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 (N-2) 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 N-2 occurs on Cu-Pd/rGO/TiO(2)via using in situ H-2 generated from water splitting in a broader pH range with achieving near 100% conversion and selectivity to N-2 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 H-2 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 H-2 and O-2 evolution and the in situ generated H-2 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 N-2 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 N-2) by coupling nitrate reduction and water oxidation.