Compósitos a base de Ag/TiO2 aplicados em processos fotocatalíticos

Abstract

The widespread use of nitrogen fertilizers increases the nitrate (NO3 - ) concentration in natural waters, causing eutrophication and presenting major risks to public health. However, cheap and efficient ways of treating water contaminated with nitrate are not yet a reality. One of the forms of remediation, which can be low-cost, is photocatalysis. In the present work, TiO2-based photocatalysts were used to reduce nitrate to ammonium or molecular nitrogen with UVC irradiation, in the presence of in the presence of formic acid, used as a hole scavenger and as a source of H+ . The Ag/TiO2 composite was prepared by the photodeposition method and its physicochemical properties were determined using colorimetry, X-ray diffraction (XRD), infrared absorption spectroscopy, Raman spectroscopy and field emission - scanning electron microscopy with energy dispersive spectroscopy (FEG-EDS). These techniques confirmed the deposition of Ag on commercial TiO2, especially UV-vis and FEG-EDS. Firstly, the photocatalytic reduction of nitrate to ammonium was done by UVC irradiation up to 24 h. During the process, adjustments were made to the amounts of solvent and photocatalyst to improve activity. Moreover, it was confirmed that Ag deposition improved the photocatalytic activity. Literature data indicate that Ag deposited on the surface of the oxide will donate electrons to O2, generating superoxides. In the present case, the reaction mechanisms were evaluated by the use of substances that avoid the formation of these reactive species, removing them from the solution and reducing photocatalytic activity during terephthalic acid photohydroxilation. To remove holes from the solution, EDTA was used; and reaction of electrons with O2 dissolved in water was avoided by the flow of N2, which was bubbled into the solution. Results indicated that the composites had no meaningful decrease in the formation of hydroxyl radicals when O2 was taken out of the solution by N2 flow, while holes withdrawn led to a meaningful decrease of activity. This indicates that Ag addition onto the semiconductor acts as electron trap avoiding electron-hole recombination, without formation of superoxide radicals.