Determination of the stability constant of cobalt-substituted mono-lacunary Keggin-type polyoxometalate and its electrocatalytic water oxidation performance

Synthesis and Characterisation of Monolacunary Keggin Monovanado-deca-tungstophosphate and Its Complexes with Transition Metal Cations

Five new complexes with metal cations (Mn²⁺, Fe³⁺, Co²⁺, Ni²⁺, and Cu²⁺) of monolacunary Keggin monovanado-deca-tungstophosphate, K₈[PVW₁₀O₃₉]·15H₂O, have been synthesised. The molar ratio of the combination between metal cations and K₈[PVW₁₀O₃₉]·15H₂O has been established to be 1:1, and its general molecular formulas were found to be: K_n[MPVW₁₀O₃₉(H₂O)]·xH₂O (n = 5 for M = Fe³⁺ and n = 6 for M = Mn²⁺, Co²⁺, Ni²⁺, and Cu²⁺). Optimal conditions for complexes' synthesis (pH, temperature, and reaction time) have been determined. The characterisation of K₈[PVW₁₀O₃₉]·15H₂O and of its compounds K_n[MPVW₁₀O₃₉(H₂O)]·xH₂O have been performed using AAS, TG-DTA-DTG, UV-VIS, IR, Raman, and powder XRD methods. In UV spectra, two maximums of absorption were obtained, at 200 and 250 nm, characteristic of Keggin polycondensate compounds. The coordination of cations Ni²⁺, Co²⁺, and Cu²⁺ through oxygen atoms from K₈[PVW₁₀O₃₉]·15H₂O in an octahedron system has been reflected with VIS spectroscopy. All these methods have proved the compositions and structures of K₈[PVW₁₀O₃₉]·15H₂O and K_n[MPVW₁₀O₃₉(H₂O)]·xH₂O, their similarity with other vanadotungstophosphates, and their achievements in the Keggin class. Additionally, all analysis methods have shown an increase in the degree of structural symmetry and the thermal stability of a polyoxoanion complex after attaching metal cations compared to the monolacunary, K₈[PVW₁₀O₃₉]·15H₂O.

A cobalt-substituted Keggin-type polyoxometalate for catalysis of oxidative aromatic cracking reactions in water

Oxidative aromatic cracking reactions in water were achieved using a catalytic system with a cobalt-substituted Keggin-type polyoxometalate as a catalyst, an Oxone® as a sacrificial oxidant and sodium bicarbonate as an additive under mild conditions.

Synthesis, Crystal Structure, Electrochemistry and Electro-Catalytic Properties of the Manganese-Containing Polyoxotungstate, [(Mn(H2O)3)2(H2W12O42)]6−

We present the synthesis and structural characterization of the manganese-containing polyoxotungstate, [(Mn(H2O)3)2(H2W12O42)]6− (1), obtained by reaction of MnCl2 with six equivalents of Na2WO4 in the presence of Zn(CH3COO)2 in acetate medium (pH 4.7). This has been assessed by various techniques (FTIR, TGA, UV-Visible, XPS, elemental analysis, single crystal X-ray and electrochemistry). Single-crystal X-ray analyses showed that, in the solid state, 1 forms a 2-D network in which [H2W12O42]10− fragments are linked in pairs via Mn2+ ions, leading to linear chains of the form [(Mn(H2O)3)2(H2W12O42)]n6n−. The connection between chains occurs also via Mn2+ ions which bind [H2W12O42]10− fragments belonging to two adjacent chains, forming an infinite 2-D network. A complete electrochemical study was done in aqueous solution where 1 is stable in the pH range 1 to 6. This complex undergoes multiple electron-transfer processes that lead to the electro-generation of manganese high oxidation state species that catalyse water electro-oxidation. 1 is also effective in the electro-catalytic reduction of nitrite and dioxygen.