Catalysis quantitative surface analysis

Over the past 10 years a multitude of new techniques has been developed to permit characterization of catalyst surfaces on the atomic scale. Low-energy electron diffraction (LEED) can determine the atomic surface structure of the topmost layer of the clean catalyst or of the adsorbed intermediate (7). Auger electron spectroscopy (2) (AES) and other electron spectroscopy techniques (X-ray photoelectron, ultraviolet photoelectron, electron loss spectroscopies, etc.) can be used to determine the chemical composition of the surface with the sensitivity of 1% of a monolayer (approximately 1013 atoms/cm2). In addition to qualitative and quantitative chemical analysis of the surface layer, electron spectroscopy can also be utilized to determine the valency of surface atoms and the nature of the surface chemical bond. These are static techniques, but by using a suitable apparatus, which will be described later, one can monitor the atomic structure and composition during catalytic reactions at low pressures (< 10-4 Torr). As a result, we can determine reaction rates and product distributions in catalytic surface reactions as a function of surface structure and surface chemical composition. These relations permit the exploration of the mechanistic details of catalysis on the molecular level to optimize catalyst preparation and to build new catalyst systems by employing the knowledge gained. 

The density functional theory and the cluster model approach enable the quantitative computational analysis of the adsorption of small chemical species on metal surfaces. Two studies are presented, one concerning the adsorption of acetylene on copper (100) surfaces, the other concerning the adsorption of ethylene on the (1(X)) surfaces of nickel, palladium and platinum. These studies support the usefulness of the cluster model approach in studies of heterogeneous catalysis involving transition metal catalysts. 

B. Limoges, J.-M. Saveant and D. Yazidi, Quantitative analysis of catalysis and inhibition at horseradish peroxidase monolayers immobilized on an electrode surface, J. Am. Chem. Soc., 125 (2003) 9192-9203. 

In heterogeneous catalysis, the term active site is also used extensively [7,8], The density of active sites per unit surface area of the catalyst is an important parameter in catalyst analysis and development [9], However, whereas the surface area is relatively easily determined experimentally [10], the number of active sites in heterogeneous catalysts is not easily estimated. Therefore, although both fields use turnover numbers (reactant converted per unit time per active site) to describe activity, only the enzymologists can be sure that the quantitation of this parameter is adequate. 

The first dynamic analysis of a working catalyst to elucidate the universally postulated structure-function correlation as a quantitative relationship still has not been achieved. The fundamental difficulties in relating bulk-sensitive XRD data to surface catalysis are one set of unmet... 

The cluster model approach and the methods of analysis of the surface chemical bond have been presented and complemented with a series of examples that cover a wide variety of problems both in surface science and heterogeneous catalysis. In has been show that the cluster model approach permits to obtain qualitative trends and quantitative structural parameters and energetics of problems related to surface chemistry and more important, provide useful, unbiased information that is necessary to interpret experiments. In this way, the methods and models discussed in the present chapter are thought to be an ideal complement to experiment leading to a complete and detailed description of the mechanism of heterogeneous catalysis. 

In the discussion of the deactivation of active surfaces in Chapter 3, we made the point that the existence of deactivation changed an entire class of steady-state phenomena into unsteady-state phenomena. This is well illustrated by extending the analysis of diffusion and chemical reaction in heterogeneous catalysis to include deactivation. On a quantitative level a precise analysis can become intricate and difficult, so we will confine the presentation here to a more qualitative level (and it s tough enough at that). 

A successful quantitative analysis of the reaction between aquated nickel(ii) ions and azo(2-pyridine)4 -N,N-dimethylaniline (46) in sodium lauryl sulphate leads to conclusions that catalysis occurs at the micellar surface and is due entirely to concentration of the reactants (Figure 4). Addition of sodium chloride (sodium ions are claimed to be completely displaced by nickel ions at the micelle surface) decreases the reaction rate marginally but tetraethyh ammonium chloride has a much more pronounced inhibitory effect. The reaction characteristics were found to be strongly dependent on the source of sodium lauryl sulphate. There is significant catalysis below the c.m.c. if both... 

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