The electrocatalytic behavior of "inactive" (Boron Doped Diamond (BDD), SnO 2 -Sb and PbO 2 ) anodes toward the oxygen evolution reaction (OER) is evaluated using dc and ac techniques, under controlled experimental conditions (e.g. air bubbling, ohmic drop correction). Tafel slopes estimated from anodic polarization curves for all catalysts are located above 100 mV dec −1 , suggesting that the rate-controlling step is similar for these materials at determined overpotentials. In agreement with the literature, it could be associated with the • OH generation since "active catalysts" displays slopes below 80 mV dec −1 . A transfer function model is derived to account for the kinetic parameters of the OER mechanism for each anode, throughout its fitting to experimental electrochemical impedance spectroscopy (EIS) spectra. The model considers the kinetics of each elementary reaction and the material balances for the rates of formation of the adsorbates (H 2 O ads , • OH ads , • O ads , • OOH ads , and O 2ads ) involved in the OER. It is found that the rate-controlling step on SnO 2 -Sb (≤2.14 V), PbO 2 (≤1.74 V) and BDD (2.54-2.71 V) is associated with the • OH formation, while this control is only modified when the potential becomes more positive for the oxide catalysts, thus, being determined by the production of adsorbed oxygen O 2ads which on its turn promotes the O 2 evolution. On the other hand, the rate-control step remains similar for BDD over the entire analyzed potential range, standing out its unique properties as inactive catalyst of the OER. In the last decades, the OER mechanism has turned out be of significant importance for environmental applications related to wastewater treatment, particularly those involving the destruction of toxic and/or recalcitrant organic compounds, using "inactive" catalysts.1-9 Under this context, active anodes contain transition elements on their surfaces with intermediate oxidations states, which enhance the OER and the partial oxidation of the contaminants; while inactive anodes favor the mineralization of the organic compounds throughout an indirect mechanism involving a strong oxidizing mediator (• OH radicals).10 Concerning inactivity, the boron doped diamond (BDD) is the state-of-the-art catalyst, since it possesses a unique role toward the formation of hydroxyl radicals.11-14 Nevertheless, overcoming its inherent high cost and manageability remain as main challenges to achieve an adequate operation at industrial levels. An intense research combining the best characteristics to electrochemically enhance • OH formation has resulted in the proposal of materials containing PbO 2 (β phase) and SnO 2 , with multiple variations during synthesis conditions. [1][2][3]12,[15][16][17] However, although the applications of these catalysts have been successful toward the electrocombustion of different organic compounds, their fundamental chemistry, rate-controlling steps and interactions occurring during the occurrence of• OH generation have not been fully understood. Most computa...

The oxidative processes of pyrite (FeS 2 ) and chalcopyrite (CuFeS 2 ) of interest for bioleaching and/or bioremediation applications are evaluated in growing medium conditions to account for differences in their reactions mechanisms proposed with chemical and electrochemical analysis, and their electronic structures calculated with density functional theory (DFT). Electrochemical (chronoamperometry, cyclic voltammetry), spectroscopic (Raman, XPS) and microscopic techniques (SEM-EDS, AFM) are used to comprehensively characterize complex surface transformations of secondary species arising during the electrochemical oxidation of these minerals. Early oxidation steps of both sulfides involve the formation of passive polysulfide species (e.g., Fe 1−x S 2 , Cu 1−x Fe 1−y S 2 ), with the additional formation of Covelite-(CuS)-like species on a more passive chalcopyrite surface. Subsequent stages indicate the formation of semiconductive compounds including elemental sulfur (S 0 ). DFT reveals that there are significant differences between pyrite and chalcopyrite densities of states (DOS), that support the fact that pyrite oxidation is more facile than chalcopyrite, as experimentally described. The DOS shows that near to the Fermi energy level of both sulfide minerals, there are few states that explain the oxidation limitations observed in the experimental region of low overpotential. At higher energies, the oxidation of pyrite is mainly due to iron species and sulfur species to a minor extent, while the chalcopyrite passivation is attributed to sulfur species and copper.

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