Controlled Synthesis of Oxidation-Insensitive Green Rust, a Mixed-Valent Iron Mineral, for Enhancing Solar Hydrogen Production via Hydrolysis of Ammonia Borane

Green rust, a mixed-valent iron [FeII-FeIII] mineral with a structure of layered double hydroxides, had been previously thought to be very unstable against oxidation, which limited materials applications. Recently, we reported the solvothermal synthesis of an exceptional type of green rust showing high oxidation and chemical stabilities and discovered its solar photocatalytic activity for hydrogen production via hydrolysis of ammonia borane. Herein, we modify the solvothermal conditions used for synthesizing the prototype green rust and try to synthesize new green rust materials with controlled properties (e.g., FeII/[FeII + FeIII] ratio) for enhancing the hydrogen production performance. In contrast to the prototype solvothermal reaction in glycerol using only FeCl3 as an iron source, FeCl2 and FeCl3 (0:25, 25:75, 30:70, 50:50, 60:40, 65:35, and 75:25 mol %) were mixed with glycerol, and the mixtures were solvothermally treated. Comprehensive analyses including 57Fe Mo''ssbauer spectroscopy confirmed the successful synthesis of green rust with controlled properties to some extent; the crystallite size and FeII/[FeII + FeIII] ratio of the products broadly semiquantitatively decreased and increased, respectively, with the increasement of the amount of FeCl2 added. The all green rust samples, like the prototype one, did not undergo structural changes when kept in air for 3 months. The green rust sample having the smallest crystallite size and moderate FeII/[FeII + FeIII] ratio gave the best solar hydrogen production performance from water containing ammonia borane and glycerol, which was not only considerably higher than that obtained on the prototype green rust but moderate (40%) in comparison with that obtained on a benchmark thermal catalyst, supported Pt nanoparticle. The H2 production mechanism of the green rust was proposed to involve solar light-assisted catalysis.

Automated summary

Researchers modified the synthesis conditions to obtain green rust materials with controlled properties and discovered a sample with excellent solar photocatalytic activity for hydrogen production.