CO-Assisted Subsurface Hydrogen Trapping in Pd(111) Films

We use low-energy electron microscopy to image CO displacing adsorbed H from the Pd(111) surface. Quantitative electron diffraction reveals that upon codosing atomic H and CO the latter assists the absorption of hydrogen, which ends up trapped between the first and second palladium layers, and blocks its desorption. Density functional calculations reproduce this effect, which is found to originate from the antibonding character of the interaction between the adsorbed hydrogen and the CO π states.