Platinum electrodes surface structures

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. 

In order to assess the role of the platinum surface structure and of CO surface mobility on the oxidation kinetics of adsorbed CO, we carried out chronoamperometry experiments on a series of stepped platinum electrodes of [n(l 11) x (110)] orientation [Lebedeva et al., 2002c]. If the (110) steps act as active sites for CO oxidation because they adsorb OH at a lower potential than the (111) terrace sites, one would expect that for sufficiently wide terraces and sufficiently slow CO diffusion, the chronoamperometric transient would display a CottreU-hke tailing for longer times owing to slow diffusion of CO from the terrace to the active step site. The mathematical treatment supporting this conclusion was given in Koper et al. [2002]. 

The potential of the stripping peak, and hence the activity of the electrode for CO oxidation, also depends on the platinum surface structure in general and on the step density in particular. Based on the chronoamperometry experiments described in Section 6.2.1.1, one would expect the stripping peak to shift to lower potential with increasing step density. That this is indeed the case is shown in Fig. 6.6. Again, this... 

Wang H, Baltruschat H. 2007. DEMS study on methanol oxidation at poly- and monocrystalline platinum electrodes The effect of anion, temperature, surface structure, Ru adatom, and potential. J Phys Chem C 111 7038-7048. 

Beden B, Juanto S, Leger JM, Lamy C. 1987. Infrared spectroscopy of the methanol adsorbates at a platinum electrode Part III. Structural effects and behaviour of the polycrystalline surface. J Electroanal Chem 238 323-331. 

We and others have been involved in the study of such systems including Cu/Au(lll),85 86 Ag/Au(lll),87 Pb/Ag(lll),88 and Cu/Pt(lll).89 The first three systems involved the use of epitaxially deposited metal films on mica as electrodes.90 92 Such deposition gives rise to electrodes with well-defined single-crystalline structures. In the last case a bulk platinum single crystal was employed. Because of the single-crystalline nature of the electrodes, polarization dependence studies could be used to ascertain surface structure. 

The processes classified in the third group are of primary importance in elucidating the significance of electric variables in electrosorption and in the double layer structure at solid electrodes. These processes encompass interactions of ionic components of supporting electrolytes with electrode surfaces and adsorption of some organic molecules such as saturated carboxylic acids and their derivatives (except for formic acid). The species that are concerned here are weakly adsorbed on platinum and rhodium electrodes and their heat of adsorption is well below 20 kcal/mole (25). Due to the reversibility and significant mobility of such weakly adsorbed ions or molecules, the application of the i n situ methods for the surface concentration measurements is more appropriate than that of the vacuum... 

As mentioned above, contradicting results were produced between smooth and dispersed platinum electrodes. It is no doubt that the surface structures play an important role in the reaction mechanism and catal rtic... 

The structure of a polycrystalline electrode depends on its preparation. Usually toe rough electrodes are prepared by electrochemical deposition of a given metal onto a suitable substrate. Microcrystals present in polycrystaUine samples are randomly oriented on the surface. Most likely, not only basal but also higher MiUer-index planes should be considered in anticipating toe final structure of the electrode surface. It was shown that the stmcture of the platinized platinum surface depends strongly on toe platinization conditions, e.g., on toe concentration of the platinization... 

Platinum electrodes are widely used as an inert electrode in redox reactions because the metal is most stable in aqueous and nonaqueous solutions in the absence of complexing agents, as well as because of its electrocatalytic activity. The inertness of the metal does not mean that no surface layers are formed. The true doublelayer (ideal polarized electrode) behavior is limited to ca. 200-300 mV potential interval depending on the crystal structure and the actual state of the metal surface, while at low and high potentials, hydrogen and oxygen adsorption (oxide formation) respectively, occur. 

Effect of Adsorption and Surface Structure on the Voltammetric Behavior of Platinum Electrodes... 

A fast repetitive triangular potential sweep (lO" V s between 0.05 and 1.5 V for 2 h) was applied to platinized platinum electrode [217]. This treatment resulted in a considerable decrease in the real surface area and in the formation of a (lOO)-type preferentially oriented surface structure. 

The similarity of the shape of the voltammetric curves of platinized system to those reported for electrodes treated by the fast repetitive potential perturbation technique or obtained by annealing electrodeposited platinum and the appearance of signs characteristic for stepped surfaces allows us to assume that these phenomena should be ascribed to the similarities in the surface structure. 

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