Enhanced Visible-Light-Induced Photocatalytic Activity in M(III)Salophen-Decorated TiO2 Nanoparticles for Heterogeneous Degradation of Organic Dyes

In this work, the construction of two heterojunction photocatalysts by coordinative anchoring of M(salophen)Cl complexes (M = Fe(III) and Mn(III)) to rutile TiO₂ through a silica-aminopyridine linker (SAPy) promotes the visible-light-assisted photodegradation of organic dyes. The degradation efficiency of both cationic rhodamine B (RhB) and anionic methyl orange (MO) dyes by Fe- and Mn-TiO₂-based catalysts in the presence of H₂O₂ under sunlight and low-wattage visible bulbs (12-18 W) is investigated. Anionic MO is more degradable than cationic RhB, and the Mn catalyst shows more activity than its Fe counterpart. Action spectra demonstrate the maximum apparent quantum efficiency (AQY) at 400-450 nm, confirming the visible-light-driven photocatalytic reaction. The enhanced photocatalytic activity might be attributed to the improved charge transfer in the heterojunction photocatalysts evidenced by photoluminescence (PL) and electrochemical impedance spectroscopy (EIS) analyses. A radical pathway for the photodegradation of dyes is postulated based on scavenging experiments and spectral data. This work provides new opportunities for constructing highly efficient catalysts for wastewater treatment.

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

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.

Nanoblackberries of {W72Fe33} and {Mo72Fe30}: Electrocatalytic Water Reduction

The reversible self-assembly of a {Mo₇₂Fe₃₀} cluster into nanoblackberries in a dilute solution of the relevant crystalline compound [Mo₇₂Fe₃₀O₂₅₂(CH₃COO)₁₂{Mo₂O₇(H₂O)}₂{H₂Mo₂O₈(H₂O)}(H₂O)₉₁]·150H₂O ({Mo₇₂Fe₃₀}_cryst) was demonstrated by Liu, Müller, and their co-workers as a landmark discovery in the area of polyoxometalate chemistry. We have described, in the present work, how these ∼2.5 nm nano-objects, {M₇₂Fe₃₀} (M = W, Mo) can be self-assembled into nanoblackberries irreversibly, leading to their solid-state isolation as the nanomaterials Fe₃[W₇₂Fe₃₀O₂₅₂(CH₃COO)₂(OH)₂₅(H₂O)₁₀₃]·180H₂O ({W₇₂Fe₃₃}_NM) and Na₂[Mo₇₂Fe₃₀O₂₅₂(CH₃COO)₄(OH)₁₆(H₂O)₁₀₈]·180H₂O ({Mo₇₂Fe₃₀}_NM), respectively (NM stands for nanomaterial). The formulations of these one-pot-synthesized nanoblackberries of {W₇₂Fe₃₃}_NM and {Mo₇₂Fe₃₀}_NM have been established by spectral analysis including Raman spectroscopy, elemental analysis including ICP metal analysis, volumetric analysis (for iron), microscopy techniques, and DLS studies. The thermal stability of the tungsten nanoblackberries {W₇₂Fe₃₃}_NM is much higher than that of its molybdenum analogue {Mo₇₂Fe₃₀}_NM. This might due to the extra three ferric (Fe³⁺) ions per {W₇₂Fe₃₀} cluster in {W₇₂Fe₃₃}_NM, which are not part of the {W₇₂Fe₃₀} cluster cage but are placed between two adjacent clusters (i.e., each cluster has six surrounding 0.5Fe³⁺) to form this self-assembly. The isolated blackberries behave like an inorganic acid, a water suspension of which shows pH values of 3.9 for {W₇₂Fe₃₃}_NM and 3.7 for {Mo₇₂Fe₃₀}_NM because of the deprotonation of the hydroxyl groups in them. We have demonstrated, for the first time, a meaningful application of these inexpensive and easily synthesized nanoblackberries by showing that they can act as electrocatalysts for the hydrogen evolution reaction (HER) by reducing water. We have performed detailed kinetic studies for the electrocatalytic water reduction catalyzed by {W₇₂Fe₃₃}_NM and {Mo₇₂Fe₃₀}_NM in a comparative study. The relevant turnover frequencies (TOFs) of {W₇₂Fe₃₃}_NM and {Mo₇₂Fe₃₀}_NM (∼0.72 and ∼0.45 s^-1, respectively), the overpotential values of {W₇₂Fe₃₃}_NM and {Mo₇₂Fe₃₀}_NM (527 and 767 mV, respectively at 1 mA cm^-2), and the relative stability issues of the catalysts indicate that {W₇₂Fe₃₃}_NM is reasonably superior to {Mo₇₂Fe₃₀}_NM. We have described a rationale of why {W₇₂Fe₃₃}_NM performs better than {Mo₇₂Fe₃₀}_NM in terms of catalytic activity and stability.

Polymer-templated supramolecular co-assemblies of proteins and metal oxide clusters as versatile platform for chemo-enzymatic catalysis

The hybridization of enzymes and inorganics in controlled manner is challenging, however, critical for the development of chemo-enzymatic cascade catalyst with high efficiency and selectivity. Here, proteins and metal oxide clusters can be facilely co-assembled on the surface of colloid of poly(4-vinylpyridine) (P4VP) via hydrogen bonding, due to their enriched surface hydrogen bonding donors. The co-assembly method can be generally applied for preparing chemo-enzymatic catalyst within the selected database of various proteins and metal oxide clusters while the assembly units retain their structures and activities. Typically, a 2.5 nm metal oxide cluster {Mo₇₂Fe₃₀}, with peroxidase-like activity, are complexed with glucose oxidase (GOX) on P4VP for the catalysis against the oxidization of o-dianisidine (ODA) with the existence of glucose. Due to the synergistic effects of chemical and enzymatic catalysis, the co-assemblies show even higher ODA oxidation activity compared to GOX/catalase bi-enzymatic system, confirming the effectiveness of the co-assembly protocol for cascade catalysis and enabling its applications in rapid glucose detection and biomass conversion.

A nanoscopic icosahedral {Mo72Fe30} cluster catalyzes the aerobic synthesis of benzimidazoles

In this study, the catalytic efficiency of amorphous {Mo₇₂Fe₃₀} nanocapsules as a safe Keplerate polyoxometalate in organic synthesis was exploited. The easy-made solid catalyst exhibited high efficiency using a very low dosage (0.02–0.05 mol%) in the catalyzed condensation of various aromatic 1,2-diamines and aldehydes for the aerobic synthesis of benzimidazoles with very small E-factor values (0.11–0.33). The superior catalytic activity of amorphous nanoclusters compared to that of its crystalline counterpart was demonstrated. The high activity and recyclability of heterogeneous catalysts in a green reaction media under oxygen atmosphere, make this environmentally benign organic process appropriate for our applied goals.

Catalytic activity of amorphous {Mo₇₂Fe₃₀} nanoclusters as a safe Keplerate polyoxometalate in aerobic synthesis of benzimidazoles was described.