Duke researchers may have found the recipe for better safe fuel

Duke researchers may have found the recipe for better safe fuel 
April 21, 2017

Natural gas may be much cleaner than coal, but it has some surprisingly filthy qualities. 

It comes from dirty places like shale beds, swamps, land fills and even hog waste lagoons. Its main ingredient, methane, always starts out with some extra ingredients mixed in. Things like stinky hydrogen sulfide, useless carbon dioxide and methane’s overbearing big brothers like ethane and propane come out of the ground with the methane and need to be removed. This impure natural gas can even start out dissolved in crude oil.

Natural Gas Fire

To get around all that mess, and the possible messes that can occur during natural gas drilling and fracking, scientists have been trying for years to figure out the best chemistry to make unnatural gas, or methane from a lab.

Currently, the process that engineers use to make methane involves high temperatures, expensive catalysts, super-precise control over the amounts of each reactant and a methane product that still has extra water and carbon monoxide milling around. 

Recently, though, Duke University chemist Jie Liu and colleagues at the Army Aviation and Missile RD&E Center discovered a new way to make nearly pure methane at room temperature, without precisely controlling the reactants. Their tools: rhodium metal nanoparticles and ultraviolet light.

Further, the reaction breaks down carbon dioxide from the air for every molecule of methane, so this clean chemistry could have a cleaning effect on the atmosphere. The research is published in the journal Nature Communications.

Every chemist who has to stand at a lab bench and perform reactions is looking for a specific product, and reactions that favor one specific product are called selective. The trouble is that the same reactants in the same conditions can give you multiple different products. In this specific reaction, which is far from selective, when the carbon dioxide and hydrogen gases float by the rhodium nanoparticles, the rhodium will grab onto the carbon dioxide and rip an oxygen atom off so hydrogen atoms can jump on to make the methane, which is just carbon and hydrogen. The nanoparticles were developed to perform this reaction at temperatures higher than 300 degrees Celsius, but under those conditions, only about half of the carbon dioxide is converted to methane. The other half becomes carbon monoxide, which makes for poor fuel.

Rhodium Light Reaction

When the researchers, instead, shined UV light on the reaction, the rate of methane production jumped by seven times and the reaction produced almost no carbon monoxide. Using a blue LED, both the selectivity and the reaction rate dipped a little, but they were still far better than the heat-based reaction.

Liu says the fact that light can tune the selectivity of this reaction so powerfully could provide chemists with a powerful new tool.

“The fact that you can use light to influence a specific reaction pathway is very exciting,” Liu said in a press release. “This discovery will really advance the understanding of catalysis.”

The nanoparticles act as a catalyst for the reaction. Many chemical reactions require a little boost of energy to get started, called an activation energy. Catalysts reduce this activation energy, which allows reactions to proceed and speed up. Once the reaction is done, the catalyst returns to its original state and is ready to repeat the reaction.

The rhodium nanoparticles catalyze the reaction by forcing electrons into the carbon dioxide, which breaks off one of the oxygen atoms. It is in this state that the carbon can take the remaining oxygen atom and run with it to make carbon monoxide or stick around to pick up the hydrogen it needs to make methane.

The light, the researchers found, removes the carbon monoxide path as an option. The nanoparticles are engineered with a specific shape and size to interact with light, and when the correct wavelength of light hits the nanoparticles, electrons in the nanoparticles get “hot” meaning they gather a large amount of kinetic energy and can move through the catalyst. This property is called plasmonic. These hot electrons attach themselves to the transitioning carbon dioxide in such a way that rearranging itself into carbon monoxide is nearly impossible.

Several Thousand Dollars Worth of Rhodium

In terms of making pure unnatural gas, this reaction is nearly unbeatable. This reaction, however, was done in a lab, and there are always issues when scaling up a reaction to an industrial scale.

Perhaps the most pressing issue is that rhodium, like its chemical brothers, cobalt and iridium, and its neighbors, platinum, palladium silver and gold, is great for catalyzing reactions, but is extremely rare and very expensive. Rhodium is in the same price range as gold, costing around between $920 and $2,750 per ounce according to different sources.

We already use rhodium as a catalyst for manufacturing fertilizer, the catalytic converters in our cars and many other industrial processes, and if we wanted to use it on a large scale for unnatural gas, the price would most likely go up. The fact that cheap, low-energy LED light can power the reaction instead of lots of expensive heat would defray the cost somewhat, but creating large factories for unnatural gas would still cost a pretty penny.

There is the possibility that another metal could do the job, but so far the chemistry has not worked out. In fact, in this study, the researchers tried replacing the rhodium with gold, but under light, the gold made sure the reaction spat out carbon monoxide instead of methane.

Even if this specific reaction cannot guarantee easy-to-make unnatural gas, the knowledge that light can adjust the specificity of this reaction could help chemists get better results on hundreds of other reactions.

—Daniel Lane

Daniel Lane covers science, medicine, engineering and the environment in North Carolina.

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