Carbon dioxide (CO2) is simultaneously a critical culprit of climate change and a compound that carries one of the world’s most valuable raw materials. When captured and processed using renewable electricity, carbon can be converted into chemicals with industrial value, depending on the catalysts involved in the reaction. Copper, for example, catalyses reactions that reduce CO2 into the simpler carbon monoxide, or transforms it into valuable multi-carbon fuels, such as ethanol and ethylene.
A key strategy to sharpen copper’s selectivity towards producing multi-carbon products is heteroatom doping, which introduces foreign elements such as fluorine into the catalyst. “It is like adding a small amount of a different ingredient to change a dish’s flavour. Fluorine slightly disturbs how electrons and atoms are arranged on the copper’s surface, in turn changing how CO2 molecules interact with the catalyst,” explained Lili Zhang, a Research Scientist at the A*STAR Institute of Sustainability for Chemicals, Energy and Environment (A*STAR ISCE2).
Zhang and A*STAR ISCE2 Research Scientist Lu Liu collaborated with researchers from the National University of Singapore; University of Hong Kong and Nanjing Tech University in China; to investigate how fluorine doping impacts copper’s performance and selectivity, as previous studies had shown inconsistent results. To better capture the dynamic nature of the catalyst, they used advanced spectroscopy techniques to see atomic-scale changes during CO2 conversion, combined with theoretical modelling to understand the underlying mechanisms.
“The catalyst only has its ‘true working form’ during the reaction, and it keeps changing very quickly. Our special tools let us watch the catalyst in real time, while electricity is flowing and CO2 is being converted, almost like filming the reaction as it happens,” said Lu Liu.
The researchers studied the behaviour of three copper pre-catalysts doped using different fluorine concentrations. The live-action ‘filming’ revealed that fluorine actively shapes the copper surface yet leaches out almost entirely within the first five minutes of the reaction. As the doping leaves behind a newly sculpted surface, the precursors are transformed into metallic copper.
The structure of the reshaped copper surface then determined the products generated through the carbon conversion reaction. The pre-catalyst with the highest fluorine content developed rougher surfaces, while moderate fluorine doping led to more defined copper surfaces that favoured multi-carbon fuels with up to 81 percent selectivity.
“We saw how fluorine sets the stage for the reaction, then exits,” said Liu “Instead of acting as permanent active ingredients, dopants can behave more like temporary tools that help restructure the material and are then removed.”
The researchers hope their work can highlight the value of using dopants to create beneficial surface features, approaching catalyst design by exploiting their dynamic nature for enabling more productive and selective reactions. They now look to enhance the long-term durability and energy efficiency of this catalytic system to make it reliable enough for large-scale industrial use.
The A*STAR-affiliated researchers contributing to this research are from the A*STAR Institute of Sustainability for Chemicals, Energy and Environment (A*STAR ISCE2).
