A strategy for stabilizing the catalyst Co4O4 in a metal–organic framework
Proceedings of the National Academy of Sciences of the United States of America
Artificial photosynthesis provides a basis for storing solar energy in chemical bonds (1, 2), with water oxidation a critical step (3, 4) . The half reaction for the oxidation of water, 2H 2 O → O 2 + 4e − + 4H + , provides protons and electrons for the production of a fuel at a cathode (4, 5) . In carrying out the reaction, it is necessary to develop highly stable molecular catalysts that cycle through multiple oxidation states during water oxidation cycles. Better mechanistic understanding of
... ic understanding of water oxidation is an important element in the correlation of structure and function in the design of high efficiency catalysts for long-term water oxidation. Since the first molecular water oxidation catalyst, the "blue dimer," described (6), many examples, based largely on complexes of Ru and Ir, have appeared that demonstrate catalytic water oxidation, including mechanistic details about how the reactions occur (7, 8). Recently, catalysts based on earth-abundant elements like Fe or Co have also been prepared with the thought of scale-up at the device level in artificial photosynthetic schemes (9-11). A structure of interest has been the Co 4 O 4 cluster, with a structure related to the reaction center in natural photosynthesis, the latter based on the Mn 4 Ca catalyst in photosystem II (PSII) (12, 13). In most cluster structures of this kind, exposure to solutions at high pH, which maximize water oxidation reactivity, leads to instability because of the loss of the active Co 4 O 4 center. In PNAS, a study by Nguyen et al. (14) provides a general strategy for stabilizing the cobalt-oxo "cubane" cluster core, Co 4 O 4 , in a metal-organic framework structure. As shown in Fig. 1 , the structure is based on a Co 4 O 4 water oxidation core, and it demonstrates durable water oxidation reactivity, even at high pH. These results are an important extension of the remarkable cobalt(III)-oxo cubane cluster Co 4 O 4 (OAc) 4 (py) 4 , which has been shown to be a functional oxygenevolution reaction catalyst (13, 15, 16). Nguyen et al. (14) show that in the rigid coordination environment of the metal-organic framework, the Co 4 O 4 core is stabilized, providing an external organic support structure. The latter prevents structural instability, with loss of the cluster from the framework as an oxide. The stability of the structure offers an opportunity for characterization of key reactive intermediates during the water oxidation cycle, especially under catalytic conditions. Stabilization of the Co 4 O 4 cluster is analogous to stabilization of the Mn 4 Ca cluster in the type II reaction center in photosynthesis. In a green plant, a relative highly tailored protein supports and encapsulates a Mn 4 Ca cluster in an environment in which electron transfer to the cluster occurs from the surrounding environment. To incorporate the catalyst, Nguyen et al. (14) utilized a simple one-step reaction with the metalorganic framework and a series of structurally carefully chosen Co 4 O 4 clusters. The incorporated clusters were characterized by ultraviolet-visible spectrophotometry, electron paramagnetic resonance, X-ray absorption near edge structure, and atomic pair distribution function analysis of X-ray diffraction data. The data were consistent with maintenance of Co 4 O 4 units in the metal-organic framework. There was no evidence for long-range crystallographic order in the structures. The final coordination network was resistant to deactivation under aqueous conditions for water oxidation, even at pH 14. Fig. 1 . Inspired by the Mn 4 Ca catalyst in the reaction center, which occurs in a highly folded membrane protein, the Co 4 O 4 cluster in a metal-organic framework also undergoes water oxidation.