Hollow-structured Pd/TiO2 as a dual functional photocatalyst for methyl orange oxidation and selective reduction of nitrate into nitrogen

Abstract

Nitrate pollution in wastewater has posed a threat to human health and the environment. Photocatalytic reduction is a promising technology to remove nitrate due to its high adaptability, low cost, and high efficiency. However, there is a great challenge to design photocatalyst with high nitrate reduction and high selectivity of nitrogen (N2) yield. In this study, a novel hollow-structured Pd/TiO2 (PHT1) photocatalyst is synthesized with large specific surface area (SBET) and pore volume, strong UV adsorption, high reduction potential than nitrate to nitrite, high charge transfer rate, and low electron-hole recombination rate. PHT1 exhibits a superior photocatalytic activity on the removal of methyl orange and nitrate. In addition, PHT1 exhibits high N2 selectivity (>95 %). A lower pH can promote the reduction of nitrate and the selectivity of N2 by PHT1. Electrons (e-) and CO2– are the main active species for nitrate reduction with OH as auxiliary radicals. Three main pathways for nitrate reduction are proposed: i) the reaction with formic acid under acidic conditions; ii) the reduction by electrons and the loaded Pd of photocatalyst to capture electrons to inhibit electron-hole recombination; iii) the reduction by CO2– generated from formic acid oxidation by OH.