Theoretical Study On The Mechanism Of Formic Acid Decomposition To Hydrogen Using Pd@UiO-66-X

As a new energy carrier,H2 has many advantages including high heat values and clean combustion production,but it is difficult to store and transport.The formic acid is energy-intensive,high stability at room temperature,and thus could be an energy carrier.Therefore,it is important to design new materials to decomposite formic acid into hydrogen with high efficiency.The byproducts such as CO may poison the catalysts,and thus it is important to achieve high reaction selectivity for the catalysts.It was reported that the loading of noble metal nanoparticles into the different temples may enhance the reaction activity for the formic acid decomposition,but the selectivity could be lower.By changing the functional groups it is possible to adjust the selectivity,however,the reaction mechanism remain unclear.In this thesis,density functional theory?DFT?based method were employed to investigate the stable structure of Pd metal nanocluster encapsulated UiO-66-X,X=-NO₂,-Cl,-H,-OH,and–NH₂,and disclose the structure-property relationship.The NEB method was employed to find the transition states and compute the energy barriers.By comparing the reaction pathways for the materials with different functional groups,the reaction mechanism was discussed.The main content of this thesis includes:1.The DFT methods were used to optimize the Pd clusters encapsulated UiO-66-X materials,and the thermodynamically stable structures with different sized of Pdn clusters were obtained.The calculations indicate that the structures of Pdn@UiO-66-X with the same metal atoms for various functional groups are similar.The framework has two different pores,i.e.,tetrahedral cage and octahedral cage.It was found that the Pdn culstere were stabilized in the tetrahedral cage,and the Pd28 was found to have highest stable thermodynamically.Bader charge analysis was used to analyze the charge population for the systems and it was found that the charges change as the functional groups vary.2.Based on the ab initio molecular dynamics and DFT calculations,the stable Pd₂₈@UiO-66-X structure were selected as the catalyst models,and the NEB method was employed to compute the decomposition of formic acid.The results suggest that the by changing the functional groups,the reaction barriers of the different reaction pathways were greatly changed.The–NO₂ functional group in Pd₂₈@UiO-66-NO₂ increases the charges of Pd cluster,and thus the reaction energy barriers of the byproduct reaction pathways were increased,leading to the high selectivity to the H2 production.In addition,the–NO₂ functional group reacts with the carboxy group of formic acid forming–NO₂H and CO₂,which greatly decrease the energy barrier from 0.62 to 0.15 eV.Thus,the reaction selectivity is greatly increased by using Pd₂₈@UiO-66-NO₂ as the catalyst.3.In addition,the hydrogenation of the–NO₂ functional group was also studied to confirm its stability.The NEB method was carried out to compute the hydrogenation of–NO₂ group to–NO₂H and–NO.The results suggest that the further hydrogenation of–NO2 group has a relatively higher energy barrier of 0.40eV,larger than the energy barrier of 0.22 eV from–NO₂H to–NO₂ group, indicating the functional groups in the catalyst is stable at mild condition.4.The decomposed H protons from formic acid move“freely”on the surface of Pd cluster,and the H₂ molecule was formed with different energy barriers at different H coverages.With 2 H protons model,the energy barrier was calculated to be 0.55eV.When more protons were generated on the surface,the energy barrier greatly reduce to 0.22 eV,indicating the formation of H₂ molecule is much easier at high H coverage.