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Nanotechnology research could take the cost out of catalysis

Chemical Engineering Professor Stacey Bent tries to improve future energy supply

Platinum jewelry looks brilliant around a neck or finger but no one luxuriates in paying hundreds of dollars to replace the catalytic converter in a car. Precious metal catalysts, which speed up chemical reactions, are good for the environment and industry–and they could enable huge improvements in our energy supply in the future–but they are rare and expensive. By exploring ways to reduce, if not replace, the need for these metals, chemical engineering Professor Stacey Bent is trying to lessen the number of greenbacks required to be green.

“Catalysis is a huge industry already and it’s going to become more important with all the new environmental and sustainable energy technologies,” she says. “So we’re trying to come up with better catalysts or at least find ways to use less of the rarer materials.”

Recently she and her collaborators, including former postdoctoral scholar Jeffrey King, were able to report a significant advance. It’s a porous, sponge-like chip of carbon called an aerogel, that they laced with tiny platinum particles. In catalysis the name of the game is to expose the reacting chemicals to the most catalyst surface area possible. A typical catalytic converter exposes platinum granules to reacting gases by suspending them in a rough coating on a ceramic honeycomb mesh. But Bent and King’s aerogels are even more porous and the platinum is better exposed, meaning they don’t have to use as much platinum to combine oxygen and carbon monoxide to make carbon dioxide. Looking beyond catalytic converters, Bent is optimistic that this will also be true for a lot of other catalyzed reactions, including ones that produce clean electricity in hydrogen fuel cells.

Better breathing through nanochemistry

In a paper published in the August issue of the journal Nano Letters, Bent’s team reported that platinum-infused carbon aerogels were able to convert virtually 100 percent of a sample of carbon monoxide to carbon dioxide using only 0.05 milligrams of platinum per square centimeter of internal surface area. Even though platinum sells for more than $2,000 an ounce, the 0.1 mg of total platinum in Bent and King’s aerogel chip would still only cost 0.7 cents.

The key to achieving so much catalysis with so little catalyst is in how the team infused platinum into the aerogel. They used a technique called atomic layer deposition (ALD), in which a vapor of platinum and other chemicals permeates the aerogel in a hot oven. The vapor is followed by oxygen, which helps fix the platinum into a metallic state. After each stage, the oven is flushed with nitrogen to remove all but the platinum, which stays bound to the aerogel surface. After just two cycles of ALD, enough platinum was in the aerogel to accomplish full catalysis.

The team included former graduate student Sandeep Giri, and also contributors from Lawrence Livermore National Lab, who made the aerogel, and researchers at the University of Bremen in Germany, who implemented the CO to CO2 reaction and measured the catalysis.

Oddly, what made ALD especially successful is the unusual way it happened to work in the aerogel. Normally ALD is useful for depositing incredibly thin sheets of material on a surface, but in the aerogel, rather than evenly coating all the spongy surfaces, it created tiny balls of platinum, each less than five billionths of a meter across. These “nanoparticles,” because of their spherical shapes, ended up exposing a lot of surface area – a fortuitous outcome.

“We weren’t sure what to expect,” Bent says. “It turned out to be a particularly nice result.”

Fueling the future

In a different device employing platinum catalysts, a hydrogen fuel cell, engineers have needed 2 to 8 milligrams per square centimeter of the metal to separate the hydrogen into a positive ion and a free electron (a reaction that generates electricity). It requires some extrapolation to say that results in the Nano Letters paper apply to fuel cells, but Bent says the goal of making fuel cells more economical by using only 1 milligram of platinum per square centimeter may now be within reach.

Carbon aerogels are promising for fuel cells not only because they are good catalyst hosts, she says, but also because they can conduct electricity, meaning little current would be lost as the electrons make their way out of the fuel cell and into the device the cell is powering, be it a car, a house or a laptop.

“That would be one of the next things we’d like to try,” Bent says. “We’ve already done some preliminary experiments on it.”

Improved catalysis could also be helpful in devices that store hydrogen for later use in fuel cells. In another area of alternative energy–renewable fuels–improved catalysis could make the chemical reactions employed to produce biofuels faster and cheaper, she adds.

Yet another environmentally beneficial idea would be to accelerate the breakdown of contaminants in wastewater, by using a light-sensitive material called titania. Placed in an aerogel, just like the platinum, titania and sunlight could work together to clean up water as it flows by.

Catalysis is so important in applying chemistry, that the possibilities that come with improving it seem endless.

“This was really a very fundamental study,” Bent says. “We just wanted to demonstrate initially that you could make a catalyst this way and we were pleasantly surprised at how catalytically active it was. Applying it to a device we can build and test would be the next step.”

Perhaps more pleasant surprises are in the near future.