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Under anaerobic conditions, ferrous iron reacts with sulfide producing FeS, which can then undergo a temperature, redox potential, and pH dependent maturation process resulting in the formation of oxidized mineral phases, such as greigite or pyrite. A greater understanding of this maturation process holds promise for the development of iron-sulfide catalysts, which are known to promote diverse chemical reactions, such as H + , CO 2 and NO 3 − reduction processes. Hampering the full realization of the catalytic potential of FeS, however, is an incomplete knowledge of the molecular and redox processess ocurring between mineral and nanoparticulate phases. Here, we investigated the chemical properties of iron-sulfide by cyclic voltammetry, Raman and X-ray absorption spectroscopic techniques. Tracing oxidative maturation pathways by varying electrode potential, nanoparticulate n (Fe 2+ S 2− ) (s) was found to oxidize to a Fe 3+ containing FeS phase at −0.5 V vs. Ag/AgCl (pH = 7). In a subsequent oxidation, polysulfides are proposed to give a material that is composed of Fe 2+ , Fe 3+ , S 2− and polysulfide (S n 2− ) species, with its composition described as Fe 2+ 1−3 x Fe 3+ 2 x S 2− 1− y (S n 2− ) y . Thermodynamic properties of model compounds calculated by density functional theory indicate that ligand oxidation occurs in conjunction with structural rearrangements, whereas metal oxidation may occur prior to structural rearrangement. These findings together point to the existence of a metastable FeS phase located at the junction of a metal-based oxidation path between FeS and greigite (Fe 2+ Fe 3+ 2 S 2− 4 ) and a ligand-based oxidation path between FeS and pyrite (Fe 2+ (S 2 ) 2− ).
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This content will become publicly available on July 23, 2026
A new quinoline-based cobalt(II) catalyst capable of bifunctional water splitting
We report on a new water-soluble cobalt(II) complex capable of water splitting bifunctionality, i.e., water reduction and water oxidation. The species [CoII(LQpy)H2O]ClO4 (1), where LQpy is the deprotonated form of the new tripodal ligand N1,N1-bis(pyridin-2-ylmethyl)-N2-(quinolin-8-yl)benzene-1,2-diamine, HLQpy, was developed aiming to replace an oxidation prone methylene group by a sturdy and redox stable quinoline. The molecular and electronic structures of 1 were evaluated by multiple spectroscopic, spectrometric, electrochemical and computational methods, and detailed pre- and post-catalytic studies were conducted to ascertain the molecular nature of the conversions. Complex 1 performs water reduction at a low onset overpotential (eta) of 0.65 V at pH 7, reaching TON3h 2900 (TOF 970 h-1) and TON18h 12 100 (TOF 672 h-1) with up to 98% faradaic efficiency (FE). Species 1 also promotes water oxidation at eta = 0.34 V under pH 8, achieving TON3h 193 (TOF 64 h-1) at 84% FE. Experimental and DFT results enabled us to propose reaction intermediates and mechanisms.
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- Award ID(s):
- 1856437
- PAR ID:
- 10628018
- Publisher / Repository:
- Royal Society of Chemistry
- Date Published:
- Journal Name:
- Inorganic Chemistry Frontiers
- ISSN:
- 2052-1553
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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