Supported metal catalysts preparation

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The present communication is concerned with dynamical phenomena occurring during supported metal catalysts preparation, i.e., during all steps prior to the establishment of reaction conditions. This includes all pretreatment steps such as drying, calcination, reduction, but we will mostly focus on the very first steps during which transition metals are deposited on the surface, most often from aqueous solutions. 

Also, the preparation and catalytic activity of some non-supported metal catalysts prepared from microemulsions will be described. 

Supported Metal Catalysts Preparation Catalysis Today 41(1998)129-137. 

Y. I. Yermakov and B. N. Kuznetzov, Supported Metallic Catalysts Prepared by Decomposition of Surface Organometallic Complexes, J. Mol. Catal., 9, pp. 13-40, 1980. 

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The second general method, IMPR, for the preparation of polymer supported metal catalysts is much less popular. In spite of this, microencapsulation of palladium in a polyurea matrix, generated by interfacial polymerization of isocyanate oligomers in the presence of palladium acetate [128], proved to be very effective in the production of the EnCat catalysts (Scheme 3). In this case, the formation of the polymer matrix implies only hydrolysis-condensation processes, and is therefore much more compatible with the presence of a transition metal compound. That is why palladium(II) survives the microencapsulation reaction... 

Mao and Mao invented a method for synthesizing supported metal catalysts with small metal nanoparticles (1-3 nm) even at high metal loadings (30-50 wt.%) [25]. The obtained metal catalysts exhibited superior electrocatalytic performance in fuel cells. In this invention, the unprotected metal nanocluster colloids prepared according... 

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 ... 

Magnetron Sputtering to Prepare Supported Metal Catalysts... 

A wide variety of solid materials are used in catalytic processes. Generally, the (surface) structure of metal and supported metal catalysts is relatively simple. For that reason, we will first focus on metal catalysts. Supported metal catalysts are produced in many forms. Often, their preparation involves impregnation or ion exchange, followed by calcination and reduction. Depending on the conditions quite different catalyst systems are produced. When crystalline sizes are not very small, typically > 5 nm, the metal crystals behave like bulk crystals with similar crystal faces. However, in catalysis smaller particles are often used. They are referred to as crystallites , aggregates , or clusters . When the dimensions are not known we will refer to them as particles . In principle, the structure of oxidic catalysts is more complex than that of metal catalysts. The surface often contains different types of active sites a combination of acid and basic sites on one catalyst is quite common. 

Recently, ultrathin evaporated films have been used as models for dispersed supported metal catalysts, the main object being the preparation of a catalyst where surface cleanliness and crystallite size and structure could be better controlled than in conventional supported catalysts. In ultrathin films of this type, an average metal density on the substrate equivalent to >0.02 monolayers has been used. The apparatus for this technique is shown schematically in Fig. 8 (27). It was designed to permit use under UHV conditions, and to avoid depositing the working film on top of an outgassing film. ... 

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. 

Recently, Chaudhari compared the activity of dispersed nanosized metal particles prepared by chemical or radiolytic reduction and stabilized by various polymers (PVP, PVA or poly(methylvinyl ether)) with the one of conventional supported metal catalysts in the partial hydrogenation of 2-butyne-l,4-diol. Several transition metals (e.g., Pd, Pt, Rh, Ru, Ni) were prepared according to conventional methods and subsequently investigated [89]. In general, the catalysts prepared by chemical reduction methods were more active than those prepared by radiolysis, and in all cases aqueous colloids showed a higher catalytic activity (up to 40-fold) in comparison with corresponding conventional catalysts. The best results were obtained with cubic Pd nanosized particles obtained by chemical reduction (Table 9.13). 

The regeneration of deactivated immobilized catalysts is not as easy as with conventional supported metal catalysts, where combustion of the deposited material is frequently used. Because such a procedure would destroy the organic ligands, one must resort to washing procedures. However, when this method fails, attempts must be made to recover the metal and the ligand, and to prepare a fresh catalyst. In principle, it is possible to recover the metal complexes from physically and ionically immobilized catalysts. This can also be done from covalently bound catalysts by using an easily hydrolyzable linker. 

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. 

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