Dumbbell-like Noble Metal-Fe₃O₄ Yolk@Shell Nanostructure And Its Performance As Nanocatalysis

Noble metal nanocatalysts have a wide range of applications in the fields of chemical synthesis,biocatalysis,energy and environment due to their universality in catalytic reactions,high catalytic activity,excellent selectivity and good stability.The construction of noble metal-Fe3O4 dumbbell-like nanoparticles via the introduction of magnetic nanoparticles(such as Fe3O4 nanoparticles)into noble metal nanocatalyst systems has emerging as an appealing area recently.On one hand,the introduction of Fe3O4 component endows the noble metal nanocatalysts with magnetic separation capability.On the other hand,the interface generated between the nobel metal and Fe3O4 in the dumbbell-like nanostructures facilitates the improvement of their catalytic activity.However,since the high surface energy and attractiveness within the magnetic components,the application of the noble metal-Fe3O4 dumbbell-like nanocatalysts were limited for their tendency of agglomeration,poor stability and short service life.Therefore,on the premise of ensuring the magnetic separation properties and high catalytic activity of the noble metal-Fe3O4 dumbbell-like nanocatalysts,solving the agglomeration and poor stability problems through rational structural design will further promote the high-performance applications of noble metal-Fe3O4 dumbbell-like nanocatalystsBased on the above consideration,in this thesis,noble metal-Fe3O4 dumbbell-like nanoparticles encapsulated in N-doped carbon hollow nanospheres(denoted as noble metal-Fe3O4@N-carbon)as novel yolk@shell nanostructures were developed.The introduction of mesoporous shells can effectively solve the problem of agglomeration of the noble metal-Fe3O4 dumbbell-like nanoparticles and greatly improve thei stability.Moreover,the yolk@shell nanostructures help to create a microenvironment for catalytic reactions which will lead to high efficient catalysis.Herein,Au-Fe3O4@N-Carbon,Pt-Fe3O4@N-Carbon and PtFe-Fe3O4@N-Carbon yolk@shell nanocatalysts were synthesized.Through transmission electron microscopy(TEM),high-resolution TEM(HRTEM),ultraviolet-visible spectroscopy(UV-Vis),nitrogen adsorption-desorption,X-ray diffraction(XRD),Raman spectroscopy(Raman),X-ray photoelectron spectroscopy(XPS),the structure and performance of the nanomaterials were well studied.Catalytic reactions,such as the liquid-phase 4-nitrophenol(4-NP)reduction,diphenylmethanol oxidation and?-ionone oxidation were selected to investigate the catalytic activity of the nanocatalysts(1)Hydrophilic Au-Fe3O4 dumbbell-like nanoparticles were employed as the core and then sequentially coated with silicon dioxide(SiO2)and polypodamine(PDA).After high-temperature carbonizing procedure and SiO2 template removal,the Au-Fe3O4@N-Carbon yolk@shell nanocatalyst was successfully synthesized.The thickness of the SiO2 shell can be modulated by varying the amount of tetraethyl orthosilicate(TEOS)silicon source used.Therefore the effective adjustment of the cavity size of Au-Fe3O4@N-Carbon yolk@shell nanostructures can be achieved.The morphology and structure were characterized by means of TEM,HRTEM,XRD,Raman,XPS,nitrogen adsorption-desorption,etc.The nitrogen adsorption-desorption results showed that the Au-Fe3O4@N-Carbon yolk@shell nanostructure has a high specific surface area(345 m2 g-1).4-NP liquid phase reduction reaction was taken as a model catalytic reaction.The kinetics of the catalytic reaction process was characterized by UV-Vis spectroscopy.The results showed that,compared with the catalytic activities of Au nanoparticles,dumbbell-like Au-Fe3O4 nanostructures and Au@N-carbon yolk@shell nanostructures,Au-Fe3O4@N-carbon yolk@shell nanocatalysts showed superior catalytic activity(k=0.119 min-1).In addition,Au-Fe3O4@N-carbon yolk@shell nanocatalysts exhibited good magnetic separation properties and high recyclability.The catalytic performance of the catalyst in catalytic oxidation of ?-ionone and diphenylmethanol oxidation was further investigated,Au-Fe3O4@N-carbon yolk@shell nanocatalysts showed the best catalytic activity among the three kinds of nanocatalysts under study(the yields were 53%and 100%,respectively).(2)In order to verify the universality of the fabrication of yolk@shell based dumbbell-like nanostructures,the hydrophilic Pt-Fe3O4 dumbbell-like nanoparticles were further selected as the core to synthesize Pt-Fe3O4@N-carbon yolk@shell nanocatalysts.The cavity size can be effectively adjusted by modulating the thickness of the SiO2 shell.The morphology and structure were characterized by TEM,HRTEM,XRD,Raman,XPS,nitrogen adsorption-desorption,etc.The results of nitrogen adsorption-desorption showed that Pt-Fe3O4@N-carbon yolk@shell nanostructures had a high surface area(370 m2 g-1).Compared with Pt nanoparticles,Pt-Fe3O4 dumbbell-like nanostructures and Pt@N-carbon yolk@shell nanocatalysts,Pt-Fe3O4@N-carbon yolk@shell nanocatalysts showed the best catalytic activity in the catalytic reactions such as 4-NP reduction(k=0.123 min-1),?-ionone selective oxidation(the yield was 57%),and diphenylmethanol oxidation(the yield was 100%).In addition,Pt-Fe3O4@N-carbon yolk@shell nanocatalysts also showed good magnetic separation properties and recycling performance.(3)In order to further improve catalytic performance and reduce the load of noble metal,hydrophilic PtFe-Fe3O4 doumbble-like particles were used as the core,and PtFe-Fe3O4@N-carbon yolk@shell nanocatalysts were successfully synthesized.The morphology and structure were characterized by TEM,HRTEM,XRD,Raman,XPS,nitrogen adsorption-desorption,etc.The nitrogen adsorption-desorption results showed that PtFe-Fe3O4@N-carbon yolk@shell nanocatalysts had a high surface area(387 m2 g-1).Compared with PtFe nanoparticles,PtFe-Fe3O4 dumbbell-like nanostructures and PtFe@N-carbon yolk@shell nanocatalysts,PtFe-Fe3O4@N-carbon yolk@shell nanocatalysts showed the best catalytic activity in the catalytic reactions such as 4-NP reduction(k=0.320 min-1),?-ionone selective oxidation(the yield was 64%),and diphenylmethanol oxidation(the yield was 100%).What is worth to mention,PtFe-Fe3O4@N-carbon yolk@shell nanocatalysts exhibited better catalytic activity than Au-Fe3O4@N-carbon yolk@shell and Pt-Fe3O4@N-carbon yolk@shell nanocatalysts.