Department of Chemistry and Cherry L. Emerson Center for Scientific Computation, Emory University , Atlanta, Georgia 30322, United States.
J Am Chem Soc. 2013 Sep 25;135(38):14110-8. doi: 10.1021/ja4024868. Epub 2013 Sep 17.
Distinguishing between homogeneous and heterogeneous catalysis is not straightforward. In the case of the water oxidation catalyst (WOC) Co4(H2O)2(PW9O34)2 (Co4POM), initial reports of an efficient, molecular catalyst have been challenged by studies suggesting that formation of cobalt oxide (CoOx) or other byproducts are responsible for the catalytic activity. Thus, we describe a series of experiments for thorough examination of active species under catalytic conditions and apply them to Co4POM. These provide strong evidence that under the conditions initially reported for water oxidation using Co4POM (Yin et al. Science, 2010, 328, 342), this POM anion functions as a molecular catalyst, not a precursor for CoOx. Specifically, we quantify the amount of Co(2+)(aq) released from Co4POM by two methods (cathodic adsorptive stripping voltammetry and inductively coupled plasma mass spectrometry) and show that this amount of cobalt, whatever speciation state it may exist in, cannot account for the observed water oxidation. We document that catalytic O2 evolution by Co4POM, Co(2+)(aq), and CoOx have different dependences on buffers, pH, and WOC concentration. Extraction of Co4POM, but not Co(2+)(aq) or CoOx into toluene from water, and other experiments further confirm that Co4POM is the dominant WOC. Recent studies showing that Co4POM decomposes to a CoOx WOC under electrochemical bias (Stracke and Finke, J. Am. Chem. Soc., 2011, 133, 14872), or displays an increased ability to reduce Ru(bpy)3 upon aging (Scandola, et al., Chem. Commun., 2012, 48, 8808) help complete the picture of Co4POM behavior under various conditions but do not affect our central conclusions.
区分均相催化和多相催化并不简单。在水氧化催化剂 (WOC) Co4(H2O)2(PW9O34)2 (Co4POM) 的情况下，最初报道的高效分子催化剂受到了挑战，因为有研究表明钴氧化物 (CoOx) 或其他副产物的形成是催化活性的原因。因此，我们描述了一系列在催化条件下彻底检查活性物质的实验，并将其应用于 Co4POM。这些实验提供了强有力的证据，证明在最初报道的使用 Co4POM 进行水氧化的条件下（Yin 等人，《科学》，2010 年，328，342），这种 POM 阴离子作为分子催化剂发挥作用，而不是 CoOx 的前体。具体来说，我们通过两种方法（阴极吸附剥离伏安法和电感耦合等离子体质谱法）定量测定 Co4POM 释放的 Co2+(aq) 的量，并表明这种钴的量，无论其存在的价态如何，都不能解释观察到的水氧化。我们证明 Co4POM、Co2+(aq) 和 CoOx 的催化 O2 释放对缓冲液、pH 值和 WOC 浓度有不同的依赖性。从水中萃取 Co4POM，但不萃取 Co2+(aq) 或 CoOx 到甲苯中，以及其他实验进一步证实 Co4POM 是主要的 WOC。最近的研究表明，Co4POM 在电化学偏压下分解为 CoOx WOC（Stracke 和 Finke，《美国化学会志》，2011 年，133，14872），或者在老化时显示出增加还原 Ru(bpy)3 的能力（Scandola 等人，《化学通讯》，2012 年，48，8808），这些研究有助于全面了解 Co4POM 在各种条件下的行为，但不影响我们的中心结论。
