Study On Photocatalytic CO₂ Reduction Activity And Selectivity Regulation Mechanism

Semiconductor-based photocatalytic CO₂ reduction technology is one of the potential strategies to alleviate the greenhouse effect caused by excessive CO₂ emission and achieve“carbon-free emissions”.However,the large-scale application of photocatalytic CO₂ reduction technology has not been achieved yet.The main reasons are chiefly as follows:（1）The higher reaction barrier(Δf G_o=-396 kJ mol^-1)results from its chemical stability of the CO₂,leading to the low efficiency of photocatalytic reduction.（2）Limited optical capture capability and the low spectral utilization are caused by the wide bandgap of most semiconductors.And photogenerated electrons easily recombine with holes,thus resulting in low efficiency of photo-energy conversion.（3）The photocatalyst surface has a strong adsorption of CO₂ reduction products and is difficult to regenerate the reactive sites,resulting in a decrease in photocatalytic efficiency.（4）Photocatalytic CO₂ reduction involves multi-electron processes（diverse reaction pathways and complex product types）,which lead to the inefficient control of product selectivity.This study develops the novel photocatalytic system based on the major challenges above,including high reaction barrier,low conversion efficiency of solar energy,difficult desorption of product and poor product selectivity in the photocatalytic CO₂ reduction process:（1）The surface of g-C₃N₄ is modified by nonpolar carbon quantum dots composed of sp²hybridized carbon,which enhanced the adsorption capability of CO₂ and photoinduced H₂ on the photocatalyst surface.The further reaction of CO₂ and H₂ was induced then,and the reaction kinetics and path of the CO₂ photoreduction process were changed,thereby improving the CO₂conversion efficiency.In addition,the narrow band gap and efficient electron transport performance of carbon quantum dots can significantly improve the spectral utilization range of composite materials and the separation efficiency of photogenerated carriers,thereby enhancing the photocatalytic reduction performance of CO₂.It showed that the optimal performance was2 wt%carbon quantum dots doped.Under this condition,the reduction activity was nearly 6times higher than unmodified sample,and the rate was increased to 60μmol g^-1 h^-1.（2）Utilizing the local photothermal effect of carbon microspheres,a new super-fast laser thermal synthesis strategy was designed.This strategy could achieve rapid heating and cross the melting stage of traditional thermal polymerization quickly.Therefore,this synthesis method could effectively solve the phenomenon of easy agglomeration during traditional thermal polymerization.On the one hand,results indicated that the incorporation of non-polar carbon microspheres enhanced the non-polar effect on the surface,which significantly improved the adsorption and activation of CO₂ and the separation efficiency of photogenerated electron hole pairs.On the other hand,full-spectrum irradiation with simulated sunlight intensity induced the local photothermal effect.And the surface temperature was controlled at about 89℃to adjust selective desorption of different products,then the selectivity of product and long-term high stable activity were strengthened.Finally,compared with the unmodified sample,the activity of CMS/C₃N₄-PTP was increased by 3 times,achieved 24-hour of long-term high activity and the selectivity of CO was enhanced to 94%simultaneously.（3）Intrinsic defects induced by hydrogenation was introduced into the intermediate energy level to design a photocatalyst with full spectral response.Meanwhile,the photocatalytic CO₂reduction performance and mechanism of the photocatalysts before and after hydrogenation were investigated.It can be found that the spectrum of WO_3-x HMSs was extended to the infrared region,which could induce the local photo-thermal effect.During the process of CO₂photoreduction,WO_3-x HMSs induced the local surface temperature soaring to 170℃with simulated solar irradiation.The intense photothermal effect activated intermediate reaction of H₂ and CO,altering the CO₂ reduction reaction kinetics and path,eventually achieving product selectivity regulation.It showed the rate was increased to 190μmol g^-1 h^-11 and CH₄ selectivity rised to 92%,which are far superior to the WO₃ HMSs.