Chemisorption of molecules

Figure 5.3 (a) Structure-determining factors in SAMs. Chemisorption of molecule onto substrate (1), intermolecular interactions (2) intramolecular conformational degrees of freedom (3) and interactions of SAM with the environment (4). 

Catalysis of metal-cluster compounds is a field of considerable interest (Lewis Green, 1982 Haggin, 1982). Metal clusters (Section 6.6) containing aggregates of metal atoms surrounded by ligands provide model systems that simulate surface intermediates formed by chemisorption of molecules on metal surfaces (Muetterties et al, 1979). An example of a cluster catalyst is Os3(CO),2, which was found to be active in the... 

However, there are other possibilities. What if instead of an unsuccessful overlap of orbitals the overlap occurs in a suitable way, and instead of repulsion, attraction between the ion and the electrode atoms results In this case strong bonds may be formed between the ion and the electrode. These bonds are the result of donation or acceptance of electrons by the ion, and are responsible for the chemisorption of molecules. 

Mineral grinding leads to distorsion of chemical and ionic bonds between atoms and ions. In the fracture areas binding and coordination states get asymmetric, and new electron and electric valences occur. Spontaneous reactions in the crystalline structure and with contact phases are the consequence of the distorsion. Surface distorsion of the crystalline structure may be diminished or completely abolished. At the same time, the free surface energy decreases due to polarization of surface ions. These ions are redistributed in the inner or outer layer of the crystalline surface and/or due to chemisorption of molecules and ions1. All these changes occur side by side, but one of them can suppress the effect of the others in a decisive manner. 

Figure 1 One-dimension potential energy diagram for precursor-mediated chemisorption of molecule R-H. The dotted line barrier above the vacuum zero is for an activated system, whereas the solid line barrier below the vacuum zero is for a facile system. Ed is the activation energy of the physisorbed state and Er is the activation barrier to the chemisorbed state.

The question of the possibility of coordinative chemisorption of molecules on a tetracoordinated silicon atom of Si02 is still extensively discussed in the literature. The assumption of the existence of such adsorption is sometimes used to explain various transformations of molecules on the surface of silica gel. Even this circumstance itself draws attention to this problem. 

There is a vast literature on the application of EHT to surface chemistry using model clusters. There are also recent reviews on this subject./104,105/ We shall discuss next specific examples relevant to chemisorption of molecules on metal surfaces for illustrative purposes. 

Much of our effort involves studies of the chemical behavior of dusters not only as a function of size, but also as a function of metal type, charge state (neutral, cationic or anionic), and reagent molecule. There are two different operating conditions for which we probe the chemisorption of molecules onto clusters as a function of duster size. The first is such that the rate of reaction is kinetically controlled. Here we obtain information about the rate at which the first reagent molecule chemisorbs onto the otherwise bare cluster. In the second case, chemisorption studies are carried out under near steady-state conditions. In this instance we attempt to determine how many molecules a particular size cluster can bind, i.e. the degree of saturation. 

Studies of the reactivity in terms of chemisorption of molecules on surfaces and clusters have been an important subject in surface and cluster science since the 60s. Knowledge of the reactivity of clusters is also an important property in the objective to use clusters as building blocks in new materials and as catalysts. We have in our group in recent years [70,71] studied the reactivity towards various diatomic molecules and compared those results with molecular calculations. 

All chemical reactions of bare metal clusters in the gas phase that have been studied to date can be characterized as addition reactions. These reactions yield products or adducts that are the result of addition, or addition followed by subsequent elimination, and are the cluster analogs of the chemisorption of molecules onto metal surfaces. The nature of these experiments precludes the detection of gas-phase reaction products that have desorbed from the cluster and to date no example of catalytic chemistry using gas-phase clusters has been reported. 

I. Chemisorption of molecules with axial anchor groups... 

Considering that the Elovich behaviour usually describes the chemisorption of molecules, one may argue that in the situations where the Elovich equation is observed, chemisorption is an activated process. 

J.E. Crowell. Chemical Modification of Surfaces The Effect of Potassium on the Chemisorption of Molecules on Transition Metal Crystal Surfaces. Ph.D. thesis. University of California, Berkeley, 1984. 

Physisorption or chemisorption of molecules and atoms has been employed to determine the fractal nature of surfaces. Two approaches can be adopted. One uses a probe molecule or atom to measure the surface area as a function of particle size. The other probes the surface area at constant particle size but with molecules or atoms of different sizes. Figure (5.20) illustrates the first approach for the chemisorption of... 

Condensation of metal atoms on metal surfaces is fairly simple, as conpared to the chemisorption of molecules. There is no activation energy for adsorption (E ac = 0 in Eq. (23) of Chap. 1) and consequently the metal atoms approaching the surface are directly attracted towards the adsorption minimum. The adsorption energy is of the order of a few eV/atom and thus bonding is substantial. The absence of i ac and the high value of lead to temperature independent sticking coefficients of 5 = 1 up to elevated... 

When dealing with electronic structure, it is important to distinguish between transition metal and non-transition metal oxides, sinee the former class presents a wider variation of electronic and fimctional behaviours, also including important aspects such as variation of the oxidation state, the formation of defects and the chemisorptions of molecules on the oxide surface. The electronic structure of the oxide strongly affect some outstanding functional properties such as the optical, the electric, the magnetic and the catalytic ones and the different models to approach their theoretical investigation have been exhaustively described in references [2,13, 14]. 

Adsorption and/or chemisorption of molecules from the gas phase to the surface, and the subsequent formation of carbon atoms. 

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