Active sites, in catalysts

Models of active sites in catalysts formed in systems such as ZnEt2 MeOCH2CH(Me)OH (2 3) [29], ZnEt2 C6H3(t - Bu)(OH)2 PhOH (2 1 1) [65] and ZnEt2-C6H3(OH)3 (2 1) [31,73] have been proposed schematically [1,74] as follows ... 

As far as the kinetics and mechanistic aspects of oxygen reduction on these non-noble metal electrocatalysts are concerned, it has been shown that these catalysts may reduce O2 to water with an apparent number of electrons transferred, n, that may reach values very close to 4. This is especially true for Fe-based electrocatalysts made either from Fe-N4 chelates or from cheaper Fe salt precursors. It seems also that the Fe-N2/C catalytic site, which is the most active site in catalysts obtained after a pyrolysis temperature > 800°C, is characterized by a low release of peroxide. Co-based catalysts release, on average, more peroxide than the corresponding Fe-based materials. Studies that were undertaken to decouple the direct 4-electron reduction of oxygen to water from the successive 2 X 2-electron reduction indicate that the direct 4-electron reduction path may be important for these catalysts. This result is in agreement with the quantum theoretical approach of Anderson and Sidik about a model of the pyrolyzed... 

In this article we critically review most of the literature concerning non-catalyzed, proton-catalyzed and metal-catalyzed polyesterifications. Kinetic data relate both to model esterifications and polyeste-rificatiom. Using our own results we analyze the experimental studies, kinetic results and mechanisms which have been reported until now. In the case of Ti(OBu)f catalyzed reactions we show that most results were obtained under experimental conditions which modify the nature of the catalyst. In fact, the true nature of active sites in the case of metal catalysts remains largely unknown. 

Figure 4.7. Schematic representation of the location of electrocatalytically and catalytically active sites in a section perpendicular to the catalyst film-solid electrolyte interface.

Microporous catalysts are heterogeneous catalysts used in catalytic converters and for many other specialized applications, because of their very large surface areas and reaction specificity. Zeolites, for example, are microporous aluminosilicates (see Section 14.19) with three-dimensional structures riddled with hexagonal channels connected by tunnels (Fig. 13.38). The enclosed nature of the active sites in zeolites gives them a special advantage over other heterogeneous catalysts, because an intermediate can be held in place inside the channels until the products form. Moreover, the channels allow products to grow only to a particular size. 

Equation (1.20) is frequently used to correlate data from complex reactions. Complex reactions can give rise to rate expressions that have the form of Equation (1.20), but with fractional or even negative exponents. Complex reactions with observed orders of 1/2 or 3/2 can be explained theoretically based on mechanisms discussed in Chapter 2. Negative orders arise when a compound retards a reaction—say, by competing for active sites in a heterogeneously catalyzed reaction—or when the reaction is reversible. Observed reaction orders above 3 are occasionally reported. An example is the reaction of styrene with nitric acid, where an overall order of 4 has been observed. The likely explanation is that the acid serves both as a catalyst and as a reactant. The reaction is far from elementary. 

The SCR catalyst is considerably more complex than, for example, the metal catalysts we discussed earlier. Also, it is very difficult to perform surface science studies on these oxide surfaces. The nature of the active sites in the SCR catalyst has been probed by temperature-programmed desorption of NO and NH3 and by in situ infrared studies. This has led to a set of kinetic parameters (Tab. 10.7) that can describe NO conversion and NH3 slip (Fig. 10.16). The model gives a good fit to the experimental data over a wide range, is based on the physical reality of the SCR catalyst and its interactions with the reacting gases and is, therefore, preferable to a simple power rate law in which catalysis happens in a black box . Nevertheless, several questions remain unanswered, such as what are the elementary steps and what do the active site looks like on the atomic scale ... 

The oxidation of phenol, ortho/meta cresols and tyrosine with Oj over copper acetate-based catalysts at 298 K is shown in Table 3 [7]. In all the cases, the main product was the ortho hydroxylated diphenol product (and the corresponding orthoquinones). Again, the catalytic efficiency (turnover numbers) of the copper atoms are higher in the encapsulated state compared to that in the "neat" copper acetate. From a linear correlation observed [7] between the concentration of the copper acetate dimers in the molecular sieves (from ESR spectroscopic data) and the conversion of various phenols (Fig. 5), we had postulated [8] that dimeric copper atoms are the active sites in the activation of dioxygen in zeolite catalysts containing encapsulated copper acetate complexes. The high substratespecificity (for mono-... 

Evaluating the results a clear kinetic picture of the catalysts has been obtained. In the steady state the active sites in Fe- and Cu-ZSM-5 are nearly fully oxidized, while for Co only -50% of the sites are oxidized. The former catalysts oporate in an oxidation reduction cycle, Fe /Fe and CuVCu. Coi in zeolites is hardly oxidized or reduced, but ESR studies on diluted solid solutions of Co in MgO indicate that Co -0 formation is possible, rapidly followed by a migration of the deposited oxygen to lattice oxygen and reduction back to Co [36]. For Fe-ZSM-5 such a migration has been observed, so a similar model can be proposed for the zeolitic systems. Furthermore, it is obvious that application of these catalysts strongly depends on the composition of the gas that has to be treated. 

A hypothesis that edge and corner sites work as active sites can explain why turn over frequency (TOF), which is defined as the reaction rate per one active site, in the case of metal catalysts, per surface exposed metal atom, increases with a decrease in the diameter of gold particles. However, it fails to explain the significant contribution of support materials and the contact structure of gold NPs. It seems to be reasonable that those edges and corners act as the sites for adsorption of one of the reactants, for example, CO in its oxidation. 

In the case of palladium particles supported on magnesium oxide, Heiz and his colleagues have shown,29 in an elegant study, a correlation between the number of palladium atoms in a cluster and the selectivity for the conversion of acetylene to benzene, butadiene and butane, whereas in the industrially significant area of catalytic hydrodesulfurisation, the Aarhus group,33 with support from theory, have pinpointed by STM metallic edge states as the active sites in the MoS2 catalysts. 

Bromley, S.T., Catlow, C.R.A. and Maschmeyer, Th. (2003) Computational modeling of active sites in heterogeneous catalysts, CatTech 7, 164. 

Centi, G., Cerrato, G., D Angelo, S. et al. (1996) Catalytic behavior and nature of active sites in copper-on-zirconia catalysts for the decomposition of N20, Catal. Today 27, 265. 

This chapter reviews the recent progress in in situ STM studies of model catalysts. From revealing reaction pathways to delineating active sites, in situ STM studies in UHV and on extended surfaces have demonstrated their power to solve fundamental questions in catalysis and enhance our understanding of the elementary steps of... 

An enzyme consists of a polypeptide chain with a particular spatial configuration specific to that sequence of amino acids. The molecule twists and turns, forming structural features that are catalytically active, these being known as active sites. There may be more than one active site per enzyme molecule. Sometimes an auxiliary catalyst, known as a coenzyme, is also needed. Apparently, only the relevant active site of the enzyme comes into contact with the substrate and is directly involved in the catalysed reaction. The active site consists of only a few amino acid residues. These are not necessarily adjacent to one another in the peptide chain but may be brought into proximity by the characteristic folding of the enzyme structure. The active site may also include the coenzyme. The remainder of the enzyme molecule fulfils the essential function of holding the components of the active site in their appropriate relative positions and orientation. 

Higher molecular weight materials are typically prepared using coordination catalysts, such as the calcium amide-alkoxide system developed by Union Carbide,932 and double metal cyanide initiators, e.g., [Zn(Fe(CN)6)].933 The nature of the active sites in these catalysts is not well understood and only a few studies have been published outside the patent literature.810,934,935... 

Arya et al. used solid phase synthesis to prepare immobilised dendritic catalysts with the rhodium centre in a shielded environment to mimic nature s approach of protecting active sites in a macromolecular environment (e.g. catalytic sites inside enzymes) [51], Two generations PS immobilised rhodium-complexed dendrimers, 6 and the more shielded 7, were synthesised.The PS resin immobilised rhodium-complexed dendrimers were used in the hydroformylation of styrene, p-methoxystyrene, vinyl acetate and vinyl benzoate using a total pressure of 70 bar 1 1 CO/H2 at 45 °C in CH2C12. 

Alkalization of iron catalysts causes two different effects. The selectivities of 1-alkenes are raised and both the growth probability a2 and the fraction f2 are markedly increased, as already shown in Figure 11.2. Detailed studies on the promoter effect of alkali have revealed the effect on 1-alkene selectivity to saturates at 1 mass% of K2C03, while the effect on f2 already begins at 0.2 mass% of K2C03.1213 This difference points to specific active sites in Fischer-Tropsch syn-... 

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