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High Durability of a 14-Membered Hexaaza Macrocyclic Fe Complex for an Acidic Oxygen Reduction Reaction Revealed by In Situ XAS Analysis.

ABSTRACT: Nonplatinum metal (NPM) catalysts for the oxygen reduction reaction (ORR) in proton exchange membrane fuel cells (PEMFCs) have been developed; however, NPM catalysts still need to be improved in terms of both their catalytic activity and durability. To overcome these problems, an Fe active site contained within a more compact ligand than conventional, porphyrinic, 16-membered ring ligands, or more specifically, a hexaaza macrocyclic ligand with a 14-membered ring (14MR), was developed. In this study, the durability of the Fe-14MR complex was compared to that of Fe phthalocyanine (FePc), which has a 16-membered ring ligand, using in situ X-ray absorption spectroscopy; demetalation of the Fe complexes was directly observed during electrochemical experiments performed under acidic ORR conditions. It was found that Fe-14MR is significantly more resistant to demetalation than FePc during the ORR.

SUBMITTER: Ohyama J

PROVIDER: S-EPMC8549110 | biostudies-literature | 2021 Oct

REPOSITORIES: biostudies-literature

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High Durability of a 14-Membered Hexaaza Macrocyclic Fe Complex for an Acidic Oxygen Reduction Reaction Revealed by In Situ XAS Analysis.

Ohyama Junya J Moriya Makoto M Takahama Ryo R Kamoi Kazuki K Kawashima Shin S Kojima Ryoichi R Hayakawa Teruaki T Nabae Yuta Y

JACS Au 20210920 10

Nonplatinum metal (NPM) catalysts for the oxygen reduction reaction (ORR) in proton exchange membrane fuel cells (PEMFCs) have been developed; however, NPM catalysts still need to be improved in terms of both their catalytic activity and durability. To overcome these problems, an Fe active site contained within a more compact ligand than conventional, porphyrinic, 16-membered ring ligands, or more specifically, a hexaaza macrocyclic ligand with a 14-membered ring (14MR), was developed. In this s ...[more]

PMID: 34723282

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Project description:Nanoparticulate iron sulfides have many potential applications and are also proposed to be prebiotic catalysts for the reduction of CO2 to biologically important molecules, thus the development of reliable routes to specific phases with controlled sizes and morphologies is important. Here we focus on the use of iron dithiocarbamate complexes as single source precursors (SSPs) to generate greigite and pyrrhotite nanoparticles. Since these minerals contain both iron(iii) and iron(ii) centres, SSPs in both oxidation states, [Fe(S2CNR2)3] and cis-[Fe(CO)2(S2CNR2)2] respectively, have been utilised. Use of this Fe(ii) precursor is novel and it readily loses both carbonyls in a single step (as shown by TGA measurements) providing an in situ source of the extremely air-sensitive Fe(ii) dithiocarbamate complexes [Fe(S2CNR2)2]. Decomposition of [Fe(S2CNR2)3] alone in oleylamine affords primarily pyrrhotite, although by careful control of reaction conditions (ca. 230 °C, 40-50 nM SSP) a window exists in which pure greigite nanoparticles can be isolated. With cis-[Fe(CO)2(S2CNR2)2] we were unable to produce pure greigite, with pyrrhotite formation dominating, a similar situation being found with mixtures of Fe(ii) and Fe(iii) precursors. In situ X-ray absorption spectroscopy (XAS) studies showed that heating [Fe(S2CNiBu2)3] in oleylamine resulted in amine coordination and, at ca. 60 °C, reduction of Fe(iii) to Fe(ii) with (proposed) elimination of thiuram disulfide (S2CNR2)2. We thus carried out a series of decomposition studies with added thiuram disulfide (R = iBu) and found that addition of 1-2 equivalents led to the formation of pure greigite nanoparticles between 230 and 280 °C with low SSP concentrations. Average particle size does not vary significantly with increasing concentration, thus providing a convenient route to ca. 40 nm greigite nanoparticles. In situ XAS studies have been carried out and allow a decomposition pathway for [Fe(S2CNiBu2)3] in oleylamine to be established; reduction of Fe(iii) to Fe(ii) reduction triggers substitution of the secondary amide backbone by oleylamine (RNH2) resulting in the in situ formation of a primary dithiocarbamate derivative [Fe(RNH2)2(S2CNHR)2]. This in turn extrudes RNCS to afford molecular precursors of the observed FeS nanomaterials. The precise role of thiuram disulfide in the decomposition process is unknown, but it likely plays a part in controlling the Fe(iii)-Fe(ii) equilibrium and may also act as a source of sulfur allowing control over the Fe : S ratio in the mineral products.

Dataset Information

High Durability of a 14-Membered Hexaaza Macrocyclic Fe Complex for an Acidic Oxygen Reduction Reaction Revealed by In Situ XAS Analysis.

Publications

High Durability of a 14-Membered Hexaaza Macrocyclic Fe Complex for an Acidic Oxygen Reduction Reaction Revealed by In Situ XAS Analysis.

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