Multicarbon molecules are critical building blocks in our society today. Decarbonizing the production of these molecules is crucial in creating an environmentally sustainable chemical industry. One such molecule is 1,3-butadiene, a key component in making synthetic rubber for pipes or tyres. However, 1,3-butadiene is commonly synthesized via the thermocatalytic cracking of naphtha or ethane, which is notorious for its high energy consumption due to the high heat and pressures required. So, how can we make 1,3-butadiene production more te sustainable?

Electrocatalysis is a promising solution, as it can be conducted under ambient conditions and the electricity required is potentially obtained from renewable sources. This makes it a viable option for an environmentally sustainable production of chemicals. We hone in on acetylene as a feedstock which can (a) form 1,3-butadiene under thermodynamically favorable conditions due to its reactive triple bond, and (b) be synthesized from greenhouse gases such as methane or carbon dioxide.  Interestingly, acetylene electrolysis has always shown the production of 1,3-butadiene but is usually considered as an unwanted by-product during ethylene production.

Thus, we sought to improve the selectivity for 1,3-butadiene from acetylene electrolysis. We hypothesized that enhancing the C-C coupling between acetylene molecules is key to increasing the selectivity. To this end, introducing partially oxidized Cu sites, which we term Cu^δ+-Cu⁰ sites, can improve C-C coupling. However, how can the oxidation state be preserved during electroreduction? In previous works on CO₂ electroreduction, iodide has been found to stabilize partially oxidized Cu, even under a negative potential. This will be a good place to start.

Starting from Cu₂O nanocubes as a catalyst for acetylene electrolysis in a flow cell, we introduced iodide to the oxide-derived catalyst simply by using a KI electrolyte. Intriguingly, we attained a selectivity for 1,3-butadiene of 93 % at an applied potential of -0.85 V vs standard hydrogen electrode (SHE) and a partial current density of -75 mA cm^-2 at -1.0 V vs SHE.  The partial current densities obtained were 20 times higher than any other previous reports (Figure 1a). Indeed, when we compared the performance of acetylene electrolysis in other anion electrolytes such as KCl, KBr, and K₂SO₄, the selectivity and current densities in KI are 20% percentage points and 1.5x higher, respectively (Figure 1b). This high selectivity for 1,3-butadiene production can be sustained for 11 hours.

Spurred by this, we look towards in-situ X-ray absorption and Raman spectroscopy to determine how the different electrolytes affect the catalyst's chemical state. The external absorption fine structure (EXAFS) results showed the obvious presence of both Cu⁺ and Cu⁰ species in the catalyst with KI electrolyte (Figure 1c). This indicates that even under a negative potential, the Cu catalyst remains partially oxidized in KI. Our Raman spectroscopy results also showed that Cu₂O was present during acetylene electrolysis in KI. In contrast, spectroscopy conducted in K₂SO₄, KCl, and KBr electrolytes showed only metallic Cu during acetylene electrolysis. 

From the experimental results, we believe a strong correlation exists between the preservation of Cu^δ+-Cu⁰ species by iodide and the increased 1,3-butadiene production rate. However, there are still unanswered questions, such as how iodide preserves the stability of the partially oxidized Cu sites, or how the 1,3-butadiene production is boosted by iodide. Thus, we turn to density function theory (DFT) calculations to answer them. The formation of  1,3-butadiene from acetylene occurs by reducing adsorbed C₂H₂ molecules to ^*C₂H₃ moieties, followed by coupling two *C₂H₃ moieties to form C₄H₆ (Figure 1d). Utilizing a square symmetry stripe with adsorbed oxygen to mimic roughened, partially oxidized Cu, we found that the co-adsorption of iodide and oxygen weakens acetylene adsorption on the catalyst surface. Based on the Sabatier principle, a weakening of acetylene adsorption energies increases the activity of reducing acetylene to 1,3-butadiene, since good catalysts are located near the top of the volcano plot. In contrast, the other analyzed anions strengthen the binding energies for acetylene, resulting in a lower acetylene reduction activity (Figure 1e). We also found that the potential to reduce oxygen in Cu₂O is 100 mV more negative with iodide than other anions, indicating that iodide does prevent the Cu⁺ species from being fully reduced. The co-adsorption of ^*I and ^*O not only results in favorable thermodynamics but also lowers the barrier for C-C coupling between ^*C₂H₃ moieties to produce 1,3-butadiene.  

The strategy raised here demonstrates the production of 1,3-butadiene with notable selectivities and current densities simply by adding iodide to copper catalysts. The electrocatalytic acetylene reduction to 1,3-butadiene can potentially result in significant energy savings compared to conventional methods, allowing us to progress toward the sustainable production of multicarbon molecules. 

The full paper can be found here: https://www.nature.com/articles/s41929-024-01250-0