Reactions Model Catalysts

Catalysis and Automotive Pollution Control III Studies in Surface Science and Catalysis, Vol. 96 1995 Elsevier Science B.V. 

Laboratoire de Genie des Procedes Catalytiques CNRS-CPE, 43 Bd dull Novembre, BP 2077 F-69616 Villleurbanne Cedex 

Numerical simulations could be advantageously substituted for some expensive experiments with an engine bench. However the reliability of the numerical results depends on the assmnptions made and on the qualify of the physical and chemical parameters involved in the model. Presently, converter models work well for specified experimental conditions, but there is no decisive proof of their predictive ability in a wide range of conditions (Pattas et al., 1994). This is because four important topics deserve further attention  

The experimental and interpretation problems related to these topics are reviewed in the next sections. 

Converter models require reliable kinetic expressions that account for the composition and temperature dependence of the reaction rates. The temperature dependence is essential for predicting light-off performance, whereas the consequences of the concentration dependence are more difficult to assess. 

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The reaction was studied in the absence, and presence, of (MeO)2AlMe as a model catalyst for the BINOL-AlMe system. The change in relative energy for the concerted hetero-Diels-Alder reaction, and formation of the hetero-Diels-Alder adduct 11 via a Mukaiyama aldol reaction, is shown in Fig. 8.13. The conclusion of the study was that in the absence of a catalyst the concerted reaction is the most... 

Both questions have been recently addressed via a surface diffusion-reaction model developed and solved to describe the effect of electrochemical promotion on porous conductive catalyst films supported on solid electrolyte supports.23 The model accounts for the migration (backspillover) of promoting anionic, O5, species from the solid electrolyte onto the catalyst surface. The... 

Scott et al. [45] prepared diimine derivatives of 2,2 -diamino-6,6 -dimethyl-biphenyl (as structure 37 in Scheme 19) as copper chelates for the catalyzed cyclopropanation reaction. All catalysts were active in this reaction but enan-tioselectivities varied importantly according to the substitution pattern of the imine aryl group only ortho-substituted ligands (by chloride or methyl groups) led to products with measurable enantioselectivity for the model test reaction (up to 57% ee with 37). 

Zhu L, Susac D, Teo M, Wong KC, Wong PC, Parsons RR, Bizzotto D, Mitchell KAR, Campbell SA (2008) Investigation of CoSa-based thin films as model catalysts for the oxygen reduction reaction. J Catal 258 235-242... 

Although much progress has been made toward understanding the nature and probable catalytic behavior of active sites on CoMo/alumlna catalysts, much obviously remains to be accomplished. Detailed explanation of the acidic character of the reduced metal sites evidently most important In HDS, and presumably In related reactions, must await the Increased understanding which should come from studies of simplified model catalysts using advanced surface science techniques. Further progress of an Immediately useful nature seems possible from additional Infrared study of the variations produced In the exposed metal sites as a result of variations In preparation, pretreatment, and reaction conditions. 

A highly detailed picture of a reaction mechanism evolves in-situ studies. It is now known that the adsorption of molecules from the gas phase can seriously influence the reactivity of adsorbed species at oxide surfaces[24]. In-situ observation of adsorbed molecules on metal-oxide surfaces is a crucial issue in molecular-scale understanding of catalysis. The transport of adsorbed species often controls the rate of surface reactions. In practice the inherent compositional and structural inhomogeneity of oxide surfaces makes the problem of identifying the essential issues for their catalytic performance extremely difficult. In order to reduce the level of complexity, a common approach is to study model catalysts such as single crystal oxide surfaces and epitaxial oxide flat surfaces. 

We have reviewed experiments on two classes of systems, namely small metal particles and atoms on oxide surfaces, and Ziegler-Natta model catalysts. We have shown that metal carbonyls prepared in situ by reaction of deposited metal atoms with CO from the gas phase are suitable probes for the environment of the adsorbed metal atoms and thus for the properties of the nucleation site. In addition, examples of the distinct chemical and physical properties of low coordinated metal atoms as compared to regular metal adsorption sites were demonstrated. For the Ziegler-Natta model catalysts it was demonstrated how combination of different surface science methods can help to gain insight into a variety of microscopic properties of surface sites involved in the polymerization reaction. 

Heterogeneously catalyzed hydrogenation of alkenes is generally considered to be a structure-insensitive reaction, as was deduced from numerous studies on more or less complex model catalyst systems [40-54]. However, the following sections will give examples of the opposite case. 

Gloaguen E, AndoUatto E, Durand R, Ozil P. 1994. Kinetic-study of electrochemical reactions at catalyst-recast ionomer interfaces from thin active layer modeling. J Appl Electrochem 24 863-869. 

Henry CR. 1998. Surface studies of supported model catalysts. Surf Sci Rep 31 235-325. Henry C. 2003. Adsorption and reaction at supported model catalysts. In Wieckowski A, Savinova ER, Vayenas CG. editors. Catalysis and Electrocatalysis at Nanoparticle Surfaces. New York Marcel Dekker. 

Kasemo B, Johansson S, Persson H, Thommahlen P, Zhdanov VP. 2000. Catalysis in the nm-regime manufacturing of supported model catalysts and theoretical studies of the reaction kinetics. Top Catal 13 43-53. 

The theoretical approach involved the derivation of a kinetic model based upon the chiral reaction mechanism proposed by Halpem (3), Brown (4) and Landis (3, 5). Major and minor manifolds were included in this reaction model. The minor manifold produces the desired enantiomer while the major manifold produces the undesired enantiomer. Since the EP in our synthesis was over 99%, the major manifold was neglected to reduce the complexity of the kinetic model. In addition, we made three modifications to the original Halpem-Brown-Landis mechanism. First, precatalyst is used instead of active catalyst in om synthesis. The conversion of precatalyst to the active catalyst is assumed to be irreversible, and a complete conversion of precatalyst to active catalyst is assumed in the kinetic model. Second, the coordination step is considered to be irreversible because the ratio of the forward to the reverse reaction rate constant is high (3). Third, the product release step is assumed to be significantly faster than the solvent insertion step hence, the product release step is not considered in our model. With these modifications the product formation rate was predicted by using the Bodenstein approximation. Three possible cases for reaction rate control were derived and experimental data were used for verification of the model. 

Table 39.6. Activity at 623 K of some model catalysts in the synthesis of MDB by reaction of pyrocatechol (PYC) and diethoxymethane (DEM) ( Yield and...

Figure 9.7 Temperature-programmed reaction (TPR) spectra for CO oxidation at a series of model catalysts prepared by the soft landing of mass-selected Aun and AunSr cluster ions on MgO(lOO) thin films which are vacancy free (typically 1 % of a monolayer), (a) MgO (b) Au3Sr (c) Au4 (d) Au8. Also shown is the chemical reactivity R of pure Aun and AunSr clusters with 1 < n < 9. (Reproduced from Ref. 21).

Johanek, V., Schauermann, S., Laurin, M. et al. (2004) On the role of different adsorption and reaction sites on supported nanoparticles during a catalytic reaction NO decomposition on a Pd/alumina model catalyst , J. Phys. Chem. B, 108, 14244. 

In order to determine the major catalytic activity of the preceding model catalyst, in the three functions of the model, the three reactions were studied separately on each catalyst (Table 5.1). The comparison of the results permits to identify the most active site, for each function, when the complete Cat I CoPd/HMOR catalyst is working. 

As we have seen in the previous chapter, the apparent topography and corrugation of thin oxide films as imaged by STM may vary drastically as a function of the sample bias. This will of course play an important role in the determination of cluster sizes with STM, which will be discussed in the following section. The determination of the size of the metallic nanoparticles on oxide films is a crucial issue in the investigation of model catalysts since the reactivity of the particles may be closely related to their size. Therefore, the investigation of reactions on model catalysts calls for a precise determination of the particle size. If the sizes of the metal particles on an oxidic support are measured by STM, two different effects, which distort the size measurement, have to be taken into account. 

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