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 (MoTiC) 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 H are 4.8, 1.6 s, and 43.5%, respectively).