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

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 (MnOx)-Mn-4 catalyst (similar to 0.84 mmol O-2 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 m(2)g(-1)) and small regions of crystallinity and layer flexibility. In contrast, the (MnOx)-Mn-S formed from Mn2+ salt gives an amorphous species of lower surface area (80 m(2)g(-1)) and lower activity (similar to 0.15 mmol O-2 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.