Pretreatment of palladium with steam could improve methane combustion
The chemical methane, as the largest component in natural gas, is a familiar fuel, but it is also a greenhouse gas many times more potent than carbon dioxide in trapping the sun’s warming rays in the atmosphere.
Vehicle engines spew lots of methane, and excess methane is burned up in flares that are a common sight above refineries, methane storage facilities and coal mines the world over. Unfortunately, that still leaves quite a large amount of natural gas emitted into the skies.
“Flares are supposed to be 99 percent efficient, but in real-world conditions, they are significantly less than that. Car engines aren’t much better,” said Matteo Cargnello, assistant professor of chemical engineering at Stanford University, who may have found a solution to this challenge.
Cargnello and a team of researchers from Stanford, the University of Wisconsin-Madison and the National Institutes of Standards and Technologies (NIST) have discovered a relatively simple pretreatment of catalysts based on palladium that can make methane combustion more efficient. It could lead to new types of catalytic flares and cleaner burning car engines that could keep untold tons of heat-trapping methane out of the skies.
Pretreatment is a process in catalysis in which the catalyst is exposed to other chemicals, like oxygen or hydrogen, to improve its reactivity or purity. The key ingredient in Cargnello’s palladium pretreatment, the researchers were surprised to learn, is steam.
In a paper describing the discovery appearing in the journal Science, the researchers compare the performance of steam to other common pretreatments with individual elements – oxygen, hydrogen and an inert gas. They found that the steam improved methane combustion rates by about 12 times overall, with the potential to stretch that improvement considerably.
“This effect is valid only with steam, not the other chemicals,” Cargnello said. “It’s also very stable. Once you have created the effect, it is permanent.”
Cargnello is an expert in catalysis – a field not accustomed to surprises – and he and his colleagues were astonished to see that their finding ran counter to the conventional wisdom that steam is detrimental to palladium catalysts in methane combustion. “Explaining how this could be was no small task,” Cargnello said. “It took us two and a half years to figure out why it was better.”
Cargnello called on the paper’s first author, Weixin Huang, who did hundreds of experiments to verify the effect was, in fact, real. He also turned to colleagues at NIST and UWM. At NIST, his collaborators included researchers Aaron Johnston-Peck and Wei-Chang “David” Yang in the Physical Measurement Laboratory.
The pair are experts in electron microscopy and helped establish visual evidence of the team’s evolving theories. Johnston-Peck and Yang were able to provide visual proof that steam introduces tiny imperfections in palladium’s crystal structure that are responsible for the outsized and important improvement in methane combustion.
“In a perfect crystal, the palladium atoms would be arranged in neatly aligned rows and ranks, like a stack of cannonballs. With the addition of steam at high temperature, the neat array undergoes strain and some of the ranks shift ever so slightly. We see the improvement in methane combustion precisely when these shifts occur,” said Yang, a materials research engineer.
The team at UWM, led by Manos Mavrikakis, professor of chemical and biological engineering and an expert in computational catalysis, used advanced computer models of atomic physics to establish the theoretical underpinnings of the steam effect on the crystalline palladium.
Mavrikakis helped the team understand how the atoms near the defects are forced into “uncomfortable” positions, a condition known as strain, which helps oxidation of palladium, and how that effect, in turn, speeds the methane combustion reaction. The strained areas around these defects can activate the carbon-hydrogen bond in methane much easier than in unstrained palladium.
“Steam pretreatment is unique among the other pretreatments in producing these key atomic-scale structural defects on palladium,” Mavrikakis said. “The combination of these two effects can explain the reactivity enhancement measured for steam pretreated catalysts almost exactly as predicted quantitatively.”
The team then showed that the improvement in methane combustion is highest precisely where these shifts in the crystal occur. The researchers estimate that at the point of greatest strain, methane combustion improves by 12 times. Additional preparation of the catalyst particles with a laser, in collaboration with Stanford’s Aaron Lindenberg, improved that performance bump further.
But, Cargnello says, if he can perfect the process to similarly strain the entirety of the palladium surface, combustion rates could improve by well more than two (and almost three) orders of magnitude – about 700 times. That quest now becomes the object of Cargnello’s future research.
“It sounded kind of crazy,” Cargnello said. “I couldn't believe it, but there it was right before my eyes. There was no other explanation.”
Additional authors include Benjamin A. Reeves, Chengshuang Zhou and Jinwon Oh of Stanford; Trenton Wolter and Lang Xu at UWM; and Megan E. Holtz and Andrew A. Herzing at NIST. Financial support for this research was provided primarily by NYSEARCH and by the Stanford Natural Gas Initiative.