Tetrahedral Keggin Core Tunes the Visible Light-Assisted Catalase-Like Activity of Icosahedral Keplerate Shell

Abstract

In this work, the effect of Keggin polyoxometalates encapsulated in Keplerate {Mo72Fe30} shell (K shell) on the visible light-assisted catalase-like activity (H2O2 dismutation) of the resulting core–shell clusters PMo12@K, SiMo12@K, and BW12@K was investigated. Superior photodismutation activity of PMo12@K compared to that of K shell and two other core–shell clusters was discovered. The homogeneity of PMo12@K and its improved oxidative stability, increased redox potential, and reduced band gap caused by a synergistic effect between the Keplerate shell and Keggin core seem reasonable to explain such a superiority. The light-dependent photocatalytic performance of PMo12@K evaluated by action spectra revealed a maximum apparent quantum efficiency (AQY) at 400 nm, demonstrating the visible light-driven photocatalytic reaction. A first-order rate constant of 2 × 10–4 s–1 and activation energy of 108.8 kJ mol–1 alongside a turnover frequency of 0.036 s–1 and a total turnover number of up to ∼3800 approved the effective photocatalytic activity and improved the oxidative stability of PMo12@K. A nonradical photocatalytic mechanism through a Fe–OOH intermediate was proposed. Thus, the structure, optical activity, and oxidative stability of a host Keplerate-type nanocluster can be tuned significantly by encapsulation of a guest, like “cluster-in-cluster” structures, which opens the scope for introducing new visible light-sensitive hierarchical nanostructures.

Melem Nanorectangular Prism-Modified {Mo72Fe30} Nanocapsule as a Visible-Light-Assisted Photocatalyst for Catalase-Like Activity

“…Given the above evidence and our recently proposed mechanism for a similar system, the following nonradical molecular mechanism on the metal centers of Keplerate or its destructed products sounds reasonable (Scheme B). In the first step, the replacement of H 2 O molecules ligated to Fe 3+ centers by H 2 O 2 molecules followed by the protonation of NH 2 groups of Melem as proton scavengers, generates a Fe-OOH intermediate, although some contribution of bridged μ-oxygen (Fe–O–Mo bonds) and terminal oxygen (MoO) could exist.…”

“…Accordingly, the metals (Fe III and Mo VI ) are expected to maintain their valence states after treatment with H 2 O 2 . 32 Thus, complicated shifts observed in the BEs of Fe and Mo (Figure S14) testify to rearrangement in the structure caused by Keplerate destruction.…”

“…The HOMO state of the monomer Melem was extracted from ref 39, and the LUMO state was estimated by the band gap of the as-prepared Melem (Figure S4A). 32 Hence, the metal centers of clusters and protonated amines of Melem act as Lewis and Brønsted acids, respectively, to facilitate O−O bond-breaking and hydrogen-transfer processes, which accelerate the rate-determining step. Accordingly, the metals (Fe III and Mo VI ) are expected to maintain their valence states after treatment with H 2 O 2 .…”

“…Just recently, we discovered that encapsulation of the Keggin H 3 PMo 12 O 40 into the Keplerate {Mo 72 Fe 30 } shell improved the photocatalytic activity of the resulting core−shell cluster. 32 Combining {Mo 72 Fe 30 } with the appropriate organic 17 or inorganic materials 33 is another solution that could modify its properties and promote its catalytic and photocatalytic activity. The electron-capturing ability of POMs makes them appropriate electron relays (reservoir) in conjunction with the electron release of organic compounds.…”

“…However, the cluster becomes unstable at higher pH (>6.6) and temperature (>70 °C) and tolerates H 2 O 2 concentrations of up to ∼20 mM, , which seriously limits its application in many environments and reactions. Just recently, we discovered that encapsulation of the Keggin H 3 PMo 12 O 40 into the Keplerate {Mo 72 Fe 30 } shell improved the photocatalytic activity of the resulting core–shell cluster . Combining {Mo 72 Fe 30 } with the appropriate organic or inorganic materials is another solution that could modify its properties and promote its catalytic and photocatalytic activity.…”