Freestanding Single-Atom-Layer Pd-Based Catalysts: Oriented Splitting of Energy Bands for Unique Stability and Activity

Freestanding and self-stabilized single-atom-layer (SAL) palladium (Pd) and palladium-cobalt (PdCo) alloy films were generated in the angstrom-sized inter-layer space of layered minerals, where the metal only grows and spreads out in two dimensions. The SAL films display strongly coordinated metallic bonds in two dimensions and dangling bonds in the z direction. Under such absolute two-dimensional coordination, their valence-empty band exhibits an unusual split into a lower-energy band in the x-y plane, which promotes in-plane stability, and a higher-energy band in the z direction, which contributes to reactivity along this axis. This gives rise to exceptional properties and functions that cannot be achieved by analogs of other dimensionalities. Specifically, the SAL PdCo alloy exhibits six times higher mass activity than commercial platinum (Pt) nanoparticles in oxygen reduction. Its mass activity is also eight times higher than that of Pd nanoparticles for formic acid oxidation.