Instabilidades em eletrocatálise: cinética complexa catalisada por superfícies modelo

Abstract

The electro-oxidation of small molecules is an area of great interest due to its application in energy converting systems, such as fuel cells. The reactions present in these systems involve several reaction pathways - which are dependent on factors such as applied potential and surface conditions, with the possibility of developing instabilities. These systems represent an excellent example of dynamic self-organization and recently, the instabilities, particularly in the form of potential or current oscillations, were used to calculate parameters such as the apparent activation energy. The present work deals with the comparative study of dynamic instabilities during the electro-oxidation of ethylene glycol (EG) in Pt(111) and Pt(110) model surfaces, as well as polycrystalline Pt in alkaline medium. For that, the EG concentration were varied between 25 and 75 mmol L-1 and the temperature between 5 and 20 °C for kinetic studies. The experiments revealed that catalytic activity follows the sequence: Pt(111) >> Pt(110)> Pt(poly). When the reaction is catalyzed by Pt(110), it does not present dynamic instabilities in the studied conditions, however such oscillations are observed when Pt(110) is replaced by Pt(111) or Pt(poly). While in the first one it was observed only period 1 oscillations, the second, more complex oscillations, starting from period 1 and evolving to the mixed mode oscillations. Regardless the system, the temperature compensation phenomenon for oscillations was observed, this is, frequency invariance in the studied temperature range. Finally, the cations of the electrolyte have an important role in the reaction catalyzed by Pt (111), where the Li +, Na + and K + sequence causes a decrease of the oscillatory region until its total extinction in the latter case.