SMALL CRYSTALLITES, CATALYTIC PROPERTIES

The electron microscope offers a unique approach for measuring individual nano-sized volumes which may be catalytically active as opposed to the averaging method employed by spectroscopic techniques. It is just this ability of being able to observe and measure directly small crystallites or nano-volumes of a catalyst support that sets the microscope apart from other analyses. There have been many studies reported in the literature over the past fifteen years which emphasize the use of analytical and transmission electron microscopy in the characterization of catalysts. Reviews (1-5) of these studies emphasize the relationship between the structure of the site and catalytic activity and selectivity. Most commercial catalysts do not readily permit such clear distinction of physical properties with performance. The importance of establishing the proximity of elements, elemental distribution and component particle size is often overlooked as vital information in the design and evaluation of catalysts. For example, this interactive approach was successfully used in the development of a Fischer-Tropsch catalyst (6). Although some measurements on commercial catalysts can be made routinely with a STEM, there are complex catalysts which require... 

It is still unclear in what way the electrons transferred to the metal-semiconductor interface can perturb the electronic structure of the metal crystallites, leading to modifications of their chemisorptive and catalytic properties. Intuitively, one expects that whatever the interaction is, it will be more intense for small metal crystallites, for which the ratio ejM was found to be significant (Figure 3) and the fraction of interface metal atoms is large. Two different modes of electronic interaction can be considered, namely ... 

Catalysts that contain metallic constituents deposited on a support are of major scientific and industrial interest ". The support permits the metal to be present on the surface as small crystallites that have a high surface area. If these crystallites are about 1 nm in diameter, most of the atoms are exposed on the metal surface and available for catalytic action. Of great significance is the interaction of the support with the metal. This interaction can modify the catalytic properties of the metal and also slow crystallite growth processes, which result in loss of area and activity. Sometimes, as in dual-function catalysts, the support is required to provide one of the catalytic functionalities, usually the acid function. 

The hydrido complex [(l,5-C8Hi2)RhH]4 can be used as a source of small rhodium crystallites. Aromatic hydrocarbon solutions of this complex are unstable when exposed to dihydrogen, allowing the formation of Z nm size, crystalline but agglomerated Rh nanoparticles, that show catalytic properties in aromatic hydrocarbures hydrogenation. ... 

Metal clusters in zeolite cages are small and structurally well defined relative to the metal crystallites (often called clusters) present in typical metal oxide supported metal catalysts used in industry. Thus, researchers have investigated zeolite supported metals in attempts to better understand the structures of supported metals, the interactions of metals with supports, and the dependence of catalytic properties on cluster size and the nature of the interactions with the support. [105, 107, 108]... 

The catalysis science of supported metal oxide catalysts, especially supported vanadia catalysts, has lagged behind their industrial development. In the 1970s, two models were proposed for the active metal oxide component a three-dimensional microcrystalline phase (e.g., small metal oxide crystallites) or a two-dimensional surface metal oxide overlayer (e.g., surface metal oxide monolayer). In the 1980s, many studies demonstrated that the active metal oxide components were primarily present as two-dimensional surface metal oxide overlayers, below monolayer coverage, and that the surface metal oxide overlayers control the catalytic properties of supported metal oxide catalysts. The synergistic interaction between the surface vanadia overlayer and the underlying oxide support prompted Ceilings to state. . that neither the problem of the structure of suppored vanadium oxide nor that of the special role of TiOa as a support have definitely been solved. Further work on these and related topics is certainly necessary. In more recent years, many fundamental studies have focused on the molecular structural determination of the surface vanadia phase and to a lesser extent the molecular structure-reactivity relationships of supported vanadia catalysts. " ... 

Figure 21 provides an example of the use of ESCA to define an oxidation state of a freshly reduced palladium-on-carbon hydrogenation catalyst exposed to the air. The metallic palladium peaks (Fig. 21a) are quite evident, indicating no bulk oxidation occurred. There is a strong peak for carbon, probably due to adsorbed CO2 from the air. The presence of a small amount of PdO is suggested at 337 eV in Fig. 21B. This peak is a shoulder on the palladium 3 5/2 peak and most likely represents a surface layer of oxide on the palladium. This information could not be conveniently obtained by XRD because small palladium (or PdO) crystallites cannot diffract X rays. Furthermore, XRD measures bulk properties and would not see surface oxides even if the crystallite sizes were sufficiently large to be XRD sensitive. We can therefore expect to see more frequent use of ESCA or other surface sensitive techniques to monitor the surface of catalytic materials.

It should be emphasized that the active sites located on the external surface, often in small amounts compared to the inner sites (<1% for crystallites of 1 //m), play a catalytic role. Generally, this leads to a selectivity decrease, the external surface lacking the shape selective properties of the inner pores.. However, recent results show that reactions which can occur only on the external surface of zeolites or just within the pore mouth are very selective, suggesting a shape selective influence of external surface depending on the nature of the substrate (Table 1.2). 

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