Research about Cu-Zn Catalysts for Urea Synthesis Depending on Structure Difference
Authors: Hwang Dongyoung , Han Gyeong Ho , Ahn Sang Hyun 
Published: December 2024

As the global population increases, there’s a growing demand for crops, leading to increased nitrogen fertilizers. In particular, urea (CO(NH3)2) accounts for over 70% of the world's nitrogen fertilizers, highlighting the importance of its production. Urea is typically produced through the Bosch–Meiser process, involving the reaction of CO2 and NH3 under harsh conditions (>180 ℃, ~150 bar). This process consumes significant energy and emits CO2 and NOx. In contrast, electrochemical urea synthesis offers a sustainable alternative when using renewable electricity from wind or solar and can be operated under mild conditions. Notably, co-reduction of CO2 and NO3 allows for the utilization of CO2 and NO3 produced in industrial processes, leading to potential energy and cost savings due to their higher reactivity and solubility compared to N2. Additionally, skipping the NH3 generation process offers advantages in reducing transportation and storage costs.

Nevertheless, the synthesis of urea through the co-reduction of CO2 and NO3 involves a complex 16-electron reaction, where C-N coupling plays a crucial role. Furthermore, undesirable by-products such as NH3 or NO2 can also be generated during the urea synthesis process, emphasizing the importance of controlling selectivity. Recent studies have proposed catalysts designed to facilitate C-N coupling by adjusting the binding energies of C and N intermediates on the catalyst surface to reduce energy barriers. Copper (Cu) and zinc (Zn) are known materials with appropriate adsorption energies for C and N intermediates, respectively. Therefore, the goal is to design catalysts that effectively perform the roles of C intermediate binding sites and N intermediate binding sites by combining these two elements appropriately. Among various catalyst synthesis methods, the galvanic displacement method is known for forming dendritic structures. Hence, we aim to synthesize CuZn binary catalysts via galvanic displacement, adjusting the catalyst's structure to understand its role in urea production.

Furthermore, in an H-type cell, it faces the challenge of CO2's low solubility in aqueous-fed systems and the difficulty of mass transfer due to the distance between the gas and the electrode. Additionally, supplying nitrate ions (NO3
–) requires additional solution. Therefore, we utilized a flow cell to directly supply CO2 gas through the gas diffusion electrode, overcoming the limitations of CO2 solubility and mass transfer. We will also supply NO3
– through the catholyte chamber. Thus, in the catholyte chamber, it is effective to establish a solid (catalysts)-liquid (NO3
–)-gas (CO2) three-phase boundary, which is predicted to promote C-N bonding.

In this process, Zn was electrodeposited onto a pretreated microporous layer on carbon paper (MPL/CP) in custom Teflon cell. After, Cu was displaced on the Zn/MPL/CP to create CuZn/MPL/CP. Cu has a higher standard reduction potential (E∘) compared to zinc, enabling the formation of CuZn binary catalysts through galvanic displacement. Four different types of CuZn catalysts were prepared by using various displacement solutions containing different Cu anions such as copper chloride (CuCl2), copper sulfate (CuSO4), copper nitrate (CuNO3), and copper acetate (Cu(CH3COO)2).

For the co-reduction reaction performance evaluation, experiments were conducted in a flow cell where CO2 was introduced as a gas, and the electrolyte contained NO3
–. Gas products were detected using gas chromatography, while liquid products were identified using 1H nuclear magnetic resonance. N-containing liquid products were detected via UV-vis spectroscopy using colorimetric methods. Specifically, urea was detected using the diacetyl monoxime method, confirming urea formation on the synthesized catalysts. It was observed that the amount of urea produced varied depending on the different CuZn catalysts.

Figure 1