Monitoring carbon isotope discrimination to unravel complicated response networks

Unraveling the pathways linking reactants and merchandise in complicated response networks is a perennial problem for researchers, as we discovered whereas learning electrochemical CO₂ discount (eCO₂R).

Electrocatalytic discount of CO₂ is a promising pathway to the extra sustainable manufacturing of chemical substances and fuels and will, if profitable, exchange the petrochemical processes which at the moment are used. Nevertheless, eCO₂R has confirmed exceedingly tough to regulate. Prototype reactors produce a mix of C₁-C₄ hydrocarbon and oxygenate merchandise; separation of the specified merchandise from this combination might be pricey. To enhance selectivity, we have to perceive the formation pathways for the specified merchandise in order that we are able to create extra selective catalysts. Nevertheless, it is a tall order: simply contemplating the C₂ and C₃ merchandise, there are over 18 doable intermediate species and many various doable response pathways.

However, we sought to shed some gentle on eCO₂R response community by working a prototype reactor below industrially related circumstances, producing, as anticipated, a mix of various merchandise. Essential questions instantly arose.

• How can we precisely classify the various analytes?
• How to pick the merchandise and the corresponding main intermediates?

We turned to different complicated response networks for clues, notably photosynthesis, which has advanced over billions of years to repair CO₂ and generate, very selectively, the molecules of life. The work of Melvin Calvin was notably inspiring [1]. Calvin used an isotopically labeled tracer, ¹⁴C, to be taught the order by which intermediates appeared within the cycle now named for him (and was awarded the Nobel Prize in 1961). We reasoned that we would be capable of use isotopic labeling in an identical means for eCO₂R.

Nevertheless, we confronted one other downside. Electrochemical reactions are a lot sooner than photosynthesis. Additionally, we didn’t need to use radioactive ¹⁴C. We then regarded extra deeply into the photosynthesis literature and realized a few very fascinating phenomenon in plant main metabolism. Photosynthesis fixes ¹²CO₂ at a barely sooner charge than ¹³CO₂  [2]. The ¹³C discrimination is quantified as δ¹³C, representing the change within the ratio of ¹³C to ¹²C (n.b. pure abundance of the steady ¹³C isotope is 1.1%). For an irreversible response, breaking or making a bond involving ¹³C is much less most well-liked than ¹²C in climbing the response vitality limitations, thus leading to ¹³C-depleted merchandise. Thus, triose-phosphates produced in chloroplasts are ¹³C depleted (adverse values of δ¹³C) in contrast with atmospheric CO₂. We additionally realized that δ¹³C measurements have been used not solely within the examine of plant main metabolism, but in addition in geosciences, climatology, and oceanography.

Turning to the system of curiosity to us, eCO₂R, we’d anticipate and have been excited to watch discrimination towards ¹³C within the merchandise (adverse values of δ¹³C) as elementary steps in eCO₂R are irreversible. Furthermore, as we anticipate the biggest kinetic isotope impact on the C-C coupling steps, we have been in a position to group merchandise sharing a given pathway by their δ¹³C values. In easy phrases, a product and its main intermediates with the identical variety of carbon atoms can be anticipated to have related values of δ¹³C, permitting us to infer key relationships within the chemical community.

Drawing inspiration from its use in atmospheric sampling [3], we utilized operando proton-transfer-reaction time-of-flight mass spectrometry (PTR-TOF-MS) for real-time detection of intermediates and merchandise in addition to their ¹³C isotopologues throughout eCO₂R. We encountered a number of hurdles in doing so. Surprisingly, the approach may be too(!) delicate; we discovered we wanted to dilute the product stream from our reactor to remain throughout the helpful dynamic vary of the instrument. Finally, we have been in a position to set up unambiguous identification of dozens of C₁-C₄ species by distinctive m/z values by coupling the PTR-TOF-MS to fuel chromatography (GC). Importantly, because of the excessive sensitivity of PTR-TOF-MS, we solely want to make use of a pure abundance feed of CO₂ (1.1% ¹³C) fairly than costly ¹³C or ¹⁴C-labelled CO₂.

Our operando measurements have been used for mechanism analysis in two methods. First, by slowly scanning the potential to extra adverse values, we are able to exactly decide the onset potential for unstable minor and main merchandise. Second, we tracked δ¹³C by means of the chemical community. For all merchandise, we noticed a adverse δ¹³C, which can be the case for the CO₂ fixation reactions in photosynthesis and Fischer-Tropsch synthesis. An sudden discovering is that the values of δ¹³C in eCO₂R are rather more adverse. Utilizing these two approaches to research the operando outcomes of various GDEs below completely different electrochemical circumstances, we discover that formaldehyde and acetaldehyde should not the key intermediates for the formation of methanol and ethanol/ethylene, respectively, and that propionaldehyde discount is on the key pathway for 1-propanol formation.

Trying ahead, we envision that use of δ¹³C measurements can be utilized to find out the intermediate pathways from reactants to merchandise in different complicated response networks involving carbon. It might even be doable to increase our methodology to a wider vary of components, in addition to a wider vary of fields.

So, when you find yourself baffled by the heaps of merchandise and intermediates and the various doable response pathways, how about contemplating δ¹³C?

For extra particulars, please learn our current publication in Nature Catalysis:

https://www.nature.com/articles/s41929-022-00891-3 

Because of all of the co-authors, and particularly Prof. Joel Ager, for his or her huge contribution to this work.

References

1. Calvin, M. Forty Years of Photosynthesis and Associated Actions. Res. 1989, 21, 3–16.
2. Tcherkez, G.; Mahé, A.; Hodges, M. ¹²C/¹³C Fractionations in Plant Main Metabolism. Tendencies Plant Sci. 2011, 16, 499–506.
3. Yuan, B.; Koss, A. R.; Warneke, C.; Coggon, M.; Sekimoto, Okay.; de Gouw, J. A. Proton-Switch-Response Mass Spectrometry: Purposes in Atmospheric Sciences. Rev. 2017, 117, 13187–13229.