How to efficiently extract and save clean energy in the sun has always been a problem for human society. The technology of mimicking the photosynthesis of plants to convert sunlight into chemical energy has long been proposed. Through the combined action of sunlight and catalysts, scientists ionize water into hydrogen and oxygen. The mixture of these two gases is flammable, easy to store, and free of pollution. However, the catalyst used in this step is expensive and cannot be used on a large scale.
Recently, a study by the American Argonne Energy Laboratory and Harvard University has made remarkable achievements in catalysts.
Fig. D. Argonne of the United States Laboratory chemists Dugan Hayes, Lin Chen, and Ryan Hadt have found a process that can accelerate the electrolysis of water through cobalt-containing catalysts. Cobalt-containing catalysts are relatively inexpensive and can replace existing precious metal catalysts in the process of generating clean energy
The team used cobalt as a catalytic step. Cobalt is relatively abundant in nature and its cost is relatively low. In the corresponding case, it can be like a partner of hydrogen atoms and oxygen atoms to match this "electronic dance".
"In fact, we can see through cobalt the instantaneous fragments of the reaction, rather than a vague chemical change. It is very important to define the nature of the catalyst on the timescale of electron exchange."
"Cobalt-containing catalysts are active ingredients of materials such as artificial leaves, and we can use this material to synthesize solar fuels," said Ryan Hadt, one of the paper's first authors, Argonne researcher.
The electrolysis water reaction can be roughly divided into two parts. The researchers focused on the first part, which is the oxidation of water. This process requires the conversion of four protons and four electrons, and the formation of a covalent bond between the two oxygen atoms, so there must be one Other atom that temporarily bonds with the oxygen atom. This is what we need for the cobalt-containing catalyst. .
The reason why this reaction is worth investigating is that the process of cobalt and oxygen bonding takes less than one billionth of a second. To understand this process, scientists used X-ray absorption spectroscopy from the photon source at Argonne to make detailed measurements.
Analytical scientists discovered that the covalent bond between the two oxygen atoms is coordinated with the orbit of cobalt ions. At this time, each cobalt ion bonds with one water molecule. The chemical nature of the compound is temporarily stable. Immediately afterwards, the water molecule seizes an electron from the cobalt ion, which is to change the valence of the cobalt ion from positive trivalent to positive tetravalent.
Such high-priced cobalt ions will squeeze out the hydrogen atoms in the hydrogen-oxygen covalent bond and replace hydrogen and oxygen ions to form a bond. At this point the hydrogen atoms have been liberated and the cobalt ions will pull away the excess two electrons on the oxygen ions, so the oxygen is successfully reduced. Finally, zero-valent hydrogen atoms and oxygen atoms bond to form hydrogen molecules and oxygen molecules, respectively; in this way, gaseous solar fuels are born.
Through Argonne's photon source, researchers can directly measure the valence of cobalt during the reaction, and theoretically calculate a quantum mechanical value called "exchange coupling." This value defines the amount of electron spin between oxygen and cobalt. The researchers believe that the spin directions of these electron pairs are reversed, that is, they have antiferromagnetic properties.
“Antiferromagnetism plays an important role in the formation of oxygen covalent bonds,†adds Hadt. "Because it makes it possible to provide two electrons simultaneously for a chemical bond."
X-ray absorption spectroscopy has successfully observed the position of high-priced cobalt ions. "Ultimately, we saw the specific location and process of the reaction. We have seen the nature of the catalyst through the transformation of electrons."
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