Preparation of Metal Catalysts

Bimetallic (Pt-Bi) or trimetallic (Pt-Pd-Bi) catalysts are available commercially from Degussa [85]. Metal promoters such as bismuth or lead, were added to platinum metals by co-impregnation, by impregnation of the supported noble metal catalyst with an aqueous promoter salt solution, or by redox surface reaction. A very simple and efficient way of loading Pt/C or Pd/C catalysts with bismuth is to add the required amount of aqueous Bi0N03 solution to a suspension 

Gold catalysts supported on active carbon and alumina have been prepared by incipient wetness impregnation or by precipitation at basic pH using aqueous solutions of HAUCI4 [82]. 

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Reduction is an unavoidable step in the preparation of metallic catalysts. It is often also a critical step, because if it is not done correctly the catalyst may sinter or may not reach its optimum state of reduction. The reduction of a metal oxide MO by H2 is described by the equation... 

In general, during the preparation of metallic catalysts, more than one type of crystallographic planes are uncovered, each of which exposes a characteristic pattern of atoms, for example, the crystallographic planes (111), (110), (100), and (010) [27],... 

Among the supports that have been used in the preparation of supported transition metal nanoparticles are carbon, silica, alumina, titanium dioxide, and polymeric supports [57], and the most frequently used support is alumina [56], These supports normally produce an effect on the catalytic activity of the metallic nanoparticles supported on the amorphous material [60], In Chapter 3, different methods for the preparation of metallic catalysts supported on amorphous solids were described [61-71],... 

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. 

In the present paragraph, three examples of the effect of the preparation of metal catalysts on the performance in the desired reactions will be discussed. In the first example, the very important effect of heat treatment after the impregnation procedure is illustrated by a study on the redox activity of Pt(O) catalysts as a function of heat treatment and particle size. 

Preparation of metal catalysts is an old field with few new preparative directions. Most supported metal catalysts are prepared by reducing the corresponding supported metal oxides. The metal oxides in turn are classically prepared by incipient wetness impregnation of solutions containing metal salts, followed by drying and calcination to form the oxides. Changes in the preparation method affect the interaction strength between the supported oxide... 

Table 1 Preparation of metal catalyst particles Microemulsion technique versus impregnation technique...

In this chapter, an overview has been presented on the common techniques for the preparation of catalysts in the laboratory. It was by no means meant as being a complete guide to the preparation of metal catalysts, but the authors hope to have given some basic principles, as well as some case studies that provide some new and useful information to the reader, especially those interested in the preparation of Pt, Au and Ag catalysts. Of the preparation techniques mentioned, impregnation is often the easiest to apply, but in the... 

The above catalysts contain about 30 per cent, of metal catalyst with 10 per cent, of metal may be readily prepared by reducing the quantity of platinum or palladium chloride used. 

Salts of neodecanoic acid have been used in the preparation of supported catalysts, such as silver neodecanoate for the preparation of ethylene oxide catalysts (119), and the nickel soap in the preparation of a hydrogenation catalyst (120). Metal neodecanoates, such as magnesium, lead, calcium, and zinc, are used to improve the adherence of plasticized poly(vinyl butyral) sheet to safety glass in car windshields (121). Platinum complexes using neodecanoic acid have been studied for antitumor activity (122). Neodecanoic acid and its esters are used in cosmetics as emoUients, emulsifiers, and solubilizers (77,123,124). Zinc or copper salts of neoacids are used as preservatives for wood (125). 

The outstanding feature of the preparation of 2,2 -bipyridine from pyridine under the influence of metal catalysts is the absence of isomeric bipyridines among the products. In this respect reactions using metal catalysts in a heterogeneous system differ from methods which have been used to prepare bipyridines in homogeneous sys-tems. ... 

Metal carbonyls form a large and important group of compounds which are used widely in the chemical industry, particularly in the preparation of heterogeneous catalysts and as precursors in CVD and metallo-organic CVD (MOCVD). 

Recent Advances in the Preparation of Supported Catalysts Containing Metal Particles of Tailored Sizes... 

Chemical reduction of metal salts in solution is the most widely used method of preparation of metal nanoparticles, especially in laboratories. In general, the reducing reagents are added into the solution of the precursor ions, but in some cases, a solvent works as a reductant. Various reducing reagents have been proposed to prepare metal nanoparticles. Ethanol or small alcohols can reduce precious metal ions such as Au, Pt", Pd, Ag, and so on [3j. Polymer-stabilized precious metal nanoparticles and their alloy particles can be used as good catalysts for various reactions. Polyols, such as ethylene glycol, were... 

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. 

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