Metal Oxides in Heterogeneous Catalysis

Jacques C. Vedrine, Ghenadii Korotcenkov

• Publisher: Elsevier
• Year: 2018
• Print ISBN: 9780128116319
• eBook ISBN: 9780128116326
• Topic: Technology & Engineering
• Subtopic: Materials Science
• Index: Technology & Engineering

Metal Oxides in Heterogeneous Catalysis is an overview of the past, present and future of heterogeneous catalysis using metal oxides catalysts. The book presents the historical, theoretical, and practical aspects of metal oxide-based heterogeneous catalysis.Metal Oxides in Heterogeneous Catalysis deals with fundamental information on heterogeneous catalysis, including reaction mechanisms and kinetics approaches.There is also a focus on the classification of metal oxides used as catalysts, preparation methods and touches on zeolites, mesoporous materials and Metal-organic frameworks (MOFs) in catalysis. It will touch on acid or base-type reactions, selective (partial) and total oxidation reactions, and enzymatic type reactionsThe book also touches heavily on the biomass applications of metal oxide catalysts and environmentally related/depollution reactions such as COVs elimination, DeNOx, and DeSOx. Finally, the book also deals with future trends and prospects in metal oxide-based heterogeneous catalysis.- Presents case studies in each chapter that provide a focus on the industrial applications- Includes fundamentals, key theories and practical applications of metal oxide-based heterogeneous catalysis in one comprehensive resource- Edited, and contributed, by leading experts who provide perspectives on synthesis, characterization and applications

1

Fundamentals of heterogeneous catalysis

Jacques C. Védrine, Laboratoire de Réactivité de Surface, Sorbonne Université, Université P. & M. Curie, Paris, France

Abstract

This chapter deals with the basic principles of catalytic reactions and focuses on gas-solid heterogeneous catalysis. It includes historical aspects of catalysis in general and heterogeneous catalysis in particular, and the main principles and main parameters of catalytic reactions. In particular, a description of metal oxide catalyst features, of active sites and of catalytic activity and selectivity is given and discussed. Kinetics and main reaction mechanisms involved in catalytic reactions are described, while main industrial processes using metal oxide catalysts in the past and present are summarized. A brief description of metal oxide materials is given with special emphasis on the importance of crystalline structure, structure sensitivity of catalytic reactions, lattice defects, ions and electrons mobility, doping, and use of promoters on their catalytic properties. At last, characterization of heterogeneous catalysts and of catalytic reaction intermediates and a description of the main physical techniques, frequently, even routinely used, are given. Special emphasis is brought about in-situ characterization, i.e. in real working conditions with mass spectrometry and/or gas chromatography analyses on line. Possible relationships between catalytic properties and physical/chemical characteristics of the catalysts and reaction intermediates are discussed, and shown to be rarely reliable.

Keywords

Principles of the catalytic phenomenon; main parameters; metal oxide catalysts; main commercial applications; historical aspects of catalysis; main catalytic reaction mechanisms; main physical techniques of characterization

1.1 Introduction to catalysis [1–3]

It was Berzelius, who first used the word “catalysis” in 1836 from the Greek word “καταλεινν” (=loose down, dissolve) by analogy to the word “analysis” in order to rationalize experimental well-known observations such as starch transformation to sugar by acids, decomposition of H₂O₂ by metals, ethanol oxidation to acetic acid on Pt, etc. By definition, catalysis is a process by which a reaction rate is enhanced by a small amount of the so-called catalyst that supposedly does not undergo any change during the reaction, at variance with surface or stoichiometric reactions. As any manager of industrial plants knows, this is a too optimistic definition as real catalysts change in structure, activity, selectivity with time on stream and deactivate more or less rapidly.

Catalysis was defined as such by Berzelius, as this phenomenon was known since early times such as wine and beer fermentation, and soap and sulfuric acid (oil of vitriol) synthesis. It was illustrated in the 1800s by Döbereiner for the conversion of alcohol to acetic acid on Pt wire (1816), for the conversion of alcohol to acetic aldehyde on Pt wire (1832), and his famous Pt lighter (H₂ from 25% H₂SO₄ on Zn and its oxidation on Pt sponge on top of the lighter), as illustrated in Fig. 1.1.

Figure 1.1 Döbereiner lighter.

Some 60 years after Berzelius, Oswald established the kinetic nature of this phenomenon and in 1895 gave the definition: “A catalyst is a substance that changes the rate of a chemical reaction without itself appearing in the products.” According to the IUPAC (1976) a catalyst is a substance that, being present in small proportions, increases the rate of attainment of chemical equilibrium, without itself undergoing chemical change.

It turned out that the catalyst acts by reducing the energy necessary to proceed along the reaction pathway, i.e. the activation energy (E_a) that needs to be surmounted to yield products, as schematised in Fig. 1.2. The activation energy is the energy required to overcome the reaction barrier and determines how fast a reaction occurs. The lower the activation barrier, the faster the reaction will be. Note that the thermodynamics of the reaction remain unchanged and that the main observation done is that the catalyst influences the reaction rate. For a catalytic reaction, the first step is the reactant(s) adsorption and the reaction energy includes the activation barrier energies of adsorbed reactants (A_ads), of adsorbed intermediates (I_ads), and of desorption of products (P_ads), as schematised in Fig. 1.2. Heterogeneous catalysis (gas or liquid phase and solid catalyst) proceeds via adsorption of one or two reactant molecule(s) on the solid surface, enhancing the reactant(s) concentration on the surface and favouring its (their) activation. For theoretical calculation it is important to know if the reactant molecules are adsorbed flat or oriented on a surface and/or are dissociated upon adsorption, as illustrated in Fig. 1.3. The degree of catalytic efficiency, gained in following a given path, is governed by the energetics of the various intermediate...

Table of contents

• Cover image
• Title page
• Table of Contents
• Copyright
• List of Contributors
• About the Editor
• About the Series Editor
• Preface to the Series
• Introduction: Editorial
• 1. Fundamentals of heterogeneous catalysis
• 2. Synthesis of metal oxide catalysts
• 3. Nanoporous oxide catalysts: A new catalyst paradigm of SYNGAS production for sustainable energy and environmental applications
• 4. Catalysts and catalysis for acid–base reactions
• 5. Gas phase heterogeneous partial oxidation reactions
• 6. Transition metal oxides for combustion and depollution processes
• 7. Photocatalytic water splitting on metal oxide-based semiconductor photocatalysts
• 8. Main industrial processes using metal oxides as catalysts or support and future trends in heterogeneous catalysis
• 9. Concluding remarks and challenges of heterogeneous catalysis on metal oxides
• Index