DOI: 10.14457/tu.the.2015.174
title: Synthesis of nitrogen-doped metal oxides and their application in photocatalytic degradation of dyes

description: Photocatalysis with metal oxides, a clean, solar-driven process, is promising for the treatment of dye wastewater. However, most metal oxides show poor visible light activity, although some of them are able to absorb visible light. Therefore, their practical applications are limited. This study aims to develop a series of nitrogen-doped metal oxide catalysts namely N-ZnO, N-ZrO2, and N-WO3 with visible light activity and to investigate their photocatalytic activities for dye degradation in batch slurry system and continuous flow system. The catalysts are synthesized by mechanochemically assisted thermal decomposition method. Their properties are characterized using XRD, FTIR, TGA, SEM, TEM, DRS, XPS, XANES, BET, and zeta potentiometry. The results show that the primary particle size of the N-doped catalysts is around 50 nm with good crystallinity. N doping occurs in both interstitial and substitutional sites and significantly enhances the spectral response up to near-infrared (NIR) region. The N-doped catalysts show higher surface area and lower crystalline size compared to the pristine counterparts, and are stable upon irradiation. No significant reduction of the photocatalytic activity in repeated runs is observed. For the degradation of amaranth in batch slurry system, the order of photocatalytic activity is found to be N-WO3 &gt; N-ZnO &gt; N-ZrO2. The light source and the catalyst determine the reactive species produced during the dye degradation. Less oxidative species such as superoxide radical and singlet oxygen play a critical role in the rhodamine 6G degradation by N-ZnO and N-ZrO2 under visible light, while hydroxyl radicals are predominant under UV light. On the contrary, hydroxyl radicals and holes play a role in the degradation of amaranth by N-WO3 under both visible and UV light. The photocatalytically treated solution of amaranth and that of methylene blue are nontoxic against Bacillus cereus, an important microorganism in agricultural soil. To deal with the issue of catalyst particle separation and to improve their applicability in a real setting, two selected catalysts namely N-WO3 and N-ZnO are immobilized on polyester fabric (PF), forming photoactive systems N-WO3@PF and N-ZnO@PF, respectively. Their properties are characterized by XRD, ATR-FTIR, SEM, and DRS. The results show that the catalyst particles are evenly coated on the PF surface. Both N-WO3@PF and N-ZnO@PF possess remarkable absorbance in the UV, visible, and NIR regions. The photoactive systems can be successfully used for the degradation of amaranth in a thin-film fixed-bed reactor (TFFBR) under solar, visible, and UV light. Under solar light, the photocatalytic activity of N-WO3@PF is higher than that of N-ZnO@PF. After repeated runs, reduced photocatalytic activity is observed, likely due to decreased stability of the catalyst particles on the PF surface. On the basis of figures-of-merit recommended by IUPAC, N-WO3@PF with a TFFBR shows a better performance compared to some existing solar photoactive systems, while N-ZnO@PF exhibits a comparable performance.