Elucidation of the active surface and origin of the weak metal-hydrogen bond on Ni/Pt(111) bimetallic surfaces: a surface science and density functional theory study

Ni/Pt(1 1 1) bimetallic surfaces were prepared by physical vapor deposition of Ni onto Pt(1 1 1) single crystals at 300 K with and without subsequent annealing at 600 K, and by direct deposition of Ni at 600 K. The structure, composition and chemical properties of these surfaces were investigated using temperature programmed desorption (TPD) of H-2, scanning tunneling microscopy (STM), low energy ion scattering spectroscopy (LEIS), X-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES). Hydrogen desorbed at a lower temperature from the annealed Ni/ Pt(1 1 1) bimetallic surfaces than from the clean Pt(1 1 1) surface or thick (> 5 ML) Ni surface, suggesting a weaker metal-hydrogen bond on the annealed Ni/Pt(1 1 1) bimetallic surfaces. In contrast, H-2 desorbed from the surface prepared by 300 K nickel deposition at a higher temperature than from Pt(1 1 1) and close to that from the thick Ni surface, suggesting a stronger metal-H bond on the unannealed Ni/Pt(1 1 1) surface. STM, LEIS and XPS results indicated that Ni atoms deposited at 300 K remain on the Pt(1 1 1) surface, but that substantial amounts of Ni atoms diffuse into the bulk in the course of annealing at 600 K. Density functional theory calculations showed that subsurface Ni atoms could leads to a weaker metal-hydrogen bond by modification of the electronic structure of the surface Pt atoms. (C) 2003 Published by Elsevier B.V.