Application Of Freezing Loading Method In The Preparation Of Single-atom Catalysts With High Photocatalytic Performance For Hydrogen Production

The rapid consumption of fossil fuels has caused serious environmental problems.It is urgent to develop and use clean and sustainable energy.Among the many alternatives to fossil fuels and their production methods,hydrogen produced from water splitting via the sunlight-driven hydrogen evolution reaction（HER）assisted by photocatalysts is regarded as an important way to generate clean and renewable energy.However,due to the wide size distribution and low atomic utilization rate,the traditional catalysts can not meet the requirements of industrial application.It is urgent to develop efficient and durable catalysts for HER.Single-atom catalysts（SACs）have attracted much attention due to their advantages of multiple active sites and high atomic utilization.SACs are monoatomic metal catalysts dispersed on specific supports with unique electronic structures that maximize the utilization of the active site-metal atoms.In addition,the active centers of SACs are unsaturated coordination,which enhances the adsorption capacity.All these factors guarantee their high catalytic activity in the field of HER.Nonetheless,there are still significant challenges in the preparation of SACs,including（i）the lack of simple synthesize methods;（ii）high preparation cost;（iii）the nucleation mechanism remains unclear.Therefore,it is of great significance to develop a simple and universal method to prepare SACs for increasing the rate of HER.In order to solve the above problems,this study creatively proposed a simple preparation method,namely freezing loading method,by freezing the precursor solution for photochemical reaction to prepare a variety of SACs for the photocatalytic hydrogen production.This method has the advantages of simple operation,clear synthesis path,low cost and environmental friendliness.In addition,the application of SACs prepared by freezing loading method in the field of HER is still in the initial stage,which has important research value and significance.Specific research contents are as follows:In the first part,a kind of Pt-C₃N₄photocatalyst with single atom and double atom co-existed was prepared by the method of freezing loading chloroplatinic acid（H₂Pt Cl₆）solution.The formation of single atom/diatom was preliminarily verified by XRD,XPS and HAADF-STEM.Compared with the normal photodeposition control group,the HER performance of the frozen experimental group was increased multiply,up to 7.29 mmol g^-1h^-1.The formation of atomic dispersion state in the optimal sample may be the reason for the improvement of HER activity.The freezing loading method proposed in this work provides a new way for the synthesis of monatomic/diatomic catalysts,and makes a contribution to the improvement of the photocatalytic hydrogen production.In order to further verify whether the freezing loading method is generally applicable to the preparation of SACs,the second part used different Pd precursor solutions and adopted the freezing loading method to prepare several groups of Pd-C₃N₄.The synthesis of single atom Pd cocatalyst on C₃N₄was confirmed by the results of HAADF-STEM and XAFS.The carrier separation efficiency,electron transfer rate,quantum efficiency and HER performance of Pd SACs prepared by freezing loading method were much higher than those obtained for the control groups prepared with equivalent liquid precursors,and the photocatalytic activity of the best sample reached8.736 mmol g^-1h^-1.Ab initio molecular dynamics（AIMD）calculations showed that the interatomic dispersion of Pd in cold solid ice increases compared with that in liquid water molecules.Gibbs free energy（ΔG_H）results showed that the synthesis of atomically dispersed Pd catalyst enhanced HER activity.This work not only extends the freezing loading method to the preparation of more SACs,but also introduces the calculations to explain the mechanism of nucleation and hydrogen production of catalyst.This work verifies the universality of freezing loading method in the field of preparation of SACs with high photocatalytic performance of hydrogen production from both experimental and theoretical perspectives.In conclusion,the proposed freezing loading method provides a promising way for simple synthesis of SACs,and further provides the possibility for regulating the solid nucleation and growth of chemical reactions.The method can be widely used to prepare SACs with excellent photocatalytic properties of hydrogen production.