CO chemisorption on two-dimensional cobalt clusters: A surface science approach to cluster chemistry

We have studied the CO chemisorption properties of small two-dimensional clusters (islands) and of monolayers and multilayers of cobalt deposited on Cu(100). The behaviour of one monolayer of cobalt is indistinguishable from the multilayers indicating weak influence of the copper support twoards the adsorption of CO. Below one monolayer of cobalt we observed the appearance of new peaks in the thermal desorption spectrum of CO related to the increasingly large number of edge atoms and small size of the cobalt islands. Of particular interest is the number of CO molecules bound per cobalt atom as a function of island size. This number was found to vary from 0.65, with the compression structure formed on the monolayer and multilayer cobalt coverages, up to approximately 3 when the cobalt coverage decreased to 0.002 monolayers.     

CO adsorption on neutral iridium clusters

The adsorption of carbon monoxide on neutral iridium clusters in the size range of n = 3 to 21 atoms is investigated with infrared multiple photon dissociation spectroscopy. For each cluster size only a single ν(CO) band is present with frequencies in the range between 1962 cm^-1 (n = 8) and 1985 cm^-1 (n = 18) which can be attributed to an atop binding geometry. This behaviour is compared to the CO binding geometries on clusters of other group 9 and 10 transition metals as well as to that on extended surfaces. The preference of Ir for atop binding is rationalised by relativistic effects on the electronic structure of the later 5d metals.     

Cluster studies of CO adsorption: II. CO on small Al clusters

Results for the free-electron-like metal Al (r_s = 2.07 bohr) are compared with previous Li (r_s = 3.25 bohr) results. From an analysis of the various contributions to the total adsorption energy (steric interaction, σ-bonding, π-backbonding) as a function of the CO height above the surface, and the adsorption site, it appears that high conduction electron density leads to strong exchange repulsion. At the top site this effect is partly cancelled by the favourable interaction possibilities with Al 3p functions. The most striking differences with Li are thus the very weak adsorption at the hollow site, and stronger adsorption at the top site.     

Cluster studies of CO adsorption: I. CO on small Li clusters

The various contributions to the adsorption energy of CO such as steric repulsion, σ bonding and π backbonding, are studied as a function of CO-cluster distance, C-O distance, adsorption site and cluster geometry. At the hollow site a double minimum is found in the adsorption energy as a function of CO-cluster distance. The effect of adsorption on the electronic structure of CO is large, in particular at the equilibrium distance close to the surface. Consequences for the C-O stretch frequency, and for the possibility of CO dissociation at the surface, are investigated.     

Hydrogen adsorption,absorption and diffusion on and in transition metal surfaces: A DFT study

Periodic, self-consistent DFT-GGA(PW91) calculations are used to study the interaction of hydrogen with different facets of seventeen transition metals—the (100) and (111) facets of face-centered cubic (fcc) metals, the (0001) facet of hexagonal-close packed (hcp) metals, and the (100) and (110) facets of body-centered cubic (bcc) metals. Calculated geometries and binding energies for surface and subsurface hydrogen are reported and are, in general, in good agreement with both previous modeling studies and experimental data. There are significant differences between the binding on the close-packed and more open (100) facets of the same metal. Geometries of subsurface hydrogen on different facets of the same metal are generally similar; however, binding energies of hydrogen in the subsurface of the different facets studied showed significant variation. Formation of surface hydrogen is exothermic with respect to gas-phase H₂ on all metals studied with the exception of Ag and Au. For each metal studied, hydrogen in its preferred subsurface state is always less stable than its preferred surface state. The magnitude of the activation energy for hydrogen diffusion from the surface layer into the first subsurface layer is dominated by the difference in the thermodynamic stability of these two states. Diffusion from the first subsurface layer to one layer further into the bulk does not generally have a large thermodynamic barrier but still has a moderate kinetic barrier. Despite the proximity to the metal surface, the activation energy for hydrogen diffusion from the first to the second subsurface layer is generally similar to experimentally-determined activation energies for bulk diffusion found in the literature. There are also some significant differences in the activation energy for hydrogen diffusion into the bulk through different facets of the same metal.     

The application of SIMS to the study of CO adsorption on polycrystalline metal surfaces

Secondary ion mass spectroscopy (SIMS) was used to study the adsorption of carbon monoxide on polycrystalline nickel, copper, iron, palladium and tungsten foils. The results demonstrate the ability of SIMS to distinguish, qualitatively, between molecular and dissociative adsorption. A correlation between SIMS results and those obtained by infra-red spectroscopy for molecular adsorption is also suggested.     

Model calculations of chemisorption on transition metal clusters

Alloying of a transition metal with a non-transition metal affects the nature of the chemical bonds between the transition metal atoms in the surface and hydrogen or other atoms chemisorbed on them. The resultant changes in strength of the chemisorption bonds are due to (1) changes in delocalization of the metal electrons involved in the chemisorption bonds, (2) changes in the number of electrons available for such bonds. Our calculations have shown that the effects of alloying on chemisorption are different for monocoordinated adsorbates (ad-atom on top of metal atom) and tricoordinated adsorbates (ad-atom equidistant to three transition metal atoms) respectively. With localized bonds, alloying increases the strength of the former bond, but weakens the latter. As a result the ratio of monocoordinated to tricoordinated adsorbates, present in equilibrium at high coverage, is drastically increased by this type of alloying. The increase is much larger than expected on a geometrical basis only.     

A computer study of ammonium adsorption on water clusters

The molecular dynamics method is used to study the adsorption of ammonia on water clusters. The adsorption of ammonia is accompanied by a decrease in the ability of the cluster system to absorb infrared radiation, a significant decline in the thermal radiation power emitted by the system, and an almost double decrease in the frequency-averaged reflection coefficient. An increase in the concentration of ammonia in the clusters causes a slight change in IR absorption coefficient, but enhances the power of emission and reflection coefficient.     

A new approach to bonding in transition metal clusters

The idea that the molecular orbitals of a cluster of atoms can be usefully classified according to their nodal structure, so that the energy increases with the number of nodes, is quantified and shown to apply in its usual form only to molecular orbitals constructed from atomic σ orbitals. The method is extended to deal with atomic π and δ orbitals by the use of vector and tensor surface harmonics respectively. In all cases the orbitals can be classified approximately in terms of angular momentum quantum numbers l and m, but in the π and δ cases there are two orbitals, with different parity, for each lm, and the energy is determined primarily by the parity. The method provides a classification scheme and an approximate energy ordering which does not depend on any point-group symmetry that the cluster may have, and therefore provides a useful framework for discussion of the bonding in cluster compounds such as the boron hydrides and the transition metal cluster carbonyls.     

Magnetic anisotropy of deposited transition metal clusters

We present results of magnetic torque calculations using the fully relativistic spin-polarized Korringa-Kohn-Rostoker approach applied to small Co and Fe clusters deposited on the Pt(111) surface. From the magnetic torque one can derive amongst others the magnetic anisotropy energy (MAE). It was found that this approach is numerically much more stable and also computationally less demanding than using the magnetic force theorem that allows to calculate the MAE directly. Although structural relaxation effects were not included our results correspond reasonably well to recent experimental data.     

Far-Infrared spectroscopy of isolated transition metal clusters

The vibrational far-infrared (IR) spectra of isolated metal clusters in the gas phase can be measured by performing photo dissociation spectroscopy of their rare gas complexes. For these experiments an intense and widely tunable source of far-IR radiation is required, and the Free Electron Laser for Infrared eXperiments (FELIX) is ideally suited for this. Vibrational spectra are obtained for vanadium cluster cations as well as for neutral and cationic niobium clusters. The comparison of the experimental vibrational spectra with theoretically calculated spectra allows for the determination of the structure of the metal clusters.     

First-principles study of transition metal linear monoatomic chains adsorption on boron nitride nanotube

Structural, electronic and magnetic properties of six 3d transition metals (TM=V, Cr, Mn, Fe, Co and Ni) linear monoatomic chains adsorbed on the (5,5) boron nitride nanotube (BNNT) at five different sites have been investigated by first-principle calculations. The results indicate all TM chains can be spontaneously adsorbed on the outer surface of the BNNT. The stable adsorption sites are different for different TM chains. All TM chains can be adsorbed on the N site, while the adsorption on the Z site is unstable. The dispersion character occurs in energy band curves of stable TM/BNNT systems and bring about the band gap disappearance in comparison with that of pure (5,5) BNNT. Interestingly, the TM/BNNT systems with nearly half-filled 3d metals V and Cr at H and N sites, as well as Mn at A site show a half-metal character and are usable in spintronics devices. The different electronic properties of BNNT can also be achieved through decorations of the same TM chain on different sites. The TM chain adsorbed BNNT systems exhibit high stability, promising electronic properties and high magnetic moments, which may be useful for a wide variety of next-generation nanoelectronic device components.     

Photoredox laser chemistry of transition metal oxides

Received: 26 May 1997/Accepted: 17 July 1997     

Symmetry and magnetic properties of transition metal clusters

The magnetic properties of 55-atom Fe, Co and Ni clusters are studied using the local spin-density formalism. The dependence of magnetic moment on cluster symmetry is found to be also cluster size dependent. The symmetry of cluster plays an important role in determining the charge distribution. The surface magnetism enhancement are found to be decreased from Fe to Ni.     

Magnetic anisotropies of late transition metal atomic clusters

We analyze the impact of the magnetic anisotropy on the geometric structure and magnetic ordering of small atomic clusters of palladium, iridium, platinum, and gold. We have employed a noncollinear implementation of density functional theory where the spin-orbit interaction has been included self-consistently. The size of the clusters ranges from two to five, six, or seven atoms, depending on the element. Our results highlight the relevance of the spin-orbit interaction in the magnetic properties of small atomic clusters made of fourth- and fifth-row elements.     

The surface chemistry of hydrocarbon fragments on transition metals: towards understanding catalytic processes

In this review the surface chemistry of hydrocarbons on transition metal surfaces is discussed, with focus on the contributions from the surface science community, and specifically the author's group, to an understanding of the mechanism of this chemistry at a molecular level. The clean production of alkyl surface moieties on well characterized metals and the study of the thermal chemistry of those moieties are described. Alpha, beta and gamma hydride eliminations, reductive elimination with hydrogen or other alkyl groups, methylene insertions, and C—C bond breaking reactions, are all introduced. Particular emphasis is given to the discussion of catalytic hydrogenation processes.     

Deformation of slow liquid and solid clusters upon deposition: A molecular-dynamics study of Al cluster impact on an Al surface

Using molecular-dynamics simulation, we investigate the self-deposition of Al_n clusters (n < 4000) on an Al substrate at velocities below the velocity of sound. Both cold crystalline and hot liquid clusters are studied. We examine the cluster deformation after impact on the surface, which we quantify by its height and base radius. At a given cluster velocity, the shape of deposited crystalline clusters is rather independent of the cluster size; only at small cluster sizes, n ? 40, the clusters are less strongly deformed. With increasing cluster size, liquid clusters are more strongly deformed than crystalline clusters. Faster projectiles become more strongly flattened by the deposition process. Even clusters depositing with vanishing velocity show a finite deformation, which is considerable for smaller clusters. At large cluster speed, clusters penetrate deeper into the (1 0 0) surface than into the (1 1 1) surface and also deform more strongly.     

Trends in the heat of adsorption on transition metal films

An LCAO-MO calculational procedure was used to determine the heat of chemisorption (Q) for an adatom monolayer on a transition metal film. Characteristic parameters of the metal such as Fermi energy (E_F), bandwidth, and position of the d atomic level (ε_d), parameters of the adatom such as energy level (ε_A), and interaction parameters (β) between adatom and metal were varied to determine the dependence of Q on metal position in the periodic table. Adatoms with one valence orbital (for simplicity, of the s type) and with occupation of zero (acceptor), one (radical), or two (donor) were considered. On-top and hole sites each led to similar conclusions. In accordance with model results, Q depends monotonically on $\text{β}\text{(ε}{}_{\mathrm{<mtext>A</mtext>}}\text{?}\text{E}{}_{\mathrm{<mtext>F</mtext>}}\text{)}$. As the d occupation increases within a given transition series, the Q value decreases for the radical and donor cases but increases for the acceptor. These curves are rather flat, in agreement with model results and experimental trends.