Ligand effects of alloying

Those who prefer speculations on the ligand effects of alloying in hydrocarbon reactions may object that what is true for CO or H2 is not necessarily... 

Experimental data on chemisorption and catalysis indicate that the different types of atoms in the surface of an alloy, such as nickel and copper, largely retain their chemical identities, although their bonding properties may be modified (6,7). At present the electronic factor in catalysis by metals generally is viewed in terms of localized chemical bonding effects similar to the "ligand effects of organometallic chemistry (6). 

Effects related to the changes in the electronic structure of a-toms after alloying or formation of bimetallics(electronic or ligand ( ) effects). 

The combined use of the modem tools of surface science should allow one to understand many fundamental questions in catalysis, at least for metals. These tools afford the experimentalist with an abundance of information on surface structure, surface composition, surface electronic structure, reaction mechanism, and reaction rate parameters for elementary steps. In combination they yield direct information on the effects of surface structure and composition on heterogeneous reactivity or, more accurately, surface reactivity. Consequently, the origin of well-known effects in catalysis such as structure sensitivity, selective poisoning, ligand and ensemble effects in alloy catalysis, catalytic promotion, chemical specificity, volcano effects, to name just a few, should be subject to study via surface science. In addition, mechanistic and kinetic studies can yield information helpful in unraveling results obtained in flow reactors under greatly different operating conditions. 

Fig. 22. TPRS spectra for CO/HCOOH from Cu/Ni alloys of varying surface composition (100). (a) 37% Ni (b) 46% Ni (c) 54% Ni (d) 61% Ni (e) 68% Ni. The peak shifts indicate weak ligand effects (see text). Reprinted with permission from Journal of Inorganic Chemistry 17, 1978. Copyright 1978, American Chemical Society.

Table 1.2 indicates that alloying platinum with tin led to important changes in the product distribution an increase in the AA chemical yield and a decrease in the AAL and CO2 chemical yields. The presence of tin seems to allow, at lower potentials, the activation of water molecules and the oxidation of AAL species into AA. In the same manner, the amount of CO2 decreased, which can be explained by the need for several adjacent platinum atoms (three or four) to realize the dissociative adsorption of ethanol into CO species, via breaking the C-C bond. In the presence of tin, dilution of platinum atoms can limit this reaction. The effect of tin, in addition to the activation of water molecules, may be related to some electronic effects (ligand effects) on the CO oxidation reaction [38]. 

To stress the localized character of chemisorption (a term surface pseudomolecules was introduced at that time), Sachtler introduced for the alloying effects discussed in paragraph (2) a term ligand effect (5). It was then a task for an experimentalist to establish how important—relatively—the effects (1) and (2) were. A general consensus now is that effect (1) is more essential than (2) in any case, but the discussion is still going on, on the reliability of some pieces of evidence which have been presented in the literature in favor of a role for effect (2). 

Bulk Pt alloys for the electrooxidation of formic acid have been less frequently studied compared to underpotential deposition (upd) modified Pt surfaces. The Pt50Ru5o surface was again found to be one of the most active Pt-Ru surfaces. Underpotentially deposited metals, such as Bi, Se, Sb, were studied as reaction modifiers for Pt surfaces and provided significant electrocatalytic activity increases. Electronic factors (ligand effects) rather than bifunctional effects were held responsible for these activity modifications, because the metal coverages that caused the activity gains were extremely small. 

Pt-Sn surface. Even if hydrogen needs more than one transition metal atom in order to dissociate, this would only explain the large decrease in hydrogen adsorption if the possibility of spillover is largely reduced on the alloys. The ligand effect, to be discussed in Section IV, would have to be present for this assumption. 

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