A team of scientists has discovered a visible light-activated single-site catalyst that converts carbon dioxide (CO2) into basic molecules that can be used to create useful chemicals. The discovery opens the possibility of using sunlight to turn a greenhouse gas into hydrocarbons.
Scientists used the National Light Source Synchrotron II, a user facility from the Department of Energy (DOE) Science Office of the Brookhaven National Laboratory, to discover the details of the effective reaction, which used a single ion cobalt to help reduce the barrier break the CO2. The team describes this single-site catalyst in an article recently published in the Journal of the American Chemical Society.
CO conversion2 simpler parts – carbon monoxide (CO) and oxygen – have valuable applications in the real world. "By breaking the CO2, we can kill two birds with one stone – eliminate CO2 from the atmosphere and manufacture basic elements for fuel fabrication, "said Anatoly Frenkel, chemist with a joint appointment to the Brookhaven Lab and Stony Brook University Frenkel led the effort to understand the catalyst activity, performed by Gonghu Li, a chemistry physicist at the University of New Hampshire.
"We now have evidence that we have manufactured a single-site catalyst.No previous work has reported solar CO2 reduction using a single ion, "said Frenkel.
Breaking the bonds that hold the CO2 together takes a lot of energy and a lot of time. Thus, Li has developed a catalyst to lower the energy barrier and speed up the process.
"The question is, between several possible catalysts, which ones are effective and practical to implement in the industry?" said Frenkel.
A key ingredient needed to break the bonds of CO2 is a source of electrons. These electrons can be generated when a material called semiconductor is activated by energy in the form of light. Light "expels" the electrons, so to speak, by putting them at the disposal of the catalyst for chemical reactions. Sunlight could be a natural source of this light. But many semiconductors can only be activated by ultraviolet light, which is less than five percent of the solar spectrum.
"The challenge is to find another semiconductor material where the energy of natural sunlight will be perfect for ejecting electrons," Frenkel said.
Scientists also needed the semiconductor to be bonded to a catalyst made from natural materials rather than expensive rare metals such as platinum. And they wanted the catalyst to be selective enough to elicit only the reaction that converts CO2 co.
"We do not want electrons to be used for reactions other than CO2"Said Frenkel.
Cobalt ions bonded to graphitic carbon nitride (C3N4), a semiconductor consisting of carbon, nitrogen and hydrogen atoms, have met all their expectations.
"The use of C3N4 as a metal-free semiconductor to capture visible light and drive chemical reactions has sparked a keen interest," said Li. "The electrons generated by C3N4 under light irradiation have sufficient energy to reduce CO2. These electrons often do not have a sufficient life span to allow them to get to the surface of the semiconductor for use in chemical reactions. In our study, we adopted a common and effective strategy to create enough energetic electrons for the catalyst using a sacrificial electron donor. This strategy allowed us to focus on the catalysis of CO2 reduction. In the end, we want to use water molecules as electron donors for our catalysis, "he added.
Peipei Huang, a postdoctoral researcher at Li's lab, made the catalyst by simply depositing cobalt ions on a C3N4 material made from commercially available urea. The team then thoroughly examined the synthesis catalyst using various techniques in collaboration with Christine Caputo of the University of New Hampshire and Ronald Grimm of the Polytechnic Institute of Worcester.
The catalyst worked in CO2 reduction under irradiation by visible light.
"This catalyst did what it was supposed to do – break down CO2 and make CO with very good selectivity in visible light, "said Frenkel. But the next goal was to see why it worked. If you can understand why this works, you can create new, more efficient materials based on these principles. "
Thus, Frenkel and Li reflected on experiments that would accurately show the structure of the catalyst. Structural studies would provide scientists with information on the number of cobalt atoms, their location relative to carbon and nitrogen atoms, as well as other features that scientists could possibly adjust to try to further improve the catalyst.
They turned to the XAS fast light absorption and scattering light (QAS) line at NSLS-II to utilize x-ray absorption spectroscopy. With the help of Steven Ehrlich, Light Line Specialist, Frenkel student Jiahao Huang took the data and analyzed the spectra.
In this technique, NSLS-II X-rays are absorbed by the atoms of the sample, which then eject electrons waves. Spectra show how these electron waves interact with surrounding atoms, in the same way that ripples on the surface of a lake are disturbed when they encounter rocks.
"To be able to do X-ray absorption spectroscopy (XAS), we have to tune and sweep the energy of the X-ray beam that hits the sample," Ehrlich said. "Each element can absorb X-rays at distinct energies, called absorption edges.On the new QAS light line, we can analyze the energy of x-rays on the edge energy of Absorption of different elements, such as cobalt, the number of photons absorbed by the sample for each value of X-ray energy.
In addition, explained Frenkel, "each type of atom produces a different type of electronic ripple, when it is excited by X-rays or by other ripples, so the Spectrum of X-ray absorption tells you the names of the surrounding atoms, how far and how much there is. "
The analysis showed that the catalyst decomposing CO2 consisted of single cobalt ions surrounded by nitrogen atoms on all sides.
"There were no cobalt-cobalt pairs, so it was evidence that it was actually simple cobalt atoms scattered on the surface," Frenkel said. .
"These data also limit the possible structural arrangements, allowing theorists to evaluate and fully understand the reactions," added Frenkel.
Although the scientific knowledge described in the document is not yet used, there are many application possibilities, said Frenkel. In the future, these single-site catalysts could be used in large-scale areas where sunlight is abundant to eliminate excess CO2 in the atmosphere, in the same way that plants break down CO2 and reuse its constituent elements to build sugars during the process of photosynthesis. But instead of making sugars, scientists could use the building blocks of CO to generate synthetic fuels or other useful chemicals.
This research was funded by the DOE Office of Science and partly by the National Science Foundation.