1.4.3. A Chemical Reaction Interposed Between Two Electron Transfers ECE the number of electrons exchanged in the two electron transfers; n2/n1 the degree of reversibility of the two electron transfers the value of the equilibrium constant of the interposed chemical reaction (K = kf/kr) the difference between the redox potential of Ox/Red and Ox’/Red’ Ox + n1e- Red Red Ox’ Ox’ + n2e- Red’ kf kr

ErCiEr ΔEo‘ negative ΔEo‘ = 0 ΔEo‘ positive 1.4.3.1. An ‘irreversible’ chemical reaction interposed between two reversible one-electron transfers (R-R) E1o‘ Ox + e- Red Red Ox’ Ox’ + e- Red’ kf The response depends on the separation between E1o‘ and E2o‘ (ΔEo‘) Ox′ is more difficult to reduce than Ox ΔEo‘ negative Ox′ is reduced at the same potential as Ox ΔEo‘ = 0 Ox′ is easier to reduce than Ox ΔEo‘ positive E2o‘ ErCiEr

kf is low kf<< n F v/R T 1.4.3.1.1 .Standard potential of the second electron transfer more cathodic than that of the first electron transfer (ΔEo' negative). kf is low kf<< n F v/R T response will be due to the first electron transfer process (fig23 – solid line) (-) kf/(nFv/RT)=0.05 (--) kf/(nFv/RT)=10 kf is large kf>> n F v/R T response will display two forward peaks, but only the second electron transfer will exhibit a return peak(fig 23 – dashed line) peak I shifts towards more anodic values and increases in height, concomitantly with the appearance of a second peak (II)

Evaluation of kf : If kf is large the ratio of the currents of the two forward peaks is directly related to the rate of the interposed chemical reaction. This enables kf to be determined using the working curve (Fig 24) 1

if the redox couples Ox/Red and Ox′/Red′ have sufficiently different standard potentials using the working curve reported in Fig 16(First-order irreversible chemical reaction following a reversible electron transfer (ErCi) -Section 1.4.2.2) measuring only ipr/ipf of the first couple return peak must be recorded before the second process begins to appear(reversing potential scan immediately after having traversed the first forward peak.) 2

1.4.3.1.2. Standard potential of the second electron transfer equal to that of the first electron transfer (ΔEo‘ = 0) A single forward peak associated with a single reverse peak If kf is large, ipf is two times greater than ipr It is not possible to obtain information on the interposed chemical reaction

1.4.3.1.3 Standard potential of the second electron transfer more anodic than that of the first electron transfer (ΔEo’ positive). If kf is small, only the first electron transfer will occur if kf is large both the electron transfers will take place if kf ≈nFv/RT , fig 25 couple Ox/Red couple ′Red′/Ox

For peak I , comparing ik (under ECE conditions is due to Ox and in part to Ox′) at a given scan rate with id (in the absence of chemical complications, hence due to only Ox – high scan rate) Evaluation of kf evaluating ip(II)/ip(I) at a given scan rate and determining kf from the working curve (Fig 16 - an irreversible chemical reaction following a reversible electron transfer-Section 1.4.2.2)

I .4.3.2. An ‘irreversible’ chemical reaction interposed between a reversible and an irreversible electron transfer (R-I) Ox + e- Red Red Ox′ Ox′ + e- Red′ kf Analogous to ErCiEr , except that the return peak is absent Evaluation of kf : determining ipr/ipf of the first reversible couple Ox/Red and approximating the process to “a reversible electron transfer followed by a first-order irreversible chemical reaction”, as R-R case. ErCiEi

1.4.3.3.An ‘irreversible’ chemical reaction interposed between an irreversible and a reversible electron transfers (I-R) Ox + e- Red Red Ox′ Ox′ + e- Red′ kf EiCiEr analogous to the case of ErCiEr except that the return peak of the first electron transfer is absent There are no simple ways to calculate the kinetic parameter kf

I .4.3.4 An ‘irreversible’ chemical reaction interposed between two irreversible electron transfers (I-I) Ox + e- Red Red Ox′ Ox′ + e- Red′ kf Analogous to the case of ErCiEr except that both the return peaks are absent There are no simple ways to calculate the kinetic parameter kf EiCiEi

1.4.3.5 Diagnostic criteria for ECE type processes difficult to assign criteria to identify every type of ECE mechanism(R-R, R-I, I-R, I-I) and for every value of ΔEo′ calculate the current function iP /v1/2 for every peak (both forward and reverse) and compare with figures 26, 27 and 28 (qualitative) not necessary to determine the trends of all the forward and reverse peaks, but only those that one judges to be the most significant

Figure 26 2 more cathodic than 1

Figure 27

Figure 28 2 more anodic than 1

ECE 1.4.4.Electrocatalysis Redox catalysis lowering the kinetic barrier of the reduction (or the oxidation) process of a species, which thermodynamically is little inclined to be reduced (or oxidized), by use of a redox mediator(carrying electrons towards (or away from) the low redox-active original species) applying a potential to the electrode to reduce the catalyst (redox mediator) upon contact with Ox, Ox is reduced to Red and the mediator reoxidized continuous cathodic reduction of the catalyst reactivates the catalytic cycle Redox catalysis

electron-transfer chain (ETC) catalytic process catalysis of a reaction triggered by electrons (through a minimal quantity of an oxidizing or reducing agent) without the occurrence of an overall change in the oxidation state of the reagent A→B is slow A in anionic radical form A- (or A+) rapidly converts to the radical anion B- in turn react rapidly with A to form B, so closing the reaction cycle A→A- (chemically and electrochemically) electron-transfer chain (ETC) catalytic process

cyclic voltammetry Example: ligand exchange in metal complexes first indication of the possible existence of an electrocatalytic process cyclic voltammetry Example: ligand exchange in metal complexes 29a - a cathodic response consistent with a reversible one-electron process 29b - first cathodic response reduces in intensity, opening the way for a new process at lower potential in the presence of L2 M(L')n

controlled potential electrolysis existence of an electrocatalytic ligand exchange In the absence of L2 In the presence of L2 the electrolysis at about -0.7 V (10 min) the consumption of one-electron per molecule if the electrocatalytic process takes place, the electrolysis current decays rapidly (a few Seconds) consuming a fraction of one-electron per molecule cyclic voltammogram of resulting solution only the cathodic peak-system at Eo' = -0.80 V

(coulombic efficiency) number of product molecules formed per electron consumed catalytic efficiency (coulombic efficiency) variation of the free energy of the overall process (negative) driving force EoB : standard potential B/B- EoA : standard potential A/A- e2/E0.r : coulombic energy term (can be neglected)