Platinum surfaces system

It is interesting to note that a dimer of Ce(IV) has also been invoked to account for observations on this exchange system at a platinum surface. The rate of exchange between the fully aquated ions of Ce(IV) and Ce(III) was concluded to be relatively slow. ... 

Experiments by Freund and Spiro/ with the ferricyanide-iodide system showed that the additivity principle held within experimental error for both the catalytic rate and potential when the platinum disk had been anodically preconditioned, but not when it had been preconditioned cathodically. In the latter case the catalytic rate was ca 25% less than the value predicted from adding the current-potential curves of reactions (15) and (16). This difference in behavior was traced to the fact that iodide ions chemisorb only on reduced platinum surfaces. Small amounts of adsorbed iodide were found to decrease the currents of cathodic Fe(CN)6 voltam-mograms over a wide potential range. The presence of the iodine couple (16) therefore affected the electrochemical behavior of the hexacyanofer-rate (II, III) couple (15). 

Iodide adsorbed on reduced platinum surfaces was found to affect several other systems. The most dramatic effect was shown when the couples U/r and O2/H2O were considered together. Addition of the current-potential curves of these two couples indicated that platinum should significantly catalyze the reaction... 

The molecular modelling approach, taking into account the pyruvate—cinchona alkaloid interaction and the steric constraints imposed by the adsorption on the platinum surface, leads to a reasonable explanation for the enantio-differentiation of this system. Although the prediction of the complex formed between the methyl pyruvate and the cinchona modifiers have been made for an ideal case (solvent effects and a quantum description of the interaction with the platinum surface atoms were not considered), this approach proved to be very helpful in the search of new modifiers. The search strategy, which included a systematic reduction of the cinchona alkaloid structure to the essential functional parts and validation of the steric constraints imposed to the interaction complex between modifier and methyl pyruvate by means of molecular modelling, indicated that simple chiral aminoalcohols should be promising substitutes for cinchona alkaloid modifiers. Using the Sharpless symmetric dihydroxylation as a key step, a series of enantiomerically pure 2-hydroxy-2-aryl-ethylamines... 

The coordination of ligands at the surface of metal nanoparticles has to influence the reactivity of these particles. However, only a few examples of asymmetric heterogeneous catalysis have been reported, the most popular ones using a platinum cinchonidine system [65,66]. In order to demonstrate the directing effect of asymmetric ligands, we have studied their coordination on ruthenium, palladium, and platinum nanoparticles and the influence of their presence on selected catalytic transformations. 

The platinum electrode is also very convenient for investigating various adsorption phenomena in electrochemical systems. The surface of platinum is very stable and reproducible. As will be shown in what follows, the true working area can be determined with high accuracy for platinum surfaces with appreciable roughness and even for electrodes with highly dispersed platinum deposits. It is comparatively easy to clean the surface of adsorbed impurities and to control the state of the surface. 

In selecting reference electrodes for practical use, one should apply two criteria that of reducing the diffusion potentials and that of a lack of interference of RE components with the system being studied. Thus, mercury-containing REs (calomel or mercury-mercuric oxide) are inappropriate for measurements in conjunction with platinum electrodes, since the mercury ions readily poison platinum surfaces. Calomel REs are also inappropriate for systems sensitive to chloride ions. 

In the late 1960s it was discovered (Entina, 1968 Binder et al., 1972) that a strong synergy effect exists in the platinum-ruthenium system. Alloys of these two metals are two to three orders of magnitude more active catalytically for the anodic oxidation of methanol than pure platinum, whereas pure ruthenium is altogether inactive for this reaction. Prolonged exploitation of such anodes is attended by a gradual decrease in catalytic activity of the alloys because of slow anodic dissolution of ruthenium from the surface layer. A similar simation is seen for platinum-tin alloys, which... 

Femandez-Vega A, Feliu JM, Aldaz A, Clavilier J. 1991. Heterogeneous electrocatalysis on well-deflned platinum surfaces modifled by controlled amounts of irreversibly adsorbed adatoms Part IV. Formic acid oxidation on the Pt(lll)-As system. J Electroanal Chem 305 229-240. 

Komanicky V, Menzel A, Chang K-C, You H. 2005. Nanofaceted platinum surfaces a new model system for nanoparticle catalysts. J Phys Chem B 109 23543-23549. 

The alkaloid adsorbed on the platinum surface could form a tridimensional space within which the hydrogenation reaction can preferentially occur, due to a close interaction with QN. This space was called chiral pocket in analogy to biological systems that show high differentiation ability due to shape discrimination (Figure 14.11). 

We then designed model studies by adsorbing cinchonidine from CCU solution onto a polycrystalline platinum disk, and then rinsing the platinum surface with a solvent. The fate of the adsorbed cinchonidine was monitored by reflection-absorption infrared spectroscopy (RAIRS) that probes the adsorbed cinchonidine on the surface. By trying 54 different solvents, we are able to identify two broad trends (Figure 17) [66]. For the first trend, the cinchonidine initially adsorbed at the CCR-Pt interface is not easily removed by the second solvent such as cyclohexane, n-pentane, n-hexane, carbon tetrachloride, carbon disulfide, toluene, benzene, ethyl ether, chlorobenzene, and formamide. For the second trend, the initially established adsorption-desorption equilibrium at the CCR-Pt interface is obviously perturbed by flushing the system with another solvent such as dichloromethane, ethyl acetate, methanol, ethanol, and acetic acid. These trends can already explain the above-mentioned observations made by catalysis researchers, in the sense that the perturbation of initially established adsorption-desorption equilibrium is related to the nature of the solvent. 

Fig. 4 Potential versus current forthe reversible O2/H2O system on a bright platinum surface (Reproduced with permission from Ref 44).

Our article has concentrated on the relationships between vibrational spectra and the structures of hydrocarbon species adsorbed on metals. Some aspects of reactivities have also been covered, such as the thermal evolution of species on single-crystal surfaces under the UHV conditions necessary for VEELS, the most widely used technique. Wider aspects of reactivity include the important subject of catalytic activity. In catalytic studies, vibrational spectroscopy can also play an important role, but in smaller proportion than in the study of chemisorption. For this reason, it would not be appropriate for us to cover a large fraction of such work in this article. Furthermore, an excellent outline of this broader subject has recently been presented by Zaera (362). Instead, we present a summary account of the kinetic aspects of perhaps the most studied system, namely, the interreactions of ethene and related C2 species, and their hydrogenations, on platinum surfaces. We consider such reactions occurring on both single-crystal faces and metal oxide-supported finely divided catalysts. 

Using this approach to study heterogeneous catalysis on the atomic scale, we have investigated the mechanism of hydrocarbon catalysis by platinum surfaces. We shall describe in detail the results of these studies, which are pertinent in determining the nature of the active sites on the surface of this metal. We shall show how the results obtained for platinum may be extrapolated to other catalyst systems. Finally, we shall present a model of metal catalysis that has been emerging from our studies of platinum surfaces. 

Aizawa and Suzuki (83,84,85,86) utilized, as an ordered system, liquid crystals in which Chi was immobilized. Electrodes were prepared by solvent-evaporating a solution consisting of Chi and a typical nematic liquid crystal, such as n-(p-methoxybenzyl-idene)-p -butylaniline, onto a platinum surface. Chl-liquid crystal electrodes in acidic buffer solutions gave cathodic photocurrents accompanied by the evolution of hydrogen gas (83). This was the first demonstration of photoelectrochemical splitting of water using in vitro Chi. Of particular interest in these studies is the effect of substituting the central metal in the Chi molecule. 

The thickness of the PPy-film has an effect on the response of the electrode towards the substrate, and can be controlled by the amount of charge passed through the system during synthesis of the conducting polymer. As can be seen in Table I, the response of the electrode to BNAH (slope of the calibration curve) increases with the thickness of the film up to about three Coulombs of charge passed. If thicker layers are deposited the response is only slightly lowered. This suggests that the transport of electrons from the (reduced) flavin to the electrode does not depend upon the diffusion of a reactive species (H to the platinum surface, which would limit the current as the film thickness is increased. 

A bare platinum electrode was cleaned as previously described and then allowed to come to adsorption equilibrium with air in the gas chamber. When dry nitrogen was passed through the chamber, the potential difference between the platinum surface and the FEP resin-coated electrode increased rapidly, reaching an equilibrium value 0.5 volt higher in approximately 2 hours. Dry oxygen was then introduced into the chamber and the surface potential of the platinum decreased to a steady-state value of + 0.2 volt relative to the reference electrode (see Figure 6). This system was allowed to remain in that condition for 24 hours, during which time there was only a 20-mv. increase in the contact... 

A comment needs to be made about the state of the platinum surface during these experiments. The solutions that were used contained 0.05 mol dm 3 HC104 to curtail iron(III) hydrolysis and most of the work was carried out at 5°C to decrease the contribution of the homogeneous reaction. Under these conditions the surface of the platinum at the catalytic potential was always in the reduced state, irrespective of its preconditioning treatment. As mentioned in Sect. 4.4, such a surface specifically adsorbs iodide ions [239, 264, 265]. In order to allow for their effect, the current-potential curves for the Fe(III)/Fe(II) couple had been determined in the presence of a small concentration (5 x 10 6 mol dm 3) of potassium iodide. It is interesting that this treatment was sufficient to maintain the validity of the additivity hypothesis, in contrast to the results obtained for the Fe(CN)g" +1 system on cathodically pretreated platinum surfaces (Fig. 25). 

Bryce and co-workers reported that the crown-annulated TTF derivatives 98 and 99 were used for UV-Vis spectroscopic and electrochemical studies of metal complexation <1996J(P2)1587>. Solution electrochemical studies showed that metal complexation to the crown unit leads to a small anodic shift in the first oxidation potential of the TTF system. Langmuir-Blodgett films of amphiphilic 99 have been assembled on solid substrates by Y-type deposition. Compounds 100-104 were used to prepare self-assembled monolayers on gold and platinum surfaces <1998AM395, 2000JOC8269>. The self-assembled monolayers of 104 were the most stable in this series of TTF-crowns. Electrochemical data for the self-assembled monolayers of 100-104 in MeCN showed two reversible one-electron waves, typical of the TTF system. The self-assembled monolayers of 102-104 exhibited an electrochemical response in aqueous electrolyes, which was observed between 50 and 100 cycles. 

The desire of obtaining a better understanding of the relative reactivities of the nickel, palladium and platinum surfaces towards ethylene reactions and the dearth of theoretical studies on the adsorption ofC2H4 on the (100) surfaces of these metals, has served as motivation for our study of these adsorption systems. 

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