Transition chemisorptive bond

The chemisorptive bond is a chemical bond. The nature of this bond can be covalent or can have a strong ionic character. The formation of the chemisorptive bond in general involves either donation of electrons from the adsorbate to the metal (donation) or donation of electrons from the metal to the adsorbate (backdonation).2 In the former case the adsorbate is termed electron donor, in the latter case it is termed electron acceptor.3 In many cases both donation and backdonation of electrons is involved in chemisorptive bond formation and the adsorbate behaves both as an electron acceptor and as an electron donor. A typical example is the chemisorption of CO on transition metals where, according to the model first described by Blyholder,4 the chemisorptive bond formation involves both donation of electrons from the 7t orbitals of CO to the metal and backdonation of electrons from the metal to the antibonding n orbitals of CO. 

The orientation of molecules at the interface depends on an interaction with both the surface and the molecules in the liquid phase, and also on the interaction within the adsorbed layer. The interaction of molecules with the electrode is stronger the weaker their interaction with other molecules in the bulk. The correlation between and 0 is linear but different for the transition metals and the sp metals. Owing to the tendency to form chemisorption bonds, transition metals bind water molecules more strongly than the sp metals. 

In these experiments, too, it is possible, therefore, to interpret the decrease in the activation energy in the light as due to excitation and loosening of the bond between noble metal and oxygen at the surface. The chemisorption bond between the oxygen atom and the noble metal atom may be described by a resonance similar to the resonance of the semiconductor bond. The bond is excited and weakened by photon absorption. The oxygen transition from the excited chemisorption bond to the CO molecule requires less energy than in the dark. 

Generally, the bonding of adatoms other than hydrogen to a metal surface is highly coordination-dependent, whereas molecular adsorption tends to be much less discriminative. For the different metals the bond strength of an adatom also tends to vary much more than the chemisorption energy of a molecule. Atoms bind more strongly to surfaces than molecules do. Here we will discuss the quantum chemical basis of chemisorption to the transition metal surfaces. We will illustrate molecular chemisorption by an analysis of the chemisorption bond of CO [3] in comparison with the atomic chemisorption of a C atom. 

It is worth noting that no overbinding seems to occur in the local density description of alkali chemisorption on nickel clusters, in contrast to findings for carbon transition metal bonds (23,27,28). At present, it would be premature to correlate this difference with the character of the various bonds (covalent vs. ionic). Clearly, density gradient corrections to the energy functional (31) would be highly useful in deciding this question. 

Bonding modifiers are employed to weaken or strengthen the chemisorption bonds of reactants and products. Strong electron donors (such as potassium) or electron acceptors (such as chlorine) that are coadsorbed on the catalyst surface are often used for this purpose. Alloying may create new active sites (mixed metal sites) that can greatly modify activity and selectivity. New catalytically active sites can also be created at the interface between the metal and the high-surface-area oxide support. In this circumstance the catalyst exhibits the so-called strong metal-support interaction (SMSI). Titanium oxide frequently shows this effect when used as a support for catalysis by transition metals. Often the sites created at the oxide-metal interface are much more active than the sites on the transition metal. 

The toxicity of an element such as sulfur is dependent on the presence, in the valency shell of the toxic element, of free electron pairs which are evidently necessary for the formation of the link with the catalyst. The toxicity—i.e., the power of forming a relatively strong chemisorptive bond—disappears if the structure of the molecule is of a shielded type in which this element is already associated with a completely shared electron octet. Thus, it appears (Maxted, 8) that the chemical bond by means of which the poison is linked to the metallic surface resembles the ordinary dative bond in which the poison is the donor. In the case of methyl sulhde adsorbed on palladium, indications have been obtained (Dilke, Eley, and Maxted, 9) by means of magnetic susceptibility measurements that electrons from the methyl sulfide enter the d-band of the adsorbing metal to give a coordinate link, the process being probably accompanied (Maxted, 10) by a filling up of the fractional deficiencies or holes in the d-band of the metal due to d- -band overlap which seem to be responmble for the catalytic activity of the transition metals (11). 

As far as data are available, with transition metals the metal-metal bond energies are quite similar in cluster compounds and in bulk metals, but—what is even more important—are also comparable to the strength of the chemisorption bond with, for example, CO. In this way, it can be rationalized why the structure of a metal surface is frequently affected by chemisorption. 

As an example of molecular adsorption, we provide in this section a detailed discussion of the chemisorption bond of CO adsorbed to a transition-metal 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. 

By alloying Pt with transition metals M (M = V, Cr, Co, Ni, Fe, Ti, etc.), the ORR activity can be enhanced remarkably in both phosphoric acid fuel cell (PAFC) and PEMFC [20-22]. The activity enhancement mechanisms have been an open question for more than three decades and ascribed to decreased Pt-ft bond distance [23], enhanced surface roughness [24], increased Pt d-band vacancy [25-27], weakened OH adsorption [28, 29], and downshifted d-band center [30-35]. Nprskov and Mavrikakis et al. combined the stmctural and electronic effects by introducing a d-band model that correlates changes in the energy center of the valence d-band density of states at the surface sites with their ability to form chemisorption bonds. 

For a meaningful discussion of electronic factors in catalysis it is necessary to briefly review the nature of chemisorption bonds. Two theories of the metallic state have been accepted, the electron band theory and the valence bond theory. Both theories recognize the existence of two separate functions for valence electrons in metals one function is to bind the atoms together and the other is to account for magnetic and conductive properties. In the electron band theory, as particularly applied to the transition metals, the s-electron energy band is broad with a low maximum... 

Two opposing theories on the nature of chemisorption bonds have been supported in recent years. Trapnell has subscribed to the view that the chemisorption bond is a covalence between electrons from the adsorbate and unpaired electrons in atomic d-orbitals. This concept easily interprets the high chemisorption activity of the transition metals and the decrease of magnetic susceptibility which can follow chemisorption. The alternative view, supported by Dowden, is that the metal-adsorbate bond is essentially similar to the metal-metal bond, i.e., a free d-s-p-orbital is employed. " In this theory, the role of unpaired d-electrons is that of forming an intermediate without which the final state cannot be attained, unless a high activation energy is overcome or the molecule is previously dissociated to atoms. 

The theory that the chemisorption bond is a covalence involving partially filled d-orbitals is not subject to quantitative test by calculation. Experimental correlations of heats of adsorption with the percentage d-character of the metal, particularly for adsorption of hydrogen on the transition metals, have led to taking the percentage d-character as a measure of unavailability of electrons in atomic d-orbitals and thus the expected strength of the chemisorption bond. For the alternative view in which the surface bond is... 

Dissociation of H2 and formation of a chemisorption bond between atomic hydrogen and a surface site consisting of a certain number of substrate atoms. H dissociative adsorption is quasi-nonactivated on most transition metals. 

Experimental heats of adsorption of H obtained on polyciystalline transition metal surfaces are shown in Figure 2a [63]. Calculated and experimental adsorption bond energies for H on the most close-packed surfaces of the transition metals are given in Figure 2b [64]. A diminution of the chemisorption bond strength is observed from center left to the right in each metal series (3d, 4d, and 5d). 

Figure 2 (a) Experimental heats of adsorption of hydrogen on polycnstalune transition metal surfaces. (From Fig. 2a in Ref 63.) (b) Calculated and experimental chemisorption bond energies for H on the most close-packed smfaces of transitions metals (1 eV= %.5 kJ mol ). (From Fig. 2 in Ref 64, with permission from Elsevier Science.)... 

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