Transition metal adatoms

Fig. 4.27 Activation energies of surface diffusion of 5-d transition metal adatoms on the W (110) surface. Squares are data obtained with a few adatoms on a plane by Bassett and circles are data with one adatom on a plane by Tsong Kellogg.

Here we discuss some representative examples in the modeling of transition metal adatoms on pristine graphene. Depending on the system, pristine graphene can be considered as a substrate or ligand for transition metals. The binding of defective graphene to transition metals is of a different nature and the topic is discussed in Section 11.3.3. 

As in the case of pristine graphene, the electronic structure of transition metal species on defective graphene may be controlled by strain. Huang and coworkers found that the strain generally affects magnetic moments of the 3d-transition metal adatoms on pristine, SV- and double vacancy (DV)-defective graphene [135]. 

Calculations showed that transition metal adatoms slide on the surface of graphene until they become bound to a defect of some kind. The unbinding is strongly thermodynamically unfavorable. It also may be kinetically unfavorable in the case of SV-sites owing to the fact that dangling bonds are formed upon unbinding. It should be noted that clustering of transition metal atoms occurs... 

The effect of electronegative additives on the adsorption of ethylene on transition metal surfaces is similar to the effect of S or C adatoms on the adsorption of other unsaturated hydrocarbons.6 For example the addition of C or S atoms on Mo(100) inhibits the complete decomposition (dehydrogenation) of butadiene and butene, which are almost completely decomposed on the clean surface.108 Steric hindrance plays the main role in certain cases, i.e the addition of the electronegative adatoms results in blocking of the sites available for hydrocarbon adsorption. The same effect has been observed for saturated hydrocarbons.108,109 Overall, however, and at least for low coverages where geometric hindrance plays a limited role, electronegative promoters stabilize the adsorption of ethylene and other unsaturated and saturated hydrocarbons on metal surfaces. 

The other mechanism involves atomic-size roughness (i.e., single adatoms or small adatom clusters), and is caused by electronic transitions between the metal and the adsorbate. One of the possible mechanisms, photoassisted metal to adsorbate charge transfer, is illustrated in Fig. 15.4. It depends on the presence of a vacant, broadened adsorbate orbital above the Fermi level of the metal (cf. Chapter 3). In this process the incident photon of frequency cjq excites an electron in the metal, which subsequently undergoes a virtual transition to the adsorbate orbital, where it excites a molecular vibration of frequency lj. When the electron returns to the Fermi level of the metal, a photon of frequency (u>o — us) is emitted. The presence of the metal adatoms enhances the metal-adsorbate interaction, and hence increases the cross... 

It is known that on transition metals, dissociation of H2 occurs readily, producing hydrogen adatoms that recombine and desorb as H2 at temperature between 300 and 900 K [9] On the other hand, a substantial activation barrier for H-H bond cleavage makes difficult the dissociation of H2 on noble-metal surfaces [9]. The effect of Pt or Pd catalysts is known to increase activity through the reduction of the activation energy [10,11]. Therefore, strategic addition of a catalyst has the potential to reduce the peak desorption temperature. 

Fig. 4 Possible adatom (xmfigurations for the coadsorption of two atomic species (e.g. C,0) on the square lattices of preferred adsorption sites on (100) surfaces of b.c.c. transition metals. The two atomic species are denoted by small open or filled circles, respectively, (a) shows the top layer of the substrate and possible adsorption sites the solid lines connect centers of the substrate atoms in this layer, (b) shows the c(2 x 2) structure with random (xxupation of the sites by the two species (c) ordered structure I (the (2x1) structure) (d) ordered structure II [ordered c(2 x 2) structure] (e) and (f) show the disordered lattice gas and lattice liquid states, respectively. (From Lee and Landau .)...

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]. 

A special case is when the electrochem-ically active components are attached to the metal or carbon (electrode) surface in the form of mono- or multilayers, for example, oxides, hydroxides, insoluble salts, metalloorganic compounds, transition-metal hexacyanides, clays, zeolites containing polyoxianions or cations, intercalative systems. The submonolayers of adatoms formed by underpotential deposition are neglected, since in this case, the peak potentials are determined by the substrate-adatom interactions (compound formation). From the ideal surface cyclic voltammetric responses, E° can also be calculated as... 

Generally, the bonding of adatoms other than hydrogen to a metal surface is highly coordination-dependent, whereas molecular adsorption tends to be much less discriminative. For the different metals the bond strength of an adatom also tends to vary much more than the chemisorption energy of a molecule. Atoms bind more strongly to surfaces than molecules do. Here we will discuss the quantum chemical basis of chemisorption to the transition metal surfaces. We will illustrate molecular chemisorption by an analysis of the chemisorption bond of CO [3] in comparison with the atomic chemisorption of a C atom. 

Let us now analyse dissociation and recombination in detail. As we saw in the previous sections dissociative adsorption is a process consisting of at least two consecutive elementary steps. Molecular adsorption proceeds dissociation. Dissociation occurs when reaction (4.4) is thermodynamically feasible. We learned in Section 4.2.1 that the surface adatom bond energy varies more than the molecular adsorption bond with variation of metal. Adsorption of atoms is favoured by metals with a partially filled d valence electron band or metals of a low work function. CO will dissociate on the first row transition metals (except... 

The dependence of the angular distribution of the adsorbate - induced photoelectron current on the initial state wave function has been recently calculated by Gadzuk (47—49). He has considered adsorption of atoms on transition metals, assuming the interaction of the adatom with the 7-type orbitals of the substrate to be responsible for the chemisorption. To avoid the difficulties associated with Penn s antiresonance effect he assumed the adsorbate level to be non-degenerate with the metal 7-band. Neglecting the variation of the optical matrix element over the width of the adsorbate level the angular distribution is in this case given by... 

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