Theoretical Investigations On The Composite Of Au/Pd Cluster Encapsulated ZIFs

Zeolitic imidazolate framework(ZIF),a new type of metal organic frameworks,has now been widely investigated in the past decade,due to its high surface area,intrinsic porous characteristics and high thermal and chemical stability.Currently,computational simulation has been widely used to study the application of metal-organic frameworks in the field of gas adsorption and separation.However,the theoretical investigations on the metal cluster encapsulated MOFs are still sparse,especially on the microscopic understanding on the interaction between the metal and MOFs,structure stability,and the role composites play in catalytic reactions.In this thesis,a combination of ab initio molecular dynamics(AIMD)simulations and density functional theory(DFT)calculations were employed to investigate the thermodynamically stable structures of Aun and Pdn(n=2,4,8)clusters encapsulated ZIF-8 and ZIF-90 composites.The calculations showed that the Aun(n=2,4,8,12,16,20)clusters interact with the ZIF-8 framework by forming C-Au-C bonding,while Aun clusters prefer to interact with the aldehyde group in ZIF-90 with Au-O bonding.In ZIF-8,the Aun clusters exhibit planar two-dimensional(2D)structures with n up to 12,while Au16 cluster shows a cage structure and Au20 possesses a tetrahedral shape.For ZIF-90,the transition from 2-dimensional to 3-dimensional structure is very similar to those in ZIF-8.Computational results showed that the Au cluster interact stronger to the ZIF-8 framework than ZIF-90,in accordance with the larger value of charge transfer between Au cluster and ZIF-8 than ZIF-90.Pd clusters(n>2)exhibit non-planar structures in ZIF-8 and ZIF-90.The results showed that Pd clusters interact with the ZIF-8 framework by forming C-Pd-C bonding,while in ZIF-90 the Pdn clusters interact with both the imidazole ring and aldehyde groups.Our calculations suggest that the Pd cluster interact stronger to the ZIF-8 framework than ZIF-90.Next,we investigated the adsorption sites and the adsorption energies of CO and O2 on Aun cluster encapsulated ZIF-8 and ZIF-90 composites,which were also compared to the isolated Aun clusters.Our calculations showed that the low-co-ordination apex Au sites in Aun@ZIF-8 and Aun@ZIF-90(n<12)are favorable sites for CO adsorption,which are similar to the isolated Au clusters.Binding energy calculations suggest that the CO adsorption on the isolated Aun clusters is stronger than that on both Aun@ZIF-8 and Aun@ZIF-90;moreover,the CO molecule interacts stronger to Aun@ZIF-8 than Aun@ZIF-90.In contrast,the CO adsorption is stonger on the Aun@ZIF-90 than Aun@ZIF-8 and isolated Au cluster,when the number of gold atoms is 16 and 20.The adsorption of O2 was also studied in these systems.Our study showed that the apex Au sites in all cases are energetically favorable for O2 adsorption when n<8,and the O2 adsorption is stronger on the isolated Au2 clusters than Aun@ZIF-8 and Aun@ZIF-90,however,the adsorption of O2 on Aun@ZIF-8 and Aun@ZIF-90 become stronger than the isolated Aun cluster with the size n=12,while in Au12@ZIF-8 the bridge Au site is the most favorable adsorption sites.When the number of atoms in Au cluster is beyond 12,the Aun@ZIF-90 has the largest adsorption affinity for O2,then followed by the isolated Au cluster and Aun@ZIF-8,and the bridge sites in favorable sites in both Aun@ZIF-8 and Aun@ZIF-90.Finally,Au8@ZIFs and Au16@ZIFs were selected as the models to understand the reaction mechanism for the CO oxidation on Aun@ZIFs.The calculations show that the barriers involved during the CO oxidation on Au8@ZIF-8 are much lower than Au8@ZIF-90 and isolated Au8 cluster.It's worth to mention that the Au16@ZIF-90 and Au16@ZIF-8 show enhanced catalytic activity for the CO oxidation reaction with a very low activation barrier as compared to the pristine Au16 cluster.