Department of Applied Chemistry, School of Engineering, The University of Tokyo, 7-3-1 Hongo, Tokyo 113-8656, Japan.
J Am Chem Soc. 2012 Jan 25;134(3):1519-27. doi: 10.1021/ja206511w. Epub 2012 Jan 13.
Manganese oxides function as efficient electrocatalysts for water oxidation to molecular oxygen in strongly alkaline conditions, but are inefficient at neutral pH. To provide new insight into the mechanism underlying the pH-dependent activity of the electrooxidation reaction, we performed UV-vis spectroelectrochemical detection of the intermediate species for water oxidation by a manganese oxide electrode. Layered manganese oxide nanoparticles, δ-MnO(2) (K(0.17)[Mn(4+)(0.90)Mn(3+)(0.07)□(0.03)]O(2)·0.53H(2)O) deposited on fluorine-doped tin oxide electrodes were shown to catalyze water oxidation at pH from 4 to 13. At this pH range, a sharp rise in absorption at 510 nm was observed with a concomitant increase of anodic current for O(2) evolution. Using pyrophosphate as a probe molecule, the 510 nm absorption was attributable to Mn(3+) on the surface of δ-MnO(2). The onset potential of the water oxidation current was constant at approximately 1.5 V vs SHE from pH 4 to pH 8, but sharply shifted to negative at pH > 8. Strikingly, this behavior was well reproduced by the pH dependence of the onset of 510 nm absorption, indicating that Mn(3+) acts as the precursor of water oxidation. Mn(3+) is unstable at pH < 9 due to the disproportionation reaction resulting in the formation of Mn(2+) and Mn(4+), whereas it is effectively stabilized by the comproportionation of Mn(2+) and Mn(4+) in alkaline conditions. Thus, the low activity of manganese oxides for water oxidation under neutral conditions is most likely due to the inherent instability of Mn(3+), whose accumulation at the surface of catalysts requires the electrochemical oxidation of Mn(2+) at a potential of approximately 1.4 V. This new model suggests that the control of the disproportionation and comproportionation efficiencies of Mn(3+) is essential for the development of Mn catalysts that afford water oxidation with a small overpotential at neutral pH.
氧化锰在强碱性条件下作为高效的水氧化电催化剂将水氧化为氧气，但在中性 pH 条件下效率较低。为了深入了解电氧化反应中 pH 依赖性活性的机制，我们通过氧化锰电极对水氧化的中间物种进行了紫外可见光谱电化学检测。在 pH 4 至 13 的范围内，沉积在掺氟氧化锡电极上的层状氧化锰纳米颗粒 δ-MnO(2) (K(0.17)[Mn(4+)(0.90)Mn(3+)(0.07)□(0.03)]O(2)·0.53H(2)O) 被证明可催化水氧化。在这个 pH 范围内，观察到 510nm 处的吸收急剧上升，同时伴随着氧气析出的阳极电流增加。使用焦磷酸盐作为探针分子，510nm 吸收归因于 δ-MnO(2)表面的 Mn(3+)。水氧化电流的起始电位在 pH 4 至 pH 8 时基本保持在约 1.5V vs SHE，但在 pH > 8 时急剧负移。引人注目的是，这种行为与 510nm 吸收起始的 pH 依赖性很好地吻合，表明 Mn(3+)是水氧化的前体。由于歧化反应导致 Mn(2+)和 Mn(4+)的形成，Mn(3+)在 pH < 9 时不稳定，而在碱性条件下，Mn(2+)和 Mn(4+)的共配体有效地稳定了 Mn(3+)。因此，中性条件下氧化锰对水氧化的低活性很可能是由于 Mn(3+)固有的不稳定性所致，其在催化剂表面的积累需要约 1.4V 的电化学氧化来实现 Mn(2+)的氧化。这个新模型表明，控制 Mn(3+)的歧化和共配体效率对于开发在中性 pH 下具有小过电势的 Mn 催化剂以实现水氧化至关重要。
