Gas‐Phase Electrocatalytic CO ₂ Reduction on Pd‐Embedded UiO‐67 Metal–Organic Framework Precatalysts Under Varying Relative Humidity Conditions

Electrocatalytic CO₂ reduction (ECO₂R) to high-value chemicals is a promising method to upcycle emitted CO₂, but it is also a fascinating scientific challenge. Catalyst materials, as well as cell configurations, play a pivotal role in the efficacy and efficiency of the ECO₂R reaction, which also dictates reaction pathways and product selectivity. In this work, we employ the isotopological Zr- and Ce-based UiO-67 metal-organic frameworks (MOFs) that contain Pd species in a zero-gap gas diffusion cathode electrode configuration, where the water content, i.e., relative humidity (RH) level, in the CO₂ gas stream can be varied. We show that only UiO-67-based MOFs containing Pd embedded in their pores can produce syngas, while the product selectivity can be controlled by varying the RH levels in the gas stream. The pristine MOFs (precatalysts) undergo chemical and structural transformation during the ECO₂R reaction, forming the active catalysts toward CO₂ electroreduction to syngas. Our work highlights the effect of water content on the selectivity during ECO₂R, but also the need for predictive catalyst design for effective electroreduction of CO₂ to high-value chemicals.