| With the rapid development of nanomaterials science,the understanding of catalyst action mechanism gradually goes to the atomic level,and a variety of atomically dispersed metal catalysts including nanocluster catalysts and single-atom catalysts have been developed.Due to the quantum confined effects and polymetallic synergies,single-atom catalysts and nanocluster catalysts show unique advantages in different types of catalytic reactions,and the connections between the two catalysts have attracted more attention.The controllable transformation of clusters and single atoms in similar material systems is a feasible way to solve this problem.However,the study of cluster-monatom transformation is not deep enough at present.Firstly,various types of cluster-monatom transformation require harsh reaction conditions,and the transformation process is not safe and efficient.Secondly,it is difficult to characterize the local structural changes of metal species in the transformation process due to the complexity of the reaction environment.Finally,due to the lack of characterization methods and the imcomplete development of theoretical calculation,the exact structure-activity relationship of nanocluster catalysts and singleatom catalysts at atomic scale is not clear enough.Synchrotron radiation spectroscopy,such as X-ray absorption fine structure spectroscopy(XAFS),can accurately characterize the neighboring coordination of active metal centers in these systems,and has a broad application prospect in the study of cluster-monatom transformation.In order to solve the above vital scientific problems,we developed a series of safe and efficient cluster-monatom transformation strategies.On the basis of domestic facilities(synchrotron radiation light sources in Beijing,Shanghai and Hefei),we conducted various kinds of advanced characterization including X-ray absorption fine structure spectroscopy(XAFS),and demonstrated the structural changes in the transformation procedures in detail.Besides,with the help of EXAFS fitting and DFT calculations,the structure-activity relationship in the actual catalytic process for the involved catalysts were studied in depth.This paper includes the following main research contents:1.Ligand-assisted low temperature transformation from Pd nanoclusters to single atoms were studied through synchrotron radiation techniques.The study of the interconversion process between Pd nanocluster catalysts and Pd single atom catalysts is of great significance for the rational design of catalysts for organic hydrogenation.In Chapter 3 of this paper,we developed a ligand-assisted synthesis strategy to achieve continuous conversion from Pd nanocluster catalysts to Pd single-atom catalysts at low activation temperatures(400℃).Through the detailed analysis towards the X-ray absorption fine structure spectroscopy and the combination usage of various kinds of characterization methods,we comprehensively described the structural changes of Pd species during the entire transformation process.In addition,based on first-principles calculations,by the means of transition state(TS)deconstruction and molecular dynamics(MD)simulation,we clarified the promoting effect of Cl ligand on the atomization of Pd clusters.This work reveals the microscopic mechanism of the ligand-assisted transformation from Pd nanoclusters to single atoms at low temperature,and provides a new idea for rational design and controllable construction of atomically dispersed Pd nanocatalysts.2.Insight into the structure-activity relationship for continuously transformed Pd nanocluster/single-atom catalysts in phenylacetylene semihydrogenation by synchrotron radiation techniques.It is of great practical significance for the development of polymer industry to study the difference of catalytic mechanism between Pd nanocluster catalysts and Pd single atom catalysts in the semi-hydrogenation of phenylene.In Chapter 4 of this paper,we slightly improved the transformation strategy of Pd nanoclusters to Pd monatomic catalysts described in Chapter 3,and synthesized comparable Pd nanocluster and Pd single-atom catalysts in a similar material system(labeled as Pd-NC@NC and PdSA@NC).By the application of various kinds of synchrotron radiation spectroscopy techniques such as X-ray absorption fine structure spectroscopy(XAFS),we characterized their electronic and atomic structures systematically.Afterwards,the author investigated the structure-activity relationship between these two catalysts in the phenylacetylene semi-hydrogenation,finding out that though both catalysts showed good styrene selectivity in this reaction,the activity of Pd-NC@NC was much higher than that of Pd-SA@NC.By means of density functional theory(DFT)calculation,we analyzed the mechanism of the two catalysts in the reaction,clarifing that the substrate adsorption barrier caused by steric effect is the main reason for the lower apparent catalytic activity of Pd-SA@NC.This work opens up a new way for the mechanism study and rational design of atomically dispersed Pd-based catalysts.3.The synchrotron radiation technique was used to study the conversion from single atoms to nanoclusters in Cu-based catalysts and their structure-activity relationship in electrochemical nitrate reduction reactions.Ammonia synthesis by electrochemical nitrate reduction is a promising synthesis strategy.The researched in the structure-activity relationship of Cu-based atomically dispersed metal catalysts in this reaction is very important for the development of high performance and low cost electrocatalysts.In Chapter 5 of this paper,we synthesized Cu single-atom catalyst by traditional impregnation method,and realized the conversion from Cu single-atom catalyst to Cu nanocluster catalyst by changing the calcination temperature,We used synchrotron radiation spectroscopy to characterize the structures of the two catalysts,and elucidated the changes on existence forms and neighboring coordination structures for Cu species during the conversion process.Subsequently,we studied the structure-activity relationship between the two catalysts in electrochemical nitrate reduction reactions,and found that Cu nanocluster catalysts showed the highest Faraday efficiency at a relatively lower reduction potential(the highest Faraday efficiency was 82.4%,the corresponding reduction potential was-0.4 V),and the overall activity of the Cu nanocluster catalyst was significantly better than that of Cu single-atom catalyst.With the help of density functional theory(DFT)calculation,we revealed that the stronger adsorption capacity of Cu nanoclusters on the substrate is the main reason for their better electrochemical activity.This work provides a new idea for rational design of electrochemical nitrate reduction catalyst for synthetic ammonia. |
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