Cu(II) Aliphatic Diamine Complexes for Both Heterogeneous and Homogeneous Water Oxidation Catalysis in Basic and Neutral Solutions

Cui Lu^†, Jialei Du^†, Xiao-Jun Su^‡, Ming-Tian Zhang^‡, Xiaoxiang Xu^†, Thomas J. Meyer^§, and Zuofeng Chen^*†

^† 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/cm² 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 pK_b ≈ 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.