The nature of the electro-catalytic response of mixed metal oxides: Pt- and Ru-doped SnO2 anodes

Abstract

[EN] The catalytic behavior of metal oxides for oxidative reactions is generally classified into active or non-active, depending on whether surface redox species participate or not. In the case of mixed metal oxides, however, this simplified scenario may be more complex. Non-active oxides containing electroactive metal species, like Pt- and/or Ru-doped SnO2 electrodes, are promising anode materials for the electrochemical treatment of waste-waters. This work analyzes the effect of Pt and Ru species on the nature of the electro-oxidative catalytic response of Ti/SnO2 anodes. For this purpose, the electro-oxidation of phenol and the competing oxygen evolution reaction (OER) in NaOH have been chosen as model reactions. The different electrodes and reactions were characterized by cyclic voltammetry, electro- chemical impedance spectroscopy, and Tafel measurements. The obtained results reveal that both Pt and Ru introduce solid-state redox processes and catalyze the OER and the phenol oxidation onto Ti/SnO2-based electrodes. Nevertheless, the dopants induce quite different active behaviors in the mixed oxides. Pt practically does not affect the OER mechanism, but enhances its kinetics, so its electrocatalytic activity is associated with a specific adsorption of hydroxyl anions or phenolate on Pt sites, without participation of the irreversible Pt/PtOx couple (i.e. a "non-redox-active" behavior). On the contrary, Ru species involve various and highly reversible redox processes that accelerate and modify the rate-determining step of the OER, and that actively mediate in the phenol oxidation.