Platinum, thermodynamic data

For complexes like PtL2X2 (X = halogen L = NH3, PR3, etc.) where cis-and trans-isomers exist, the trans-isomer is usually thermodynamically more stable. The c/s-isomer may be formed first in a reaction and, in the case of platinum, may be relatively inert to substitution. (Thermodynamic data are relatively scarce trans-Pt(NH3)2Cl2 is some 13kJmol-1 more stable than the cis-isomer.)... 

From the thermodynamic data of Appendix C, show that the product of the reaction of ammonia gas with oxygen would be nitrogen, rather than nitric oxide, under standard conditions and in the absence of kinetic control by, for example, specific catalysis of NO formation by platinum. (Assume the other product to be water vapor.)... 

The thermodynamic data for the platinum oxides are not well established. However, a reasonable value for the free energy of formation of the lower oxide, PtO, at 527°C. is —1 kcal./mole (5, 25). This corresponds to an oxygen dissociation pressure at 527°C. of 0.28 atm.—i.e., bulk PtO is unstable toward decomposition to bulk Pt for oxygen pressures below 0.28 atm. Bulk Pt02 is, of course, even less stable. Nevertheless, it has been reported that at this temperature and an oxygen pressure... 

In contrast to these promising thermodynamic data, the kinetics of the electrochemical mineralization is very slow and in practice it can be achieved close to the thermodynamic potentials only in very limited cases. In fact, only platinum-based electrodes can allow EM of simple Ci organic compounds. A typical example is the use of Pt-Ru catalyst in the electrochemical mineralization of methanol. 

The effects of pH and Cl ion concentration on the species distribution of platinum compounds have been used to fashion the following plausible argument for the chemistry of cw-DDP in vivo. With the use of thermodynamic data for the ethylenediamine (en) analogue [Pt(en)Cl2], the relative concentrations of hydrolyzed species at pH 7.4 were estimated (see Table 9.3) for blood plasma and cytoplasm (Figure 9.7). The higher chloride ion concentration in... 

Table 10. Thermodynamic data for platinum(II)-olefin complex formation for the reaction PtCl r olefin PtCI3 (oiefinj + Cl ...

Table 1 summarizes some of the most relevant results from thermodynamic studies on platinum single-crystal surfaces. In order to facilitate the comparison of thermodynamic data corresponding to different adsorption reactions, values of the thermodynamic properties at the standard state ( AG , A/T and AS ) are provided. Values of the lateral interaction parameter (a>), and its temperature dependence (dm/dT) are also given in Table 1. While the lateral interaction parameter measures the magnitude of the lateral interactions, its temperature dependence reflects the entropic contributions to the lateral interactions. Therefore, the enthalpic contribution to the lateral interaction energy, can be obtained from - T da/dT.

The early thermodynamic data are effective quantities related simultaneously to hydrogen adsorption and numerous accompanying phenomena, but they are still not hopeless for deeper analysis. In addition we should not underestimate the importance of this early period for accumulating the experimental experience, as platinum metals electrochemistry requires rather specific techniques of surface and solution preparation, to avoid contaminations. 

The four-electron reduction of oxygen [reaction (I)] is very irreversible and therefore experimental verification of the thermodynamic reversible potential of this reaction is very difficult. The exchange current densities for reactions (I) and (II) are typically 10" -10" A/cm of real surface area for Pt and other noble metals at room temperatures. Any other side reaction, even if slow and otherwise difficult to detect, may compete with reaction (I) or (II) in establishing the rest potential. Indeed, unless special experimental procedures are used, the thermodynamic potential cannot be obtained at ambient temperature in aqueous electrolytes. Even on the most active platinum electrode in pure acid or alkaline aqueous solution under ordinary conditions, the rest potential in the presence of oxygen at 1 atm and ambient temperature usually does not exceed 1.1 V vs. the NHE and most often has a value close to 1.0 V. In early work on O2 electrochemistry, before reliable thermodynamic data were available, the potential 1.08 V vs. RHE was considered as the reversible value for reactions (I) and (II). 

Phases and Phase Equilibria. Selected phase diagrams are shown in Figs. 3.1-252-3.1-257 [1.219]. Pd forms continuous solid solutions with all other noble metals and with Co, Cu, Fe, and Ni. Miscibility gaps exist in alloys with C, Co, Ir, Pt, Rh, and ternary Pd—Ag—Cu alloys (Fig. 3.1-257) [1.220]. All platinum-group metals (PGM) lower the y-a transition temperature in Fe-alloys considerably (Fig. 3.1-343). Thermodynamic data are given in Tables 3.1-190-3.1-194. Numerous intermediate phases exist also in alloys with rare earth metals [1.216,217,217,222]. The solubility of... 

Thermodynamic data are available for the alloy ZrPt and so it is possible to determine the conditions under which platinum and zirconia would be expected to react, as follows. 

Microcalorimetric measurements at elevated temperatures of the thermolysis of several cis-[PtX2L2] [L = NH3, amine, or pyridine(py) X = Cl, Br, or I] allowed derivation of standard enthalpies of formation and other thermodynamic data. Calculations of standard enthalpies of formation of diamine (dicarboxylate)platinum(II) and [Pt(acac)2] led to estimates of mean Pt-O bond dissociation enthalpies. " ... 

Some half-cells are given with platinum as the inert electrode however, this is only taken as an example for an inert electrode and it does not mean that there is any dependence of the standard potentials on the electrode material. The standard potentials of dissolved redox systems are independent of the electrode material. This is opposite to the standard rate constants of electron transfer, which are very dependent on the electrode material. Please note also that many of the given standard potentials cannot be obtained by electrochemical measurements. They are calculated from thermodynamic data obtained, e.g., from calorimetry. The system Pt MnO, Mn +, is irreversible not only on platinum but also on all other elec-... 

The role of hydrous oxides in metal oxidation reactions has already been briefly outlined here. Where such materials are involved the thermodynamic data summarized in most current Pourbaix diagrams are inadequate. In such diagrams, at present, lines relating to redox processes where both the oxidized and reduced form of the couple are insoluble are generally assumed to involve a conventional, 2.303(RT/F) V/pH unit, potential variation with pH. It is only necessary to consider one example, namely, the difficulty of reducing hydrous oxide films on platinum in base (where there is evidently little inhibition of kinetic origin), to appreciate the need to extend such diagrams to take hydrous oxide behavior into account... 

There is little data available to quantify these factors. The loss of catalyst surface area with high temperatures is well-known (136). One hundred hours of dry heat at 900°C are usually sufficient to reduce alumina surface area from 120 to 40 m2/g. Platinum crystallites can grow from 30 A to 600 A in diameter, and metal surface area declines from 20 m2/g to 1 m2/g. Crystal growth and microstructure changes are thermodynamically favored (137). Alumina can react with copper oxide and nickel oxide to form aluminates, with great loss of surface area and catalytic activity. The loss of metals by carbonyl formation and the loss of ruthenium by oxide formation have been mentioned before. 

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