Theoretical study of small Mo clusters and molecular nitrogen adsorption on Mo clusters

This paper studies the small molybdenum clusters of Mo_n (n=2--8) and their adsorption of N₂ molecule by using the density functional theory (DFT) with the generalized gradient approximation. The optimized structures of Mo_n clusters show the onset of a structural transition from a close-packed structure towards a body-centred cubic structure occurred at n=7. An analysis of adsorption energies suggests that the Mo₂ is of high inertness and Mo₆ cluster is of high activity against the adsorption of N₂. Calculated results indicate that the N₂ molecule prefers end-on mode by forming a linear or quasi-linear structure Mo--N--N, and the adsorption of nitrogen on molybdenum clusters is molecular adsorption with slightly elongated N--N bond. The electron density of highest occupied molecular orbital and lowest unoccupied molecular orbital, and the partial density of states of representative cluster are also used to characterize the adsorption properties of N₂ on the sized Mo_n clusters.     

Adsorption of thiophene on Al(1 1 1) and deposition of aluminum on condensed thiophene

The adsorption of thiophene on clean Al(1 1 1) at 90 and 130 K has been studied with X-ray and ultraviolet photoelectron spectroscopy (XPS and UPS) and work function measurements. Relatively weak chemisorption compared to adsorption on transition metals is indicated by minor changes in the valence spectrum in progressing from monolayer to multilayer thiophene, a modest work function change of −0.50 eV due to saturation dosing, and return of the work function and valence spectrum to that of clean Al(1 1 1) upon annealing at 210 K. The complementary experiment in which aluminum is thermally deposited on multilayer thiophene condensed on gold at 130 K has also been performed. XPS peak area analysis shows that metal doses less than 14×10¹⁵ atoms/cm² result in penetration through the physisorbed thiophene, but higher doses lead to the gradual build up of metal throughout the organic layer. Persistence of the thiophene UPS valence features for metal doses of 50×10¹⁵ atoms/cm² is consistent with penetration and aluminum island formation. For aluminum deposition on thiophene, charge transfer from aluminum is evidenced by metal-induced low binding energy components in the C 1s and S 2p spectra at 282.6 and 162.5 eV, respectively, and a shift in the Al 2p spectrum of 0.5 eV to higher binding energy compared to metallic aluminum. UPS also indicates progression of the frontier orbital toward the Fermi level as aluminum is deposited.     

Adsorption of interacting monomers on spanning clusters of polyatomic species

The adsorption-desorption process occurring on heterogeneous surfaces is studied by considering a special case where a fractal is used as adsorbent. The fractal surface is the spanning cluster corresponding to the random deposition of objects that occupy more than one site (k-mers) on a square lattice. Such a surface is characterized according to the deposited k-mer. Then, the adsorption of repulsively interacting particles adsorbed on the fractal surface is studied by using Monte Carlo simulations. Different thermodynamic quantities (adsorption isotherms, coverage susceptibility, etc.) are calculated and explained in terms of the characteristics of the substrate. A scheme to characterize the structure of the substrate by just considering the adsorption isotherm is presented and discussed.     

Interplay of valence-shell clusters and the vibrational field in94Mo and95Mo

The properties of⁹⁴Mo and⁹⁵Mo nuclei, which are typical representatives of N=52 and N=53 nuclei, respectively, are described in the cluster-vibrational field coupling model in natural representation (without parameter fitting). The interplay of few valenceshell neutrons and vibrational degree of freedom gives rise to the coexistence of quasivibrational, quasirotational, and clustering phenomena.     

Adsorption of O on Mo(110) surface from first-principles calculation

First-principles calculations based on density functional theory (DFT) have been performed to study O adsorption in on-surface and subsurface sites. For different coverages, hollow site is found to be the most stable on-surface adsorption site. For the subsurface adsorption at the bare Mo surface, the adsorption energies are found to be higher than those at the on-surface sites, suggesting that these sites are less stable. However, the presence of preadsorbed O overlayer enhances the binding energy of subsurface adsorption, particularly for the adsorption of O at octahedral site. Further, vibrational frequencies, work-function and density of states are presented for O adsorption in on-surface sites.     

Adsorption of CO on a pseudomorphic palladium monolayer on Mo(110)

Adsorption of CO on a Pd monolayer (ML) supported on Mo(110) has been studied using low energy electron diffraction (LEED), temperature programmed desorption (TPD), and high resolution electron energy loss spectroscopy (HREELS). Three ordered CO substructures denoted as are observed with LEED. The binding energy of C0 on the 1.0 ML Pd/Mo(110) surface is reduced by 12 kcal/mol relative to the Pd(111) surface, consistent with previous results for supported palladium monolayers on other substrates. Two vibrational states of C0 are observed near 1950 and 2050 cm⁻¹, with the feature at the lower wavenumber having the smaller binding energy.     

A thermal desorption study of thiophene adsorbed on the clean and sulfided Mo(100) crystal face

The adsorption of thiophene (C₄H₄S) on the clean and sulfided Mo(100) crystal surface has been studied. A fraction of the adsorbed thiophene desorbs molecularly while the remainder decomposes upon heating, evolving H₂ and leaving carbon and sulfur deposits on the surface. The reversibly adsorbed thiophene exhibits three distinct desorption peaks at 360, 230–290 and 163–174 K, corresponding to binding energies of 22, 13–16 and 7–9 kcal/mol respectively. Sulfur on the Mo(100) surface preferentially blocks the highest energy binding state and causes an increase in the amount of thiophene bound in the low binding energy, multilayer state. The thiophene decomposition reactions yield H₂ desorption peaks in the temperature range 300–700 K. We estimate that 50–66% of the thiophene adsorbed to the clean Mo(100) decomposes. The decomposition reaction is blocked by the presence of c(2 × 2) islands of sulfur and is blocked completely at θ_s = 0.5, at which point thiophene adsorption is entirely reversible.     

Adsorption parameters of Cu and Ni on the {110} plane of W and Mo

This paper reports on the embedded-atom method (EAM, developed by Johnson) calculations of adsorption parameters for Cu and Ni on {110} unrelaxed substrates of Mo and W. The following are calculated: (i) the equilibrium height h _c of an adatom; (ii) the optimum coefficients W ₀ A _hk of a truncated Fourier representation of the adatom-substrate interaction potential; (iii) the desorption energies E _des ; and (iv) the activation energies Q of surface migration. It is shown that (a) the calculated values of E _des agree satisfactorily with available empirical data; (b) the scale factor W ₀ is approximately proportional to the bonding as measured by E _des , whereas the normalized coefficients A _hk are determined by the {110} substrate symmetry; (c) E _des and Q are respectively dominated by the embedding energy and pair potential of the EAM; and (d) W ₀ and E _des for an atom in a monolayer are only about half as much as the corresponding quantities for an isolated atom.     

Adsorption and diffusion of OH on Mo modified Pt(111) surface: First-principles theory

A first-principles study of adsorption and diffusion of OH on Pt and PtMo(111) surfaces is described. It confirms that the dissociation of water is much easier on PtMo than on pure Pt. Furthermore, we also found that OH binds most strongly at Mo atop site with adsorption energy of −3.32 eV, which is ∼1 eV stronger than binding to the pure Pt(111) surface. OH is much more localized on the PtMo alloy surface than on pure Pt. Both the stranger bond and the higher localization of OH contribute to the enhanced fuel cell performance with PtMo electrodes compared to pure Pt.     

Adsorption studies of trichloroethylene (TCE) on MgO(100)/Mo(100)

Employing ultraviolet photoelectron spectroscopy (UPS, He I), the more surface sensitive metastable impact electron spectroscopy (MIES) and temperature programmed desorption (TPD) measurements of the adsorption properties of the pollutant trichloroethylene (TCE) on thin MgO(100) films, grown on a Mo(100) single crystal, have been investigated. From TPD spectra of different coverages it is concluded that TCE interacts only weakly with MgO, which is attributed to physisorption. For increasing coverages a change from one peak to two peaks in the TPD spectra, one at higher, the second at lower temperatures with respect to the single peak is detected. Additionally, the observation of a local minimum for the work function (WF) for both MIES and UPS spectra is presented. Such a local minimum has been reported previously for the adsorption of metals with outer s valence electrons on transition metal substrates and adsorption of metals with outer s valence electrons on metal oxide films. Herein, we present the first WF minimum observed for a system of organic molecules adsorbed on an insulating surface. Two different models are discussed in order to understand the presented results.     

Determination of fine and hyperfine structure in Fe−Mo(W) dinuclear clusters

The fine and hyperfine structure of two dinuclear sulfide bridged Fe?Mo complex anions and their W homologues have been studied by magnetic susceptibility and Mössbauer measurements. It is shown that, by following a stepwise methodology, it is possible to derive from the low temperature magnetization data the value and sign of the fine structure parametersD andE/D. These parameters are further confirmed by an independent analysis of the Mössbauer data. Magnetic and electric hyperfine interaction parameters are also determined from the Mössbauer results. Both fine and hyperfine parameters point to a valence scheme, for all complexes, of Fe^II?Mo^VI(W^VI) with a varying degree of charge delocalization from the iron to the molybdenum (tungsten) site. The parameterD is negative with an orientation of itsz axis close to theV _zz axis.     

Adsorption of Nd on the Mo(1 1 0) surface

The adsorption of Nd on the Mo(1 1 0) surface has been studied by low energy electron diffraction and Auger electron spectroscopy. It has been found that at low coverages Nd adatoms form a rich amount of dilute (n × 2) commensurate structures, which can be explained as forming zigzag chains oriented along the [1 1 0] direction. Monte Carlo simulations indicate that the formation of the zigzag chain structures is initiated by the indirect lateral interaction between Nd adatoms.     

Experiments on clusters

In this contribution we discuss four different types of experiments that have been conducted at molecular beams of neutral clusters. The size of particularly stable sodium chloride clusters and their corresponding geometrical structure is inferred from intensity anomalies in mass spectra. This information is obtained either for charged or for neutral clusters depending on whether the clusters are ionized by electron impact or by multiphoton absorption. The important role of fragmentation in mass spectrometry of xenon clusters is revealed by multiphoton ionization; dissociative reactions occurring on the time scale of 10⁻⁷ s with respect to the ionizing event can be analyzed. The solvation energy of negatively charged carbon dioxide clusters as a function of cluster size is obtained from electron attachment spectra. A resonance in the ion yield close to zero eV electron energy signifies that all clusters except for the monomer feature a positive electron affinity. An analysis of the kinetic energy of fragment ions, originating from delayed dissociation of triply charged carbon dioxide clusters, reveals that the size distribution of their fission fragments is extremely symmetric.     

Adsorption of water monomer and clusters on platinum(111) terrace and related steps and kinks II. Surface diffusion

Surface diffusion of water monomer, dimer, and trimer on the (111) terrace, (221) and (322) stepped, and (763) and (854) kinked surfaces of platinum was studied by density functional theory using the PW91 approximation to the energy functional. Monomer diffusion on the terrace is facile, with an activation barrier of 0.20 eV, while dimer and trimer diffusions are restricted due to their high activation barriers of 0.43 and 0.48 eV, respectively. During monomer diffusion on the terrace the O–Pt distance increases by 0.54 Å, about 23% of the initial distance of 2.34 Å. The calculated rate of monomer diffusion hops is in good agreement with the onset temperature of diffusion measurements of Daschbach et al., J. Chem. Phys., 120 (2004) 1516. Alternative monomer diffusion pathways, in which the molecule rolls or flips, were also found. These pathways have diffusion barriers of 0.22 eV. During dimer diffusion on the terrace, the donor molecule rises 0.4 Å at the saddle point, while the acceptor rises by only 0.03 Å. Monomer diffusion up to steps and kinks, with activation barriers of 0.11–0.13 eV, facilitate chain formation on top of step edges. The energy landscape of monomer diffusion from terrace to step to kink sites is downhill with a maximum activation barrier of 0.26 eV. A model for water adsorption is presented in which monomer diffusion leads to concurrent formation of terrace clusters and population of steps/kinks, the latter consistent with the STM measurements of Morgenstern et al., Phys. Rev. Lett., 77 (1996) 703.     

The effect of the proton on electropolymerization of the thiophene

Electropolymerization of thiophene was investigated in neutral and acidic medium from the standpoint of film formation and conductivity. Results obtained from voltammetric investigations have shown that proton affects both the electropolymerization of thiophene and its conductivity. This effect was explained by theoretical calculations using molecular mechanic (MM+, MM2) and semi-empirical (AM1) methods. From these calculations, it is seen that proton is significantly important in the stabilization and growth of the polymer. Proton is added to the sulfur atom in the polymer, which has a basic character because of its lone-pair electrons. With the addition of proton, the effect of the lone-pair electrons to the delocalization existing in the ring and the contribution to the conductivity of polythiophene is removed. Thus, the conjugation in the ring is reduced to the π-system in carbon skeleton and the structure of the protonated polythiophene resembles to that of polyacetylene.     

Aquasonolysis of thiophene and its derivatives

The rate constants and products of the aquasonolysis of thiophene, tetrahydrothiophene, 2-methylthiophene, 2,5-dimethylthiophene, and 2-ethylthiophene have been investigated. The sonolysis of the selected compounds in aqueous solution follows pseudo-first-order kinetics. The aquasonolytical rate constants correlate very well with the water solubility and Henry's Law constants of thiophenes. Surprisingly, vapour pressures and heats of formation of thiophenes have little effect on their aquasonolytical rates. The pseudo-first-order rate constant for the decay of thiophene decreases as its initial concentration increases. These results indicate that the transfer process of organic solutes between cavitation bubbles and the bulk liquid can strongly affect the aquasonolysis. Furthermore, carbon disulfide, diacetylene, and dimers can be detected as common products during ultrasonic irradiations. Hence, the predominant pathway of the aquasonolysis of thiophenes is the pyrolysis during the collapse of cavities.     

Adsorption and reactions of ethanol on Mo2C/Mo(1 0 0)

The adsorption, desorption and dissociation of ethanol have been investigated by work function, thermal desorption (TPD) and high resolution electron energy loss (HREELS) spectroscopic measurements on Mo₂C/Mo(1 0 0). Adsorption of ethanol on this sample at 100 K led to a work function decrease suggesting that the adsorbed layer has a positive outward dipole moment By means of TPD we distinguished three adsorption states, condensed layer with a Tp = 162 K, chemisorbed ethanol with Tp = 346 K and irreversibly bonded species which decomposes to different compounds. These are hydrogen, acetaldehyde, methane, ethylene and CO. From the comparison of the Tp values with those obtained following their adsorption on Mo₂C it was inferred that the desorption of methane and ethylene is reaction limited, while that of hydrogen is desorption limited process. HREEL spectra obtained at 100 K indicated that at lower exposure ethanol undergoes dissociation to give ethoxy species, whereas at high exposure molecularly adsorbed ethanol also exists on the surface. Analysis of the spectral changes in HREELS observed for annealed surface assisted to ascertain the reaction pathways of the decomposition of adsorbed ethanol.     

Adsorption of nitrogen oxide molecules to the surface of nanosized nickel clusters formed on the (111) surface of a magnesium oxide film

The properties of the systems formed on deposition of Ni atoms on the (111) surface of a MgO film of thickness equal to six monomolecular layers grown on a Mo(110) crystal face and the adsorption of NO nitrogen oxide molecules to the system surface have been studied by methods of electron spectroscopy (AES, XPES, LEED, LEIBSS) and reflective infrared absorption spectroscopy. On deposition of Ni atoms on the surface of MgO at a substrate temperature of 600 K, three-dimensional islands of Ni are formed. The subsequent adsorption of NO results in molecule dissociation even at 110 K. The efficiency of this process depends on the morphology of the Ni layer.