Synthesis And Application Of TiO₂ Nanosheet Based Single-Atom Catalysts

Single-atom catalyst(SACs)utilizes much less catalytic reactive components,which are dispersed on the substrates and facilities 100%atom efficiency.It is a high efficeint design routine bridging advantages of both heterogeneous and homogeneous catalysts.Unlike typical Ti O₂ such as anatase and rutile,the single layered two-dimensional titanium oxide nanosheets are synthesized by mature soft chemistry methods,which show promising performance as substrates for catalysts.By combing two-dimensional nanosheets and single catalysts,we developed three kinds of single atom/two-dimensional single-layered titanium oxide nanosheets systems,which the catalyst performances are tested,and reaction mechanisms are studied by various methods.The details of the thesis are the following:(1)By the photo deposition method,atomically dispersed unsaturated Ru oxide/monolayered two-dimensional Ti O₂nanosheet has been synthesized which shows great performance on the visible light hydrogen evolution reaction.Under visible light irradiation,1 wt%Ru content shows 1.96 mmol h^-1 g^-1 hydrogen production efficiency and 6.42 mmol h^-1 g^-1 for full light spectrum.By various characterization methods,we found that the atomically dispersed unsaturated Ru O_x is the key component for such efficient hydrogen production performance.The band gap of the two-dimensional Ti O₂nanosheets is shrunk and elevated by the heterojunction effect,which broaden the light absorption range up to 800 nm.Meanwhile the oxygen vacancy from Ru O₅ promotes electron-hole separation and electron transportation,allowing more utilization of light and promoting the hydrogen evolution reaction.(2)By the solid state reaction method,the single atom catalyst of Ru doped titania is one-step synthesized and further exfoliated to produce single atom Ru doped at Ti site/monolayered two-dimensional Ti O₂nanosheet.As tested as visible light hydrogen evolution reaction photocatalysts,Ti NS doped by 0.33 wt%Ru₁ enables superior mmol h^-1 g^-1 scale H₂ production rate(4.81)under visible light irradiation,comparable to 8.95mmol h^-1 g^-1 under full light spectrum.Various characterizations and theoretical calculations discovered that both Ru₁ dopant and the induced oxygen vacancies play the key roles in promoting visible light hydrogen evolution reaction.Firstly,single-atom Ru₁ replacing Ti induces an isolated impurity energy level in between valence band and conduction band,which extends light absorption range up to 700 nm and lowers the hydrogen evolution reaction barrier by promoting H⁺adsorption at O site from Ru-O-Ti.Secondly,V_O created by the introduction of Ru dopants extends light absorption from 360 nm to 470 nm and serves as an electron trapping site,which promotes photogenerated electron separation and transportation.(3)By using a cation-deficient from monolayered Ti O₂ nanosheet,atomically dispersed Pd/monolayered two-dimensional Ti O₂nanosheet catalyst has been fabricated by electrostatic adsorption.This catalyst exhibited an extraordinarily high catalytic activity(TOF up to 11110 h^-1)for Suzuki coupling at room temperature,surpassing conventional Pd catalysts under the same conditions.Compared to conventional homogeneous catalysts,this heterogeneous catalyst can be easily filtered out and reused without significant loss of activity.DFT calculations revealed that Pd₁ in Pd₁^IIO₄ slab held an enriched charge distribution,which was caused by the charge transfer from the support,greatly accelerating the C-Br bond cleavage.In-situ FTIR and EXAFS indicated Pd₁ acted as a highly active site to rapidly break the C-Br bond in bromobenzene.