Water reduction on the facets of Fe(OH)2: an experimental and DFT study

Abstract

H₂ production during the transformation of Fe(OH)₂ into magnetite (i.e., Schikorr reaction), has a significant impact on many environmental applications, including the long-term effectiveness of permeable reactive barriers composed of zerovalent iron and the stability of geological radioactive waste repository. However, the reactivity of the Schikorr reaction is not systematically evaluated and its chemical mechanism still remains controversial. Here, we investigate water reduction on the Fe(OH)₂ surfaces with the help of an interface-reaction model between water and Fe(OH)₂. The Fe(OH)₂ hexagonal nanosheets prepared using a coprecipitation method possess dominantly exposed (001) and (100) facets. As unveiled by the DFT simulations, the (001) Fe(OH)₂ facet only exposes OH, which is, therefore, inert to water reduction. Moreover, the active Fe-exposed (100) Fe(OH)₂ facet is more thermodynamically favourable for water adsorption and dissociation than the (101) and (102) facets. However, the high energy barrier for water dissociation (2.79 eV) may restrict the rate of water reduction on the Fe(OH)₂ facet. In addition, the capacity of (100), (101), and (102) Fe(OH)₂ facets to adsorb OH⁻ is stronger than that for H₂O, which further inhibits water reduction. Furthermore, DOS calculations manifest that Fe d orbitals play a significant role in H₂O adsorption, indicating that manipulating the electronic structure of the Fe site is a critical path to mediate water reduction in Fe(OH)₂. Our study provides new insights into the understanding of the mechanism of the phase transformation of Fe(OH)₂ under anaerobic conditions in environmental and earth sciences.