Cobalt based water oxidation catalysis with photogenerated Ru(bpy)⁠3⁠3+: Different kinetics and competent species starting from a molecular polyoxometalate and metal oxide nanoparticles capped with a bisphosphonate alendronate pendant

Polyoxometalates (POMs) are a class of inorganic compounds that have been extensively considered in the development of robust water oxidation catalysts (WOCs). In particular, cobalt based POMs have shown remarkable activity as WOCs under both dark and light activated cycles, although current studies focus on their possible evolution to different forms of cobalt oxides under operating conditions. In this paper, we investigate the activity in water oxidation catalysis of a heptanuclear cobalt based POM [{(B-α-PW⁠9O⁠34)Co⁠3(OH)(H⁠2O)⁠2(O⁠3PC(O)(C⁠3H⁠6NH⁠3)PO⁠3)}⁠2Co]⁠14−, hereafter Co⁠7(Ale)⁠14− and of 5 nm cobalt oxide nanoparticles, Co⁠3O⁠4(Ale), both carrying the same bisphosphonate alendronate ligand. Under the application of an anodic bias in borate buffer pH 8, both species lead to the formation of an active cobalt oxide layer at the working electrode, although in the case of Co⁠7(Ale)⁠14− participation of the POM scaffold in the electrodeposition is evidenced by the presence of W(VI) in the active film. In particular, Co⁠7(Ale)⁠14− and Co⁠3O⁠4(Ale) were investigated in a photocatalytic cycle with Ru(bpy)⁠3⁠2+ (bpy = 2,2′-bipyridine) as the photosensitizer and persulfate as the electron acceptor, where the photogenerated oxidant is Ru(bpy)⁠3⁠3+. Key findings are: (i) ion pairing and aggregate formation of anionic Co⁠7(Ale)⁠14− and Co⁠3O⁠4(Ale) with cationic Ru(bpy)⁠3⁠2+ are limited under photocatalytic regime (20 mM aqueous buffer, pH 8), as confirmed by the solubility product constant K⁠SP = 2.1 ± 0.4 × 10⁠−15 M⁠4 for a Co⁠7(Ale)⁠14−:Ru(bpy)⁠3⁠2+ 1:3 adduct, emission quenching experiments, and bimolecular electron transfer (ET) kinetics; (ii) fast ET to Ru(bpy)⁠3⁠3+, approaching diffusional rates, occurs from Co⁠7(Ale)⁠14− (k⁠ET = 9.4 ± 0.3 × 10⁠8 M⁠−1 s⁠−1) overarching by four orders of magnitude the Co⁠3O⁠4(Ale) nanoparticles; (iii) Co⁠7(Ale)⁠14− enables up to 18 photoinduced ET events in 90 ms, while in the same timeframe, a non-complete bleaching recovery (ET events < 1) results from the Co⁠3O⁠4(Ale) nanoparticle analogs; (iv) photocatalytic oxygen evolution is observed with similar rates for both cobalt based species (TOF per cobalt = 1.2–1.7 × 10⁠−3 s⁠−1); (v) dark O–O bond formation is the likely rate determining step of water oxidation for both systems, as the O⁠2 evolution kinetics do not depend on photoinduced ET rates and oxygenic activity, and light irradiation intensity (λ = 450 nm, 0.4–1.6 × 10⁠−7 einstein s⁠−1); (vi) leaching of Co(II) ions from Co⁠7(Ale)⁠14− in catalytic conditions is estimated below 2% by ⁠31P NMR line broadening experiments and supported by persistent flash photolysis kinetics; (vii) the ultimate fate of Co⁠7(Ale)⁠14− under oxygenic turnover involves a polyoxometalate structural rearrangement, while no evidence of cobalt oxide nanoparticles formation is provided by TEM images; (viii) conversely, Co⁠3O⁠4(Ale) nanoparticles preserve their size along the light driven cycle. Therefore, this study supports the involvement of POM based intermediates in water oxidation catalysis, while future perspectives should be directed toward the identification of such main characters, and of the mechanism of O⁠2 construction