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Researchers unravel the complex reaction pathways in zero-carbon fuel synthesis

Researchers have used isotopes of carbon to trace how carbon dioxide emissions could be converted into low-carbon fuels and chemicals. The result could help the chemical industry, which is the third largest subsector in terms of direct CO2 emissions, recycle its own waste using current manufacturing processes.

When the eCO2EP: A chemical energy storage technology project started in 2018, the objective was to develop ways of converting carbon dioxide emitted as part of industrial processes into useful compounds, a process known as electrochemical CO2 reduction (eCO2R)

While eCO2R is not a new technique, the challenge has always been the inability to control the end products. Now, researchers from the University of Cambridge have outlined how carbon isotopes can be used to trace intermediates during the process, which will allow scientists to create more selective catalysts, control product selectivity, and promote eCO2R as a more promising production method for chemicals and fuels in the low-carbon economy. Their results are reported in the journal Nature Catalysis.

The project was led by Professor Alexei Lapkin, from Cambridge’s Centre for Advanced Research and Education in Singapore (CARES Ltd) and Professor Joel Ager, from the Berkeley Education Alliance for Research in Singapore (BEARS Ltd). Both organisations are part of the Campus for Research Excellence and Technological Enterprise (CREATE) funded by Singapore’s National Research Foundation.

In the 1950s, Berkeley’s Melvin Calvin identified the elementary steps used in nature to fix carbon dioxide in photosynthesis. Calvin and his colleagues used a radioactive form of carbon as a tracer to learn the order in which intermediates appeared in the cycle now named after him, work which won him the Nobel Prize in Chemistry in 1961.

The eCO2EP team found that with a sensitive enough mass spectrometer, they could use the small differences in reaction rates associated with the two stable isotopes of carbon, carbon-12 and carbon-13, to perform similar types of analyses.

First, a mixture of products such as methanol and ethylene were generated by a prototype reactor that was built to operate under industrial conditions. To detect both major and minor products in real time as the operating conditions were changed, high-sensitivity mass spectrometry was used.

Since high-sensitivity mass spectrometry is more commonly used in biological and atmospheric sciences, co-authors Dr Mikhail Kovalev and Dr Hangjuan Ren adapted the technique to their prototype system. They developed a method to directly sample the reaction environment with high sensitivity and time response.

The researchers used the difference in reaction rates of carbon-12 and carbon-13 to group a product such as ethanol and its major intermediates sharing the same pathway, to deduce key relationships in the chemical network.

The researchers found that there are substantial differences in the mechanisms at work in smaller reactors versus larger reactors, a finding which will enable them to better control product selectivity.

The team also discovered that the reaction used less of the heavier carbon-13 isotope than carbon-12. This difference in usage was found to be five times greater than that observed in natural photosynthesis, where carbon-13 is fixed at a slower rate than carbon-12. This is inspiring efforts in Professor Ager’s lab to better understand fundamental physics and the chemical origins of this large and unanticipated effect. An international patent application has also been filed.

“The set-up of the project within CREATE Campus allowed Joel and I to create an environment of creativity and ambition, to enable the researchers to excel and to target the really complex and interesting problems,” said Lapkin. “The monitoring of multiple species in such a complex reaction is, by itself, a significant breakthrough by the team, but the ability to further dig into the mechanism by exploring the isotope enrichment effect has made all the difference.”

“This work required an interdisciplinary approach drawing on expertise from both Cambridge and Berkeley,” said Ager. “CREATE campus provided an ideal environment to realise this collaborative research with a skilled and motivated team.”

The eCO2EP project was funded by the National Research Foundation, Prime Minister’s Office, Singapore under its Campus for Research Excellence and Technological Enterprise (CREATE) programme.

Hangjuan Ren et al. ‘Operando proton-transfer-reaction time-of-flight mass spectrometry of carbon dioxide reduction electrocatalysis.’ Nature Catalysis (2022). DOI: 10.1038/s41929-022-00891-3.”

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