Surfaces of transition metals

The performance of VASP for alloys and compounds has been illustrated at three examples The calculation of the properties of cobalt dislicide demonstrates that even for a transition-metal compound perfect agreement with all-electron calculations may be achieved at much lower computational effort, and that elastic and dynamic properties may be predicted accurately even for metallic systems with rather long-range interactions. Applications to surface-problems have been described at the example of the. 3C-SiC(100) surface. Surface physics and catalysis will be a. particularly important field for the application of VASP, recent work extends to processes as complex as the adsorption of thiopene molecules on the surface of transition-metal sulfides[55]. Finally, the efficiciency of VASP for studying complex melts has been illustrate for crystalline and molten Zintl-phases of alkali-group V alloys. 

A. A. Davydov, Infrared spectroscopy of adsorbed species on the surfaces of transition metal oxides, Wiley, Chichester, 1990. 

The possibility for electropolymerization on the top surface of Prussian blue films was probably first shown in [126] describing the high oxidizing ability of Berlin green, the fully oxidized form of Prussian blue. Afterwards non-conducting polymers were synthesized on the top surface of transition metal hexacyanoferrate-modified electrodes for immobilization of the enzyme [127],... 

Decomposition of methane to H2 and carbon over Ni/Si02 was carried out in a membrane reactor (membrane 90Pd-10Ag) [106]. The use of the membrane reactor allowed increasing the H2 yield by shifting the reaction equilibrium toward the products. An excellent review of the literature data on nonoxidative methane activation over the surface of transition metals was recently published by Choudhary et al. [107]. 

A.A. Davydov and C.H. Rochester, Infrared Spectroscopy of Adsorbed Species on the Surface of Transition Metal Oxides, Wiley, New York, 1990. 

Surfaces of transition metals that have been passivated by electrochemical oxidation or by the action of chemical oxidizing agents have been extensively investigated by in situ STM/AFM these investigations... 

Using perturbation theory. Hammer and Nprskov developed a model for predicting molecular adsorption trends on the surfaces of transition metals (HN model). They used density functional theory (DFT) to show that molecular chemisorption energies could be predicted solely by considering interactions of a molecule s HOMO and LUMO with the center of the total d-band density of states (DOS) of the metal.In particular. 

By applying this technique, it is not only possible to prepare relatively well-defined catalysts that may be alloys of a given composition but also catalysts in which adatoms of main group elements may be located on the surface of transition metal particles or organometallic fragments that are likely adsorbed (coordinated) at some particular crystallographic positions of the metallic particles. Each of these three different types of materials exhibits interesting and unusual selec-tivities in many catalytic reactions [33, 34]. 

Aromatic radical cation formation on the intracrysta 1s surfaces of transition metal layers lattice silicates, J. Phys. Chera., 78 (1974) 994-999. 

The possibility of hydrogen activation on the surface of transition metal carbides has been supported by experiments in which WC was used as a promoter for the hydrogen oxidation reaction over the oxide catalyst V205.1 WC additions to V2O5 were shown to critically accelerate the reaction of H2 + 02 (so that under the same conditions the activities of V205 and WC were separately much lower). Moreover, WC additions to V205 boosted the reduction of vanadium oxide by hydrogen. Qualitatively,... 

In summary, it has been demonstrated that surface morphology is critically important in determining the performance of solar cells with layered compound semiconductors. Steps on structured surfaces of transition metal dichalcogenides have been identified as carrier recombination sites. The region defined by the depth of the space charge layer parallel to the van der Waals planes can be considered as essentially "dead" in the sense that its photoresponse is negligible. As the "step model" predicts, marked improvement in solar cell performance is found on samples with smooth surfaces. 

Oxides are always present on the surface of transition metals in alkaline solution. At open circuit they are intermediates in the mechanism of corrosion. The resistance of Ni towards corrosion in base is better than Fe or mild steel, especially at high caustic concentration and high temperature [23, 24]. The role of surface oxides in the cathodic range of potentials depends on the conditions of their formation. Thus, a reducible layer of hydroxide Ni(OH)2 or even oxohydroxide NiOOH has been found [385] to be beneficial for the electrocatalytic activity. It has even been claimed [386] that some good performances are specifically due to the formation of oxide layers during the preparation (Fig. 19). An activation of the Ni surface by the application of anodic current pulses has been reported [387] to be beneficial owing to the formation of Ni(OH)2 layers. This has been confirmed by impedance studies of the mechanism [388]. 

Pinnavaia, T. J., Hall, P. L., Cady, S. S., and Mortland, M. M. (1974). Aromatic radical cation formation on the intracrystal surfaces of transition metal layer silicates. /. Phys. Chem. 78, 994-999. 

Important oxidation processes involving the metal centers can occur at the surfaces of transition metal oxides (e.g, -Cr203) upon dissociative oxygen adsorption (Scheme 3). In some cases the (nondissociative) adsorption of oxygen can lead to the formation of superoxide ( )2 or peroxide 02 species with simultaneous oxidation of surface metal cation centers. 

It is clear that much work remains to be done to extend our understanding to polax surfaces of transition metal oxides in which the cations have partially filled d orbitals. An especially challenging issue is related to mixed valence metal oxides, such as Fe304, in which the cations exist under two oxidation states. In addition, considering the rapid development of ultra-thin film synthesis and characterization, a simultaneous effort should be performed on the theoretical side to settle the conditions of stability of polar films. More generally, on the experimental side, it seems that one of the present bottlenecks is in a quantitative determination of the surface stoichiometry, an information of prominent interest to interpret the presence or absence of reconstruction. 

Fig. 6. Surface segregation energies of transition metal impurities (solute) for the closed-packed surfaces of transition metal hosts.

Vines F, Sousa C, Liu P, Rodriguez JA, lllas F (2005) A systematic density functiona theory study of the electronic structure of bulk and (001) surface of transition-metals carbides. JChem Phys 122 174709... 

We shall here discuss studies in which carbon is first deposited on the surface of transition metal catalysts after subsequent removal of coproducts of the deposition the carbon-covered surface is exposed to hydrogen. 

The bulk crystal form of the alkaline earth oxides is the NaCl structure. The bulk termination at the (100) surface consists of a stack of coplanar bilayers each containing one cation and an anion. CaO(lOO) has been the subject of a LEED study which found a contraction of top interlayer spacing by —1.2% (Prutton et al., 1979). Buckling of the top bilayer was not investigated. Buckling of the top bilayer was found in a recent LEED study of MgO(lOO) (Blanchard et al. 1990). In this case, the oxygen atom moves out of the surface by 0.05 0.025 A whilst the Mg atom sinks into the surface the same distance. The center-of-mass plane of the top MgO bilayer is unrelaxed. The (100) surface of transition metal oxide CoO, which also has... 

Figure 3.21. (a) Heats of adsorption of CO on polycrystalline transition metal surfaces, (b) Heats of adsorption of CO on various single crystal surfaces of transition metals. 

POTENTIAL ENERGY SURFACES OF TRANSITION-METAL-CATALYZED CHEMICAL... 

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