Mechanism of dihydrogen formation in the magnesium-water reaction

Abstract

The thermodynamically favored reaction between water and magnesium, Mg + 2H2O → Mg(OH)2 + H2, is normally sluggish, but it becomes reasonably rapid when a milled composite of powdered magnesium metal and powdered iron (1-10 mol %) is used with sodium chloride solutions. Iron functions as an activator, and chloride functions as a catalyst that depassivates the outermost oxide/hydroxide layer and allows water to penetrate to the activated magnesium surface. Adding solutes such as sodium nitrate, copper(II) chloride, and sodium trichloroacetate to the reaction mixture suppresses the yield of dihydrogen. Manometric and calorimetric studies on the stoichiometry and kinetics of the reaction between Mg(Fe) powders and aqueous solutions demonstrate that short-lived, partially, and fully solvated electrons (ep- and es-) are precursors of dihydrogen and that they and the hydrogen atoms (H.) formed from them can be scavenged, resulting in suppressed dihydrogen yields.