Development of Composite Photocatalysts for Solar Denitrification

DoctorNitrate is a ubiquitous aquatic pollutant that cannot be easily controlled and the imbalance between the anthropogenic discharge and the natural removal in the global nitrogen cycle causes the accumulation of nitrate in the environment. Solar photocatalytic reduction of recalcitrant nitrate has attracted interests as a potential control method but achieving high efficiency and selectivity to N2 without generating harmful byproducts such as nitrite and ammonium is still a challenging issue. Although cocatalyst-loaded photocatalysts for nitrate removal have been heavily reported, they require the addition of reducing reagents such as organic compounds and H2, which limits their practical applications. To overcome this drawback, developing an electron donor-free denitrification system is highly desired. An ideal approach is utilizing water as a reductant but, due to the kinetic mismatch between nitrate reduction and water oxidation, finding an ideal photocatalyst is still difficult to be realized. Furthermore, the previously reported (photo)catalysts always demand anoxic condition to avoid the rapid repetitive reaction with oxygen, which is a major bottle neck for its practical applications as a proper nitrate treatment technology. In this research, utilizing solar energy as a sustainable mean of controlling nitrogen pollutant is newly proposed. The aim of this study is (1) the demonstration of a single photocatalytic system that combines photocatalytic denitrification and the oxidation of water or aquatic pollutants (i.g., water splitting or degradation of pollutants), which utilizes in situ generated H2 as a reductant for nitrate and nitrite reduction and (2) to provide breakthrough strategies for the commercialization of solar denitrification. In the first work, to understand the interaction between nitrate/nitrite and other aquatic pollutants commonly present in wastewater, the simultaneous removal of aquatic pollutants and nitrite (i.g., a well-known intermediate product in denitrification process) was studied. Pd was selected among various hydrogenation metals (Ni, Pd, Pt, Cu, Ag, and Au) and further studied as a suitable cocatalyst for NO2− removal. NO2− is readily oxidized NO3− by reactive radical species (hydroxyl radical, •OH) on Pd loaded TiO2 (Pd/TiO2) in the absence of chemical electron donors. The selectivity to N2 was highly enhanced in the presence of arsenite (As(III)) and 4-chlorophenol (4-CP) by maximizing H2 production and utlizing H2 as an in situ reductant. The underlying reason for the enhanced N2 yield is that the photooxidation of NO2− can be totally hindered by prompt consumption of photogenerated holes to degrade As(III) and 4-CP, in turn, facilitating charge separation and transportation of charge carriers in the presense of As(III) and 4-CP. The co-presence of aquatic pollutants and nitrite can enhance the recyclability of this proposed system on which the oxidation of Pd, a chronic problem of catalytic NO2−/NO3− treatment, is prevented by efficient hole extractions. Instead of the coupling of solar denitrification with polluant degradation, the second study demonstrated 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/TiO2 via overall water splitting in a broader pH range with achieving near 100% conversion and selectivity to N2. 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. The in situ generated H2 was immediately consumed in the presence of nitrate. However, the conversion of nitrate to N2 in the presence of dissolved oxygen in the reactor seems to be almost impossible. In the third work, the special-purpose ternary composite (Cr2O3/Ag-Pd/TiO2) was synthesized and firstly evaluated as a suitable photocatalyst for the ambient aerated denitrification. The oxygen-tolerant nitrate conversion proceeded through overall water splitting by utilizing in situ generated H2 as a sole reductant and NO3− can be subsequently transformed to N2. The introduction of Cr2O3 shell perfectly inhibited accumulation of bulky O2 molecules around the catalyst, and accelerated the protons and electrons coupled transfers. The reduction of NO3− and NO2− was possible because of electrons transferred from silver nano particles, which reduces protons to H2 under the light irradiation and subsequently utilizes H+ and H2 as reductants. Silver cocatalysts were significantly involved in O2 generation on Cr2O3/Ag-Pd/TiO2 composite by their intrinsic redox-mediated shuttles. The proposed photocatalytic processes using three different photocatalyts represent viable treatment options for nitrogen-contaminated water containing numerous of aquatic pollutants. Also, this study is a step forward towards the development of solar-driven technologies as sustainable and efficient routes to overcome global development goals related to water availability and renewable energy sources