Ensemble Effects in Adsorption

The cases we have discussed until now have only considered situations where the adsorbate has bonded to one single elemental metal. The variations in the adsorption strength have been induced by second-order effects, meaning that only the surroundings or the atoms in the vicinity of the adsorption site have changed, giving rise to an indirect effect on the adsorbate. 

The simplest model would be to use data of pure metal surfaces alone and not taking into account strain and ligand effects induced by the host material. Let AB JB be the model surface alloy system we want to describe. Here, B is the host material and A is the solute and x is the fractional amount of A in the surface layer. The adsorption energy of oxygen on the surface of our model system can now be approximated as 

FIGURE 8.17 Oxygen binding energies for a series of surface alloys obtained using two simple interpolation schemes and compared to fnll DFT calculations. The dashed line indicates perfect agreement. Adapted from Andersson et al. (2006). (See insert for color representation of the figure.) 

---

Liu P, Nprskov JK. 2001b. Ligand and ensemble effects in adsorption on alloy smfaces. Phys Chem Chem Phys 3 3814. 

Evidence for ensemble effects in VIIIC/IB alloys has been obtained by examination of carbon monoxide adsorption by infrared spectroscopy. This technique has been applied to the systems Pd-Ag, Ni-Cu, and Pd-Au. It is generally accepted that carbon monoxide may chemisorb in bridged or linear forms, the former providing an absorption band in the region 1900-1950 cm and the other in the region 2000-2050 cm . There may be a distinguishable contribution to the former from CO bonded... 

J. A. Rodriguez, The chemical properties of bimetallic surfaces importance of ensemble and electronic effects in the adsorption of sulfur and S02, Prog. Surf. Sci., 2006, 81, 141. 

Describe the differences between ensemble effects and ligand effects (in relation to bimetallic surfaces), and how these may play a role in the adsorption activity of a surface. 

An effect other than this ensemble effect has to be invoked in order to explain the increase in M and S for the alloys with 40-70% Cu. Although the origin of this apparent energetic effect is not clear, the effect could be due to differences in size of the nickel clusters in the surface caused by differences in bulk concentration of the alloy, or to adsorption-induced enrichments of nickel in the surface. The latter can also depend on the bulk concentration of the alloys. 

The geometry of the ensembles of A atoms in the surface can, however, influence the adsorption complexes not only by changing the number of single bonds to different atoms of the adsorbed molecule, as illustrated above a second type of ensemble effect can be visualized for any given atom of the adsorbate. Taking the adsorption complexes of CO on a transition metal as an example, we can discern linear, bridged, and multisite complexes ... 

Weak adsorption is understood to be chemisorption simply determined by charge transfer, in Mulliken s sense (137) for instance, adsorption of xenon to transition metals (138) as compared to physical adsorption determined by dispersion forces. In terms of this secondary ensemble effect the influence of alloying is smaller for strong adsorbates than for weak adsorbates, if adsorption on the same sites is considered. 

In this paper we have introduced the secondary ensemble effect, which ascribes changes in heat of chemisorption of multiply bonded atoms to a decrease in the coordination of these atoms to the surface metal atoms. This effect will in general lead to a decrease in heat of adsorption upon alloying. 

It can often happen that the active site in an alloy is composed of more than one species of metal atom. Adsorbates then bond to different metals simultaneously, and consequently the adsorption energy or reaction barriers are different than if the active site was composed of only one type of metal. This effect is called an ensemble effect. Fig. Ic and Fig. 2b shows one example where ensemble effects can be probed in a DFT calculation. We note that another somewhat different use of the term ensemble effect is to describe scenarios where a critical number or geometry of... 

A somewhat similar case of an ensemble effect was proposed for CO adsorption on a Pt25Ni75(lll) alloy surface [63]. Whereas CO adsorbs on-top of Pt atoms [64], on Ni(lll) it adsorbs in the threefold hollow sites [65]. The HREELS (high-resolution electron energy loss spectroscopy) peak associated with CO adsorbed on threefold Ni sites almost vanishes if the sample prepara-... 

It may be argued that the low reactivity of the alloy with respect to tin is due to the fact that the bimetallic system does not have adsorption sites with two or three adjacent tin atoms ( ensemble effects [31,32]). But the differences in reactivity... 

Ensemble effects are useful when adsorption requires a special grouping of surface atoms. To explain this, let us examine the simple example of ethylene adsorption on nickel, which occurs in a dt-adsorbed mode. Two nickel atoms, the right distance apart, are needed to bond a pair of carbon atoms. The bonds must be stable, but not too strong or subsequent reaction is difficult. Figure 4.2 shows symmetry and distances for tow index planes of the face centered cubic nickel surface. 

---