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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 , TiO₂, 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(Mn⁴⁺,Mn³⁺)₂O₄ ⋅ 1.5 H₂O. 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,

	2H₂O → O₂ + 4H⁺ + 4e⁻	(water oxidation)
	4H⁺ + 4e⁻ → 2H₂	(reduction and generation of dihydrogen)
	2H₂O → 2H₂ + O₂	(photolysis reaction)

Maple leaf, acer rubrum

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 Mn₄CaO₄ 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 Mn⁴⁺, and then photoreduced to Mn²⁺. Just as in the case of titania photolysis, a voltage is applied to oxidize the Mn²⁺ to Mn⁴⁺.

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 Mn₄O₄ 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.

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