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EPR spectroscopy of iron- and nickel-doped [ZnAl]-layered double hydroxides: modeling active sites in heterogeneous water oxidation catalysts

Sayler, Richard I. and Hunter, Bryan M. and Fu, Wen and Gray, Harry B. and Britt, R. David (2020) EPR spectroscopy of iron- and nickel-doped [ZnAl]-layered double hydroxides: modeling active sites in heterogeneous water oxidation catalysts. Journal of the American Chemical Society, 142 (4). pp. 1838-1845. ISSN 0002-7863. https://resolver.caltech.edu/CaltechAUTHORS:20200102-104558690

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Abstract

Iron-doped nickel layered double hydroxides (LDHs) are among the most active heterogeneous water oxidation catalysts. Due to inter-spin interactions, however, the high density of magnetic centers results in line-broadening in magnetic resonance spectra. As a result, gaining atomic-level insight into the catalytic mechanism via electron paramagnetic resonance (EPR) is not generally possible. To circumvent spin-spin broadening, iron and nickel atoms were doped into non-magnetic [ZnAl]-LDH materials and the coordination environments of the isolated Fe(III) and Ni(II) sites were characterized. Multifrequency EPR spectroscopy identified two distinct Fe(III) sites (S = 5/2) in [Fe:ZnAl]-LDH. Changes in zero field splitting (ZFS) were induced by dehydration of the material, revealing that one of the Fe(III) sites is solvent-exposed (i.e. at an edge, corner, or defect site). These solvent-exposed sites feature an axial ZFS of 0.21 cm⁻¹ when hydrated. The ZFS increases dramatically upon dehydration (to -1.5 cm⁻¹), owing to lower symmetry and a decrease in the coordination number of iron. The ZFS of the other (“inert”) Fe(III) site maintains an axial ZFS of 0.19-0.20 cm⁻¹ under both hydrated and dehydrated conditions. We observed a similar effect in [Ni:ZnAl]-LDH materials; notably, Ni(II) (S = 1) atoms displayed a single, small ZFS (±0.30 cm⁻¹) in hydrated material, whereas two distinct Ni(II) ZFS values (±0.30 and ±1.1 cm⁻¹) were observed in the dehydrated samples. Although the magnetically-dilute materials were not active catalysts, the identification of model sites in which the coordination environments of iron and nickel were particularly labile (e.g. by simple vacuum drying) is an important step towards identifying sites in which the coordination number may drop spontaneously in water, a probable mechanism of water oxidation in functional materials.


Item Type:Article
Related URLs:
URLURL TypeDescription
https://doi.org/10.1021/jacs.9b10273DOIArticle
ORCID:
AuthorORCID
Hunter, Bryan M.0000-0001-8559-9304
Gray, Harry B.0000-0002-7937-7876
Britt, R. David0000-0003-0889-8436
Additional Information:© 2019 American Chemical Society. Received: September 23, 2019; Published: December 31, 2019. This work was supported by the NSF CCI Solar Fuels Program (CHE-1305124) and the Arnold and Mabel Beckman Foundation. The authors declare no competing financial interest.
Group:CCI Solar Fuels
Funders:
Funding AgencyGrant Number
NSFCHE-1305124
Arnold and Mabel Beckman FoundationUNSPECIFIED
Issue or Number:4
Record Number:CaltechAUTHORS:20200102-104558690
Persistent URL:https://resolver.caltech.edu/CaltechAUTHORS:20200102-104558690
Official Citation:EPR Spectroscopy of Iron- and Nickel-Doped [ZnAl]-Layered Double Hydroxides: Modeling Active Sites in Heterogeneous Water Oxidation Catalysts. Richard I. Sayler, Bryan M. Hunter, Wen Fu, Harry B. Gray, and R. David Britt. Journal of the American Chemical Society 2020 142 (4), 1838-1845; DOI: 10.1021/jacs.9b10273
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:100455
Collection:CaltechAUTHORS
Deposited By: Tony Diaz
Deposited On:02 Jan 2020 19:15
Last Modified:31 Jan 2020 21:03

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