The Chemisorption Bond

Can one further enhance the performance of this classically promoted Rh catalyst by using electrochemical promotion The promoted Rh catalyst, is, after all, already deposited on YSZ and one can directly examine what additional effect may have the application of an external voltage UWR ( 1 V) and the concomitant supply (+1 V) or removal (-1 V) of O2 to or from the promoted Rh surface. The result is shown in Fig. 2.3 with the curves labeled electrochemical promotion of a promoted catalyst . It is clear that positive potentials, i.e. supply of O2 to the catalyst surface, further enhances its performance. The light-off temperature is further decreased and the selectivity is further enhanced. Why This we will see in subsequent chapters when we examine the effect of catalyst potential UWR on the chemisorptive bond strength of various adsorbates, such as NO, N, CO and O. But the fact is that positive potentials (+1V) can further significantly enhance the performance of an already promoted catalyst. So one can electrochemically promote an already classically promoted catalyst. 

The chemisorptive bond A-M is a chemical bond, thus chemisorption is reactant- and catalyst-specific. The enthalpy, AH, of chemisorption is typically of the order of -1 to -5 eV/atom (-23 to -115 kcal/mol, leV/molecule=23.06 kcal/mol). 

It therefore becomes important first to examine the chemisorption of promoters on clean catalyst surfaces and then to examine how the presence of promoters affects the chemisorptive bond of catalytic reactants. 

Adsorbates acting as promoters usually interact strongly with the catalyst surface. The chemisorptive bond of promoters is usually rather strong and this affects both the chemical (electronic) state of the surface and quite often... 

The key of the promotional action is the effect of electropositive and electronegative promoters on the chemisorptive bond of the reactants, intermediates and, sometimes, products of catalytic reactions. Despite the polymorphic and frequently complex nature of this effect, there are two simple rules always obeyed which can guide us in the phenomenological survey which follows in this chapter. 

Similar is the effect of S coadsorption on the CO TPD spectra on Pt(lll) as shown in Figure 2.29. Sulfur coadsorption weakens significantly the chemisorptive bond of CO. 

Rule 1 Electropositive adsorbates strengthen the chemisorptive bond of electron acceptor (electronegative) adsorbates and weaken the chemisorptive bond of electron donor (electropositive) adsorbates. 

Indirect ( through the metal ) interaction due to the redistribution of electrons in the metal. In this case an electropositive promoter decreases the work function of the surface and this in turn weakens the chemisorptive bond of electropositive (electron donor) adsorbates and strengthens the chemisorptive bond of electronegative (electron acceptor) adsorbates. 

One of the most striking results is that of C2H4 oxidation on Pt5 where (xads,o ctact = -1, i.e. the decreases in reaction activation energy and in the chemisorptive bond strength of oxygen induced by increasing work function ethylene epoxidation and deep oxidation on Ag.5... 

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. 

FI. Increasing work function 0 (e.g. via addition of electronegative promoters) strengthens the chemisorptive bond of electron donor adsorbates (D) and weakens the chemisorptive bond of electron acceptor adsorbates (A). 

The double layer is described by its effective thickness, d, and by its field strength E (Fig. 6.15). The adsorbed moleculeJias a dipole moment P. It is well documented100 that the local field strength E can affect strongly not only the chemisorptive bond strength but also the preferred orientation of the adsorbate (Fig. 6.16). 

The enhancement in the catalytic activity is due to the electrochemical supply of H+to the catalyst which decreases the catalyst work function and thus strengthens the chemisorptive bond of electron acceptor N while at the same time weakening the bonds of electron donor H and NH3. 

This destabilization of surface Rh oxide formation with increasing catalyst potential or work function has been shown to be due to strong lateral repulsive interactions of the backpspillover O2 species and normally chemisorbed oxygen33 which causes a pronounced, up to leV, decrease in the chemisorptive bond strength of normally chemisorbed o.35,36... 

The Characterization and Properties of Small Metal Particles. Y. Takasu and A. M. Bradshaw, Surf. Defect. Prop. Solids p. 401 1978). 2. Cluster Model Theory. R. P. Messmer, in "The Nature of the Chemisorption Bond G. Ertl and T. Rhodin, eds. North-Holland Publ., Amsterdam, 1978. 3. Clusters and Surfaces. E. L. Muetterties, T. N. Rhodin, E. Band, C. F. Brucker, and W. R. Pretzer, Cornell National Science Center, Ithaca, New York, 1978. 4. Determination of the Properties of Single Atom and Multiple Atom Clusters. J. F. Hamilton, in "Chemical Experimentation Under Extreme Conditions (B. W. Rossiter, ed.) (Series, "Physical Methods of Organic Chemistry ), Wiley (Interscience), New York (1978). 

If molecules or atoms form a chemical bond with the surface upon adsorption, we call this chemisorption. To describe the chemisorption bond we need to briefly review a simplified form of molecular orbital theory. This is also necessary to appreciate, at least qualitatively, how a catalyst works. As described in Qiapter 1, the essence of catalytic action is often that it assists in breaking strong intramolecular bonds at low temperatures. We aim to explain how this happens in a simplified, qualitative electronic picture. 

Pt(ll 1) reveals that the contribution from the 5chemisorption bond is small, whereas the 2n-d interaction clearly strengthens the bond, as only the bonding region of this orbital is occupied. [Adapted from B. Hammer, Y. Morikawa and J.K. Norskov, Phys. Rev. Lett. 76 (1996) 2141.]... 

Figure 2 displays a qualitative correlation between the increase or decrease in CO desorption temperature and relative shifts in surface core-level binding energies (Pd(3d5/2), Ni(2p3/2), or Cu(2p3/2) all measured before adsorbing CO) [66]. In general, a reduction in BE of a core level is accompanied by an enhancement in the strength of the bond between CO and the supported metal monolayer. Likewise, an opposite relationship is observed for an increase in core-level BE. The correlation observed in Figure 2 can be explained in terms of a model based on initial-state effects . The chemisorption bond on metal is dominated by the electron density of the occupied metal orbital to the lowest unoccupied 27t -orbital of CO. A shift towards lower BE decreases the separation of E2 t-Evb thus the back donation increases and vice versa. 

Bagus PS, Illas F (1992) Decomposition of the chemisorption bond by constrained variations order of the variations and construction of the variational spaces. J Chem Phys 96 8962... 

By capturing an electron or a hole the chemisorbed particle passes from the electrically neutral to the charged state. It is very important that the trapped electron or hole is forced to take part in the chemisorption bonding. 

It is important that these forms differ in the strength of the chemisorption bonding, i.e., in the heat of adsorption. The charged form is always stronger than the neutral form. Indeed, in the first case, unlike the second, desorption must be accompanied by the delocalization of an electron or hole this is always an endothermic process. 

Figure la shows the weak (electrically neutral) form of chemisorption of a H atom the chemisorption bond, as can be illustrated, is provided in this case by an electron of the H atom which is drawn, to a greater or lesser extent, from the atom into the lattice this is the radical form of chemisorption. The strong acceptor and donor forms are presented in Fig. 1 (b and c, respectively) these are electrically charged and valency-saturated forms. 

Occupied molecular orbitals of the adsorbate with ionization potentials between 0 and hv- (p become visible. If one compares their binding energies with those in a UPS spectrum of a physisorbed multilayer of the same gas, one readily recognizes which of the molecular orbitals are involved in the chemisorption bond. For example, the adsorbate level in Fig. 3.19 has shifted a few eV with respect to its position indicated in the density of states picture (taken as the position in a physisorbed gas), indicating that the level is involved in the chemisorption bond. 

Although the resonant level model successfully explains a few general aspects of chemisorption, it has nevertheless many shortcomings. The model gives no information on the electronic structure of the chemisorption bond it does not tell where the electrons are. Such information is obtained from a more refined model, called the density functional method. We will not explain how it works but merely give the results for the adsorption of Cl and Li on jellium, reported by Lang and Williams [20]. 

Figure A.14 Energy diagram for the adsorption of an atom on a d-metal. Chemisorption is described with molecular orbitals constructed from the d-band of the metal and atomic orbitals of the adatom. The chemisorption bond in b) is weaker, because the antibonding chemisorption orbital is partially filled (compare Fig. A.5).

If the interaction between the atomic orbital and the d-band is weak, the splitting between the bonding and the antibonding orbital of the chemisorption bond is small. The antibonding orbital falls below the Fermi level and is occupied. This represents a repulsive interaction and does not lead to bonding. 

Although UPS indicates that the CO 5a level shifts to lower energy upon chemisorption, this does not automatically imply that the 5a level is responsible for the chemisorption bond. It depends on the filling of its antibonding partner whether the interaction of the metal with the 5a level is attractive or not. 

The picture sketched above disagrees with the frequently used donation/back donation model of adsorbed CO. This model describes the chemisorption bond in terms of donation of electrons from the CO 5a orbital into empty orbitals at the surface of the metal, and back donation of d-electrons from the metal to the unoccupied 2n level of CO. The back donation is essentially correct, but the donation is not. 

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