Oxygen Reduction Reaction Promoted by Manganese Porphyrins

Abstract

The oxygen reduction reaction (ORR) is catalyzed by manganese­(II) porphyrins in the presence of Brønsted acids (HAs). Analyses of the catalytic cyclic voltammetric profiles have permitted the ORR mechanism to be constructed and rate constants to be extracted for both the formation of the initial oxygen adduct and the O–O bond cleavage event for a series of HAs. The dependence of the formation rate constant of the oxygen adduct on reactant concentrations reveals a rate law that is first order in Mn porphyrin and oxygen substrate. A second order dependence in HA is observed for unadorned Mn porphyrin platforms whereas with Mn hangman porphyrin, a proton is provided intramolecular to the oxygen adduct and consequently the HA order is reduced to unity. The stabilization of the oxygen adduct with an additional hydrogen bond from HA engenders a rate-determining step involving O–O bond cleavage, resulting in the rare instance where the activation of the O–O bond is directly observed.

Combining scaling relationships overcomes rate versus overpotential trade-offs in O 2 molecular electrocatalysis

“…The Nernst equation for this ORR reaction (Eq. 1) shows that a one-unit increase in acid pK a decreases  eff by 0.059 V: ∆ eff = −(0.059 V)∆pK a (15,16). In iron porphyrin ORR catalysis, protonation is the rate-determining step, and the TOF max varies with pK a according to the Brønsted equation: log(TOF max ) = −(pK a ), where  ≈ 0.3 (5,7,11).…”

Significantly improved electrocatalytic oxygen reduction by an asymmetrical Pacman dinuclear cobalt(ii) porphyrin–porphyrin dyad

“…16 As inspired by nature, metal porphyrins have been extensively studied as ORR catalysts. 4, [19][20][21][22][23][24][25][26][27][28][29][30][31][32][33] In addition to biologically relevant Fe porphyrins, Co porphyrins have also received great attention because of their high activity and stability. [19][20][21][22][23][24][25][26][32][33][34] However, mononuclear Co porphyrins are much less selective than Fe analogues to catalyze the 4e reduction of O 2 to water.…”

Bioinspired molecular catalysts for homogenous electrochemical activation of dioxygen

“…The comparison of the results reported with the [Mn III Cl(TPP)] complex, i.e. ORR [31] vs oxygen atom transfer [17] (Figure 3), illustrates the kinetic dilemma to resolve if one wants to drive the fate of the M:O 2 intermediate to a reaction or the other. This problem can a priori be easily addressed by using a large amount of a not too inert substrate.…”