Kinetic and Mechanistic Characterization of Low-Overpotential, H₂O₂-Selective Reduction of O₂Catalyzed by N₂O₂-Ligated Cobalt Complexes

Abstract

A soluble, bis-ketiminate-ligated Co complex [Co(NO)] was recently shown to catalyze selective reduction of O to HO with an overpotential as low as 90 mV. Here we report experimental and computational mechanistic studies of the Co(NO)-catalyzed O reduction reaction (ORR) with decamethylferrocene (Fc*) as the reductant in the presence of AcOH in MeOH. Analysis of the Co/O binding stoichiometry and kinetic studies support an O reduction pathway involving a mononuclear cobalt species. The catalytic rate exhibits a first-order kinetic dependence on [Co(NO)] and [AcOH], but no dependence on [Fc*] or [O]. Differential pulse voltammetry and computational studies support Co-hydroperoxide as the catalyst resting state and protonation of this species as the rate-limiting step of the catalytic reaction. These results contrast previous mechanisms proposed for other Co-catalyzed ORR systems, which commonly feature rate-limiting protonation of a Co-superoxide adduct earlier in the catalytic cycle. Computational studies show that protonation is strongly favored at the proximal oxygen of the Co(OOH) species, accounting for the high selectivity for formation of hydrogen peroxide. Further analysis shows that a weak dependence of the ORR rate on the p K values of the protonated Co(OOH) species across a series of Co(NO) catalysts provides a rationale for the unusually low overpotential observed for O reduction to HO.

Brønsted Acid Scaling Relationships Enable Control Over Product Selectivity from O2 Reduction with a Mononuclear Cobalt Porphyrin Catalyst

“…58,59 The ORR rates were monitored by following the growth of the optical absorption band at 780 nm, corresponding to the conversion of Fc* to Fc* + during the reaction (see Figure S5 in the Supporting Information). 15,51,52 Upon completion of the reaction, the ORR selectivity was established by using iodometric titration and a Ti IV (O)SO 4 colorimetric assay to quantify the amount of H 2 O 2 present in the reaction mixture, similar to that described previously 49,54,61−63 (see also Section VII in the Supporting Information). The product selectivity is high in all cases (≥93%, Table 1), but the product identity changes sharply, from H 2 O 2 to H 2 O, between the p K a values of 2,6-(HO) 2 BA (3.6) and TFAH (4.9).…”

“…When DCAH is used rather than [DMF-H][ClO 4 ], the cathodic peak associated with the Co III (O 2 • ) species shifts to slightly higher potential (0.17 V, Figure 7b), an effect attributed to hydrogen bonding between the acid and the Co III (O 2 • ) species, similar to observations made previously with a N 2 O 2 -ligated Co III (O 2 • ) complex. 52…”

“…46−52 We recently studied a series of Co complexes bearing pseudo-macrocyclic N 2 O 2 -type ligands [e.g., bis(ketiminate), salen derivatives] that led to highly selective H 2 O 2 production in methanol with decamethylferrocene (Fc*) as the reductant and acetic acid as the proton source. 51,52 A parallel study showed that H 2 O is the sole product when p- hydroquinone was used as a combined source of electrons and protons. 53,54 Mechanistic studies of the latter reaction showed that p -hydroquinone intercepts a Co II (HOOH) intermediate, facilitating O–O cleavage prior to release of H 2 O 2 .…”

Influence of the distal guanidine group on the rate and selectivity of O2 reduction by iron porphyrin

“…The factors that affect the rate and selectivity of the catalytic oxygen reduction reaction (ORR) are of fundamental interest as it is an essential process for several energy conversion technologies ranging from fuel cells to metal air batteries. 1 The ORR can proceed either via a 2e À /2H + pathway to produce H 2 O 2 or via a 4e À /4H + pathway to produce water. 2 The latter pathway is favoured both from a thermodynamic point of view 2 and because it avoids partially reduced oxygen species (PROS) such as H 2 O 2 and O 2 À which are very reactive and oen detrimental to the catalyst.…”

High‐Efficiency Oxygen Reduction to Hydrogen Peroxide Catalyzed by Nickel Single‐Atom Catalysts with Tetradentate N2O2 Coordination in a Three‐Phase Flow Cell

“…At a current density of 70 mA cm −2 , the Ni‐N 2 O 2 /C catalyst maintained a Faradaic efficiency to H 2 O 2 of 91 % (Figure c; Supporting Information, Figure S17). This current density was far superior to those of homogeneous molecular systems in organic media (<10 mA cm −2 , Figure d; Supporting Information, Table S4), whose performance is limited by the poor solubility of the O 2 in the solvent systems frequently used (organic solvents are typically needed to dissolve the homogeneous catalysts). Achieving a high current density during O 2 reduction to H 2 O 2 was one of our main motivations for pursuing a Ni‐N 2 O 2 /C catalyst system.…”