Differentiating homogeneous and heterogeneous water oxidation catalysis: Confirmation that [Co₄(H₂O)₂(α-PW ₉O₃₄)₂]^10- is a molecular water oxidation catalyst

Distinguishing between homogeneous and heterogeneous catalysis is not straightforward. In the case of the water oxidation catalyst (WOC) [Co ₄(H₂O)₂(PW₉O₃₄) ₂]^10- (Co₄POM), initial reports of an efficient, molecular catalyst have been challenged by studies suggesting that formation of cobalt oxide (CoO_x) 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 CoO_x. 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, Co²⁺(aq), and CoO_x have different dependences on buffers, pH, and WOC concentration. Extraction of Co₄POM, but not Co²⁺(aq) or CoO_x into toluene from water, and other experiments further confirm that Co₄POM is the dominant WOC. Recent studies showing that Co ₄POM decomposes to a CoO_x WOC under electrochemical bias (Stracke and Finke, J. Am. Chem. Soc., 2011, 133, 14872), or displays an increased ability to reduce [Ru(bpy)₃]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.