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Manganese Photolysis of Water
June 1, 2011
Photocatalysis is one method of
solar energy harvesting. In photocatalysis, a
chemical reaction is enabled by the simultaneous presence of light and a
catalyst. In a
previous article (Titania Photocatalysis, February 16, 2011), I summarized research at
Lawrence Berkeley National Laboratory and the
University of California at Berkeley on using titania for photocatalysis.
Titanium dioxide, TiO
2, also called titania, is an abundant and inexpensive material that's used as a
pigment in
opaque white
paint. Although titania, itself, is just barely photocatalytic, the Berkeley team found that
hydrogenation of titania to produce surface disorder allowed absorption of nearly a quarter of the
solar spectrum, and the material showed no sign of degradation after 22 days. One possible reaction on titania is the
electrolysis of water to create
hydrogen and
oxygen.
An international research team with members from
Monash University (
Victoria, Australia), the
High Energy Accelerator Research Organization (KEK,
Tsukuba, Japan), the
Australian Synchrotron (Victoria, Australia) and
The University of California - Davis, has shown that another mineral,
birnessite, is an effective photocatalyst.[1-2]
Birnessite is a manganese-bearing mineral with composition Na
0.3Ca
0.1K
0.1(Mn
4+,Mn
3+)
2O
4 ⋅ 1.5 H
2O. This common mineral is also the crystal form of deep sea
manganese nodules that I mentioned in
another article (The Manganese Conspiracy, June 9, 2010).
The key to this mineral's activity can be seen in the formula unit, which shows two
charge states for
manganese. This is also the reason for the catalytic activity of many compounds of its neighbor in the
periodic table,
iron. The scientists were led to manganese and this particular mechanism by
nature, through the reaction mechanisms of
photosynthesis.
The first important step in the photolysis of water is
water oxidation, one of the
half-reactions of photolysis,
When "green" energy really is green.
Leaf of Freeman maple, a hybrid of Acer rubrum and Acer saccharinum.
Photo by John A. Knouse,
via Wikimedia Commons)
The
Photosystem II, a
protein complex used by photosynthetic organisms in the oxidation half-reaction, includes a Mn
4CaO
4 catalyst. In experiments with synthetic variations of this system, the research team found that their material was
decomposing to birnessite and was still photoactive. As they write in their paper's abstract, "The original manganese cluster serves only as a precursor to the catalytically active material."
The water splitting occurs in alternating
oxidation and reduction cycles in which manganese in birnessite
nanoparticles is oxidized to form Mn
4+, and then photoreduced to Mn
2+. Just as in the case of titania photolysis, a
voltage is applied to oxidize the Mn
2+ to Mn
4+.
There are some technical issues for practical application. I don't have a copy of the full article ($32 is
too pricey for my meager budget), but the
electrical conductivity of the material might be a problem; and a
cathode is required.
This discovery proves that working in one research area for many years pays
unexpected dividends. The principles you find in early
experiments will direct your efforts in later experiments.
The Monash group has been working with manganese photolysis of water for many years. A 2008 paper of their's in collaboration with
Princeton University was on the water-splitting ability of a manganese–oxo complex of Mn
4O
4 contained in the aqueous channels of a Nafion membrane.[3] In that case, a silver chloride solution was used with a
silver cathode, and photocurrents were achieved with
visible light illumination and an applied potential of 1.2 volts. Their further investigation of this material is what precipitated their discovery.
References:
- Rosalie K. Hocking, Robin Brimblecombe, Lan-Yun Chang, Archana Singh, Mun Hon Cheah, Chris Glover, William H. Casey and Leone Spiccia, "Water-oxidation catalysis by manganese in a geochemical-like cycle," Nature Chemistry, Published online May 15, 2011.
- Splitting water to create renewable energy simpler than first thought?, Monash University Press Release, May 16, 2011.
- Robin Brimblecombe, Gerhard F. Swiegers, G. Charles Dismukes and Leone Spiccia, "Sustained Water Oxidation Photocatalysis by a Bioinspired Manganese Cluster," Angewandte Chemie International Edition, vol. 47, no. 38 (September 8, 2008) pp. 7335-7338.
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Linked Keywords: Photocatalysis; solar energy; chemical reaction; catalyst; Lawrence Berkeley National Laboratory; University of California at Berkeley; Titanium dioxide; pigment; opaque; paint; hydrogenation; solar spectrum; electrolysis of water; hydrogen; oxygen; Monash University; Victoria, Australia; High Energy Accelerator Research Organization; Tsukuba, Japan; Australian Synchrotron; The University of California, Davis; birnessite; manganese nodules; oxidation state; charge state; manganese; periodic table; iron; nature; photosynthesis; water oxidation; half-reaction; Acer rubrum; Acer saccharinum; Wikimedia Commons; Photosystem II; protein complex; chemical decomposition; redox; oxidation and reduction; nanoparticle; voltage; open access journal; electrical conductivity; cathode; serendipity; experiment; Princeton University; silver; visible light.