Solar denitrification coupled with in situ water splitting

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.