The motion of adsorbate molecules across surfaces is fundamental to self-assembly, material growth, and heterogeneous catalysis. Recent Scanning Tunneling Microscopy studies have demonstrated the electron-induced long-range surface-migration of ethylene, benzene, and related molecules, moving tens of Angstroms across Si(100). We present a model of the previously unexplained long-range recoil of chemisorbed ethylene across the surface of silicon. The molecular dynamics reveal two key elements for directed long-range migration: first 'ballistic' motion that causes the molecule to leave the ab initio slab of the surface traveling 3-8 Å above it out of range of its roughness, and thereafter skipping-stone 'bounces' that transport it further to the observed long distances. Using a previously tested Impulsive Two-State model, we predict comparable long-range recoil of atomic chlorine following electron-induced dissociation of chlorophenyl chemisorbed at Cu(110).
CO adsorption on ${\mathrm{NO}}_{2}$-predosed $\mathrm{Au}{111}$ reveals an unexpected attractive coadsorbate interaction, associated with an unprecedented blueshift of the CO stretch frequency, a sizeable attenuation of the infrared ${\mathrm{NO}}_{2}$ symmetric stretch band, and a $(\sqrt{7}\ifmmode\times\else\texttimes\fi{}\sqrt{7})R19\ifmmode^\circ\else\textdegree\fi{}$ structure characterized by scanning tunneling microscopy and low energy electron diffraction. Density functional calculations allow us to rationalize these observations, and point towards a general pattern of behavior for electronegative coadsorbates on coinage metals, with important implications for catalytic promotion.
This chapter reviews the state of knowledge relating to chirality within the electronic structure of otherwise non-chiral two-dimensional systems. One important consequence of the electronic chirality within each individual Dirac cone may be found in the phenomenon of Klein tunnelling. The Dirac cones associated with graphene arise in a fundamentally two-dimensional material, but in principle the mathematical treatment that predicts their existence can potentially apply in any situation where electrons are confined within a two-dimensional sub-space. One example is to be found in the surface states of at least one half-metallic ferromagnet. There exists a class of materials in which Dirac cones arise not through the symmetry properties of the atomic geometry, but rather through the topological properties of the electronic structure itself. These materials are known as topological insulators, and the Dirac cones are to be found in surface-localised electronic states at their surfaces.
This work presents results from density functional theory calculations which are used to elucidate the reduction of pyruvic acid to lactic acid by direct hydrogenation over Cu{110} in vacuo. We propose a plausible pathway from reactants to products that crucially relies upon an intramolecular tunneling step to circumvent energetically unfavorable hydrogen exchange with the surface. The conclusions are further augmented by analyzing the electron density and frontier orbitals of key reaction intermediates. This reveals the origin of the predicted activity to be intimately linked to the electronic structure, which in turn is dependent upon the asorption geometry of pyruvic acid. Through the use of equilibrium thermodynamics, we are able to show the influence of temperature and pressure on the reaction profile. Importantly showing, that as the temperature is raised at low pressure (1 × 10−10 mbar), so the rate-determining step switches from being the carbonyl reduction to the reprotonation of the carboxylate group (leading to the desorption of lactic acid). At ambient pressure of 1 bar, the influence of temperature on the relative barrier heights is much less significant. This is an important step in attempting to bridge the so-called "pressure gap" and opens up the possibility of understanding the reactivity of small biologically relevant molecules at metal surfaces.
A c(2×4) LEED pattern is observed for methylidyne (CH) chemisorbed on Pt{110}(1×2) at a saturation coverage of 0.25 ML. Density functional calculations reveal that methylidyne is preferentially adsorbed in the fcc three-fold hollow site on the {111} microfacet of the reconstructed surface. A structure for the ordered overlayer is thus proposed, and both through-metal and through-space interactions are considered as possible causes for this unexpected long-range coherence. We argue that entropic effects may be implicated.
Microscopic reaction pathways for the transformation of chemisorbed methyl to atomic carbon on Pt{110}(1 × 2) have been identified using calculations based on density functional theory in combination with a constrained minimization technique. For CH3 and CH2 dehydrogenation, calculated activation energies of 0.34 eV (33 kJ mol-1) and 0.56 eV (54 kJ mol-1) are obtained, respectively. For CH dehydrogenation, the calculated activation barrier of 1.20 eV (116 kJ mol-1) is in excellent agreement with the experimentally determined barrier of 1.25 eV (121 ± 3 kJ mol-1).1 The resulting calculated reaction-energy profile for the conversion of CH3 to CH and ultimately carbon on Pt{110}(1 × 2) is reported and discussed in terms of previous experimental results obtained for this system.
Chiral surfaces offer great potential as a medium for enantioselective synthesis or separation, yet their dynamic enantiospecific interactions with adsorbates are not well understood. Here, the influence of chiral surfaces on the molecular rotations of desorbing molecules is investigated. Formic acid desorption from $\mathrm{Cu}{531}$ and $\mathrm{Cu}{110}$ serve as model systems for desorption processes of an achiral adsorbate from a chiral and an achiral surface. Our first-principles molecular dynamics study reveals a much larger and more directed angular momentum for molecules desorbing from the chiral surface and a clear preference for one sense of rotation. This result provides new insight into desorption and adsorption processes and propensities on chiral surfaces.
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