Preparation Of Atomically Precise Ni₆ Clusters And Study On Their Catalytic Properties

The preparation and application of metal clusters with precise atomic structures have been extensively studied.In some catalytic reactions,the precise spatial structure of the metal clusters make the catalytic active sites easier to identify relative to metal nanoparticles with various sizes and structures,and it is easier to correlate the structure of catalyst with the catalytic properties.However,active sites of the metal clusters are occupied by external ligand molecules,thereby reducing catalytic activities of the metal clusters.Therefore,reducing the steric effect of ligands,maintaining the precise atomic structure of metal clusters while improving their catalytic ability are the main research purpose of this thesis.In this thesis,NiCl2·6H2O was used as the precursor.Ni6(SC2H4Ph)12 clusters with six nickel atoms protected by twelve sulfhydryl ligands were synthesized by size etching,and their performances in catalytic reactions were systematically investigated.The research content mainly includes the following parts:1.Atomically precise Ni6(SC2H4Ph)12 clusters were prepared,and their crystal structure were analyzed.The atomic spatial arrangement,electronic structure and optical performance of Ni6(SC2H4Ph)12 clusters were analyzed by ultraviolet spectroscopy,mass spectrometry,thermogravimetric analysis and other characterization methods.The results show that Ni6(SC2H4Ph),2 clusters have unique double crown structure and thermal stability characteristics compared with the corresponding traditional nickel-based catalysts;2.Ni6(SC2H4Ph)12 clusters were used as catalyst to catalyze the hydrogenation of nitrile.The effects of NH3 on the activity of Ni6 clusters were explored by FT-IR,UV-vis and the DFT calculation method.It has been proved that the presence of NH3 can effectively inhibit steric effect of attached ligands,and NH3 molecules can compete with the sulfhydryl molecules for adsorption to Ni sites,thereby exposing more active sites of Ni and increasing activities of Ni6(SC2H4Ph)12 cdusters in this reaction.However,other bases do not give similar results in place of NH3 molecules.In addition,the adsorption of NH3 is also beneficial to stabilize the precise structure of Ni6 clusters;In this reaction,NH3 molecules play an important and irreplaceable role in improving the catalytic activity of Ni6(SC2H4Ph)12;3.Using pretreated supported Ni6(SC2H4Ph)12 clusters to catalyze methane oxidation with H2O2 or O2 as oxidant.The effects of supports,reaction temperature,reaction pressure and reaction time on the catalytic performances were studied.Under lower temperature conditions,Ni6(SC2H4Ph)12 clusters can catalyze oxidation of methane to methanol with high selectivity.In this thesis,the mechanism of the reaction was also investigated by EPR,ATR-IR and other characterization methods.It is believed that the pretreated Ni6(SC2H4Ph)12 clusters can weaken the steric effect of the ligands in the clusters,thus exposing more active sites while ensuring the complete structure of Nib clusters.Further,its catalytic activity is enhanced,methoxy group is the main intermediate of the reaction process.