Bioinspired catalyst splits water

August 8, 2014
Catalysts based on manganese (black substance) can mimic the splitting of water into oxygen and hydrogen that occurs in plants inside the photosystem II protein cluster responsible for photosynthesis. Credit: Reproduced from Ref. 1 and licensed under CC BY-NC-ND 4.0 © 2014 A. Yamaguchi et al.

Plants use photosynthesis to convert carbon dioxide and water into sugars and oxygen. The process starts in a cluster of manganese, calcium and oxygen atoms at the heart of a protein complex called photosystem II, which splits water to form oxygen gas, protons and electrons.

Researchers have attempted to develop synthetic catalysts that mimic this cluster, using light or electricity to convert into fuels such as hydrogen gas. Unlike plants, however, these artificial catalysts can only split alkaline water, which makes the process less sustainable.

Ryuhei Nakamura and colleagues at the RIKEN Center for Sustainable Resource Science have now developed a manganese oxide-based catalyst system that can split water efficiently at neutral pH. "Nature utilizes a safe, clean and abundant form of water to make fuels, thereby realizing sustainable ecosystems in the true sense," says Nakamura. "Catalysts that utilize water at a neutral pH as a resource for renewable energy would become the foundation for sustainable human societies."

In photosystem II, charged manganese (Mn) ions gradually give up electrons as they tear protons away from water molecules. This causes manganese in the 2+ and 3+ valence states to become oxidized, resulting in Mn4+ ions. Although the less-oxidized Mn3+ ions are quite stable in photosystem II, Nakamura and his colleagues previously found that they are unstable in synthetic manganese oxide catalysts at neutral pH.

To overcome this instability, the researchers sped up the regeneration of Mn3+ ions, which usually occurs when a water–Mn2+ complex loses a proton and an electron in two separate steps. Nakamura's team realized that ring-shaped organic molecules called pyridines could help those steps to happen at the same time—a process likely promoted by amino acids in photosystem II. They found that the manganese oxide catalyst produced 15 times more oxygen at neutral pH when used in conjunction with a pyridine called 2,4,6-trimethylpyridine.

The team also tested the reaction in deuterated water, which contains a heavier isotope of hydrogen than normal water. The catalyst generated oxygen much more slowly in the presence of 2,4,6-trimethylpyridine, suggesting that removal of a proton from the water–Mn2+ complex is the key step that determines the overall rate of the water-splitting reaction.

As pyridines would not be suitable for large-scale water splitting because they are potential environmental pollutants, the team now hopes to identify safer alternative proton-removing molecules that could be immobilized onto the surface of the catalyst to enhance its activity.

Explore further: Picking apart photosynthesis: New insights could lead to better catalysts for water splitting

More information: Yamaguchi, A., Inuzuka, R., Takashima, T., Hayashi, T., Hashimoto, K. & Nakamura, R. Regulating proton-coupled electron transfer for efficient water splitting by manganese oxides at neutral pH. Nature Communications 5, 4256 (2014). DOI: 10.1038/ncomms5256

Related Stories

Insights from nature for more efficient water splitting

June 30, 2014

Water splitting is one of the critical reactions that sustain life on earth, and could be a key to the creation of future fuels. It is a key in the process of photosynthesis, through which plants produce glucose and oxygen ...

Chemists develop novel catalyst with two functions

July 9, 2014

Chemists at the Ruhr-Universität Bochum have made a decisive step towards more cost-efficient regenerative fuel cells and rechargeable metal-air batteries. They developed a new type of catalyst on the basis of carbon, which ...

Molecular snapshots of oxygen formation in photosynthesis

July 11, 2014

Researchers from Umeå University, Sweden, have explored two different ways that allow unprecedented experimental insights into the reaction sequence leading to the formation of oxygen molecules in photosynthesis. The two ...

Improving the cost and efficiency of renewable energy storage

July 17, 2014

A major challenge in renewable energy is storage. A common approach is a reaction that splits water into oxygen and hydrogen, and uses the hydrogen as a fuel to store energy. The efficiency of 'water splitting' depends heavily ...

Recommended for you

3-D printed structures that 'remember' their shapes

August 26, 2016

Engineers from MIT and Singapore University of Technology and Design (SUTD) are using light to print three-dimensional structures that "remember" their original shapes. Even after being stretched, twisted, and bent at extreme ...

New method developed for producing some metals

August 25, 2016

The MIT researchers were trying to develop a new battery, but it didn't work out that way. Instead, thanks to an unexpected finding in their lab tests, what they discovered was a whole new way of producing the metal antimony—and ...

New electrical energy storage material shows its power

August 24, 2016

A powerful new material developed by Northwestern University chemist William Dichtel and his research team could one day speed up the charging process of electric cars and help increase their driving range.

2 comments

Adjust slider to filter visible comments by rank

Display comments: newest first

Shootist
2 / 5 (3) Aug 08, 2014
Nuclear fission reactors can split H2O as well. Get cracking..
wasp171
3 / 5 (2) Aug 08, 2014
From the article above:
"Although the less-oxidized Mn3+ ions are quite stable in photosystem II"

This is quite wrong!
Mn+3 is MORE oxidized than Mn+2!

Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.