Chemists develop new catalyst that turns CO2 into gasoline

CAMBRIDGE, US: MIT chemists have created a new catalyst material that gives key vision into the design requirements for producing liquid fuels from carbon dioxide, the leading component of greenhouse gas emissions.

The results advise a route toward using the world’s existing infrastructure for fuel storage and distribution, without adding net greenhouse emissions to the atmosphere.

The new catalyst takes the process only through its first stage — converting carbon dioxide (CO2) to carbon monoxide (CO), explained assistant professor of chemistry Yogesh Surendranath, the senior researcher on a new study describing the advance.

But that’s a key early step toward converting CO2 to other chemicals including fuels; there are already established methods for converting CO and hydrogen to a variety of liquid fuels and other products.

The study appears in the journal Angewandte Chemie. Its authors are Youngmin Yoon, a graduate student at MIT; Anthony Shoji Hall, a former MIT postdoc who is now a professor of materials science at Johns Hopkins University; and Surendranath, assistant professor at MIT.

“The problem in CO2 conversion is how to selectively convert it,” Surendranath said.

While this basic molecule can form the basis of virtually any carbon-based chemistry, the tricky part is to create a system in which CO2 consistently converts to a single end-product that can then be further processed into the desired material. The new system, he says, provides just that kind of selective, specific conversion pathway — and, in fact, a whole range of such pathways. And if the hydrogen and CO are produced using solar or wind-generated power, the entire process could be carbon neutral.

Tunable conversion

“What you want is a tunable catalyst,” he said, and that’s just what this team developed, in the form of a highly porous silver electrode material. Depending on the exact formulation of this material it’s possible to design variations of this catalyst where “each one may be designed for a different application.”

The scientists learned that by tuning the dimensions of the material’s pores they could get the system to produce the desired proportion of CO in the end-product.

Most efforts to tune the selectivity of silver catalysts for CO production have focused on varying the surface-active site chemistry. However, with this formulation, a material called a silver inverse opal, it is the pore structure of the material that determines the effect. “What we found was very simple,” Surendranath said. “You can tune the pore dimensions to tune the selectivity and activity of the catalyst, without modifying the surface-active site chemistry.”

Honeycomb structure

The porous material can be made by depositing tiny polystyrene beads on a conductive electrode substrate, then electrodepositing silver on the surface, then dissolving away the beads, leaving pores whose size is determined by that of the original beads. Because of the way spheres naturally organize themselves when packed together, this method produces a honeycomb-like structure of hexagonal cells, Surendranath explained.

It turns out that varying the thickness of this porous catalyst produces a double effect: As the porous inverse opal get thicker, the catalyst more strongly promotes the production of CO from CO2 by up to three times, while also suppressing an alternative reaction, the production of H2 (hydrogen gas), by as much as tenfold. Using this combined effect, production of CO can be easily varied to make up anywhere from 5 to 85 percent of the reaction’s output. The study’s results provide fundamental insights that may be applicable to designing other catalyst materials for fuel production from CO2.

Finally, conversion plants could be connected directly to the emissions flow from fossil-fuel power plants, for example, to turn the CO2 into fuel instead of releasing it into the atmosphere at all.

“We’re very optimistic” that the process can be successfully developed, Surendranath said. If so, that could represent “the closing of the anthropogenic carbon cycle,” through the use of renewably generated electricity to turn greenhouse gas emissions into fuel.

In essence, the net process would be doing the same thing that plants and cyanobacteria did on Earth millions of years ago to produce fossil fuels in the first place: taking carbon dioxide out of the air and converting it into more complex molecules. But in this case, instead of taking place over millennia, the process needs to be replicated very quickly in a lab or factory. “It’s the same thing that got us these fuels in the first place,” he said, “but we need to do it faster and more efficiently than natural photosynthesis.”

The research was supported by the Air Force Office of Scientific Research and the MIT Department of Chemistry, and is part of the research taking place through the MIT Energy Initiative’s Low-Carbon Energy Centers, established as part of the Institute’s Plan for Action on Climate Change.

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