Phosphate‐Templated Encapsulation of a {Co^II₄O₄} Cubane in Germanotungstates as Carbon‐Free Homogeneous Water Oxidation Photocatalysts

Abstract

The ever‐growing interest in sustainable energy sources leads to a search for an efficient, stable, and inexpensive homogeneous water oxidation catalyst (WOC). Herein, the PO43− templated synthesis of three abundant‐metal‐based germanotungstate (GT) clusters Na15[Ge4PCo4(H2O)2W24O94] ⋅ 38H2O (Co4), Na2.5K17.5[Ge3PCo9(OH)5(H2O)4W30O115] ⋅ 45H2O (Co9), Na6K16[Ge4P4Co20(OH)14(H2O)18W36O150] ⋅ 61H2O (Co20) with non‐, quasi‐, or full cubane motifs structurally strongly reminiscent of the naturally occurring {Mn4Ca} oxygen evolving complex (OEC) in photosystem II was achieved. Under the conditions tested, all three GT‐scaffolds were active molecular WOCs, with Co9 and Co20 outperforming the well‐known Na10[Co4(H2O)2(PW9O34)2] {Co4P2W18} by a factor of 2 as shown by a direct comparison of their turnover numbers (TONs). With TONs up to 159.9 and a turnover frequency of 0.608 s−1 Co9 currently represents the fastest Co‐GT‐based WOC, and photoluminescence emission spectroscopy provided insights into its photocatalytic WOC mechanism. Cyclic voltammetry, dynamic light scattering, UV/Vis and IR spectroscopy showed recyclability and integrity of the catalysts under the applied conditions. The experimental results were supported by computational studies, which highlighted that the facilitated oxidation of Co9 was due to the higher energy of its highest occupied molecular orbital electrons as compared to Co4.

Immobilization of a [CoIIICoII(H2O)W11O39]7– Polyoxoanion for the Photocatalytic Oxygen Evolution Reaction

“…In addition to theseand in contrast to the homogeneous WOC casea linear O 2 evolution is obtained for the {Co III Co II W 11 }-APTES-TiO 2 composite, which corresponds to a constant reaction rate. Long-term experiments further validate this point: no saturation in WOC activity is observed for at least 10 h (Figure S9), while the amount of generated O 2 approximates a TON of 82.5, one of the highest values among other Co-based POMs tested under similar homogeneous WOC conditions …”

“…Long-term experiments further validate this point: no saturation in WOC activity is observed for at least 10 h (Figure S9), while the amount of generated O 2 approximates a TON of 82.5, one of the highest values among other Co-based POMs tested under similar homogeneous WOC conditions. 17 Considering the number of {Co III Co II W 11 } co-catalyst clusters on TiO 2 , the WOC performance of the {Co III Co II W 11 }-APTES-TiO 2 can be translated to an average TOF of around 0.39 min −1 . Figure 6b plots instant TOF values of both photosystems and shows that despite the initial TOF of the POM cluster in the solution being at least 4-fold higher, a strong stabilization effect is in place, which renders the {Co III Co II W 11 }-APTES-TiO 2 composite a superior light-driven WOC already after 5 min of turnover conditions.…”

“…After the first all-inorganic WOC-active [{Ru 4 O 4 (OH) 2 (H 2 O) 4 }(γ-SiW 10 O 36 ) 2 ] 10− cluster had been reported in 2008, 14 the first earth-abundant Co-based [Co 4 (H 2 O) 2 (PW 9 O 34 ) 2 ] 10− has triggered extensive interest in Co-containing POM-WOCs. 15 To date, many other Co-based POMs have been reported for homogeneous (light-driven) water oxidation, and their overall performance was investigated with regard to POM's structural type, 16 accessibility of Co centers, 17 and pH values. 18 Despite these developments, two issues still hinder a broader implementation of POM-WOCs: self-aggregation of POM clusters under the turnover conditions and the poor stability of molecular photosensitizers.…”

A Hexadecanuclear Cobalt-Added Tungstogermanate Containing Counter Cobalt Hydrates: Synthesis, Structure and Photocatalytic Properties

“…Due to the complexity of total water splitting reactions, they have often been studied in separate half-parts: oxygen evolution reactions (OERs) and hydrogen evolution reactions (HERs) [ 31 ]. To date, the CoAPs have been widely investigated as water oxidation catalysts in both photo-/electrochemical systems [ 9 , 20 , 25 , 32 , 33 , 34 , 35 , 36 , 37 , 38 ]. The well-defined structures of CoAPs are beneficial for comprehensive studies on the relationship between the catalytic mechanism and structure properties.…”

Synthesis and characterization of the `Japanese rice-ball'-shaped Molybdenum Blue Na4[Mo2O2(OH)4(C6H4NO2)2]2[Mo120Ce6O366H12(OH)2(H2O)76]∼200H2O

“…The early transition-metal ions (M n+ ) are commonly Mo V/VI , W V/VI , V IV/V , Nb V , or Ta V , which form {MO x } (x = 4-7) polyhedra that are typically linked together via corner-and edge-shared O atoms (Pope, 1987). Their structural diversity (size, shape, and composition) and unique properties give rise to a plethora of potential applications from medicine (Stephan et al, 2013;Yamase, 2005;Tanuhadi et al, 2020) to catalysis (Al-Sayed et al, 2021;Chen et al, 2021) and macromolecular crystallography (Bijelic et al, 2015;Mauracher et al, 2014a,b;Breibeck et al, 2019).…”