Electrochemical Nitrate Reduction through Cooperative Redox Participation of the CoMo _<i>x</i> Cluster-Anchored γ-FeO(OH) Nanocore

Polyoxoanions can be used as well-defined molecular metal-oxo cluster ligands for the stabilization of reactive metal-oxide cores with tunable active sites and redox properties, and these unique materials can behave as potential electrocatalysts. Herein, an Anderson-type heteropolyoxoanion, (NH₄)₃[Co(OH)₆Mo₆O₁₈], is used to synthesize a crystalline ∼5 nm γ-FeO(OH) nanocore on which several in situ-derived {CoMo_<i>x</i>} cluster anions are covalently attached through [Fe-O-Mo] linkages, making them highly aqueous dispersed ionic nanoparticles {CoMo_<i>x</i>@FeO(OH)}, carrying a negative charge on the surface, as precedented from the zeta (ζ) potential, vibrational, and X-ray photoelectron spectroscopies. The presence of redox-active Fe^III and Mo^VI centers in this complete inorganic nanostructure allows sequential electroreduction of Fe^III centers of the FeO(OH) core at 0.81 V (vs reversible hydrogen electrode (RHE)), followed by {CoMo_x} cluster reduction at 0 V vs RHE. Under an applied potential of -0.6 V vs RHE, a cooperative electron injection from both the reduced γ-FeO(OH) core and the POM ligand to the aqueous nitrate at pH 3 leads to the formation of 17.56 μmol h^-1 cm^-2 NH₃ (65%) and 26.68 μmol h^-1 cm^-2 H₂ (25%). Under similar electrochemical conditions, the analogous material [Mo₇O₂₄]^6--protected γ-FeO(OH) {Mo_<i>x</i>O_<i>y</i>@FeO(OH)} shows almost comparable NH₃ and H₂ yields, while bare γ-FeO(OH) produces nearly half of it. The better electrocatalytic performance of the POM-protected γ-FeO(OH) materials is due to a higher number of active sites and the cooperative redox participation of the core and the POM ligands. The ¹⁵N labeling experiment confirms the source of nitrogen in the produced ammonia. D₂O labeling with a kinetic isotope effect of 1.2, electrokinetic study, and in situ IR spectroscopy indicate nitrate adsorption as the rate-limiting step for the electrochemical nitrate reduction reaction (eNO₃RR) and the hydroxylamine pathway as the possible reaction pathway of the eNO₃RR. This study, therefore, opens up scope in the design of a variety of POM-anchored metal oxides, which can have potential applications in electrochemical conversions.