The
four-electron oxygen reduction reaction (4e–-ORR)
is the mainstay in chemical energy conversion. Elucidation
of factors influencing the catalyst’s reaction rate and selectivity
is important in the development of more active catalysts of 4e–-ORR. In this study, we investigated chemical and electrochemical
4e–-ORR catalyzed by Co2(μ-O2) complexes bridged by xanthene (1) and anthracene
(3) and by a Co2(OH)2 complex bridged
by anthraquinone (2). In the chemical ORR using Fe(CpMe)2 as a reductant in acidic PhCN, we found that 1 showed the highest initial turnover frequency (TOFinit = 6.8 × 102 s–1) and selectivity
for 4e–-ORR (96%) in three complexes. The detailed
kinetic analyses have revealed that the rate-determining steps (RDSs)
in the catalytic cycles of 1–3 have
the O2 addition to [CoII
2(OH2)2]4+ as an intermediate in common.
In the only case that complex 1 was used as a catalyst, k
cat depended on proton concentration because
the reaction rate of the O2 addition to [CoII
2(OH2)2]4+ was so fast
as compared to that of the concerted PCET process of 1. Through X-ray, Raman, and electrochemical analyses and stoichiometric
reactions, we found the face-to-face structure of 1 characterized
by a slightly flexible xanthene was advantageous in capturing O2 and stabilizing the Co2(μ-O2)
structure, thus increasing both the reaction rate and selectivity
for 4e–-ORR.
Ethane is directly converted to ethanol by the photochemical reaction at temperatures lower than 100 °C in atmospheric pressure. The conversion reaction is initiated by the hydroxyl radicals formed in the photolysis of water vapor. The conversion coefficient of ethane was 7.7% at 5h, and the selectivity 33.3%.
Four-electron oxygen reduction reaction (4e−-ORR) is the foundation of both natural and artificial energy conversion systems. Mechanism studies and catalysis improvements of 4e−-ORR are important research for the actualization of a sustainable society. In this study, we present a dinuclear cobalt complex containing mono-deprotonated forms of 6,6′-dihydroxy-2,2′-bipyridine (6DHBP-H+), [Co2(OH)2(6DHBP–H+)2(btpyxa)](PF6)2 (2) is a highly active 4e−-ORR catalyst in a low acid concentration solution. When ferrocene (Fe(Cp)2) was used as a reductant in PhCN containing a low concentration of perchloric acid (1.0 mmol L−1), 2 showed higher selectivity (99%) and reaction rate (kcat = 6.0 × 103 M−1 s−1) for 4e−-ORR than the bpy analog 1 (kcat = 6.2 × 10 M−1 s−1) and 4DHBP analog 3 (kcat = 1.5 × 102 M−1 s−1). A high catalytic current in the cyclic voltammetry (CV) of 2 indicates a high reaction rate for electrochemical ORR under low acid concentration conditions. Moreover, X-ray crystallography of the corresponding monomeric analog [Co(OH2)(6DHBP–2H+)(trpy)](PF6) (4, 6DHBP–2H+: a doubly deprotonated form of 6DHBP) suggests that OH groups of 2 can form hydrogen bonds with a μ-O2 ligand. Hydroxy groups at the 6,6′-position of bpy would deliver protons to the μ-O2 ligand of the intermediate, thereby promoting O–O bond cleavage in the proton-coupled electron transfer (PCET) process.
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