Abstract:A series of cerium ion-doped titanium dioxide (Ce 3+ -TiO 2 ) catalysts with special 4f electron configuration was prepared by a sol-gel process and characterized by Brunauer-EmmettTeller method, X-ray diffraction, X-ray photoelectron spectroscopy (XPS), UV-visible diffuse reflectance spectroscopy (DRS), and also photoluminescence (PL) emission spectroscopy. The photocatalytic activity of Ce 3+ -TiO 2 catalysts was evaluated in the 2-mercaptobenzothiazole (MBT) degradation in aqueous suspension under UV or visible light illumination. The experimental results demonstrated that the overall photocatalytic activity of Ce 3+ -TiO 2 catalysts in MBT degradation was signigicantly enhanced due to higher adsorption capacity and better separation of electron-hole pairs.The experimental results verified that both the adsorption equilibrium constant (K a ) and the saturated adsorption amount ( max ) increased with the increase of cerium ion content. The results of XPS analysis showed that the Ti 3+ , Ce 3+ , and Ce 4+ ions reside in the Ce 3+ -TiO 2 catalysts. The results of DRS analysis indicated that the Ce 3+ -TiO 2 catalysts had significant optical absorption in the visible region between 400-500 nm because electrons could be excited from the valence band of TiO 2 or ground state of cerium oxides to Ce 4f level. In the meantime, the dependence of the electron-hole pair separation on cerium ion content was investigated by the PL analysis. It was found that the separation efficiency of electron-hole pairs increased with the increase of cerium ion content at first and then decreased when the cerium ion content exceeded its optimal value. It is proposed that the formation of two sub-energy levels (defect level and Ce 4f level) in Ce 3+ -TiO 2 might be a critical reason to eliminate the recombination of electron-hole pairs and to enhance the photocatalytic activity.

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Two types of lanthanide ion-doped titanium dioxide (Ln3+-TiO2) catalysts including La3+-TiO2 and Nd3+-TiO2 were prepared by a sol-gel method. The effects of the lanthanide ion doping on the crystal structure, surface area, adsorption properties, pore size distribution, and surface chemical state of the catalysts were investigated by means of XRD, BET, and XPS. As results, the crystal size decreased significantly, while the specific surface area, t-plot total surface area, micropore volume, and the total pore volume increased owing to the lanthanide ion doping. The nitrogen adsorption-desorption isotherms of the catalysts showed that the N2 adsorption ability of the Ln3+-TiO2 catalysts was better than the TiO2 catalyst. Among them, the 0.7% Ln3+-TiO2 catalysts demonstrated the highest adsorption ability. The photocatalytic activity of the catalysts was investigated in the experiments of the photocatalytic degradation of benzene, toluene, ethylbenzene and o-xylene (BTEX) in a gaseous phase. The photocatalytic efficiency of the TiO2 catalysts with the lanthanide ion doping was remarkably enhanced by BTEX removal. The 1.2% Ln3+-TiO2 catalysts achieved the highest photocatalytic activity. The enhanced photodegradation of BTEX is possibly due to the improved adsorption ability and the enhanced electron-hole pairs separation due to the presence of Ti3+ on the surface of Ln3+-TiO2 catalysts and the electron transfer between the conduction band/defect level and lanthanide crystal field state.

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Urban pluvial flooding now occurs more frequently than it has in past decades, mainly due to an increasing number of extreme precipitation events occurring in the context of a changing climate. To limit the evolving risks of urban pluvial flooding in a more environmentally friendly manner, the research community has recently paid increasing attention to Nature-Based Solutions (NBS), which are based on new green technologies. To meet the urgent demand for a comprehensive review of the most recent literature, this review conducts a systematic survey of the literature to characterize various NBS adopted in different regions of the world and to elaborate on the benefits and limitations of such NBS. The review highlights the role of NBS in urban flood risk management under ongoing climate change and rapid urbanization. It shows that NBS could effectively mitigate urban flooding caused by high-frequency precipitation events, with additional economic, ecological, and social benefits. However, NBS are less effective at helping cope with pluvial flooding caused by extreme precipitation events over a short period of time, while gray infrastructures also have limitations as a mitigation measure against extreme pluvial flooding. We thus recommend identifying flood risk management strategies by evaluating the performance of alternative combinations of NBS with gray infrastructures in preventing pluvial flooding in the cities. Finally, recent advances made in the applications of NBS are presented with suggestions (e.g., long-term monitoring) to improve urban flood adaptive management. This article is categorized under: Engineering Water > Planning Water Engineering Water > Sustainable Engineering of Water Science of Water > Water Extremes K E Y W O R D S extreme precipitation, nature-based solution, risk management, urban pluvial flood Yijing Huang and Zhan Tian equally contributed to this study.[Correction added on 27 April 2020, after first online publication: the unit of second affiliation has been updated and funding information has been reordered.]