Preparation of Supported Catalysts

As already mentioned, in the overwhelming majority of heterogeneous catalytic reactions, the zone where the chemical reaction occurs is limited to the interface between the solid and fluid phases, thus the material of the bulk solid, which is not in direct contact with the fluid phase, is not involved. This means that, in general, it is necessary to prepare a catalyst with a large proportion of active metal atoms exposed at the surface. In order to increase their number, it is possible to support the metal on a high surface-area oxide (alumina, silica, zeolites). It is then 

For this purpose, a series of catalysts was prepared with the aim of increasing the dispersion of the active metal (ruthenium and palladium ) on the inert material under the influence of the solvent jets produced by the bubble collapse and a higher reactivity (due to the depassivating effects and the reduction of particle size)xhe latter effect is especially exploited in such preparations. 

The ruthenium catalysts have been supported on a commercial alumina (Akzo, surface area 155 m g-i) using two different precursors RuCla and Ru(acac)3. The interesting point is that the proposed preparation method (vacuum impreg-nation 7) jg of common use in preparing supported catalysts. The ruthenium salt dissolved in a suitable solvent is put in contact with alumina at room temperature. Ultrasound is used during the reduction step performed with hydrazine or formaldehyde at 70°C. The ultrasonic power (30W cm 2) was chosen to minimize the destructive effects on the support (loss of morphological structure, change of phase). 

36 Campbell, I.M. Catalysis of Surfaces Chapman and Hall, London, 1988. 

39 Bianchi, C.L. Carli, R, Fontaneto C. Ragaini, V. in Preparation of Catalysts VI (Poncelet, G. Ed.), Elsevier Science, Amsterdam, 1995,1095. 

Pellets or tablets (1.5-10 mm in diameter), rings (6-20 mm) and multichanneled pellets (20-40 mm in diameter and 10-20 mm high) are used when a high mechanical strength is required. They are produced by compressing a mixture of the support powder and several binders (kaolin day, stearic acid) and lubricants (graphite) in a press. 

In many cases supports are shaped into simple cylinders (1-5 mm in diameter and 10-20 mm in length) in an extrusion process. The support powder is mixed with binders and water to form a paste that is forced through small holes of the desired size and shape. The paste should be sufficiently stiff such that the ribbon of extmded material maintains its shape during drying and shrinking. When dried, the material is cut or broken into pieces of the desired length. Extrusion is also applied to make ceramic monoliths such as those used in automotive exhaust catalysts and in DeNOx reactors. 

There are in principle two ways to make supported catalysts  

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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). 

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

The data in the Figs. 9.1,9.2 and 9.4 nicely illustrate the complementarity of XPS and SIMS and the possibilities that thin film oxide supports offer for surface investigations. Owing to the conducting properties of the support, charging is virtually absent and typical single crystal techniques such as monochromatic XPS and static SIMS can be applied to their full potential to answer questions on the preparation of supported catalysts. 

B. Kraeutler, A.J. Bard, Heterogeneous photocatalytic preparation of supported catalysts. Photodeposition of platinum on titanium dioxide powder and other substrates, /. Am. Chem. Soc. 100 (1978) 4317-4318. 

The use of hetero-metallic (MM )carbonyl complexes as precursors can lead to the preparation of supported catalysts having weU-defined bimetallic entities in which the intimate contact between M and M remains in the final catalyst and the atomic ratio M/M of the aggregates is that of the bimetallic carbonyl precursor used. This is illustrated in Figure 8.1, in which the definite interaction of the MjM (CO) complex with the functional group (F) of a surface (S) produces a new anchored surface species. This new surface species could evolve with an appropriate treatment producing tailored bimetallic particles. 

Sections 8.3.1-8.3.3 present the use of iron, mthenium and osmium carbonyls, respectively, in the preparation of supported catalysts. Over non-inert supports, besides the characteristics of carbonyl compounds, the reactivity of the surface and that of the specific element, mainly related with its redox properties, will be covered for each metal. 

Fe(CO)s and Fe3(CO)i2 are the most frequently used carbonyl iron complexes in the preparation of supported catalysts Fe2(CO)g is not soluble in hydrocarbon... 

Ru3(CO)i2 is the binary carbonyl of ruthenium that has been most used in the preparation of supported catalysts [90-99]. 

Carbonyl compounds of metals of group 9 have been extensively used in the preparation of supported catalysts, mainly for two reasons the metals are active in several reactions of industrial interest and there are stable carbonyls of easy decarbonylation of the three metals of this group. Besides the stable neutral homo-nuclear carbonyl compounds CojjCOjg, Co4(CO)i2, Rh4(CO)i2, Rh,s(CO)iis and Ir4(CO)i2, other heteronuclear carbonyl species, mainly containing Co-Rh, Co-Ru and Rh-Fe, have been used in the preparation of catalysts. Both metals, Co and... 

Exemplarily, Zapf et al. [145] described a manual wash coating/impregnation method, a typical procedure for the preparation of supported catalysts on microstructures. Micro structured stainless-steel plates (X2CrNiMol7 12 2 and MoTil7 12 2) structured by photochemical etching were used. The micro channels had a semi-circular cross-section and were fabricate with the dimensions 500 pm x 300 pm, 750 pm x 300 pm and 500 pm x 70 pm (width x depth). The first... 

Impregnation and ion-exchange methods for preparation of supported catalysts are discussed in detail in Section 2.2.1.1. Only ion exchange in the solid state as a novel method to prepare zeolite catalysts is described here. 

Another important consideration in preparing mixed-oxide catalysts is the spontaneous monolayer dispersion of oxides and salts onto surfaces of support substrates on calcination. Both temperature and duration of calcination are important here, as discussed in the reviews by Xie and Tang [63] and by Knozinger and Taglauer [64]. If this dispersion step is inadequate or incomplete, the resulting oxide layer, and any reduced metal surface from it, will not be reproducible from the same catalyst system therefore, one can then have different catalysts prepared at different times and, of course, from one laboratory to another. Spreading and wetting phenomena in preparation of supported catalysts is discussed in Section A.2.2.1.3. 

Although the preparation of supported catalysts is one, if not the most, important step in the course of a catalytic process, it is interesting to observe that little attention has been given to this subject in most textbooks written on catalysis By contrast, the number of documents published on this subject is very large (Fig 2(a)) This apparent contradiction lies in the fact that most, if not all, papers and patents give preparation procedures rather than rationalizations and concepts, making any synthesis, usually required in textbooks, rather difficult It is thus no surprise that the term catalyst design has been little used... 

The preparation of supported catalysts by anchoring/ grafting usually involves several basic steps. 

Earlier reviews devoted to the use of metal allyls and other organometallics for the preparation of supported catalysts have been written by Ballard [45], and Candlin and Thomas [46] extensive work has been performed by Yermakov and co-workers [11, 47-50], and also by Iwasawa and co-workers [15, 16, 51, 52] on difficult-to-reduce elements (e.g. Ti, Zr, Hf, Cr, Mo, W) and, for example, by Basset and co-workers [53-55] on easy-to-reducc elements (e.g. Ni, Pd, Pt). 

Many reviews have been devoted to the use of carbonyl metal complexes for the preparation of supported catalysts [2, 4, 17, 18, 61-67]. 

Geus JW, van Dillen AJ. Preparation of supported catalysts by deposition - precipitation. Handbook of Heterogeneous Catalysis. Weinheim Wiley-VCH Verlag GmbH Co. KGaA 2008. p. 428 167. 

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