Core-shell catalysts dissolution

Chapters 18-21 discuss core-shell and advanced Pt alloy catalysts (which also can be considered to have a core-shell structure). Chapter 18 studies the fundamentals of Pt core-shell catalysts synthesized by selective removal of transition metals from transition metal-rich Pt alloys. Chapter 19 outlines the advances of core-shell catalysts synthesized by both electrochemical and chemical methods. The performance, durability, and challenges of core-shell catalyst in fuel cell applications are also discussed. Chapter 20 reviews the recent analyses of the various aspects intrinsic to the core-shell structure including surface segregation, metal dissolution, and catalytic activity, using DFT, molecular dynamics, and kinetic Monte Carlo. Chapter 21 presents the recent understanding of activity dependences on specific sites and local strains in the surface of bulk and core-shell nanoparticle based on DFT calculation results. 

Next, we turn our attention to core-shell stmctures, assuming they would be formed either to minimize the amount of Pt present in the catalyst, or be naturally formed either because of segregation effects or because of metal dissolution in acid medium. ... 

In this chapter, we review dealloyed core-shell nanoparticle catalysts, which were synthesized by selective dissolution of transition metals from the surface of a transition-metal-rich Pt alloys (e.g., PtMs). Figure 18.1 shows an illustration of the... 

The enhanced ORR activities of the dealloyed Pt binary catalysts could be related to a similar lattice-strain effect as revealed in the dealloyed PtCus catalysts. However, it is still unclear what the origin of the different activities of different dealloyed PtM3 catalysts is. Regarding their comparable atomic radius, the alloy elements Co, Ni, and Cu are assumed to induce a similar extent of lattice strain. Nevertheless, due to different redox chemistry of the transition metals, different extent of metal dissolution may exist [47], which might result in different core-shell fine structures and hence different activities. 

We have reviewed the family of dealloyed Pt-based nanoparticle electrocatalysts for the electroreduction of oxygen at PEMFC cathodes, which were synthesized by selective dissolution of less-noble atoms from Pt alloy nanoparticle precursors. The dealloyed PtCua catalyst showed a promising improvement factor of 4-6 times on the Pt-mass ORR activity compared to a state-of-the-art Pt catalyst. The highly active dealloyed Pt catalysts can be implemented inside a realistic MEA of PEMFCs, where an in situ voltammetric dealloying procedure was used to constructed catalytically active nanoparticles. The core-shell structural character of the dealloyed nanoparticles was cmifirmed by advanced STEM and elemental line profile analysis. The lattice-contracted transition-metal-rich core resulted in a compressive lattice strain in the Pt-rich shell, which, in turn, favorably modified the chemisorption energies and resulted in improved ORR kinetics. 

Another promising way to improve the activity and durability of Pd-based nanocatalysts is to deposit a Pt layer on them. Recently, Pd/C and PdM/C catalysts modified by a Pt monolayer were found to possess higher activity than that of Pt/C due to the strain and electronic effects from the Pd-based cores, and the durability of the catalysts is improved significantly and comparable to Pt/C [70, 93-95]. The Pd-based core materials are expected to be partially dissolved xmder the fuel cell operation conditions due to some defects in the Pt monolayer. In the meantime, the diffusion of Pt atoms on the surface results in a more compact shell. Thus, further dissolution of Pd-based core is greatly reduced. 

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