Theory for Influence of the Metal Electrolyte Interface on Heterogeneous Electron Transfer Rate Constant: Fractional Electron Transferred Transition State Approach

Abstract

We develop a theory for outer sphere heterogeneous electron transfer (ET) rate constant (k 0 ) and exchange current density (i 0 ). The model hypothesizes that the transition state is attained by alignment of Fermi and reactant energy levels through exchange of fractional electronic charge (δ ≠ ). This approach accounts the contributions from: (i) work function and Fermi energy of metal, (ii) solvent polarity and size, (iii) electronic nature and size of electroactive species, and (iv) outer Helmholtz plane (OHP) potential-dependent composition. At the outset, we develop a model for the potential ϕ 1 at the inner Helmholtz plane accounting the influence of electronic and inner dipolar layer screening on the metal. The equation for ϕ 1 is used to obtain the potential ϕ 2 at OHP through a modified Gouy−Chapman−Stern approach. The concentration of electroactive species at OHP (c i OHP ) under the influence of the Frumkin effect is obtained by substituting ϕ 2 in Kornyshev's packing density restriction model. Our theory of the Frumkin effect highlights its dependence on metal, ionic strength, and applied potential. Further, free energy of activation (ΔG ≠ ) for the ET reaction is formulated as a product of δ ≠ and the work function of solvated metal. δ ≠ varies linearly with the energy of lowest unoccupied or highest occupied molecular orbitals of electroactive species and the work function of metal. The standard rate constant is obtained in terms of ΔG ≠ , and the exchange current density is expressed in terms of k 0 , c i OHP , and ϕ 2 . The theory unravels that a range of more than 10 orders of magnitude of kinetic reactivity is encompassed through 4−20% variation in δ ≠ . Finally, the theory captures the experimental data for different metals, solvents, supporting electrolytes, and electroactive species.

Hexadecyl trimethyl ammonium bromide‐modified 4‐mercaptopyridinde mixture for enhancement of corrosion resistance: Experimental and molecular dynamics simulation study

“…The inhibitor molecules adsorbed on the anodic and cathodic sites of the metal, the corrosion rate constant got decreased, and therefore corrosion reactions on these sites were impeded. 68,69 In fact, the contribution of the adsorbed inhibitor molecules to the hindering of the cathodic reactions are greater. The R ct values of the inhibited is bigger than that in the uninhibited and the inhibition efficiency increase with increasing of the inhibitor concentration (25-200 mg L −1 ).…”

“…This is beneficial to the coadoption of C 16 TAB and 4MP molecules, that is to say, the higher E HOMO value of 4MP is in favor of the bonding process through electron donating from inhibitor molecule to the metal surface and the lower E LUMO value of C 16 TAB is in favor of the bonding process through electron accepting from metal surface to the inhibitor molecule. The inhibitor molecules adsorbed on the anodic and cathodic sites of the metal, the corrosion rate constant got decreased, and therefore corrosion reactions on these sites were impeded 68,69 . In fact, the contribution of the adsorbed inhibitor molecules to the hindering of the cathodic reactions are greater.…”

Why halides enhance heterogeneous metal ion charge transfer reactions

“…Different mechanisms and explanations for anion enhancements may also be valid. Theories have been developed for how electrostatics can be used to explain outersphere electron transfer, 42 and how electrode modication can be used to enhance rates by changing the potential of zero charge. 43,44 Rate enhancements can also arise from a change in mechanism from outer-sphere to inner-sphere, as is reported to occur for Cr 2+ / Cr 3+ when halides are added to the electrolyte.…”

Theory for Potential of Zero Charge and Capacitance on Metals with Nanocorrugated Steps

“…20 Such surface local curvature fluctuations tailored the inhomogeneity in the electronic properties 21 and consequently the electronic WF and PZC. 15,22−25 The atomic scale nanostructuring-driven alteration in WF and PZC controls the interfacial electrontransfer phenomena 26 and the formation of an electric double layer. 27−29 The electronic WF represents the energy barrier to free space that prevents an electron at the Fermi level from escaping the solid, defined as 30,31 ϕ E 0 = E vac − E F , where E vac corresponds to the energy of electron at rest just outside the solid surface at the vacuum level and E F is the energy of Fermi level in eV.…”

“…Therefore, a generic, systematic, and the microscopic level theoretical approach for the local and average WF variation at the atomically stepped surface is required, which can be further linked to the experimentally observable physical quantities, viz., PZC 53−55 and the exchange current density. 26 An analytical expression of the electronic WF (ϕ E 0 ) in terms of electronic screening length and dielectric is defined as 33…”