Probing Edge Site Reactivity of Oxidic Cobalt Water Oxidation Catalysts

Abstract

Differential electrochemical mass spectrometry (DEMS) analysis of the oxygen isotopologues produced by (18)O-labeled Co-OEC in H2(16)O reveals that water splitting catalysis proceeds by a mechanism that involves direct coupling between oxygens bound to dicobalt edge sites of Co-OEC. The edge site chemistry of Co-OEC has been probed by using a dinuclear cobalt complex. (17)O NMR spectroscopy shows that ligand exchange of OH/OH2 at Co(III) edge sites is slow, which is also confirmed by DEMS experiments of Co-OEC. In borate (Bi) and phosphate (Pi) buffers, anions must be displaced to allow water to access the edge sites for an O-O bond coupling to occur. Anion exchange in Pi is slow, taking days to equilibrate at room temperature. Conversely, anion exchange in Bi is rapid (kassoc = 13.1 ± 0.4 M(-1) s(-1) at 25 °C), enabled by facile changes in boron coordination. These results are consistent with the OER activity of Co-OEC in Bi and Pi. The Pi binding kinetics are too slow to establish a pre-equilibrium sufficiently fast to influence the oxygen evolution reaction (OER), consistent with the zero-order dependence of Pi on the OER current density; in contrast, Bi exchange is sufficiently facile such that Bi has an inhibitory effect on OER. These complementary studies on Co-OEC and the dicobalt edge site mimic allow for a direct connection, at a molecular level, to be made between the mechanisms of heterogeneous and homogeneous OER.

In situ characterization of cofacial Co(IV) centers in Co 4 O 4 cubane: Modeling the high-valent active site in oxygen-evolving catalysts

“…In this regard, insights into the details of the OER mechanism in Co-OECs have been greatly aided by molecular model complexes. Complementary electrochemical studies of binuclear cobalt complexes and Co-OECs are consistent with the contention that OER occurs at a dicobalt edge site (23,(25)(26)(27). As illustrated in Fig.…”

“…Such exchange coupling provides a mechanism for charge localization within cubanes, as shown here, and by analogy within the metalate clusters of Co-OEC. Isotope labeling studies of oxidic cobalt OER catalysts (23,24) and computational mechanistic studies of Co-OECs (42,43), together with model studies of dicobalt complexes (23), indicate that O-O bond formation requires an adjacent Co(IV) 2 active edge site; it is necessary that two Co(IV) centers localize to the edge site of a cobaltate cluster preceding turnover, as opposed to hole equivalents delocalized over the cluster active site. As we show here, antiferromagnetic coupling provides a mechanism to drive hole localization within the cluster core.…”

“…Electrochemical kinetics (19) and spectroscopic measurements (20,21) support a mechanism consisting of a minor equilibrium proton-coupled electron transfer process to generate effectively a Co(III)Co(IV) precatalyst, followed by a subsequent oxidation to generate a doubly oxidized state that drives the turnover-limiting O-O bond-forming step (22). Isotope labeling studies of active oxidic cobalt OER catalysts (23,24) establish direct coupling of oxygens on neighboring sites, thus identifying one path for O-O bond formation from a Co(IV) 2 state,…”

Self-healing catalysis in water

“…The transport of the Co 4+ in CoP i films (i.e., the oxidizing hole equivalent) is predicted to be fast based on the Co 3+/4+ selfexchange electron transfer rate constant of k ET (Co 3+/4+ ) = 3 × 10 5 M -1 ·s -1 measured in cobalt cubane model complexes (31). Delivery of the hole equivalent to the surface of the catalyst allows Co 4+ in CoP i to react with Co 2+ in solution to yield Co 3+ , which adds to edges exposed from dissociation of P i from the cobalt cluster, as observed in substitution kinetics studies of native CoP i (26) and Co 7 metallate clusters (32). These results account for the first order in [Co 2+ ] and inverse order in [P i ].…”

“…Isotopic oxygen labeling studies have shown that OER occurs at edge sites of the cobaltate clusters, with a dicobalt active site as the minimal structural unit that supports catalysis ( Fig. 3) (26). OER catalysis proceeds from a Co(III) "Co(IV)" precatalyst (P) state, as shown by X-ray absorption spectroscopy (XAS) (22,27) and…”