Molecule chemisorbed

The presence of a precursor breaks the dynamical motion into tliree parts [34], First, there is the dynamics of trapping into the precursor state secondly, there is (at least partial) thennalization in the precursor state and, thirdly, the reaction to produce the desired species (possibly a more tightly bound chemisorbed molecule). 

Stdhr J and Jaeger R 1982 Adsorption-edge resonances, core-hole screening and orientation of chemisorbed molecules CO, NO and N2 on Ni(IOO) Rhys. Rev. B 26 4111-31... 

Oxide-supported metals constitute one of the most important classes of heterogeneous catalysts, and for this reason they have been investigated by many techniques adsorption isotherms, IR of chemisorbed molecules, electron microscopy, EXAFS, etc. Flowever, the fact that they have been studied by so many methods proves that no one technique is totally satisfactory. 

A type of molecular resonance scattering can also occur from the formation of short-lived negative ions due to electron capture by molecules on surfrices. While this is frequently observed for molecules in the gas phase, it is not so important for chemisorbed molecules on metal surfaces because of extremely rapid quenching (electron transfer to the substrate) of the negative ion. Observations have been made for this scattering mechanism in several chemisorbed systems and in phys-isorbed layers, with the effects usually observed as smaU deviations of the cross section for inelastic scattering from that predicted from dipole scattering theory. 

If it is assumed that 2,2 -bipyridine is bonded to the catalyst by both nitrogen atoms, then the position of the chemisorbed molecule on the metal is rigidly fixed. Unless two molecules of this base can be adsorbed at the required distance from each other and in an arrangement which is close to linear, overlap of the uncoupled electrons at the a-position cannot occur. The failure to detect any quaterpyridine would then indicate that nickel atoms of the required orientation are rarely, if ever, available. Clearly the probability of carbon-carbon bond formation is greater between one chemisorbed molecule of 2,2 -bipyridine and one of pyridine, as the latter can correct its orientation relative to the fixed 2,2 -bipyridine by rotation around the nitrogen-nickel bond, at least within certain limits. 

Electronic Interaction between Metallic Catalysts and Chemisorbed Molecules R. Suhrmann... 

Metal atom matrix chemistry. Correlation of bonding with chemisorbed molecules. G. A. Ozin, Acc. Chem. Res., 1977,10, 21-26 (32). 

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. 

Fundamental is that the atoms in the surface pha.se are not fully co-ordinated. These sites are often called Co-ordinatively Unsaturated Sites (CUS) . These sites chemisorb molecules because upon formation of bonds with the adsorbing molecules the Gibbs free energy is lowered. 

Temperature Programmed Desorption (TPD). Chemisorbed molecules are bonded to the surface by forces dependent on the nature of the sites. For instance, ammonia will be strongly adsorbed on acid sites, whereas it is only weakly adsorbed on basic sites. Consequently, the adsorbate complex formed with the basic sites will decompose at lower temperatures than that formed with the acid sites. The following example regarding the NH.i-zeolite H-ZSM-5 system will illustrate this. 

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... 

A-A bond. As a result it may be more reactive than in the gas phase. In molecules such as CO and NO this weakening of the bond is readily observed with infrared spectroscopy. This partial filling of the antibonding orbital of a chemisorbed molecule is often called (not entirely correct) back donation . 

Such processes assume that molecules from a fluid phase in contact with a solid catalytic surface combine chemically with catalyst surface molecules and reaction subsequently proceeds between chemisorbed molecules followed by desorption of the products. A large number of different rate equations with varying numbers of constants can be derived by making various auxiliary assumptions and tested against experimental rate data. Since a more or less plausible mechanism is postulated, the feeling is that a chosen rate equation is somewhat extrapolatable outside an experimental range with greater... 

We describe in some detail the techniques of nuclear magnetic resonance which are used for studying alumina-supported platinum catalysts. In particular, we describe the spin-echo technique from which the Pt lineshape can be obtained. We also discuss spin echo double resonance between surface Pt and chemisorbed molecules and show how the NMR resonance of the surface Pt can be separately studied. We present examples of experimental data and discuss their interpretation. 

Furthermore, as mentioned above the screening of the dipole field by the conduction electrons can be represented by an image dipole inside the metal. This complex of the chemisorbed molecule and its image has a vibration frequency different from that of the free molecule. The electrodynamic interaction between a dipole and its image has been discussed in many works. The theoretical problem is that the calculated frequency shift is extremely sensitive to the position of the image plane (Fig. 3a). One can with reasonable parameter values obtain a downward frequency shift of the order of 5-50 cm S but the latest work indicates that the shift due to this interaction is rather small. 

When the chemisorbed molecule is vibrationally excited this influences not only the metal electrons but also the ion cores in the neighbourhood. The vibrating ion cores can then in turn couple to other molecules and give rise to a short range interaction mediated via the substrate lattice. However, as Cl is much larger than the highest substrate phonon frequency the effect of this interaction is very small , but it can be important for low frequency modes . 

If the interaction with the substrate is weaker the situation becomes different. For a chemisorbed molecule an electronic rearrangement or even charge transfer between the substrate and the molecule has taken place, as discussed in the previous section. Physisorbed molecules on the other hand are only bonded via the fluctuating polarization, that is the van der Waals interaction. The problem when investigating physisorbed or weakly chemisorbed... 

Suppose now that the chemisorbed molecule AB, composed of two atoms or two atomic groups A and B connected by a simple bond, is in a state of weak bonding with the surface. When a free valence of the surface comes into play, the valence bond inside the molecule is broken that is, the chemisorbed molecule dissociates into two radicals A and B, the valence of one of them becoming free, while that of the other is saturated by the free valence of the surface. Thus, one of the dissociation products is in a state of weak and the other in a state of strong bonding with the surface. The law of conservation of valencies is satisfied the free valence of the surface is saturated and reappears as a free valence of the newly produced radical. 

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. 

In conclusion the experimental support given by the use of molecular probes to the molecular approach to chemisorption considered as a localized interaction of chemisorbed molecules with metal atoms on the surface of metals or metal oxides [2, 5] was at the origin of the development of a more precise experimental identification of surface species involved in some aspects of heterogeneous catalysis. 

This approach allowed more precise identification of the nature of the bonds between many chemisorbed molecules and different kind of surface metallic sites (cations in zeolites or on the surface of metal oxides, metal atoms on the surface... 

This molecular picture of the surface states of chemisorbed molecules was consolidated by the analogy of the bonding of such molecules on molecular metal cluster and on small metal particles [11-13]. 

Self-assembled surface layers of 6-thioguanme (6TG) on mercury electrode have been described and electrochemicaUy characterized by Arias etal. [108, 109]. Several condensed phases of chemically adsorbed 6TG have been described. It has been found that under conditions of complete coverage, the films of chemisorbed molecules significantly inhibit mercury oxide formation at the electrode [108]. 

The inner part of the potential energy surface describes the motion of the chemisorbed molecule. 

The polarization induced by the chemisorbed molecules can be transmitted to the hydrogen molecules in the upper part of the layer, and polarizable substances other than hydrogen can also serve as mediators of polarization transmission as shown in Fig. 27. 

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