(Invited) Reconstruction of Metal-Organic Framework-Based Heterogeneous Catalysts during CO 2 Electroreduction

Electrochemical CO 2 reduction reaction (CO 2 RR), which enables CO 2 conversion into value-added fuels and chemical feedstocks, provides an avenue toward carbon neutrality and renewable energy storage. In the CO 2 RR, heterogeneous electrocatalysts are essential to activate substrate species and interact with reaction intermediates for efficient and selective CO 2 conversion. Especially, Cu-based active sites in the catalysts enable C-C coupling via *CO dimerization for multi-carbon (C 2+ ) chemicals production. Although judiciously controlled surface of catalysts is required to control the binding of CO 2 RR intermediates, the active sites of the catalysts undergo severe reconstruction during electrochemical reaction. This hinders the identification of active sites because it induces structure and phase evolution of electrocatalysts during CO 2 RR. Therefore, reconstruction is a major bottleneck in the development of CO 2 RR, and it has remained as a challenge to unveil the mechanism of reconstruction. Here, I will present our recent efforts to understand the reconstruction of Cu-based metal-organic framework (MOF) during electrochemical CO 2 RR. MOFs have porous structures where the organic linkers are bonded with metal node in secondary building unit (SBU). We verified that oxidation tendency of metal ions in SBU is a key to determine the stability of MOFs. Pourbaix diagram helps to predict the trend of metal reduction in MOFs during CO 2 RR. Operando X-ray absorption spectroscopy (XAS) during CO 2 RR enabled to track the states of metal node in MOFs. We found that Cu ions in Cu-MOFs can be reduced to pure Cu states during CO 2 RR. This reveals the reconstruction of Cu-MOFs under reductive potential. To unveil the reconstruction behaviors of Cu active sites in the MOFs, we investigated the CO 2 RR of Cu benzene-1,3,5-tricarboxylate (Cu-BTC) and Cu benzene-1,4-dicarboxylate (Cu-BDC) MOFs. Unlike the Cu-BTC which resulted in the formation of C 1 chemicals as major products, Cu-BDC produced C 2 chemicals such as ethylene (C 2 H 4 ) in CO 2 RR. We elucidate how the catalytic active sites of Cu-MOFs undergo the reconstruction and evolve during CO 2 RR. Also, we explain the how the reconstruction of Cu active sites in MOFs affects the steering of CO 2 RR pathway. In the concluding remarks, the future perspective in the catalyst reconstruction during CO 2 RR will be discussed.