Core-shell catalysts activity enhancement

Abstract In this chapter, we review recent works of dealloyed Pt core-shell catalysts, which are synthesized by selective removal of transition metals from a transition-metal-rich Pt alloys (e.g., PtMs). The resulted dealloyed Pt catalysts represent very active materials for the oxygen reduction reaction (ORR) catalysis in terms of noble-metal-mass-normalized activity as well as their intrinsic area-specific activity. The mechanistic origin of the catalytic activity enhancement and the stability of dealloyed Pt catalysts are also discussed. 

Electrochemical leaching of Pt alloy catalysts has been deliberately applied to form core-shell materials with enhanced activities, either in half-cell tests [24, 25] or even... 

Bezerra et al extensively reviewed heat treatment and stability effects of various Pt/C, Pt-M/C, and C-supported Pt-free alloy catalysts, taking into account particle sizes and stiuctural parameters. Appropriate heat treatment of Pt/C catalysts improves ORR activity by stabilizing the carbon support against corrosion, which in turn increases the cathode life time. Depositing mixed-metal Pt monolayers on carbon-supported metal nanoparticles or Pt monolayers on noble/non-noble core-shell nanoparticles leads to enhanced electrode performance. RRDE experiments on the catalytic activity of Pt-M (M = Au, Pd, Rh, Ir, Re or Os) monolayers on carbon-supported Pd nanoparticles showed that an 80 20 PtiM ratio for the nuxed monolayers performs better than commonly nsed Pt/C catalysts. ... 

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. 

So far, we have presented several types of dealloyed Pt binary and ternary nanoparticle catalysts, which showed substantially enhanced ORR activities compared with pure Pt. The activity enhancement originated from a lattice-strain-controUed mechanism. However, it is still unclear why different transition metals resulted in different activities and stabilities and how particle structural characteristics such as size, shape, and composition would come into play. To understand these issues, it is important to achieve an atomic-scale understanding of the core-shell fine structures... 

Many commercial heterogeneous catalysts are not impregnated in a uniform fashion. For example, various precious-metal catalysts consist of an exterior active shell and an inert core in order to enhance the effectiveness factor. Several automobile-muffler catalysts have a carbon-monoxide-oxidation catalyst in one shell and an NOx-reducing catalyst in another shell. Our understanding of the reaction-diffusion interaction facilitated this rational design of the optimal profile of catalyst-activity distribution and shape. It would be of both practical importance and academic interest to develop a rational procedure for enhancing the performance of metallocenes by their nonuniform impregnation on the support. 

As explained above, it is possible to enhance the MOR activity of the Pt-Ru bimetallic catalysts with both core/shell and well-mixed microstructures by controlling their surface compositions around PtsoRuso. In addition to the enhancement of the MOR activity, durability is an important issue for practical catalysts. In this section, an importance of the well-mixed Pt-Ru bimetallic catalyst is demonstrated in order to improve the durability. 

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