Chemisorbed intermediates surface bonding

Chemisorption is vital in catalysis. Transition metals such as Fe, Pd, Pt, Ir, Ni, Co, Cr, Mn, Ti, Hf, Zr, V, Nb, Mo, W, Ru, and Os have the ability to chemisorb simple molecules such as 02, CO, H2, C02, C2H4, and N2 [24,25,27], However, if chemisorption is very strong, the catalytic sites are blocked. Therefore, it is necessary that an intermediate between weak chemisorption, when there is no reaction, and strong chemisorption, when the sites are blocked [24,25], is available. In this sense, the first d-block metals form especially stable surface bonds, while the noble metals form weak bonds. These properties are unfavorable to catalysis. Hence, the best metallic catalysts are in between these two groups [27],... 

NH, NHj, NH3, and H species are together larger than the free-site fraction so that Langmuir-Hinshelwood conditions, with only one significant chemisorbed intermediate, do not obtain. In fact, quite early work had already indicated 54) that, in technical catalysis for NH3 synthesis, it is the bonding of Nj (as N) to the catalyst surface which determines the overall rate of the reaction. Correspondingly (55), at moderate temperatures at W, NH3 decomposes giving imide and nitride species on the surface. The rate of decomposition of the nitride species (chemisorbed N) as an intermediate in the NH3 synthesis reaction at Fe was shown by Mittasch et al. (5(5) to be equal to that of NH3 production. 

Transmission infrared spectroscopic investigations of the interaction of PH3 with Alg03-supported Rh surfaces indicated that PH3 initially adsorbs in a molecular state at 90 K and decomposes to PHg and PH at 100 K, probably via an intermediate hydrogen-bonded chemisorbed state of molecular PH3 [44]. PH3 can displace CO on these surfaces, and the formation of Rh(CO)PH3 was verified spectroscopically [45]. Displacement of CO or NO by PH3 was also found on Rh(IOO) surfaces [43]. 

Pre-treatment of the cathode produces a finely divided copper surface and this catalyses the hydrogenolysis of phenylhydroxylamine to aniline which becomes the principal reaction product. The reaction probably involves promotion of nitrogen-oxygen bond cleavage in the chemisorbed phenylhydroxylamine intermediate. 

Another PES topology for molecular dissociation occurs when an intermediate molecularly chemisorbed state lies parallel to the surface between the physisorption well and the dissociated species as shown in Figure 3.2(b). This molecular state is usually described in terms of a diabatic correlation to a state formed by some charge transfer from the surface to the molecule [16]. In this case, there can be two activation barriers, V] for entry into the molecular chemisorption state of depth Wx and barrier V2 for dissociation of the molecularly chemisorbed state. This PES topology is relevant to the dissociation of some it bonded molecules such as 02 on metals, although this is often an oversimplification since distinct molecularly adsorbed states may exist at different sites on the surface [17]. In some cases, V < 0 so that no separate physisorbed state exists [18]. If multiple molecular chemisorption... 

Metal cluster complexes containing vinylidene ligands have been considered as models of species present when olefins or alkynes are chemisorbed on metal surfaces (114). Vinylidene has been detected in reactions of ethylene or acetylene with Fe(100), Ni( 111), and Pt(l 11) surfaces (115), and was shown to be an intermediate by theoretical studies on a manganese surface (116). The facile cleavage of C-H bonds which occurs in these systems, together with hydrogen addition or abstraction, also occurs on metal clusters. Typical of the reactions considered is the hydrogen transfer reaction... 

Figure 6 The rotation mechanism occurs via a allylic intermediate. a) The hydrogen attached to C3 can either bond to a surface Pt atom or recombine with a chemisorbed D atom to form gaseous HD.

In cases where two bonds are cleaved, it is often suggested that the cleavage occurs in one step. This assumption is based on the fact that a stepwise reaction would lead to the formation of either cyclopropyl or cyclobutyl derivatives which are usually inert under the conditions employed. However, it is well established that the pervasive hydrogen species chemisorbed on transition metal surfaces is the hydrogen atom . Thus the mechanism could still be stepwise since the cyclic intermediates are obtained as highly reactive radicals which can further add hydrogen in a series of successive steps. 

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