Pt based alloys

Jacob T. 2006b. Water formation on Pt and Pt-based alloys a theoretical description of a cata-l3dic reaction. ChemPhysChem 7 992-1005. 

Teliska M, Murthi VS, Mukeqee S, Ramaker DE. 2007. Site-specific vs specific adsorption of anions on Pt and Pt-based alloys. J Phys Chem C 111 9267-9274. 

To evaluate the catalytic activity or to investigate the reaction mechanism, planar electrodes with well-defined characteristics such as surface area, surface and bulk compositions, and crystalline structure have often been examined in acidic electrolyte solutions. An appreciable improvement in CO tolerance has been found at Pt with adatoms such as Ru, Sn, and As [Watanabe and Motoo, 1975a, 1976 Motoo and Watanabe, 1980 Motoo et al., 1980 Watanabe et al., 1985], Pt-based alloys Pt-M (M = Ru, Rh, Os, Sn, etc.) [Ross et al., 1975a, b Gasteiger et al., 1994, 1995 Grgur et al., 1997 Ley et al., 1997 Mukeijee et al., 2004], and Pt with oxides (RuO cHy) [Gonzalez and Ticianelli, 2005 Sughnoto et al., 2006]. 

Figure 10.4 Area-normalized CL spectra of Pt4/7/2 for the pure Pt (dotted Une), Pt5gCo42 (solid line), and PtgoRu4o (dashed line) alloys with respect to p (a) as-prepared (h) after electrochemical stabilization. The samples were thin film pure Pt or Pt-based alloys (diameter 8 mm and thickness 80 nm) prepared on Au disks by DC sputtering. Electrochemical stabilization of Pt58 C042 was performed by repeated potential cycling between 0.075 and 1.00 V at a sweep rate of 0.10 V s in 0.1 M HCIO4 under ultrapure N2 (99.9999%) until CV showed a steady state. PtgoRu4o was stabilized by several potential cycling between 0.075 and 0.80 V at 0.10 V s in 0.05 M H2SO4 under ultrapure N2. (From Wakisaka et al. [2006], reproduced by permission of the American Chemical Society.)...

This is the first experimental demonstration of changes in the strength of CO adsorption at Pt-based alloy electrodes. Nprskov and co-workers theoretically predicted a similar linear relation between changes in ads(CO) and shifts in the (i-band center [Hammer et al., 1996 Hammer and Nprskov, 2000 Ruban et al., 1997]. Because the Pt4/7/2 CL shift due to alloying can be more easily measured by XPS than the li-band center can, this should be one of the most important parameters to aid in discovering CO-tolerant anode catalysts among Pt-based alloys or composites. 

Min M, Cho J, Cho K, Kim H. 2000. Particle size and alloying effects of Pt-based alloy catalysts for fuel cell applications. Electrochim Acta 45 4211-4217. 

The current-write anisotropy-field cut-off is believed to be about HKmax = 20-30 kOe for perpendicular recording, which requires most advanced writer head technology and includes enhancements such as tilted [15, 16] and/or recently proposed exchange-coupled composite media [54, 55], Hence, most of the materials listed in Table 2 are excluded from perpendicular media applications. Co-rich Co-Pt based alloys and Co/Pd type multilayers look promising from the perspective of near term extensions to higher write coercivities. Future HAMR media candidates are mainly Co/Pt based multilayers (Dp 6.1-8.3 nm) (entry level) and ultimately FePt-Zl0-compounds (Dp 2.4-4.4 nm). 

Different alloys display reactivities that, due to ion adsorption, vary with the nature of the electrolyte. In HCIO4, the catalytic activity order for Pt-based alloys with cobalt and nickel was PtsCo... 

An archetypal MEA consists of an electrolyte membrane sandwiched between two catalyst layers and two gas diffusion layers (GDLs) as shown in Fig. 1. The fuel and oxidant gases diffuse through the GDL to react in the catalyst layer between the electrode and electrolyte. The catalyst, typically Pt or Pt based alloy, are nanoparticles residing on carbon particles. In addition to its primary purpose as the center of reactivity, the catalyst must participate in the effective adsorption of the reactants, conduction of the electrons to/from the electrode and diffusion of protons to/from... 

For many years, the oxygen reduction reaction (ORR) on Pt and Pt-based alloys has been studied extensively by experimental " and computational methods, and it has been shown that the formation of a Pt-skin layer is accompanied with a Pt-depleted layer underneath for many Pt-3d alloys. Notably, several studies of O adsorption on Pt-skin surfaces have revealed that the binding strength is weaker than that on the pure Pt(lll) surface and this may facilitate the removal of adsorbed O, therefore in-... 

To quantily the metal dissolution trends, and to offer comparisons of the stability of surface Pt atoms in different environments, we reported the development and application of a computational approach based on first-principles calculations on metal slabs, using the methodologies explained in this chapter. The method allows us to evaluate the electrochemical potential shift AU (V) for the dissolution of Pt atoms in an alloy surface, relative to the potential at which the same reaction would take place on pure Pt(lll) surfaces. Recent investigations in our lab have found interesting correlations between the potential shift for the onset of surface oxidation of Pt in Pt-based alloys with respect to the same potential in pure Pt surfaces and the d-band shift of the surface atoms, reflecting the changes in the electronic structure due to alloying. The results will be published elsewhere. 

The electro-oxidation of H2/CO mixtures is a very complicated reaction with many variables, e.g., temperature, CO concentration, and pH. From a practical standpoint, the structure sensitivity is not so interesting, since at any realistic level of CO, e.g., >10 ppm CO, and temperature the pure-Pt surface is highly poisoned by adsorbed CO, and the electrode polarization is impractically large. It is, however, still of fundamental importance to know the structure sensitivity of the reaction on pure Pt in order to understand the properties of Pt-based alloy catalysts that are not so highly poisoned by the CO, i.e., so-called CO-tolerant catalysts. For our purposes here, we discuss only one characteristic measure of the structure sensitivity, shown in Figure 14. For a wider range of results, we refer the interested reader to [54] and references therein. Figure 14 shows the current for H2 oxidation at 50-mV... 

The dissociative adsorption of the oxygen molecule is unlikely and the first electron transfer to the oxygen molecule [Eq. (63)] is considered to be the rate determining step. The formation of H2O2 and various forms of platinum oxides on the surface reduces the theoretical potential obtainable by ORR. In fact, potential losses in PEMFC arise predominantly because of the sluggishness of the ORR. The structural sensitivity of ORR has been discussed over the Pt-based electrodes. Pt/C is commonly used for the cathode material, and Pt-based alloys, such as PtCr and PtNi, have also been investigated. 

The PE MFC has a solid ionomer membrane as the electrolyte, and a platinum, carbon-supported Pt or Pt-based alloy as the electrocatalyst. Within the cell, the fuel is oxidized at the anode and the oxidant reduced at the cathode. As the solid proton-exchange membrane (PEM) functions as both the cell electrolyte and separator, and the cell operates at a relatively low temperature, issues such as sealing, assembly, and handling are less complex than with other fuel cells. The P EM FC has also a number of other advantages, such as a high power density, a rapid low-temperature start-up, and zero emission. With highly promising prospects in both civil and military applications, PEMFCs represent an ideal future altemative power source for electric vehicles and submarines [6]. 

Table 1. Properties of state of the art Pt based alloys and desired properties of potential candidates...

Satellite thruster combustion chambers manufactured by EADS Space Transportation are machined from massive Pt based alloys, which are capable of operating at about 1600°C in continuous mode. Main inconvenient of such chambers is the elevated raw material cost of about 30 /gr. Other qualified materials for the application are silicide coated Nb,... 

Despite the experience of the EADS with Pt-based alloys and their good acceptance, the increasingly competitive satellite market demands structural materials with lower prices that nevertheless must remain chemically stable (no coating). This motivated research in the field of Cr based alloys. 

The CLs of the early PEMFCs were prepared from the noble metal blacks and thns contained very high metal loadings per geometric MEA area. Later, it became apparent that precions metals in these CLs were not utilized efflciendy and the new generation of PEMFCs emerged, based on carbon-supported precious-metal catalysts (usually Pt or Pt-based alloy). The MEA preparation techniques have undergone continuous evolution since the early days of PEMFCs. For a historical overview, the reader is referred to refs. 1 and 5. In this chapter we focus on carbon materials as supports for PEMFCs and DMFCs. 

Because of the great potential of methanol as a fuel for low-temperature fuel cells, the electro-oxidation of methanol on Pt or Pt-based alloy electrodes has been studied extensively in the past decades [112-115]. It is generally accepted that methanol is oxidized to CO2 by the so-called dual-path mechanism [112] via adsorbed CO (poison) and non-CO reactive intermediates. The formation of CO by dehydrogenation of methanol has been well confirmed, but no consensus has been reached so far on the nature of the reactive intermediates in the non-CO pathway. Various adsorbates such as CHxOH [116], -COH [116], formyl (-HCO), [117] carboxy (-COOH) [117], a dimer of formic acid [35], and COO [38] have been claimed to be the reactive intermediates from IRAS and other physicochemical measurements. However, the spectra of the reaction intermediates are not well reproduced by other groups. 

U.A. Paulus, A. Wokauna, G.G. Scherer, TJ. Schmidt, V. Stamenkovic, N.M. Markovic, P.N. Ross, Oxygen reduction on high surface area Pt-based alloy catalysts in comparison to well defined smooth bulk alloy electrodes, Electrochim. Acta 47 (2002) 3787-3798. 

Several reports indicate that Pt-based alloys are at least as good as pure Pt, and in many cases the alloyed material shows a better performance for the oxygen electroreduction (OER) [26, 27, 33, 46, 47, 48]. Experimental data on the catalytic activity of bimetallic surfaces regarding the OER is controversial. It is clear that certain bimetallic catalysts (Pt-Cr, Pt-Fe, Pt-Co, Pt-Ni) yield a slightly enhanced oxygen reduction current, but the reported degree of enhancement differs among researchers [2]. 

Table 17.1 DFT calculated binding energies (BE) of oxygenated reactants and intermediates of the ORR to Pt3 and Pt-based alloy clusters.

This bifunctional mechanism is typically invoked to explain CO oxidation on Pt-based alloys as well. While this model has been successful in describing several experimental results on Pt-based electrodes, a complete first-principle theoretical evidence to this mechanism does not exist. 

The most promising ones are based upon (supported) Pt-based alloys. Numerous carbon supported bimetallic catalysts for the ORR have been studied, amongst them, PtCo, - PtAu, - PtV, PtFe, PtZn further PtM/C can be found in Refs. 12 and 178. 

At the current stage of technology, carbon-supported Pt and Pt-based alloy catalysts are the most active and stable catalysts for an ORR, which have been used for fuel cell cathodes. The major research effort for Pt and Pt-based alloy cattdysts is to optimize (1) the size and dispersion of nanoparticles, (2) interaction between Pt catalyst and supporting materials, and (3) Pt-alloying strategy. 

In addition to Pt—Au/C catalysts, several other Pt-based alloy catalysts, such as PtCo/C, PtNi/C, PtV/C, and PtPd/C were also reported, and the mechanism of enhancing ORR activity was investigated using both RDE and RRDE techniques. The mechanism of ORR improvement by alloying is ascribed to (1) increase in the catalyst surface roughness,(2) decrease in the coverage of surface oxides and an enrichment of the Pt-active sites of the catalyst surface,(3) increase in the d-orbital vacancy, which strengthened the Pt—O2 interaction, and (4) decrease in the Pt—Pt distance and the Pt—Pt coordination numbers. Table 7.4 lists the properties of PtCo/C and PtNi/C catalysts and their ORR performance parameters measured by RDE techniques for comparison. 

For transition metal catalysts, two-electron reduction was reported for less active metals such as Au and Hg. For the most active catalyst, Pt and Pt-based alloys, four-electron reduction steps are generally believed. However, the detailed mechanism and reduction pathways are not clear and much debate remains. Even for the first electron transfer step, different views still exist [38-40]. Examples of plausible first steps include the following (1) splitting of the 0-0 bond upon oxygen adsorption on two Pt sites (S) in abridge configuration, O2 + 2S —> O + O (2) formation of... 

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