Photocatalytic Transformation Of Nitrogen And Phenol Contaminants By Ag Enhanced TiO₂ And G-C₃N₄ Composites

In aqueous environment, nitrogen and phenol contaminants pose serious threats to plants, animals and human beings. Photocatalytic technology can utilize solar or UV energy to mineralize the organic and inorganic contaminants. However, titanium dioxide（TiO₂）, one of the typical semiconductor photocatalysts, has some disadvantages such as narrow range of photoresponse, poor quantum efficiency and easy recombination of photogenerated electron-hole pairs. It is urgent to develop high efficient photocatalysts. In recent years, the development and application of TiO₂ and graphitic carbon nitride（g-C₃N₄） based composite photocatalysts, and new type of TiO₂ with highly reactive facets have attracted more attention.In this study, the problems involved in the development TiO₂ and g-C₃N₄ and their applications in wastewater treatment were pointed out after traditional methods for the removal of nitrogen and phenol contaminants were reviewed. On this basis, a new sight of constructing Ag enhanced composite photocatalytic materials was proposed, and the performances and mechanisms during the transformation the nitrogen and phenol contaminants were systematically investigated.On the carrier of commercial P25, a facile method of chemical precipitation with pH adjustment was used to prepare Ag₂O/P25 composite photocatalyst for the first time. The as-prepared Ag₂O/P25 was applied in the oxidation of ammonia and nitrite, and the catalyst structure, photocatalytic activity and reaction mechanism were investigated. During the oxidation of ammonia, 1% Ag₂O/P25 catalyst showed an optimal reactivity at pH 12.0. The results demonstrated that ammonia oxidation took place via both parallel and sequential steps. Ammonium was oxidized to nitrite, and then a fraction of the newly formed nitrite was further oxidized to nitrate by hydroxyl radicals. In the oxidation of nitrite, the conversion could reach 100% after 8 h UV irradiation by 1% Ag₂O/P25 at pH 9.0 and 10.0. In addition, the presence of nitrite on ammonia oxidation and the presence of ammonia on nitrite oxidation were also investigated.In order to broaden the application of Ag₂O/P25, selective reduction of nitrate catalyzed by Ag₂O/P25, with formic acid as the hole scavenger was explored. Under UV irradiation, 5% Ag₂O/P25 showed a high conversion ratio and a high N2 selectivity in nitrate reduction. Photoluminescence（PL） spectra analysis confirmed the improved separation efficiency of electron-hole pairs on 5% Ag₂O/P25. In this study, the mechanism involved in selective reduction of nitrate by Ag₂O/P25 was specifically discussed, and two pathways were proposed: i) via reduction with 2COproduced from the reaction between photo-generated holes and formic acid, and ii) by direct interaction of the nitrate with electrons reaching the surface of Ag₂O/P25. In addition, the activity and stability of Ag/P25 and Ag₂O/P25 in nitrate reduction were compared, and the differences were because of the different hetero-structures of Ag₂O-Ag and Ag-Ag₂ O on P25. Ag₂O/P25 has a broad application prospect due to its good reusability. This work provides a new sight into the use of Ag₂O/P25 catalyst in nitrate reduction.By means of ascorbic acid, an environmental friendly molecule as the morphology control agent, TiO₂ with controllable phases and more highly reactive {001} facets was prepared through a one-pot hydrothermal method. The phase content and the percentage of {001} facets were analyzed by X-ray diffraction（XRD） and transmission electron microscope（TEM）. The contribution of ascorbic acid as control agent was discussed. The product containing anatase（with 18.4% {001} facets） and rutile was selectived and applied in the oxidation of ammonia and nitrite, and the reduction of nitrate. This work opens a door to prepare TiO₂ with highly reactive {001} facets.g-C₃N₄ was prepared via a thermal polymerization with dicyandiamide（C₂H₄N₄） used as precursor. Ag₂O/g-C₃N₄ composite photocatalyst was then constructed using a chemical precipitation method with pH adjustment. The structure and composition of the as-prepared samples were characterized. Under UV and visible light irradiation, photocatalytic degradation of phenol by Ag₂O/g-C₃N₄ was evaluated. The enhanced reactivity of g-C₃N₄ by Ag₂ O were attributed to: i) the improved dispersibility of Ag₂ O on the surface of g-C₃N₄; ii) the enhanced optical absorption property arising from the hetero-structures between Ag₂ O and g-C₃N₄; and iii) the synergetic effects of the inner electric field and the matched energy band structure that improved the separation efficiency of the photogenerated electrons and holes. The oxidative species in the photodegradation process were detected through the trapping experiments of radicals. The structures of the catalyst before and after reaction were analyzed by XRD, and the photodegradation mechanism was then put forward. The +h,2 O-and OH could efficiently degrade phenol into intermediate products, and finally, into CO₂ and H2 O. The as-prepared Ag₂O/g-C₃N₄ composite provides a new method for the removal of phenol contaminant in wastewater under visible light, and can therefore be used as a promising photocatalyst in the remediation of other organic contaminants.Finally, on the basis of the obtained Ag₂CO₃/g-C₃N₄, Ag₂O/Ag₂CO₃/g-C₃N₄ composite was prepared for the first time by phase transformation method and its microstructure was characterized. The photocatalytic performance of Ag₂O/Ag₂CO₃/g-C₃N₄ was evaluated by the degradation of phenol under UV and visible light irradiation, and the oxidative species in the photodegradation process were discussed. This work provides a facile phase transformation approach to fabricate other novel heterostructured catalysts.