Vacancy-Rich SnO2 Quantum Dot Stabilized by Polyoxomolybdate as Electrocatalyst for Selective NH3 Production

The pronounced conductivity of tin dioxide (SnO₂) nanoparticles makes it an ideal multifunctional electrode material, while the challenge is to stabilize the quantum dot (QD) SnO₂ nanocore in water. An Anderson-type polyoxomolybdate, (NH₄)₆[Mo₇O₂₄], is employed as an inorganic ligand to stabilize a ca. 6 nm SnO₂ QD (Mo_x@SnO₂). X-ray scattering and diffraction studies confirm the tetragonal SnO₂ nanocore in Mo_x@SnO₂. Elemental analyses are in good agreement with the mass spectrometric detection of the [Mo₇O₂₄]^6- cluster present in Mo_x@SnO₂. The ionic POMs attached to the SnO₂ surface through [Mo-O-Sn] covalent linkages have been established by surface zeta potential, shift of the [Mo = O]_t Raman vibration, and extended X-ray absorption fine structure (EXAFS) analyses. The presence of the [Mo₇O₂₄]^6- cluster in the Mo_x@SnO₂ is responsible for the remarkable aqueous stability of Mo_x@SnO₂ in the pH range of 3-9. Dominant oxygen vacancy in the SnO₂ core, identified by EXAFS data and the anisotropic electron paramagnetic resonance (EPR) signals (g ∼ 2.4 and 1.9), results in facile electronic conduction in Mo_x@SnO₂ while being deposited on the electrode surface. Mo_x@SnO₂ acts as an active catalyst for the electrocatalytic nitrate reduction (eNOR) to ammonia with 94% faradaic efficiency (FE) at -0.2 V vs RHE and a yield rate of 28.9 mg h^-1 cm^-2. The stability of Mo_x@SnO₂ in acidic pH provides scope to reuse the Mo_x@SnO₂ electrode at least four times with notable NH₃ selectivity and a superior production rate (239.06 mmol g^-1_(cat) h^-1). This study demonstrates the essential role of POM in stabilizing SnO₂ QD, harnessing its electrochemical activity toward electrocatalytic ammonia production.