First author: Jing Gao
Corresponding authors by Dan Ren, Michael Grätzel
Correspondence: Ecole Polytechnique Fédérale de Lausanne
【Research Highlights】
Converting CO2 into valuable products and achieving this goal with renewable electricity promises to help solve current climate problems. The electrochemical reduction of CO2 to propylene, an important raw material, requires a multi-step C-C coupling reaction, and each propylene molecule needs to transfer 18 electrons, so the kinetics is relatively slow. Here, the authors demonstrate the electrochemical synthesis of CO2 onto Cu nanocrystals with a peak geometric current density of -5.5 mA/cm². Metal Cu nanocrystals formed from CuCl precursors mainly have Cu(100) and Cu(111) crystal plane structures, which may be beneficial for adsorbing key C1 and C2 intermediates. When CO is used as a reactant, propylene yield is significantly reduced. Inferred by electrochemical reduction of isotope-labeled mixed carbon dioxide and carbon monoxide, a critical step in propylene formation may be the coupling between the adsorption/molecular CO2 or carboxyl group and the *C2 intermediate involved in the ethylene pathway.
【Main content】
Using renewable electricity to drive the electrochemical conversion of carbon dioxide (CO2) into high value-added products is a promising strategy to mitigate the negative effects of excessive carbon emissions caused by human activities. Using Cu-based catalysts, electrochemical CO2 reduction has shown considerable activity in the production of a wide range of C1 and C2 chemicals. While C3+ terminal oxygen-containing compounds such as n-propanol and n-butanol can be produced by CO2 reduction, C3+ hydrocarbons, such as propylene (CH2=CH-CH3), rarely become CO2 reduction products. Propylene, a key chemical feedstock, reached an annual capacity of 1.3 million tonnes in 2019, requiring an energy input equivalent to about 190 million barrels of crude oil and resulting in approximately 80 million tonnes of CO2 emissions. Electrosynthesis of propylene from CO2 with a negative carbon footprint is an attractive strategy for the production of raw materials that are indispensable for the polymer industry, but it has not yet been achieved.
Electroreduction of CO2 to propylene involves 18 electron transfers per propylene molecule and requires multiple C-C coupling steps, which creates kinetic and thermodynamic hurdles to drive this reaction:
3CO2 + 12H2O + 18e- ⇄ C3H6 + 18OH- Eo = 0.13 V versus RHE
where E° is the thermodynamic equilibrium potential and RHE represents the reversible hydrogen electrode.
Lee et al. observed that on chloride-induced biphasic Cu2O-Cu catalysts, electrochemical CO2 reduction yielded propylene, but at a lower yield of only 72 μA cm-2 and a Faraday efficiency (FE) of 0.9% (-1.8 V relative to RHE). Recently, Pablo-García et al. proposed that the production of propylene can be traced back to allyl alkoxy (CH2=CHCH2O) intermediates, which are prone to desorption in the alkaline microenvironment, resulting in the adverse production of propylene. This conclusion helps explain why propylene is rarely produced/detected in CO2 reduction, in contrast to ethylene production. It is necessary to have an in-depth understanding of the reaction pathways that form propylene in order to design catalysts for this reaction.
Based on this, Dan Ren and Michael Grätzel of the Ecole Polytechnique Fédérale de Lausanne reported the synthesis of Cu nanocrystals (CuNCs), whose surfaces are mainly composed of Cu(100) and Cu(111) surfaces, which significantly improves the selectivity and yield of electrosynthesis of propylene from CO2 reduction. By conducting well-designed controlled experiments including the reduction of CO, CO2/CO, CO2/He, and 13CO2/CO mixtures, the authors propose that the formation of propylene shares a highly protonated *C2 intermediate with the formation of ethylene, and that *CO cannot be a *C1 intermediate coupled with *C2 species to form propylene. This is in contrast to the n-propanol pathway, in which *CO is a key C1 precursor and participates in the *C1-*C2 coupling reaction.
Figure 1. Structural and chemical characterization of Cu CuNC.
Figure 2. Generation of C3 products on CuNC catalysts during CO2 reduction in electrochemical flow cells
Figure 3. Identification of key intermediates for the production of propylene by CO2 reduction
Figure 4. GC-MS analysis of CO2 reduction under different feed conditions
Figure 5. Identification of *C3 intermediates in propylene formation
Figure 6. Mechanism of electroreduction of CO2 to propylene on Cu catalyst
Literature information
Jing Gao, Alimohammad Bahmanpour et al. Electrochemical synthesis of propylene from carbon dioxide on copper nanocrystals. Nature Chemistry (2023).
https://doi.org/10.1038/s41557-023-01163-8