Nanostructured materials for the photocatalytic abatement of N-containing pollutants from waste water

INTRODUCTION The development of effective methods for the abatement of some harmful pollutants from waste waters and from hydric resources is a challenging task. Nitrogen containing compounds, such as inorganic ammonia, nitrites and nitrates, and some organic N-containing compounds (dyes, pesticides, drugs, etc.), are harmful contaminants for drinking water, inducing acute and/or chronic diseases, especially affecting infants and children. Furthermore, when released in waste waters they contribute to eutrophication, or possibly contaminate ground water. This is particularly relevant in agriculturally intensive zones and in the case of some relevant industrial processes involving e.g. nitration reactions. In the present work, we developed nanostructured photocatalysts for the abatement of N-containing compounds, focusing on selectivity towards innocuous N2, to be applied for the treatment of waste waters to meet legislative specifications. EXPERIMENTAL/THEORETICAL STUDY The photocatalysts have been prepared in nanosized form by using an innovative flame pyrolysis (FP) approach, able to synthesise in one step single or mixed oxide nanoparticles, characterized by homogeneous particle size and good phase purity. In the present investigation TiO2 and SrTiO3 have been prepared by means of a home made apparatus, starting from Titanium isopropoxide, dissolved in o-xylene and added 1:1 with propionic acid (in case containing Sr nitrate for the preparation of SrTiO3). Different pyrolysis conditions have been set in order to optimize materials properties. Noble metals have been added to the samples by post synthesis deposition-precipitation (Au), photodeposition (Pt) and/or directly during the FP synthesis (Ag). The role of the metal nanoparticles was the enhancement of the lifetime of the photoproduced charges by electron trapping and/or the enhancement of the materials absorption in the visible region, which is one of the most practically relevant goals of this research. The same materials formulations have been prepared starting from commercial nanostructured TiO2 supports for comparison purposes. The characterization of the synthesized nanomaterials was carried our by N2 adsorption/desorption, FT-IR, XPS, SEM, TEM, XRD, DR-UV-Vis. The photocatalytic performance of the samples, to be correlated with nanomaterials properties, has been checked for the photoreduction of nitrate ions by means of two batch photoreactors. A first design is based on external irradiation through a quartz window, a second prototype is based on an immersed UV lamp. RESULTS AND DISCUSSION The FP procedure proved an interesting method for the preparation of nanostructured Ti-based photocatalysts. Particle size ranged between 5 and 60 nm, depending on synthesis conditions and the resulting phase was a mixture of anatase and rutile, their proportion depending mainly on flame temperature and residence time in the hottest zone of the flame. Doping of the sample with Au (0.1-0.5 wt%) induced the appearance of a plasmonic band with increased absorption in the visible portion of the spectrum. No significant Au leaching or variation of absorption spectrum was assessed after used. Doping with Ag also resulted important to improve the harvesting of visible light, the sample becoming light purple. In this case the FP technique provided a very rapid and effective mean for the incorporation of this dopant in the nanomaterial. CONCLUSION Photocatalysts have been synthesized by means of flame spray pyrolysis and compared with commercial nanostructured TiO2 supports. The FP synthesis proved flexible and versatile also for catalyst doping and to widely tune materials properties. ACKNOWLEDGMENTS The financial support of Fondazione Cariplo through the measure “Ricerca sull’inquinamento dell’acqua e per una corretta gestione della risorsa idrica”, grant no. 2015-0186, is gratefully acknowledged.