Catalyst sphere, metals distribution

There are also catalyst formulations which have highly dispersed metals which are deliberately heterogeneously distributed on a support. If the microscopist is aware of the situation, he can take precautions in the sample preparation. This type of sample is the worst possible case to analyze because not only does the analyst have a complex mixture of components to sort out, but the analysis statistics are very poor. Consequently, additional time is usually required to survey the catalyst particles in order to establish a consensus of how it was constructed. Specialized specimen preparation such as ultramicrotoming and scraping the exterior of a sphere or extrudate may alleviate some of the interpretation problems. Additional aid may be solicited from a scanning electron microscope wherein an elemental distribution of a polished cross section of the catalyst sphere or extrudate can be made. 

In order to study the effect of metals distribution on activity and stability, test catalysts were prepared by diluting y-alumina sphere-based catalysts of fixed concentration (a Pt Pd ratio of 2.5 1.0) with bare spheres in varying amounts. The T50 data for these test catalysts for CO oxidation are plotted in Figure 8. Each curve represents the data at a constant total metals concentration when calculated over the entire catalyst—bare sphere combination. The curves demonstrate the effect... 

Figure 16.11 SEM-EDX topical analysis of a spherical SILP WGS catalyst cut into half and polished- (a) SEM image of the catalyst sphere cut into half and (b) EDX map of the catalyst sphere showing ruthenium (purple representing precursor) and sulfur (blue representing ionic liquid) distribution, metal support liquid [BMMIM][OTf], a = 0.1 nil mlp. g, support material agglomer-...

Supported platinum and palladium catalysts were prepared by soaking a preformed alumina support (Kaiser KC/SAF gel-derived alumina, 250 m2/g, %-inch spheres) in concentrated aqueous solutions of the appropriate metal chloride. The catalysts were calcined at 600 °C in air before use. Specific metal surface areas were measured by titration of chemisorbed oxygen with hydrogen (I, 2). Prior to the adsorption measurement, each sample was reduced 1 hr in H2 at 500°C, evacuated 1 hr at 500°C, and exposed overnight to 02 at room temperature. Elemental distributions in the catalyst pellets were determined by electron microprobe. 

In this specific case, the colloid stabilizers are dendrimers, for instance, polyamidoamine (PAMAM), which are hyperbranched polymers that ramify from a single core and form a porous sphere [103, 104] (Scheme 17.1). Dendrimer-encapsulated nanoparticles (DENs) are synthesized by sequestering metal ions within appropriate dendrimers, and then by chemically reducing the resulting composite. They can be synthesized in various media, such as water or ethanol. The size of the nanoparticles is usually nearly monodisperse, and can be tuned by varying the metal-to-dendrimer ratio prior to reduction. Supported catalysts can then he prepared by immobilizing DENs onto a sohd support. As in the case of colloids, the last step, which consists in the removal of dendrimers hy thermal treatment, may lead to an increase in both the metal-particle size and particle-size distribution. 

EXAFS analysis of colloidal metals has begun to shed light on this complex state of affairs. In the few examples reported so far for bimetallic colloids, a nonuniform distribution of metals has been observed. Ibshima [164, 165] has studied a series of bimetallic PtPd catalysts, and concluded that the distribution of the two metals in the particles is nonuniform on the basis of differences in their respective mean coordination numbers. In our laboratory, colloidal 4.0 nm PdCu/PVP has been analyzed by this method and the distribution of the metals in the alloy particles shown to be nonuniform. However the component which is enriched at the surface appears to be palladium, in contrast to the segregation of copper to the surface of bulk copper-palladium alloys. [203] In addition, the ability of surface deposited copper to dissolve in a preformed palladium particle has been clearly demonstrated on the basis of the analysis of the coordination sphere of the copper. [204]... 

The drying process can affect the catalyst distribution within the support. The crystallite size of a supported metal catalyst may also be altered if a considerable portion of the soluble metal is occluded rather than adsorbed. Initially, evaporation occurs at the outer surface, but the liquid evaporated from small pores is replaced by liquid drawn from large pores by capillarity, possibly causing a nonuniform distribution of catalyst. In the precipitation method, the dried catalyst particles are formed into granules, spheres, tablets, and extrudates. In either method, the dried material is calcined to activate the catalyst. 

The objective of our woik is to provide mechanically strong caibon support bodies of a narrow size distribution from renewable biomass resources. A second objective is to provide supported base metal catalysts without using Itydrogen gas as a reducing agent We start from hydrophihc bodies of carbohydrates, such as Micro Crystalline Cellulose (MCC) spheres or partially Carbonized Sucrose (CS) spheres prepared by hydrothermal treatment of an aqueous sucrose (common table sugar) solution. 

Fig. 9. Microprobe analysis of the molybdenum distribution over silica spheres of Mo/Si02 catalysts with various metal loadings.

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