Redox transitions of chromium, manganese, iron, cobalt and nickel protoporphyrins in aqueous solution

Abstract

The electrochemical redox behavior of immobilized chromium, manganese, iron, cobalt, and nickel protoporphyrins IX has been investigated over the pH 0-14 range. In the investigated potential domain the metalloporphyrins were observed in four different oxidation states (M(I), M(II), M(III) and M(IV)). The metalloporphyrins differ in the potentials at which the redox transitions occur, but the observed pH dependence of the redox transitions was similar for the different metalloporphyrins and revealed that the M(II)/M(III) and M(III)/M(IV) transitions were accompanied by a hydroxide transfer at high pH. The fact that the metalloporphyrins are immobilized on graphite does not seem to have a large influence on their redox behavior, as can be deduced from the comparable behavior of immobilized metalloporphyrins on gold and of watersoluble metalloporphyrins in solution. We also performed density functional theory (DFT) calculations on the metalloporphyrins in different oxidation states. The geometries and spin states predicted by these calculations agree well with experimentally determined values; the calculations were also able to predict the electrochemical potentials of the [M(II)]/[M(III)-OH] redox transition to within about 300 mV.

Electrocatalytic Nitrate Reduction by a Cobalt Protoporphyrin Immobilized on a Pyrolytic Graphite Electrode

“…25,26 The main product at low pH was hydroxylamine with a small amount of N 2 O. A possible explanation for the discrepancy with the results reported here is the different working potential window.…”

“…The selectivity of the overall nitrate reduction then depends on the consecutive reduction of NO 2 − . However, in acidic solution (pH <5), Duca et al 26 …”

“…Gaseous product N 2 O is formed by the reaction of the M−HNO adduct with the NO in solution, a reaction whose mechanism on iron-based porphyrins has been studied in detail in our earlier work. 25,26 The overall reaction for the formation of N 2 O is shown in eq 6 below. N 2 O will consecutively react to N 2 , but this reaction is very slow.…”

“…26,32 It is therefore important to investigate the formation of NO at different pH values on the immobilized cobalt protoporphyrin. Figure 5(d−f) shows the formation of NO at pH 1 to 3 as detected by OLEMS during the electrochemical reduction of nitrate.…”

“…25,26 Building on these previous results from our group, in this work, a simple metal-containing complex, cobalt protoporphyrin (CoPP), directly adsorbed on a pyrolytic graphite (PG) electrode, was utilized as an electrocatalyst for the reduction of nitrate in acidic solution. Co-PP was chosen among a series of metal PPs because it has the highest reactivity and selectivity toward hydroxylamine.…”