<i>In Situ</i> Study of Atomic Structure Transformations of Pt–Ni Nanoparticle Catalysts during Electrochemical Potential Cycling
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Abstract
When exposed to corrosive anodic electrochemical environments, Pt alloy nanoparticles (NPs) undergo selective dissolution of the less noble component, resulting in catalytically active bimetallic Pt-rich core–shell structures. Structural evolution of PtNi<sub>6</sub> and PtNi<sub>3</sub> NP catalysts during their electrochemical activation and catalysis was studied by <i>in situ</i> anomalous small-angle X-ray scattering to obtain insight in element-specific particle size evolution and time-resolved insight in the intraparticle structure evolution. <i>Ex situ</i> high-energy X-ray diffraction coupled with pair distribution function analysis was employed to obtain detailed information on the atomic-scale ordering, particle phases, structural coherence lengths, and particle segregation. Our studies reveal a spontaneous electrochemically induced formation of PtNi particles of ordered Au<sub>3</sub>Cu-type alloy structures from disordered alloy phases (solid solutions) concomitant with surface Ni dissolution, which is coupled to spontaneous residual Ni metal segregation during the activation of PtNi<sub>6</sub>. Pt-enriched core–shell structures were not formed using the studied Ni-rich nanoparticle precursors. In contrast, disordered PtNi<sub>3</sub> alloy nanoparticles lose Ni more rapidly, forming Pt-enriched core–shell structures with superior catalytic activity. Our X-ray scattering results are confirmed by STEM/EELS results on similar nanoparticles