Flipping molecular attachments amps up activity CO2 catalyst

Flipping molecular attachments amps up activity of CO2 catalyst

9:26 AM, 6th October 2015
Flipping molecular attachments amps up activity of CO2 catalyst
The research team (left to right): Etsuko Fujita, Dave Szalda, Zahid Ertem, Jim Muckerman and Komal Garg.

NEW YORK, US: New research by chemists at the US department of energy’s Brookhaven National Laboratory and their collaborates offers clues that could help scientists design more effective catalysts for transforming carbon dioxide (CO2) to useful products.

The study, published in Angewandte Chemie International Edition, reveals how a simple rearrangement of molecular attachments on an iridium hydride catalyst can greatly improve its ability to coax notoriously stable CO2 molecules to react.

The research, which combined laboratory experiments with theoretical analysis, shows that, in the dark, only one of the two molecular arrangements can effectively transform CO2 to formate (HCOO),a precursor of methanol.

In the presence of light, however, both species form a common intermediate that can transform CO2 to carbon monoxide (CO), a useful raw material for making fuels and industrial chemicals.

“There is strong interest in finding ways to reuse CO2 to create a carbon-neutral society,” said Brookhaven chemist Etsuko Fujita, who led the experimental portion of this work. “Reactions to produce products such as methanol or hydrocarbons from CO2 would be very useful. But if you think about the energy input and output of these reactions, it's really very difficult.”

“If you understand how a catalyst works, you can often devise ways to modify its function to make it work even better,” said Zahid Ertem, whose theoretical analyses provided the framework for understanding the experimental results.

Based on earlier research, the scientists had suspected there might be two varieties of these particular catalyst different molecular arrangements of the same atoms, known as isomers. And indeed their experiments allowed them to isolate the two varieties.

The only structural difference between the two isomers is a simple flip in positioning of two connected rings of atoms relative to the rest of the molecule-one linked to the central iridium atom by a negatively charged carbon atom and the other linked by a neutral nitrogen atom. But that simple flip in the positions of these two rings has a dramatic effect on the respective molecules' properties.

Another aim of the study was to explore the role of iridium hydride as a proposed key intermediate in the conversion of CO2 to CO. But as it turns out, the intermediate is a form of the molecule that lacks the hydride but has the carbon-linked ring in the position opposite where the hydride would attach.

"In the ground state, the length of the metal-hydride bond is significantly longer in the isomer where the carbon is opposite the hydride than it is in the flipped isomer where the nitrogen atom is opposite the hydride," said Ertem. "Unlike the neutral nitrogen atom, the negatively charged carbon 'pushes' electrons through the metal atom toward the hydride ion, lengthening the metal-hydride bond and increasing the hydricity. That, in turn, makes it easier for the hydride to be given up during reactions when the carbon is in this position."

A next step might be trying to design an even more reactive catalyst by adding strong electron-donating groups.

Fujita and Ertem collaborated with these additional co-authors on this study: Komal Garg, Yasuo Matsubara, Anna Lewandowska-Andralojc and James Muckerman from Brookhaven Lab, Shunsuke Sato from Toyota Central R&D Labs, David Szalda from Baruch College, City University of New York and Japan Science and Technology Agency (JST).

© Brookhaven National Laboratory News 

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