Vibrational lifetimes of molecular adsorbates on metal surfaces

We report density functional theory calculations of electron-hole pair induced vibrational lifetimes of diatomic molecules adsorbed on metal surfaces. For CO on Cu(100), Ni(100), Ni(111), Pt(100), and Pt(111), we find that the C-O internal stretch and the bending modes have lifetimes in the 1-6 ps range, and that the CO-surface stretch and the frustrated translational modes relax more slowly, with lifetimes >10 ps for all cases except CO on Ni(111). This strong mode selectivity confirms earlier calculations for CO on Cu(100) and demonstrates that the trends carry over to other metal substrates. In contrast, for NO adsorbed on Pt(111), whereas we still find that the bending mode has the shortest lifetime, about 1.3 ps, we predict the other three modes to have almost equal lifetimes of 8-10 ps. Similarly, for CN adsorbed on Pt(111), we calculate that the internal stretching and molecule-surface stretching modes have approximately equal lifetimes of about 15 ps. Our results are in reasonable agreement with experiment, where available. We discuss some of the underlying factors that may contribute to the observed mode selectivity with adsorbed CO and the altered selectivity with NO and CN.     

Carbon on transition metal surfaces

The review surveys the conditions of formation and properties of four forms of surface carbon on transition metals, to wit, adsorbed atoms and clusters, surface carbide and graphite, and their role in the physical and chemical processes on the surface. The first-order phase transition in the adlayer, when graphite islands coexist with carbon gas, are considered. The effect of intercalation, when atoms (Cs, K, Na, Ba, Pt, Si) penetrate spontaneously under the graphite islands physisorbed on the metal, and its mechanism are discussed. An analysis is made of the poisoning of platinum-group metal catalysts in the reaction of dissociation, when graphite islands characterized by extreme adsorption and catalytic passivity form in the adlayer. The method of CsCl dissociation to probe the surface carbon is treated. Attention is drawn to the adsorption of a number of atoms (Cs, K, Ba, Pt) on a graphite monolayer on metals, and the properties of such systems are discussed. The effects observed in coadsorption of CsCl molecules with K, Na, Ba, Tm atoms on a graphite monolayer on metals are covered. By analogy with the bulk carbides, surface carbides of fixed stoichiometry and very strong metal-carbon bonding have been revealed to form on the surface of transition metals (W, Re, Mo). The effect of displacement of surface carbon into the bulk of the metal stimulated by the adsorption of some atoms (Si, S, O) is discussed. The carbon clusters adsorbed on metals are considered. The transport of surface carbon, its desorption and diffusion between the surface and the bulk of the metal with a single- and double-phase adlayer are reviewed.     

Adsorption of carbon monoxide and oxygen on transition metal surfaces: A comparative study of the results from electron spectroscopy and theoretical calculations

Results of investigations on the adsorption of CO andO ₂ on transition metal surfaces by employinguv and x-ray photoelectron spectroscopy and electron energy loss spectroscopy (eels) are presented. Results of molecular orbital calculations on adsorbed CO and O₂ are also discussed. Some of the interesting aspects discussed are, satellites in the O(ls) region due to adsorbed CO, vibrationaleels of adsorbed O₂ and dissociation energy profiles of adsorbed O₂ on clean surfaces as well as surfaces covered with potassium or presorbed atomic oxygen. Contribution No 245 from the Solid State and Structural Chemistry Unit.     

Electrodeposition of noble metal nanoparticles on carbon nanotubes

Noble metal nanoparticles can be electrodeposited on carbon nanotubes under potential control. The nanotube sidewalls serve both as the electrodeposition template and as the wire electrically connecting the deposited nanoparticles.     

Polyoxometalates containing late transition and noble metal atoms

This review describes the state of the art in the field of polyoxometalates containing noble metal atoms (ruthenium, rhodium, palladium, silver, osmium, iridium, platinum and gold). The structures of the various species are listed together with their applications (mainly in catalysis).     

Activation of carbon dioxide and carbon monoxide at aluminium surfaces

Dynamic photoelectron spectroscopy has shown that the adsorption of carbon dioxide at aluminium surfaces is followed by a dissociative reaction leading to the formation of a metastable surface carbonate in the temperature range 80-120 K. The carbonate is subsequently reduced (120–475 K) (deoxygenated) to generate two different forms of surface carbon, one carbidic C^δ- (a) and the other less ionic C⁰(a) possibly graphitic. Quantification of the C(ls) and O(ls) spectra enable each of the species O^2-(a), CO₃^2-(a), C^δ-(a) and C⁰ (a) to be distinguished and their surface concentrations calculated over a wide temperature range. The temperature and pressure dependences of CO₂ reduction suggest the participation of a precursor dimer state (CO₂^---CO₂)(a) which then disproportionates. Furthermore studies of the coadsorption of ammonia and carbon dioxide in analogous systems indicate that a discrete and specifically reactive species, O^- (s), is formed during carbonate formation. The results are discussed in the context of recent theoretical studies of FREUND and MESSMER and also comparisons made with metal-CO₂ complexes.The facile surface reduction of CO₂ via a surface carbonate suggested that a possible route to carbon-oxygen bond cleavage in carbon monoxide interaction with an sp-metal surface (aluminium) was a step-wise oxidation to CO₂ leading to surface carbonate which was then readily deoxygenated. Studies of carbon monoxide: dioxygen mixtures (100: I) confirmed that this indeed occurred. A modified ELEY-RIDEAL type mechanism involving a hopping "non-adsorbed" CO molecule and a short-lived surface O^- (s) species is suggested.     

Vibrational center-ligand couplings in transition metal complexes

The mode-tracking principle [J. Chem. Phys. 2003, 118, 1634] for the direct quantum chemical calculation of preselected, characteristic molecular vibrations makes vibrational analyses of very large molecules feasible. This is demonstrated here for the [(Ph(3)PAu)(6)C](2+) complex, in which 18 phenyl groups in the ligand sphere are explicitly taken into account. We are aiming at the motion of the endohedral carbon atom, which is in an extraordinary bonding situation because it is surrounded by an octahedral core of gold atoms in this cluster. Secondary effects of the full ligand sphere on the vibrations of the [Au(6)C] core embedded in [(R(3)PAu)(6)C](2+) clusters are investigated. For this purpose, local vibrations of the octahedral core are generated, and their long-range couplings with the phosphine ligand sphere become visible in the mode-tracking iterations. The exact normal modes of these characteristic vibrations of the cluster are then obtained after convergence of the mode-tracking refinement. This protocol allows us to assess the coupling of the outer ligand sphere with the inner core of the cluster in terms of changes of the vibrational frequencies and of the collective motions of the atomic nuclei. The vibrational frequencies of the octahedral [Au(6)C] core split due to symmetry breaking in the C(1)-symmetric [(Ph(3)PAu)(6)C](2+) cluster. Our study demonstrates how effects of the periphery of a large molecule on local vibrations can be quantified. Furthermore, we predict the first set of characteristic vibrational frequencies obtained with first-principles methods for this gold cluster, whose vibrational spectra have not yet been recorded experimentally.     

Adsorption of noble gases on metal surfaces and the scanning tunneling microscope

Physisorption on metal surfaces, and the tunneling currents through the adsorbed species, are calculated using a unified formalism that presents both problems on the same footing. Our method is based on a self-consistent LCAO approach whereby the different interaction parameters defining the bonds, and the tunneling currents, are calculated using the atomic properties of the atomic species forming the interface. Green function methods and the Keldish formalism are used to calculate the different physical properties. We present results for xenon adsorbed on aluminum.     

Hydrogen-induced reconstruction of transition metal surfaces

This study compares the results of a number of recent papers on hydrogen adsorption on Rh(110), Rh(311) and Fe(211) as well as on Ni(111) and Fe(110) surfaces. It particularly deals with the structural aspect of these low energy electron diffraction (LEED) investigations and correlates them, if available, with respective thermodesorption data. Upon dissociative adsorption by a non activated process hydrogen induces local displacements of the atoms about the adsorption sites. With increasing coverage these displacements order to form a sequence of weakly reconstructed phases and gradually lift the surface layer relaxation of the formerly clean surface. Along close packed rows of metal surface atoms hydrogen atoms tend to occupy threefold coordinated adsorption sites which, in turn, arrange in single or double chains. The coverage dependent periodicity of these adlayer structure elements together with the respective shift buckling of the substrate surface generates the observed superstructures. Since not only open but also close packed surfaces show this weak (and sometimes strong) reconstruction upon hydrogen adsorption it should be generally considered in all adsorption systems.     

Entropy of adsorption of carbon monoxide on energetically heterogeneous surfaces

Standard entropies of adsorption (Δs ⁰) of CO on different materials (Cu catalysts, Au catalysts, ZnO and to TiO₂) are obtained from static adsorption microcalorimetry, adsorption isobars and temperature-programmed desorption, based on the thermodynamics of adsorption on energetically heterogeneous surfaces. Vibrational entropies of the surfaces s _vib^α are normally between the rotational and the standard translational entropy of CO in gas phase, and decrease with increasing adsorption energy, which agrees with the explanation of statistical thermodynamics. Δs ⁰ reflects both the mobility of adsorbates and the specific adsorbate-adsorbent interaction. Limits for reasonable values of the entropy of adsorption are proposed.     

Dehydrogenation of cycloalkanes over noble metal catalysts supported on active carbon

Dehydrogenation of decalin to naphthalene through tetralin and that of dicyclohexyl to biphenyl through phenylcyclohexane with Pt/C and Pd/C was investigated mainly under the liquid film state, in which the catalyst was just wet but not suspended and covered with a thin film of liquid substrate. To improve the catalytic activities, the effects of the addition of tellurium into Pd/C were investigated to reveal that the combination of tellurium, palladium and conjugated systems produced during the dehydrogenation of cycloalkames was important to the improvement of the activities.     

Reactions of carbon monoxide with transition metal-carbon bonds

Summary Insertions of carbon monoxide into transition metal-carbon bonds are among the most selective methods for the catalytic introduction of oxygen into organic compounds. A variety of processes are known today and most of them are of great industrial importance. Depending on the nature of the organic substrate, the catalyst and additional reaction components products range from aldehydes, alcohols, and acids to esters, anhydrides, acid chlorides, amides etc. An attempt is made here to show that all these reactions proceed by similar mechanisms.From the book Coordination and Catalysis by G. Henrici-Olive and S. Olive, to be published shortly by Verlag Chemie.     

Determination of carbon monoxide in metal carbonyl complexes

Carbon monoxide is liberated from carbonyl-containing metal complexes by treatment with various reagents such as pyridinium hydrobromide perbromide in dichloromethane, hydrogen peroxide in acetone, or iodine in pyridine, and oxidized to carbon dioxide by the Schütze iodine pentoxide reagent. The carbon dioxide is determined gravimetrically following absorption on soda asbestos.     

Catalytic reduction of dinitrogen monoxide by carbon monoxide over various metal oxides

A volcano-shaped relationship was obtained between the temperature Tc, where a given conversion (per unit surface area of the catalyst) attained, and the heat of formation, Q, of metal oxide, suggesting that the mechanism of the reaction depended upon Q-value.

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T_c, ( ), , Q, , Q.

     

Synthesis of a first-row transition metal parent amido complex and carbon monoxide insertion into the amide N-H bond

The parent amido iron complex (dmpe)2Fe(H)(NH2) (dmpe = 1,2-bis(dimethylphosphino)ethane), the first such first-row transition metal complex, has been synthesized and characterized spectroscopically and crystallographically. This complex has been found to insert carbon monoxide into the amide N-H bond (rather than the M-N bond) to give trans-(dmpe)2Fe(H)(NHCHO). The mechanism of this transformation has been studied and is believed to occur through an unusual mechanism in which CO behaves as an apparent electrophile.     

Structure and bonding of ethoxy species adsorbed on transition metal surfaces

The interaction of the ethoxy radical with Cu(111), Ag(111), Pd(111) and Au(111) has been studied using a periodic density functional approach. The most stable adsorption site is the fcc with adsorption energies in the 1.1–2.2 eV interval. All analyses consistently indicate that ethoxy becomes negatively charged, that the presence of ethoxy slightly perturbs the electronic structure of the metallic surface, that the interaction is essentially of electrostatic character and not directional predicting a rather mobile species. The calculated adsorption energies are found to correlate almost linearly with the total net charge on the ethoxy moiety thus confirming that the electrostatic interactions dominate the bonding between this organic species and the underlying metallic surfaces.     

Self-consistent meta-generalized gradient approximation study of adsorption of aromatic molecules on noble metal surfaces

The adsorption of benzene, pyridine, and two nucleobases on the Au(111) surface has been investigated using a fully relaxed, self-consistent meta-generalized gradient approximation (meta-GGA) density functional theory setup with the M06-L functional. The meta-GGA based molecule-surface separations are shortened and the adsorption bond strengths of the molecules are greatly improved over the virtually non-interacting results obtained when using a plain GGA exchange-correlation functional. The nucleobases containing oxygen atoms show higher corrugation with adsorption site and orientation than the other aromatic molecules considered. The adsorption of pentacene is studied on Au, Ag, and Cu surfaces. In agreement with experiment, the adsorption energies are found to increase with decreasing nobleness, but the dependency is underestimated. We point out how the kinetic energy density can discriminate between covalent and non-covalent bonding regions of orbital overlap.