Yue Wu (right) with two members of his research group – doctoral student Xiaopeng Liu (left) and postdoctoral research associate Fan Yang (center). Photo: Christopher Gannon/Iowa State University.
Yue Wu (right) with two members of his research group – doctoral student Xiaopeng Liu (left) and postdoctoral research associate Fan Yang (center). Photo: Christopher Gannon/Iowa State University.

Methane, which produces more warming than other greenhouse gases and is the subject of newly announced US emission restrictions, is hard to break down and keep out of the atmosphere. It’s not that the primary component of natural gas is chemically complex: a methane molecule is just one carbon atom bonded to four hydrogen atoms. But those carbon-hydrogen bonds are hard to break.

Typically, breaking these bonds involves high temperatures and mixing the flammable gas with oxygen to produce syngas to make methanol or with hydrogen to make ammonia. That’s expensive and potentially explosive. Other conversion reactions aren’t very efficient and also produce the most abundant of the greenhouse gases, carbon dioxide (CO2).

So, is there another way to use methane and keep it out of the atmosphere? One that’s safe and efficient? One that returns value? One that helps fight climate change?

Yue Wu and his research group at Iowa State University have been looking for just such a way over several years of experimental failures and discoveries. Now, they and a group of collaborators have found and tested a catalyst technology that appears to be meet all the requirements.

“The results provided a potential solution to this long-time challenge and represented the best stability, conversion rate and selectivity to convert methane to ethane or ethylene, two main precursors for the modern petrochemical industry,” said Wu, a professor in chemical engineering at Iowa State, in a project summary. Wu and his group report this novel catalyst in a paper in Nature Catalysis, while the Iowa State University Office of Innovation Commercialization is currently seeking a patent for it.

The catalyst consists of one or two layers of platinum, each layer just an atom thick, deposited on two-dimensional metal carbide structures called 'MXenes'. In this case, the structures are made from carbon, molybdenum and titanium.

According to Wu, his research group discovered that the thin MXene layers essentially allow every platinum atom to be used as a catalyst and help prevent the formation of residues that can cover and deactivate the platinum. That means less platinum is required to make the catalyst.

Wu's group started studying carbides – combinations of carbon and metals – about five years ago, with support from the US Office of Naval Research. The original work was to identify the electrical and thermal properties of various carbides. But the work didn’t go as expected – the material’s thermal conductivity was much lower than predicted. “You can think of this as a failure,” Wu said.

But the researchers also discovered that the MXene surfaces are very active and able to absorb many molecules. And so, with support from Iowa State College of Engineering, Wu’s research group began studying MXenes as a potential catalyst.

“We had never seen carbide so active,” Wu said. “It’s usually very inert. It’s used, for example, for high-speed drill bits – the surface is hard and inert.”

They started by using this catalyst technology to remove hydrogen from shale gas, the natural gas found in shale rock formations. That work evolved to study other reactions involving natural gas. “Nobody tried to use these carbides for these high-volume reactions before,” Wu said.

Key to the methane-to-ethane/ethylene conversion is making sure the carbides are pure enough and their surfaces clean enough to support the reactions. Get it all right, and those reactions exhibit about 7% methane conversion with around 95% selectivity toward ethane/ethylene in a continuously operating fixed-bed reactor. The products can be turned into plastics and resins, such as the common and ubiquitous plastic polyethylene.

“Remarkably, these novel catalysts run for 72 hours of continuous operation without any signs of deactivation, indicating a promising start toward technologies suitable for exploitation on the industrial scale,” Wu wrote.

Methane emissions are such a contributor to climate change that world leaders took steps to restrict them during COP26, the recent United Nations summit on climate change in Glasgow, UK. President Joe Biden announced that the US will take steps to restrict methane emissions from existing oil and gas operations. More than 100 countries also signed a Global Methane Pledge to reduce methane emissions by 30% over the next nine years.

The researchers’ new catalyst technology could advance these efforts to keep methane out of the atmosphere. Wu calls the technology “revolutionary”, saying it “opens the door to reducing the emission of methane and its combustion product, CO2, in the future”.

This story is adapted from material from Iowa State University, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.