Water Oxidation Catalysis Beginning with 2.5 μM [Co₄(H₂O)₂(PW₉O₃₄)₂]^10–: Investigation of the True Electrochemically Driven Catalyst at ≥600 mV Overpotential at a Glassy Carbon Electrode

Abstract

Evidence for the true water oxidation catalyst (WOC) when beginning with the cobalt polyoxometalate [Co4(H2O)2(PW9O34)2]10– (Co4–POM) is investigated at deliberately chosen low polyoxometalate concentrations (2.5 μM) and high electrochemical potentials (≥1.3 V vs Ag/AgCl) in pH 5.8 and 8.0 sodium phosphate electrolyte at a glassy carbon working electrodeconditions which ostensibly favor Co4–POM catalysis if present. Multiple experiments argue against the dominant catalyst being CoO x formed exclusively from Co2+ dissociated from the parent POM. Measurement of [Co2+] in the Co4–POM solution and catalytic controls with the corresponding amount of Co(NO3)2 cannot account for the O2 generated from 2.5 μM [Co4(H2O)2(PW9O34)2]10– solutions. This result contrasts with our prior investigation of Co4–POM under higher concentration and lower potential conditions (i.e., 500 μM [Co4(H2O)2(PW9O34)2]10–, 1.1 V vs Ag/AgCl, as described in Stracke, J. J.; Finke, R. G. J. Am. Chem. Soc. 2011, 133, 14872) and highlights the importance of reaction conditions in governing the identity of the true, active WOC. Although electrochemical studies are consistent with Co4–POM being oxidized at the glassy carbon electrode, it is not yet possible to distinguish a Co4–POM catalyst from a CoO x catalyst formed via decomposition of Co4–POM. Controls with authentic CoO x indicate conversion of only 3.4% or 8.3% (at pH 8.0 and 5.8) of Co4–POM into a CoO x catalyst could account for the O2-generating activity, and HPLC quantification of the Co4–POM stability shows the postreaction Co4–POM concentration decreases by 2.7 ± 7.6% and 9.4 ± 5.1% at pH 8.0 and 5.8. Additionally, the [Co2+] in a 2.5 μM Co4–POM solution increases by 0.55 μM during 3 min of electrolysisfurther evidence of the Co 4 -POM instability under oxidizing conditions. Overall, this study demonstrates the challenges of identifying the true WOC when examining micromolar amounts of a partially stable material and when nanomolar heterogeneous metal-oxide will account for the observed O2-generating activity.

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).…”

Visible‐Light‐Induced Water Oxidation Mediated by a Mononuclear‐Cobalt(II)‐Substituted Silicotungstate

“…[16,18] Because those dissolved metal anions were catalytically active themselves or prone to form catalytically active species (e.g., CoOx, NiOx), [19] the hydrolytic stability of polyoxometalates should be of major importance. As reported in the literature, polyanions with all-inorganic ligands were robust enough to sustain the harsh oxidizing environment; however, their hydrolytic stability was sometimes questionable and was influenced by many factors (e.g., pH, buffers).…”

POM & MOF‐based Electrocatalysts for Energy‐related Reactions

“…Using ag old electrode and 0.1 m sodium phosphate buffer (pH 8), aT S= 120 mV dec À1 was determined and the value of half-wavep otential (potential at which current reaches 50 %) E 1/2 = 0.750 Vw as found, which is closed to the thermodynamic value for water oxidation (E8 = 0.760 Vv s. RHE, pH 8). [120] The close proximity of the Ru-Ru atoms of the POM was pointed out by the authors as ak ey factor for the electrocatalytic performance, and the results were validated by at est with am ono-substituted Ru-POM, [PW 11 Ru III (H 2 O)O 39 ] 4À ,f irstly synthesized by Pope et al, [108] which had no OER electrocatalytic activity.M ore recently, Stracke andF inke [121,122] tested at etra-Co-substituted POM (Co 4 -POM), [Co 4 (H 2 O) 2 (PW 9 O 34 ) 2 ] 10À ,a sO ER electrocatalyst. This compound had already been reported as an efficient catalyst for the chemical water oxidation [103] and, in this work, the authors found that depending on the reaction conditions (e.g.…”

“…[122] However, the precise distinctionb etween Co 4 -POM or CoO x as dominant catalystw as not attained. [122] Nonetheless, Goberna-Ferrón and co-workers studied the OER activity of [Co 9 (H 2 O) 6 (OH) 3 (HPO 4 ) 2 (PW 9 O 34 ) 3 ] 16À (more stable at ah igh pH than Co 4 -POM), and also verifiedt he formationo faCoO x film on the fluorine-doped tin oxide coated glass electrode, for a POM concentration of 1mm and an applieda nodico verpotential of 1.41 Vv s. RHE. [123] In order to trap free Co 2 + ions and, hence,p reventt he CoO x film deposition, ac helating ligand (2,2'-bipyridyl) was added during the electrolysis experiments.…”