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Catalysis

Nanoparticle catalysts communicate across distances

Reactions at one active site influence reactions at other sites within the same particle and between nearby particles

by Mitch Jacoby
March 30, 2018 | APPEARED IN VOLUME 96, ISSUE 14

 

09614-leadcon-randp.jpg
Credit: Chen group/Cornell
Active sites on metal nanoparticles can communicate (blue antennas) via two mechanisms. Positively charged species carry messages within a particle (dashed blue line). Negatively charged species drive communication between nearby particles (green line).

Nanoparticle catalysts obviously can’t talk but a new study suggests they’ve found a way to communicate with each other. By using a method that tracks single-molecule reactions on individual nanoparticles, researchers have shown that catalytic reactions occurring at one active site on a particle can influence reactions occurring at another site (Nat. Chem. 2018,DOI: 10.1038/s41557-018-0022-y). This finding broadens chemists’ understanding of catalysis and may lead to ways of improving catalyst performance.

Researchers have long known about so-called allosteric effects in enzymes. This type of cooperative chemical communication, in which a molecule binding at one site influences chemical activity at another site on the enzyme, is driven by structural and bonding changes and affects the function of the biocatalysts.

To determine whether solid nanoparticle catalysts exhibit a related type of chemical communication, a team led by Peng Chen of Cornell University scrutinized nanoparticles made of either gold or palladium as the materials mediated various types of model catalytic reactions. The reactions converted nonfluorescent reactants to highly fluorescent products, allowing the researchers to watch the process unfold using high-resolution fluorescence microscopy methods.

09614-leadcon-micro.jpg
Credit: Nat. Chem.
By coupling electron microscopy (left) and fluorescence microscopy (right), researchers can map the locations at which thousands of reactions occurred on a Pd nanorod. Light colored squares correspond to regions with highest catalytic activity.

By analyzing single-molecule data pooled from a large number of particles, the team found that a catalytic reaction that occurred at one active site can affect the probability of a reaction occurring at another site on the same particle or on a nearby particle.

Specifically, the researchers observed that within single particles, this communication, in which a reaction at one site tends to be followed by a reaction at a given nearby site, can span hundreds of nanometers and tens of seconds. The correlation decreases exponentially with increasing separation between sites and increasing time between pairs of reactions. The researchers also observed communication over many micrometers from one gold particle to another, but not between gold and palladium particles.

Based on data collected during further experiments, including some that involved applied voltages, the team proposes that interparticle communication is driven by diffusion of negatively charged product molecules that promote reactions on active sites on other particles. In contrast, intraparticle communication likely proceeds via migration of positively charged electron vacancies known as holes.

“These are intriguing findings,” says Prashant K. Jain, a catalysis specialist at the University of Illinois, Urbana-Champaign. It is quite striking that such cooperativity is not limited to adjacent or nearby sites but involves relatively distant sites and even well separated nanoparticles, he remarks.

“I wonder if communication between active sites separated by a few hundred nanometers is a general feature of heterogeneous catalysts.”

It would be interesting, Jain says, to learn if during multistep reactions, a product formed at one site diffuses to another, where it is transformed to a different species that can no longer react at the first site. Detailed answers to those types of questions could help chemists tailor catalysts to improve product selectivity.


This article has been translated into Spanish by Divulgame.org and can be found here.

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