Bulk platinum

The expression in the square brackets is exactly the formation energy of the Pt bulk oxide out of bulk platinum and molecular O2, which can easily be evaluated by DFT calculations [Jacob, 2007b]. Together with the experimental value for AGq, we can finally formulate the following stabUity ranges at which the three known Pt bulk oxides are thermodynamically stable ... 

We and others have been involved in the study of such systems including Cu/Au(lll),85 86 Ag/Au(lll),87 Pb/Ag(lll),88 and Cu/Pt(lll).89 The first three systems involved the use of epitaxially deposited metal films on mica as electrodes.90 92 Such deposition gives rise to electrodes with well-defined single-crystalline structures. In the last case a bulk platinum single crystal was employed. Because of the single-crystalline nature of the electrodes, polarization dependence studies could be used to ascertain surface structure. 

Table 3.2 The results of Bagotzky and Vassilyev (1967) on the adsorption of methanol at bulk platinum in acid solution at 0.4 V vs. RHE.

Development of methods related to DFT that can treat this situation accurately is an active area of research where considerable progress is being made. Two representative examples of this kind of work are P. Rinke, A. Qteish, J. Neugebauer, and M. Scheffler, Exciting Prospects for Solids Exact Exchange Based Functional Meet Quasiparticle Energy Calculations, Phys. Stat. Sol. 245 (2008), 929, and J. Uddin, J. E. Peralta, and G. E. Scuseria, Density Functional Theory Study of Bulk Platinum Monoxide, Phys. Rev. B, 71 (2005), 155112. 

The thermodynamically stable form of bulk platinum in oxygen saturated water at ambient conditions is the completely hydrated platinum(IV)oxide, Pt02 4H20, also referred to as platinic acid H2Pt(OH) 14yl5) with a standard Gibbs energy of formation of -84 kJ mol" The formation of this compound will be even more favoured in the case of incompletely coordinated platinum. 

Fig. 4. Reflectivity of parallel- and perpendicular-polarized light at 1600 cm for the system Ge/Pt/ vacuum determined for platinum films of various thicknesses. In the calculations, the optical constants of bulk platinum were used nce = 4.0, npt = 5.71 + Z23.35. The angle of incidence was 45°. Calculations were performed according to the matrix formalism described in the text.

Note particularly that film electrodes make it economically practical to use very expensive materials as electrodes because of the very small quantities required to form the film. For example, a platinum film of 300 nm thickness and 1 cm2 cross-sectional area contains 640 pg of platinum. A macroscopic wire electrode of the same surface area constructed with 26-gauge platinum wire (0.4 mm diameter, 7.9 cm long) contains 210 mg of platinum, or 330 times as much platinum. At a price of 60/g, the cost of the platinum in the film ( 0.04) is much lower than the cost of the wire ( 12.80). Despite the great reduction in raw materials cost, the resulting electrode may not actually be proportionally less expensive than bulk platinum for small numbers of film electrodes, due to significantly greater fabrication costs. However, when produced on a large scale, costs of film electrodes may be dramatically lower. 

A way to stretch or compress metal surface atoms in a controlled way is to deposit them on top of a substrate with similar crystal symmetry, yet with different atomic diameter and lattice constant. Such a single monolayer of a metal supported on another is called an overlayer. Metal overlayers strive to approach the lattice constant of their substrate without fully attaining it hence, they are strained compared to their own bulk state [24, 25]. The choice of suitable metal substrates enables tuning of the strain in the overlayer and of the chemisorption energy of adsorbates. A Pt monolayer on a Cu substrate, for instance, was shown to bind adsorbates much weaker than bulk platinum due to compressive strain induced by the lattice mismatch between Pt and Cu, with Cu being smaller [26]. 

What are transition metal clusters and why are they interesting For example what are the properties of a 5-atom platinum cluster Is it similar to bulk platinum or does it behave more like the atom What we are discovering is that below a certain size each small n-atom duster has unique chemical and electronic properties, it behaves neither iike the atom, the bulk, nor even 1i ke other clusters of the same metal. Thus there are opportunities to exploit their novel properties by making it possible to create new materials with potential applications in such diverse areas as solid state physics, electronics, chemistry and catalysis. 

In contrast to the ferromagnetic metals, it has been established that carbon solubility in bulk platinum is very low (ref. 46) and probably not a critical factor in the overall carbon deposition process. Carbon will dissolve in platinum, but segregates to the surface on cooling without any apparent disruption of the surface structure (refs. 32,47). 

A set of parameters fitted to experimental data for bulk platinum has been given in Table 7 of [58]. There are two typographical errors in that table. The value of Rorb = 0.39 and Xtotai = Xexp = 325 x 10 . The experimental susceptibility has been corrected for an estimated diamagnetic contribution of 41 x 10 for the Pd data in Table 8 a similar correction of 57 x 10 has been applied. 

On the other hand, Chen et al. developed polypyrrole film electrodes containing nanodispersed platinum particles and investigated their catalytic properties for the electro-oxidation of hydrogen [13]. They confirm the remarkable electrocatalytic activity of these PPy/Pt films compared to bulk platinum electrodes, as the film thickness increases to 5 pm. 

X-Ray Diffraction Studies. The dependence of the lattice parameter of bulk platinum-iridium alloys on composition is shown in Figure 4.22 (4,45). Lattice parameters are commonly obtained from X-ray diffraction measurements. For platinum-iridium catalysts, X-ray diffraction measurements provide a way of demonstrating the presence of bimetallic clusters of platinum and iridium, if the metal dispersion is not too high. 

---