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 fil led d-bands, and Ni provides unfilled d-states, this allows study of t he influence of the d-electrons on the hydrogen adsorption mechanism. The band structures of Cu and NiAl are rather similar and we can addre ss the problem of whether the adsorption is dominated by the local cha racter 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 e V and 3.0 eV in the order Al, NiAl, Cu, Ni in qualitative agreement wi th the available experimental data. On NiAl(110), hydrogen adsorbs wit h approximately the same energy above the Ni and Al rows, although the components of the adsorption energy vary significantly. The reduced a dsorption energy on NiAl compared to Cu and Ni is traced back to the i ncreased sp-electron density and the more diffuse character of the d-o rbitals in NiAl. Adsorption strength is determined by the competing ef fects of energy gain due to polarization of metal electrons toward the attractive proton potential and energy loss due to Pauli repulsion. C omparison 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 chemisorpti on on metal and alloy surfaces. (C) 1996 American Institute of Physics .