A comparative theoretical study of atomic hydrogen adsorption on the (110) faces of Al, Cu, Ni, and NiAl

The interaction of atomic hydrogen with Al(110), Cu(110), Ni(110), and NiAl(110) is investigated using a spin-unrestricted screened Hartree-Fock method. As Al is a pure sp-metal, Cu and NiAl have completely filled d-bands, and Ni provides unfilled d-states, this allows study of the influence of the d-electrons on the hydrogen adsorption mechanism. The band structures of Cu and NiAl are rather similar and we can address the problem of whether the adsorption is dominated by the local character or the band structure. A detailed analysis of the chemisorption bond is given, based on a density matrix partitioning technique. The adsorption energy is found to increase from 1.8 eV via 2.1 eV to 2.4 eV and 3.0 eV in the order Al, NiAl, Cu, Ni in qualitative agreement with the available experimental data. On NiAl(110), hydrogen adsorbs with approximately the same energy above the Ni and Al rows, although the components of the adsorption energy vary significantly. The reduced adsorption energy on NiAl compared to Cu and Ni is traced back to the increased sp-electron density and the more diffuse character of the d-orbitals in NiAl. Adsorption strength is determined by the competing effects of energy gain due to polarization of metal electrons toward the attractive proton potential and energy loss due to Pauli repulsion. Comparison with the mechanism of formation of gas phase chemical bonds reveals a novel mechanism of the chemisorption bond which is uncommon in the gas phase. The long-range interaction with the image potential, the high polarizability, and the spillover of metal electrons play a decisive role in determining the new mechanism of hydrogen chemisorption on metal and alloy surfaces. (C) 1996 American Institute of Physics.