Green synthesis and photocatalytic properties of manganese-based oxides
Date
2020
Authors
Gagrani, Ankita
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Abstract
In recent years, manganese-based oxides, particularly calcium manganese oxides, have attracted significant interest as a new class of bio-mimetic catalysts, due to their elemental and structural similarity to the natural photocatalytic centre in plant cells. In natural photosynthesis, bio-electricity decomposes water into hydrogen and oxygen within a unique catalytic centre composed of a CaMn4O5 cluster, in the oxygen evolving complex (OEC) enzyme hidden in the Photosystem II protein. Inspired by this nature's solar technology, calcium manganese oxides have been explored as photo catalysts in sustainability-related application including the generation of clean hydrogen fuel through water splitting.
For this application, calcium manganese oxides have some unique advantages over other common photocatalysts, (i) bang gap energy allowing the absorption of visible light to utilise a wide range of solar spectrum, and (ii) constitution of low cost, Earth abundant and environment friendly elements. The combination of these unique properties makes calcium manganese oxides a suitable choice as a photocatalyst for a large-scale operation in an eco-friendly manner.
However, there are still considerable research gaps in the catalytic applications of calcium manganese oxides. For example, investigation into the catalysis of calcium manganese oxides has been carried out mostly without light irradiation and there is little knowledge about how and which calcium manganese oxides behave as a photocatalyst or as an electrode material in photo-electrochemical cells. Furthermore, information critical to these light-driven applications, such as band edge position and bandgap energy, is still lacking for calcium manganese oxides. Moreover, the relationship between catalytic performance under light and different structural parameters such as manganese oxidation states, structural similarity to the natural OEC cluster, and morphology of the catalyst, is not fully understood. Also, the role of calcium in the catalysis is not known. There is also a strong need to develop new techniques to synthesise nanoscale catalysts that have less environmental impacts than conventional solvent-based methods.
In this study, a solvent-free method to synthesize manganese-based oxide catalysts was developed. MnO2, Ca2Mn3O8, CaMn2O4 and CaMnO3 ultrafine particles were successfully synthesized in two steps; mechanochemical processing and subsequent heat treatment. The study of band-edge position and bandgap energy revealed that theoretically the selected manganese-based oxide catalysts can absorb the visible solar spectrum, degrade organic pollutants by free-radical generation, and generate oxygen through photo-electro-catalytic water splitting. However, experimentally, at pH 7, manganese oxides and calcium manganese oxides showed poor charge transfer efficiency, leading to poor photo-catalytic dye degradation. For the study of photo-electrochemical water splitting, polypyrrole, a conducting polymer, was used along with catalyst powders in the anode of a photo-electrochemical cell, to increase charge separation efficiency. This study also identified that oxygen evolution catalysis by CaMn2O4 is affected more by surface area than particle morphology.
This work has added new knowledge into the research field of bio-inspired photocatalysts. In this work, potentially green and scalable techniques to produce manganese-based oxide catalysts were developed. The photocatalytic performance of manganese-based oxides in dye decolourisation were investigated under various conditions. Their catalytic and photocatalytic water oxidation performance was investigated using a photo-electrochemical cell and the effects of particle morphology, manages oxidation states and crystal structures on the water splitting reaction were elucidated. These findings will be helpful in designing sustainable technologies for the environment.
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