Platinum single crystals with oriented

A detailed comparison of the voltammetric behavior of platinum single crystal and faceted electrodes was also given. A mechanistic model for the electrochemical faceting of metals with development of preferred crystallographic orientations was proposed. ... 

The last topic to comment is the difference between the optimum compositions in Ru of the Pt-Ru alloys for the oxidation of CO (50 at.% Ru) and methanol (10-20 at.% Ru). Gasteiger et al. " proposed an explanation based on a statistical modelization of the surface of an alloy containing x at.% Ru. The stmctural atomic model proposed by these authors needs a maximum number of 3-fold platinum sites for the adsorption of the CH3OH molecule, and adjacent to 1 Ru atom to adsorb the OH species. This would correspond to a surface with 8-14 at.% Ru depending on the orientation of the low index single crystal planes, 8% for (111), 10% for (100)... 

While attempting to use platinum in fuel cells, it has been demonstrated that its surface exhibits important electrocatalytic activities toward the oxidation of organic compounds. However, this effect can sometimes be enhanced by the use of bimetallic surfaces [1-10]. The physical mixture and the electronic interaction of the alloy components lead to a modification in the interaction between the adsorbate and the substrate in an electrocatalytic reaction. As a consequence of the structural changes at the single crystal surfaces during the electrochemical activation (examined with in situ STM) [11], it has been demonstrated that most of the catalysts are constituted by randomly oriented islands [12-14]. 

Most of the work in the previous sections of this chapter has dealt with mercury electrodes for the reasons discussed in Section 13.2.1. However, electrochemists are also interested in studying the interfacial structure of solids, because most electrochemical studies are carried out with solid electrodes (e.g., platinum or carbon). Such studies are difficult, because there are problems in reproducing a surface and in keeping it clean. Impurities in solution can diffuse to the electrode surface and adsorb, thereby significantly changing the interfacial properties. Moreover, the surfaces of solids, unlike those of mercury, are not atomically smooth, but have defects, such as dislocation lines, with a density of at least 10 to 10 cm. In comparison, a typical metal surface density has about 10 atoms cm. Especially important to the understanding of solid electrodes has been the use of so-called well-defined metal electrodes, that is, single crystal metals with very carefully prepared surfaces of known orientation (35). 

Structure Sensitivity of Hydrocarbon Conversion Reactions on Platinum Surfaces How does the reaction rate depend on the atomic structure of the platinum catalyst surface To answer this question, reaction rate studies using flat, stepped, and kinked single-crystal surfaces with variable surface structure were very useful indeed. For the important aromatization reactions of n-hexane to benzene and Ai-heptane to toluene, it was discovered that the hexagonal platinum surface where each surface atom is surrounded by six nearest neighbors is three to seven times more active than the platinum surface with the square unit cell [155, 156]. Aromatization reaction rates increase further on stepped and kinked platinum surfaces. Maximum aromatization activity is achieved on stepped surfaces with terraces about five atoms wide with hexagonal orientation, as indicated by reaction rate studies over more than 10 different crystal surfaces with varied terrace orientation and step and kink concentrations (Figure 7.38). 

A different approach to the preparation of preferentially oriented surfaces on polycrystalline systems has been demonstrated by Sumino and Shibata. In References 57-59 it has been shown that a fast-cycling treatment is not required for obtaining deposits with preferential orientation. A platinum electrode with a single-crystal (100) surface was obtained by annealing a thin platinum film electrodeposited on a polycrystalline platinum foil. 

Actually, the results of a comparative study of the voltammetric behavior of NO3 ion at a platinized platinum electrode with (100) preferred orientation and at a polycrystalline one prove the higher catalytic activity of the former system over the latter one, as expected on the basis of single crystal studies. It should be mentioned that this kind of difference was not found in the reduction of CIO4. 

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