Making green fuels, no fossils required

Making green fuels, no fossils required

9:42 AM, 6th November 2015
Making green fuels, no fossils required
Nitrogen-doped carbon nanotubes proved to be efficient, and potentially inexpensive, catalysts for reducing carbon dioxide.

COLUMBIA, US: Using solar or wind power to produce carbon-based fuels, which are commonly called fossil fuels, might seem like a self-defeating approach to making a greener world.

But when the starting material is carbon dioxide, which can be dragged out of the air, the approach is as green as it gets. The technology that makes it economically feasible isn't available yet, but a recently published paper presents nice step forward in the effort to not just sequester CO2, but turn it into a useful fuel that is part of a carbon-neutral future.

Solar panels and wind turbines are most typically used to produce electricity, but on a large scale, electricity from sources like these poses problems.

An alternative that has been long talked about is to use that green electricity to kick CO2 up the energy ladder. Carbon dioxide, the combustion by-product that comes out of power plant smokestacks and is getting too plentiful in the Earth’s atmosphere, is at the bottom of the hill when it comes to carbon-based fuels.

If you could add some energy to it, though, you could convert CO2 into carbon compounds that are fuels, not a waste product. In chemical parlance, it’s called reducing CO2 when you convert it to less-oxidized forms of carbon, all of which have actual fuel value. Some single-carbon molecules to aim for would include (in increasing energy content) carbon monoxide (CO), methanol, and methane.

Any of these could be stored for a cloudy or windless stretch of time, and in most situations a lot more readily than electricity. Methane is the primary component of natural gas, for which there is already plenty of existing infrastructure. Methanol, or wood alcohol, is a close relative of ethanol, or grain alcohol, and is routinely used as a liquid fuel. Carbon monoxide might seem unusual in this context, but it has chemical value as a fuel, both in and of itself and as a precursor to other fuels.

The trick, of course, is to be able to do the CO2 reduction economically. That means not just efficiently converting the electrical energy into chemical energy, but also making the device that does the job a cost-effective one.

Xiao-Dong Zhou, an associate professor of chemical engineering at the University of South Carolina and his research team published their research in the journal in Angewandte Chemie.

As a starting point for making the catalysts, they used a model of carbon nanotubes, which are made purely of carbon atoms. But in making their catalysts for CO2 reduction, they departed from the carbon-only motif by sprinkling in a few nitrogen atoms to create a different kind of geometric and electronic structure.

The resulting “nitrogen-doped carbon nanotubes” proved to be adept at reducing CO2 to CO, and the team reports that the catalysts are more stable than metal-based catalysts reported in the literature for the same reaction.

When a nitrogen atom is substituted into a position where a carbon atom belongs in a carbon nanotube, it turns out that several distinct chemical bonding patterns can result. The team showed that one of them, termed the pyridinic structure, was the most effective as an electrocatalyst, and was competitive even with much more expensive precious metal catalysts that have been reported for CO2 reduction.

“We are working in conjunction with other institutions, and they are developing the other side, the water side, using photovoltaics to split water, and eventually we want to couple those two reactions together,” said Zhou. “So one side will be water splitting, generating protons from the anode that travel through the electrolyte to reach the cathode side and then react with carbon dioxide and with incoming electrons to convert carbon dioxide to fuels.”

© University of South Carolina News



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