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Abstract
The oxygen evolution reaction (OER) is considered to be the limiting step for the water splitting process. OER catalyst optimization is hindered because in the desirable acidic conditions the sole active catalysts are the expensive RuO2 and IrO2-based materials. Thus, the understanding of the factors controlling the reactivity of IrO2 is mandatory. In this contribution, we carried out spin polarized DFT (PBE-D2) periodic calculations to analyze the catalytic activities of the main ((110), (011), (100) and (001)) IrO2 surfaces. We considered the oxo-coupling (I2M) and water nucleophilic attack (WNA) mechanisms and computed the energy barriers of the chemical processes. Results show that the oxo-coupling and the water attack should be viewed as homolytic couplings and thus, the two processes only occur if the Ir=O species on the surfaces exhibit oxyl character. In these cases, the WNA mechanism is always easy and it becomes the most favorable pathway on the (110), (100) and (001) surfaces. In contrast, for the (011) facet the oxo-coupling is preferred. The required overpotentials for the four IrO2 surfaces depends on the feasibility to oxidize the Ir-OH to Ir-O species. However, if the oxidation is too favorable the resulting Ir=O species has no oxyl character and thus it does not further react. Therefore, the optimal catalyst shows a trade-off between the Ir-OH oxidation feasibility and the oxyl character of the surface Ir=O species. These two factors are tuned by the coordination of the unsaturated iridium sites: the (100) and (001) surfaces are more active than the (110) and (011). The key role of oxyl species has been shown to be important for molecular catalysts. However, the implications of oxyl species on IrO2 have rarely been mentioned. Present results show that the homogeneous and heterogeneous processes seem to have important similarities, thus suggesting that strategies used in one of the two fields could be transferred to the other.
