Renewable fuel from vehicle exhaust: Oak Ridge’s catalytic breakthrough

Researchers from Oak Ridge National Laboratory have come up with a highly-efficient process to make liquid fuels directly from carbon dioxide and water.

Because burning fuels in an internal combustion engine produces CO2 and water as its byproducts, the new discovery is, essentially, a form of reverse combustion. It’s a simple concept, but over the years it has proven to be difficult science.

The best news? Namely this: the solution of carbon dioxide dissolved in water turned into ethanol with a yield of 63 percent. Typically, this type of electrochemical reaction results in a mix of several different products in small amounts.

And let’s add this: the process relies on low-cost materials, and operates at room temperature in water — so there’s a distinct hope that the process will scale cost-effectively.

Applications

Ideas abound for applications for the low-cost technology.

Wind and solar. The ORNL research team proposed that the process could be used to store excess electricity generated from variable power sources such as wind and solar. The intermittent nature of wind and solar energy production has been a well-known barrier to using these forms of renewable energy to provide base load to the electrical grid. So, there’s that.

All aboard! One of these days, though, why not an onboard system? After all, we use onboard catalytic converters in vehicles every day to reform carbon monoxide, volatile organic compounds  and nitrogen oxides into non-toxic CO2 and oxygen emissions before they leave the engine exhaust pipe.

Something nearer-term? Think biogas from dairy operations. We suggested in “Bossie the Climate Warrior” earlier this month that methane emissions can be flared to convert methane to CO2, and herein combined with on-site water to make ethanol. That’s a second value stream for a dairy farmer who, at best, right now is generally looking at burning methane to produce power or compressing it for CNG vehicles. Here’s a higher value use.

The catalyst

The key is in the nanotechnology-based catalyst which contains multiple reaction sites, The ORNL team use tiny spikes of carbon and copper to turn carbon dioxide, a greenhouse gas, into ethanol. Their finding, which involves nano-fabrication and catalysis science, was serendipitous.

The catalyst’s novelty lies in its nanoscale structure, consisting of copper nanoparticles embedded in carbon spikes. This nano-texturing approach avoids the use of expensive or rare metals such as platinum that limit the economic viability of many catalysts. ORNL researchers developed a catalyst made of copper nanoparticles (seen as spheres) embedded in carbon nanospikes that can convert carbon dioxide into ethanol.

“We discovered somewhat by accident that this material worked,” said ORNL’s Adam Rondinone, lead author of the team’s study published in ChemistrySelect. “We were trying to study the first step of a proposed reaction when we realized that the catalyst was doing the entire reaction on its own.”

“We’re taking carbon dioxide, a waste product of combustion, and we’re pushing that combustion reaction backwards with very high selectivity to a useful fuel,” Rondinone said. “

“By using common materials, but arranging them with nanotechnology, we figured out how to limit the side reactions and end up with the one thing that we want,” Rondinone said. “Ethanol was a surprise — it’s extremely difficult to go straight from carbon dioxide to ethanol with a single catalyst.”

How it works

It appears that the spiky textured surface of the catalysts provides ample reactive sites to facilitate the carbon dioxide-to-ethanol conversion. “They are like 50-nanometer lightning rods that concentrate electrochemical reactivity at the tip of the spike,” Rondinone said.

The energy limitation

It’s not a perpetual energy machine. The process consumes energy in the form of electric power, so the primary applications will be cases where there is excess electricity available, or whether the of social cost CO2 emissions are worth more than the costs of running the process.  Hence the scenario proposed with excess energy from wind or solar energy production.

“A process like this would allow you to consume extra electricity when it’s available to make and store as ethanol,” Rondinone said. “This could help to balance a grid supplied by intermittent renewable sources.”

The researchers plan to refine their approach to improve the overall production rate and further study the catalyst’s properties and behavior.

More on the story

The study is published as “High-Selectivity Electrochemical Conversion of CO2 to Ethanol using a Copper Nanoparticle/N-Doped Graphene Electrode.” and you can find it here.

The research team included Yang Song, Rui Peng, Dale Hensley, Peter Bonnesen, Liangbo Liang, Zili Wu, Harry Meyer III, Miaofang Chi, Cheng Ma, Bobby Sumpter and Adam Rondinone.