Strained-metal overlayers

The first deals with small islands of silver on a ruthenium substrate. One may look at this sample as a, perhaps somewhat far-fetched, model of a supported catalyst or a bimetallic surface. As metal layers are almost never in perfect registry with the substrate, they possess a certain amount of strain. Goodman and coworkers [46] used these strained metal overlayers as model systems for bimetallic catalysts. Here we look first at the electronic properties of the Ag/Ru(001) system as studied by UPS. 

The catalytic activity of strained-layer Ni on W(llO) for methanation and ethane hydrogenolysis has been studied as a function of Ni coverage. The activity per Ni atom site for methanation, a structure-insensitive reaction, is independent of the Ni coverage and similar to the activity found for bulk Ni. The activation energy for this reaction is lower on the strained-metal overlayer, however, very likely reflecting the lower binding strength of CO on the bimetallic system. 

Table 1. Comparison of Strained-Metal Overlayer Systems...

That CO chemisorption is perturbed on strained-layer Ni is not surprising in view of CO chemisorption behavior on other metal overlayer systems. For example, on Cu/Ru it has been proposed that charge transfer from Cu to Ru results in decreased occupancy of the Cu 4s level. This electronic modification makes Cu more nickel-like , and results in an increase in the binding energy... 

Similar effects can be found for metal overlayers, where a monolayer of one metal is deposited on top of another metal. Here there is an additional effect relating to the fact that the overlayer usually takes the lattice constant of the substrate. For metal overlayers we therefore find a combination of ligand and strain effects. 

A way to stretch or compress metal surface atoms in a controlled way is to deposit them on top of a substrate with similar crystal symmetry, yet with different atomic diameter and lattice constant. Such a single monolayer of a metal supported on another is called an overlayer. Metal overlayers strive to approach the lattice constant of their substrate without fully attaining it hence, they are strained compared to their own bulk state [24, 25]. The choice of suitable metal substrates enables tuning of the strain in the overlayer and of the chemisorption energy of adsorbates. A Pt monolayer on a Cu substrate, for instance, was shown to bind adsorbates much weaker than bulk platinum due to compressive strain induced by the lattice mismatch between Pt and Cu, with Cu being smaller [26]. 

DF slab calculations have been used to study in a systematic way the effects of bimetallic bonding on the valence band of Pd and many other metals [14,36,101,102]. For metal overlayers, the strain induce by the metal substrate on... 

This expression indicates that the change in hybridization energy is opposite and proportional to the shift of the d band center. Thus, if the d band shifts upwards the hybridization energy increases and vice versa. Strain and the associated shift of the d band can be brought about by growing the desired metal pseudomorfically on another material with a different lattice constant. The term pseudomorfic means that the overlayer grows with the same lattice constant as the substrate. The overlayer may thereby be strained or compressed depending on the lattice constants of the two materials. 

Pt surfaces tend to restructure into overlayers with an even higher density of Pt atoms than the close-packed (111) surface [21]. The Pt atoms are closer to each other on the reconstructed surfaces than in the (111) surface. The overlap matrix elements and hence the bandwidth are therefore larger, the d bands are lower and consequently these reconstructed surfaces bind CO even weaker than the (111) surface. The reconstructed Pt surfaces are examples of strained overlayers. The effect of strain can be studied theoretically by simply straining a slab. Examples of continuous changes in the d band center and in the stability of adsorbed CO due to strain are included in Figure 4.10. The effect due to variations in the number of layers of a thin film of one metal on another can also be described in the d band model [22,23]. 

The chemical behavior of monolayer coverages of one metal on the surface of another, i.e., Cu/Ru, Ni/Ru, Ni/W, Fe/W, Pd/W, has recently been shown to be dramatically different from that seen for either of the metallic components separately. These chemical alterations, which modify the chemisorption and catalytic properties of the overlayers, have been correlated with changes in the structural and electronic properties of the bimetallic system. The films are found to grow in a manner which causes them to be strained with respect to their bulk lattice configuration. In addition, unique electronic interface states have been identified with these overlayers. These studies, which include the adsorption of CO and H2 on these overlayers as well as the measurement of the elevated pressure kinetics of the methanation, ethane hydrogenolysis, cyclohexane dehydrogenation reactions, are reviewed. 

Properties of supported catalysts by bimetallic substrates depend on the changes in geometry of the catalyst material by the strain of the substrate. Using a bimetallic substrate multiphes the possibilities to tune the catalyst to specific requirements. The chemistry of the nanosized overlayer is affected by the different orbital overlaps of atoms from the catalyst cluster and those from the substrate. Additionally, small supported metallic islands show low coordination and reduced near-neighbor distances thus their chemical properties are different with respect to those of flat surfaces. " Reactivity of several bimetallics were also studied by Balbuena et al., including bimetallics systems . Norskov et al. found several relations for the bimetallic systems considering local and nonlocal effects have also been reported. ... 

This vast number of possibilities calls for a systematic procedure to identify a subset of the most likely interface matchings of the parent crystals. This subset will then be the starting point for atomistic modeling. The question about unit cell size and shape is relatively simple to address. Many related procedures based on linear elasticity theory and lattice strain estimates may be adopted. The basic situation is sketched in Fig. 4 an overlayer unit cell A needs to be matched together with a substrate unit cell B. Matching pairs of unit cells are, in general, multiples of primitive cells in the interface plane for the metal and ceramic, respectively. 

The formation of an overlayer of adatoms or molecules can lead to reconstruction of the surface metal layers. This will reduce strain in the surface layer due to the altered metal-metal atom interactions. Often ordered surface phases are formed, in which the adatoms have reduced reactivity, because of the increased interaction with the reconstructed surface atom overlayer. The reactivity of the adsorbate overlayer is then limited to the boundary atoms of the overlayer surface islands. Once ordered overlayers are formed and the surface concentration of adatoms or molecules is further increased, bonding in... 

Similar effects can be found for metal overlayas. Overlayers of one metal on another are often found for alloy catalysts because one of the components usually segregates to the surface. In such systems, the overlayer atoms have Ugand effects from the second layer as in the NSAs, but they also have to adapt to the lattice constant of the host metals. Hence, we have a combination of the Ugand and strain effects. Figure 8.15 shows a systematic theoretical study of shifts in d-band centers as different late transition metals are deposited on host materials consisting of other late transition metals. 

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