Platinum surfaces evidence

BaldeUi S, Markovic NM, Ross PN, Shen YR, Somoijai GA. 1999. Sum frequency generation of CO on (111) and polycrystalUne platinum electrode surfaces Evidence for SFG invisible surface CO. J Phys Chem B 103 8920-8925. 

Chemisorption of oxygen at Pt(lll) has been studied in detail by Ertl s group25 and the STM evidence is for complex structural features present in the temperature range 54M60K (Figure 4.14). The limitations of the Langmuir model, frequently invoked for reactions at platinum surfaces, is obvious from... 

Figure 3.35 shows the potential dependence of the integrated band intensity of the linear CO observed in the experiment described above and the corresponding variation in the methanol oxidation current. The latter was monitored as a function of potential after the chemisorption of methanol under identical conditions to those employed in the IRRAS experiments. As can be seen from the figure the oxidation of the C=Oads layer starts at c. 0.5 V and the platinum surface is free from the CO by c. 0.65 V. The methanol oxidation current shows a corresponding variation with potential, increasingly sharply as soon as the CO is removed strong evidence in support of the hypothesis that the adsorbed CO layer established at 0.4 V acts as a catalytic poison for the electro-oxidation of methanol. 

A direct evidence of the way of tetrahedral anion adsorption at three-fold sites and the degree of hydratation is not available at present. However, a strong indication of such adsorption of sulphates is found in voltammetry on gold (14) and in our data for platinum surfaces (12). A pronounced difference between the sulphate and perchlorate adsorption effects is... 

A striking feature of the template model is the restriction of the role of the modifier to that of a template, which does not take into account direct binding interactions of the reactant with the modifier. Furthermore, there exists no experimental evidences for the formation of ordered arrays of cinchona molecules on a platinum surface. In 1995, Margitfalvi and Hegedus [235] criticized this model showing that the model is too idealistic and oversimplified. 

The paraffins adsorb with their chain axis parallel to the platinum substrate. Thus their surface unit cell increases smoothly with increasing chain length as shown in Fig. 5.3. The n-butane molecules, unlike the larger molecules, form several monolayer surface structures as the experimental conditions are varied. It appears that the smaller the paraffin the more densely packed it is on the surface. Evidently, as the packing becomes too dense for n-butane in one surface structure it forms a different one. 

Immeasurable ions are formed when hydrocarbons in an air stream are oxidized in the presence of a heated platinum filament (3,10). The experimental evidence suggests that these ions are produced by the chemical oxidation reactions catalyzed by the platinum surface (10). That this process is one of chemionization is supported by the fact that the extent of ionization observed depended greatly on the molecular structure of the particular hydrocarbon being oxidized. This is in contrast to the ionization measured in flames, where the extent of ionization for saturated hydrocarbons, for example, does not appear to depend in any way on the molecular structure but only on the number of carbon atoms (9). 

That oxygen is present on a platinum surface exposed to gaseous oxygen is well known hence the presence of the monatomic layer on the electrode is practically certain. The evidence as to the presence of a layer of hydrogen, on the surface evolving hydrogen, is less complete, but it is probably present. 

Evidence has long accumulated to show that surfaces of supposedly inert metals readily undergo oxidation. For example, Hickling was able to achieve a cyclic oxidation and reduction of a platinum surface, which could be repeated indefinitely without permanent changes in the metal. (See Figure 14-4.) Kolthoff and Tanaka showed that the oxidation could be carried out either electrochemically or chemically with such strong oxidants as dichromate, Ce(IV), and permanganate. 

Penetration of the acid through the recast film all the way to the platinum surface was supported by voltammetric evidence and was apparently assisted by the potential multicycling routine employed to maintain an impurity-free platinum surface [1]. Such a study of a filmed platinum electrode immersed in aqueous acid solution is, therefore, less than perfect for providing good data on the rate of ORR at the interface between platinum and hydrated ionomer in a fuel cell cathode, where the only interfacial liquid is distilled water. 

R. Zaera, D. Goodbey, and G.A. Somoiiai, Methylcyclopentane Conversion Over Platinum Single Crystal Surfaces Evidence for the Cyclic Mechanism of n-Hexane Isomerization, J. Catal., 101 (1986) 73. 

There is clear evidence that adsorbate and alloyed metal atoms on platinum surface promote CO electro-oxidation. The reduced overpotential is primarily a result of the promotion of the activation of water. The subsequent kinetics are determined by the details of a Langmuir-Hinshelwood reaction between the adsorbed oxidant (OH) and adsorbed CO. Evidence is also presented that relates this promotion (or poisoning) of CO electro-oxidation to tolerate CO in hydrogen feeds in the hydrogen electro-oxidation reaction. An alternative mechanism that may operate at low potentials [79,113] may be that the reduction in CO adsorption energy on platinum induced by Ru [86,113,114] results in a higher equilibrium concentration of nonpoisoned sites. The relative importance of these mechanisms is a function... 

NH3/O2 reactions should be studied at low pressures on both clean and contaminated platinum surfaces to determine whether the relative reactivities at 1100 °C are in agreement with the hypothesis. Such experiments would also help to determine whether the methane reacts wholly at the surface or not. The balance of evidence reviewed above does slightly favour the reaction of methane solely at the surface, but definitive experiments are desirable on this point. 

Since Steps (32)—(34) are in quasi-equilibrium and follow the rds. they are not proven mechanistic paths. However, evidence of H2O2 in solution (164-166) and the presence of surface [OH] discussed earlier support this mechanism. Kinetic evidence is consistent with the above mechanism, assuming Temkin adsorption on oxygen-free platinum surfaces at low potentials, < 1 V (77). Similar steps can be written for alkaline electrolytes, with reactions (29), (31), (32), or (34) involving HjO instead of H. ... 

Different voltammograms were recorded at various concentrations of metal adatoms (lead, bismuth, and thallium) in the presence of 10 mM lactose in 0.1 M Na2C03-NaHC03- Their optimized concentrations gave evidences of the electrocatalytic effect by the ratio / t rvi//pt for the current densities, with and without adatoms, versus electrode potential (Figure 21.22). In fact, the presence of metal adatoms at the platinum surface induces a shift of the lactose oxidation peaks toward lower potential and an increase in the current densities. 

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