Rate-determining heterogeneous processes in the combustion of metal alkyls

Abstract

Studies have been made of the formation of solid products during the oxidation of Group III metal alkyls and of the effect of such solids on the subsequent combustion of the alkyls and on the alkyl-induced oxidation of hydrocarbons. The solid products of the combustion of aluminium trimethyl do not affect the rate or course of subsequent combustion, which appears to involve the homogeneous dcomposition of a transient peroxide. This latter reaction produces free radicals which are capable of inducing the oxidation of hydrocarbons at ambient temperatures. In contrast, the solid products of the combustion of indium triethyl have a profound effect on subsequent combustion. The present results show that the peroxidic intermediates formed during the combustion of the latter metal alkyl are sufficiently stable to reach the vessel walls where they decompose heterogeneously. This surface reaction, which is catalyzed by metal-alkyl and metal-alkoxy groups in the solid products, does not yield reactive free radicals and, thus, hydrocarbons present are not oxidized. In the combustion of certain metal alkyls, then, the rate-determining steps are heterogeneous. In general, the relative importance of homogeneous and heterogeneous rate-determining processes appears to be a function of the electron deficiency of the metal alkyl.