Five metals combine in 2D to create novel catalyst

A two-dimensional alloy material – made from five metals as opposed to the traditional two – has been developed by a collaboration between researchers at Washington University in St. Louis and the University of Illinois at Chicago (UIC). In a first for such a material, it has been shown to act as an excellent catalyst for reducing carbon dioxide (CO₂) to carbon monoxide (CO), giving it potential applications in environmental remediation.

The research, from the lab of Rohan Mishra, assistant professor in the Department of Mechanical Engineering & Materials Science at Washington University, is reported in a paper in Advanced Materials.

"We're looking at transforming carbon dioxide, which is a greenhouse gas, into carbon monoxide," Mishra said. "Carbon monoxide can be combined with hydrogen to make methanol. It could be a way to take CO₂ from the air and recycle it back into a hydrocarbon."

The basis of this innovation is a class of materials known as transition metal dichalcogenides (TMDCs), which combine a transition metal with a chalcogen such as sulfur, selenium or tellurium. And when an alloy contains more than three metals at near equal ratios, it's said to be 'high entropy'. Hence the wordy name of the material developed in Mishra's lab: high-entropy transition metal dichalcogenides.

TMDCs are not new. There has been great interest in two-dimensional forms of these materials due to their unique optical and electronic properties, but Mishra had a suspicion they could be used for something else. "We've been looking at these, too," he said, "but exploring their potential for electrocatalysis."

As TMDCs are effectively two-dimensional (about three atoms thick), they make for efficient catalysts. Reactions occur on the surface of a catalyst, and a two-dimensional material has a lot of surface area, and not much else.

In a paper in Advanced Materials in 2020, the group had shown that two-metal TMDC alloys showed improved catalytic activity over individual TMDCs. "This led us to the question, can adding more metals to these alloys make even better catalysts?" Mishra said.

With 10 applicable transition metals and three chalcogens, there are 135 two-metal and 756 five-metal possible TMDC alloys. However, just like oil and water, not all combinations will mix together to form a homogenous alloy.

"Without guidance from computations, experimentally determining which elemental combinations will give an alloy becomes a trial-and-error process that is also time consuming and expensive," Mishra explained.

Fortunately, Mishra could take advantage of previous work by John Cavin, a graduate student in Washington University's Department of Physics, who had shown which two transition metals can be combined, and at what temperatures, to form binary TMDC alloys.

"The question was, 'Could we even synthesize a TMDC alloy that had that many components?'" Cavin said. "And will they improve the reduction of CO₂ into CO?"

To find out, he used quantum mechanical calculations to predict which combinations were most likely to improve the material's ability to catalyze CO₂. Then he had to determine if the resulting material would be stable, but had no tools to do so. So, he developed one himself.

"I had to develop a thermodynamic model for predicting stable high-entropy TMDC alloys from the quantum mechanical calculations," Cavin said. These calculations were carried using considerable supercomputing resources, made available by the Extreme Science and Engineering Discovery Environment network, which is supported by the US National Science Foundation.

After years of development, the resulting analysis was sent to experimental collaborators at UIC. "At UIC, they could synthesize the materials that we predicted would form a high-entropy TMDC alloy," Mishra said. "Furthermore, one of them showed exceptional activity."

These TMDC alloys may have other uses too. The UIC researchers synthesized three of the four different TMDC alloys and will continue to analyze them. "These are new materials, they have never before been synthesized," Mishra said. "They may have unanticipated properties."

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