An Atomically Dispersed Pd Sub‐Metallene: Intermediate State of Single Atoms and Metal Bonds

Despite the metal coordination and single-atom catalyst (SAC) have been extensively investigated in surface science over the past decade, their overall activity in involving multi-step reactions remains unsatisfactory owing to the metal bond and single atom being irreconcilable. Here, a stable atomically dispersed Pd sub-metallene (Pd ADSM) layer supported on the 2D MXene (Mo₂TiC₂) is reported, which combines the advantages of 2D structures, single atoms, and metal bonds. Pd ADSM shows covalent structures along the z-coordination and highly coordinated metal bonds in the 2D direction. During the alkaline hydrogen evolution reaction (HER), Pd ADSM shows 7- and 112-times higher mass activity than the SAC (Pd SAC) and commercial Pt/C at the overpotential of -108 mV, respectively. Operando characterizations and theoretical calculations reveal that the Pd─Pd interface not only makes the adsorbed water form a flexible hydrogen-bonded skeleton closer to the catalytic center but also reduces the energy barrier for the HER rate-determining step. Moreover, the moderate adsorption energy of Pd─Pd bonds in ADSM can rapidly activate, dissociate, and desorb hydrogen molecules at room temperature, resulting in record-high hydrogen sensing performances (Response time, Recovery time, and Sensitivity for 100 ppm H₂ are 4.8, 1.6 s, and 43.5%, respectively).