Ammonia\nSynthesis with Visible Light and Quantum Dots

Light-assisted synthesis of ammonia\nfrom nitrate and nitrite sources\nis a sustainable approach to reduce the burden of the energy-intensive\nHaber-Bosch process. However, poor selectivity and the need for UV-active\nphotocatalysts are the current bottlenecks in the synthesis of ammonia\nfrom nitrate and nitrite sources. Herein, we introduce selective visible-light-driven\nammonia production from nitrate and nitrite ions with indium phosphide\nquantum dots (InP QDs) as the photocatalyst. The presence of catalytic\nindium sites and microenvironment modulation through an interplay\nof catalyst-reactant interactions resulted in efficient and selective\nammonia formation under visible light. Ammonia was produced in an\nattractive yield of ∼94% in both aqueous and gaseous phases\nwithin 2 h of visible-light irradiation at room temperature. A decent\nformation of ammonia was observed under sunlight as well, strengthening\nthe translational prospects of InP QD photocatalysts. Mechanistic\ninvestigations ascertained a negligible role of competing hydrogen\nevolution in direct nitrate reduction, confirming the active participation\nof photoexcited charge carriers from InP QDs in the ammonia synthesis.\nKinetic studies revealed the energetically challenging nitrate-to-nitrite\nconversion as the rate-determining step, with subsequent reactions\nproceeding with ∼100% conversion to yield ammonia. A series\nof experiments concluded that water is the proton source in the InP\nQD-photocatalyzed synthesis of ammonia. Our study shows the impact\nof the rationally designed core and surface of InP QD-based photocatalysts\nin developing sustainable routes to produce ammonia beyond the Haber-Bosch\nprocess.