Efficient Photochemical, Thermal, and Electrochemical Water Oxidation Catalyzed by a Porous Iron-Based Oxide Derived Metal–Organic Framework

Iron-based catalysts are of particular interest for water oxidation because of their high abundance, low toxicity, and rich redox properties. Herein, we report low cost porous iron-based oxides derived from calcining precursors of Prussian blue analogue (PBA) M x [Fe(CN) 6 ] y (M = Fe, Co, Ni). This synthesis approach involves a simple self-assembly technology and a low-temperature annealing procedure. These catalysts were investigated for photocatalytic, cerium(IV)-driven, and electrochemical water oxidation, and they exhibited superior activity. It is noteworthy that this photocatalytic water oxidation was conducted under neutral conditions that are similar to the natural photosystem II. The high initial turnover frequency (TOF) of ∼5.4 × 10 –4 s –1 per transition metal atom at the first 60 s is obtained under neutral pH using porous Co x Fe 3– x O 4 in photocatalytic water oxidation reaction, which is comparable with those published iron-based catalysts. Under cerium(IV)-driven water oxidation conditions, the TOF of porous Co x Fe 3– x O 4 is 5.2 × 10 –4 s –1 per transition metal atom, which is the highest value among all the documented iron oxides. In the electrochemical water oxidation, the porous Ni x Fe 3– x O 4 catalyst exhibits a low overpotential of 402 mV at 10 mA cm –2 . Meanwhile, the porous iron-based oxides possess beneficial ferromagnetic properties and excellent stability so that they were used repeatedly without loss in activity.