Theoretical analysis of hydrogen chemisorption on Pd(111), Re(0001) andPdML/Re(0001),

Gradient-corrected density-functional theory (DFT-GGA) periodic slab calculations have been used to analyze the binding of atomic hydrogen on monometallic Pd(111), Re(0001), and bimetallic ${\mathrm{Pd}}_{\mathrm{ML}}/\mathrm{R}\mathrm{e}(0001)$ [pseudomorphic monolayer of Pd(111) on Re(0001)] and ${\mathrm{Re}}_{\mathrm{ML}}/\mathrm{P}\mathrm{d}(111)$ surfaces. The computed binding energies of atomic hydrogen adsorbed in the fcc hollow site, at 100% surface coverage, on the Pd(111), Re(0001), ${\mathrm{Pd}}_{\mathrm{ML}}/\mathrm{R}\mathrm{e}(0001),$ and ${\mathrm{Re}}_{\mathrm{ML}}/\mathrm{P}\mathrm{d}(111)$ surfaces, are -2.66, -2.82, -2.25, and -2.78 eV, respectively. Formal chemisorption theory was used to correlate the predicted binding energy with the location of the d-band center of the bare metal surfaces, using a model developed by Hammer and N\o{}rskov. The DFT-computed adsorption energies were also analyzed on the basis of the density of states (DOS) at the Fermi level for the clean metal surfaces. The results indicate a clear correlation between the d-band center of the surface metal atoms and the hydrogen chemisorption energy. The further the d-band center is from the Fermi level, the weaker is the chemisorption bond of atomic hydrogen on the surface. Although the DOS at the Fermi level may be related to the location of the d-band, it does not appear to provide an independent parameter for assessing surface reactivity. The weak chemisorption of hydrogen on the ${\mathrm{Pd}}_{\mathrm{ML}}/\mathrm{R}\mathrm{e}(0001)$ surface relates to substantial lowering of the d-band center of Pd, when it is pseudomorphically deposited as a monolayer on a Re substrate.