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Catalysts heterogeneous reaction mechanisms

Development of powerful spectroscopy and microscopy techniques, which allow us to study underlying chemical transformations that govern the performance of catalysts, including reaction mechanisms and the evolution of catalyst structure, with high spatial and temporal resolutions and at relevant conditions [2-6]. Development of density functional theory (DFT) methodology, which is utilized to study chemical transformations at the elementary step level with reasonable accuracy and efficiency [7]. DFT is particularly well suited for the treatment of extended metallic structures, which are often ideal model systems for heterogeneous catalytic processes [8-11]. [Pg.276]

Besides the heavy chemical industry, where catalysis is a dominant feature of most conversion processes, enzyme catalysis is a critical component of bio-chemical processes. All that was said about mechanisms of catalytic reactions applies to enzyme catalysis. As can be expected, there are additional factors in enzyme catalysis that complicate matters. Many enzymatic reactions depend on factors such as pH, ionic strength, co-catalysts and so on that do not normally play a role in conventional heterogeneous catalysis. Despite this, the understanding of mechanisms in enzyme catalysis has outpaced that in heterogeneous catalysis and can now serve as a guide to the search for heterogeneous reaction mechanisms. [Pg.57]

There are of course also parallels between homogeneous and heterogeneous transition metal catalysts. Many reaction mechanisms of homogeneous and heterogeneous catalysts exhibit similarities with regard to the intermediates and the product distribution. [Pg.12]

Syndiotactic polystyrene (SPS) can be readily polymerized using homogeneous or heterogeneous metallocene catalysts, based on group 4 metal compounds, especially titanium compounds like T1CI4, CpTiClj, and Cp Ti(OCH3)3 with methyl aluminoxane (MAO) as cocatalyst [1-3]. The recent developments of transition metal catalysts and reaction mechanisms are discussed in earlier chapters. This chapter will be focused on the quantitative aspects of SPS polymerization kinetics and related physical and chemical phenomena. [Pg.140]

As demonstrated above, PHIP can be a very useful tool in the studies of homogeneous catalytic processes that involve activation of molecular hydrogen. However, as most of the industrial catalytic processes are heterogeneous, it would be highly desirable to extend PHIP to the hypersensitive NMR studies of heterogeneous catalysts and catalytic reactions. The requirements for PHIP observations imposed on the catalysts and reaction mechanisms are rather stringent (see Section 7.2). Therefore, an obvious first step in this direction was to address immobilized metal... [Pg.155]

This is an exothermic reaction, and both homogeneous (radical or cationic) and heterogeneous (soHd catalyst) initiators are used. The products range in molecular weight from below 1000 to a few million (see Olefin polymers). Reaction mechanisms and reactor designs have been extensively discussed (10-12). [Pg.432]

The chemical properties of oxide surfaces have been studied by several methods, including oxygen exchange. This method has been used to investigate the mechanisms of heterogeneous reactions for which oxides are active catalysts [36]. The dimerization step does not necessarily precede desorption and Malinin and Tolmachev [634], in one of the few reviews of decomposition kinetics of solid metal oxides, use this criterion to distinguish two alternative reaction mechanisms, examples being... [Pg.146]

The catalyst deactivates, but after four runs the conversion is still significantly higher (> 99% after 2 h) as compared with the uncatalyzed reaction. Moreover, the Z-selectivity in all four runs is higher than 80%, whereas in the uncatalyzed reaction, it is typically only 30% (Z). The fact that the solid powder can be used several times furthermore supports the fact that the reaction mechanism is heterogeneous. The reason for the deactivation is unknown. A disadvantage of the nanoparticles is the difficulty of separation. Thus, in some cases the particles form col-... [Pg.290]

A reason for using microkinetics in heterogeneous catalysis is to have comprehensive kinetics and a transparent reaction mechanism that wonld be useful for re or design or catalyst development. Furthermore, in the long run, the exparimental effort to develop a microkinetics scheme can be less than that for a Langmuir-Hinshelwood (LH) or powa--law scheme because of the more fundamental nature of the reaction kinetics parameters. [Pg.677]

Heterogeneous reactions are complicated because the reacting species must be transferred from one phase to another before the reaction can occur. Despite much research, chemists still have limited knowledge about the mechanisms of reactions that involve heterogeneous catalysts. However, it is known that heterogeneous catalysis generally proceeds in four steps, as illustrated in Figure 15-21 for the conversion of NO into N2 and O2. ... [Pg.1106]

Both heterogeneous and homogeneous catalysts have been found which allow the hydroamination reaction to occur. For heterogeneously catalyzed reactions, it is very difficult to determine which type of activation is involved. In contrast, for homogeneously catalyzed hydroaminations, it is often possible to determine which of the reactants has been activated (the unsaturated hydrocarbon or the amine) and to propose reaction mechanisms (catalytic cycles). [Pg.93]

Heterogeneous catalysts can thus have a major influence on selectivity. Changing the catalyst can change the relative influence on the primary and by product reactions. This might result directly from the reaction mechanisms at the active sites or the relative rates of diffusion in the support material or a combination of both. [Pg.116]

It is true, however, that many catalytic reactions cannot be studied conveniently, under given conditions, with usual adsorption calorimeters of the isoperibol type, either because the catalyst is a poor heat-conducting material or because the reaction rate is too low. The use of heat-flow calorimeters, as has been shown in the previous sections of this article, does not present such limitations, and for this reason, these calorimeters are particularly suitable not only for the study of adsorption processes but also for more complete investigations of reaction mechanisms at the surface of oxides or oxide-supported metals. The aim of this section is therefore to present a comprehensive picture of the possibilities and limitations of heat-flow calorimetry in heterogeneous catalysis. The use of Calvet microcalorimeters in the study of a particular system (the oxidation of carbon monoxide at the surface of divided nickel oxides) has moreover been reviewed in a recent article of this series (19). [Pg.238]

Moreover, the use of heat-flow calorimetry in heterogeneous catalysis research is not limited to the measurement of differential heats of adsorption. Surface interactions between adsorbed species or between gases and adsorbed species, similar to the interactions which either constitute some of the steps of the reaction mechanisms or produce, during the catalytic reaction, the inhibition of the catalyst, may also be studied by this experimental technique. The calorimetric results, compared to thermodynamic data in thermochemical cycles, yield, in the favorable cases, useful information concerning the most probable reaction mechanisms or the fraction of the energy spectrum of surface sites which is really active during the catalytic reaction. Some of the conclusions of these investigations may be controlled directly by the calorimetric studies of the catalytic reaction itself. [Pg.260]

Dioxygen and oxidized substances react on the surface of the catalyst only. The pure heterogeneous reaction occurs only after diffusion of reactants to the catalytic surface and back diffusion of products from the surface into the solution. A combination of a few mechanisms of such types are possible. [Pg.421]

A reaction mechanism may involve one of two types of sequence, open or closed (Wilkinson, 1980, pp. 40,176). In an open sequence, each reactive intermediate is produced in only one step and disappears in another. In a closed sequence, in addition to steps in which a reactive intermediate is initially produced and ultimately consumed, there are steps in which it is consumed and reproduced in a cyclic sequence which gives rise to a chain reaction. We give examples to illustrate these in the next sections. Catalytic reactions are a special type of closed mechanism in which the catalyst species forms reaction intermediates. The catalyst is regenerated after product formation to participate in repeated (catalytic) cycles. Catalysts can be involved in both homogeneous and heterogeneous systems (Chapter 8). [Pg.155]


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See also in sourсe #XX -- [ Pg.22 , Pg.1253 ]




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Catalyst , reaction mechanism

Catalyst mechanism

Catalysts heterogeneity

Catalysts heterogeneous

Catalysts heterogeneous reactions

Catalysts heterogenous

Heterogeneous reaction

Heterogeneous reactions mechanism

Heterogenized catalysts

Reaction heterogeneous reactions

Reaction mechanisms heterogenous

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