Adsorption on Transition Metals

Studies of oxygen adsorption on other transition metals have been reported [40—43], Table 5.1 summarizes the calculated molecular oxygen adsorption properties. 

Z distance to the surface. D distance between oxygen atoms. Magnetic moment. 

Study of intermediates and reaction paths for the ORR on another transition metal, gold, has been conducted as well [43]. In this theoretical investigation, 

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The stability of the adsorption of a hydrocarbon on the active site prior to C-H bond dissociation apparently strongly depends on the nature of the site and has not yet been unequivocally established. None of the studies on oxidative coupling of methane on non-transition metal oxides reported a stable methane preadsorption.34 Transition metal oxides, however, may reveal different behaviour and there exists already theoretical evidence for stable methane adsorption on transition metal atoms and complexes.35 Also the stability of the so-called encounter complexes has been qualitatively predicted to increase the reactivity of transition metal MO species... 

A very detailed comparison of CO adsorption on transition metals in UHV has been given in [21]. The large amount of data on well-ordered metal surfaces enables the comparison of surfaces having identical structures. 

Structural parameters for metal adsorption on transition metals involving the formation of monolayer alloys. The given bond length is the shortest distance between the substitutional atom and the host atom, da is the spacing between the first and second layers of the substituted surface, b is the amplitude of the buckling in the mixed top layer. 

Formal chemisorption theory has also been used to described a number of other important chemisorption phenomena, such as the stabilizing effects of neighboring electropositive adsorbates (K, Na), the destabilizing effects of electronegative adsorbates (Cl, F), surface relaxation and surface reconstruction [18], More recently. Hammer and Nbrskov [17] applied formal theory to elegantly explain the results from a series of large-scale periodic density functional quantum chemical calculations for adsorption on transition metal and bimetallic surfaces. Chemisorption theory will undoubtedly continue to play an important role in describing relevant concepts in chemisorption and surface reactivity well into the future. More quantitative results from theory, however, will require more sophisticated quantum mechanical methods. 

The opposite situation from weak interaction of inert gases with the surface space charge is surface ionization, when the adsorbate is ionized by the substrate. This typically occurs in alkali-metal adsorption on transition-metal surfaces. In the more usual situation with chemisorbed molecules, only partial charge transfer occurs to or from the substrate to the molecule. If the negative pole of the molecule points toward the vacuum, the induced electric fields cause an increase in the work function. Table 5.4 lists the work-function changes obtained by the chemisorption of several molecules on rhodium. 

It is well known from catalysis that electropositive (e.g. Na, Cs, K) and electronegative (e.g. S, O, C, Cl) adatoms decrease or increase the reaction rate and thus poison or promote the reaction, respectively [153-155]. Alkali-metal influenced adsorption on transition metals was reviewed by Bonzel [154]. Coadsorption of alkali metals and H, or D, on Al(lOO) revealed that the sticking coefficient and dissociation rate are extremely weak ( 10 at all alkali coverages [156]). Upon exposing alkali-covered metal substrates to a beam of atomic H or D, alkali hydride formation was observed. 

Finally, a recent STM study of Br2 adsorption on Ni(llO) indicates that this system is unique amongst halogen adsorption on transition metal surfaces [99FJ. It was found that Br2 adsorption removes Ni atoms from the (110) terraces forming added Ni-Br rows on the surface. Thus the reconstruction produces a mixed surface layer containing both Br and Ni atoms. While similar reconstractions have been observed for O adsorption on (110) surfaces [93B4], bromine on Ni( 110) is the only known case for any of the low-index planes where the halogen does not form a simple chemisorbed layer where the adatoms sit atop a (1x1) surface. 

The work function of a solid is also sensitive to the presence of adsorbates. In fact, in virtually all cases of adsorption the work function of the substrate either increases or decreases the change being due to a modification of the surface dipole layer. The formation of a chemisorption bond is associated with a partial electron transfer between substrate and adsorbate and the work function will change. Two extreme cases are (i) the adsorbate may only be polarized by the attractive interaction with the surface giving rise to the build up of a dipole layer, as in the physisorption of rare gases on metal surfaces and (ii) the adsorbate may be ionized by the substrate, as in the case of alkali metal adsorption on transition metal surfaces. If the adsorbate is polarized with the negative pole toward the vacuum the consequent electric fields will cause an increase in work function. Conversely, if the positive pole is toward the vacuum then the work function of the substrate will decrease. 

Figure 6. Ionization potentials of aromatic molecules, grouped according to their behavior of adsorption on transition-metal ion-exchanged montmorlllonites,...

Consider oxygen adsorption on transition metal oxide surfaces. The role of the narrow bands of metal fi -states in the transition metals is now played by localized surface resonances and surface states, typically in the gap of semiconducting or insulating transition metal oxides. 

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