A First-Principles Model for Hydrogen Uptake Promoted by Sulfur on Ni(111)

First-principles calculations have been employed to assist in the development of a three-zone model for hydrogen uptake promoted by sulfur on the (111) surface plane of Ni. The three zones suggested by previous work of Protopopoff and Marcus [J. Vac. Sci. Technol. A, 5, 944 (1987).] consist of (1) a site-blocking zone, where sulfur prevents hydrogen occupation at four adsorption sites per sulfur atom, (2) a promotion zone, where nearby sulfur raises the chemical potential of hydrogen in the six next-nearest-neighbor sites and beyond these two zones, (3) a free zone in which there is no influence of the adsorbed sulfur. The model is based on adsorption energies of hydrogen on Ni(111) as well as the sulfur-hydrogen interaction energies calculated from density functional theory. The results from the model indicate that the maximum promotion for hydrogen uptake occurs when up to 0.1 monolayer (ML) coverage of sulfur is present, and beyond this limit the site-blocking features of sulfur begin to dominate, thus reducing the available surface area down to 0 at 0.25 ML of sulfur. (C) 2011 The Electrochemical Society. [DOI: 10.1149/1.3536470] All rights reserved.