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.

The effects of a wide range of aromatic additives on the vapour-phase slow oxidation of diisopropyl ether at 360" C are reported. It is found possible to correlate the efficacy of the inhibiting action with the electronic properties of the benzene ring in the inhibitor. Experiments are also reported on the effects of aromatic additives on the slow oxidation of diisopropyl ether in the cool flame temperature range and on the ignition of diethyl ether t o both cool and hot flames. It appears that aromatic inhibitors react with radical chain carriers, the reaction not being limited t o a single type of radical and being greatly facilitated by increasing spread of the electronic cloud of the ring.A number of reactions in solution are known which involve aromatic compounds and where changes in rate can be correlated with electronic properties. This pa.per is thought to report the first case of such an effect in a gas-phase reaction.In a series of papers 1, 2, 3, an extensive investigation of the vapourphase oxidation of ethers has been described, including the effect of a wide range of additives upon the oxidation in various pressure-temperature regions. Such a systematic study of the effect of additives has the Chamberlain and Walsh, Colloque SUY la cine'tique et le me'canisme des re'acitons d'znj7ammation et de combustion en phase gazeuse (Paris, April, 1948).Chamberlain and Walsh, Third Symfiosium on Combustion and Flame and Explosion Phenomena (Madison, U.S.A., Sept., 1948).3 Chamberlain and Walsh, ibid. 4 Chamberlain and Walsh (in course of publication).

The effect of lead tetraethyl on various pressure-temperature regions of ether combustion is described. The inhibiting effect was found to persist after prolonged evacuation of the apparatus. Various experiments suggested that this was due to the deposition of lead oxide. Work has therefore been carried out to determine the effects of the oxides of lead on the slow oxidation and on the ignition of ethers. It is found that lead monoxide gives an inhibiting effect profound, persistent and in every way similar to that of lead tetraethyl. Experiments show that it does this by a surface destruction of chain centres. It is further shown that, on oxidation, lead tetraethyl gives rise to a fog of (probably lead monoxide) particles. Its inhibiting effect is then exerted either by these colloidal particles or by the film of lead monoxide to which they give rise on the walls of the vessel. Lead tetraethyl and lead monoxide inhibit, at both ‘low’ and ‘high ’ temperatures, both the slow oxidation of di is opropyl ether and the process leading to hot flames of diethyl ether. They have no effect on the induction period preceding a cool flame in diethyl ether and only a slight effect on the cool-flame ignition limit. They both reduce the pressure kick due to a cool flame. These results are briefly discussed and are related to other published work.

No abstract

No abstract