Electrochemical Nitrate Reduction through Cooperative Redox Participation of the CoMo x 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, (NH 4 ) 3 [Co(OH) 6 Mo 6 O 18 ], is used to synthesize a crystalline ∼5 nm γ-FeO(OH) nanocore on which several in situ-derived {CoMo x } cluster anions are covalently attached through [Fe–O–Mo] linkages, making them highly aqueous dispersed ionic nanoparticles {CoMo x @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 NH 3 (65%) and 26.68 μmol h –1 cm –2 H 2 (25%). Under similar electrochemical conditions, the analogous material [Mo 7 O 24 ] 6– -protected γ-FeO(OH) {Mo x O y @FeO(OH)} shows almost comparable NH 3 and H 2 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 15 N labeling experiment confirms the source of nitrogen in the produced ammonia. D 2 O 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 (eNO 3 RR) and the hydroxylamine pathway as the possible reaction pathway of the eNO 3 RR. 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.