Phase-controlled Synthesis And Catalytic Performance Of Ni-based Nanomaterials

Metal nanomaterials with unique physical and chemical properties have attracted great research interest and been widely used in catalysis,energy and environmental protection.The crystal phases of metal nanomaterial have a significant effect on their physicochemical properties,especially the catalytic performance due to the different spatial distributions and electronic structures.To figure out the relationship between crystalline structure and catalytic performance of Ni-based catalysts,a group of Ni nanomaterials with well-defined crystalline phases were synthesized for the investigation of structure-activity relationship.The specific findings include:(1)By simply tuning the initial concentration of Ni2+,two typical Ni nanocrystals with well-defined phases are synthesized using solvothermal method,including a hexagonal prism with a hexagonal close-packed(hcp)phase and an icosahedron with a face-centered cubic(fcc)phase.It is found that hcp Ni NCs are formed via an unusual phase transformation process from the thermodynamically stable fcc phase to metastable hcp phase.Both the experimental study and density functional theory(DFT)calculations reveal that the initial reduction rate of Ni2+ could control the internal structure of Ni at the nucleation stage,while the preferential adsorption of polyethylenimine(PEI)on hcp Ni facets plays a key role in the phase transformation and facet evolution of Ni NCs at the growth stage.Compared with fcc Ni nanocrystals,hcp Ni exhibits superior catalytic activity in the reduction of 4-nitrophenol.(2)Supported hcp Ni catalyst is synthesized by mechanical mixing hcp Ni nanocrystals with ?-Al2O3 nanosheets in aqueous solution.The phase transformation from hcp to fcc phase and unique redispersion phenomenon are observed during high-temperature annealing treatment under hydrogen atmosphere.The experimental study reveal that higher temperature and long-time treatment are beneficial for the dispersion of Ni nanocrystals,and thus enhance their catalytic activity for CO methanation.The effects of phases of Ni and ?-Al2O3,and annealing atmosphere are thoroughly investigated.It is found that the "melting and solidification" of Ni nanocrystals during the phase transformation and appropriate metal-support interaction between Ni and ?-Al2O3 nanosheets play vital roles in the redispersion of Ni.(3)Well-defined heterophase Ni nanocrystals with varying proportions of stable fcc phase and metastable hcp phase are synthesized by a one-pot solvothermal method with N,N-dimethylfomamide(DMF)as both the solvent and the reduction agent,PEI as the capping agent,and graphene as the support.The heterostructure of Ni nanocrystals is generated via the epitaxial growth of hcp Ni from fcc Ni(111)facets.The PEI is demonstrated to control the formation of the metastable hcp phase,while graphene plays a vital role in reducing the particle size and stabilizing the heterophase structure.Compared with fcc Ni nanocrystals,the heterophase(fcc/hcp)Ni nanocrystals grown on graphene show a greatly improved catalytic activity toward 4-nitrophenol(4-NP)reduction.The enhanced catalytic activity could be attributed to the stronger interaction between 4-NP and fcc/hcp interfaces.