Platinum electrodes, adsorption

This rather simple experiment demonstrates quite clearly the poisoning of the oxygen reduction process by copper. Incidentally, Cu(II) is a rather common impurity in distilled water and mineral acids, and this experiment demonstrates that underpotential deposition of a monolayer of copper from solutions containing as little as 1 ppm Cu can drastically affect the behavior of a platinum electrode. Adsorption of small amounts of other impurities (i.e., organic molecules) can also have an effect on solid-electrode behavior. Thus, electrochemical experiments often require making great efforts to establish and maintain solution purity. 

From the results obtained with in situ reflectance spectroscopy and on-line analytical methods, investigators at Universite de Poitiers proposed a complete mechanism for the electrooxidation of methanol at a platinum electrode. The first step of the electrooxidation reaction is the dissociative adsorption of methanol, leading to several species according to the following equations ... 

Adsorption due to the operation of chemical forces is called chemisorption. A typical example of chemisorption is also the adsorption of methanol on platinum electrodes, which is accompanied by a deep destruction of the methanol molecule. 

Adsorption of Reaction Components In many cases, adsorption of a reactant is one of the hrst steps in the electrochemical reaction, and precedes charge transfer and/or other steps of the reaction. In many cases, intermediate reaction products are also adsorbed on the electrode s snrface. Equally, the adsorption of reaction products is possible. The example of the adsorption of molecular hydrogen on platinum had been given earlier. Hydrogen adsorption is possible on the platinum electrode in aqueons solntions even when there is no molecular hydrogen in the initial system at potentials more negative than 0.3 V (RHE), the electrochemical reaction... 

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. 

On the surface of metal electrodes, one also hnds almost always some kind or other of adsorbed oxygen or phase oxide layer produced by interaction with the surrounding air (air-oxidized electrodes). The adsorption of foreign matter on an electrode surface as a rule leads to a lower catalytic activity. In some cases this effect may be very pronounced. For instance, the adsorption of mercury ions, arsenic compounds, or carbon monoxide on platinum electrodes leads to a strong decrease (and sometimes total suppression) of their catalytic activity toward many reactions. These substances then are spoken of as catalyst poisons. The reasons for retardation of a reaction by such poisons most often reside in an adsorptive displacement of the reaction components from the electrode surface by adsorption of the foreign species. 

Nakabayashi, S., Sugiyama, N., Yagi, 1. and Uosaki, K. (1996) Dissociative adsorption dynamics of formaldehyde on a platinum electrode surface onedimensional domino Chem. Phys., 205, 269-275. 

We have also discussed two applications of the extended ab initio atomistic thermodynamics approach. The first example is the potential-induced lifting of Au(lOO) surface reconstmction, where we have focused on the electronic effects arising from the potential-dependent surface excess charge. We have found that these are already sufficient to cause lifting of the Au(lOO) surface reconstruction, but contributions from specific electrolyte ion adsorption might also play a role. With the second example, the electro-oxidation of a platinum electrode, we have discussed a system where specific adsorption on the surface changes the surface structure and composition as the electrode potential is varied. 

Claviher J, Orts JM, Gomez R, Pehn JM, Aldaz A. 1996. Comparison of electrosorption at activated polycrystaUine and Pt(531) kinked platinum electrodes. Surface voltammetry and charge displacement on potentiostatic CO adsorption. J Electroanal Chem 404 281-289. 

Lebedeva NP, Rodes A, Feliu JM, Koper MTM, van Santen RA. 2002b. Role of crystalline defects in electrocatalysis CO adsorption and oxidation on stepped platinum electrodes as studied by in situ infrared spectroscopy. J Phys Chem B 106 9863-9872. 

Smith PE, Ben-Dor KF, Abruna HD. 2000. Poison formation upon the dissociative adsorption of formic acid on bismuth-modified stepped platinum electrodes. Langmuir 16 787-794. 

Clavilier J, Feliu JM, Aldaz A. 1988. An irreversible structure sensitive adsorption step in bismuth underpotential deposition at platinum electrodes. J Electroanal Chem 243 419-433. 

Perez JM, Beden B, Hahn F, Aldaz A, Lamy C. 1989. In situ infrared reflectance spectroscopic study of the early stages of ethanol adsorption at a platinum electrode in acid medium. J Electroanal Chem 262 251-261. 

Kunimatsu K, Golden WG, Seki H, Philpott MR. 1985a. Carbon monoxide adsorption on a platinum electrode studied by polarization modulated FT-IRRAS. 1. Co Adsorbed in the double-layer potential region and its oxidation in acids. Langmuir 1 245 -250. 

Loucka T, Weber J. 1968. Adsorption and oxidation of formaldehyde at the platinum electrode in acid solutions. J Electroanal Chem 21 329-344. 

Perez JM, Munoz E, Moralldn E, Cases F, Vazquez JL, Aldaz A. 1994. Formation of CO during adsorption on platinum electrodes of methanol, formaldehyde, ethanol and acetaldehyde in carbonate medium. J Electroanal Chem 368 285-291. 

Biegler T, Koch DFA. 1967. Adsorption and oxidation of methanol on a platinum electrode. J Electrochem Soc 114 904-909. 

Cyclic voltammetry studies of single-crystal platinum electrodes in acidic aqueous electrolytes showed that the two characteristic peaks of hydrogen adsorption/desorption on platinum (see Fig. 5.40) correspond in fact to reactions at two different crystal faces the peak at lower potential to Pt(100) and the other one to Pt(lll). 

Electrode processes can be retarded (i.e. their overpotential is increased) by the adsorption of the components of the electrolysed solution, of the products of the actual electrode reaction and of other substances formed at the electrode. Figure 5.43 depicts the effect of the adsorption of methanol on the adsorption of hydrogen at a platinum electrode (see page 353). 

The dependence of the relative coverage of the platinum electrode with methanol on its concentration in solution indicates that the adsorption of methanol obeys the Temkin isotherm (4.3.46). 

Large platinum carbonyl clusters have been investigated as models for the adsorption of carbon monoxide on platinum surfaces and on platinum electrodes. An issue is how large the clusters must be before they adopt the properties of the bulk metal. Teo et al. have investigated the magnetic properties of the clusters [Pt6(CO)12]2+, [Pt9(CO)18]2+, [Pt y(CO)22f+, and... 

The authors found that as CO is more strongly adsorbed than hydrogen, the introduction of CO into the electrochemical cell was immediately accompanied by its adsorption at the platinum electrode. This was shown by the decrease in the charge under the hydride udsorption features, and ulso the appearance of the oxidative stripping peaks 1 and II. 

Figure 2.19 Linear sweep voltammograms of a platinum electrode immersed in N -saturated 0.5 M H2SO t showing the anodic stripping of adsorbed CO. The CO was adsorbed from the CO-saturated electrolyte for 10 minutes at the designated potential. The scan rate was 1 mV s The adsorption potential was (a) 0.05 V and (b) 0.4 V vs. NHE. Note the different electrode potential scales for the two plots. From Kunimatsu et at. (1986).

IRRAS spectrum collected at 0.4 V. In this way the build-up of C=Onds with adsorption time was monitored, as shown in Figure 3.36(b). From Figures 3.36(a) and (b) it is clear that the deactivation of the platinum electrode is closely related to the increase in C=Oads. 

Yoshida has studied anodic oxidations in methanol containing cyanide to elucidate the electrode processes themselves.288 He finds that, under controlled potential ( 1.2 V), 2,5-dimethylfuran gives a methoxynitrile as well as a dimethoxy compound (Scheme 57). Cyanide competes for the primary cation radical but not for the secondary cations so that the product always contains at least one methoxy group. On a platinum electrode the cis-trans ratio in the methoxynitrile fraction is affected by the substrate concentration and by the addition of aromatic substances suggesting that adsorption on the electrode helps determine the stereochemistry. On a vitreous carbon electrode, which does not strongly adsorb aromatic species, the ratio always approaches the equilibrium value. 

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