Atomic resolution microscopy shows the single platinum atoms on copper nanoparticles that can split hydrogen for the efficient and selective hydrogenation of butadiene. Image: Tufts University.
Atomic resolution microscopy shows the single platinum atoms on copper nanoparticles that can split hydrogen for the efficient and selective hydrogenation of butadiene. Image: Tufts University.

A new generation of platinum-copper catalysts that require very low concentrations of platinum to perform important chemical reactions is reported today by Tufts University researchers in Nature Communications.

Platinum is used as a catalyst in many applications, from fuel cells to chemical production, because of its remarkable ability to promote a wide range of chemical reactions. However, its future potential uses are significantly limited by its scarcity and cost, as well as by the fact that platinum readily binds with carbon monoxide, which can ‘poison’ the desired reactions. This is what happens in polymer electrolyte membrane (PEM) fuel cells, which are the leading contenders for small-scale and mobile power generation not based on batteries or combustion engines.

The Tufts researchers have now discovered that dispersing individual, isolated platinum atoms on the surface of copper, which is much cheaper than platinum, can create a highly effective and cost-efficient catalyst for the selective hydrogenation of 1,3 butadiene. Produced by steam cracking of naphtha or by catalytic cracking of gas oil, butadiene is an impurity in propene streams that must be removed through hydrogenation in order to facilitate downstream polymer production. The current industrial catalyst for butadiene hydrogenation uses palladium and silver.

Copper, while a relatively cheap metal, is not nearly as catalytically powerful as platinum, noted Charles Sykes, professor of chemistry and one of the senior authors on the paper. "We wanted to find a way to improve its performance," he said.

The researchers first conducted surface science experiments to study precisely how platinum and copper metals mix. "We were excited to find that the platinum metal dissolved in copper, just like sugar in hot coffee, all the way down to single atoms," said Sykes. "We call such materials single atom alloys."

The Tufts chemists used a specialized low temperature scanning tunneling microscope to visualize the single platinum atoms and study their interaction with hydrogen. "We found that even at temperatures as low as -300°F these platinum atoms were capable of splitting hydrogen molecules into atoms, indicating that the platinum atoms would be very good at activating hydrogen for a chemical reaction," Sykes said.

Armed with this knowledge, Sykes and his fellow chemists turned to long-time Tufts collaborator Maria Flytzani-Stephanopoulos, professor in energy sustainability at the School of Engineering, to identify a hydrogen-based reaction of importance to the chemical industry. She chose butadiene hydrogenation.

After showing that the model catalyst could promote butadiene hydrogenation in vacuum conditions in the laboratory, Flytzani-Stephanopoulos's team took the study to the next level. They synthesized small quantities of realistic catalysts, such as platinum-copper single atom alloy nanoparticles supported on an alumina substrate, and then tested them under industrial pressures and temperatures.

"To our delight, these catalysts worked very well and their performance was steady for many days," said Flytzani-Stephanopoulos. "While we had previously shown that palladium would do related reactions in a closed reactor system, this work with platinum is our first demonstration of operation in a flow reactor at industrially relevant conditions. We believe this approach is also applicable to other precious metals if added as minority components in copper."

The researchers also found that the reaction actually became less efficient when they used more platinum, because clusters of platinum atoms have inferior selectivity compared with individual atoms. "In this case, less is more," said Flytzani-Stephanopoulos, "which is a very good thing."

Because platinum is at the center of many clean energy and green chemicals production technologies, the new, less expensive platinum-copper catalysts could facilitate broader adoption of such environmentally friendly devices and processes, she added. In addition, the general design approach used to produce this platinum-copper catalyst could be applied to other catalysts.

"Traditionally catalyst development happens by trial and error and screening many materials," said Flytzani-Stephanopoulos. "In this study we took a fundamental approach to understanding the atomic scale structure and properties of single atom alloy surfaces and then applied this knowledge to develop a working catalyst. Armed with this knowledge, we are now ready to compare the stability of these single atom alloy catalysts to single atom catalysts supported on various oxide or carbon surfaces. This may give us very useful criteria for industrial catalyst design."

This story is adapted from material from Tufts 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.