Shock-Tube Studies on the Decomposition of Tetramethyl-Lead and the Formation of Lead Oxide Particles

The decomposition of tetramethyl-lead (TML) in dilute (< 0.2% vol.) mixtures with argon, argon + oxygen, and air, has been examined behind incident shock waves. The rate of decomposition of TML has been measured over a temperature range of 890 to 1060 K and found to be first order with a rate constant, k in units of s$^{-1}$, given by lg$k$ = (13.25 $\pm $ 0.4) - (9.0 $\pm $ 0.4) $\times $ 10$^{3}/T$. The rate was unaffected by the presence of an 800-fold excess of oxygen. A sensitive turbidimetric technique was used to follow the rate of condensation of lead-containing particles. Particles were formed rapidly on a millisecond time scale and at a temperature of about 900 K, provided that the lead vapour exceeded a critical saturation ratio of 50 $\pm $ 10. In the presence of air the same critical condition held and the sequence leading to the formation of lead monoxide was found to be TML $\rightarrow $ Pb(g) $\rightarrow $ Pb(s) $\overset ^{\text{O}_{2}}\to{\rightarrow}$ PbO(s). The difference in the optical properties of smokes of lead and lead monoxide provided a means of detecting the oxidation of lead particles to lead monoxide particles. Gaseous lead monoxide formed in abundance only at temperatures around 1600 K, where the combustion of the hydrocarbon portion of TML occurred, and the reaction Pb + OH $\rightarrow $ PbO + H is a plausible one to explain its appearance. These processes are briefly discussed with reference to the manner in which lead alkyls control spontaneous ignition in gasoline engines. The results add weight to the theory that lead oxide smoke can form sufficiently early in the engine cycle to influence the process of spontaneous ignition.