Scientists focus on pairs of atoms that increase the activity of a catalyst

Replacing the expensive metals that decompose the exhaust gases in catalytic converters with cheaper and more effective materials is a priority for scientists, for both economic and environmental reasons. Catalysts are required to perform chemical reactions that would not otherwise occur, such as converting polluting gases from automobile exhaust to clean compounds that can be released into the environment. To improve them, researchers need a deeper understanding of exactly how catalysts work.

Now, a team from Stanford University and the Department of Energy’s SLAC National Accelerator Laboratory have identified exactly which pairs of atoms in a palladium-platinum nanoparticle, a combination commonly used in converters, are the most active in breaking down those gases. .

They also answered a question that has puzzled catalyst researchers: Why do larger catalyst particles sometimes work better than smaller ones, when the opposite is expected? The answer has to do with how the particles change shape during the course of the reactions, creating more of those highly active sites.

The results are an important step toward engineering catalysts for better performance in both industrial processes and emission controls, said Matteo Cargnello, an assistant professor of chemical engineering at Stanford who led the research team. Their report was published on June 17 in procedures of the National Academy of Sciences.

“The most exciting result of this work was identifying where the catalytic reaction occurs: at what atomic sites can this chemistry take that takes a polluting gas and turns it into harmless water and carbon dioxide, which is incredibly important and incredibly difficult to do , “Cargnello said. “Now that we know where the active sites are, we can design catalysts that work better and use less expensive ingredients.”

Catalysts are required to perform chemical reactions that would not otherwise occur, such as converting polluting gases from automobile exhaust to clean compounds that can be released into the environment. In a car’s catalytic converter, precious metal nanoparticles like palladium and platinum bond to a ceramic surface. As the emission gases pass, the atoms on the surface of the nanoparticles adhere to the passing gas molecules and encourage them to react with oxygen to form water, carbon dioxide, and other less harmful chemicals. A single particle catalyzes billions of reactions before it runs out.

Today’s catalytic converters are designed to work best at high temperatures, Cargnello said, which is why the most damaging exhaust emissions come from vehicles just starting to heat up. With more engines designed to work at lower temperatures, there is a pressing need to identify new catalysts that will perform better at those temperatures, as well as on ships and trucks that are unlikely to switch to electrical operation any time soon.

But what makes one catalyst more active than another? The response has been elusive.

In this study, the research team analyzed catalyst nanoparticles made of platinum and palladium from two perspectives: theory and experiment, to see if they could identify specific atomic structures on their surface that contribute to increased activity.

Rounded particles with jagged edges.

On the theory side, SLAC staff scientist Frank Abild-Pedersen and his research group at the SUNCAT Center for Science and Catalysis Interface created a new approach to model how exposure to gases and steam during chemical reactions affects the shape and atomic structure of a catalytic nanoparticle. This is computationally very difficult, Abild-Pedersen said, and previous studies had assumed that particles existed in a vacuum and never changed.

His group created new and simpler ways to model particles in a more complex and realistic environment. Calculations by postdoctoral researchers Tej Choksi and Verena Streibel suggested that as the reactions progress, the eight-sided nanoparticles become more round and their flat, faceted surfaces become a series of small irregular steps.

By creating and testing nanoparticles of different sizes, each with a different ratio of jagged edges to flat surfaces, the team hoped to focus on exactly which structural configuration, and even which atoms, contributed the most to the catalytic activity of the particles.

A little help from the water.

Angel Yang, Ph.D. A student in Cargnello’s group, he made precision-controlled nanoparticles containing a uniformly distributed mixture of palladium and platinum atoms. To do this, he had to develop a new method of making the largest particles by sowing around the smallest ones. Yang used X-rays from SLAC’s Stanford synchrotron radiation light source to confirm the composition of the nanoparticles she made with the help of Simon Bare of SLAC and her team.

Yang then conducted experiments where nanoparticles of different sizes were used to catalyze a reaction that converts propene, one of the most common hydrocarbons present in exhaust gases, to carbon dioxide and water.

“The water here played a particularly interesting and beneficial role,” he said. “It usually poisons or disables catalysts. But here exposure to water caused the particles to round up and open more active sites.”

The results confirmed that the larger particles were more active and that they became rounder and more irregular during reactions, as computational studies predicted. The most active particles contained the highest proportion of a particular atomic configuration, one in which two atoms, each surrounded by seven neighboring atoms, form pairs to carry out the reaction steps. It was these “7-7 pairs” that allowed large particles to perform better than smaller ones.

Going forward, Yang said, he hopes to discover how to seed nanoparticles with much cheaper materials to lower their cost and reduce the use of rare precious metals.

Industry interest.

The research was funded by BASF Corporation, a leading manufacturer of emission control technology, through the California Research Alliance, which coordinates the research between BASF scientists and seven West Coast universities, including Stanford.

“This document addresses fundamental questions about active sites, with experimental theories and perspectives that come together in a really nice way to explain experimental phenomena. This has never been done before, and that is why it is quite significant,” said Yuejin Li, a senior BASF senior scientist who participated in the study.

“In the end,” he said, “we want to have a theoretical model that can predict which metal or combination of metals will have even better activity than our current state of the art.”