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.
Fig. 1. Asymmetric C-C coupling pathways in biological carbon fixation and inorganic CO2 electroreduction. In the Wood-Ljungdahl pathway, CO2 is efficiently converted to *CO (“Western” branch) and further combined with the *CH3 (“Eastern” branch) and coenzyme-A (CoA), leading to the synthesis of acetyl-CoA (CH3COS-CoA) and finally the generation of acetic acid. However, asymmetric C-C coupling in inorganic CO2 electroreduction has been largely suggested but without definitive experimental evidence.
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).
Fig. 2. Isotope labeled co-reduction experiments and Cu-Ag nanoparticle assembly based tandem CO2 electroreduction. Both mechanism exploration and more practical tandem CO2 electrolysis demonstrated the feasibility of the asymmetric C-C coupling in electrochemical CO2 reduction with inorganic catalysts.
Although previous density functional theory (DFT) calculations provided a positive support for the C-C asymmetric coupling in Cu-catalyzed CO2 electrolysis7-8, 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 *CHX 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) assembly9-10 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 *CHX 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 *CHx 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.
For more details, please read our recent publication in Nature Catalysis:
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- Su, Y., et al. Close-Packed Nanowire-Bacteria Hybrids for Efficient Solar-Driven CO2 Fixation. Joule, (2020).
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