Ensemble and Ligand Effects

Crucial to the understanding of the selectivity patterns discussed in the previous section is the concept of an ensemble of surface metal atoms. 

Inasmuch as parallel catalytic reactions differ in the number of adjacent surface atoms of the active metal that are required for forming the respective chemisorption complexes, it is clear that the reaction requiring the largest ensemble of these atoms will be the most sensitive to alloying with a second metal unable to form such chemisorption bonds (62, 63, S5/as illustrated by the following considerations. 

In every catalytic reaction involving a hydrocarbon molecule, an important intermediate is the monoadsorbed complex, e.g., for n-hexane on a surface containing platinum atoms, the complex 

In practice this will occur at high temperature, resulting in a complex that according to Anderson (118) is a necessary prerequisite for skeletal isomerization. 

From this state, nondestructive desorption might be difficult, so that hydro-genolysis (metal cracking) might become the preferred way to regenerate the free sites of the catalyst. 

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Sachtler WMH. 1997. Factors influencing catalytic action—Ensemble and ligand effects in metal catalysis. In Ertl G, Kndzinger H, Weitkamp J, eds. Handbook of Heterogeneous Catalysis. Volume 3. Weinheim VCH-Wiley. 

Measurements of the infrared spectra of carbon monoxide on supported palladium and Pd-Ag atoms (75a) shed light on the relative importance of the ensemble and ligand effects. Three CO absorption bands were observed on palladium and its alloys at 2060,1960, and 1920 cm-1. 

Fig. 2 Illustration of strain, ensemble and ligand effects, (a) Adsorbate bonding is affected by a change in metal-metal bond distances, (b) Adsorbate bonding is affected by bonding to two different metals instead of one type of metal, (c) Adsorbate bonding is affected by hetero-metallic bonding of the active site atoms with neighboring atoms.

The carbides of the early transition metals exhibit chemical and catalytic properties that in many aspects are very similar to those of expensive noble metals [1], Typically, early transition metals are very reactive elements that bond adsorbates too strongly to be useful as catalysts. These systems are not stable under a reactive chemical environment and exhibit a tendency to form compounds (oxides, nitrides, sulfides, carbides, phosphides). The inclusion of C into the lattice of an early transition metal produces a substantial gain in stability [2]. Furthermore, in a metal carbide, the carbon atoms moderate the chemical reactivity through ensemble and ligand effects [1-3]. On one hand, the presence of the carbon atoms usually limits the number of metal atoms that can be exposed in a surface of a metal carbide (ensemble effect). On the other hand, the formation of metal-carbon bonds modifies the electronic properties of the metal (decrease in its density of states near the Fermi level metal—>carbon charge transfer) [1-3], making it less chemically active... 

The model is consistent with observed patterns in adsorption and reactivity across the periodic table. Both spacing and orbital occupancy are involved so that combinations of ensemble and ligand effects are necessary to explain dependencies. 

Chemisorption studies with probe molecules could provide valuable information on metal dispersion and metal-promoter interactions and as shown in the present work, cou d help in identification of the cative phase/alloy composition. Both ensemble and ligand effects could simultaneously be operative, as has been demonstrated in Pt-Sn system. While the ensemble effect promotes activity, ligand effect could improve selectivity and stability. The major role of alkali promoters is to control acidity and retard acid catalysed side reactions and thus improve selectivity and stability. 

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. 

Fig. 1 Side view of slab models of various bimetallic structures often used in computational studies. In each case, the bottom layers of the material are defined using the structure of a specified bulk material. The number of surface and bulk layers varies in different studies, (a) In the sandwich structure the surface is one component, often the same component as the bulk material and the second layer is another component. This structure is often used to determine ligand effects, (b) The pseudomorphic monolayer structure combines strain and ligand effects in one structure by placing a second component on top of a bulk material, (c) The near surface alloy combines strain, ligand and ensemble effects in one structure by considering an alloy film defined by just a few atomic layers on top of an ordered bulk material.

Surface heterogeneity may merely be a reflection of different types of chemisorption and chemisorption sites, as in the examples of Figs. XVIII-9 and XVIII-10. The presence of various crystal planes, as in powders, leads to heterogeneous adsorption behavior the effect may vary with particle size, as in the case of O2 on Pd [107]. Heterogeneity may be deliberate many catalysts consist of combinations of active surfaces, such as bimetallic alloys. In this last case, the surface properties may be intermediate between those of the pure metals (but one component may be in surface excess as with any solution) or they may be distinctly different. In this last case, one speaks of various effects ensemble, dilution, ligand, and kinetic (see Ref. 108 for details). 

This section deals with changes in the heat of chemisorption caused by the secondary ensemble effect and the ligand effect, which are used to determine the factors governing the changes in bond strength upon alloying. 

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