Solar denitrification coupled with in situ water splitting

Abstract

Utilizing solar energy as a sustainable means of controlling the nitrogen pollutant is proposed. The photochemical conversion of nitrate (NO₃⁻) to dinitrogen (N₂) without using chemical reductants is an ideal solution but difficult to be realized. Here we demonstrate a successful case of solar denitrification (with 0.1–10 mM nitrate) coupled with in situ water splitting (without chemical reductants) by developing a ternary composite photocatalyst composed of TiO₂, Cu–Pd bimetals, and reduced graphene oxide (rGO) (Cu–Pd/rGO/TiO₂). Direct transformation of NO₃⁻ to N₂ occurs on Cu–Pd/rGO/TiO₂via using in situ H₂ generated from water splitting in a broader pH range with achieving near 100% conversion and selectivity to N₂ while it is not possible at all with rGO/TiO₂ and Cu–Pd/TiO₂. The unique activity is ascribed to the synergic action of Cu as a co-catalyst for nitrate-to-nitrite conversion, Pd for nitrite-to-dinitrogen conversion, and rGO for the enhanced charge separation/transfer and H₂ production. The combined roles of Cu–Pd and rGO in retarding the charge recombination and accelerating the electron transfer from TiO₂ to NO₃⁻ are confirmed by monitoring the time-resolved photoluminescence and slurry-type photocurrent generation, respectively. The in situ water splitting on Cu–Pd/rGO/TiO₂ was confirmed by the concurrent H₂ and O₂ evolution and the in situ generated H₂ was immediately consumed in the presence of nitrate. The introduction of rGO enabled the denitrification even under visible light (up to 450 nm) and the apparent quantum yield (AQY) of N₂ production reached a maximum of 4.9% at 320 nm. The proposed composite photocatalytic system realizes the selective solar conversion for chemical reductant-free denitrification (nitrate to N₂) by coupling nitrate reduction and water oxidation.