Sample preparation supported-metal catalysts

The use of EM (except in the special case of SEM) demands that the catalyst, whether mono-or multi-phasic, be thin enough to be electron transparent. But, as we show below, this seemingly severe condition by no means restricts its applicability to the study of metals, alloys, oxides, sulfides, halides, carbons, and a wide variety of other materials. Most catalyst powder preparations and supported metallic catalysts, provided that representative thin regions are selected for characterization, are found to be electron transparent and thus amenable to study by EM without the need for further sample preparation. 

This method of preparation of supported metal catalyst requires a closed reactor to perform the preparation in the absence of water, so both the organic solvent and the oxide support must be carefully dehydrated. The method is based on the following principle the metal is evaporated and co-condensed with the organic to 77 K on the walls of the reactor. Under dynamic vacuum, the co-condensate is then warmed up to 195 K, and melted. The oxide support is impregnated with the solvated metal atom (cluster) at the same temperature, After a given time of contact, the slurry is warmed up to ambient temperature, and the solvent is eliminated, after which the sample can be dried. 

Regarding the preparation of praseodymia and teibia supported metal catalysts, the information available is rather scarce. All the reported studies have dealt with dispersed noble metal samples. Though metal vapor deposition has been applied in some cases (231), the impregnation techniques have coirstituted the most usual preparation procedure. Chlorine-containing (53,82,85,127,175,278), and chlorine-free (53,84,232,278) metal precursors have been used. As already reported, PrOCl and Tb(3CI have been identified in praseodymia and terbia supported catalysts prepared from chlorinated precursors (82). Water (82,85,127,175), and non-aqueous solvents. 

Kinetic data for the hydrogenation of ethene by the series of catalysts are presented in Figure 1. The presence of the discontinuity appears to depend on the structure of the catalyst, and consequently must either be associated with the preparation route or with particle size. The discontinuity between two kinetic regimes is observed for samples Pt-A and Pt-B, the two samples with the smallest particle sizes. The independent data of Jackson et al, obtained for the hydrogenation of ethene by a range of supported metal catalysts, also show a correlation between the presence of a sharp discontinuity and small metal particle size. These data, taken together with the results of the NMR characterisation of the materials would tend to support the conclusion that it is metal particle size, not support interactions that are responsible for the presence of the kinetic discontinuity. 

An alternative to the oxidative pre-treatment of the support is the oxidative treatment of the supported metal catalyst. This has been explored in ternary catalysts, such as the Pt-Ru-Mo system. Oxidation with aqueous H2O2 during the preparation of catalysts mainly affects the atomic ratio of Pt/Mo and increases the extent of oxidation of surface with no influence on the sample nanostructure. 

Atomic absorption spectroscopy (AAS) is used to determine the chemical composition of the metal loading of a supported catalyst. In a sample preparation procedure the catalyst is treated with very strong and often oxidizing acids to extract all metal atoms as ions in solution. This solution is injected into a spectrometer that gives a quantitative analysis of all metal components in the solution based on the spectral absorption (or emission in the case of Auger electron spectroscopy, AES) in a flame. Note that in this method all dissolvable metal atoms are analyzed not only the catalytic active surface atoms. 

In this case, we used the traditional method of impregnation, carried out in conditions leading to the formation of highly dispersed Ag particles on the support surface (1) samples were prepared with a low content of Ag ( 2 wt.%) (2) Ag was supported by adsorption on SiC>2 surface of the ammonia complex of the diluted silver nitrate solutions. In this case, the formation of the supported particles at the later stages of the sample preparation was mainly performed from the adsorbed silver complex. Contribution of this complex being in volume of support pores was practically excluded. (3) samples with supported silver complex were dried by the method of sublimation or by the adsorption-contact method which preserved the uniformity of adsorbed silver complex distribution on the support surface. This contributed to the obtention of a more homogeneous distribution of metal particles after subsequent reduction. The application of the adsorption-contact drying method for the preparation of the supported metal catalysts has not been found in literature. 

MgO-supported model Mo—Pd catalysts have been prepared from the bimetallic cluster [Mo2Pd2 /z3-CO)2(/r-CO)4(PPh3)2() -C2H )2 (Fig. 70) and monometallic precursors. Each supported sample was treated in H2 at various temperatures to form metallic palladium, and characterized by chemisorption of H2, CO, and O2, transmission electron microscopy, TPD of adsorbed CO, and EXAFS. The data showed that the presence of molybdenum in the bimetallic precursor helped to maintain the palladium in a highly dispersed form. In contrast, the sample prepared from the monometallie precursors was characterized by larger palladium particles and by weaker Mo—Pd interactions. ... 

It is well established that commercially important supported noble metal catalysts contain small metal crystallites that are typically smaller than a few nanometers. The surface of these crystallites is populated by different types of metal atoms depending on their locations on the surface, such as comers, edges, or terraces. In structure sensitive reactions, different types of surface metal atoms possess quite different properties. For example, in the synthesis of ammonia from nitrogen and hydrogen, different surface crystallographic planes of Fe metal exhibit very different activities. Thus, one of the most challenging aspects in metal catalysis is to prepare samples containing metal particles of uniform shape and size. If the active phase is multicomponent, then it is also desirable to prepare particles of uniform composition. 

---