# Kinetics of Heterogeneous Catalytic Reactions

Michel Boudart
G. Djéga-Mariadassou
Copyright Date: 1984
Pages: 240
https://www.jstor.org/stable/j.ctt7zv441

## Table of Contents

1. Front Matter
(pp. i-vi)
2. Table of Contents
(pp. vii-viii)
3. FIGURES
(pp. ix-xii)
4. TABLES
(pp. xiii-xiv)
5. PREFACE TO THE FRENCH EDITION
(pp. xv-xvi)
M. Boudart and G. Djéga-Mariadassou
6. INTRODUCTION
(pp. xvii-2)

During its lifetime, any catalyst produces a large number of product molecules per molecule of active phase. This quantity is frequently called theturnover. The rate of the catalytic act, called theactivityof the catalyst, is theturnover frequency, or the number of turnovers per second.

To understand the activity at the molecular level or to build a catalytic reactor on an industrial scale, the first necessary information is the turnover frequency and its variation with process variables, temperature, total pressure, and composition. This variation is described by arate equation. To obtain the latter is an art based...

7. Chapter 1 CONCEPTS AND DEFINITIONS
(pp. 3-37)

Heterogeneous catalysis is much more than a subfield of chemical dynamics and chemical kinetics. It is related to other disciplines, as shown in the triangular representation below.

In particular, thanks to the recent development of the chemical physics of metallic surfaces, kineticists have reconsidered earlier views and theories concerning catalysis by metals and alloys. New techniques had yielded new results, and new concepts had to be incorporated in the kinetic framework of heterogeneous catalysis.

Catalyst preparation is responsible for the composition, structure and texture of catalytic materials. Today, the synthesis of new metallic catalysts is achieved in a more rational...

8. Chapter 2 KINETICS OF ELEMENTARY STEPS: ADSORPTION, DESORPTION, AND SURFACE REACTION
(pp. 38-76)

A major development in heterogeneous catalysis is the very recent accumulation of data on the kinetics of elementary steps on well-defined surfaces. These are atomically clean, smooth planes on large single crystals. Cleanliness is achieved by high temperature evacuation, treatment with reactive gases, and/or ion bombardment. The structure of the surface is established by low-energy electron diffraction. Its composition is checked and monitored by Auger electron spectroscopy. All observations are made with the sample in an ultrahigh vacuum chamber with a background pressure of the order of 10−8to 10−10Pa. Residual gas composition is monitored by mass spectroscopy. Very...

9. Chapter 3 KINETICS OF OVERALL REACTIONS
(pp. 77-117)

The value of pre-exponential factors of rate constants can be used to assess the authenticity or validity of presumed elementary steps. Nothing much can be said a priori about activation energies. For adsorption equilibrium constants$K=\exp (\Delta S_{a}^{0}/R)\exp (-\Delta H_{a}^{0}/RT)$, the rule is that$\Delta H_{a}^{0}$a should be normally negative, as adsorption is in general exothermic, while entropy normally decreases upon adsorption, although the loss cannot exceed what was available to the fluid phase molecule, namely$S_{\text{fluid}}^{0}$(Vannice et al., 1979):\[0<-\Delta S_{a}^{0}

Yet endothermic adsorption and a gain of entropy upon adsorption are conceivable. In the latter case, imagine dissociative chemisorption of H2...

10. Chapter 4 KINETICS OF TWO-STEP REACTIONS ON NON-UNIFORM SURFACES
(pp. 118-154)

In the preceding chapter, general results on the kinetics of surface-catalyzed reactions have been obtained with the usual simplification of a Langmuir surface with catalytic sites of uniform thermodynamic and kinetic properties. This restriction will now be relaxed for the case of two-step reactions, which were shown to be much more widespread than might be anticipated, thanks to the frequent existence of a rate-determining step and of a most-abundant-reaction intermediate.

Beyond the theory of Temkin (1957, 1965), the foundation of which was discussed by Khammouma (1972), we will consider the problem of the optimization of catalysts by simply replacing a...

11. Chapter 5 STRUCTURE-INSENSITIVE AND STRUCTURE-SENSITIVE REACTIONS ON METALS
(pp. 155-193)

Consider a series of catalyst samples, each one consisting of metal particles of sizedon an inert support, withdvalues varying from 1 to 10 nm from sample to sample. Asdincreases, the relative concentrations of surface atomsCiand sitesBjchange, asiincreases from low to high values. This means that the surfacestructurechanges (see §2.1). If, then, the turnover frequency for a reaction varies withdas shown on Fig. 5.1, the reaction is said to be structure-sensitive (Boudart, 1969). It must be shown of course that the variation invtis...

12. Chapter 6 PARASITIC PHENOMENA
(pp. 194-214)

The necessity to eliminate the existence of substantial gradients of temperature and concentration in kinetic studies of heterogeneous catalytic reactions has been emphasized in the preceding chapter. To do justice to these effects requires the well-developed science of chemical reaction engineering (Denbigh and Turner, 1970; Froment and Bischofif, 1979). Our purpose here is simply to attract attention to some consequences of these effects and suggest ways to avoid them in the laboratory.

The nature of the problem is suggested by Fig. 6.1, which represents schematically a typical grain, of the order of 1mm in size, of a supported metal catalyst....

13. AUTHOR INDEX
(pp. 215-217)
14. SUBJECT INDEX
(pp. 218-222)