Solution Speciation and Stability of Cobalt‐Polyoxometalate Water Oxidation Catalysts by X‐ray Scattering

Goberna‐Ferrón, Sara; Soriano-López, Joaquín; Galán‐Mascarós, José Ramón; Nyman, May

doi:10.1002/ejic.201500404

Cited by 44 publications

(38 citation statements)

References 56 publications

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“…Existing experiments show that most of the catalytic processes utilizing cobalt-containing POMs occur in neutral or basic media (often at pH=8). [22][23][24][25] Therefore, we focus the discussion of our results considering that the Co 4 -catalyzed water oxidation occurs at pH=8. Here, we estimate the effect of pH on a single S i  S j transformation by using standard Nernst eq.…”

Section: Water Oxidation By the Pco 4 Anionmentioning

confidence: 99%

“…Note that the POM is unstable above pH=10. [25] Despite experimental evidences, we decided to compute the deprotonation of S 0 species in order to confirm that the pK a value could be higher than 10, and consequently, that the true nature of the catalyst corresponds to the Co(II)aqua species S 0 . For the [S 0 (Co II -OH 2 )S 0 '(Co II -OH)] step we have estimated a pK a value of 19.7 and an endergonic free energy of +16.1 kcal•mol -1 at pH=8 ( Figure 4).…”

Section: Water Oxidation By the Pco 4 Anionmentioning

confidence: 99%

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Tetracobalt-polyoxometalate catalysts for water oxidation: Key mechanistic details

Soriano-López

Musaev

Hill

et al. 2017

Journal of Catalysis

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A mechanism of water oxidation catalyzed by the carbon-free tetra-Co containing polyoxometalates [Co 4 (H 2 O) 2 (PW 9 O 34) 2 ] 10-(PCo 4) and [Co 4 (H 2 O) 2 (VW 9 O 34) 2 ] 10-(VCo 4) is elucidated by DFT calculations. Computational analysis for PCo 4 suggests that a first PCET step may proceed via a sequential electron-then-proton transfer (ET+PT) pathway and leads to one electron oxidize species S 1 (POM-Co III-OH). In contrast, the second PCET, which controls the potential required to form POM-Co III-O  active species S 2 is clearly a concerted process. The overall S 0 S 2 transformation is estimated to require less than 1.48 V and 1.62 V applied potential at pH=8 for PCo 4 and VCo 4 anions, respectively. At operando conditions, with the presence of a buffer and with an applied potential above the threshold potential the two H-atom removal could take place via concerted pathways. These steps represent rapid pre-equilibria before the rate determining step, which corresponds to the O-O bond formation. The key chemical step occurs via nucleophilic attack of an external water molecule to intermediate S 2. We assume that this step governs the kinetics of the reaction. Comparison of the calculated energetics and electronic structures of intermediate species in the PCo 4 and VCo 4 catalyzed water oxidation cycle shows that coupling of d orbitals of V and Co atoms in VCo 4 increases the oxidation potential of the Co-center. The orbital coupling is also responsible for the higher catalytic activity of VCo 4 because it increases the electrophilicity of Co III-O  moiety in the key S 2 species. Recently, numerous homogeneous molecular water oxidation catalysts have been reported,[4-19] among which carbon-free polyoxometalate (POM) complexes are promising. [20, 21] These complexes act as WOCs in both homogeneous and heterogeneous conditions,[22, 23] and yet, are allinorganic species with high stability towards oxidative degradation.[24, 25]

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Section: Water Oxidation By the Pco 4 Anionmentioning

confidence: 99%

Section: Water Oxidation By the Pco 4 Anionmentioning

confidence: 99%

Tetracobalt-polyoxometalate catalysts for water oxidation: Key mechanistic details

Soriano-López

Musaev

Hill

et al. 2017

Journal of Catalysis

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“…As mentioned above,d etermination of the true catalyst for the water oxidation to dioxygenh as become an important issue because extra caution needs to be taken to clarify whether nanoparticles are produced from the homogeneous metal complexes throughout the oxidation reactions, acting as the actualc atalysts or not. [81][82][83][84][85][86][87][88][89][90][91][92][93] Water-soluble cobalt complexes (Figure 1a-d) have been reportedt oa ct as precatalysts rather than actual catalysts in the photocatalytic water oxidation by persulfate with [Ru(bpy) 3 ] 2 + . [94] The TON and yield of O 2 are listed in Table 2.…”

Section: Homogeneous Catalystsmentioning

confidence: 99%

Homogeneous and Heterogeneous Photocatalytic Water Oxidation by Persulfate

Fukuzumi

Jung

Yamada

et al. 2016

Chemistry An Asian Journal

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Photocatalytic water oxidation by persulfate (Na2 S2 O8 ) with [Ru(bpy)3 ](2+) (bpy=2,2'-bipyridine) as a photocatalyst provides a standard protocol to study the catalytic reactivity of water oxidation catalysts. The yield of evolved oxygen per persulfate is regarded as a good index for the catalytic reactivity because the oxidation of bpy of [Ru(bpy)3 ](2+) and organic ligands of catalysts competes with the catalytic water oxidation. A variety of metal complexes act as catalysts in the photocatalytic water oxidation by persulfate with [Ru(bpy)3 ](2+) as a photocatalyst. Herein, the catalytic mechanisms are discussed for homogeneous water oxidation catalysis. Some metal complexes are converted to metal oxide or hydroxide nanoparticles during the photocatalytic water oxidation by persulfate, acting as precursors for the actual catalysts. The catalytic reactivity of various metal oxides is compared based on the yield of evolved oxygen and turnover frequency. A heteropolynuclear cyanide complex is the best catalyst reported so far for the photocatalytic water oxidation by persulfate and [Ru(bpy)3 ](2+) , affording 100 % yield of O2 per persulfate.

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“…Before electrocatalytic essays, the films were conditioned at a constant applied potential of 1.541 V vs NHE during 5 min in a pH 5 p-toluene sulfonic solution (Figure 1), where Co 9 is still stable. [29] This is important, since ppy films are reversibly oxidized, with anion uptake from solution. This conditioning avoids major contributions from this process to the total current in water oxidation conditions.…”

Section: Resultsmentioning

confidence: 99%

“…In water oxidation electrocatalysis, it is fundamental to determine if there is any evolution of the catalyst during turnover conditions. [7,[32][33][34][35][36][37] Depending on reaction conditions, the catalysts, including POMs, may evolve into the corresponding oxides, which are also competent catalysts. In order to gather experimental evidence about the genuine activity of the Co 9 species in these films, we characterized the films before and after water oxidation.…”

Section: Resultsmentioning

confidence: 99%

Conducting Organic Polymer Electrodes with Embedded Polyoxometalate Catalysts for Water Splitting

2017

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Polyoxometalates (POMs) from earth‐abundant metals are promising electrocatalysts to promote the oxygen evolution reaction in a green process to obtain hydrogen from water and a renewable energy source. Taking advantage of their high anionic charge, we have incorporated a cobalt‐based POM as dopant into an organic conducting polymer matrix (polypyrrole). This heterostructure combines the catalytic water oxidation features from the POM, together with the unique processability of polypyrrole. We have optimized the electrocatalytic performance by studying the effect of several parameters, such as the POM content, film thickness, counter anions, and so forth. It is remarkable that less than 6 % POM content in a polypyrrole film multiplies oxygen evolution reaction currents by one order of magnitude, reducing the required overpotentials by approximately 200 mV. Despite the moderate efficiency (ca. 65 %), this strategy, being based on inexpensive and readily available raw materials, may be useful for the preparation of water splitting anodes that can be adapted to any surface, geometry, or support.

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Solution Speciation and Stability of Cobalt‐Polyoxometalate Water Oxidation Catalysts by X‐ray Scattering

Cited by 44 publications

References 56 publications

Tetracobalt-polyoxometalate catalysts for water oxidation: Key mechanistic details

Tetracobalt-polyoxometalate catalysts for water oxidation: Key mechanistic details

Homogeneous and Heterogeneous Photocatalytic Water Oxidation by Persulfate

Conducting Organic Polymer Electrodes with Embedded Polyoxometalate Catalysts for Water Splitting

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