Probing the Fen+/Fe(n−1)+ redox potential of Fe phthalocyanines and Fe porphyrins as a reactivity descriptor in the electrochemical oxidation of cysteamine

Abstract

The M n + /M (n − 1)+ redox potential of MN4 macrocyclic molecular catalysts is a very good reactivity descriptor for several electrochemical reactions. One important feature about this reactivity descriptor is that it can be determined experimentally under the same conditions of the kinetic measurements in contrast to other descriptors like intermediate binding energies that are estimated from DFT calculations. However a linear correlation between both descriptors seems to exist. Plots of activity as (logj) E at constant E versus the M n + /M (n − 1)+ redox potential gives volcano correlations. Another important aspect about this parameter is that it is possible to tune the M n + /M (n − 1)+ redox potential of the MN4 catalyst by manipulating the structure of the macrocyclic complex and tailoring the electron-withdrawing power of the ligands to obtain the maximum activity. In this work we have probed the redox potential as a reactivity descriptor for the oxidation of cysteamine studying a series of substituted Fe phthalocyanines and Fe porphyrins adsorbed on glassy carbon and pyrolytic graphite in alkaline media. As expected the catalytic activity of these FeN4 species varies strongly with the Fe (II)/(I) redox potential of the different Fe phthalocyanines and a plot of activity as (logj) E versus E°Fe(II/I) gives a volcanoshaped correlation so a formal potential value exists for which the highest activity can be achieved demonstrating that the formal potential of the complexes seems to be an universal reactivity descriptor for electrochemical reactions.

Atomically dispersed Fe 3+ sites catalyze efficient CO 2 electroreduction to CO

“…The pyrrolic N ligands may stabilize Fe 3+ relative to Fe 2+ , whereas the pyridinic N ligands have the opposite effect. Thermodynamically, the respective reduction potentials support this hypothesis: The standard reduction potential of [Fe(phen) 3 ] 3+/2+ is 1.06 V versus SHE, whereas that of Fe 3+/2+ couple in Fe-porphyrin complexes can reach as low as -0.4 V versus SHE (31). Preservation of the +3 oxidation state during CO 2 electroreduction is counterintuitive because the formation of highly reduced, low-valent Fe species is necessary for molecular Fe catalysts (32).…”

Redox Potentials as Reactivity Descriptors in Electrochemistry

“…Equation (26) explains the linear correlation in the region of strong adsorption (ΔG°O 2 is negative) and is essentially independent of the concentration of the reactant, in this case, O 2 .…”

“…The surface coverage of active sites varies as 0 < θ M(II) < 1 depending on the electrode potential. Figure 10 shows a simulation of the variation of θ M(II) with potential using the Nernst equation for adsorbed species [26].…”

“…So, Eqs. (26) and (27) describe both linear correlations in the volcano plots with slopes of different signs, i.e.+β'/RT for the strong adsorption region and Àβ'ΔG°O 2 /RT for the weak adsorption region. According to this, the volcano plot should be symmetrical since both legs of the volcano have the same absolute value of the slope.…”

“…possibly for other reactions [12,13,26]. The literature in this subject is very abundant, and more details and discussion about this pyrolysed MNx catalysts are beyond the scope of this chapter.…”