Tackling CO2 conversion challenge

ENERGY SAVER: Brown University chemistry professor Shouheng Sun is part of the team that demonstrated that a unique core-shell nanoparticle is a cheaper, more active and longer-lasting fuel-cell catalyst than commercially available platinum products. / COURTESY BROWN UNIVERSITY/MIKE COHEA
ENERGY SAVER: Brown University chemistry professor Shouheng Sun is part of the team that demonstrated that a unique core-shell nanoparticle is a cheaper, more active and longer-lasting fuel-cell catalyst than commercially available platinum products. / COURTESY BROWN UNIVERSITY/MIKE COHEA

A group of Brown University scientists hopes it has uncovered a climate-change-fighting breakthrough hiding in gold dust.
Or gold nanoparticles, more precisely, the subject of one series of experiments within a joint Brown-Yale University research partnership investigating ways to sequester and convert earth-warming carbon dioxide into valuable industrial chemicals.
“These chemicals are made on a multimillion-ton scale and are all sourced from petroleum,” said G. Tayhas R. Palmore, professor of engineering, chemistry, and medical science at Brown and director of the joint research group, the Center for the Capture and Conversion of CO2.
“We have a number of chemical engineers with interests in taking carbon dioxide from the atmosphere or exhaust plumes and making something useful,” she said. “We think we can develop a new chemical methodology from converting CO2 instead of converting from oil.”
The Center for Capture and Conversion was launched a year and a half ago with $1.7 million in seed money from a National Science Foundation grant.
It includes seven university faculty members, six of them from Brown and one from Yale, plus students and post-doctoral researchers, for a total of 45 people in areas including molecular chemistry and carbon engineering.
Experts have known that gold, when combined with electricity, can catalyze a chemical reaction turning carbon dioxide into carbon monoxide, but no one has yet been able to do it on a large-enough scale to be commercially or environmentally viable.
Existing processes have either used too much electricity – requiring more carbon burned than is sequestered – or required too much gold. Larger pieces of gold are also insufficiently selective, producing carbon monoxide, but also unwanted hydrogen.
But Brown chemistry professor Shouheng Sun and engineering professor Andrew Peterson discovered that reducing gold to very specific size pieces – 8 nanometers – produced much better results than expected. Sun expected that the smaller they made gold nanoparticles, the more efficient they would be at converting carbon dioxide to carbon monoxide, because a greater number of smaller particles would have a larger surface area.
They were surprised that when they went smaller than 8 nanometers, however, the reactions became less efficient.
“This baffled us for a while,” Sun said. “We thought 4 nanometers should be more active, but when we tested it, 8 nanometers was the best.”
It was only when Peterson, the “chemical computational” expert, began studying the shape of the nanoparticles that he discovered that the edges of gold particles are crucial to their efficiency for attracting carbon. Particles at 8 nanometers had the maximum edge length while anything smaller had too many corners and not enough edges, Sun said.
When electricity was applied to the 8 nanometer gold nanoparticles the resulting reaction produced 90 percent carbon monoxide, with only 10 percent comprised of other molecules. The energy required to produce that reaction was also reduced dramatically.
The group has chosen to focus on converting carbon monoxide, because it is a good “starter material” that can be easily turned into other compounds through relatively simple reactions.
Formic acid, acrylic acid and methylene are the three compounds the group is focused on making from the carbon monoxide, as they in turn are building blocks to make things like synthetic natural gas and plastics.
The discovery of an ideal size and shape for gold nanoparticle catalysts will not in itself result in a commercial solution to sequestering carbon, but could be significant when combined with other work on the problem, including separate efforts at the Center for the Capture and Conversion.
Sun said the next steps in the research will include testing materials other than gold to see whether a better catalyst can be produced using the nano-science processes and knowledge used in the first experiments. In conjunction with the Brown Technology Ventures Office, they are patenting discoveries as they go and hope at some point to draw the interest of large companies, entrepreneurs or investors able to commercialize it.
At the same time, other scientists at the Center for Capture and Conversion are working on other aspects of the sequestration challenge, including chemistry at the particle and bulk-metal level, as well as the nano level.
Then there is investigation into the best forms with which to convey electricity to the catalysts, through different materials and shapes; and how to best concentrate CO2 from the atmosphere from exhausts or from the diffuse amounts in the atmosphere, through to something more like sparkling water.
The ultimate goal is to improve all of these processes enough that they draw interest from the major chemical-producing companies, like BASF or Proctor & Gamble, which now rely on petroleum as a primary raw material.
While previous National Science Foundation grants for carbon conversion have focused on alternative fuels, Palmore said the round that her group secured targeted chemicals, at least partially in response to demand from the industry.
In addition to the Brown-Yale group, researchers at the University of California at Santa Barbara and University of Wisconsin are also working on carbon with NSF grants.
To grow the Capture and Conversion Center further, Palmore said she is looking at a round of “Phase 2” grants about one year from now. Those grants provide about $4 million per year for a decade.
“What [Sun and other researchers] have done is open a new door to catalysis and these are the things NDF looks at to see that this is a group of people who could solve this problem,” Palmore said. “CO2 conversion to useful chemicals and loosening this carbon-usage loop is a grand challenge.” •

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