Preparation of supported metal catalysts

The goal of this work was to develop a support independent synthetic technique for the preparation of supported metal catalysts. There were three criteria that had to be simultaneously achieved ... 

In the last few years remarkable progress has been made in the preparation of supported metal catalysts. Entirely new methods have been developed, comprising precipitation of the metal as an insoluble salt or hydroxide on the support under controlled conditions, or loading the support with the metal by means of ion exchange. A feature of catalysts prepared according to the former method (I, 2) is that, after reduction, they have a high metal content (50% by weight, or more), while the metal crystals are still small (20-40 A) and distributed very uniformly over the support. The latter approach yields catalysts with metal crystallites of approximately 10 A however, the metal content is rather low [about 2% (3-5)]. 

An alternative approach for the preparation of supported metal catalysts is based on the use of a microwave-generated plasma [27]. Several new materials prepared by this method are unlikely to be obtained by other methods. It is accepted that use of a microwave plasma results in a unique mechanism, because of the generation of a nonthermodynamic equilibrium in discharges during catalytic reactions. This can lead to significant changes in the activity and selectivity of the catalyst. 

The above exposition shows that homo- and heteronuclear metal carbonyl precursors have been used successfully in the preparation of supported metal catalysts that behave better than those prepared by conventional methods. This is frequently... 

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. 

It has been reported that the precursors of catalyst components over promoted Cu catalysts influence reaction behavior of CO2 hydrogenation. Nitta et al. has reported that the precursor components of the precipitated Cu-ZrOz catalysts have a great influence on the methanol selectivity as well as CO2 conversion [4]. Based on these findings through CO2 hydrogenation, it is considered that appropriate choice of the precursors in the preparation of supported metal catalyst might improve ethanol formation in CO2 hydrogenation. 

Surface organometallic chemistry (SOMC) has shown high potential for the preparation of supported metal catalysts with desired composition and good dispersion [3]. For example, the controlled hydrogenolysis of tetra-n-butyltin (Sn(/i-C4H9)4) on the surface of group VIII metals leads to well-defined bimetallic catalysts [3-6]. In SOMC on metal supported on oxide, judicious selection of reaction conditions (temperature, initial complex concentration etc.) allows the reaction to occur preferentially between organometallic complexes and metal surface [3,5,6]. 

Metallic glasses have been used in catalysis in two ways, namely, in investigations carried out on as-quenched glassy metals and in those where the glassy metals were subjected to different pretreatments and served merely as precursors to catalytically active materials. The use of glassy metals as catalyst precursors has been shown to open up new possibilities for the preparation of supported metal catalysts with unusual chemical and structural properties. This potential resides mainly on the high reactivity and isotropic nature of these materials compared to their crystalline counterparts. Several efficient supported metal catalysts are compared to conventionally prepared supported metal catalysts in Chap. 3. 

In this chapter, the most common techniques for preparation of supported metal catalysts will be discussed, including impregnation, coprecipitation, homogeneous deposition precipitation, and precipitation at constant pH. In principle, these techniques can all be used to attach the active phase to supports, some preferably in the form of a powder, others in the form of a pre-shaped body. First, a general description of the techniques will be presented. Then, the techniques are illustrated by specific examples of the preparation of metallic catalysts. In view of the expertise of the authors of this chapter, Pt, Au, and Ag as the active metal phases will be emphasized. The last two examples are focused on the production of propene oxide and, as a consequence, they refer to an unresolved research issue. The results on the Ag catalysis have not been published elsewhere, and are therefore treated extensively. 

R.D. Gonzalez, T. Lopez, R. Gomez, Sol-Gel preparation of supported metal catalyst, Catal. Today 35 (1997) 293. 

The use of glassy metals as catalyst precursors has opened new routes for the preparation of supported metal catalysts with unusual chemical and structural... 

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