Photocatalytic CO₂ Reduction By Electrostatic Assembly Of CdS Nanocrystals With A Dinuclear Cobalt Catalyst

With the development of global industry and the progress of society,environmental problems,including energy shortages and greenhouse effect,are becoming serious.By simulating the natural photosynthetic system,we are committed to building an efficient photocatalytic system which can convert CO2 into valuable chemicals.Such strategy can alleviate the above issues.At present,some bottlenecks still remain in the development of photocatalytic systems for the reduction of CO2.First,many types of products,including CO,CH4,CH3OH and HCOOH,can be generated from CO2 reduction.Thus,the achievement of high selectivity of CO2 reduction is one of the challenges.Second,the rate of CO2 reduction is relatively low,which demands the design of high-efficiency catalysts and photosensitizers.Third,the stability of photosensitizers and catalysts in the catalytic systems still remain problematic.Fourth,in most cases,electrons are transferred from photosensitizers to catalysts by bimolecular collision,which results in low electron transfer efficiency in the photocatalytic system.Fifrth,the reported photosensitizers or catalysts are mostly noble metal complexes and the reaction media are mostly organic solvents,inconsistent with the principle of economic applicability.In this research work,negatively charged CdS nanocrystals?CdS-MPA?were used to assemble with positively charged dinuclear cobalt complexes?Co2L?through electrostatic interactions.This assembly can efficiently catalyze the CO2-to-CO conversion under visible light??>420 nm?in aqueous medium.The main research contents are as follows:1.For comparison,uncharged CdS NCs capping with neutral oleic acid ligands?CdS-OA?,and positively charged,surface-ligand-free NCs?CdS-BF4?,are used in parallel investigations on the influence of different surface charges on photocatalysis.The results show that,in terms of photocatalytic reduction of CO2,CdS-MPA achieves a high selectivity of 95%,a high yield of 34.51?mol CO and a large turn-over number?TON?of 1380 based on the cobalt catalyst.This catalytic performance is superior to those of CdS-OA and CdS-BF4.2.Dynamic light scattering measurements were employed to track the changes of the assembly size during the photocatalytic process.When the reaction proceeded for 10 h,the detected diameter of the photocatalytic system reached 20.6 nm,suggesting the formation of the assembly.The rate of CO production was increased simutaneously.As the reaction time increased,the size of the assembly reached up to 278.0 nm,while the rate of CO production slowed down and eventually ceased after 120 h.3.A series of experiments?LC-MS,XPS,ICP-MS,etc.?were deliberately carried out on the solid isolated from the centrifugation of the reaction mixture to further verify the assembly between CdS-MPA and co-catalyst Co₂L.4.Photoluminiscence measurements confirmed that the assembly between CdS-MPA and Co₂L can facilitate the electron transfer between CdS-MPA and co-catalyst Co₂L.