Researchers recycle GHG into fuel

A team of researchers at the Pennsylvania State University has discovered a method of turning captured carbon dioxide into methane, a combustible fuel, using solar energy. Led by Craig Grimes, a professor of electrical engineering and materials science and engineering, the team described a highly efficient photocatalyst that can yield significant amounts of methane, other hydrocarbons, and hydrogen in a simple, inexpensive process.

Figure 1: A proposed flow-through reactor for more efficient conversion of carbon dioxide to methane.

The researchers used arrays of nitrogen-doped titania nanotubes sputter-coated with an ultra-thin layer of either a platinum or copper co-catalyst. The titania captures high-energy ultraviolet wavelengths, while the copper shifts the bandgap into the visible wavelengths to better utilise the part of the solar spectrum where most of the energy lies. In addition, the thin-walled nanotubes increase the transport ability of the charge carriers by reducing the chance for recombination of the electron with the hole.

The nanotube arrays were placed inside a stainless steel chamber filled with carbon dioxide infused with water vapour. The chamber was then set outdoors in sunlight. After a few hours, the team measured the amount of carbon dioxide converted into useful fuels. The results showed 160µL of methane per hour per gram of nanotubes, a conversion rate approximately 20 times higher than previous efforts done under laboratory conditions using pure UV light.

Figure 2: A batch reactor for converting carbon dioxide to methane using sunlight.

“Copper oxide and titanium dioxide are common materials,” said Grimes. “We can tune the reaction using platinum nanoparticles or ideally other, less expensive catalysts.” Grimes believes that the conversion process can readily be improved by several orders of magnitude, which could make the process economically feasible.

“You could have a small-scale solar condenser and a concentrated source of carbon dioxide in a closed loop cycle to make a portable fuel. It’s a good way of storing energy for when the sun goes down,” he suggested. Inexpensive solar concentrators could improve the process, as the photocatalytic carbon dioxide conversion appears to scale with the intensity of sunlight.

Capturing carbon dioxide at source points, such as fossil fuel (coal, natural gas, etc.)-burning power plants, and turning it into a transportation fuel in a cheap, sunlight-driven process could dramatically improve the economics of carbon dioxide capture. “Then maybe we could figure out how to capture and reuse the carbon dioxide in our vehicles and none of it would go back into the atmosphere,” Grimes said.

Future research will look into increasing conversion rates by modifying the co-catalysts and changing the reactor design from a batch reactor to a flow-through photocatalytic design. “We are now reaching for low hanging fruit,” said Grimes. “There is plenty of opportunity for dramatic improvements.”

Fossil fuel use, ranging from electricity-generating power plants to automobiles, pumps billions of tons of greenhouse gases into the atmosphere annually, changing the climate in ways that are likely to be detrimental to future generations. The rising use of fossil fuels, driven by population growth and rising standards of living across the globe, adds to the urgency of finding a solution to the problem of rapidly increasing atmospheric carbon dioxide, the major greenhouse gas. At Penn State, a team of researchers led by Craig Grimes has come up with an ingenious method of turning captured carbon dioxide into methane, a combustible fuel, using the energy of the sun.