Microwave Synthesis, Characterization And Oxygen Reduction Electrocatalytic Performance Of Platinum Alloy Nano-octahedrons

Due to the high catalytic activity for oxygen reduction reactions,the PtNi alloy octahedron catalysts with surfaces all composed of(111)facets have been extensively studied.However,the octahedral catalytic activity of PtNi alloy is not only affected by its special geometry,but also the surface component,an important factor determining its activity.For example,the typical component segregation structure,Pt enriched in edges and corners and Ni enriched in(111)face,in the PtNi alloy nano-octahedron will cause the Ni-rich(111)surface to dissolve during the ORR catalysis process,resulting in the decrease of its catalytic activity.Therefore,controlling the surface composition of the Pt alloy nano-octahedron is crucial for improving its catalytic activity.At present,the postannealing method is commonly used internationally.In fact,the anisotropic growth mechanism of different elements in the Pt alloy nano-octahedron is the real reason of the segregation of its components.Therefore,turning the surface compositional segregation structure of PtNi alloy nano-octahedron can also be obtained by adjusting its growth mechanism.Here,adjusting the growth mechanism of Pt and Ni by various means,we attempt to evade this typical segregation structure of the PtNi alloy during the growth stage.First,we use microwaves to enhance the diffusion ability of Pt and Ni atoms in the PtNi alloy nano-octahedron to modulate the surface elemental segregation of it.Using high-resolution STEM and EELS spectrum imaging,we studied the effects of reaction temperature on the structure,surface composition and ORR catalytic activity of Pt-Ni nano-octahedrons and investigated the effect of microwave on promoting the surface diffusion process.Compared with the traditional solvothermal synthesis,the microwave significantly increases the bulk diffusion rate of nanoparticles during the synthesis and promotes the degree of alloying.And,with the increase of the reaction temperature,the microwave method can significantly promote the diffusion of surface atoms,enabling the direct synthesis of Pt-Ni octahedrons with Pt-rich {111} surface and Ni-rich subsurface and as a result high ORR activity.Second,by ternary doping of Mo,we obtained MoPtNi nano-octahedron catalysts with better catalytic activity.We synthesized the MoPtNi nanocatalysts containing different Mo doping amounts by a microwave synthesizer,and studied the effect of Mo doping on the morphology,composition and catalytic activity of the PtNi nanocatalyst.We found that the doping element Mo enhances the alloying degree of the PtNi catalyst.And,when doped with less Mo,the catalytic activity is significantly improved by changing the oxygen adsorption capacity of the PtNi octahedral near surface,with the MoPtNi nanocatalyst maintaining a regular octahedral morphology and the Pt,Ni atomic ratio without any change;while doped with more Mo,Mo reduces catalytic activity due to adsorption on the PtNi catalyst surface,as the morphology of MoPtNi nano-catalyst impaired and the atomic ratio of Pt to Ni obviously turned.Therefore,when the ternary doping method is used to improve the catalytic performance of PtNi nano-octahedron,the doping content of the doping element should be controlled at a lower level.These works have greatly enriched the way of regulating the surface composition of Pt alloy nano-octahedron,and have a profound effect on obtaining high activity and high stability of Pt alloy nano-catalyst.