Dissociative chemisorption diatomic molecules

Alkali-metals are frequently used in heterogeneous catalysis to modify adsorption of diatomic molecules over transition metals through the alteration of relative surface coverages and dissociation probabilities of these molecules.21 Alkali-metals are electropositive promoters for red-ox reactions they are electron donors due to the presence of a weakly bonded s electron, and thus they enhance the chemisorption of electron acceptor adsorbates and weaken chemisorption of electron donor adsorbates.22 The effect of alkali-metal promotion over transition metal surfaces was observed as the facilitation of dissociation of diatomic molecules, originating from alkali mediated electron enrichment of the metal phase and increased basic strength of the surface.23 The increased electron density on the transition metal results in enhanced back-donation of electrons from Pd-3d orbitals to the antibonding jr-molecular orbitals of adsorbed CO, and this effect has been observed as a downward shift in the IR spectra of CO adsorbed on Na-promoted Pd catalysts.24 Alkali-metal-promotion has previously been applied to a number of supported transition metal systems, and it was observed to facilitate the weakening of C-0 and N-0 bonds, upon the chemisorption of these diatomic molecules over alkali-metal promoted surfaces.25,26... 

If we move the chemisorbed molecule closer to the surface, it will feel a strong repulsion and the energy rises. However, if the molecule can respond by changing its electron structure in the interaction with the surface, it may dissociate into two chemisorbed atoms. Again the potential is much more complicated than drawn in Fig. 6.34, since it depends very much on the orientation of the molecule with respect to the atoms in the surface. For a diatomic molecule, we expect the molecule in the transition state for dissociation to bind parallel to the surface. The barriers between the physisorption, associative and dissociative chemisorption are activation barriers for the reaction from gas phase molecule to dissociated atoms and all subsequent reactions. It is important to be able to determine and predict the behavior of these barriers since they have a key impact on if and how and at what rate the reaction proceeds. 

Here we shall be concerned with the interaction of inacming diatomic molecules (H-/ 0.) with either types of potential energy wells The molecular InteractJjon (responsible for elastic and direct-inelastic scattering with extremely short residence times of the irpinglng molecules in the potential) and the chemisorptive interaction (leading to dissociative adsorption and associative desorption, reflectively, and associated with H (D) atoms trapped in the chemisorption potential for an appreci le time). 

Dissociative chemisorption was considered to be either direct, when the incoming diatomic molecule has sufficient energy to surmount the barrier without being trapped into the molecular state, or indirect, when it passes via the molecular (precursor) state into the dissociated state. If the dissociated state is not immediately equilibrated with the lattice, the fragments will move across... 

The classical and traditional view is that for dissociative chemisorption of diatomic molecules to occur at metal surfaces, it is essential that two adjacent (vacant) sites are available ... 

Figure 4.4 shows a section of a perfect single crystal surface [such as Pt(lll)] which is approached by a diatomic molecule (say 02) undergoing dissociative chemisorption. The progress of this process is illustrated by a contour plot of the energetics as a function of the distance x of the molecule from the surface and of the separation y between the two atoms, together with the well-known one-dimensional Lennard-Jones potential diagram. (The molecular axis is assumed to be parallel to the surface... 

If we wish to understand the conditions under which a diatomic molecule such as H2, N2, or CO dissociates on a surface, we need to take two orbitals of the molecule into account - the highest occupied and the lowest unoccupied molecular orbital (the HOMO and LUMO of the so-called frontier orbital concept). Let us take a simple case to start with the molecule A2 with occupied bonding level a and unoccupied antibonding level a. We use jellium as the substrate metal and discuss the chemisorption of A2 in the resonant level model. What happens is that the two levels broaden due to the rather weak interaction with the free electron cloud of the metal. 

Note that this result would reduce to the Langmuir isotherm, Eq. (8-6), if only A were adsorbed. Equations (9-14) and (9-15) are applicable when A occupies one site. Often in chemisorption a diatomic molecule, such as oxygen, will dissociate upon adsorption with each atom occupying one site. Formally, dissociative adsorption may be written... 

Figure 26. Diatomic molecule and primary zone atoms used for GLE simulations of dissociative chemisorption on a BCC (110) surface. The atoms labeled by 2 are in the second layer.

So far the 3D flat-surface model has been quite successful in providing qualitative and even some quantitative dynamics information for hydrogen dissociation on metals such as the role of hydrogen vibration and rotation in dissociative adsorption on Cu(lll) (104,114,117-119). However, the inherent limitation of the flat-surface model dictates that it cannot provide information on surface corrugation and its effect on molecular adsorption. One would like to investigate the effect of rotational orientation of diatomic molecules on chemisorption in the presence of surface corrugation. In order to obtain... 

Earlier we described the catalytic reaction as a series of consecutive steps at the surface, in which adsorbate and adsorbate-surface bonds are formed and/or broken on the reaction path towards the product molecule. The forces between surface atoms and adsorbate atoms responsible for rearrangement of the chemical bond are similar to those responsible for strong adsorption (E > 10 kcal/nx)l). The adsorption process dominated by such interaction is called chemisorption. Even on a single crystal metal surface, several adsorption modes are conceivable and for dissociation of a diatomic molecule many different reaction paths can be envisioned. However, usually only one particular surface atom configuration is preferred to lead to the idea of catalytic active site. If catalysis of a molecule is studied that has several reaction possibilities, some desirable and others not, a selective reaction usually requires a particular surface atom composition and rearrangement. 

Finally, it should be recalled that in our DFT results most structures reach at least one early minimum showing no dissociation of the molecule before the chemisorption minimum is reached. These structures may be considered as precursors to dissociation, related to physisorption of the diatomic molecule on the metal cluster. 

One of the first predictions made on the basis of steric effects was that the ease of chemisorption of diatomic molecules should strongly depend on the lattice dimensions of the metallic catalysts. The reasoning was that for large interatomic distances, diatomic molecules would have to dissociate to be completely chemisorbed, while for closely packed lattices, repulsion effects would hinder chemisorption. This is exemplified by our first example, the dehydrogenation of cyclohexane. 

The bond energy AHbond is readily extracted from the heat of adsorption. In the case of the chemisorption of a diatomic molecule X2 onto a site on a uniform solid surface M the molecule may adsorb without dissociation... 

Dissociative chemisorption of a diatomic molecule can also happen through the dissociation in a gas phase and a creation of two gas phase atoms these two atomic species can be then adsorbed on the surface (this way is almost always non-activated). If the curves describing molecular and atomic adsorption intersect at or below the zero potential energy line, then the precursor physisorbed molecule can experience non-activated dissociation, followed by chemisorption (Fig. 4. la). In contrast, if the energetic for these two pathways are such that the intersection occurs above the zero eneigy plane, then chemisorption wiU be activated with activation energy, Ead, as indicated in Fig. 4. lb. 

Fig. 4.1 Potential energy curves for (7) physical and (2) chemical adsorption (a) non-activated (b) activated. Epot - potential energy, Qc - heats of chemisorption, Qp - heats of physisorption, Ead -energy of activation for desorption, Ediss - dissociation energy for the diatomic molecule. The sum AEdes = Ead + Qc is the the heat of hemisorption, in the activated processes [8]...

The slopes of these relations depend on the reaction studied. For dissociative adsorption processes involving simple diatomic molecules, the slope of the transition state energy as function of the dissociative chemisorption energy is often close to 1. This implies that the electronic structure of the transition state is similar to that of the final state, and hence, it is indicative of a late tfansition state. This behavior can be observed directly in the transition state structures for NO dissociation, as is shown in Figure 6.8b. 

As far as phenomenological modeling is concerned, an excellent review of earlier thermodynamic approaches to chemisorption and surface reactivity was given by Benziger (156), who also developed some general thermodynamic criteria for dissociative versus nondissociative adsorption of diatomic and polyatomic molecules on transition metal surfaces (137, 156). In particular, for quantitative estimates of QA, A = C, N, or O, Benziger (156) used the heats of formation of bulk metal carbides, nitrides, and oxides. The BOC-MP approach is different, however, not only analytically but also in making direct use of experimental values of QA. 

Chemisorption dissociates the molecules of diatomic gases, such as hydrogen or oxygen, transforming them into adsorbed atoms. 

Figure 6.3. Chemisorption and dissociation of a diatomic AB molecule, (a) The traditional Lennard-Jones one-dimensional potential energy diagram of E vs. R, where R is the reaction coordinate.

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