The Modulation Of Electronic Structure Of Catalysts Applied To Photocatalytic CO₂ Reduction Reaction

Excessive CO₂ emission has caused serious climate change,and how to effectively solve this issue has aroused great attention among scientists.Solar-driven CO₂reduction is considered a green and promising solution for achieving the goal of"carbon peaking and carbon neutrality".However,there are still great challenges to achieve this goal due to the thermodynamic stability of CO₂ molecular and the slow multi-electron transfer kinetic process of CO₂ reduction and the competitive hydrogen evolution reaction.In principle,the modulation of the electronic properties of catalysts can improve the adsorption and activation of CO₂ and its reaction intermediates,thus promoting the reaction kinetics and regulating the product selectivity.In addition,accelerating the interfacial charge transfer between the photosensitizer and the catalyst can improve the utilization efficiency of photogenerated electrons by adsorbed CO₂molecules on catalyst surface,thus promoting the photoreduction of CO₂.Based on the theoretical guidance,in this thesis,oxygen vacancy-functionalized nickel hydroxide（OVs-Ni（OH）₂）,nitrogen-doped carbon-encapsulated nickel hydroxide（Ni（OH）₂/NC）,and bipyridyl cobalt/nitrogen vacancy-modified polymeric carbon nitride（Co（bpy）₃/NVs-PCN）catalysts were prepared by surface engineering design,and photocatalytic CO₂ reduction performance of the prepared catalysts were investigated.The main contents are as follows:1.Oxygen vacancy functionalized nickel hydroxide nanosheets（OVs-Ni（OH）₂）catalysts were synthesized by a novel photochemical method.Characterization analysis and theoretical calculations confirmed that OVs can promote the electron transfer from active Ni atoms to CO₂,thus greatly improving the interaction strength between OVs-Ni（OH）₂ and CO₂,while lowering the energy barrier for COOH*formation,which can preferentially promote the reduction of CO₂.In addition,the photo-electrochemical properties showed that the OVs modification can boost the OVs-Ni（OH）₂ catalyst to capture photogenerated electrons efficiently,and promoting the charge transfer between the photosensitizer and the catalyst.The photocatalytic CO₂ reduction performance study showed that the OVs-Ni（OH）₂ catalyst exhibited excellent activity and selectivity for CO₂-to-CO photoreduction with visible light,with CO evolution rate up to 31.58μmol h^-1(0.35 mg catalyst,90228μmol h^-1 g^-1)and CO selectivity of 98%,outperforming most of analogous catalysts in literatures.Moreover,even under ultralow CO₂ concentration of 0.04%（representative CO₂ concentration of air）and low reaction temperature（273 K）,this catalyst can still trigger CO₂ reduction.Our work provided a new methodology to synthesize OVs-Ni（OH）₂catalyst for efficient CO₂ reduction.More significantly,the relationship between the OVs and the catalytic performance has been established,which may guide the design of highly-selective CO₂ reduction catalysts.2.A nitrogen-doped carbon-encapsulated Ni（OH）₂（Ni（OH）₂/NC）hybrid catalyst was prepared using a photochemical method.Under visible light irradiation,the Ni（OH）₂/NC catalyst exhibited improved CO selectivity and catalytic stability,achieving a CO evolution rate of 14.93μmol h^-1(0.30 mg catalyst,49766μmol g^-1 h^-1)and 83%CO selectivity,which was significantly better than that of the pristine Ni（OH）₂(8.51μmol h^-1 and 45%CO selectivity).The structures and physicochemical property of the Ni（OH）₂/NC were carefully analyzed by various characterization techniques and density functional theory（DFT）calculations.Results suggested that the NC modification can modulate the electronic structures of Ni（OH）₂,and thus consolidate the adsorption and activation of CO₂.Furthermore,the interface charge transfer between photosensitizer and Ni（OH）₂ can also be promoted after NC modification.As a result,the Ni（OH）₂/NC catalyst exhibited superior activity for CO₂-to-CO photoreduction.Moreover,we also successfully prepared NC-encapsulated Ni O（Ni O/NC）under the same preparation conditions.This Ni O/NC catalyst also exhibited enhanced selectivity and activity for CO₂ reduction with visible light,suggesting the effectiveness of carbon-modification for managing CO₂ reduction.This work provided a new avenue to modulate the selectivity and activity of CO₂ reduction over transition metal hydroxides or oxide catalysts.3.A nitrogen vacancy-modified polymeric carbon nitride（NVs-PCN）material was successfully prepared by a novel formic acid vapor-assisted etching protocol.Characterization analysis and theoretical calculations determined that the NVs modification can optimize the electronic properties of Co（bpy）₃ catalyst,and thus considerably strengthening the adsorption of CO₂,while improving the electron transfer from the Co（bpy）₃ catalyst to the adsorbed CO₂,consequently accelerating the activation and reduction of CO₂.As a result,the Co（bpy）₃/NVs-PCN photosystem exhibited a 279-fold improvement in CO₂ reduction activity over the pristine Co（bpy）₃/PCN.Under the same preparation conditions,we also successfully prepared another NVs-modified Co（bpy）₃/PCN-T（trithiocyanuric acid as a precursor）.This Co（bpy）₃/NVs-PCN-T photosystem also showed exceptional performances for photocatalytic CO₂ reduction with visible light.This work provided a new approach to catalyst structure modulation and also elucidated the role of defective structures in the CO₂ reduction process.