The mode of action of lead tetraethyl as an inhibitor of combustion processes

Abstract

Older theories of action of lead tetraethyl are briefly discussed. New experiments are summarized, showing (i) lead tetraethyl, on oxidation, first forms PbO as a colloidal fog, (ii) lead tetraethyl almost certainly inhibits by virtue of forming PbO, (iii) PbO is an inhibitor because it causes a surface destruction of chains. A simple way is described of coating a quartz reaction vessel with a thin film of PbO. Such a vessel is then used for the study of the effect of PbO on particular combustion reactions. This procedure obviates the uncertainty and difficulty in kinetic studies of the essential inhibiting action of lead tetraethyl, of starting with the alkyl itself. In a PbOcoated vessel " degenerate branching" in methane oxidation near 500°C may be eliminated. An implication is that the secondary chains which are responsible for the degenerate branching probably do not involve the formation of H atoms, 0 atoms or OH radicals. The reason is that, under the conditions used, the rates of reaction of these atoms or radicals with methane are not slow compared to their rates of diffusion to, and reaction with, a PbO surface ; and therefore that a PbO surface could not cut out degenerate branching if these atoms or radicals were formed in the secondary chains. Another implication is that inhibition by PbO in the methane + oxygen system is most probably due to a particular destruction of HO;! radicals. Only these (or possibly HC03), radicals appear sufficiently unreactive in the gas phase to be supposed capable of diffuslng, under the experimental conditions used, to a PbO surface and there being predominantly destroyed. These conclusions are bound up with further implications concerning the relative rates of certain free radical reactions. Inhibition by PbO of hydrocarbon oxidation generally appears to be limited to those conditions where the secondary oxidation of aldehydes is important; and is probably due to the same action as is involved in methane oxidation. PbO also inhibits the hydrogen -toxygen and carbon monoxide -1-oxygen reactions in such a way as to show that its ability to destroy chain carriers is not limited to HO2 radicals. It so happens, however, that in the conditions used for many oxidation systems only comparatively inert radicals, such as HO2, have much chance of reaching the surface and there being destroyed.

Surface Effects in Butane Oxidation

“…From indirect evidence LValsh and co-urorliers have suggested that the radical HOz plays such a role in the oxidation of methane a t about 500" C (20,22). In the oxidation of higher paraffins a t low temperatures, many other radicals have been postulated as participating in the reaction mechanism (10,27,32).…”

“…Furthermore, the chemical trallsformatioll is liltely to release ethyl radicals into the combustion mixture and so cause solne promoting action. An explanation is then available for the observation that lead tetraethyl has a net promoting effect ~lllder certain esperilnental conditions, for example, a t very high temperatures (17) or with fuels that are believed to oxidize via very short chains (10,20).…”

“…T h e work of Icing and co-workers (17, 18, 19) and of Walsh and co-worlcers (20,21,22) inay be cited as illustrating both the lack of agreement 011 mechanism and the possible iinportance of heterogeneous processes.…”

“…Walsh and co-worlters (20,21,22) ascribe the antiltnock action of lead tetraethyl to the formation of a colloidal smolce of lead monoxide, which is assumed to inhibit the lcnoclciilg reaction by virtue of a surface destructioil of free radical chains. Each group of investigators claims supporting evidence from combustion experiments.…”

Oxydation lente du cétène en phase gazeuse. I. Réaction à "hautes températures"

“…For personal use only. [22] .H + CHzCO = Hz + C H C O La terminaison des chaines, compte-tenu des effets de surface observCs, est sans doute hCtCrogene et, parmi les radicaux, HO, est le plus susceptible de se detruire sur la paroi, en particulier recouverte de chlorure de potassium ou d'oxyde de plomb (21).…”