The pursuit of an ideal coordination environment of the catalytic site for water splitting

Abstract

The construction of catalysts from cheap materials and exquisite tuning of the coordination environment of the active site is pivotal to the development of a highly active sustainable water-splitting catalyst. Although recent years have seen tremendous growth in the application of Prussian Blue Analogues (PBAs) as non-noble catalysts for water splitting, the effect of the structural coordination of the active sites on the activity of a Prussian blue (PB) catalyst is yet to be explored. Herein, using two simple synthetic strategies, we show that manipulating the coordination environment of the catalytic sites affects the morphology, electronic properties, and eventually the catalytic activity of PBAs. Moreover, this study mimics natural photosynthesis by using solar light as an energy source. First, we demonstrate that the water oxidation activity and stability of a Co–Fe PBA can be tuned by coordinating bidentate capping ligands to the catalytic cobalt sites. Structural characterization studies reveal that the ligand decorated structures are of relatively lower dimensionality and they retained their network structures even after photocatalysis. Photocatalytic water oxidation studies indicate that coordination of one equivalent ligand group to the catalytic cobalt sites (CoL–Fe) results in an enhancement of about 50 times in upper-bound turnover frequency (TOF), while coordination of two equivalent ligand groups to the catalytic cobalt sites (CoL2–Fe) lead to an inactivity, which is attributed to the lack of coordination of water molecules to the catalytic sites. In addition, computational studies support experimental observation by showing that bidentate pyridyl groups enhance the susceptibility of the rate-determining Co(IV)-oxo species to the nucleophilic water attack during the critical O−O bond formation. We found in the second study that the replacement of [Fe(CN)6]3− unit with a square planar [Ni(CN)4]2− building block drastically changes the electronic environment and catalytic properties by converting the PB structure from 3D to a 2D layered structure, and we utilized it for the first time for photocatalytic hydrogen evolution reaction. We synthesized a 2D cyanide-coordination compound [Co–Ni] and performed a complete structural and morphological characterization that fully supports our synthetic claim. Relying on its exposed facets, layered morphology, and abundant surface-active sites, [Co–Ni] can efficiently convert water and sunlight to H2 in the presence of a ruthenium photosensitizer with an optimal evolution rate of 30,029 μmol g−1 h −1, greatly exceeding that of 3D PBA frameworks and top-ranked catalysts operating under the same condition. Furthermore, [Co–Ni] retains its structural integrity throughout a 6-hour photocatalytic cycle, which is confirmed by XPS, XRD and Infrared analysis. Overall, these two strategies signify the importance of the coordination environment of the active sites in exploiting structure/morphology and optimizing the activity of the catalyst.