Researchers Unravel the Complex Reaction Pathways in Zero Carbon Fuel Synthesis

(Cambridge Centre for Advanced Research and Education in Singapore/Phys.Org) Photosynthesis is the natural process of converting carbon dioxide (CO₂) to useable chemical compounds. In contrast, carbon capture and utilization technologies through processes such as electrochemical CO₂ reduction (eCO₂R) are the man-made equivalents that could enable the chemical industry to convert its current CO₂ waste to useful products.

Researchers from the Cambridge Center for Advanced Research and Education in Singapore (CARES) have used carbon isotopes to trace intermediates during eCO₂R, thereby shedding light on the complex formation pathways for the desired products. The research is published in the journal Nature Catalysis.

This will allow scientists to create more selective catalysts, exert control over product selectivity, and promote eCO₂R as a more promising production method for chemicals and fuels in the low-carbon economy.

The principal investigators for the eCO₂EP carbon utilization project were Professor Alexei Lapkin, from (CARES Ltd) and Professor Joel Ager from the Berkeley Education Alliance for Research in Singapore (BEARS Ltd).

Both organizations are part of the Campus for Research Excellence and Technological Enterprise (CREATE); CREATE became the crucial link to conceive and execute the three-year eCO₂EP program. While the project successfully completed in June 2021, ongoing research outcomes such as this paper continue to highlight the project's impact.

Professor Lapkin says, "The setup 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."

In the 1950s, Melvin Calvin (a professor at Berkeley) deduced the elementary steps 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 (Calvin won the Nobel Prize in Chemistry in 1961 for this work).

The eCO₂EP team reasoned that with a sufficiently sensitive 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 was generated by a prototype reactor that was built to operate under industrially relevant conditions. In order to detect both major and minor products in real-time as the operating conditions were changed, high-sensitivity mass spectrometry was used.

As high-sensitivity mass spectrometry is more commonly used in biological and atmospheric sciences, Research Fellows Dr. Mikhail Kovalev and Dr. Hangjuan Ren had to adapt the technique to the prototype system. They developed a new methodology to directly sample the reaction environment with unprecedented sensitivity and time response. They 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. This led to several discoveries.

The first discovery is that the mechanism in reactors being scaled up to commercial size has substantial differences compared to what had previously been known in smaller reactors operating at lower conversions. This knowledge will enable researchers to better control product selectivity.

The second discovery is that the discrimination against the heavier of the two stable carbon isotopes, i.e., carbon-13, is five times larger than that in natural photosynthesis. This is inspiring efforts in Professor Ager's lab to better understand fundamental physics and its chemical origins of this large and unanticipated kinetic isotope effect.

This research provides insight into how the chemical industry, which is the third largest subsector in terms of direct CO₂ emissions, can recycle its own waste within current manufacturing processes.

Professor Lapkin adds, "The operando 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."

An international patent application has been filed on the large carbon isotope discrimination effect that was discovered in the project.

Professor Ager concludes, "This work required an interdisciplinary approach drawing on expertise from both Cambridge and Berkeley. CREATE campus provided an ideal environment to realize this collaborative research with a skilled and motivated team." READ MORE

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