Balancing Active Au Cluster Formation and Surface Hydroxy Groups on Mg–Al Layered Double Hydroxides for Preferential CO Oxidation Under H ₂ ‐Rich Conditions

ABSTRACT Preferential oxidation of CO (CO‐PROX) is essential for supplying CO‐free hydrogen to polymer electrolyte fuel cells (PEFCs). We investigated the structure–performance relationship of Au catalysts supported on Mg─Al layered double hydroxides (LDHs) under H 2 ‐rich conditions. By combining catalytic measurements with in situ X‐ray absorption fine structure (XAFS), we demonstrated that cationic Au species and atomically dispersed Au(0) are not responsible for appreciable CO‐PROX activity. Instead, catalytic activity emerged after generating Au(0) clusters induced by thermal treatment under CO‐PROX conditions. The temperature required for Au cluster formation depended on the LDH crystallite size. According to in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), LDH nanoparticles (NPs) possessing abundant edge hydroxy groups stabilized atomically dispersed Au species more effectively than bulk LDH, but simultaneously promote H 2 O accumulation, blocking the active sites. In contrast, bulk LDH enabled the formation of active Au clusters at lower temperatures, while a moderate number of hydroxy groups allows regulation of the surface H 2 O quantity, thereby facilitating efficient removal of carbonate intermediates and accelerating CO oxidation. As a result, Au/bulk‐LDH achieved high CO oxidation activity and selectivity. These results provide mechanistic insights and design principles for Au‐based CO‐PROX catalysts operable at PEFC‐relevant temperatures.