Preparation Of Bi-H_x And Study On CO₂ Reduction By Mimicking Photosynthesis

Nowadays,fossil fuels are the main energy source in the world.With the rapid development of society and economy,human demand for energy continues to increase.Excessive consumption of fossil fuels will release a large amount of carbon dioxide,which will lead to the greenhouse effect.Mimicking photosynthesis for reducing carbon dioxide is an effective way to alleviate the greenhouse effect.In this thesis,we study Bi-Hx to mimick photosynthesis combined with thermal catalysis to achieve CO2 conversion,and to explore the performance and reaction mechanism of reducing carbon dioxide under the synergistic effect of heat and light.In addiction,through combining Bi-Hx with metal Sn,the activity of the catalyst in reducing carbon dioxide is further improved,and the catalyst is characterized by a series of tests.The main research contents and results of this paper are as follows:(1)Bi-Hx/NF mimicking photosynthesis for carbon dioxide reductionBi is loaded on nickel foam(NF)via electrodeposition approach(denoted as Bi/NF),then splitting of H2O into H atoms and O2 and storage of the H atoms in Bi nanoparticles to form Bi-Hx(denoted as Bi-Hx/NF)were simultaneously achieved.For comparison,Bi/NF catalyst is prepared at the same time.Cyclic voltammetry(CV)curves proves the hydrogen storage characteristics of Bi-Hx/NF.According to X-ray diffraction(XRD),scanning electron microscopy(SEM)and other related characterizations,it is found that the synthesized two catalysts are both hexagonal nanosheets with a size of 0.5-2 μm and light absorption reach～600 nm.The results show that hydrogen storage does not obviously change the crystal,morphology and light absorption of Bi,proving that Bi/NF is a good candidate for absorbing the visible light.The results of thermal photocatalytic reduction of CO2 over Bi-Hx/NF catalyst show that the rate of CO production reach 283.8 μmol·h-1·g-1 without sacrificial reagents and cocatalysts under 420 nm irradiation at 180℃,which is 9 times that on pristine Bi in H2 gas.After 81 h of cyclic reaction,Bi-Hx/NF can still maintain high activity and good stability.This excellent activity is attributed to the formed Bi-Hx intermediate that improves charge separation and reduces reaction barriers in CO2 reduction.Theoretical and experimental studies confirm that the mechanism of CO2 reduction reaction.First,H2O splitting into H atoms and O2 and storage of the H atoms in Bi nanoparticles(denoted as Bi-Hx)are simultaneously achieved by an electrochemical approach.Then,the obtained Bi-Hx is used as a reducing equivalent to reduce CO2 to CO by in situ generating H+/e-pairs under light irradiation.Meanwhile,Bi-Hx recovers to Bi for reversible storage and release H,completing the catalytic cycle.(2)Bi-HxSn/NF thermal photocatalytic carbon dioxide reductionThe Bi-HxSn/NF composite catalyst is prepared by electrodeposition.The TEM image shows that the morphology of the Bi-HxSn catalyst has a nanorod with a length of 1 μm.The results of CO2 reduction test show that when the deposition time of Sn is 7 min,the Bi-HxSn/NF catalyst has the highest catalytic activity.The CO production rate increases with temperature rising and reaches a maximum of 359.8 μmol·g-1·h-1at 180℃ under 420 nm light irradiation.Studies demonstrate that the excellent activity is attributed to the introduction of Sn,Sn can effectively improve the separation efficiency of photo-generated charges,accelerate electron transfer,and promote the thermal photocatalytic reduction of CO2 to CO.Based on the analysis of experimental results,the reaction mechanism is proposed.CO2 is adsorbed on the active sites of Bi-HxSn.Under light irradiation,photoinduced electron-hole pairs are generated on Bi-HxSn.Meanwhile,the holes and the active H stored on Bi generate H+,the electrons on Bi will be transferred to Sn.With the assistance of thermal energy,the more adsorbed CO2 molecules would be activated,which may prone to react with e-and H+to produce CO.