Photoelectrocatalytic Reduction Of CO₂ Through WO₃-based Semiconductor Functional Material

Photoelectrocatalytic（PEC）reduction of CO₂ might be an environment-friendly strategy to alleviate the green-house effect caused by excessive emission of CO₂ and provide potential solutions for sustainable energy.It is well known that many semiconductors have been applied to PEC CO₂reduction because they can supply photoelectrons and active sites for CO₂.Among many semiconductors,WO₃ has simple synthesis process,feasible modification of structure and chemically stability,which is suitable for photoelectrocatalytic reduction of CO₂.However,it has drawbacks like wide band-gap（～3.0 V）and low conduction band（-0.15 V）that prohibit its application in CO₂ reduction.The above disadvantages can be overcome by certain modification methods.This thesis studies the modification of semiconductors for PEC reduction of CO₂in water.N/Pdoping method is controlled by changing the temperature of the N doping and the content of the P doped precursor to adjust the band gap structure of WO₃.Campared with pristine WO₃,the chemical and physical properties of N/P doped WO₃ were greatly altered with the suitable doping amount compared with the pristine WO₃ semiconductor.Four different N doped and P doped catalysts were confirmed by XRD,XPS and SEM,TEM,EPR etc.The existence of oxygen vacancies were solidly proved.Oxygen vacancies promting the photoelectrocatalytic activity of WO₃ was characterized by LSV,Mott-Schottky plots,and EIS experiments.The results of photoelectrocatalytic CO₂ reduction in water exhibited that N/P doped WO₃ have high activity due to rich oxygen vacancies.Otherwise,their CB positions and external voltage significantly affect the selectivity of catalysts.The high CB level or high external voltages are favor to the acidic products like formic and acetic acid.On other hand,the low CB level and external voltages are prior to generation of alcohols.The effect of oxygen vacancies on catalytic activtity and oxygen vacancies on catalytic activtity and selectivity was proved by quenching reaction with H₂O₂ oxidation process.The ability of catalyst achieved to149μmol·g^-1·h^-1 for methanol,210μmol·g^-1·h^-1 for ethanol.We continue to explore different semiconductor-generated heterojunctions for photoelectrocatalytic reduction of CO₂.g-C₃N₄ is a semiconductor with wide band gap,and its high valence band position can be well correlated with the energy level of WO₃.Through in-situ annealed process,N-doping process.Finally,we deposited a layer of Pd nanocrystals on the surface of the catalyst by electrodeposition.Similarly,we proved the heterojunction by XPS,SEM,TEM and PL,and characterized them by LSV,Mott-Schottky and EIS experiments.The photoelectric performanc concluded that the formation of a heterojunction can effectively enhance the separation of photogenerated electrons and holes.The ability of catalyst achieved to 77.5μmol·g^-1·h^-1 for methanol,1250μmol·g^-1·h^-1 for formic acid,and 162.5μmol·g^-1·h^-1 for acettic acid.