Homogeneous deposition

Homogeneous deposition of ultrafine metal particles on the surfaces of fine powder is not easy using PVD. A device for stirring the powder support in a vacuum chamber is needed to avoid heterogeneous deposition. Sputter deposition units equipped with stirring powder supports have already been adapted for the industrial production of Ti02 and carbon-supported gold catalysts by 3M [35]. 

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. 

Kuzniatsova, T., Kim, Y., Shqau, K., Dutta, P.K., and Verweij, H. (2007) Zeta potential measurements of zeolite Y application in homogeneous deposition of particle coatings micropor. Mesopor. Mater., 103,102-107. 

In the case of Bi and T1 phases only very short reaction times (of the order of minutes) are possible because of the evaporation of Bi and Tl. This may be compensated by using excess Bi and T1 in the solution or by modification of the solvent (mixture of water and glycerol) that enhances the solubility of Bi and allows the use of less nitric acid in the solution. A more homogeneous deposition results in a better product after heat treatment. Figure 9 shows a typical resistivity measurement for a thallium film that has, however, a very weak critical current (50 A/cm2 at 77K). 

Notes Potassium thiocyanate can produce actual reduction in the size of individual silver halide grains by virtue of its dissolving action. As a result of physical development, it produces a more homogeneous deposition of silver. It is used in concentrations of 1.0 to 1.5g/liters in certain developers, such as Kodak DK20 (DK-20 is not included in the formulas found in this edition). 

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... 

Fukuyama, K., Wier, K., Epstein, W. L., Dense Homogeneous Deposits... 

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]. 

High substantivity and homogeneous deposition onto fabrics. 

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... 

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