Heterogeneous catalytic processes methods

Pulse radiolysis and pulsed laser methods are powerful tools to study the reactions at the interface between clusters and solution, such as fast heterogeneous catalytic processes, or transfer of the charges generated inside the particle. 

The heterogeneous reactors with supported porous catalysts are one of the driving forces of experimental research and simulations of chemically reactive systems in porous media. It is believed that the combination of theoretical methods and surface science approaches can shorten the time required for the development of a new catalyst and optimization of reaction conditions (Keil, 1996). The multiscale picture of heterogeneous catalytic processes has to be considered, with hydrodynamics and heat transfer playing an important role on the reactor (macro-)scale, significant mass transport resistances on the catalyst particle (meso-)scale and with reaction events restricted within the (micro-)scale on nanometer and sub-nanometer level (Lakatos, 2001 Mann, 1993 Tian et al., 2004). 

To analyze reaction mechanisms in complex catalytic systems, the application of micropulse techniques in small catalytic packed beds has been used. Christoffel [33] has given an introduction to these techniques in a comprehensive review of laboratory reactors for heterogeneous catalytic processes. Mtlller and Hofmann [59,61] have tested the dynamic method in the packed bed reactor to investigate complex heterogeneous reactions. Kinetic parameters have been evaluated by a method, which employs concentration step changes and the time derivatives of concentration transients at the reactor outlet as caused by a concentration step change at the reactor inlet. 

This book aims to illustrate and discuss the subject of heterogeneous catalysis and to show the current capabilities of the theoretical and computational methods for studing the various steps (diffusion, adsoprtion, chemical reaction, etc.) of a heterogeneous catalytic process. 

Metallic particles displaying a small particle size, around a few nanometres, and narrow size distribution could easily be obtained via the microemulsion technique. Such particles have proved to be active in several classic heterogeneous catalytic processes, in some cases displaying a higher performance than the catalysts prepared following more traditional impregnation methods. Major... 

The disparate time and length scales that control heterogeneous catalytic processes make it essentially impossible to arrive at a single method to treat the complex structural behavior, reactivity and dynamics. Instead, a hierarchy of methods have been developed which can can be used to model different time and length scales. Molecular modeling of catalysis covers a broad spectrum of different methods but can be roughly categorized into either quantum-mechanical methods which track the electronic structure or molecular simulations which track the atomic structme (see the Appendix). 

Heterogeneous catalytic systems offer the advantage that separation of the products from the catalyst is usually not a problem. The reacting fluid passes through a catalyst-filled reactor m the steady state, and the reaction products can be separated by standard methods. A recent innovation called catalytic distillation combines both the catalytic reaction and the separation process in the same vessel. This combination decreases the number of unit operations involved in a chemical process and has been used to make gasoline additives such as MTBE (methyl tertiai-y butyl ether). 

Heterogeneous catalytic gas-phase reactions are most important in industrial processes, especially in petrochemistry and related fields, in which most petrochemical and chemical products are manufactured by this method. These reactions are currently being studied in many laboratories, and the results of this research can be also used for synthetic purposes. The reactions are usually performed [61] in a continuous system on a fixed catalyst bed (exceptionally a fluidized bed). 

It is in the very nature of the catalytic process that the intermediate compound formed between catalyst and reactant is of extreme lability therefore not many cases are on record where the isolation by chemical means, or identification by physical methods, of intermediate compounds has been achieved concomitant with the evidence that these compounds are true intermediaries and not products of side reactions or artifacts. The formation of ethyl sulfuric acid in ether formation, catalyzed by HjSO , and of alkyl phosphates in olefin polymerization, catalyzed by liquid phosphoric acid, are examples of established intermediate compound formation in homogeneous catalysis. With regard to heterogeneous catalysis, where catalyst and reactant are not in the same... 

Research tools and fundamental understanding New catalyst design for effective integration of bio-, homo- and heterogeneous catalysis New approaches to realize one-pot complex multistep reactions Understanding catalytic processes at the interface in nanocomposites New routes for nano-design of complex catalysis, hybrid catalytic materials and reactive thin films New preparation methods to synthesize tailored catalytic surfaces New theoretical and computational predictive tools for catalysis and catalytic reaction engineering... 

In heterogeneous catalysis by metal, the activity and product-selectivity depend on the nature of metal particles (e.g., their size and morphology). Besides monometallic catalysts, the nanoscale preparation of bimetallic materials with controlled composition is attractive and crucial in industrial applications, since such materials show advanced performance in catalytic processes. Many reports suggest that the variation in the catalyst preparation method can yield highly dispersed metal/ alloy clusters and particles by the surface-mediated reactions [7-11]. The problem associated with conventional catalyst preparation is of reproducibility in the preparative process and activity of the catalyst materials. Moreover, the catalytic performances also depend on the chemical and spatial nature of the support due to the metal-support interaction and geometrical constraint at the interface of support and metal particles [7-9]. 

This chapter begins, after this brief Introduction, by considering the different designs of HP IR cell, with particular emphasis on more recent developments. Applications of HP IR spectroscopy to mechanistic studies of catalytic reactions will then be discussed, illustrated by examples of both in situ catalytic investigations and model stoichiometric reactions. The chapter will concentrate on homogeneous catalytic processes. The reader is referred elsewhere for coverage of in situ IR spectroscopic methods in heterogeneous catalysis [1]. 

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