Imagine a machine that could turn carbon dioxide, a harmful greenhouse gas, into a different substance –– something useful, like fuel. This device would not only slow climate change by reducing the amount of carbon dioxide in the atmosphere, but also provide a new energy source. Even better, imagine that this innovation is powered by the sun, using solar energy to convert one molecule into another.
This might seem like something out of a climate scientist's dream, an unattainable magical contraption. But chemists have already begun building the foundation of knowledge for this machine to become a reality. Most recently, one study made a breakthrough: for the first time, researchers have mapped the molecular details of how the solar-powered reaction occurs.
“CO2 is a very stable molecule,” said Dr. Tonü Pullerits, Professor of Chemical Physics at Lund University and an author of the study. Pullerits said that the first step to turning carbon dioxide into a more reactive, malleable substance, is transforming it to carbon monoxide.
“It will always cost you energy to go uphill from CO2 to CO. So, where can that energy come from?” said J. Houston Miller, a researcher not affiliated with the study and Professor of Chemistry at George Washington University. “The best answer is the sun.”
The study, published last month in Nature, comes from a large multi-national scientific collaboration of researchers from Denmark, Sweden, China and Germany.
“What makes our work special is that we explain how this reaction goes on… in quite a detailed way,” Pullerits said. “It was an exciting day when we realized that we can actually explain this.”
The researchers used two materials to drive the solar-powered reaction. The first is a covalent organic framework, or COF, that absorbs light, and serves as a structure where the conversion takes place. Pullerits described COF as a “micro porous material.” The shape of the material gives it a large surface area, allowing it to efficiently absorb light, for example from the sun. As a light source, Pullerits and colleagues shone laser pulses onto the COF. The second material is a catalytic complex containing the element rhenium, one of the rarest elements on earth. The complex is embedded in the COF, harnessing the light the COF absorbed to drive the reaction. Pullerits said knowledge of the catalytic complex is not new, but the addition of the COF is.
The study found its answers through spectrometry, a method of analyzing a material using waves of light. This work determined precisely how the catalytic complex and COF work together to capture light and harness it to convert carbon dioxide into carbon monoxide, a feat no study had yet reached.
“We were using short laser pulses to follow the process. We triggered the reaction with one pulse of a laser, and with a second pulse slightly later, we could figure out what happened.” Pullerits said.
In the study, light passed through the COF, and researchers observed what colors of light were absorbed and which weren’t. What colors a molecule will absorb reveal a lot about the molecule’s structure, Miller said.
The conversion from carbon dioxide to carbon monoxide requires adding electrons. The COF captures the light, and the light particles excite electrons, creating carbon monoxide. Then, researchers studied the way the light particles triggered the electrons to join the reaction.
However, researchers were puzzled by some of their observations at first. They knew one light pulse generated only half the electrons necessary for the conversion, but the reaction was occurring nonetheless. Where did the other half of the electrons come from?
Then, the team had a breakthrough.
“We all of a sudden realized… we can store the extra electrons needed in the porous COF material. The previous pulses made a few additional electrons which are stored. The COF is charged,” Pullerits said.
Finally, the team understood how the reaction occurs. The COF was not only absorbing light, but also absorbing electric charge by way of storing electrons. The new porous material they introduced was the key to the reaction. And through their use of light spectrometry, they knew the exact mechanism of how it all worked. This detailed knowledge allows researchers to recreate the reaction and find ways to make it more efficient, moving closer to the dream device.
Pullerits is modest about the implications of his work.
“My general picture of how science works and how it can hopefully help mankind… It’s like building a house. Everybody brings in their brick and at the end somewhere there is going to be something that is very important, solving some practical question,” Pullerits said. “I don’t dare to claim that our work is more than a brick in a wall, but it was important that we covered this area.”
Pullerits envisions a future with a fully built house, or “a device that lets the sun shine on it and starts converting CO2 to something more stable or usable.”
Now, researchers can draft next steps.
Pullerits continued: “Now that we understand what is going on, we can start asking the question, ‘What could be done to make it better?’”
"This is an evolutionary field. People need to keep doing it because somebody's going to make it work," said Miller. "We could have solar powered fans that essentially scrub the air all the time, but we need to do it soon."