† Shanghai
Key Lab of Chemical Assessment and Sustainability, Department of Chemistry, Tongji University, Shanghai 200092, China
‡ Center
of Basic Molecular Science (CBMS), Department of Chemistry, Tsinghua University, Beijing 100084, China
§ Department
of Chemistry, University of North Carolina
at Chapel Hill, Chapel Hill, North Carolina 27599, United States
ACS Catal., 2016, 6, pp 77–83
DOI: 10.1021/acscatal.5b02173
Publication Date (Web): November 12, 2015
Abstract
Simply
mixing a Cu(II) salt and 1,2-ethylenediamine (en) affords precursors
for both heterogeneous or homogeneous water oxidation catalysis,
depending on pH. In phosphate buffer at pH 12, the Cu(II) en complex
formed in solution is decomposed to give a phosphate-incorporated
CuO/Cu(OH)
2 film on oxide electrodes that catalyzes water oxidation. A current density of 1 mA/cm
2
was obtained at an overpotential of 540 mV, a significant enhancement
compared to other Cu-based surface catalysts. The results of
electrolysis studies suggest that the solution en complex decomposes by
en oxidation to glyoxal, following Cu(II) oxidation to Cu(III). At pH 8,
the catalysis shifts from heterogeneous to homogeneous with a
single-site mechanism for Cu(II)/en complexes in solution. A further
decrease in pH to 7 leads to electrode passivation via the formation of a
Cu(II) phosphate film during electrolyses. As the pH is decreased, en,
with p
Kb ≈ 6.7, becomes less coordinating and the
precipitation of the Cu(II) film inhibits water oxidation. The
Cu(II)-based reactivity toward water oxidation is shared by Cu(II)
complexation to the analogous 1,3-propylenediamine (pn) ligand over a
wide pH range.
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