Platinum, interaction with water

It suggests that the ox3rgeii source for this methanol oxidation is not Pt-OH but water, which might be weakly interacted with the platinum siuface. 

These examples show that adsorption of water molecules on platinum electrodes depends on the solution components. If the energy of the solute adsorption is higher than that of water molecules, water tends to adsorb on the top of the primary solute layer, which is directly bound to the platinum adsorption sites. If the interaction of organic molecules with platinum is weak, water adsorbs directly onto the electrode surface. In the... 

From this point of view, a brief comparison of acyloxylation of cis- or irany-stilbenes in electrochemical and chemical conditions is also relevant. Oxidation of cis- or irany-stilbene at the platinum anode in the presence of acetic or benzoic acid gives predominantly meyo-diacylates of hydroxy-benzoin or, if some water is present, t/treo-monoacylate. None of the stereoisomeric erythro-mono-acylate and rac-diacylate were obtained in either case. There was no evidence of isomerization of cis- to trany-stilbene nnder the electrolytic conditions employed (Mango and Bonner 1964, Koyama et al. 1969). The sequence of reaction steps in Scheme 2.27 was proposed. Adsorption-controlled one-electron oxidation of the snbstrate takes place. Then the cis-stilbene cation-radical interact with acetate to form an oxonium ion. The phenyl groups in the oxoninm adopt the trans mntnal disposition which is thermodynamically preferential. The trany-benzoxoninm ion is the common intermediate for conversions of both cis- and trany-stilbenes and, of conrse, for all the final prodncts (Scheme... 

Oxidized sulfur species occurring in natural waters (sulfate, sulfite, thiosulfate] do not interact with the platinum electrode when in the presence of H2S and the pH-Eh-E52- relations found were similar to the above relations. Thus, the unambiguous relations found between pH, Eh and E22- in aqueous solutions of hydrogen sulfide can be employed to characterize solutions and water samples where hydrogen sulfide is the only reduced sulfur species present. 

Tungsten Hexafluoride, WFg, is obtained by the action of anhydrous hydrogen fluoride upon tungsten hexaehloride in platinum vessels, or by the interaction of antimony peiitafluoride with the hexa-chloride. It is a solid at low temjreratures just above 0° C. it sublimes to a heavy gas which fumes in moist air it reacts with water with production of tungstic acid. It attacks both glass and mercury by alkalies it is decomposed, and with alkali fluorides it forms double salts. 

The salt reacts rapidly with water precipitating some of the platinum as hydrated dioxide, most of it remaining in solution as hexafluoroplatinate(iv). This behaviour resembles the reductive interaction of the hexafluoro-rhenates(v)-osmates(v) and -iridates(v), with water to give the quadripositive fluorosalts. Since the latter cannot be synthesized in aqueous media, they are probably thermod5mamically unstable with respect to hydrolysis. If so, evidently the hexafluoroplatinate(v) ion must remain intact during reduction to the hexafluoroplatinate(iv) ion. 

This conversion is catalyzed by [Ru(Hedta)(H20)] (Hedta = trianion of eth-ylenediaminetetraacetic acid) at 30 °C and 10 Pa in the presence of a solid semiconductor mixture (CdS/Pt/RuO,). The photocatalytic production of ammonia is initiated by absorption of visible light (505 nm) by the CdS semiconductor (Fig. 13.12). Presumably, the incoming photons promote electrons from the valence band (VB) of CdS to its conducting band (CB), a process that leaves holes in the valence band. Water is photooxidized by RuOi, releasing electrons which are trapped by holes in the valence band of CdS. The electrons in the conducting band are transferred to the nithenium complex via platinum metal. Protons from the water oxidation are attracted to the reduced ruthenium complex, interact with coordinated N, in some unknown fashion, and are expelled as NH3. The cycle is complete when the coordination site left by NH3 becomes occupied once again by HjO. It remains to be seen whether proposed cycles such as this one measure up to their promise. 

The local interactions between the metal atoms and water molecules or ions have been obtained from semiempirical and ab initio quantum chemical cluster calculations (see below). This technique has been used by the groups of Spohr and Heinzinger, and later by Berkowitz and coworkers, for platinum surfaces with (100) and (111) surface geometry [48, 49, 52-54, 76] and also for mercury surfaces [40, 77-81]. 

The (pair) potential o-Pt attractive for larger Pt-O distances only if p is small, leading to the preferential adsorption site on top of a platinum atom with an adsorption energy of 35.7kJmol on the (100) surface (see below), in agreement with the extended Hiickel calculations [82]. The distance dependence of the total interaction energy of one water molecule with an entire platinum crystal is shown in Fig. 6 for several adsorption sites and orientations. Quantum chemical calculations... 

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