1.5 Consecutive Electron Transfer Processes - PowerPoint Presentation

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1.5 Consecutive Electron Transfer Processes

High intensity of the

return peak relative to the second reduction process originates from the fast reoxidation of the metallic copper deposited on the electrode surface during the Cu+/Cuo reduction

Anodic stripping

acetonitrile

a series of one-electron transfers, more or less separated from each

other

Shape of the cyclic voltammogram

2) separation between the respective standard potentials

1) degree of reversibility of the successive electron transfers

Ox + e- Red1

Red1 + e- Red2

E

1o′E2o′

1.5.1.Two reversible one-electron transfers

(EE)

Fig 31a

Fig 31b

Fig 31c

Fig 31d

Fig 31

small

Δ

Eo′ make the curve be distorted, in that the two peak-systems are ill-resolveddifficult determining of

Eo′2 and E

o′1Δ

E

o

′

is a

function of the difference between the potential of the most cathodic and the most anodic peaks

electronic interaction (through-bond or through-space)or electronic communication between the redox sites

Ox + Red

2

2Red

1

Comproportionation in EE :

Fig 32

Δ

E

o

′

without

communication

(fig 31c)kcom=4 , ΔEp= 60 , ΔEo

′=35.6 mVsimilar voltammogram with that of a reversible one-electron processHigher peak current than that of a one-electron process

With communicationΔEo′

increases and K

com

increases

the greater the distance

between redox centers,

the smaller

their interaction

delocalization

of the first added electron between the two

centers

makes

the addition of the successive electron electrostatically more

difficult

Δ

E

o

′

, K

com

increase

Amount of communication

1.5.2.Two one-electron transfers with different extents of reversibilityIn the case where not all the processes of a

consecutive electron transfer sequence are reversible, the irreversibility of a particular step becomes evident by the absence of the reverse peak in its pertinent response

(Fig 34)

1.5.2.1.Diagnostic

criteria for two-electron transfers with

different extents of reversibility

The voltammetric responses of EE processes can be qualitatively confused with processes complicated by coupled chemical reactions.The only distinctive criterion to define

these processes is the constancy of the parameter ip/v1/2 relative to each step with the scan rate.

1.6. Adsorption ProcessesThe transport of electro active species from the bulk of the solution tothe electrode may be governed not only by diffusion but also by

adsorption of the species on the electrode surfaceOx

(solution) Ox(adsorbed) Ox + e- RedRed(solution) Red(adsorbed)

weak adsorption

a significant increase in the height of the forward peak, when the reagent Ox is the adsorbed species(fig 35a)

a significant increase in the height of the reverse peak, when the product Red is the adsorbed species(fig 35b)weak adsorption of the reagent make ip/v1/2 increase (this is the only case in which such an effect is observed) under the effects of adsorption what remains constant with the

scan rate is ip/vfor weak adsorption of the product, increase in the height of the reverse peak with the scan rate makes the current ratio, ipr

/ipf , become greater than 1

Fig 35a – 35b

strong adsorption

pre- and post-peaks appear in the cyclic voltammetric response

a pre-peak appears when the adsorbed species is the product Red (fig 35d)a post-peak appears when the adsorbed species is the reagent Ox (fig 35c)The separation between the adsorption peak (pre- or post-peak) and the peak due to the diffusive electron transfer can be taken as a qualitative measure of the adsorption energy

The larger the separation, the stronger the adsorptionFig 35c – 35d

A simpler case occurs when only the adsorbed species on the electrode surface are electro active.

the species in solution Ox/Red are not electro active in the potential range consideredthe concentration of Ox in solution is so low (with respect to the amount

of Ox adsorbed) that a negligible diffusive current is producedincreasing the scan rate the intensity of the adsorption peak (which linearly increases with v) tends to become larger than that of the diffusive peak (which linearly increases with v1/2) that at high scan rates the diffusive peak may completely disappearDiffusion or adsorption?Such a situation may be found when:

the electron transfer of the adsorbed species is reversible:

the cyclic voltammetric response is quite sharp and the forward and reverse peaks are

completely symmetric (ΔEp ≈ 0)both the forward and reverse currents start at zero, reach a maximum value then return to zerothe adsorption phenomenon follows the so-called Langmuir isotherm

Fig 36

current intensity at the maximum of reverse peakquantity of adsorbed Ox at the electrode surface

the charge flown in the course of the reduction process of OX

(adsorbed)

the degree of absorption of Ox and Red

the width of the peak at half heightthe number of electrons spent in the process OX(adsorbed)/Red

(adsorbed)

Some quantities:

the reduction process of the adsorbed species is irreversiblethe peak shape is still quite sharp, however, not only the

reverse peak results are now absent, but the forward peak also loses itsSymmetry(Fig 37)

the curve will exhibit the reverse peak, but the peak system will not be completely symmetricthe reduction process of the adsorbed species is quasireversibleFig 37