Kinetics at single crystal electrodes

Abstract

In this chapter, an approach to kinetic studies on single crystal platinum electrodes is attempted. The selected reactions are those called structure-sensitive, involving chemisorption steps of reactants and/or intermediates that clearly reflect site-dependent adsorption energies. For this reason, it is important to define the type of sites involved in the reaction and describe how it is possible to characterize them under electrochemical conditions. The reactivity of the different surfaces is tested against a classical probe reaction: the CO stripping in acidic and alkaline solutions. The changes observed when dissolved CO is also present are also shown. The oxidation of formic acid is then discussed, taking into account the existence of a dual-path mechanism that leads to surfaces poisoned by CO. In order to extract relevant kinetic information, the two reactions should be separated, in such a way that the experimental current could be safely assigned to the active intermediate route alone. This is discussed for different orientations and different acidic solutions. The more complex case of ethanol oxidation, which also involves CO poisoning as a result of the C-C bond breaking, a reaction step that is sensitive to the surface structure, anion adsorption and pH, is briefly described. Finally, ammonia oxidation in alkaline solution, an extreme electrocatalytic reaction that only occurs at surfaces having platinum sites with square symmetry, is addressed.