Applications Of Titanium Dioxide/metal Composites In Photocatalytic Carbon Dioxide Reduction And Nitrogen Fixation And Related Mechanism Studies

With the rapid development of society,humans’excessive dependence on non-renewable fossil fuels has caused serious environmental pollution problems.Therefore,it is necessary for us to find a more sustainable and environmentally friendly energy conversion system to solve the huge demand for energy and environmental problems of social development.As a new chemical technology,photocatalysis has attracted more and more attention due to its features such as green,environmentally friendly and sustainable.Among them,titanium dioxide（TiO₂）,as the most classical semiconductor photocatalyst,has been widely studied in the fields of sewage purification,air purification,and energy synthesis due to its good chemical stability,high catalytic activity,and no secondary pollution.In the process of research,researchers still found many problems in the process of studying TiO₂:（1）TiO₂has a large forbidden band width,which makes it have weak light absorption capacity;（2）Due to the easy recombination of photogenerated electrons and holes,the quantum efficiency of TiO₂is low;（3）Pure TiO₂can only absorb ultraviolet light and has a low utilization efficiency of sunlight.Due to the emergence of these problems,the further development and practical application of TiO₂materials have been restricted.Based on the traditional photocatalyst TiO₂,we optimize and modify the TiO₂photocatalyst through crystal face adjustment and noble metals loading.With a series of testing methods,we analyze the morphology,phase structure,catalytic performance of the catalyst.Meanwhile,we simulate the catalytic reaction path and charge density through theoretical calculations,and study the performance and mechanism of the composite photocatalyst from various aspects.While improving the photocatalytic carbon dioxide reduction and nitrogen fixation capabilities of TiO₂,we also revealed the role of crystal faces,interfaces,and active sites in the photocatalytic process of TiO₂.This work provides experimental and theoretical basis for the design and synthesis of metal-semiconductor composite photocatalysts in the future.The specific research contents of this paper are as follows:1.Interfacial facet engineering on the Schottky barrier between plasmonic Au and TiO₂in boosting the photocatalytic CO₂reductionHybrid photocatalytic nanostructures composed of plasmonic metal and wide-band-gap semiconductor components have been widely developed,in which metal not only acts as a cocatalyst to trap the photogenerated electrons from semiconductor for improved charge separation and provide highly active sites for accelerated reaction kinetics,but also serves as a light-harvesting antennae to extend the light absorption region based on the injection of plasmonic hot electrons into the semiconductor.In both circumstances,rational design of metal/semiconductor interface is highly desirable to smooth the migration of electrons and promote the separation of carriers.Herein,based on the deposition of Au on TiO₂nanocrystals with different exposed facets,it is found that the formation of Au/TiO₂（101）interface lowers the height of Schottky barrier in comparison with Au/TiO₂（001）interface,enhancing the transfer of conduction band（CB）electrons from TiO₂to Au cocatalysts under ultraviolet light irradiation and promoting the hot electron injection from plasmonic Au into the CB of TiO₂with the excitation of Au by visible light.The more efficient interfacial charge transfer and separation enable more electrons participating in the conversion of CO₂to CO and CH₄.As a result,at both excitation wavelengths,the Au-TiO₂sample with exclusive Au/TiO₂（101）interfaces significantly ameliorates the photocatalytic activities in CO and CH₄production compared to other samples containing Au/TiO₂（001）interfaces.The interfacial facet engineered Schottky barrier may open a new window to rationally designing metal-semiconductor hybrid structures for photocatalysis.2.Quantifying photocatalytic N₂fixation on the edge sites of Pd co-catalystsTo identify the roles and quantitatively evaluate the activities of edge and surface atoms of metal co-catalysts in photocatalytic N₂fixation,a protocol has been developed based on the rational choice of well-defined Pd nanocubes with tunable size and specific exposed facet as co-catalysts.The key to this strategy is the approximate number of Pd atoms exposed for surface reaction and in contact with TiO₂supporter for electron transfer based on the precise control of the loading amount of Pd.As the result,the number of edge atoms is only the variable,which increases with the decrease of the size of Pd.Based on the accurate calculation of the number of surface and edge Pd atoms in the Pd nanocubes as well as the correlation between edge atoms and photocatalytic activity,the turnover frequency of the edge and surface atoms toward NH₃formation have been determined to be 18.9 h^-1and 0.0436 h^-1,respectively.The superior activity of edge atoms is agreement with the DFT calculations,in which the rate determined step of N₂hydrogenation on edge atoms is in lower barrier compared with that on surface atoms.