Studies On The Properties And Mechanism Of Selective Photocatalysis Over Ag/TiO₂ Composites

Photocatalytic technology which based on solar energy is regarded as one of the most promising ways to solve energy and environmental problems.Up to now,the rational design of photocatalysts with selective photocatalytic reduction or oxidation performances and the corresponding mechanisms of selective photocatalytic reactions still remain many problems and formidable challenges,and it become the hot topics and attracted the intensive attentions in photocatalysis and organic synthesis.Particularly,the photocatalytic reactions in the aqueous phase have many advantages,such as mild reaction conditions,environment friendly,high economic value,but it is difficult to perform the selective photocatalytic reactions normally occur through a mechanism with uncontrol free radicals（e.g.·OH、·O₂^-、·OOH or·O₂H,etc）mechanism in an aqueous solution.In fact,the investigation on selective photocatalysis has been largely neglected because the free radical reaction has been always considered as an unselective process,so that the development of photocatalytic selective reaction in the water is slow.Therefore,the studies on the photocatalytic properties and mechanism are important for their theory value and practical significance by the rationally design of photocatalysts with photocatalytic reduction or oxidation process,and it is helpful to understand the appropriate conditions of controlling the photocatalytic reduction or oxidation in water phase,explore the formation law of various reactive oxygen radicals（ROS）and improved the conversion efficiency of the selective photocatalytic reaction,forthermore realize the photocatalytic mechanism of the model organics.This work was supported by the National Natural Science Foundation of China.The modified Ag nanoparticles（NPs）were loaded on the amorphous TiO₂（a-TiO₂）substrates with a large specific surface area,by this way,a lot of photocatalysis which have good reduction,highly hydrogen generation efficiency,excellent selective oxidation and conversion ratio were designed and synthesized respectively.The photocatalytic properties of Ag/TiO₂-based composites for hydrogen production and the effect of local surface plasmon resonance（LSPR）effect of Ag NPs on the hydrogen production performance of composites were studied.Moreover,the modulation of reactive oxygen radicals in the process and the photocatalytic degradation mechanism of model organic Rhodamine B（RhB）were investigated.The main contents are listed as follows:1.Through ultraviolet（UV）-induced technology,with a large specific surface area of the a-TiO₂ as substrate,a series of Ag NPs with different organic coating thickness were respectively synthesied,on this basis,strongly coupled CdS quantum dots（QDs）and CuS QDs can modified on Ag/a-TiO₂ by the Ag-S bonds to form strong coupling CdS/Ag/Ti O₂ and CuS/Ag/TiO₂ composites,and an improved UV light induced reduction control methods used for the synthesis of Ag/TiO₂ composites were set up.Specifically speaking,a series of Ag nanoparticles with the organic layer thickness from 0.8 nm to 0.3 nm were modified on the TiO₂ substrates by adjusting the pH of synthesis system.2.When the Ag/TiO₂ composites were used as photocatalyst under UV light irradiation and N₂ gas protection,the paranitroaniline（4-NA）can be reduced to form p-phenylenediamine（4-PPD）at 60 min.Barely TiO₂ can also catalyze the reduction of 4-NA,but p-phenylenediamine can not be obtained at the same condition.On this basis,we used NaBH₄as reducing agent,the catalytic performances（non-photocatalytic reaction）of Ag/TiO₂composites were systematically investigated by the reduction of p-nitroaniline at 288.15,293.15,296.15,298.15,and 303.15 K.The kinetic（rate）constants are obtained at these temperatures.According to these rate constants（k）,the apparent activation energy（E_a）of NaBH₄ reducing 4-NA was abtained,and the E_a is about 102.0 kJ mol^-1.3.A series of Ag/TiO₂ composites which have different thickness of organic coating between AgNPs and TiO₂ were used as light catalyst for hydrogen production,the influence factors of Ag NPs LSPR effect were systematically investigated and analyzed in the photocatalytic hydrogen evolution,and the hydrogen production can reach 476.5μmol g^-1 h^-1.It was found that the non-conductive organic coating can suppress the direct electron charge transfer between AgNPs and TiO₂.In this case,the a-TiO₂ will be activated by the LSPR of Ag NPs,and the efficiency of photocatalytic hydrogen evolution will be significant improved.4.Only Ag/a-TiO₂ can photocatalytic selective oxidize RhB under visible light when the a-TiO₂,c-TiO₂,Ag/a-TiO₂,and Ag/c-TiO₂ were respectively used as photocatalyst under UV and visible light irradiation.According to the results of the radical scavenging experiments,the transition state protection mechanism of RhB was firstly proposed to explain the photocatalytic selective oxidation of RhB.That is,only when the influence（or amount）of·O₂^-is bigger than that of the·OH during the photocatalytic reaction of RhB,RhB can be selectively oxidized to form rhodamine 110（Rh-110）,if not,RhB will be complete oxidized.The reason is that the·O₂^-can firstly react with RhB to form a transition state of RhB（RhB*）at the intial stage of photocatalytic reaction,so that the destruction of the conjugated oxygen oxime structure of RhB can be avoided,and the N-deethylation reaction of RhB can be performed.If not,when the effect of·OH is bigger than that of·O₂^-,the conjugated oxygen oxime structure of RhB will be destroyed,resulting in that the photocatalytic oxidation of RhB is an unselective reaction.5.In order to explore the affecting regularity of ROS,a series of the photocatalytic experiments of RhB and the scavenging experiments of corresponding ROS were performed using Ag/a-TiO₂,Ag/c-TiO₂,CdS/a-TiO₂,CdS/Ag/a-TiO₂,and CdS/Ag/c-TiO₂ composites as photocatalyst.According to the results,it can be found that:1)the selective photocatalytic oxidation of RhB can be performed when the effect（or amount）of·O₂^-is bigger than the·OH in the reaction system,if not,RhB will be complete oxidized.2)the effect of·OH cannot be neglected during the selective photocatalytic oxidation of RhB.When the effect of·OH is eliminated,the N-deethylation reaction of RhB*can not be performed so that the selective photocatalytic oxidation of RhB disappears.Therefore,the rational controls of various ROS are an essential prerequisite for highly selective oxidation reactions.3)The effects of h⁺can be considered together with the·O₂^-and·OH during the selective photocatalytic oxidation of RhB.4)the results of photocatalytic oxidation of RhB can be adjusted by the frequency（wavelength）of the irradiation light source during the photocatalytic process when the Ag/a-TiO₂–based composites as catalysis.6.Ag/a-TiO₂ composites were loaded on the flower-like CuS with a solid structure,and the CuS/Ag/TiO₂ composites were synthesized,as well as the photocatalytic degradation of RhB can also be performed by the as-perpared CuS/Ag/TiO₂ composites,it was found that the adsorptive property of CuS/Ag/TiO₂ composites was more signficantly.In summary,photocatalytic performance and catalytic mechanism of Ag/a-TiO₂-based composites were studied through changing the nanoarchitectures of composites,the results are important for designing the rational structure of photocatalysts,obtaining the highly catalytic activity of photocatalysts,promoting the development of selectivity photocatalytic reaction in organic synthetic chemistry,controlling the condition of the selective photocatalytic oxidation,improving the conversion efficiency of the selective photocatalytic oxidation,and promoting the practical application of selectivity photocatalytic oxidation reaction.All of results could provide some reference strategies and plans for the future design and development of photocatalyst or photocatalytic reactions,such as designing photocatalyst with targeted function,improving the conversion efficiency of the selective photocatalytic oxidation,promoting the development of photocatalysis in the organic synthesis,and so on.