Host-Guest Ensemble Effect on Dual-Pt atom-on-Rh Nanosheets Enables High-Efficiency and Anti-CO Alkaline Hydrogen Oxidation

Explicit atomic coordinations of binary metal atoms on low-dimensional host-guest-type bimetallic nanomaterials can arouse unique ensemble effects and significantly improve the catalysis performances, but they confront a formidable challenge in chemical synthesis. Here, we draw on a thermoinduced secondary crystallization process of preprepared amorphous/crystalline hybrid ultrathin Rh nanosheets (NSs) to capture Pt atoms into the two-dimensional (2D) Rh lattice, by which precisely controlled Pt atomic dispersions from a single atom to dual atoms and to clustered atoms can be facilely elected and embedded into the ultrathin Rh hosts. Among them, the dual-Pt atom-on-Rh (denoted as Pt2-Rh) NSs exhibit an extremely high activity and an excellent CO tolerance for the alkaline hydrogen oxidation reaction (HOR), achieving a 18.5 times higher mass activity at 50 mV overpotential than that of commercial Pt/C. Theoretical studies indicate that the host-guest ensemble effect aroused by the specific pairwise Pt2-on-Rh coordination feature can moderately enhance both the surface *H and *OH bindings, which synergistically lower the free energy of the rate-determining Volmer step and thus promote the combination of neighboring *H and *OH to form H2O. Enhanced *H adsorption can also prevent the main available sites from being occupied by hydroxyl groups in alkaline conditions. The free energy diagram and anti-CO experiments coherently indicate that the oxidation of *CO by neighboring *OH to form *COOH is kinetically more favorable to proceed on the Pt2-Rh NSs, effectively preventing the catalysts from being poisoned. This work sheds light on the ensemble effect of host-guest heteroatom coordinations for designing high-efficiency alkaline HOR catalysts.