Catalytically active MnOx 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 MnOx catalysts form without supports in situ under photocatalytic conditions. Our most active (4)MnOx catalyst (∼0.84 mmol O2 mol Mn(-1) s(-1)) forms from a Mn4O4 bearing a metal-organic framework. (4)MnOx is characterized by pair distribution function analysis (PDF), Raman spectroscopy, and HR-TEM as a disordered, layered Mn-oxide with high surface area (216 m(2) g(-1)) and small regions of crystallinity and layer flexibility. In contrast, the (S)MnOx formed from Mn(2+) salt gives an amorphous species of lower surface area (80 m(2) g(-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 Mn2p3/2 core levels detects enriched Mn(3+) and Mn(2+) content on the surfaces, which indicates possible disproportionation/comproportionation surface equilibria.
Keywords: birnessite structure; manganese oxide; metal-organic frameworks; water oxidation catalyst; water splitting.
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