The Nature of the Electro‐Oxidative Catalytic Response of Mixed Metal Oxides: Pt‐ and Ru‐Doped SnO₂ Anodes

Abstract

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 SnO 2 electrodes, are promising anode materials for the electrochemical treatment of wastewaters. This work analyzes the effect of Pt and Ru species on the nature of the electro-oxidative catalytic response of Ti/SnO 2 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 solidstate redox processes and catalyze the OER and the phenol oxidation onto Ti/SnO 2 -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.[a] Dr.

The generation of hydroxyl radicals and electro-oxidation of diclofenac on Pt-doped SnO2–Sb electrodes

“…In other words, the promotion and acceleration of OER and/or organic oxidation induced by Pt may come from destabilization of surface-• OHs, instead of a faster • OHs generation. This agrees with the true electrocatalytic effects observed for Pt doping on the OER and phenol electro-oxidation [35].…”

“…The effect of Pt on the electrocatalytic response of Ti/SnO 2 -Sb anodes found for diclofenac oxidation has many similarities with that previously observed for phenol [33][34][35]. Independently of the electrolyte, in both cases Pt improves the direct (non-• OHs-mediated) oxidation of the pollutant prior OER and the pollutant removal by galvanostatic treatment, at least up to 3 % Pt.…”

“…The incorporation of Pt into the SnO 2 -Sb anode decreases the rate constants for • OH generation (Table 1). This is a new interesting result that cannot be expected from the literature, since Pt doping was found to thermodynamically and kinetically promote the OER [35,36], and the electrocatalytic activity towards organics oxidation [33][34][35]. Considering the mechanism proposed by Comninellis et al [17], which regards the • OHs as reaction intermediates for both the OER and organic oxidation, and the changes on ݇ observed in this work, it is hypothesized that the presence of Pt may alter (weaken) the physisorption strength of • OHs (reduce the potential needed) to form O 2 or react with organic molecules, but not the rate of • OH formation (water discharge).…”

“…The composition and nomenclature of the studied electrodes was Ti/SnO 2 -Sb-Pt(x%), with Sb = 13 at% and x = 3 and 13 at% Pt (metal atomic composition); the Ti/SnO 2 -Sb electrode without Pt has been also studied. Details on the preparation and physico-chemical properties of this type of electrodes can be found in the literature [31,[34][35][36][37][38]. The Ti/Pt mesh (INAGASA S.A.) and BDD (Si/BDD, Adamant Technologies) plate electrodes were commercially supplied.…”

“…Then, considering the observed catalytic effects of Pt [33,34], a promotion of • OHs generation would be expected by incorporating Pt into the SnO 2 -Sb anodes. Nevertheless, in a recent study [35] the electrocatalytic effect of Pt has been associated to a strong specific adsorption of phenol on Pt/PtOx sites. Despite this controversy, the influence of Pt doping on the capability of SnO 2 -Sb anodes to produce • OHs has been never investigated.…”