In Situ Clustering of Copper Nanoparticles in Mesoporous Cerium-Based Metal−Organic Frameworks for Efficient Electrocatalytic Nitrate Reduction to Ammonia
Authors: Shen Cheng-Hui , Zhao Yingji , Zhu Liyang , Kung Chung-Wei , Yusuke Yamauchi 
Published: December 2024

Owing to their tunable intra-framework functionalities as well as their exceptional chemical stability in water, cerium(IV)-based metal–organic frameworks (Ce-MOFs) have been utilized in a range of applications in aqueous solutions. By performing post-synthetic modifications (PSM), the high-density and spatially separated active units for the targeted reactions can be introduced within the entire framework structure, which makes Ce-MOFs particularly appealing for catalytic applications. Despite the low electrical conductivities of most MOFs hinder their direct use for electrochemical applications, electrons can be transported in the intrinsically insulating Ce-MOFs by node-to-node redox hopping during the electrochemical processes. We thus reasoned that chemically robust Ce-MOFs incorporated with spatially separated electrochemically active moieties should be intriguing materials for electrocatalytic systems. Nevertheless, the inherent microporous nature of most Ce-MOFs severely hinders the mass transfer and restricts the accessibility of reactants, which results in not only the limited utilization of active units but also poor catalytic activity. Recently, porosity engineering of MOFs has been regarded as a promising strategy to solve the issue. By selecting suitable polymeric surfactants as soft templates to guide the growth of MOF crystals, the morphology and ordered mesoporous arrays of MOFs could be facilely regulated. It is believed that the design of ordered mesoporous Ce-MOFs can improve the diffusion of guest molecules and enhance the electrocatalytic performances.
Among various electrocatalytic reactions, the electrochemical reduction of nitrate (NO3
-) to ammonia (NH3) is particularly of interest. Nevertheless, the competitive hydrogen evolution reaction (HER) suppresses the Faradaic efficiency and ammonia yield. With the electronic energy level matching to the molecular orbital of nitrate, copper is regarded as a potential candidate that can rapidly adsorb nitrate at the catalytic sites and efficiently reduce nitrate. While the low porosity and limited surface-to-volume ratio result in the low metal utilization of bulk copper-based materials. By serving a Ce-MOF as a porous platform, copper ions can be decorated on the hexa-cerium nodes, and spatially separated Cu particles within nanopores of the Ce-MOF can be further obtained after electrochemical treatment. We thus reasoned that the use of chemically robust and mesoporous Ce-MOFs for the in-situ formation of Cu nanoparticles would be a promising strategy to facilitate the mass transfer of nitrate for the electrochemical ammonia production.
In this study, a pluronic poly(ethylene oxide)−poly(propylene oxide)−poly(ethylene oxide) (PEO−PPO−PEO) surfactant was employed as a soft template for the formation of Ce-MOF crystals with ordered mesopores. The immobilization of copper ions on hexa-cerium nodes was further conducted by a solvothermal deposition in MOFs (SIM) method. Thereafter, in situ construction of Cu nanoparticles confined within the Ce-MOF skeleton (denoted as Cu@mesoMOF) was performed by the electroreduction method. The electrocatalytic performance for nitrate reduction to ammonia was investigated in 0.5 M Na2SO4 with 10 mM NaNO3, and chronoamperometric techniques were used to evaluate the activity. The catalyst loading of modified electrodes and the applied potential for nitrate reduction were first optimized. As a comparison, modified electrodes with Cu nanoparticles confined within the Ce-MOF without mesopores (Cu@MOF) were also performed. By providing the ordered mesopores as channels for mass transfer, the diffusion of nitrate from the external solution to the internal catalytic sites can be facilitated, and the experimental results here demonstrate that the Cu@mesoMOF thin film can achieve a higher Faradaic efficiency and better selectivity. Furthermore, a 1.8-fold ammonia production rate can be obtained for Cu@mesoMOF thin film compared to the Cu@MOF thin film, demonstrating the electrocatalytic activity is deeply influenced by the diffusion of reactants. Findings here shed light on the use of mesoporous MOFs for facilitating the mass transfer and suggest the rational design of mesoporous property can be a promising strategy to enhance the activity of MOF-based catalysts for electrocatalysis.

Figure 1