Pt-metallization

The second form consists of Pt metal but the iridium is present as iridium dioxide. Iridium metal may or may not be present, depending on the baking temperature (14). Titanium dioxide is present in amounts of only a few weight percent. The analysis of these coatings suggests that the platinum metal acts as a binder for the iridium oxide, which in turn acts as the electrocatalyst for chlorine discharge (14). In the case of thermally deposited platinum—iridium metal coatings, these may actually form an intermetallic. Both the electrocatalytic properties and wear rates are expected to differ for these two forms of platinum—iridium-coated anodes. 

A common property of coordinated alkenes is their susceptibility to attack by nucleophiles such as OH , OMe , MeC02, and Cl , and it has long been known that Zeise s salt is slowly attacked by non-acidic water to give MeCHO and Pt metal, while corresponding Pd complexes are even more reactive. This forms the basis of the Wacker process (developed by J. Smidt and his colleagues at Wacker Chemie, 1959-60) for converting ethene (ethylene) into ethanal (acetaldehyde) — see Panel overleaf. 

This colour change can be observed with the ions of Mg, Mn, Zn, Cd, Hg, Pb, Cu, Al, Fe, Ti, Co, Ni, and the Pt metals. To maintain the pH constant (ca 10) a buffer mixture is added, and most of the above metals must be kept in solution with the aid of a weak complexing reagent such as ammonia or tartrate. The cations of Cu, Co, Ni, Al, Fe(III), Ti(IV), and certain of the Pt metals form such stable indicator complexes that the dyestuff can no longer be liberated by adding EDTA direct titration of these ions using solochrome black as indicator is therefore impracticable, and the metallic ions are said to block the indicator. However, with Cu, Co, Ni, and Al a back-titration can be carried out, for the rate of reaction of their EDTA complexes with the indicator is extremely slow and it is possible to titrate the excess of EDTA with standard zinc or magnesium ion solution. 

Measurement of E° and Activities Electrochemical cells can be constructed to measure E° and thermodynamic properties such as K, AG, AH, AS, A V, and ACP for a reaction. Consider as an example the cell shown schematically in Figure 9.4.x The cathode consists of an Ag metal rod coated with AgCl(s). The anode is a Pt metal rod around which H2(g) is bubbled. The two electrodes are... 

C.G. Vayenas, S. Bebelis, I.V. Yentekakis, P. Tsiakaras, and H. Karasali, Non-Faradaic Electrochemical Modification of Catalytic Activity on Pt Metals, Platinum Metals Review 34(3), 122-130 (1990). 

Figure 11.6. Galvanostatic catalytic rate transients showing the equivalence of electrochemical promotion when using YSZ30 (a) or TiOj31 (b) as the Pt metal film support. See text for discussion.22 Reprinted with permission from Academic Press.

H2 and He and bulk Pt as a function of temperature is shown in figure 4. (There is no measurable difference in Pt metal over this temperature range.) " ere is a continuous, reversible change for the... 

J = 5/2 and J = 3/2 states in the catalyst are similar to each other as they are in Pt metal but they are significantly different from Pt metal. 

Figure 5. Difference spectra of the data in figure 4 where Pt metal Ln m has been subtracted appropriately from the catalyst data. In E) the difference areas Cat. (He, 773 K) - Cat. (He,...

Is as follows Apply the Pt-Pt phase shift (derived from Pt metal) to the 1st forward transform of Pt02 This will partially smear the Pt-0 peak. Then take the back transform of this smeared Pt-0 peak. Extract a new phase shift from this back transform using the known Pt-0 distance, 2.07 X. This phase shift can now be used on the catalysts to focus the Pt-0 peak region. 

Platinum was added to Nation before Incorporating CdS In order to avoid the reduction of CdS during the platlnlzatlon process. Nation (DuPont 117, 0.018 cm thick) films were soaked In Pt(NH2)2l2 (0.1 mM) solution for 4 hr. The amount of the Pt complex Incorporated was determined by measuring the optical absorption change In the liquid phase. The films were subsequently reduced with NaBH (0.1 M) solution for one day to produce Pt metal dispersed throughout the polymer film. The amount of Pt was found to be about 0.02 mg cm 2. 

The efficiency of semiconductor PCs in some reactions (such as dehydrogenation of organics, splitting of HjO and H2S, etc.) can be enhanced by depositing tiny islands of additional catalysts, which facilitate certain reactions stages that may not require illumination. For example, islands of Pt metal are deposited on the surface of the composite photocatalyst in Fig.6 with the aim to facilitate the step of H2 formation. 

Several aspects should be considered in the selection of appropriate Pt precursors. Solubility of precursor is important as this parameter determines how well a precursor can be dissolved in a given solvent so as to facilitate the nucleation and growth. Reduction potential is another parameter, which governs the ease with which the precursor can be reduced to Pt metal. Finally, the thermal stability of the precursor partly determines the reaction temperature for the formation of Pt nanoparticles. 

First, the selective deposition method was developed. It is the novel preparation technique, where the maximum loading around 20 wt% with keeping the particle size below 2nm [14]. Figure 6 shows Pt metal particles supported on monodispersed spindle titania particles. 

The main issue of the book is application of nanosized particles in both homogeneous and heterogeneous catalysis. A variety of reactions catalyzed by metal colloids or supported nanosized metals is discussed. The most intriguing reaction seems to be ethane hydrogenolysis catalyzed by Pt clusters on porous carrier and studied by G. A. Somorjai and his group. Another challenging observation by this group is shape isomerization of Pt metal particles affected by the addition of silver ions. 

Table 3.4 illustrates that the composition of the gas phase easily can lead to sintering. For example, in air regeneration Pt metal crystallites will be transformed in to the oxides, and the Tamman temperature decrea,ses to below 420 K. 

Figure 1.2 Free energy map forthe associative mechanism of ORR at Pt metal and the steps considered in this mechanism [Nprskov et al., 2004].

Paradoxically, all these significant recent contributions to the theory of the ORR, together with most recent experimental efforts to characterize the ORR at a fuel cell cathode catalyst, have not led at aU to a consensus on either the mechanism of the ORR at Pt catalysts in acid electrolytes or even on how to properly determine this mechanism with available experimental tools. To elucidate the present mismatch of central pieces in the ORR puzzle, one can start from the identification of the slow step in the ORR sequence. With the 02-to-HOOads-to-HOads route appearing from recent DFT calculations to be the likely mechanism for the ORR at a Pt metal catalyst surface in acid electrolyte, the first electron and proton transfer to dioxygen, according to the reaction... 

Can an ORR mechanism at Pt metal in an acid electrolyte with the Reaction (1.2) as the first and rate-limiting step be defended in light of the recently reported apparent Tafel slope and reaction order for ORR in the PEFC cathode ... 

The lesson to be taken from this report by Paik et al. [2004] is that a Pt catalyst in contact with a hydrous electrolyte is so active in forming chemisorbed oxygen at temp-eramres and potentials relevant to an operating PEFC, that the description of the cathode catalyst surface as Pt, implying Pt metal, is seriously flawed. Indeed, that a Reaction (1.4) acmally takes place at a Pt catalyst surface, exposes, Pt to be less noble than usually considered (although it remains a precious metal nevertheless. ..). Such a surface oxidation process, taking place on exposure to O2 and water and driven by electronically shorted ORR cathode site and metal anode site, is ordinarily associated with surface oxidation (and corrosion) of the less noble metals. 

---