Transition adatoms

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. 

This is illustrated in Figure 1.6 for the dissociation of CO [3]. As a consequence of the high value of a, the proportionality constant of recombination is usually approximately 0.2, reflecting a weakening of the adatom surface bonds in transition state by this small amount. It implies that typically one of the six surface bonds is broken in the transition state compared to the adsorption state of the two atoms before recombination. 

Typical surfaces observed in Ising model simulations are illustrated in Fig. 2. The size and extent of adatom and vacancy clusters increases with the temperature. Above a transition temperature (T. 62 for the surface illustrated), the clusters percolate. That is, some of the clusters link up to produce a connected network over the entire surface. Above Tj, crystal growth can proceed without two-dimensional nucleation, since large clusters are an inherent part of the interface structure. Finite growth rates are expected at arbitrarily small values of the supersaturation. 

Note that the results obtained are in accordance with the lability principle. The smaller U is, the more labile are the electrons in the adatom and the stronger is the distortion of the shape of the free energy surfaces, leading to a decrease of the activation free energy and to an increase of the transition probability. 

The modulation of the charge of the adsorbed atom by the vibrations of heavy particles leads to a number of additional effects. In particular, it changes the electron and vibrational wave functions and the electrostatic energy of the adatom. These effects may also influence the transition probability and its dependence on the electrode potential. 

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 .)...

Experimental data of Gibson and Sibener appears to confirm qualitatively these predictions at least for monolayers. The phonon linewidths were broadened around T up to half of the Brillouin zone. The hybridization splitting could not be resolved, but an increase of the inelastic transition probability centered around the crossing with the Rayleigh wave and extending up to 3/4 of the zone has been observed and attributed to a resonance between the adatom and substrate modes. 

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]. 

Figure 6.10 Illustration of a Cu adatom hopping from a hollow site to an adjacent hollow site on Cu(100). The diagram on the left shows the adatom (grey) before the hop, the middle shows the transition state (TS), and the diagram on the right shows the adatom after the hop.

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... 

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.

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