On surface phonons and reconstruction: Comparison of the W and the Fe (001) surface

A phenomenological surface phonon model of the (001) surface of W and Fe is used to clarify some questions related to the problem of which high-symmetry surface phonon mode is the most perspective candidate for the tungsten (001) surface reconstruction observed experimentally. Conditions are derived for the appearance and softening of the relevant modes. The ¯M ₁,¯M ₅ and ¯X ₃ modes seem to be the most promising. A few points from the current literature on the reconstruction problem are discussed. The results seem to support the idea about the general instability of the tungsten (001) surface.     

A theoretical study of photoelectron spectra of NbO2, MoO2 and RuO2 by cluster models

DV-Xα calculations have been applied to various small clusters of rutile-family dioxides (NbO₂, MoO₂ RuO₂). It appears that by taking into consideration the potential due to the atomic charges, the density, the ionization cross sections of the energy levels, and by summing the density of states (DOS) of the two different clusters representing surface structures, computations on even small clusters provide information which compares well with the experimental XPS spectra.     

Insights into the structures,stabilities, electronic and magnetic properties of X2Aun (X = La,Y, and Sc; n = 1–9) clusters: comparison with pure gold clusters

A systematic study of the X₂Au_n (X = La, Y, Sc; n = 1–9) clusters are performed by using the density functional theory at TPSS level. The structures, stabilities, electronic, and magnetic properties are investigated in comparison with pure gold clusters. The results show that the transition points of the doped clusters from two-dimensional to three-dimensional structure are obviously earlier than gold clusters. The impurity X atoms tend to occupy the most highly coordinated position and form the largest probable number of bonds with gold atoms. In addition, the impurity atoms can strongly enhance the stabilities of gold clusters. It indicates that the impurity atoms dramatically affect the geometries and stabilities of the Au_n clusters. The highest occupied molecular orbital–lowest occupied molecular orbital gap, vertical ionisation potential, and chemical hardness show that the X₂Au₆ clusters have higher stabilities than the others. In La₂Au_1–9, Y₂Au_1–7, and Sc₂Au_1–4 clusters, the charges transfer from X atoms to the Au_n frames. The total magnetic moments of X₂Au_n clusters exist distinctly odd–even alternation behaviours except for La₂Au₄ and Sc₂Au₄ clusters.     

Basis set limit binding energies of hydrogen bonded clusters

The utility of the recently developed extrapolation method to estimate the binding energies of weakly bound clusters at the basis set limit exploiting the similar basis set convergence behaviour of correlation energies of the monomer and cluster in correlated calculations (J. chem. Phys., 116, 5389 (2002)) was tested for small to medium (HF)_n and (H₂O)_n (n = 2–5) clusters using various correlation consistent cc-pVXZ (X= D,T,Q,5) basis sets containing different numbers of diffuse functions and 6–31G type basis sets at the MP2 and CCSD(T) level for which accurate basis set limits are available for comparison. It is shown that the basis set limit binding energies estimated by this extrapolation method with modest size of basis sets (cc-pVDZ/cc-pVTZ or 6–3 1 G(d,p)/6-3 IG(2df,2pd)) are much closer to the exact basis set limits than the estimates by commonly used X ^?3 extrapolation or counterpoise corrected binding energies, signifying the importance of this extrapolation method for the study of large weakly bound clusters. It is also shown that the inclusion of appropriate diffuse functions in the basis sets can significantly improve the accuracy of the estimated basis set limits by this extrapolation method. For (HF)_n clusters the MP2 and CCSD(T) basis set limits estimated by this extrapolation method with aug-cc-pVDZ and aug-cc-pVTZ basis sets are 18.4 (18.5) and 18.9 (18.9) for the dimer, 61.8 (62) and 63.2 (63) for the trimer, 113.5 and 114.7 (116) for the tetramer, and 155.2 and 156.3 (158) for the pentamer, respectively, with the values in parentheses representing the apparent basis set limits, with the numbers in units of kJ mol^?1. The corresponding results for (H₂O)_n clusters are 20.5 (20.5) and 20.6 (20.7) for (H₂O)₂, and 60.5 (61) and 60.1 (60) for (H₂O)₃, respectively.     

Calculations with the CNDO method on clusters of silver atoms

An attempt has been made to explore the applicability of the complete neglect of differential overlap (CNDO) method for investigating clusters of silver atoms. A new parametrization for silver has been obtained by comparing charge distributions, as well as local and total density of states, from CNDO calculations with those from the X_α scattered wave (X_αSW) method for an Ag₇ cluster which represents a fragment of the silver lattice. These parameters have then been used for making CNDO calculations on four further clusters of the same type, namely Ag₆, Ag₁₀, Ag₁₃ and Ag₁₉, and the results are compared with previous X_αSW calculations. These CNDO calculations give d-band widths in broad agreement with those from the X_αSW method. The most significant difference is that the CNDO method gives less localization on central atoms with high coordination numbers than is found from the X_αSW calculations. It is suggested that this apparent deficiency of the CNDO calculations may be less serious when the clusters are being used for modelling part of a solid metal rather than for specifically investigating the properties of small particles.     

Structures,energetics and vibrational spectra of (H2O)32 clusters: a journey from model potentials to correlated theory

Empirical model potentials are found to be very useful for generating most competitive minima of large water clusters, whereas correlated (e.g. second order-Møller–Plesset perturbation (MP2) theory or higher) calculations are necessary for predicting their accurate energetics and vibrational features. The present study reports the structures and energetics of (H₂O)₃₂ clusters at MP2 level using aug-cc-pvDZ basis set, starting with low-lying structures generated from model potentials. Such high-end and accurate calculations are made feasible by the cost-effective fragment-based molecular tailoring approach (MTA) in conjunction with the grafting procedure. The latter is found to yield electronic energies with a sub-millihartree accuracy with reference to their full calculation counterparts. The vibrational spectra of nine low-lying (H₂O)₃₂ isomers are obtained from the corresponding MTA-based Hessian matrix. All these low-lying isomers show almost similar spectral features, which are in fair agreement with the experiment. The experimental spectrum of (H₂O)₃₂ is thus better understood from the vibrational features of this set of very closely spaced isomers. The present case study of (H₂O)₃₂ clearly demonstrates the efficacy in obtaining accurate structures, energetics and spectra at correlated level of theory by combining model potential-based structures with fragmentation methods.     

Interaction between two hydrogen atoms approaching a metal surface

Two regimes of HH interaction outside a metal surface are considered: (i) Beyond the spill-out region of the metal electrons (physisorption) (ii) Embedded protons in the electron spill-out. Using an extension of the Heitier-London model to treat (i), the H₂ binding energy for the molecule parallel to the surface is reduced, whereas for the perpendicular configuration stronger binding obtains. In (ii), the asymptotic form of the screening charge round a single proton is considered, in a linear response framework, and for an infinite barrier model of the surface. If the screened potential has no singularities in k space, then the interaction energy ΔE(X), X being the distance between the protons, falls off as X^?5 times an oscillatory function, for H₂ parallel to the planar metal surface. The effect of self-consistency on this result is then examined and it is concluded that the asymptotic interaction energy is unchanged in form, though the amplitude is altered.     

Density function study transition metal chromium-doped alkali clusters: the finding of magnetic superatom

The structures, stabilities and magnetic properties of CrX_n (X = Na, Rb and Cs; n up to 9) clusters are studied using density functional theory to search for the stable magnetic superatoms. The geometrical optimisations indicate the ground-state structures of CrX_n evolve toward a close packed structure with an interior Cr atom surrounded by X atoms as the cluster size increase. Their stabilities are analysed by the relative energy, gain in energy (ΔE(n)) and the highest unoccupied molecular orbital and lowest unoccupied molecular orbital gaps. Furthermore, the magnetic moments of CrX_n clusters show an odd–even oscillation. Here, we mainly focus on the CrX₇ (X = Na, Rb and Cs) clusters due to the same valence count as the known stable magnetic superatoms VNa₈, VCs₈ and TiNa₉. Although these clusters all have a filled electronic configuration 1S²1P⁶ and large magnetic moment 5 μ_B, our studies indicate that only CrNa₇ is highly stable compared to its nearest neighbours, while CrRb₇ and CrCs₇ clusters are less stable. This suggests that Cr-doped Na₇ is most appropriate for filled electronic configuration and CrNa₇ is shown to be a stable magnetic superatom. More interesting, we find CrRb₈ and CrCs₈ with the filled electronic configuration 1S²1P⁶ have higher stability and large magnetic moment 6 μ_B in their respective series.     

Synergistic effects in toluene/C3H4 isomers co-pyrolysis: Formation of indene and naphthalene

A combination of experimental and kinetic modeling study is performed to explore synergistic effects between toluene and C₃H₄ isomers on the formation of polycyclic aromatic hydrocarbons (PAHs) and pyrolysis reactivity. Co-pyrolysis of toluene-allene and toluene-propyne is investigated in a flow reactor employing synchrotron vacuum ultraviolet photoionization mass spectrometry (SVUV-PIMS) at 0.04 bar and 1 bar. Mole fraction profiles of fuels and intermediates up to two-ring PAHs are obtained. A kinetic model for co-pyrolysis of toluene-C₃H₄ isomers is established and examined against the present data. Sampled mass-specific photoionization efficiency (PIE) curves are employed to identify the presence of aliphatic aromatic species, favoring specific perception into interactions between phenyl/benzyl radicals and C₃ species. The synergistic effects observed in this work are not sensitive to the molecular structure of allene and propyne but quite sensitive to the experimental pressures. The reason being that the interactions between phenyl/benzyl radicals and small molecules like CH₃, C₂H₂ and C₃H₃ are pressure dependent. Both experimental and simulation results indicate the essential role of the aliphatically substituted aromatic in the growth reactions. Indene and naphthalene are identified as the predominant C₉H₈ and C₁₀H₈ products respectively, in all cases studied. Channels leading to the formation of indene and naphthalene vary with pressure, according to rate-of-production (ROP) analyses. The phenyl + C₃H₄/C₃H₃ channel and benzyl + C₂H₂ channel make comparable contributions to the formation of indene at 0.04 bar, while the latter channel dominates the formation of indene at 1 bar. Both C₇H₅ + C₃H₃ channel and benzyl + C₃H₃ channel can lead to the formation of naphthalene at 0.04 bar, while the latter channel is more competitive at 1 bar.     

Study of morphology and composition of Pt-black catalysts IV. SIMS study of Pt-black

Various Pt-black samples were studied by SIMS. Yields of light ions (such as H, C, O, Si or CO), Pt and Pt-containing clusters are reported for samples of various pretreatment. Hydrogen content and depth profile of all Pt-black provides direct experimental evidence for the presence of hydrogen on the surface and in subsurface layers of Pt. Apart from clusters containing alkali metal ions (PtNa, PtK) having extremely high yields, PtC, PtO, PtC₂H, etc., clusters indicate that, in agreement with previous results, the surface of Pt-break samples is largely covered by partly oxidized, hydrogen containing carbonaceous deposits. The appearance of PtC₂H⁺₃ clusters in ethylene treated platinum has been tentatively identified with ethylidyne reported recently on single crystal surfaces. Present and previous results permitted us to propose a model for the atomic structure of surface layers of disperse Pt-black.     

Effects of buried high-Z layers on fast electron propagation

Density functional study is performed on the stabilities, aromaticity, infrared spectra, and optical properties of exohedral fullerene derivatives C₇₆X₁₈(X = H, F, Cl, and Br). The bond dissociation energy and energy gap between HOMO and LUMO of C₇₆H₁₈ are larger than those of the isolated C₇₆F₁₈ and C₇₆Cl₁₈, indicating the possibility for synthesising C₇₆H₁₈ from the viewpoint of thermodynamics and kinetics. C₇₆X₁₈(X = H, F, Cl, and Br) show strong aromaticity, suggesting their stabilities are correlative with the conjugation. The tensors of static linear polarisabilities, mean static linear polarisabilities, polarisability anisotropy, and first-order hyperpolarisabilities of C₇₆X₁₈(X = F, Cl, and Br) increase as X goes from F to Br. We rationalise the nonlinear properties by studying the low-energy optical absorption band obtained by employing time-dependent density functional theory.     

Reactions of small gold cluster cations with propylene, methane, and hydrogen: permissive and competitive coadsorption effects

Coadsorption effects of molecular hydrogen and small hydrocarbons, CH₄ and C₃H₆, on free Au₃ ⁺ and Au₅ ⁺ were investigated in an octopole ion trap under multi-collision conditions. For hydrogen and methane the observations indicate that both molecules coadsorb on the same adsorption site, i.e., the same atom of the cluster. This type of molecular adsorption on free clusters is termed permissive coadsorption, in contrast to competitive coadsorption, in which two molecules compete for the same adsorption site. The latter case was observed for hydrogen and propylene: already trace amounts of propylene were able to completely saturate the clusters preventing the coadsorption of H₂. The size dependent adsorbate coverage is discussed and implications on the cluster structure are deduced from time and temperature dependent reaction measurements.     

On the interaction of halogen atoms with (111) and (100) surfaces of silicon

The chemisorption of F, Cl, Br, and I atoms on the (111) and (100) surfaces of silicon has been studied by the MNDO method and using clusters of Si atoms to simulate the substrate. In the case of the adducts F-Si₄H₉ and Cl-Si₄H₉ the MNDO results are in close agreement with previous ab-initio ones concerning both the equilibrium distances and the chemisorption energies. For all the cases considered, the binding energy decreases in the order F>Cl>Br>I. The most stabel adduct is always obtained upon chemisorption at the bridge position on the (100) surface. Chemisorption at on-top positions leads to slightly less stabel adducts and is nearly isoenergetic on (100) and (111) surfaces. All the results are essentially insensitive to the dimensions of the clusters used to simulate the substrate.     

Nonempirical quantum-chemical calculation of the electric field gradient at the <Superscript>51</Superscript>V nucleus sites in alkali metavanadates

The electric-field gradient tensor at the vanadium nucleus site was calculated ab initio within a cluster model for chained vanadates XVO₃ (X=Li, Na, K). A comparison with experiment showed that it suffices to consider only small (VO₄)^3? and (V₃O₁₀)^5? clusters in crystals of this type. The calculation scheme stability with respect to increasing cluster size was analyzed.     

Orientation dependence of surface enhanced raman intensities: results from abinitio calculations

Time dependent Hartree—Fock methods have been used to calculate Raman intensifies for H₂ adsorbed onto a model lithium cluster as a function of the orientation of H₂ relative to the cluster surface. The intensity is found to be largest for perpendicular adsorption, dropping to a small value at an intermediate angle, and then rising to a second but smaller maximum for parallel adsorption. The results are interpreted using a model which correlates enhancement to lithium cluster orbital energy shifts induced by the static quadrupole field Of H₂.     

The growth of ice clusters on the Si(100)(2 × 1)-H(D) surface: Electron energy loss spectroscopy and thermal desorption studies

The growth of ice clusters on the Si(100)(2 × 1)-H surface has been investigated mainly by the use of high-resolution electron energy loss spectroscopy and thermal desorption spectroscopy. At 90 K, H₂O molecules are adsorbed on the (2 × 1)-H surface to form ice clusters by the hydrogen-bonding interaction. Four H₂O peaks are observed at 165, 185, 215 and 270 K in the thermal desorption spectra for the ice-covered surface. The peaks at 165 and 185 K correspond to the ice clusters and the peaks at 215 and 270 K to the strongly-bound H₂O species which play a role as the nucleation centers of the ice clusters desorbing as H₂O at 185 and 165 K, respectively.     

Theoretical vibrational frequencies for NHX and CHX reactive intermediates on nickel(100) and nickel(111) surfaces

Theoretical studies have been performed on CH_X and NH_X species chemisorbed on threefold and fourfold sites of nickel clusters. From this work we have obtained geometries and vibrational frequencies that can be compared with experimental data concerning these species chemisorbed on Ni(100) and Ni(111) surfaces. These results can be used to help assign the observed losses in EELS spectra for CH on the Ni(111) surface.     

Chemisorption of hydrogen on titanium: Embedding theory and comparisons with small clusters

The chemisorption of H₂ on Ti(0001) is treated using an ab initio CI theory for the surface region. Dissociation of H₂ occurs above the surface but more stable 3-fold coordination sites lie closer to the surface at ～ 1.3 Å. Adsorption in adjacent 3-fold sites is less stable than in separated sites sharing only one surface atom. The calculated adsorption energy of 45 kcal/mol H₂ compares favorably with experiment. Bonding involves mainly the 4s electrons of the metal leading to hydridic hydrogens and a polarized lattice electron distribution, but d bonding and correlation effects significantly increase the binding energy. Calculations on small metal clusters also show dissociative adsorption but much larger hydrogen binding energies are obtained.     

Hydrogen adsorption and desorption on the Pt and Pd subnano clusters — a review

In this review, we present our recent first principles studies on the sequential H₂ dissociative chemisorption and H desorption on the Pt _n and Pd _n clusters (n=2–9, 13). Upon full saturation by H atoms, the calculated H2 dissociative chemisorption energy and H desorption energy on Pt₁₃ and Pd₁₃ clusters are similar to the corresponding values on smaller close-packed clusters. Indeed, the catalytic performances of these subnano clusters do not vary significantly with the particle sizes or shapes. Instead, they are dependent on the surface metal atoms which can be accessed by H atoms. In addition to the coverage dependency of the H₂ dissociative chemisorption and H sequential desorption energies, the phase transition of both Pt₁₃ and Pd₁₃ from the icosahedral to fcc-like structures at certain H coverage was also investigated.      

Adsorption of thiophene on silica-supported Mo clusters

The adsorption/decomposition kinetics/dynamics of thiophene has been studied on silica-supported Mo and MoS_x clusters. Two-dimensional cluster formation at small Mo exposures and three-dimensional cluster growth at larger exposures would be consistent with the Auger electron spectroscopy (AES) data. Thermal desorption spectroscopy (TDS) indicates two reaction pathways. H₄C₄S desorbs molecularly at 190–400 K. Two TDS features were evident and could be assigned to molecularly on Mo sites, and S sites adsorbed thiophene. Assuming a standard preexponential factor (ν = 1 × 10¹³/s) for first-order kinetics, the binding energies for adsorption on Mo (sulfur) sites amount to 90 (65) kJ/mol for 0.4 ML Mo exposure and 76 (63) kJ/mol for 2 ML Mo. Thus, smaller clusters are more reactive than larger clusters for molecular adsorption of H₄C₄S. The second reaction pathway, the decomposition of thiophene, starts at 250 K. Utilizing multimass TDS, H₂, H₂S, and mostly alkynes are detected in the gas phase as decomposition products. H₄C₄S bond activation results in partially sulfided Mo clusters as well as S and C residuals on the surface. S and C poison the catalyst. As a result, with an increasing number of H₄C₄S adsorption/desorption cycles, the uptake of molecular thiophene decreases as well as the H₂ and H₂S production ceases. Thus, silica-supported sulfided Mo clusters are less reactive than metallic clusters. The poisoned catalyst can be partially reactivated by annealing in O₂. However, Mo oxides also appear to form, which passivate the catalyst further. On the other hand, while annealing a used catalyst in H/H₂, it is poisoned even more (i.e., the S AES signal increases). By means of adsorption transients, the initial adsorption probability, S₀, of C₄H₄S has been determined. At thermal impact energies (E_i = 0.04 eV), S₀ for molecular adsorption amounts to 0.43 ± 0.03 for a surface temperature of 200 K. S₀ increases with Mo cluster size, obeying the capture zone model. The temperature dependence of S₀(T_s) consists of two regions consistent with molecular adsorption of thiophene at low temperatures and its decomposition above 250 K. Fitting S₀(T_s) curves allows one to determine the bond activation energy for the first elementary decomposition step of C₄H₄S, which amounts to (79 ± 2) kJ/mol and (52 ± 4) kJ/mol for small and large Mo clusters, respectively. Thus, larger clusters are more active for decomposing C₄H₄S than are smaller clusters.