O 3 ) to even (O 2 ) oxygen. Alternatively, O atoms that have been consumed in the formation of ozone are just recycled when ozone reacts with excited molecular oxygen O 3 + normalO 2 false( normala Δ g 1 false) → O + 2O 2 In terms of O to O 2 conversion, reaction followed by reaction thus has an efficiency of , while reaction followed by reaction is a null cycle.…”

Intramolecular isotope distribution in heavy ozone (16O18O16O and 16O16O18O)

“…This is due to two reasons. First, for a wide range of ionization and dissociation conditions ozone gross production in plasma reactors is dominated by reactions of the neutral ground state constituents [ Jacobs et al , 1996]. Second, the similar isotopic composition of ozone [ Morton et al , 1990] from electric discharge experiments and from photolysis recycling studies, the latter ensuring reactants to be in their electronic ground state, indicates that ground state kinetics if not completely, at least largely, determines the isotopic composition of ozone, even under discharge conditions.…”

A new feature in the internal heavy isotope distribution in ozone

“…This is because the ozone formation in a Tesla discharge has been shown to be dominated by neutral O-atoms and O 2 molecules in their ground state. 19 In addition, the ozone experiments by Bhattacharya et al 7 demonstrated that photolysis and discharge experiments lead to similar internal isotopic distribution. In their study, the UV photolysis of oxygen was done by varying pressure whereas the Tesla discharge was done by varying both pressure (P) and temperature (T).…”