The first atomic-level video of a catalytic reaction reveals hidden pathways

For the first time, a team of university-led scientists in the northwest has directly observed catalytic catalysts at the atomic level.

In a fascinating new video, a single atom moves and shakes during a chemical reaction that removes hydrogen atoms from alcohol molecules. By looking at the process in real time, researchers have discovered several short-lived intermediate molecules involved in the reaction and previously hidden reaction pathways.

Observations were made possible by single molecule atomic resolution time-resolved electron microscopy (SMART-EM), a powerful instrument that allows researchers to see individual molecules react in real time.

Observing the reaction in this way helps scientists understand how catalysts work. These new insights could lead to designs for more efficient and sustainable chemical processes.

A study titled “Atomic Resolution Imaging as a Mechanical Tool for Studying Single Site Heterogeneous Catalysts” was published in Journal Chem.

“By visualizing this process and following the reaction mechanism, we can get the most detailed and accurate understanding of what is happening,” said Yoshi Kratish of Northwestern, the first and common author of the study.

“In the past, we couldn’t see how atoms move. Now we can. When I realized what we had achieved, I had to close my laptop and take a break for a few hours. I never did this with catalytic work.

“Catalysts enable modern life,” said Tobin J. Marks of Northwestern, a senior author of the study. “It is used to make everything from fuels and fertilizers to plastics and medicines. To make chemical processes more efficient and environmentally friendly, we need to understand exactly how catalysts work at the atomic level. Our research is a huge step towards achieving that.”

Catalyst expert Marks is a professor of chemistry Charles E. and Emma H. ​​Morrison, Professor of Chemistry Vladimir N. Ipatiev of Catalytic Chemistry at Weinberg University of Arts and Sciences in the Northwest, and Professor of Chemistry and Bioengineering at the McCormick School of Engineering in Northwestern.

Kratish is an assistant professor of chemistry research at Marks Group. Marks and Kratish led the research with Michael Betsick, McCormick’s professor of materials science and engineering, George C. Schatz, professor of chemistry at Weinberg, and Professor Nakamura eiichi, University of Tokyo, George C. Schatz, who invented SmartM and invented SmartNakuiki.

Catching fleeting molecules in “Movie Chemistry”

Researchers have long been seeking to observe live catalytic events at the atomic level. A chemical reaction is like a journey between the starting material and the final product. Along the journey, temporary and sometimes unexpected molecules form and suddenly transform into other molecules. These so-called “intermediate” molecules are unpredictable and fleeting, making them difficult to detect.

However, by viewing the reactions directly, scientists can determine the exact sequence of events, revealing the complete response pathway and displaying their elusive intermediates. Until recently, however, it was impossible to observe these secret dynamics.

While traditional electron microscopes can image atoms, their beams are too strong to image the soft organic matter used in the catalyst. High-energy electrons easily break down carbon-based structures and destroy them before scientists can collect data.

“Most traditional transmission electron microscopy technologies operate under conditions that easily damage organic molecules,” says Kratish. “This makes it very difficult to directly observe sensitive catalysts or organic matter during reactions using traditional TEM methods.”

To overcome this challenge, the team turned to Smart-EM, a new technique that allows them to capture images of sensitive organic molecules. Released in 2018 by Nakamura and his team, Smart-Em uses much lower electronic doses to minimize the amount of energy transferred to the sample and damage. By capturing a series of rapid sequences, Smart-EM generates videos of the dynamic processes Nakamura calls “film chemistry.”

“Since 2007, physicists have been able to realize their dreams of over 200 years: the ability to see individual atoms,” Nakamura said in a 2019 statement. “But it didn’t end there. Our research group was able to create videos of molecules beyond this dream and see the chemical reactions in unprecedented detail.”

Measuring from messy things

When applying Smart-EM to the catalyst for the first time, a team in the northwest chose a simple chemical reaction: removing hydrogen atoms from alcohol molecules. But first they had to choose the right catalyst. Approximately 85% of industrial catalysts are heterogeneous, meaning that they are solid materials that react with liquids and gases.

Heterogeneous catalysts are stable and efficient, but are messy with many different surface sites where the reaction can occur.

“There are many benefits to heterogeneous catalysts,” Kratish said. “But there are major disadvantages. Often, they are black boxes. There are unknown numbers of sites where the reaction can occur. Therefore, we don’t fully understand where and how the reaction occurs. This means we can’t know exactly which part of the catalyst is most effective.”

To make the catalyst easier to study, a team in the northwest designed a single-site heterogeneous catalyst with well-defined active sites. The single-site catalyst consisted of molybdenum oxide particles fixed to cone-shaped carbon nanotubes. The team then used Smart-EM to investigate how those catalysts facilitated the conversion of ethanol, a clean alternative to fossil fuels, into hydrogen gas.

“It’s much more convenient to have one site,” Kratish said. “You can choose a good site to monitor and really zoom in.”

Announce hidden routes

Before the study, scientists assumed that alcohol went straight into the catalyst, where it became hydrogen gas and aldehydes (the molecule formed when alcohol molecules oxidize). From there, aldehyde, a room temperature gas, escaped into the air. But seeing the process unfolds, another story became clear.

Using Smart-EM, researchers found that aldehydes do not float, but instead stick to catalysts. They also discovered that aldehydes bond to form short chain polymers. In another surprise, the researchers discovered that aldehydes also react with alcohols to form the intermediate molecule, hemiacetal.

To confirm these findings, the team used a variety of microscope techniques, x-ray analysis, theoretical models, and computer simulations. Everything matched Smart-EM data.

“This is a big breakthrough,” Kratish said. “Smart-EM is changing the way we see chemistry. We want to finally separate those intermediates, control the amount of energy we put into the system, and study the kinetics of living organic catalytic conversions. That’s incredible. This is just the beginning.”