| Redefining the Electrochemical Kinetics of Redox Reactions on Passive Surfaces Digby Macdonald1; 1UNIVERSITY OF CALIFORNIA AT BERKELEY, Berkeley, United States; PAPER: 381/Corrosion/Regular (Oral) SCHEDULED: 14:00/Mon. 28 Nov. 2022/Andaman 2 ABSTRACT: The mechanisms for Redox reactions (R O + ne-) occurring on metal substrates generally assume a bare metal surface but bare metal surfaces only exist on the platinum group metals and then only at sufficiently negative potentials (e.g., for Pt < -0.15 VSHE) that a barrier oxide layer cannot form, even as a metastable phase. At higher potentials, the surfaces are invariably covered with a point-defective, barrier oxide layer (bl) upon which may exist a generally non-point defective, precipitated outer layer [e.g., Pt/PtO/Pt(OH)2. For Pt, the barrier layer is found to be a n-type semi-conductor having the general formula Pt1+xO1-y, where positive values of x, y < 0.1 indicate the presence of metal interstitials (〖Pt〗_i^(2+)) and oxygen vacancies (V_O^(∙∙)), both of which are electron donors. No hint of p-type behavior was observed, demonstrating the metal vacancy (V_Pt^2') is a minority species. The surface vacancies have been postulated to act as reaction sites at the barrier layer/solution interface and the surface oxygen vacancy has been postulated as providing the adsorption sites for the oxygen electrode reaction (OER) on passive titanium. This invariably links the reaction mechanism of the redox reaction to the defect structure of the substate bl oxide. However, the bl also represents a barrier to electron transfer. For sufficiently thin bl oxides (< 3 nm), charge carrier transfer between the metal and the redox reaction site that occurs at the bl/solution interface is postulated to occur by direct (resonant) quantum-mechanical tunneling but for thicker bls (> 5 nm), the oxygen vacancy, which is present in all oxide films that grow into the metal, acts as a quantum well having quantized energy levels. In these cases, tunneling between neighboring oxygen vacancies is indirect and because of the presence of a high electric field (commonly (1-3)x106 V/cm precludes occupied/empty energy levels between neighboring vacancies of the same energy noting that tunneling is an isoenergetic process. Accordingly, tunneling is envisioned to involve “virtual” states in which the tunneled electron temporarily reside as it loses energy to the lattice and relax to a quantized state in a neighboring vacancy that in turn is isoenergetic with the next virtual state. In essence, this is a “hopping” mechanism in which the hopping sites are alternating oxygen vacancies and the “virtual states”. Clearly, the probability of quantum-mechanically mediated hopping between an oxygen vacancy and the virtual state that is at the same location as the neighboring oxygen vacancy is a very sensitive function of the concentration of oxygen vacancies in the bl as indicated by the expression for the probability of transfer, Pr~exp(-β ̂L), where β ̂ is the tunneling constant, and L is the tunneling distance. The theory of the electrochemical kinetics of redox reactions has been modified to accommodate reaction on passive surfaces and to involveme surface defects (primarily oxygen vacancies) in the reaction mechanism. This requires to use of the Point Defect Model (PDM) to calculate the concentration of the appropriate vacancy at the bl/s interface and to estimate the thickness of the bl as a function of voltage, which are then used to modify the Generalized Butler-Volver equation, to define the exchange current density in terms of the standard exchange current density, and to define the standard state as the hypothetical bare metal surface. The application of this revised theory for redox reactions on passive surfaces is illustrated by the electrochemistry of carbon steel in hydrogenated alkaline solutions. |
