Deoxygenation of IrO2(1 1 0) surface: Core-level spectroscopy and density functional theory calculation

Deoxygenation of the IrO2(1 1 0) surface is investigated at 403–493 K, using the core-level spectroscopy and density functional theory (DFT) calculation. The Ir-4f7/2 signals of 1f-cus-Ir with and without on-top oxygen (Otop) emerge as surface features of the baked-out surface, whose positive and negative shifts in binding energy are in line with the DFT computation results. Progressively increasing the reduction temperature, the 1f-cus-Ir feature quickly disappears and the signal of 2f-cus-Ir emerges at 403 K. Meanwhile the feature of 1f-cus-Ir + Otop diminishes but persists when the Ir metal signal is evident. The intriguing coexistence of 1f-cus-Ir + Otop and Ir metal at 433–443 K is elucidated in the theoretical pathway study. DFT calculation reveals that O2 desorption via pairing two neighboring Otop atoms is the rate-determining step of surface deoxygenation. Under the UHV conditions, Otop is replenished via migration of the surface oxygen species, including the threefold coordinated oxygen (O3f) of a reduced surface. Hence the Otop atom is an active and long-lived surface species, which does not vanish until O3f is consumed and surface Ir begins to cluster. Under the realistic pressure conditions, Otop can also be refreshed via the dissociative adsorption of gas-phase oxygen. In either pathway, Otop is a critical intermediary of IrO2(1 1 0) oxidation catalysis.