Pd diffusion on MgO(100): The role of defects and small cluster mobility

Department of Chemistry 351700, University of Washington, Seattle, WA 98195-1700, United States; Department of Chemistry and Biochemistry, University of Texas at Austin, 1 University Station Stop A5300, Austin, TX 78712-0165, United States; Faculty of Science, VR-II, University of Iceland, 107 Reykjavík, Iceland
Surface Science (Impact Factor: 1.87). 01/2006; DOI: 10.1016/j.susc.2006.01.034

ABSTRACT Density functional theory is used to explore the energy landscape of Pd atoms adsorbed on the terrace of MgO(1 0 0) and at oxygen vacancy sites. Saddle point finding methods reveal that small Pd clusters diffuse on the terrace in interesting ways. The monomer and dimer diffuse via single atom hops between oxygen sites with barriers of 0.34 eV and 0.43 eV respectively. The trimer and tetramer, however, form 3D clusters by overcoming a 2D–3D transition barrier of less than 60 meV. The trimer diffuses along the surface either by a walking or flipping motion, with comparable barriers of ca. 0.5 eV. The tetramer rolls along the terrace with a lower barrier of 0.42 eV. Soft rotational modes at the saddle point lead to an anomalously high prefactor of 1.3 × 1014 s−1 for tetramer diffusion. This prefactor is two order of magnitude higher than for monomer diffusion, making the tetramer the fastest diffusing species on the terrace at all temperatures for which diffusion is active (above 200 K). Neutral oxygen vacancy sites are found to bind Pd monomers with a 2.63 eV stronger binding energy than the terrace. A second Pd atom, however, binds to this trapped monomer with a smaller energy of 0.56 eV, so that dimers at defects dissociate on a time scale of milliseconds at room temperature. Larger clusters bind more strongly at defects. Trimers and tetramers dissociate from monomer-bound-defects at elevated temperatures of ca. 600 K. These species are also mobile on the terrace, suggesting they are important for the ripening observed at ⩾600 K during Pd vapor deposition on MgO(1 0 0) by Haas et al. [G. Haas, A. Menck, H. Brune, J.V. Barth, J.A. Venables, K. Kern, Phys. Rev. B 61 (2000) 11105].

  • [Show abstract] [Hide abstract]
    ABSTRACT: The diffusion of isolated adatoms and small clusters is reviewed for transition and noble metals adsorbed on the (001) surface of magnesium oxide. While isolated adatoms diffuse by hopping among adsorption sites, small clusters such as dimers, trimers and tetramers already display a variety of diffusion mechanisms, from cluster hopping to rotation, sliding, leapfrog, walking, concertina, flipping, twisting, rolling and rocking. Since most of the available results are computational, the review is mostly related to theoretical work. Connection to experiments is discussed where possible, mostly by dealing with the consequences that adatom and small cluster mobility may have on the growth of larger aggregates on the MgO(001) surface.
    Journal of Physics Condensed Matter 07/2009; 21(26):264001. · 2.22 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: We investigate the effect of ferroelectric polarization direction on the geometric properties of Pd deposited on the positive and negative surfaces of LiNbO(3) (0001). We predict preferred geometries and diffusion properties of small Pd clusters using density functional theory, and use these calculations as the basis for kinetic Monte Carlo simulations of Pd deposition on a larger scale. Our results show that on the positive surface, Pd atoms favor a clustered configuration, while on the negative surface, Pd atoms are adsorbed in a more dispersed pattern due to suppression of diffusion and agglomeration. This suggests that the effect of LiNbO(3) polarization direction on the catalytic activity of Pd [J. Phys. Chem. 88, 1148 (1984)] is due, at least in part, to differences in adsorption geometry. Further investigations using these methods can aid the search for catalysts whose activities switch reversibly with the polarization of their ferroelectric substrates.
    Physical Review Letters 08/2011; 107(7):076102. · 7.73 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: The stepwise and concerted reaction mechanisms of NO with CO on Pd1 atom and Pd4 cluster adsorbed on the MgO surface have been studied by using the DFT/B3LYP method with the embedded cluster model. The reaction barriers have been calculated by using the IMOMO method at the CCSD level. The results suggest that the rate-controlling step barrier of the NO dissociation on the supported Pd4 cluster for the stepwise reaction mechanism is about 57 kcal/mol and lower by about 33 kcal/mol than that on the single supported Pd atom. However, the concerted reaction pathways on the single supported Pd atom and four supported Pd atoms are energetically unfavorable with the barriers of about 98 kcal/mol and 71 kcal/mol, respectively. Compared to the concerted reaction mechanism, the stepwise reaction mechanism of NO + CO is a possible pathway of CO2 molecule formation on the supported Pd catalysts.
    Chemical Physics 02/2012; 395:108–114. · 1.96 Impact Factor

Full-text (2 Sources)

Available from
May 26, 2014