Surface and Structural Investigation of a MnO_x Birnessite-Type Water Oxidation Catalyst Formed under Photocatalytic Conditions

Catalytically active MnO_x species have been reported to form in situ from various Mn-complexes during electrocatalytic and solution-based water oxidation when employing cerium(IV) ammonium ammonium nitrate (CAN) oxidant as a sacrificial reagent. The full structural characterization of these oxides may be complicated by the presence of support material and lack of a pure bulk phase. For the first time, we show that highly active MnO_x catalysts form without supports in situ under photocatalytic conditions. Our most active ⁴MnO_x catalyst (∼0.84 mmol O₂ mol Mn^-1 s^-1) forms from a Mn₄O₄ bearing a metal-organic framework. ⁴MnO_x is characterized by pair distribution function analysis (PDF), Raman spectroscopy, and HR-TEM as a disordered, layered Mn-oxide with high surface area (216 m²g^-1) and small regions of crystallinity and layer flexibility. In contrast, the ^SMnO_x formed from Mn²⁺ salt gives an amorphous species of lower surface area (80 m²g^-1) and lower activity (∼0.15 mmol O₂ mol Mn^-1 s^-1). We compare these catalysts to crystalline hexagonal birnessite, which activates under the same conditions. Full deconvolution of the XPS Mn2p_3/2 core levels detects enriched Mn³⁺ and Mn²⁺ content on the surfaces, which indicates possible disproportionation/comproportionation surface equilibria.