Homogeneous deposition precipitation

Successful applications of the oxygen-modified CNFs are reported on immobilization of metal complexes ]95], incorporation of small Rh particles [96], supported Pt and Ru CNFs by adsorption and homogeneous deposition precipitation ]97, 98], Co CNFs for Fischer-Tropsch synthesis ]99], and Pt CNFs for PEM fuel cells [100]. 

Bezemer, G. L., Radstake, P.B., Koot, V., van Dillen, A. J., Geus, J. W., and de Jong, K. P. 2006. Preparation of Fischer-Tropsch cobalt catalysts supported on carbon nanofibers and silica using homogeneous deposition-precipitation. Journal of Catalysis 237 291-302. 

A relatively easy method of placing an active phase homogeneously on a monolithic support is by deposition precipitation (76). The advantage of this method is that an insoluble metal salt is deposited on the support, which usually cannot spread readily during the drying step. Satisfactory results are obtained when supersaturation of the liquid occurs everywhere in the monolith at the same rate. This goal can be achieved by a homogeneous deposition precipitation procedure in which both the... 

Homogeneous deposition precipitation (HDP) is explored for the preparation of carbon nanofiber supported ruthenium catalysts. First, carbon nanofibers (CNF, 177 m /g) are oxidized using nitric acid thus activating the graphitic carbon surfiice. Second, ruthenium (hydr)oxide is deposited homogeneously onto the CNF by hydrolysis of urea at 363K. 

In this section preparation and characterization of catalytic materials are briefly reviewed with respect to their applications in environmental catalysis. A number of techniques for the preparation of the supports and catalysts are emphasized. Techniques such as impregnation, homogeneous deposition precipitation, grafting, hydrolysis, sol-gel, and laser-activated pyrolysis are used for the preparation of catalysts for fundamental studies. 

Wet impregnation is not feasible for all catalyst types. For instance, vanadia on silica cannot be prepared by means of wet impregnation. With homogeneous deposition precipitation (HDP), however, it is possible to prepare vanadia on silica catalysts [21], The principle of this method is to use a lower valence of the metal, which may be produced by cathodic reduction of the respective metal ion. The reason for using lower valence state metal ions is the lower acidity compared to that of the higher valence state and the higher solubility of the metal ions. This technique was used for the preparation of silica-supported vanadia, titania, and molyb-dena catalysts [22, 23]. 

Mariscal et al. [29] prepared a series of titania-silica honeycombs by impregnation and homogeneous deposition precipitation. Only small amounts of titania are deposited on the silica matrix by homogeneous deposition precipitation. It was shown that incipient wetness impregnation was the only method to control both the amount of deposited titania and the concentration profile across the wall of the honeycombs [29]. 

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. 

This preparation technique is related to homogeneous deposition precipitation, but here the base and the precursor solution are injected simultaneously to the suspension of the catalyst support at a fixed pH. In this way, the interaction of the precipitating compound with the support can be optimised, while at the same time the crystallisation of unwanted compounds, such as large crystallites of basic copper nitrate in the case of copper catalysts, can be prevented.11,12... 

AuCf. was used as the Au source. Incipient wetness impregnation was performed by adding the AuCfs solution to the appropriate amount of Ti02 support, followed by drying at 375 K (5K/min), and calcination at 673 K (5K/min) for 1 hour. In a homogeneous deposition precipitation method (method HDP-I), a gradual increase of the pH of a AuClo/TiOo suspension... 

Please note that equation (5.1) holds for an acidic medium with limited CO2 solubility. For more elevated pH, bicarbonate and carbonate will be formed and buffering will occur, leading to a maximal pH of about 6. An elegant aspect of the urea method is that the reactant can be added at low temperature and the uniform suspension can be heated to typically 363 K, where hydrolysis of urea takes place uniformly throughout the solution. The urea method is therefore sometimes referred to as homogeneous deposition precipitation (HDP). In the case of noble metals such as palladium or gold, some workers add sodium hydroxide to the suspension to increase the pH. In the latter case, HDP is not fully possible and one should refer to that as DP. 

Ni supported on silica-alumina using three different techniques homogeneous deposition precipitation, impregnation and coprecipitation... 

Nickel incorporated onto silica-alumina using various procedures impregnation, coprecipitation and homogeneous deposition precipitation, was tested in propene oligomerization. Homogeneous deposition precipitation was found to be the most active. At a conversion of around 90% of the propene, the products were mainly dimers ca. 60%) and trimers ca. 25%). The product spectrum was unaffected by pressure changes, while higher temperatures resulted in more heavy products. 

After the homogeneous deposition precipitation of the cobalt species the pH was pH approximately 6.7. In this regard it is important to note that the first pKa... 

In contrast, employing equilibrium deposition-filtration or homogeneous deposition-precipitation the deposition takes place during the equilibration step (Figure 2.1) inside the interface developed between the surface of the support particles and the impregnating solution. This results in relatively small supported nanoparticles. The small size of the supported nanoparticles is largely imposed by the relatively small size of the interface (Figure 2.3). 

The role of the support is also important. Thus, y -alumina and silica favor the surface polymerization and then the surface precipitation of the inner-sphere complexes formed upon deposition of the cobalt or nickel aqua complexes. This allows the successful application of the homogeneous deposition-precipitation... 

Therefore, we examined the properties of the bare carbon fibrils, as well as the properties of catalysts supported on carbon fibrils. To obtain carbon fibril-supported palladium catalysts, a homogeneous deposition-precipitation procedure was utilized. Varying preparation conditions, such as, the atmosphere during drying and the pretreatment of the carbon fibrils, affect the activity in the liquid-phase hydrogenation of nitrobenzene significantly. 

Carbon fibrils can be produced rather easily, e.g., by exposing supported, finely dispersed iron or nickel particles to reducing carbon containing gas flows. To this end, one has to produce first finely dispersed iron or nickel particles on a support material, such as alumina or silica. The desired catalyst can be prepared, e.g., by incipient wetness impregnation of the support material with a suitable metal salt solution or by means of homogeneous deposition-precipitation of the metal ions onto the carrier. 

Application of small palladium particles on the carbon fibrils is viable. The homogeneous deposition-precipitation method used gves best results when air (oxygen) is excluded. Anchoring of palladium on the carbon fibrils is possible afier activation of the surface of the carbon fibrils with boiling nitric acid. XRD patterns do not exhibit diffiaction maxima of palladium. Only after reduction up to 250 C of the palladium (oxide), it could be detected by TEM due to sintering. The palladium particle size was small approximately 4 nm. The activity of the carbon fibril-supported palladium catalyst obtained was comparable to that of a commercial, activated carbon supported palladium catalyst. 

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