Unraveling Dynamic Structural Evolution of Single Atom Catalyst <i>via In Situ</i> Surface-Enhanced Infrared Absorption Spectroscopy

Metal-nitrogen-carbon (M-N-C) single-atom catalysts (SACs) have been widely applied in catalyzing electrochemical redox reactions. However, their long-term catalytic stabilities greatly limit their practical applications. This work investigates the dynamic evolution of two model Cu-N-C SACs with different Cu-N coordinations, namely the Cu₁/N_pyri-C and Cu₁/N_pyrr-C, in electrochemical CO reduction reaction (CORR), based on a collection of <i>in situ</i> characterizations including <i>in situ</i> attenuated total reflectance surface-enhanced infrared absorption spectroscopy, <i>in situ</i> X-ray absorption spectroscopy, <i>quasi-in situ</i> electron paramagnetic resonance spectroscopy and <i>in situ</i> ultraviolet-visible spectroscopy, complemented by theoretical calculations. Our findings reveal that the Cu nanoparticle formation rate over Cu₁/N_pyrr-C is more than 6 times higher than that over Cu₁/N_pyri-C during the electrochemical CORR. <i>Quasi-in situ</i> electron paramagnetic resonance and <i>in situ</i> UV-vis spectroscopy measurements demonstrate that hydrogen radicals can be <i>in situ</i> produced during electrochemical CORR, which will attack the Cu-N bonds in the Cu-N-C SACs, causing leaching of Cu²⁺ followed by subsequent reduction to form Cu nanoparticles. Kinetic calculations show that Cu₁/N_pyri-C displays a better catalytic stability than Cu₁/N_pyrr-C resulting from the stronger Cu-N_pyri bonds. This study deepens the understanding of the deactivation mechanism of SACs in electrochemical reactions and provides guidance for the design of next-generation SACs with enhanced durability.