Research: Designing better catalysts for the conversion of polluting gases into useful products –

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 details of the effective reaction, which used a single ion cobalt to help reduce the energy 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 in simpler parts – carbon monoxide (CO) and oxygen – has 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 the manufacture of fuel, "said Anatoly Frenkel, chemist with a joint appointment to the Brookhaven Lab and Stony Brook University. Frenkel led efforts to understand the catalyst activity, which was carried out by Gonghu Li, a chemistry physicist at the University of New Hampshire.

"We now have proof 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 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" electrons, so to speak, making them available to the catalyst during 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 is perfect for removing electrons," said Frenkel.

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 CO₂Said 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 conduct chemical reactions has attracted a lot of interest," said Li. "The electrons generated by C3N4 under light irradiation have a sufficiently high energy to reduce CO₂. These electrons often do not have enough life to go 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.

"In order to perform X-ray absorption spectroscopy (XAS), we must tune and analyze the energy of the X-ray beam reaching the sample," Ehrlich said. "Each element can absorb X-rays under different energies, called" absorption edges ". On the new QAS light line, we can analyze the X-ray energy on the absorption edge energy of different elements, such as cobalt in this case. We then measure the number of photons absorbed by the sample for each value of the X-ray energy. "

In addition, explained Frenkel, "each atom type produces a different type of electronic ripple, when it is excited by X-rays or other ripples, so that the X-ray absorption spectrum you indicates which are the surrounding atoms. how far and how much is there. "

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, that was the proof that it was actually simple cobalt atoms scattered on the surface, "Frenkel said.

"These data also limit the possible structural arrangements, providing theorists with information to assess and fully understand the reactions," Frenkel added.

Although the science described in the paper is not yet used in a practical way, there are many potential applications, 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.