Researchers at the University of Texas at Arlington (UTA) have developed an energy-efficient one-step process that converts carbon dioxide and water into useable liquid hydrocarbon fuels. Using concentrated light, heat and high pressures, the process also yields oxygen as a byproduct.
“Our process has an important advantage over battery or gaseous-hydrogen powered vehicle technologies as many of the hydrocarbon products from our reaction are exactly what we use in cars, trucks and planes, so there would be no need to change the current fuel distribution system,“ notes Frederick MacDonnell, UTA interim chair of chemistry and biochemistry who worked on the project.
Operating at 180° to 200°C and pressures up to 6 atmospheres, a photothermochemical flow reactor drives the conversion.
“We are the first to use both light and heat to synthesize liquid hydrocarbons in a single stage reactor from carbon dioxide and water,” explains Brian Dennis, UTA professor of mechanical and aerospace engineering who partnered with MacDonnell on the project. “Concentrated light drives the photochemical reaction, which generates high-energy intermediates and heat to drive thermochemical carbon-chain-forming reactions, thus producing hydrocarbons in a single-step process.”
For the experiment, the team used a hybrid photochemical and thermochemical catalyst based on titanium dioxide, a white powder that can’t absorb the entire visible light spectrum. An article in the Proceedings of the National Academy of Sciences contains more detail.
“The current catalyst is very stable but does not absorb visible light. We need to move to catalysts which absorb visible light and are generally less stable, so this will be a major thrust of our future work,” notes MacDonnell.
“Our next step is to develop a photo-catalyst better matched to the solar spectrum,” he adds. “Then we could more effectively use the entire spectrum of incident light to work towards the overall goal of a sustainable solar liquid fuel.”
The two researchers envision using parabolic mirrors to concentrate sunlight on the catalyst bed to provide both heat and photo-excitation for the reaction. Excess heat potentially could even drive related operations such as product separations and water purification, they point out.
To test the new catalyst, as well as alternative methods for immobilizing it, the team designed a new photoreactor, which just completed beta testing, note MacDonnell and Dennis.
“The capacity is low, on the order of a mL/day, but that’s enough to test productivity, selectivity and quantum efficiency of a new catalyst at higher pressures,” explains MacDonnell. “The new photoreactor is a microreactor configuration so it’s able to hold a relatively high pressure of 300 psig at 250°C under continuous flow, he adds.
Once the team has confidence in the new reactor’s performance, they plan to increase the reactor’s runtimes, letting it run overnight unattended, compared to the previous system’s 5–8 hour limit.
The team notes it’s too soon to tell what productivity and selectivity to higher hydrocarbons might be achievable, but believes the engineering challenges, while significant, are solvable.
“In practice, we would use a continuous flow reactor that lies coincident with the focal line of a long parabolic trough mirror. The reactants are under 20-atm pressure and are heated to 200°C while in contact with a solid catalyst. The light must be able to enter the reactor and strike the catalyst bed. Light below 1.5 eV (>800-nm wavelength) will not be absorbed in a manner that drives the photochemical reaction but can and should help provide the thermal energy needed to maintain the reaction temperature. Any excess thermal energy is absorbed by a working fluid circulating through the reactor. This heat energy can then be used in secondary processes such as seawater distillation (to feed the CO2 reduction reaction) or product separation. Lastly, the reactor needs to withstand the thermal cycling inherent in all solar processes,” says MacDonnell.