Clean Fuels from Greenhouse Gas

A new catalyst material could eventually offer a way to produce fuels—including gasoline, jet fuel, and diesel—from carbon dioxide. Such conversion, if it can be done at large scale, could make it possible to continue using the world’s existing infrastructure for fuel storage and distribution without adding any net greenhouse-gas emissions to the atmosphere.

The innovation, a porous silver electrode material that acts as a catalyst, makes it feasible to convert carbon dioxide and water to carbon monoxide and hydrogen. That reaction represents the first step toward converting the greenhouse gas into other chemicals, including liquid fuels, explains Yogesh Surendranath, PhD ’11, an assistant professor of chemistry, who was the senior author on a study describing the work. Established methods exist for converting carbon monoxide and hydrogen to valuable fuels or other products. “The problem in CO2 conversion is how to selectively convert it,” Surendranath says—that is, how to end up with carbon monoxide and hydrogen gas in the desired proportion to provide the feedstock for the next chemical processing steps. The new system, he says, provides just that kind of selective, specific conversion pathway.

Surendranath says that by simply “changing the mesostructure” of the porous material—that is, by tuning the lengths of its pores—it’s possible to get the system to produce the desired proportion of carbon monoxide in the output gas.

Surendranath and his MIT colleagues made the catalyst material by depositing tiny polystyrene beads onto a conductive base. After electrodepositing silver on the surface, they dissolved away the beads, leaving pores whose size and length are determined by the diameter of the beads and the thickness of the porous silver film.

Varying the lengths of the pores produces a double effect. As the pores get longer, the catalyst promotes the production of carbon monoxide from carbon dioxide up to three times more strongly. It also suppresses by a factor of up to 10 the production of hydrogen gas from the water that is mixed with the carbon dioxide. By changing the pore dimensions, the production of carbon monoxide can be varied to make up anywhere from 5 to 85 percent of the reaction’s output. For some applications, the hydrogen can also be a useful product.

Surendranath says the work provides key insights to help develop carbon-­neutral replacements for fossil-fuel systems—without requiring changes in the existing infrastructure of gas stations, delivery vehicles, and storage tanks.