Chemical 'dance' of cobalt catalysis could pave way to solar fuels

Chemical 'dance' of cobalt catalysis could pave way to solar fuels
Argonne chemists Dugan Hayes, Lin Chen, and Ryan Hadt have identified a rapid electronic process that could aid the water-splitting reaction in cobalt-containing catalysts. Cobalt catalysts are relatively inexpensive and could replace more expensive precious metal catalysts in the production of clean energy, most notably solar fuels. Credit: Argonne National Laboratory

By splitting a water molecule into two atoms of hydrogen and one of oxygen, scientists can use the boundless energy of the sun to make a clean fuel. In a new study from the U.S. Department of Energy's (DOE) Argonne National Laboratory and Harvard University, scientists have for the first time been able to see an especially important step in the water-splitting process, which may bring us closer to abundant solar energy for all.

Splitting a water molecule requires a metal catalyst to get the reaction going. Recently, much scientific attention has focused on cobalt, a relatively abundant and inexpensive catalyst that - in the right circumstances - can serve as an escort to an electronic dance between hydrogens and oxygens.

"Essentially, it allows you to have a focused snapshot, as opposed to just seeing a chemical blur. It's important that we determine the characteristics of the catalyst on the timescale the electrons are moving."

"Cobalt -evolving catalysts are the active components in technologies like artificial leaves and other materials in which you can harvest light to drive the synthesis of solar fuels," said Argonne postdoctoral researcher Ryan Hadt, a co-first author of the study.

The overall water-splitting reaction actually has two halves. The researchers focused on the first half, called water oxidation, which requires the transfer of four protons and four electrons and eventually results in the formation of an oxygen-oxygen bond. For this process, the oxygens need a temporary dance partner, which is played by the cobalt catalyst.

But the reason this dance isn't yet well understood is that the transfers and the formation of the bond happen in a flash - the whole process takes less than a billionth of a second. To understand the nuances of the bonding action, the researchers needed to perform X-ray absorption spectroscopy measurements at Argonne's Advanced Photon Source.

In their analysis, the researchers focused on a particularly intriguing chemical twist. At the beginning of the process, a bridge of two connects two cobalt ions. Each of the cobalt ions, in turn, is connected to its own water molecule. At this point, things are pretty stable.

The electronic dance is ready to begin when a cobalt ion adds an additional positive charge, temporarily increasing a characteristic number that scientists term an "oxidation state." In the case of cobalt, the oxidation state changes, just for an instant, from three to four.

When two cobalt ions with an oxidation state of four come into contact, the process begins in earnest. The charge transfers cause the hydrogen atoms of the to dissociate from their oxygen bonds, leaving the bonded just to oxygen ions.

The key moment follows immediately afterwards, when the cobalt centers each receive an extra electron from the newly exposed oxygen atoms. When this happens, a bond is formed between the two oxygens, creating a molecular intermediate stage called a peroxide, which can be rapidly oxidized to release a dioxygen molecule. The electrons obtained from water during this process can be used to make solar fuels.

By using the Advanced Photon Source, a DOE Office of Science User Facility, the researchers were able to directly measure cobalt oxidation states and then use theory to a calculate a quantity known as "exchange coupling," a quantum mechanical value that identifies the relationship between the spins of the electrons that are shuttled between the oxygen and cobalt atoms. The researchers found that these electrons spins are in opposite directions - in scientific parlance, they are antiferromagnetically coupled.

"Antiferromagnetism plays an important role in the formation of the oxygen-oxygen bond," said Hadt, "as it provides a way to simultaneously transfer two electrons to make a chemical bond."

Argonne postdoctoral researcher and study author Dugan Hayes also pointed to the unique ability of the Advanced Photon Source to resolve the location of the extra-oxidized atoms. "Essentially, it allows you to have a focused snapshot, as opposed to just seeing a chemical blur," he said. "It's important that we determine the characteristics of the catalyst on the timescale the electrons are moving."

A paper based on the research, "In situ characterization of cofacial Co(IV) centers in Co4O4 cubane: Modeling the high-valent active site in oxygen-evolving catalysts," appeared in the March 27 edition of the Proceedings of the National Academy of Sciences.


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More information: Casey N. Brodsky et al, In situ characterization of cofacial Co(IV) centers in CoOcubane: Modeling the high-valent active site in oxygen-evolving catalysts, Proceedings of the National Academy of Sciences (2017). DOI: 10.1073/pnas.1701816114
Citation: Chemical 'dance' of cobalt catalysis could pave way to solar fuels (2017, June 3) retrieved 21 July 2019 from https://phys.org/news/2017-06-chemical-cobalt-catalysis-pave-solar.html
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Jun 03, 2017
This poorly written (anonymous) article is really about Argonne NL and Harvard scientists using Argonne's Advanced Photon Source to capture the mechanics of a cobalt catalyzed H2O cracking so clearly that they identified the antiferromagnetic spins of the oxygen atoms. This investigation is great work, and does enable better exploitation of the cobalt catalyst by tweaking its application in light of the actual mechanics.

The article is not so great. No wonder it's anonymous, but still that's a disservice to the reader.

Jun 04, 2017
They will never split water with less energy than the resultant product hydrogen can deliver as a fuel or do it cheaply. In some ways, it's a bigger pie-in-the-sky than fusion.

Jun 04, 2017
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Jun 04, 2017
They will never split water with less energy than the resultant product hydrogen can deliver as a fuel or do it cheaply.
And how do we know how much it will cost to split water in the future?

Jun 04, 2017
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Jun 04, 2017
Of course: the primary problem of solar plants is their installation cost, which cannot be further lowered by choice of new materials
Why not? Here is a cost curve on solar panels for the past 40 years or so. http://costofsola...rything/ What crystal ball do you have - that tells us they cannot get any cheaper? Trump is not the player in this massive game. I am not surprised that you would cancel research into a very promising area. Cede the future to other countries - that is real smart. Here is a very interesting article on this subject - from an Australian perspective - https://cleantech...oal-gas/

Jun 04, 2017
Because of science: thermodynamics. Never heard of it?
Yes I have heard of it. So how does thermodynamics - tell us the cost of splitting water in the future? If the cost of solar panels continues to fall, then the cost of the electricity will fall, and the cost of splitting water will fall. Nothing to do with thermodynamics.

Jun 04, 2017
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Jun 04, 2017
Nope, the cost of splitting of water is given by its energy demands - and the cost of energy production will increase rather than decrease
You don't know what will happen to the cost of energy. If the cost of solar panels continues to fall (it is a technology rather than a mined resource - so the odds are excellent that the downward cost curve will continue) - then the cost of splitting water will get cheaper. It has nothing to do with thermodynamics - it is simply about money.

Jun 04, 2017
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Jun 04, 2017
Nope, the cost of splitting of water is given by its energy demands - and the cost of energy production will increase rather than decrease
No - the cost of electricity will continue down. You are just wrong on this one.
the low cost of solar energy at the market can be also indicia of low interest about it, which can be consequence of low effectiveness of this solution.
Except we are installing gigawatts of the stuff all over the world. Tesla just started selling their solar tiles - and they are sold out into 2018. And their product is stupid expensive compared to other products. This company is planning a new giga watt scale plant in Egypt - https://cleantech...y-egypt/ Again - you are wrong. Like I keep trying to rebut Willie - the past is not necessarily the future.

Jun 04, 2017
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Jun 04, 2017
At least I'm not delusional.
Do you have any actual support for making that kind of statement? I can show you cost curves on wind turbines and solar panels. How does it make me delusional to read articles like this (and there are multiple articles every week showing the FACTS). https://cleantech...ue-fall/
Let me see your support for believing that the cost of solar and wind are going to stop going down.

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