Theoretical studies of hydrogen molecule adsorption on flat and stepped platinum surfaces

Theoretical calculations have been applied to the adsorption of hydrogen molecule on flat and stepped platinum surfaces. The method of large unit cell is modified to deal with the stepped surface as well as the flat one. This method is free from the boundary effect which is inevitable in the cluster method. The results calculated for the d-band width and the highest occupied level are in good agreement with the experiments. For the dissociative adsorption, the bottom of the step site is the most favorable, and the result is attributed to the extra orbital interactions at this site.     

The decomposition of ammonia on the flat (111) and stepped (557) platinum crystal surfaces

Ammonia adsorption, desorption and decomposition to H₂ and N₂ has been studied on the flat (111) and stepped (557) single crystal faces of platinum using molecular beam surface scattering techniques. Both surfaces show significant adsorption with sticking coefficients on the order of unity. The stepped (557) surface is 16 times more reactive for decomposition of ammonia to N₂ and H₂ than the flat (111) surface. Kinetic parameters have been determined for the ammonia desorption process from the Pt(111) surface. The mechanism of ammonia decomposition on the (557) face of platinum has been investigated.     

On the local densities of states on flat and stepped Pt surfaces

The electronic structure of the d band of both flat and stepped Pt surfaces is investigated within the tight-binding approximation, using a moment method. A sharp surface virtual bound state peak is found in the local density of states at the protruding edge of the stepped surfaces and the symmetry of states near the Fermi level are found to be rather dependent on the geometry of the surface. Possible connections with experiments are briefly discussed.     

First-principles study of the water structure on flat and stepped gold surfaces

The geometric structure and electronic properties of flat and stepped gold–water interfaces have been addressed by periodic density functional theory (DFT) calculations. This work was motivated by a recent electron energy loss spectroscopy study [H. Ibach, Surf. Sci. 604 (2010) 377] indicating that the structure of a water layer on stepped Au(511) differs significantly from the one on Au(100). Based on ab initio molecular dynamics simulations, the measured spectra have been reproduced and linked to the geometric arrangement of the water molecules. Furthermore, we find a strong polarization of the water layers which contributes to the water-induced work function change of the substrate.     

Density functional slab model studies of water adsorption on flat and stepped Cu surfaces

Density functional and periodic slab model calculations are performed to study adsorption of water on various Cu surfaces, focusing on monomers and dimers at the planar Cu surfaces and monomers at stepped ones. Single water molecules tend to weakly bind to atop positions with the molecular plane basically parallel to the substrate surface on the planar surfaces or the step plane on the stepped surfaces with negligible structural deformation of water. The experimental adsorption energies of water on the (1 1 1) and (1 0 0) surfaces are about twice as large as the theoretical values of monomerically adsorbed water. This phenomenon is demonstrated to be due to formation of water clusters and/or existence of surface defects. It is revealed that the most favorable hexagonal ring superstructure on Cu(1 1 0) is a four-layer structure, not the commonly accepted bi-layer configuration. We found that the adsorption energy of monomeric water correlates linearly with following quantities, respectively: the bond length and the stretching frequency of the Cu-O bond, the coordination number of the surface Cu atom, the surface work function of the clean surface and the 1b₁ MO energy shift with respect to the value in the gas phase.     

Low energy electron diffraction studies of chemisorbed gases on stepped surfaces of platinum

The chemisorption of hydrogen, oxygen, carbon, carbon monoxide and ethylene was studied by low-energy electron diffraction on ordered stepped surfaces of platinum which were cut at angles less than 10° from the (111) face. The chemisorption characteristics of stepped platinum surfaces are markedly different from those of low index platinum surfaces and they are also different from each other. Hydrogen and oxygen which do not chemisorb easily on the (111) and (100) crystal faces chemisorb readily and at relatively low temperatures and pressures on the stepped platinum surfaces used in this study. In contrast to the ordered adsorption of carbon monoxide and ethylene on low index faces, the adsorption was disordered on the stepped surfaces and there is evidence for dissociation of the molecule. Carbon formed several ordered surface structures and caused faceting on the stepped surface, which are not observed on low index platinum surfaces. There appears to be a much stronger interaction of chemisorbed gases with stepped surfaces than with low index planes that must be caused by the differing atomic structures at the steps. Evidence for the differing reactivities of the two stepped surfaces are also discussed.     

Kinetics of ammonia oxidation on stepped platinum surfaces. I. Experimental results

The kinetics of ammonia oxidation with oxygen have been investigated in the 10⁻⁵ and 10⁻⁴ mbar range on Pt(5 3 3) and Pt(4 4 3). Only N₂ and NO but no N₂O were detected as reaction products. The dependence of product formation on temperature and on the partial pressures of the reactants has been studied under steady state conditions. The reactive sticking coefficients were determined under reaction conditions with s_reac of ammonia reaching nearly 0.2. The Pt(5 3 3) surface was found to be catalytically more active than Pt(4 4 3) by a factor of 2-4.     

Coupled harmonic oscillator models of carbon monoxide adsorbed on stepped,platinum surfaces

Harmonic oscillator models are used to explain recent experimental data on infrared absorption by CO molecules adsorbed on two stepped platinum surfaces. These data reveal only a lower frequency band at low coverage and only a higher frequency band at high coverage. Both bands exist over a range of intermediate coverages. The data are explained by a coupled-dipole model which includes the effects of electronic polarizability, the tilted orientation of CO molecules at step sites, and the electric field enhancement at step sites. The lower-frequency band is associated with CO molecules adsorbed on step sites and the higher-frequency band is associated with two-dimensional islands consisting of both step and terrace CO. The model explains the observed variation of frequency and intensity with coverage for CO adsorption on Pt(533) and Pt(432) surfaces. The model calculations indicate that the wavenumber for a single, linearly bonded CO molecule is about 9 cm⁻¹ higher on a terrace site than on a step site.     

A model calculation on adsorption of CO on the flat,stepped and kinked nickel surfaces

The adsorption of CO on Ni was investigated by quantum chemical calculations using the CNDO/2 tight binding method. The surfaces used as models are the (111), 4(111) × (111), 3(111) × (110) and 3(111) × (100) surfaces. The CO bond is weakened in this sequence of surfaces. The active sites for the CO bond fission are the trench regions of the step and kink structures. The Ni 3d orbitals play an important role for the weakening of the CO bond, though their contribution is small for the Ni-C bond formation.     

DFT study of CO and NO adsorption on low index and stepped surfaces of gold

Adsorption energies and vibrational frequencies of CO and NO adsorbed on gold (1 1 1), (1 0 0), (1 1 0) and (3 1 0) surfaces, as well as on adatoms on Au(1 0 0) have been calculated using density functional theory. The results clearly show that the adsorption energy of the molecules increases considerably with increasing the degree of coordinative unsaturation of the gold atoms to which the molecules bind, and thus support the view that defects, steps and kinks on the surface determine the activity of gold catalysts.     

Comparative study of thermal and photo-induced reactions of NO on particulate and flat silver surfaces

Adsorption states, thermal reactions, and photoreactions at photon energies 2.3–4.7 eV of NO dimers and monomers have been compared between 8-nm silver nanoparticles (Ag NPs) formed on an Al₂O₃/NiAl(110) substrate and flat Ag(111) surfaces, by thermal desorption (TPD) and by photodesorption using mass selected time-of flight measurements. On the Ag NPs, the (NO)₂ and NO species are bound more weakly and with broader variation of adsorption states, compared to Ag(111). For (NO)₂ excitation of the Mie plasmon of the Ag NPs with p-polarized 3.5-eV photons enhances the photodesorption cross section (PCS) of NO from (NO)₂ by a factor 15 compared to Ag(111); even off the plasmon resonance up to 3-fold PCS enhancement is obtained which we ascribe to hot electron confinement. However, since translational energy distributions of photodesorbed NO are roughly the same on Ag NPs and on Ag(111), common mechanisms of photoexcitation and photoreactions apply on both types of surfaces, and neither enhancement modifies the photoinduced dynamics. Stronger particle-induced influences are observed for the photoinduced NO monomer by changes in its properties, chemical environments, and formation/decay kinetics.Our results show that NPs can lead to considerable changes of efficiency and, under favorable cases, also of branching of photoinduced surface reactions.     

The oxidation of CO by O2, NO and N2O on polycrystalline platinum under knudsen conditions: A comparison

The oxidation of CO by O₂, NO and N₂O can be described by quite similar elementary reaction sequences which differ only in the way atomic oxygen is generated on the surface. This difference is sufficient to explain the observed experimental results.     

Desorption of O2 from a stepped platinum(113) surface during 308 nm laser irradiation

Angular and velocity distributions of desorbing O₂ during irradiation of 308 nm laser pulses were studied on a stepped Pt(1 1 3) surface. With increases in the coverage, three desorption components collimated at around 12°, 30° and 50° successively appeared when the desorption angle was changed in a plane along the step edge. The translational temperature also showed maxima at these collimation angles, and the values were slightly lower than previous results for 193 nm irradiation. Some possible desorption mechanisms are discussed.     

Theoretical studies of CO and NO on CuO and Cu2O(110) surfaces

The characteristics of CO and NO adsorption on surfaces of CuO(110) and Cu₂O(110) have been studied by using the self-consistent-charge discrete variational X_a method (SCC-DV-X_a). The calculated results show that the CO and NO molecules are perpendicularly adsorbed on cuprous ions of Cu₂O and cupric ions of CuO, respectively and with oxygen pointing upward in both cases. The order of chemisorption energy of the four adsorbed systems is: CuO-NO > Cu₂O-CO > Cu₂O-NO > CuO-CO. In all chemisorptions discussed d orbitals of Cu do play an important role.     

LEED studies on stepped W surfaces

Steps on single crystal surfaces have been shown to cause changes of various physical properties and to influence the behaviour towards chemical reactions. A proper knowledge of the step structure is required for the understanding of these phenomena. The following investigation concentrates on a detailed evaluation of the LEED patterns of various stepped tungsten surfaces. A formula is given for determining the terrace width of ordered step arrays from any diffraction order. Step height, step orientation, terrace width and the sample orientation have been deduced from the LEED patterns and the experimental errors involved are being discussed. The step height can be determined within 1% and the terrace width within 1 to 3% depending on the step density. It is concluded that the determination of the sample orientation as obtained from the LEED pattern is at least as precise as by using the Laue X-ray back reflection technique.     

Structural phase transitions on stepped surfaces

We predict a new class of structural phase transitions which occur on a microscopic length scale on stepped surfaces. These include commensurate and incommensurate “step-step” transitions. They can occur either on clean equilibrium surfaces or can be adsorbate induced. We also present a Landau-Lifshitz classification of some of these structural phase transitions. Critical exponents characterizing these transitions can be experimentally determined by LEED measurements.     

Atomistic structure of stepped surfaces

Atomistic computer simulation with embedded atom method (EAM) interatomic forces was used to study the structure of surface steps on the {111} unreconstructed surface in fcc metallic materials. The energetics and local atomic relaxation behavior of ledges parallel to the 110 direction were studied using a potential describing lattice properties of Au. The vacancy formation energies in the stepped surfaces was also studied, and it was found that the energy of formation of a vacancy in a terrace is the same as that in the perfect unstepped surface. This value is 30% lower than that of the bulk. The vacancy formation energy in the ledge is reduced by a factor of two with respect to that of the terraces. The structure of the “up ledge” (A step) is different from the “down ledge” (B step). These differences do not significantly affect the energy of the ledges, although they do affect the vacancy formation energies in sites in the second surface layer near the ledge. The implications of the results for the formation of kinks and the general structure of high index surfaces are discussed.