Adsorption and diffusion energetics of hydrogen atoms on Fe(110) from first principles D.E. Jiang, Emily A. Carter * Department of Chemistry and Biochemistry, Box 951569, University of California, Los Angeles, CA 90095-1569, USA Received 10 August 2003; accepted for publication 4 October 2003 Abstract Spin-polarized density functional theory (DFT) has been used to characterize hydrogen atom adsorption and dif- fusionenergeticsontheFe(110)surface.TheKohn-Shamequationsaresolvedwithperiodicboundaryconditionsand within the all-electron projector-augmented-wave (PAW) formalism, using a generalized gradient approximation (GGA) to account for electron exchange and correlation. We investigate the site preference of H on Fe(110) for 0.25, 0.50, and 1.0 ML coverages and find that the quasi three-fold site is the only stable minimum (in agreement with experiment).WefindthelongandshortbridgesitestobetransitionstatesforHdiffusiononFe(110),whiletheontop site is a rank-2 saddle point. The preference of the three-fold site is rationalized via an analysis of the site- and orbital- resolveddensityofstates.AnanalysisofchargedensitydifferencessuggeststhattheH–Feinteractionisquitecovalent, with only 0.1 electron transferred from Fe atoms to H in the three-fold site of Fe(110). We also compare two ex- perimentallyobserved0.50MLphasesforH/Fe(110):agraphitic(2 · 2)-2Handa(2 · 1)phase.WeconfirmtheLEED datathattheFe(110)-(2 · 2)-2H superstructure is more stable at low temperature. The predicted adsorption structure and weak substrate reconstruction for the Fe(110)-(2 · 2)-2H phase roughly agree with experiment, though discrep- ancies do exist regarding the H-surface height and the H–H distance. Moreover, trends in work function with coverage are predicted to be qualitatively different than older measurements, with even the sign of the work function changes in question.Lastly,azig–zagdiffusionpathforHatomsonFe(110)isproposed,involvingaverylow(<0.2eV)barrier. Ó 2003 Elsevier B.V. All rights reserved. Keywords: Density functional calculations; Chemisorption; Iron; Hydrogen atom; Surface diffusion 1. Introduction The interaction of hydrogen with transition metal surfaces is of great fundamental and practical interest in both heterogeneous catalysis and metallurgy. Transition metal surfaces cata- lyze numerous hydrogenation reactions. Hydrogen chemisorption on iron is particularly important, not only as a reactant in Fe-catalyzed ammonia synthesis [1], the Fischer–Tropsch reaction [2] (and many others), but also because hydrogen is known to embrittle steels [3,4], causing fracture and ulti- mately failure of the steel component. The adsorption of hydrogen on low-index Fe surfaces has been investigated experimentally for * Corresponding author. Tel.: +1-310-206-5118; fax: +1-310- 267-0319. E-mail address: eac@chem.ucla.edu (E.A. Carter). 0039-6028/$ - see front matter Ó 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.susc.2003.10.007 Surface Science 547 (2003) 85–98 www.elsevier.com/locate/susc