Preparation Of Carbon-Nitride-Based Titanium Single-Atom Catalyst And Bismuth Subcarbonate Nanosheet Catalyst And Their Catalytic Performances On Photo/Electroreduction Of CO₂

Carbon dioxide（CO₂）is one kind of low-cost and widespread carbon resource.Developing high-performance catalysts for the conversion of CO₂ to fuel or chemical resources can not only efficiently reduce the emission of CO₂ but also effectively relieve the energy crisis and environmental issue.Especially,photo-/electrocatalytic CO₂ reduction are two prospective strategies.In this thesis,based on low-cost two-dimensional（2D）carbon materials,we prepared a single titanium-oxide species implanted in g-C₃N₄ matrix（Ti O–CN）by one-step thermal polymerization for efficient visible-light photoreduction of CO₂ to CO and further investgated the electron transfer path during the photocatalytic CO₂ reduction;We prepared the ultrathin2D bismuth subcarbonate nanosheets by hydrothermal method and subsequently decorated graphdiyne on them for highly electrocatalytic CO₂ to formate and revealed the component transformation on the catalyst during the electrocatalytic CO₂ reduction.These works provide ways to rationally design efficient photo/electrocatalysts for CO₂ reduction and deeply understand their catalytic mechanism.The details are as follows:1.A highly efficient visible-light CO₂ reduction photocatalyst,Ti O–CN（single titanium-oxide species implanted in g-C₃N₄ matrix）,was synthesized through thermal polymerization by using Ti Cl₄ and dicyandiamide as precursors.In the Me CN/H₂O/TEOA（v:v:v=3:1:1）solution,the as-prepared Ti O-CN shows extremely high CO generation rate of 283.9μmol h^–1g^–1,which is5.7,6.8 and 292.2 times larger than those of Ti O₂/CN hybrid material,CN and commercial Ti O₂,respectively.PL,photocurrent response,TRF and ESR results show that the introduction of single titanium oxide species into the framework of CN greatly enhances the separation of photo-generated carriers in CN and prolongs the lifetime.The visible-light generated electrons in CN can efficiently transfer to Ti⁴⁺and reduce Ti⁴⁺to Ti³⁺,which can further efficiently reduce[Co（bpy）₃]²⁺to[Co（bpy）₃]¹⁺,resulting in highly efficient photo-reduction activity for CO₂-to-CO conversion.2.An ultrathin BOC nanosheet was synthesized through temperature-controlled hydrothermal synthesis method by using sodium oleate and Bi（NO₃）₃·5H₂O as precursors,and further used as an effective CO₂ electrocatalyst.TEM images exhibit that the evolution process of morphology for BOC changes from irregular morphology（110℃）to ultrathin square shape（140℃）and then to thick square shape（180℃）.AFM shows the average thickness of the ultrathin BOC nanosheet is about 2.1 nm.Ultrathin BOC shows the best electrochemical CO₂reduction activity towards formate production.The formate current density reaches 11.7 m A cm^–2 at low potential of–0.85 V vs.RHE and gets 23.7 m A cm^–2at–1.00 V.Meanwhile,87.4%of the Faradic efficiency for formate was obtained at–0.90 V.Further characterizations show that the ultrathin BOC has a lower electron transfer resistance and more active sites.3.By using ultrathin BOC nanosheet as substrate,graphdiyne（GDY）was partially grow on BOC（BOC@GDY）through controlling the amount of GDY monomer and the reaction time,and further used as a highly efficient CO₂ electrocatalyst.Electrochemical CO₂ reduction test shows that BOC@GDY possesses enhanced activity for formate production compared with that of bare BOC.The partial current density for formate of BOC@GDY reaches 13.5 m A cm^–2merely at–0.80 V and 38.2 m A cm^–2 at 1.00 V vs.RHE.Meanwhile,high selectivity is obtained with FE_formate up to 82%at a wide potential window ranging from–0.80 to–1.00 V,and got the highest FE_formate of 95.5%at–0.85 V.The catalytic activity is further great enchanced by using gas diffusion cell.The current densities reach to 100 m A cm^–2 and 200 m A cm^–2 at potentials of–0.71 V and–1.10 V.What’s more,over 91%FE for formate were obtained at wide potential range between–0.70 and–1.10 V.The CO₂ adsorption results show that BOC@GDY has a preferable affinity toward CO₂,and thus facilitate CO₂ reduction.In-situ XAS and Raman spectra show that the BOC@GDY is more easily to be reduced to metallic Bi under the same potential relative to bare BOC,possibly benefit from the electron-rich nature of GDY.More content of metallic Bi can provide more active sites for CO₂ reduction with better electrical conductivity,thus bringing to much better performance for the electrochemical CO₂ reduction.