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Brown team develops new model for effect on strain on catalyst performance; potential to optimize catalysts for different reactions

Research in recent years has shown that applying a strain to metal catalysts—either compression or tension—can in some cases change the way they perform. Now, Brown University researchers have developed a new model to explain why stretching or compressing metal catalysts can make them perform better. The theory, described in the journal Nature Catalysis, could open new design possibilities for new catalysts with new capabilities.

The recent literature contains abundant examples showing how the use of strain can tune the electronic structure of catalysts and thus their reactivity, presumably under the assumption that the basic rules of scaling relations still apply. In this work, we first developed a mechanics-based eigenstress model to qualitatively predict the response of binding energy on surface sites to strain, and used this model to rationalize how specific adsorbate–surface combinations naturally break scaling relations.

We then showed how strain itself can be engineered to break the response of adsorbate and transition-state scaling relations, through examples of surface processes under both biaxial and uniaxial loading, showing that strain does not just tune catalysts within existing design criteria, but can open up important new kinetic possibilities. The vast majority of literature studies have employed biaxial strain, yet techniques to experimentally apply anisotropic strain (via uniaxial loading, for example) are just emerging.

Given recent experimental demonstrations of catalysis under anisotropic strain, such approaches may become possible for the future design of heterogeneous catalytic systems that are not subject to the constraints of typical catalysts.

—Khorshidi et al.

A metal catalyst works by causing reactants to bind to its surface, a process known as adsorption. Adsorption breaks chemical bonds of the reactant molecules, enabling various stages of a chemical reaction to take place on the metal’s surface. After the reaction stages are complete, the final product is released from the catalyst through the reverse process, called desorption.

A catalyst’s key property is its reactivity, meaning how tightly it binds chemical molecules to its surface. Catalysts need to be somewhat reactive for binding to happen, but not too reactive. Too much reactivity causes the catalyst to hold molecules too tightly, which may hinder some stages of the reaction or make it so the final products can’t desorb.

It’s been shown in recent years that applying a strain to a catalyst can tune its reactivity, and there’s a well-established theory for how it works. Generally speaking, the theory predicts that tensile strain should increase reactivity, while compression should reduce it. However, Andrew Peterson, an assistant professor in Brown’s School of Engineering and co-author of the research, and his group kept encountering systems that aren’t easily explained by the theory.

That impelled the researchers to think about a new way to view the problem. The traditional theory describes things on the level of electrons and electron bands. The new theory focuses instead on the mechanics of how molecules interact with a catalyst’s atomic lattice.

Peterson and his team showed that molecules bound to a catalyst’s surface will tend to either push atoms in the lattice apart or pull them closer together, depending upon the characteristics of the molecules and the binding sites. The different forces produced by molecules have interesting implications for how external strain should affect a catalyst’s reactivity.

It suggests that tension, which stretches a catalyst’s atomic lattice, should make a catalyst more reactive to molecules that naturally want to push the lattice apart. At the same time, tension should decrease reactivity for molecules that want to pull the lattice together. Compression—squeezing the lattice—has an inverse effect.

The new theory not only helps explain previously puzzling results, it makes important new predictions. Specifically, it predicts a way to break traditional scaling relations between catalysts and different types of molecules.

Scaling relations mean that, under normal circumstances, when you increase a catalyst’s reactivity for one chemical, it increases the reactivity for other chemicals as well. Similarly, if you decrease reactivity for one chemical, you decrease it for others.

—Andrew Peterson

Those scaling relations cause troublesome tradeoffs when trying to optimize a catalyst. Getting the perfect reactivity for one chemical could cause another chemical to bind too tightly (or too loosely), potentially inhibiting some stages of a reaction. The new theory suggests that strain can break those scaling relations—enabling a catalyst to simultaneously bind one chemical more tightly and another more loosely, depending on the chemical’s natural interaction with the catalyst’s atomic lattice and the way that the strain field is engineered on the catalyst surface.

Now you can start to think about really fine tuning catalysts to perform better throughout different reaction steps. That could dramatically improve a catalyst’s performance, depending on the chemicals involved.

—Andrew Peterson

Peterson’s team has started putting together a database of common reaction chemicals and their interactions with different catalyst surfaces. That database could serve as a guide for finding reactions that could benefit from strain and the breaking of scaling relations.

The research was supported by the U.S. Army Research Office (W911NF-11-10353). Other authors on the paper were Alireza Khorshidi, James Violet and Javad Hashem.

Resources

  • Alireza Khorshidi, James Violet, Javad Hashemi & Andrew A. Peterson (2018) “How strain can break the scaling relations of catalysis” Nature Catalysis volume 1, pages 263–268 doi: 10.1038/s41929-018-0054-0

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