In the quest for alternative energies, there is one treasure that many scientists have tried and failed to find. That treasure is the secret of photosynthesis, the chemical reaction that allows plants to convert light into energy, releasing nothing but harmless oxygen in the process. But now a chemistry graduate student at CalTech, Emily Tsui, has figured out a previously unknown aspect of this process. Her discovery could one day lead to cities powered entirely by artificial photosynthesis.
As any chemistry or physics student can attest, photosynthesis is a giant mess. It's a complicated chemical process whose many components are called "photosystem II." The upshot is this: Plants use photons to smash water molecules into their constituent parts, grabbing energy from electrons and releasing oxygen along the way. What Tsui discovered has to do with the part where electrons are converted into energy. She found that calcium, a metal found in seashells and your bones, is key to this process. Previously, nobody had been sure what role calcium played in photosynthesis.
According to a release from CalTech:
Emily Tsui prepared a series of compounds that are structurally related to the oxygen-evolving complex. She built upon an organic scaffold in a stepwise fashion, first adding three manganese centers and then attaching a fourth metal. By varying that fourth metal to be calcium and then different redox-inactive metals, such as strontium, sodium, yttrium, and zinc, Tsui was able to compare the effects of the metals on the chemical properties of the compound.
"When making mixed-metal clusters, researchers usually mix simple chemical precursors and hope the metals will self-assemble in desired structures," Tsui says. "That makes it hard to control the product. By preparing these clusters in a much more methodical way, we've been able to get just the right structures."
It turns out that the redox-inactive metals affect the way electrons are transferred in such systems. To make molecular oxygen, the manganese atoms must activate the oxygen atoms connected to the metals in the complex. In order to do that, the manganese atoms must first transfer away several electrons. Redox-inactive metals that tug more strongly on the electrons of the oxygen atoms make it more difficult for manganese to do this. But calcium does not draw electrons strongly toward itself. Therefore, it allows the manganese atoms to transfer away electrons and activate the oxygen atoms that go on to make molecular oxygen.
A number of the catalysts that are currently being developed to drive artificial photosynthesis are mixed-metal oxide catalysts. It has again been unclear what role the redox-inactive metals in these mixed catalysts play.
Translation: We now are one ingredient closer to making our own photosynthetic reactions. One day, buildings could essentially function like plants, transforming light and water into energy and oxygen. All we need to do is create an artificial photosynthesis system, building on research like Tsui's. Here's to the future of cities that behave like forests.
Read the full scientific article in Nature Chemistry.
Image by Emily Tsui.