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Electrocatalysis
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Controlling Oxygen Reduction Selectivity through Steric Effects:
Electrocatalytic Two-Electron and Four-Electron Oxygen Reduction
with Cobalt Porphyrin Atropisomers
Bin Lv+, Xialiang Li+, Kai Guo, Jun Ma, Yanzhi Wang, Haitao Lei, Fang Wang, Xiaotong Jin,
Qingxin Zhang, Wei Zhang, Ran Long,* Yujie Xiong, Ulf-Peter Apfel, and Rui Cao*
Abstract: Achieving a selective 2eꢀ or 4eꢀ oxygen reduction
reaction (ORR) is critical but challenging. Herein, we report
controlling ORR selectivity of Co porphyrins by tuning only
steric effects. We designed Co porphyrin 1 with meso-phenyls
each bearing a bulky ortho-amido group. Due to the resulted
steric hinderance, 1 has four atropisomers with similar
electronic structures but dissimilar steric effects. Isomers
abab and aaaa catalyze ORR with n = 2.10 and 3.75 (n is
the electron number transferred per O2), respectively, but aabb
and aaab show poor selectivity with n = 2.89–3.10. Isomer
abab catalyzes 2eꢀ ORR by preventing a bimolecular O2
activation path, while aaaa improves 4eꢀ ORR selectivity by
improving O2 binding at its pocket, a feature confirmed by
spectroscopy methods, including O K-edge near-edge X-ray
absorption fine structure. This work represents an unparalleled
example to improve 2eꢀ and 4eꢀ ORR by tuning only steric
effects without changing molecular and electronic structures.
a promising fuel.[8–10] Recently, many molecular ORR cata-
lysts have been reported.[1–5,11–28] Although achievements have
been made in improving the 4eꢀ ORR by providing rapid
electron and proton transfers,[17,29,30] introducing hydrogen-
bonding[31,32] and electrostatic interactions,[33] and using
dinuclear cooperation,[34–38] realizing these functionalities is
challenging from both design and synthesis points of view.
Unlike 4eꢀ ORR, few studies have been dedicated to
improving the 2eꢀ ORR.[27,39,40] Thus, achieving 2eꢀ and 4eꢀ
ORR in a controllable manner is very rare, especially using
one catalyst system.
In principle, O2 tends to be reduced by 2eꢀ at single Co
sites (Scheme 1a), because one Co ion is not efficient to
ꢀ
mediate O O bond cleavage to form Co-oxo species, which is
high in energy.[41] However, Co complexes can also catalyze
4eꢀ ORR through a bimolecular path (Scheme 1a).[42,43] Due
to the presence of both paths, Co complexes usually show
poor or moderate selectivity for either 2eꢀ or 4eꢀ ORR. For
examples, Co porphyrins have been largely studied as ORR
catalysts by displaying high activity but poor selectivity.[36] To
improve selectivity, extensive efforts have been made to
synthesize Co porphyrin cages with isolated Co sites[40] for
2eꢀ ORR and dinuclear Co porphyrins[34–38] for 4eꢀ ORR, in
which peroxo-bridged Co porphyrins are proposed as inter-
mediates (Scheme 1b).
O
2 can be reduced by two electrons (2eꢀ) to H2O2 or by four
electrons (4eꢀ) to H2O.[1–3] The 4 eꢀ ORR is required in many
energy conversion techniques,[4–7] while the 2eꢀ ORR makes
H2O2, which is a widely used industrial oxidant and also
[*] B. Lv,[+] Dr. X. L. Li,[+] K. Guo, Y. Z. Wang, Dr. H. T. Lei, F. Wang,
X. T. Jin, Q. X. Zhang, Prof. Dr. W. Zhang, Prof. Dr. R. Cao
Key Laboratory of Applied Surface and Colloid Chemistry, Ministry of
Education, School of Chemistry and Chemical Engineering,
Shaanxi Normal University
Xi’an 710119 (China)
E-mail: ruicao@snnu.edu.cn
J. Ma, Prof. Dr. R. Long, Prof. Dr. Y. J. Xiong
Hefei National Laboratory for Physical Sciences at the Microscale,
Collaborative Innovative Center of Chemistry for Energy Materials
(iChEM), School of Chemistry and Materials Science, and National
Synchrotron Radiation Laboratory,
Scheme 1. a) Earlier proposed 2eꢀ and 4eꢀ ORR paths with Co
catalysts. b) Proposed peroxo-bridged bimolecular Co porphyrin
species.
University of Science and Technology of China
Hefei, Anhui 230026 (China)
E-mail: longran@ustc.edu.cn
Prof. Dr. U.-P. Apfel
Ruhr-Universitꢀt Bochum, Fakultꢀt fꢁr Chemie und Biochemie,
Anorganische Chemie I
Universitꢀtsstrasse 150, 44801 Bochum (Germany)
and
Fraunhofer UMSICHT
Herein, we report achieving 2eꢀ and 4eꢀ ORR in
a controllable manner by tuning only steric features of
a catalyst system. Co porphyrin 1 was used (Figure 1). Due to
the steric hinderance of the bulky ortho-amido group at each
meso-phenyl substituent, 1 has four atropisomers with similar
electronic structures but dissimilar steric effects (Fig-
ure 1b).[44] Isomers abab and aaaa display significantly
improved 2eꢀ and 4eꢀ ORR, respectively, but aabb and
aaab have poor ORR selectivity. Note that aaaa has been
Osterfelder Strasse 3, 46047 Oberhausen (Germany)
[+] These authors contributed equally to this work.
Supporting information and the ORCID identification number(s) for
Angew. Chem. Int. Ed. 2021, 60, 1 – 6
ꢀ 2021 Wiley-VCH GmbH
1
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