On the Origin of the Improvement of Electrodeposited MnOx Films in Water Oxidation Catalysis Induced by Heat Treatment. Issue 11 (4th May 2015)

• Record Type: Journal Article
• Authors: Khan, Munirah; Xiao, Jie; Zhou, Fengling; Yablonskikh, Mikhail; MacFarlane, Douglas R.; Spiccia, Leone; Aziz, Emad F.
• Is Part Of: ChemSusChem. Volume 8:Issue 11(2015:Jun.)
• Page Start: 1980
• Page End: 1985
• Publication Date: 2015-05-04
• Subjects: electrochemistry -- heat treatment -- manganese oxides -- water oxidation -- x‐ray absorption; Green chemistry -- Periodicals; Sustainable engineering -- Periodicals; Chemistry -- Periodicals; Chemical engineering -- Periodicals; 660
• DOI: 10.1002/cssc.201500330
• Languages: English
• ISSNs: 1864-5631

Abstract

Abstract: Manganese oxides (MnO x ) are considered to be promising catalysts for water oxidation. Building on our previous studies showing that the catalytic activity of MnO x films electrodeposited from aqueous electrolytes is improved by a simple heat treatment, we have explored the origin of the catalytic enhancement at an electronic level by X‐ray absorption spectroscopy (XAS). The Mn L‐edge XA spectra measured at various heating stages were fitted by linear combinations of the spectra of the well‐defined manganese oxides—MnO, Mn3 O4, Mn2 O3, MnO2 and birnessite. This analysis identified two major manganese oxides, Mn3 O4 and birnessite, that constitute 97 % of the MnO x films. Moreover, the catalytic improvement on heat treatment at 90 °C is related to the conversion of a small amount of birnessite to the Mn3 O4 phase, accompanied by an irreversible dehydration process. Further dehydration at higher temperature (120 °C), however, leads to a poorer catalytic performance. Abstract : Show me the lost water: Heat treatment of manganese oxide (MnO x ) films leads to successive losses of adsorbed water on the material surface at 50 °C and structural water in bulk at 90 and 120 °C. The loss of the structural water embedded in birnessite phase at 90 °C triggers the transformation of a small amount of birnessite into an Mn3 O4 phase, and results in a significant improvement in catalytic performance.