Chemisorption dissociative

When a molecule adsorbs to a surface, it can remain intact or it may dissociate. Dissociative chemisorption is conmion for many types of molecules, particularly if all of the electrons in the molecule are tied up so that there are no electrons available for bonding to the surface without dissociation. Often, a molecule will dissociate upon adsorption, and then recombine and desorb intact when the sample is heated. In this case, dissociative chemisorption can be detected with TPD by employing isotopically labelled molecules. If mixing occurs during the adsorption/desorption sequence, it indicates that the mitial adsorption was dissociative. 

Li Y L ef a/1995 Experimentai verification of a new mechanism for dissociative chemisorption atom abstraction Phys. Rev. Lett. 74 2603... 

NakatsujI H and Nakal H 1990 Theoretical study on molecular and dissociative chemisorptions of an O2 molecule on an Ag surface dipped adcluster model combined with symmetry-adapted cluster-configuration interaction method Chem. Phys. Lett. 174 283-6... 

H] Asscher M, Haase G and Kosloff R 1990 Tunneling mechanism for the dissociative chemisorption of Nj on metal surfaces Vacuum 41 269... 

Kroes G J, Wiesenekker G, Baerends E J, Mowrey R C and Neuhauser D 1996 Dissociative chemisorption of H2 on Cu(IOO)—a four-dimensional study of the effect of parallel translational motion on the reaction dynamics J. Chem. Phys. 105 5979... 

Alford J M, Weiss F D, Laaksonen R T and Smalley R E 1986 Dissociative chemisorption of H2 on niobium cluster ions. A supersonic cluster beam FT-ICR experiment J. Rhys. Chem. 90 4480... 

Jensen J A, Yan C and Kummel A C 1995 Energy dependence of abstractive versus dissociative chemisorption of fluorine molecules on the silicon (111)-(7 7) surface Science 267 493-6... 

The microcalorimetric measurements of Della Gatta and his co-workers in their investigation of the interaction of water vapour with highly dehydroxylated y-alumina confirm that in this system also, the nondissocia-tive chemisorption of water is nonactivated, whilst the dissociative chemisorption is always activated. Thus the pseudo-equilibrium between the two chemisorbed states is displaced towards dissociative chemisorption as the temperature is increased above 150 C. 

Fischer-Tropsch ohgomerization of CO -1- H9 to make hydrocarbons and oxygenated compounds was originally catalyzed by cobalt, which forms the active carbonyl, but now iron promoted by potassium is favored. Dissociative chemisorption of CO has been observed in this process. 

The molecular chemisorption of CO on various alkali-modified metal surfaces has been studied extensively in the literature. It is well established that alkali modification of the metal surface enhances both the strength of molecular chemisorption and the tendency towards dissociative chemisorption. This effect can be attributed to the strongly electropositive character of the alkali, which results in donation of electron density from the alkali to the metal and then to the adsorbed CO, via increased backdonation into the... 

Many molecules undergo partial oxidation on adsorption and many alkanes and alkenes are believed to yield an adsorbed CHO group on adsorption (Petrii, 1968). These processes usually lead to the complete oxidation of the organic molecule to carbon dioxide and few workers have attempted to halt the reaction at an intermediate stage. Hence, although there are undoubtedly possibilities for using dissociative chemisorption for synthetic reactions, this chapter will not consider these processes further. 

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. 

Methane is a stable molecule and therefore hard to activate. As a result the sticking probability for dissociative chemisorption is small, of the order of 10 only, and ruthenium is more reactive than nickel. However, a stretched overlayer of nickel is significantly more active than nickel in its common form, in agreement with expectation. 

Studying the kinetics of the interaction of N2 with the Ru catalysts revealed that the Cs promoter enhances both the rate of dissociative chemisorption and the rate of recombinative desorption. Ru catalysts were found to be rather inactive for NH3 synthesis without alkali... 

The influence of the Ni atoms becomes clear from a comparison of the actual reaction path, which consists of physical adsorption and subsequent dissociative chemisorption, with the theoretical alternative reaction path, consisting of dissociation of H2 followed by the formation of two Ni-H bonds. H2 is a very stable molecule and, as a consequence, the potential energy of the dissociated H-atoms is very high. In moving to the adsorbed state, Ni-... 

In the foregoing it has been discus.sed how a metal can dissociate H2. Fig. 3.6 explains the principle of catalysis with an example of the hydrogenation of ethylene, for which dissociative chemisorption of hydrogen is an elementary step in the catalytic cycle. The adsorption of alkenes, on the other hand, is non-dissociative. 

Kardash D, Korzeniewski C. 2000. Temperature effects on methanol dissociative chemisorption and water activation at polycrystalhne platinum electrodes. Langmuir 16 ... 

Shin J, Tomquist WJ, Korzeniewski C, Hoaglund CS. 1996. Elementary steps in the oxidation and dissociative chemisorption of ethanol on smooth and stepped surface planes of platinum electrodes. Surf Sci 364 122. 

Thermal accommodation 1st stage of chemisorption molecular transient Dissociative chemisorption with formation of hot transient O 5 (s)... 

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