Differentiating Homogeneous and Heterogeneous Water Oxidation Catalysis: Confirmation that [Co₄(H₂O)₂(α-PW₉O₃₄)₂]^10– Is a Molecular Water Oxidation Catalyst

Abstract

Distinguishing between homogeneous and heterogeneous catalysis is not straightforward. In the case of the water oxidation catalyst (WOC) [Co4(H2O)2(PW9O34)2](10-) (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](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.

Quantum Chemical Modeling of Homogeneous Water Oxidation Catalysis

“…An (Figure 22). [58,128] Recently,S oriano-Lópeze tal. reported aD FT study on the water oxidation mechanism catalyzed by this complex.…”

Improving Singlet Oxygen Resistance during Photochemical Water Oxidation by Cobalt Porphyrin Catalysts

“…[12][13][14][15][16] We recently demonstrated that three water-soluble Co-porphyrins, CoTPPS, CoTCPP, and CoTMPyP (Figure 1), are all highly active WOCs when using [Ru(bpy) 3 ] 2 + as a photosensitizer and Na 2 S 2 O 8 as a sacrificial electron acceptor; [17] Wang and Groves have also demonstrated that Co-porphyrins are also active as electrocatalysts for WO. [18] In the [Ru(bpy) 3 ] 2 + /Na 2 S 2 O 8 photoinitiated system some degradation of the sensitizer and catalyst cannot be avoided owing to the formation of highly oxidizing sulfate radical anions (SO 4 À C, E 1/2 > 3.15 V vs. NHE).…”

Molecular complexes in water oxidation: Pre-catalysts or real catalysts

“…Both cathodic adsorptive stripping voltammetry and inductively coupled plasma mass spectrometry measurements suggested that the possibly in situ generated Co(aq) 2+ species is not capable of catalyzing water oxidation, and the dominant WOC is still Co-POM [103].…”