Oscillatory reactions and temperature: From bulk effects to the local monitoring

Author(s)

Alana Aragón Zülke

• Document type: Doctoral Thesis
• Press: São Carlos.
• Institution: Universidade de São Paulo (USP). Instituto de Química de São Carlos
• Defense date: 21 nov. 2017

Examining board members

Hamilton Brandão Varela de Albuquerque; Antonio Carlos Dias Angelo; Elton Fabiano Sitta; Edson Antonio Ticianelli

• Advisor: Hamilton Brandão Varela de Albuquerque

Abstract

The oscillating electro-oxidation of formic acid on polycrystalline platinum in acidic media, as a model system, was employed to investigate the temperature effects on the coupling of fast and slow dynamics processes belonging to its oscillatory dynamics, i.e. the core oscillator (fast dynamics) and the slow term deactivation of surface caused by the oxygen place-exchange process. Using a temperature range from 5 to 45°C, we observed two disparate regions of tendencies upon temperature increment on the galvanostatic oxidation. The system exhibits conventional Arrhenius behavior for T up to 25°C and, on the other hand, T > 25° revealed the occurrence of temperature (over)compensation. The system was characterized by means of conventional electrochemical techniques and electrochemical impedance spectroscopy. Clearly, we observed an inflexion point at 25°C marked by a break on the tendency of oscillatory frequency ( f, hopf, Sosc) and poisoning rates. Results were discussed in terms of the key role of PtO species, which chemically couple slow and fast dynamics. In summary we were able to: (i) identify the competition between two reaction steps as responsible for the two temperature domains; (ii) compare the relative activation energies of these two steps; and (iii) suggest the role of a given reaction step on the period-increasing set of reactions involved in the oscillatory dynamics. In addition, we performed experiments to monitor the local temperature of the interface during oscillatory dynamics. Two experimental strategies were applied: low cost thermometers-electrodes and a more sophisticated experimental set up based on pyroelectric detection. It should be noted that both configurations were able to monitor temperature differences during oscillatory dynamics (in the range of 0.1 ~ 0.5mK per cycle). The results obtained for the reconstruction of the heat fluxes (ø) corroborate with the current mechanistic model of the oscillating electro-oxidation of formic acid in Pt in acidic medium. We observed that the poisoning stages of the electrode are accompanied by the increase in ø while the reactivation of the electrode is accompanied by the decrease in ø. (AU)

• FAPESP's process: 13/06205-3 - Kinetic instabilities at the electrode/solution interface: measurement and variation of the surface temperature
• Grantee: Alana Aragon Zulke
• Support type: Scholarships in Brazil - Doctorate