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) 3Co (OH) 6Mo6O18, is used to synthesize a crystalline ∼5 nm γ-FeO (OH) nanocore on which several in situ-derived CoMox cluster anions are covalently attached through Fe-O-Mo linkages, making them highly aqueous dispersed ionic nanoparticles CoMox@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 FeIII and MoVI centers in this complete inorganic nanostructure allows sequential electroreduction of FeIII centers of the FeO (OH) core at 0. 81 V (vs reversible hydrogen electrode (RHE) ), followed by CoMox 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 Mo7O246--protected γ-FeO (OH) MoxOy@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.
Samanta et al. (Mon,) studied this question.