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

Deibert, BJ, Zhang J, Smith PF, Chapman KW, Rangan S, Banerjee D, Tan K, Wang H, Pasguale N, Chen F, Lee KB, Dismukes GC, Chabal YJ, Li J.  2015.  

Journal:

Chemistry

Volume Number:

21

Issue Number:

40

Pages:

14218-14228

Abstract:

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.
Citation:
Deibert, BJ, Zhang J, Smith PF, Chapman KW, Rangan S, Banerjee D, Tan K, Wang H, Pasguale N, Chen F et al..  2015.  Surface and Structural Investigation of a MnOx Birnessite-Type Water Oxidation Catalyst Formed under Photocatalytic Conditions. Chemistry. 21(40):14218-14228.