Oxygen-evolution reaction in the presence of cerium(iv) ammonium nitrate and iron (hydr)oxide: old system, new findings

Abstract

Solar fuel production by photosynthetic systems strongly relies on developing efficient and stable oxygen-evolution catalysts (OECs). Cerium(IV) ammonium nitrate (CAN) has been the most commonly used sacrificial oxidant to investigate OECs. Although many metal oxides have been extensively investigated as OECs in the presence of CAN, mechanistic studies were rarely reported. Herein, first, Fe(III) (hydr)oxide (FeO_xH_y) was prepared by the reaction of Fe(ClO₄)₃ and KOH solution and characterized by some methods. Then, changes in Fe oxide in the presence of CAN during the OER were tracked using in situ Raman spectroscopy, in situ X-ray absorption spectroscopy, in situ visible spectroscopy, and in situ electron paramagnetic resonance spectroscopy. FeO_xH_y in the presence of CAN and during the OER converted to γ-Fe₂O₃ and [Fe(H₂O)₆]³⁺, and a small amount of oxygen was formed. A maximum turnover frequency and turnover number of 10⁻⁶ s⁻¹ and 1.3 × 10⁻³ mol(O₂)/mol(Fe) (for half an hour) in the presence of CAN (0.20 M) and FeO_xH_y were observed.