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 ID: 781456
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Slide1
Slide2Slide31.5 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
Slide4a 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
Slide5Ox + 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
Slide6small
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
′
Slide7without
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
Slide81.5.2.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)
Slide91.5.2.1.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.
Slide101.6. 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)
Slide11weak 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
Slide12strong 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
Slide13A 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:
Slide14the 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
Slide15the 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:
Slide16the 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
Slide17