Photocatalytic and photoelectrocatalytic oxidation of nitrogen compounds in wastewater

Abstract

In past decades, the intensification of human activities has created an alarming rise in environmental pollution levels. The use of photocatalysis or photoelectrocatalysis present alternatives for the decentralized treatment of domestic wastewater compounds, with the potential coupling to energy recovery. This study aims to explore the use of these technologies for the oxidation of urea and the possible coupling to hydrogen production.

The use of two TiO2 nanostructured electrodes, P25 and titanium nanotubes (TiNT), was studied for the photoelectrochemical oxidation of urea. The TiNT showed better performance with higher photocurrent, an incident photon-to-current conversion efficiency (IPCE) reaching 55.11 % at 340 nm and an oxidation rate of 0.0037 min -1 . The better performance of the TiNT electrode was attributed to the longitudinal pathway provided by the nanotubes, aiding charge transfer.

Subsequently, the enhancement in the TiO2 (P25) performance through the deposition of Cux O clusters was explored using atomic layer deposition. The photoelectrocatalytic results showed that the addition of Cux O clusters rectified cathodically the TiO2 generated current. Alternatively, when used in photocatalytic suspension, the Cu 2O clusters with low loadings (<1 wt.%) showed improved urea oxidation achieving up to 1.7 times the rate of unmodified TiO2 (P25).

Additionally, the oxidation of urea coupled to hydrogen production was investigated, using WO3 as photoanode. The WO3 electrode exhibited superior performance than P25, with a visible light activity up to 470 nm, 10 times higher photocurrent, an IPCE of 43 % at 360 nm and a urea oxidation rate constant of 1.47×10−2 min−1 . Hydrogen was simultaneously produced with a Faradaic efficiency of 87.3 % and a solar-to-hydrogen conversion efficiency of 1.1%.

This thesis showed the potential for the development of technologies based on the photoelectrochemical treatment wastewater pollutants coupled to energy recovery. Further work is needed to address the selective oxidation of urea to dinitrogen and testing under real conditions.

Date of Award	Nov 2023
Original language	English
Supervisor	Pilar Fernandez-Ibanez (Supervisor) & John Byrne (Supervisor)