Catalysis of radical-recombination reactions in flames by alkaline earth metals

Abstract

The rate of recombination of hydrogen atoms in fuel-rich hydrogen/oxygen/nitrogen flames containing metal additives has been measured using the lithium/lithium hydroxide method to monitor hydrogen atom concentrations. It is found that the rate of the recombination reaction is accelerated by the addition of alkaline earth metals. The preferred mechanism for catalysis is via the cycle of reactions: MOH + H →MO + H₂(7), MO + H₂O(+X)→M(OH)₂(+X)(8), M(OH)₂+ H⇌MOH + H₂O (–5), and the rate of the catalyzed recombination is given by the expression [graphic omitted] Values of k₇ and lower limits of k₈ have been determined for calcium, strontium and barium at 1570 K and 1800 K. The upper limit for the catalysis to be effective is about 2000 K. This is determined by the increase of K₅ with temperature.