Abstract
The work describes a study of the oxidation power of N-doped and undoped anatase TiO2, as well as TiO2 Degussa P25 suspensions for photocatalytic degradation of the herbicides RS-2-(4-chloro-o-tolyloxy)propionic acid (mecoprop) and 3,6-dichloro-pyridine-2-carboxylic acid (clopyralid) using visible and UV light. Undoped nanostructured TiO2 powder in the form of anatase was prepared by a sol-gel route. The synthesized TiO2, as well as TiO2 Degussa P25 powder, were modified with urea to introduce nitrogen into the structure. N-doped TiO2 appeared to be somewhat more efficient than the starting TiO2 (anatase) powder when visible light was used for mecoprop degradation. N-doped TiO2 Degussa P25 was also slightly more efficient than TiO2 Degussa P25. However, under the same experimental conditions, no degradation of clopyralid was observed in the presence of any of the mentioned catalysts. When the kinetics of mecoprop degradation was studied using UV light, more efficient were the undoped powders, while in the case of clopyralid, N-doped TiO2 Degussa P25 powder was most efficient, which is probably a consequence of the difference in the molecular structure of the two herbicides.
Highlights
The photocatalytic degradation of pollutants in water and air has attracted much interest in the last several decades, as can be seen from the recent reviews [1,2,3]
The X-ray diffraction (XRD) patterns of the powders are presented in Figure 2, where A stands for anatase and R for rutile phases
The pattern of TiO2 Degussa P25 powder exhibited both anatase and rutile crystalline polymorphs in a wt % ratio A: R = 88: 12 (Table 1), which is in accordance with recently reported values in the literature [14,15]
Summary
The photocatalytic degradation of pollutants in water and air has attracted much interest in the last several decades, as can be seen from the recent reviews [1,2,3]. Among the semiconductor photocatalysts (oxides, sulfides, etc.), TiO2 has been most extensively investigated due to its high photocatalytic activity, chemical and biological stability, insolubility in water, acidic and basic media, non-toxicity and availability. Photocatalysis by semiconductors is a result of the interaction of electrons and holes, generated in a solid by light absorption, with the surrounding medium. The holes at the TiO2 surface are scavenged by surface hydroxyl groups and water molecules to generate OH radicals. The resulting OH radical, being a very strong oxidizing agent (standard redox potential +2.8 V) [5], can oxidize all organic compounds to the mineral final products, i.e. CO2, H2O and corresponding inorganic ions, if the compound contains heteroatoms
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