Pt skin catalyst

Figure 6.21. Schematics of currently pursued Pt-based electrocatalyst concepts for the ORR. (A) Pt bulk alloys (B) Pt alloy monolayer catalyst concepts (C) Pt skin catalyst concept (D) De-alloyed Pt core-shell catalyst concept.

Catalyst Structure C in Figure 6.21 is commonly referred to as the Pt-skin catalysts [87,95,107,124,125]. The term Pt skin catalysts will here be used to refer to a monolayer of pure Pt sitting on a Pt-depleted Pt alloy core in contrast, a Pt monolayer catalyst was referred above to as a monolayer of pure Pt on top of... 

Pt skin catalysts are prepared by high-temperature annealing and are therefore expected to be thermodynamically stable structures Thermochemical studies of the metal segregation energies of various metal alloys [114] suggest that Pt-rich alloys prefer to segregate Pt atoms to the surface and form Pt skins. 

Xu et al. [57] performed self-consistent periodic DFT calculations (GGA-PW91) to smdy the adsorption of O and O2 and the dissociation of O2 on the (111) facets of ordered PtsCo and Pt3Fe alloys and on monolayer Pt skins covering these two alloys. They also investigated explicitly the strain effect by a 2 % compression of Pt(l 11). They discovered that the Co atoms on the PtsCo) 11) surface allow O2 to dissociate more easily than on Pt(lll) [the lowest activation energy on Pt3Co(l 11) is 0.24 eV/02, compared to 0.77 eV/02 on Pt(l 11)] and also bind O and O2 more strongly (—4.29 eV/O vs. —3.88 eV/O, —0.92 eV/02 vs. —0.62 eV/02). While for monolayer Pt-skin catalysts, the authors showed that... 

This chapter deals with aspects of the synthesis of fuel cell catalysts. Practical catalysts for low-temperature fuel cells are typically in the nano-size range and are frequently formed or deposited on high-surface-area supports. Pt is the most eommonly used eatalyst for both cathode and anode in proton exchange membrane fuel eells (PEMFCs). In the case of the cathode, combined catalyst systems such as Pt nanoparticles supported on Au or Pt alloy catalysts, as well as Pt-skin catalysts formed in combination with the iron group metals have also attracted attention. Much work has been carried out on the development of non-noble metal (Pt-ffee) catalysts, the synthesis of which will be discussed in Section 9.5. In the case of the anode, bi-metallie eatalysts are typically employed unless the fuel is neat H2. Pt-Ru is the state-of-the-art catalyst for both methanol and reformate fuel cells. For the latter, other anode catalysts such as Pt/MoOx and Pt/Sn are also considered promising. 

Jiang, X., T. M. Gur, F. B. Prinz, and S. F. Bent. 2010. Atomic layer deposition (ALD) co-deposited Pt-Ru binary and Pt skin catalysts for concentrated methanol oxidation. Chem. Mater. 22 3024-3032. 

More recently, Stamenkovic et al. [95,107] reported on the formation of Pt skins on Pt alloy electrocatalysts after high-temperature annealing. Pt skins were reported to exhibit strongly enhanced ORR activity. It was argued that the electronic properties of the thin Pt layer on top of the alloy alter its adsorption properties in such a way as to reduce the adsorption of OH from water and therefore to provide more surface sites for the ORR process (see Section 5.2 in Chapter 4 for a detailed discussion of skin catalysts, compare also Section 4.1.5 in the present Chapter). 

Besides activity, durability of metal electrode nano-catalysts in acid medium has become one of the most important challenges of low-temperature fuel cell technologies. It has been reported that platinum electrode surface area loss significantly shortens the lifetime of fuel cells. In recent years, platinum-based alloys, used as cathode electrocatalysts, have been found to possess enhanced stability compared to pure Pt. The phenomenon is quite unusual, because alloy metals, such as Fe, Co and Ni, generally exhibit greater chemical and electrochemical activities than pure Pt. Some studies have revealed that the surface stmcture of these alloys differs considerably from that in the bulk A pure Pt-skin is formed in the outmost layer of the alloys due to surface segrega-... 

PtCo, and PtNi, and this was related to the formadon of a Pt-skin layer on the alloy pardcles. However, for Pt alloys with precious metals such as Ru, Os, or Re, the bifunctional mechanism is operative because of the stability of these elements in the Pt surface. With the introduction and evolution of more powerful characterization techniques such as XAS, it has been possible to perform more detailed studies of the crystalline phases present in a catalyst. The work of McBreen and Mukeijee has shown clearly that in Pt-Ru alloys, the Ru increased the Pt d-band vacancies and decreased the Pt-Pt bond distances. The wider conclusion of this work was that a fine-tuning of the electronic structure and the electrocatalysis is necessary in order to design an even more CO tolerant and active catalyst. 

E ure 9.10 Correlation between specific activity for the ORR at 0.9 V and the d-band center for different Pt-skin and Pt-skeleton type catalysts. Note how PtjCo is atop both curves. ... 

Figure 5. A x magnitudes for PtM (M = Fe, Cr, Ni, and Co) bimetallic catalysts and pure Pt in FI2S04 at 0.84 V prepared as described in Ref. 28. The region where the maximum is observed for OFi and O on Pt is indicated. Note the presence of the additional shoulder below 0 V when M atoms are on the surface (case for PtNi and PtCo), and the absence of these shoulders in the case of a Pt skin (case for PtFe and PtCr). Thus OH/Pt near a M island can be distinguished from OH/Pt distant from these islands.

Another approach to carbonless electrode is to use a non-carbon catalyst-support that meets the requirements of a support. Since metallic supports will corrode faster than carbon, they can hardly be used, unless they can be completely coated by a thin layer of Pt skin to avoid corrosion. Many ceramic materials can meet the stability requirement, but they lack of electronic conductivity. Again, this will not be a problem if these particles can be coated by an electronically conducting Pt thin layer. Chhina used semi-conducting indium tin oxide (ITO) particles to support Pt and achieved an average Pt crystallite size of 13 nm. Pt supported on ITO showed extremely high thermal stability and only lost 1% wt. of materials versus 57% wt. for Pt supported on Hispec 4000 carbon upon heating to 1000 °C. 

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