Chemisorbed molecules structures

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. 

XANES, which can be used to determine molecular structure and orientation of chemisorbed molecules on well-characterised single-crystal surfaces and is able to discriminate between the same atoms in different bonding situations, has been used to examine the supramolec-ular organisation adopted by the dye Reactive Red 3 1 physisorbed and chemisorbed on to cotton and cellophane substrate materials [315]. A distinct difference in the nature of the dye/cotton interaction was observed for different preparative methods. The mode by which... 

Besides, the structure, nature and reactivity of the chemisorbed molecule could not be unambiguously identified because the physical tools used could not lead easily to a complete understanding of the quasi molecular character of surface chemisorbed species and move precisely to the definition of the elementary steps occurring during the molecular transformations taking place on the surfaces. 

Interpretation of the spectra of chemisorbed molecules presents some difficulties because the surface compounds formed during chemisorption have no exact counterparts among conventional compounds. Although some general principles can be applied, interpretations of the spectra of unknown species are usually based on empirical comparison with spectra of compounds of known structure. Experience has shown, however, that these difficulties are more philosophical than practical. Interpretations of spectra of chemisorbed molecules by comparison with the spectra of compounds of known structure have produced results which are self-consistent and reasonable in a wide range of applications. 

The very language I have used here conceals a trap. It puts the burden of motion and reactive power on the chemisorbed molecules, and not on the surface, which might be thought of as passive, untouched. Of course, this can t be so. We know that exposed surfaces reconstruct, i.e., make adjustments in structure driven by their unsaturation.20 They do so first by themselves, without any adsorbate. And they do it again, in a different way, in the presence of adsorbed molecules. The extent of reconstruction is great in semiconductors and extended molecules, and generally small in molecular crystals and metals. It can also vary from crystal face to face. The calculations I will discuss deal with metal surfaces. One is then reasonably safe (we hope) to assume minimal reconstruction. It will turn out, however, that the signs of eventual reconstruction are to be seen even in these calculations. 

A variation of XANES or NEXAFS has been used to determine the structure of molecules chemisorbed on surfaces. In this approach photoemitted electrons excite molecular orbitals in the chemisorbed molecules. By varying the polarization of the incident photons, molecular orientation can be determined from selection rules for excitation. The bond lengths can be determined from a quasi-empirical correlation between bond-length and the shift in the molecular orbital excitation energy. This technique has been used to study the chemisorption of several hydrocarbon molecules on different metal surfaces./17/... 

The infrared spectra of chemisorbed molecules provide relatively clear and direct evidence concerning the structure of these molecules. Most of the problems to which the infrared techniques have been applied have been stimulated by an interest in heterogeneous catalysis. Since chemisorption is vital to catalysis and since the structure of chemisorbed molecules can be determined by infrared, it is reasonable to ask what has been learned about catalytic activity from these spectra. The number of cases where even a tenuous relationship between the spectra and activity is seen is not large. However, the infrared experiments were not designed specifically to seek such relationships. Despite this, interesting observations concerning catalytic activity have been made and will be described here to illustrate the type of reasoning involved rather than to claim well-defined relationships. 

Itaya s group presented images of benzene, naphthalene and anthracene on Cudll), and naphthalene and anthracene on Rh(l 11)/ Wandlowski and coworkers monitored adsorption of uracil on gold surfaces They reported imaging chemisorbed molecules as well as physisorbed molecules and determined their adlattice structures. They also made a correlation between the structure and lateral interaction forces of adsorbed molecules. They showed that application of sufficiently positive electrode potentials results in uracil deprotonation, leading to different surface structure and geometric orientation. 

The type of adsorption that affects the rate of a chemical reaction is chemisorption. Here, the adsorbed atoms or molecules are held to the surface by valence forces of the same type as those that occur between bonded atoms in molecules. As a result the electronic structure of the chemisorbed molecule is perturbed significantly, causing it to be extremely reactive. Interaction with the catalyst cau.ses bonds of the adsorbed reactant to be stretched, making them easier to break. 

Although the Langmuir theory of adsorption is used frequently for technical process development it is a crude approximation, as surface reconstruction frequently occurs. Adsorbed molecules change the structure of the surface layer and the catalytic properties of surface sites are not equal in the ability to bind chemisorbed molecules. The rate is dependent on spatial arrangement and the heat of adsorption depends on coverage (Figures 2.27, 2.28). 

Stohr J, Baberschke K, Jaeger R, Treichler R, Brennan S (1981) Orientation of chemisorbed molecules from surface-absorption fine-structure measurements CO and NO on Ni(100). Phys Rev Lett 47 381-384... 

The preferred adsorption site of CO depends at least on three factors the metal, the crystallographic face, and the CO coverage. For example, on the Ni(l 11) face, CO occupies the bridge sites first, while on Rh(l 11) [99] and Pt(l 11) [100] the top sites are preferred at ]ow coverages. But the threefold site is occupied first on Pd(l 11). At higher coverages there are often two or more adsorption sites occupied simultaneously. The repulsive adsorbate-adsorbate interaction often forces the CO molecules onto unusual sites of lower symmetry to maximize the distance between the adsorbed molecules. Table 2.7 (p. 261) lists the two-dimensional surface structures of small chemisorbed molecules (CO and NO) that were observed. 

In Figure 2.30 the bond-breaking sequences for ethylene and benzene chemisorbed on the Rh(l 11) surface are compared. At low temperatures the structures of the chemisorbed molecules are different. As the temperature is increased, benzene appears to break into three short-lived acetylene molecules, which become C2H spe-... 

Zeolites form another class of materials useful for fundamental studies . As mentioned earlier, zeolites are microporous silica-aluminates with micropores of dimensions comparable to organic molecules. The materials are unique, because these micropores are determined by the three-dimensional crystallographic structure of the material and catalytic events occur at the interphase of zeolite micropore and zeolite lattice. As a result the catalytically active sites are well defined. Zeolites are used in practice in the acidic form or promoted with metal or sulfide particles. High Resolution Electron Microscopy, Neutron Diffraction and Solid State NMR are techniques that arc applied for structural characterization and to study the behaviour of chemisorbed molecules. 

Solid-state NMR is proving to be a powerful technique for the study of reactions at surfaces. For example, NMR has been used in catalysis studies for determining the structure of chemisorbed molecules and for monitoring changes occurring in those stmctures as a function of temperature. 

Thus the electronic structure of the metals is decisive for their catalytic activity. The transition metals, with their partially filled d orbitals, are particularly good catalysts. These orbitals are responsible for the covalent binding of gases on metal surfaces in chemisorption and catalysis. Whereas transition metals have one or more unpaired d electrons in the outer electron shell, the weakly chemisorbing main group elements have only s or p electrons. It is postulated that unpaired d electrons are necessary to hold the chemisorbed molecules in a weakly bound state, from which they can then be transferred into a strongly bound state. 

Stohr J, Gland JL, Eberhard W, Outka D, Madix RJ, Sette F, Koestner RJ, Doebler U (1983) Bonding and bond lengths of chemisorbed molecules from near-edge X-ray-absOTption fine-structure studies. Phys Rev Lett 51(26) 2414—2417... 

---