Coprecipitation supported metals

Even though the component and size of metals and metal oxide support are defined, the catalytic activity for CO oxidation often markedly changes depending on the contact structure of noble metal particles with the supports. In particular, Pd, Ir, and Au exhibit high catalytic activity when they are deposited on reducible metal oxides by coprecipitation, deposition-precipitation, and grafting. Goulanski has classified supported metal catalysts for low-temperature oxidation into three groups [72], There are three possible active sites metal surfaces with metal oxide as a simple support metal oxide thin layer underneath of which metal particles are buried and the perimeter interfaces around noble metal particles. 

In the first case where metal surfaces provide active oxygen species to the support contact structure is not critical. The second case is often observed when supported metal catalysts are prepared by coprecipitation or sol-gel methods. Noble metals whose oxides are more stable than Pt oxides such as Pd and Ir are more readily buried in the bulk of metal oxide supports, and the metal oxide overlayers of a thickness of about a few monolayers are modified in their electronic and redox properties by underlying noble metal nanoparticles to become active at lower temperatures. 

To prepare supported metal catalysts for low-temperature CO oxidation, coprecipitation, deposition-precipitation, and grafting methods are effective, because they can give strongly interacting metal particles with the support. 

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. 

The preceding paragraphs have introduced the various preparation methods leading eventually to supported metal particles. These methods fall obviously into two categories depending on whether the metal is basically in its zerovalent state (decomposition of metal cluster compound, chemical deposition, ion implantation, and vapor-phase deposition) or in an oxidized state (coprecipitation, impregnation, and ion ex-change)(Table I). 

Several methods are available for preparing supported metal catalysts. Commonly used methods include impregnation to incipient wetness, deposition-precipitation (DP), coprecipitation, and deposition/immobilization of colloidal metals. A useful description of such techniques is reported for gold catalysts [43]. 

Disproportionation of the praseodymium and terbium oxides may also be induced by prolonged exposure to air, at room temperature [26,189]. As a result, Ln(OH)3 and Ln02 phases are formed. It would be therefore important to verify the likely occurrence of this phase segregation effect, because of its influence on the actual structural constitution and chemical behavior of the resulting material. Likewise, the formation of Ln(OH)3 (Ln Pr, Tb) may strongly favor the occurrence of dissolution/coprecipitation phenomena during the preparation of praseodymia (terbia) supported metal catalysts. These phenomena may induce nanostructural effects similar to those commented on above. 

NiOl-3), and Be(0H)2 ). These gold catalysts are active in the oxidation of CO at a temperature as low as -70 C. However, coprecipitation is valid only for a selected group of metal oxides as mentioned above, because the precipitation rates of support metal hydroxide and gold hydroxides and their affinity might determine in the dispersion of gold. 

A variety of mixed metal catalysts, either as fused oxides (42 7 8) or coprecipitated on supports (25 0) or as physical mixtures of separate catalysts (5P), have been tested in aniline reductions. In the hydrogenation of ethyl p-aminobenzoate, a coprccipitated 3% Pd, 2% Rh-on-C proved superior to 5% Rh-on-C, inasmuch as hydrogenolysis to ethyl cyclohexanecarboxylate was less (61) (Table 1). 

Besides supported (transition) metal catalysts, structure sensitivity can also be observed with bare (oxidic) support materials, too. In 2003, Hinrichsen et al. [39] investigated methanol synthesis at 30 bar and 300 °C over differently prepared zinc oxides, namely by precipitation, coprecipitation with alumina, and thermolysis of zinc siloxide precursor. Particle sizes, as determined by N2 physisorpt-ion and XRD, varied from 261 nm for a commercial material to 7.0 nm for the thermolytically obtained material. Plotting the areal rates against BET surface areas (Figure 3) reveals enhanced activity for the low surface area zinc... 

The possible strategies are coprecipitation to prepare mixed hydroxides or carbonates [5], cosputtering of gold and the metal components of the supports by Ar containing O2 to prepare mixed oxides [23], and amorphous alloying to prepare metallic mixed precursors [24]. These... 

Fig. 6.5 Syntheses of metal loaded nanoparticles (Au) on metal oxide supports using impregnation, coprecipitation, deposition-precipitation, and photo-deposition methods. For Pt loaded nanoparticles H2PtCl6 (aq) is used.

Phosphorus, and particularly the availability of P04, is well known to limit the productivity in most lakes (Schindler, 1977). Important abiotic interactions exist between the DOM-Fe complex and P. These interactions may reduce the availability of phosphate, thus resulting in the exacerbation of P limitation in humic-rich lakes. In oxygenated waters and in the absence of DOM, Fe hydroxide particles react with P04 to form a complex that is quick to settle to bottom sediments. Indeed, this coprecipitation can be a primary sink for phosphates in lakes (Stumm and Morgan, 1996). Given the important role of Fe hydroxides in binding P04, Fe hydroxides associated with DOM are suspected of binding P. No clear evidence exists to support the direct binding of P04 to DOM (Stewart and Wetzel, 1981), and it is believed that the P04 must be associated with a metal oxide bound to the DOM. 

For Pt supported on a co-precipitated tin oxide-alumina catalyst, alloy formation occurs to a much smaller extent than it does on a material prepared by impregnation with the chloride complex of the two metals (40,41). Since most commercial catalyst formulations are based on tin-alumina coprecipitated support materials, it appears that the studies using Pt and Sn coimpregnation techniques, while interesting, are not directly applicable to the commercial catalysts. 

The success of Haruta s early work lay in his choice of preparation method and support. Gold particles of the necessary small size were first obtained by coprecipitation (COPPT) and later by deposition-precipitation (DP) (see Sections 4.2.2 and 4.2.3) classical impregnation with HAuCLj does not work. The choice of support is also critical transition metal oxides such as ferric oxide and titania work well, whereas the more commonly used supports, such as silica and alumina, do not work well or only less efficiently. This strongly suggests that the support is in some manner involved in the reaction. 

The use of various methods (coprecipitation, impregnation and deposition-precipitation) confirmed the superiority of the transition metal oxides as supports,69 these being more easily reducible than the ceramic oxides that gave low activities (Table 6.6). With mixed Fe2C>3-MgO supports activity increased with iron content, not withstanding a growth in gold particles size.69... 

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