The effect of the heteroatom (X=P, As, Si and Ge) on the geometrical and electronic properties of α-Keggin polyoxometalates (M=Mo, W and Nb) – A DFT investigation

Factors governing the protonation of Keggin-type polyoxometalates: influence of the core structure in clusters

Atomic substitution is a promising approach for controlling structures and properties for developing clusters with desired responses. Although many possible coordination candidates could be deduced for substitution, not all can be prepared. Therefore, predicting the correlation between structures and physical properties is important prior to synthesis. In this study, regarding Keggin-type polyoxometalates (POMs) as a model cluster, the dominant factors affecting the protonation were investigated by atomic substitutions and geometry changes. The valence of Keggin-type POMs and the constituent elements of the cluster shell structure affect the charge and potential distribution, which change the protonation sites. Furthermore, the valence of Keggin-type POMs and the bond length between the core and shell structure determine the protonation energy. These factors also affect the HOMO-LUMO gap, which governs photochemical and redox reactions. These governing factors derived from actual parameters of the α-isomer of Keggin-type POMs enabled us to deduce the protonation energy of the β-isomer, which is more difficult to prepare and isolate than the α-isomer.

Heteroatom-Modulated Assembly of Hexalanthanoid-Containing Polyoxometalate-Based Coordination Networks

Two series of lanthanoid (Ln)-containing polyoxometalates (POMs) {[Ln₆(ampH)₄(H₂O)_24-n(ampH₂)_n(PW₁₁O₃₉)₂]·21H₂O (Ln = Tb, n = 0 (1), Ln = Er, n = 1 (2)) and K₂[Ln₆(ampH)₄(H₂O)₂₂(SiW₁₁O₃₉)₂]·23H₂O (Ln = Tb (3), Er (4)) (ampH₂ = (aminomethyl) phosphonic acid)} have been synthesized with tri-lacunary Keggin-type POMs containing different types of heteroatoms. Compounds 1 and 2 display neutral organic-inorganic hybrid POM molecules containing {Ln₆(ampH)₄} ({Ln₆}) cores sandwiched by two {PW₁₁O₃₉} units. By changing the heteroatoms from P^V to Si^IV, the extended 2D networks of 3 and 4 were successfully isolated where the adjacent {Ln₆} clusters were connected by {SiW₁₁O₃₉} moieties. Luminescence performances and magnetic properties of 1-4 have been systematically surveyed. The solid-state fluorescence spectra of 1-4 display characteristic emissions of Ln components resulting from the 4f-4f transitions, and energy transfer from the POM segments to Ln³⁺ centers in 1 and 3 has been observed based on the lifetime decay behaviors. Furthermore, all compounds can be utilized as electrocatalysts toward reduction of nitrite with high stability.

Tuning the redox activity of polyoxometalate by central atom for high-efficient desulfurization

The efficient removal of hydrogen sulfide (H₂S) is of great importance for various industrial processes such as the sewage stream depollution and syngas upgrade. Oxidative desulfurization with polyoxometalates (POMs) has been proved one of the most attractive ways to remove H₂S from the systems, while the role of the central atom in POMs has not been well evaluated. Herein, we demonstrate the desulfurization activity of POMs could be well internally switched by the central atoms. In particular, the S^VI-centered POM of [Himi]SMo, exhibited greatly enhanced desulfurization performance compared to its structural analogs with Ge^IV or P^V as central atoms, with a breakthrough H₂S capacity of 627.0 mg g^-1 compared to 39.5 and 54.9 mg g^-1 respectively, well surpassing state-of-the-art H₂S desulfurizes. In addition, its activity was well maintained at a wide range of temperature (0-50 °C) and pH (4-9). More interestingly, electrochemical re-oxidation of the H₂S laden [Himi]SMo was found much more active than the fresh one, achieving H₂S capacity up to 2174 mg g^-1. Air involved in-situ re-oxidation and S-O metathesis mechanisms were proposed and experimentally evidenced to explain the high capacity. This work opens a new concept for the rational design of POMs in terms of H₂S removal.

A first-principles understanding of the CO-assisted NO reduction on the IrRu/Al2O3 catalyst under O2-rich conditions

The IrRu alloy offered optimal energetics for NO reduction by CO. The ensemble effect plays a key role in promoting the reactivity of the IrRu alloy. Making the IrRu surface alloy is better for CO-SCR than forming an alloy over the bulk structure.