CONCEPTS AND DEFINITIONS

1.1 Introduction

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 manner than heretofore, by means of solution theory, colloidal chemistry, solid state chemistry, and organometallic chemistry. At any rate, the most striking recent advance in catalysis by metals is the control of catalyst preparation and characterization, so that reproducible surfaces yield reproducible reactivity in different laboratories.

Besides helping in the characterization of catalytic solids, new physical tools (spectroscopy, diffraction) also contribute to the identification of reaction intermediates responsible for the elementary steps that constitute the catalytic cycle.

Ultimately, the indispensable quantity for the elaboration of theories in catalysis is the correct and reproducible measurement of the true reactivity of molecules at the solid-fluid interface. This measurement is the difficult art of the chemical kineticist. The measurement must be checked so that it can be shown to be exempt from all artifacts: adventitious poisoning, improper contacting between solid and fluid, and all physical processes of heat and mass transfer. But if the kinetic data are correct, and if the catalyst characterization is adequate, the available information can lead to the progressive transformation of catalysis from an art to a science.

In fact, in the case of metals, surface science and catalysis science have progressed side by side in the past ten years. There now exist several examples where kinetic data on clean, well-defined macroscopic single crystals agree very well with those obtained on reproducible and characterized supported metallic clusters between 1 and 10 nm.

This remarkable agreement justifies the limitation of this book to metallic catalysts. Nevertheless, our attempt will be to develop the general kinetic principles involved in heterogeneous catalysis, with metals selected as an example.

The theories underlying these principles are relatively few. In spite of the post-Langmuirian era of surface science starting twenty years ago, the Langmuir (1916) isotherm remains one of the pillars that support surface catalytic kinetics. Nevertheless, it is now necessary to take into account corrosive chemisorption in which the adsorbate forms with the atoms of the metal surface a new coincidence lattice over the metal lattice itself (Bénard, 1970; Ponec and Sachtler, 1972; Hanson and Boudart, 1978). The theoretical developments of Horiuti (1957, 1967) are of wide applicability. But the two most essential theories of almost universal applicability remain that of the transition state or activated complex, elaborated by Eyring and his school (Glasstone et al., 1941) and the quasi-stationary state approximation popularized and defended by Bodenstein. Bodenstein's powerful method was further systematized by Christiansen (1953) for both catalytic and chain reactions.

In the first chapters of this book, the metal surface will be considered as made up of sites that are identical thermodynamically and kinetically without any interaction between adsorbed species. This Langmuirian view will be amended later as the early recognition by Taylor (1925) of the importance of active centers will be embodied in the theory of non-uniform surfaces following Temkin (1957, 1965, 1979) and Wagner (1970). Temkin's formalism, which rests on the Brønsted relation between rate constants and equilibrium constants (see Boudart, 1968), is very general. Since a collection of sites on a non-uniform surface can be assimilated to an array of different catalysts, Temkin's theory helps in understanding what determines the optimum catalyst for a given reaction. The finding of an optimum catalyst is the most common goal of applied catalysis.

1.2 Definitions

1.21 Stoichiometric Equation and Stoichiometric Coefficients

Generally, for any chemical reaction, whether it be an overall reaction or an elementary step, we can write:

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (1.2.1)

where vi is the stoichiometric coefficient of component Bi taken as positive if Bi is a product, or negative if Bi is a reactant.

1.22 Extent of Reaction

This quantity, introduced by the Brussels school of thermodynamics, is defined by:

[xi] (mol) = [ni - n0i]/vi (1.2.2)

where n0i and ni are the quantity of substance of Bi, expressed in mole, at time zero or at any time respectively.

1.23 Reaction Rate

Following the recommendations of IUPAC (1976, 1979), reaction rate is defined in the most general way by:

[xi] = d[xi]/dt mol s-1 (1.2.3)

But in practice, the rate is referred to unit volume, mass or area of the catalyst. Thus we can have a volumic rate:

1/V d[xi]/dt mol cm-3 s-1 (1.2.4)

where V is the volume of the solid catalyst, or a specific rate:

1/m d[xi]/dt mol g-1s-1 (1.2.5)

where m is the mass of the catalyst, or an areal rate:

1/A d[xi]/dt mol cm-2 s-1 (1.2.6)

where A is the area of the catalyst. These expressions for the rate of reaction will be designated by v, the meaning of which will be made clear by the context.

It is clear that the areal rate (1.2.6) is the best one of the three. Yet, two catalysts can have the same surface area but different concentrations of active sites. A definition of the rate in terms of the number of active sites would seem to be preferred.

1.24 Number of Turnovers

This is the number n of times that the overall reaction takes place through the catalytic cycle. The rate is then:

rate = dn/dt s-1 (1.2.7)

where n = [xi] × NA with NA = 6.0225 × 1023 mol-1. The areal rate can also be expressed as:

areal rate = 1/A dn/dt cm-2 s-1 (1.28)

1.25 Turnover Frequency or Rate of Turnover (Formerly Called Turnover Number, Boudart 1972)

This is the number of turnovers per catalytic site and per unit time, for a reaction at a given temperature,...