Bacteria do asymmetric coupling in CO₂ fixation, but can inorganic catalysts do it?

Converting CO₂ into value-added liquid products through renewable electricity is a promising research direction that can significantly contribute to realizing a carbon-neutral society. Researchers have long discovered that copper-based catalysts can convert CO₂ into multicarbon aldehydes, alcohols, or acids through inorganic aqueous electrolysis¹. However, a lack of comprehensive understanding of the CO₂ transformation mechanisms, especially including the critical C-C coupling step, has hindered the development of more efficient catalyst design strategies.

In the pursuit of more efficient artificial photo-/electro-synthesis, our research group has pioneered the concept of inorganic/bio hybrids for “photon-in, chemical bond-out”, interfacing light-absorbing inorganic materials (such as Si nanowire arrays², CdS nanoparticles³, Au clusters⁴) with micro-organisms such as bacteria capable of digesting CO₂ to produce liquid fuels (so-called, “liquid sunlight”)^5-6. Nearly quantitative CO₂ to CH₃COOH conversion has been achieved under optimized conditions thanks to the highly efficient asymmetric CH₃-CO coupling pathway, that is, the Wood–Ljungdahl metabolic pathway of the bacteria. This is almost unimaginable in the current inorganic electrolysis.

Having been inspired by the Wood-Ljungdahl (W-L) pathway, we set out to explore whether a similar asymmetric coupling pathway is possible in inorganic CO₂ electrocatalysis. We also started to investigate if this reaction pathway could regulate the product spectrum, potentially towards multi-carbon oxygenates, just as the exquisite selectivity of the W-L pathway towards acetic acid (Fig. 1).

Although previous density functional theory (DFT) calculations provided a positive support for the C-C asymmetric coupling in Cu-catalyzed CO₂ electrolysis^7-8, there has been no direct experimental evidence regarding the electrocatalytic asymmetric C-C coupling over Cu surface. Therefore, we designed isotope labeled ¹³CH₃I and ¹²CO co-reduction experiments to directly address this decade-long question.

Here, the CH₃I was utilized as a precursor that can in situ generate *CH₃ on Cu surface under applied negative bias, representing deeply-hydrogenated *CH_X intermediates in CO₂ reduction reaction (CO₂RR) while the externally introduced CO can adsorb on Cu surface (*CO) as the commonly acknowledged most important CO₂RR intermediate for multi-carbon formation (Fig. 2 left). ¹H NMR spectroscopy was used to analyze liquid products, distinguishing whether asymmetric coupling occurs on the surface of negatively biased Cu. If C-13 labeled multicarbon products exist as a result of the asymmetric ¹³CH₃–¹²CO coupling, a characteristic peak splitting would be easily observed.

With this approach, we found that common multi-carbon liquid products in CO₂RR, including ethanol, acetaldehyde, acetic acid, and acetone, are generated through the asymmetric C-C coupling process. This experimental result tells us that (1) W-L pathway-analogous asymmetric C-C coupling is indeed a viable reaction mechanism in inorganic CO₂ electrocatalysis, and (2) potentially it can be harnessed to control catalytic selectivity by promoting such asymmetric coupling pathway in CO₂RR.

We then further moved on to more practical CO₂ electrolysis with nanocatalysts to advance our understanding of asymmetric coupling. To this end, a Cu-Ag tandem nanoparticle (NP) assembly^9-10 was employed as a tandem approach¹¹ to utilize the asymmetric coupling by tuning the availability of intermediates species during the CO₂ electrolysis (Fig. 2 right). In this catalytic system, Ag NPs serve as a CO generator (“Western” or carbonyl branch) while Cu NPs work both as a *CH_X generator (“Eastern” or methyl branch) and the catalytic center for the asymmetric coupling. We first performed CO₂RR in a pure Cu NP system, and a significant amount of CH₄ was observed. Within this potential window, we then replaced a fraction of the Cu NPs with Ag NPs, therefore tuning the intermediate species ratio of *CO and *CH_x in the catalytic microenvironment. We found that the final generation rates of multicarbon oxygenates are strongly correlated with the co-existence of both intermediates. Furthermore, maximized oxygenate generation rate was achieved when those two intermediates were broadly aligned.

Based on our findings, we envision that future development of advanced inorganic “Western” and “Eastern” catalysts (i.e., tandem CO₂ electrolysis) will enable asymmetric C-C coupling at much lower overpotentials, leading to energy-efficient and selective multi-carbon oxygenate production, just like enzymatic biochemical reactions.

Author: Dr. Chubai Chen and Dr. Sunmoon Yu

Link: Nature Portfolio Chemistry Community