Chemisorption complexe

R. J. Kokes and A. L. Dent Chemisorption Complexes and Their Role in Catalytic Reactions on Transition Metals Z. Knor... 

Inefficient reaction pathway to bond cleavage and chemisorption Complex formation... 

Chemisorption Complexes and Their Role in Catalytic Reactions on Transition Metals... 

Passing in this scheme from the left to the right side we can formulate several problems which have to be studied (1) the transition from state A to B (2) the identification of the chemisorption complexes (3) the reactivity of these complexes (4) their role in the particular catalytic reaction (e.g., blocking of the surface, their mutual interaction) (5) the mechanism of the reaction (the transition from state B to C) (e.g., does the reaction proceed in the chemisorbed layer or can some components react directly from the gas phase, impinging on the chemisorbed species ) and (6) the liberation of the reaction products from the surface into the gas phase or their stability in the surface. We consider as a fundamental -problem the identification of those chemisorption complexes that are responsible for the reaction in the desired direction. 

We ivill discuss the reaction of hydrogen and oxygen on transition metals first. This reaction has been extensively studied in our laboratory 18-32) using evaporated metal films as a catalyst. From our previous considerations it follows that as a consequence of the choice of this particular system we must restrict ourselves to certain problems only. We cannot identify the surface species (we can indirectly indicate only some of them) nor understand completely their role in the reaction. Because of the polycrystalline character of the film, all the experimental results are averaged over all the surface. Several new problems thus arise, such as grain boundaries, and, consequently, the exact physical interpretation of these results is almost impossible it is more or less a speculative one. However, we can still get some valuable information concerning the chemical nature of the active chemisorption complex. The experimental method and the considerations will be shown in full detail for nickel only. For other metals studied in our laboratory, only the general conclusions will be presented here. 

In this section we have discussed several chemisorption complexes from the point of view of the chemical nature of one part of this complex—the one resulting from the gas molecule. However, it follows from the above-mentioned results that the metal atoms, constituting the second part of... 

In the Introduction the problem of construction of a theoretical model of the metal surface was briefly discussed. If a model that would permit the theoretical description of the chemisorption complex is to be constructed, one must decide which type of the theoretical description of the metal should be used. Two basic approaches exist in the theory of transition metals (48). The first one is based on the assumption that the d-elec-trons are localized either on atoms or in bonds (which is particularly attractive for the discussion of the surface problems). The other is the itinerant approach, based on the collective model of metals (which was particularly successful in explaining the bulk properties of metals). The choice between these two is not easy. Even in contemporary solid state literature the possibility of d-electron localization is still being discussed (49-51). Examples can be found in the literature that discuss the following problems high cohesion energy of transition metals (52), their crystallographic structure (53), magnetic moments of the constituent atoms in alloys (54), optical and photoemission properties (48, 49), and plasma oscillation losses (55). 

Nalewajski, R. F.1997. Consistent two-reactant approach to chemisorption complexes in charge sensitivity analysis. In Developments in the Theory of Chemical Reactivity and Heterogeneous Catalysis. (Eds.) W. M. Mortier, and R. A. Schoonheydt, pp. 135-196. Trivandrum Research Signpost. 

C complexes, 32 185-186 CFjHCFjH, 39 340 chemisorption complexes, 32 170-172 CjH, enthalpies, 37 141, 143 "C-labeling studies, 25 166-172 commercial, 6 197 complex molecules, 30 58-72 medium-sized rings, 30 68-72 polymethylcycloalkanes, 30 59-65 substituted aromatics, 30 65-68 cyclic-acyclic product ratio, 30 8-9 cycloalkanes, 30 68-69 function, hydrogen pressure, 30 12, 15-16 hydrocarbon reaction models, 32 202-205 hydrogenolysis and, 23 93, 103 interconversion, 30 81-82 isopentane, 30 17 label scrambling, 30 7, 12-13 mechanism, 30 5-16 bifunctional, 30 4 catalyst particle size and, 30 72-85 concerted, 30 20... 

Inasmuch as parallel catalytic reactions differ in the number of adjacent surface atoms of the active metal that are required for forming the respective chemisorption complexes, it is clear that the reaction requiring the largest ensemble of these atoms will be the most sensitive to alloying with a second metal unable to form such chemisorption bonds (62, 63, S5/as illustrated by the following considerations. 

Fig. 31. (Continued). The complementary reactive (delocalized) IRM for the molecular adsorption (Part A) and transition-state (Part B) chemisorption complexes of Fig. 30. The numerical data are reported in the same order as in Fig. 21. The CT parameters have been calculated for the assumed water (base, B) - rutile (acid, A) electron transfer...

It has been suggested that virtually all samples of MgO (and probably silicates as well) contain small amounts of HjO and CO2 (Freund, 1981). These impurities lead to formation of O species. The species CO4 " has also been postulated to occur on the surface of MgO exposed to COj. This species has recently been studied by ab initio 8CF Hartree-Fock-Roothaan MO calculations, both in its anion form and as the protonated cluster C(0H)4 (Gupta et al., 1981). Its calculated equilibrium bond distance is intermediate between those observed for B emd N in tetrahedral coordination with oxygen, and there seems to be no intrinsic source of instability. Thus, such a species seems stable. However, the calculations indicate a charge on C in 04" " very similar to that in COj, arguing for the formulation O 04 as first approximation to the electronic structure, rather than the C°(04 ) formulation suggested by Freund (1981). Perhaps such a species is better formulated as a chemisorption complex of a anion (a bent, 18-valence-electron system) and an O. . . 0 ... 

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