Adsorption of diatomic molecules

The nondissociative adsorption of diatomic molecules (CO, N2, NO, and H2) on the surface ions of oxides and halides is accompanied by distinct perturbations of the vibrational spectra. This statement is documented in detail for CO in this review. At this stage of the discussion, it is sufficient to mention the following points. 

Still most dynamical simulations of reactions at surfaces are limited to rather simple systems, such as the adsorption of diatomic molecules on low-index single crystal surfaces. With the development of more efficient algorithms and the improvement of computer power, more and more complex systems will be able to be addressed. One recent example is the ab initio molecular dynamics simulation of the soft-landing of Pd clusters on oxide surfaces [127] where up to n = 13 Pd atoms have been taken into account in the calculations. 

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... 

The adsorption of diatomic molecules on a metal surface may be considered as a competition between molecular and dissociative adsorption... 

Many years before reliable information was available concerning the actual structure of any cataij st, attempts were made to consider the disposition of a reacting molecule on the active surface. So long as these conjectures were confined to reactants which were adsorbed by single atoms of the catalyst surface, little progress could be made by such hypotheses. Burk (1), however, studied the implications of two-point adsorption of diatomic molecules and came to the conclusion that some of the known facts concerning catalyst poisons could be accounted for by the assumption that this mechanism was involved. 

Table3.1-189 Heat of adsorption of diatomic molecules on different single crystals planes of various transition metals (kcal/mol) [1.218, p. 267]...

Juaristi Jl, Alducin M, Diez-Muino R, Busnengo HF, Salin A (2008) Role of electron-hole pair excitations in the dissociative adsorption of diatomic molecules on metal surfaces. Phys Rev Lett 100 116102... 

Some calculated values for the heat of adsorption of diatomic molecules, Qab, are listed in Table 6.6 and compared to experimental values obtained from the literature [16]. The effect of the type of coordination and the bonding end of a molecule can readily be observed. Illustration 6.2 provides some sample calculations resulting in these values for weak chemisorption. Additional examples are given elsewhere [16-18]. 

The adsorption of diatomic molecules (CO and H2) on metals, based on the orbital theory and, according to the above scheme, must take into account the dissociation of molecules and two orbitals of the molecule which, as mentioned earlier, can occupy the highest levels (HOMO) and the lowest level (LUMO) which allows the adsorption on surfaces. 

Prior to 1970 our understanding of the bonding of diatomic molecules to surfaces, and in many cases the type of adsorption (i.e., molecular or dissociative) was almost entirely dependent on indirect experimental evidence. By this we mean that deductions were made on the basis of data obtained from monitoring the gas phase whether in the context of kinetic studies based on gas uptake or flash desorption, mass spectrometry, or isotopic exchange. The exception was the important information that had accrued from infrared studies of mainly adsorbed carbon monoxide, a molecule that lent itself very well to this approach owing to its comparatively large extinction coefficient. 

The adsorption of diatomic or dimeric molecules on a suitable cold crystalline surface can be quite realistically considered in terms of the dimer model in which dimers are represented by rigid rods which occupy the bonds (and associated terminal sites) of a plane lattice to the exclusion of other dimers. The partition function of a planar lattice of AT sites filled with jV dimers can be calculated exactly.7 Now if a single dimer is removed from the lattice, one is left with two monomers or holes which may separate. The equilibrium correlation between the two monomers, however, is appreciable. As in the case of Ising models, the correlation functions for particular directions of monomer-monomer separation can be expressed exactly in terms of a Toeplitz determinant.8 Although the structure of the basic generating functions is more complex than Eq. (12), the corresponding determinant for one direction has been reduced to an equally simple form.9 One discovers that the correlations decay asymptotically only as 1 /r1/2. 

If a homonuclear diatomic molecule (e.g., H2, 02, N2, etc.) is adsorbed with dissociation into atoms, the adsorption isotherms (121) and (123) give 8 as the functions of partial pressure of atoms in gas phase, pat, that would correspond to dissociation equilibrium at the partial pressure of diatomic molecules, p. These values are linked by... 

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. 

Modification of Vibrational Spectra of Diatomic Molecules Induced by the Adsorption on Oxide and Halide surfaces A method for Probing the Structures of the Adsorption Sites and the Surface Morphologies of Sintered Materials... 

Second, at low coverages, the vibrational perturbation induced by adsorption on cationic sites located on different faces of the same microcrystal is primarily determined by the coordinative unsaturation of the cation (which in turn is a complex function of the structure of the face). This statement implies that the vibrational spectra of diatomic molecules adsorbed on low-surface-area materials (in which the crystallites exhibit only a few dominant faces) are usually characterized by the presence of a small number of narrow peaks—one for each exposed face. Therefore, vibrational spectra of adsorbed species provide morphological information that can be compared with information derived from HRTEM and SEM studies of the same microcrystals. 

On the virgin sample, the surfaces of the microcrystals are fully covered by adsorbed water and CO2 consequently, they do not show any adsorption by diatomic molecules such as CO and H2. 

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... 

Low-activation energy dissociation of diatomic molecules with k bonds requires a specific topology of the adsorption site. Not only is a large ensemble of surface atoms necessary but an arrangement in a step-edge topology is also needed. This is discussed in Section 10.6.2.1. 

Rudenko A, Keil EJ, Katsnelson MI, Lichtenstein AI (2010) Adsorption of diatomic halogen molecules on graphene a van der Waals density functional study. Phys Rev B 82 035427 Vydrov OA, Wu Q, Voorhis TV (2008) Self-consistent implementation of a nonlocal van der Waals density functional with a Gaussian basis set. J Chem Phys 129 014106 Grimme S, Antony J, Ehrlich S, Krieg H (2010) A consistent and accurate ab initio parametrisation of density functional dispersion correction (DFT-D) for the 94 elements H-Pu. J Chem Phys 132 154104... 

Table 2.1 lists the Langmuir adsorption isotherms for several important cases, including dissociative adsorption of a diatomic molecule, and competitive adsorption of two molecules. Note that the fraction of unoccupied sites, 0, equals 1 over the denominator of the Langmuir expression. Isotherms can always be written in the form 9 = Kji A 9, which will appear very useful in solving the rate equations of complex catalytic mechanisms. 

In contrast to molecular adsorption, the interaction energy of atoms as C, O, or N with a metal surface is a strong function of coordination number. Adsorbed atoms almost always prefer bonding in sites of threefold or fourfold coordination. Molecules can also adsorb on top or bridge sites. As will be seen, this has major consequences for the dissociation paths of diatomic molecules on metal surfaces. 

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