Silica coprecipitates

Fig. 4. Magnetization vs. temperature for a partially sintered 40% nickel-silica coprecipitate before and after adsorption of hydrogen at room temperature. This permits a comparison of electrons taken in per atom of nickel with hydrogen atoms adsorbed per atom of nickel.

Fig. 5. Effect of hydrogen adsorbed at room temperature on a 26% nickel-silica coprecipitate showing diminishing influence of hydrogen on smallest particles of nickel (i.e., those observed at low temperatures only).

Florisil Magnesia-silica coprecipitate 2 Strongly Acidic General-Purpose Adsorbent... 

If both silica and magnesium are present, they mutually precipitate thus, salts of either mineral may be added to the BW to provide a balance and ensure coprecipitation as serpentine ... 

In these chimneys, coprecipitation of barite and amorphous silica is taking place from the solution characterized by lower temperatures and lower flow rate than the black smoker. 

The neutron activation method for the determination of arsenic and antimony in seawater has been described by Ryabin et al. [66]. After coprecipitation of arsenic acid and antimony in a 100 ml sample of water by adding a solution of ferric iron (10 mg iron per litre) followed by aqueous ammonia to give a pH of 8.4, the precipitate is filtered off and, together with the filter paper, is wrapped in a polyethylene and aluminium foil. It is then irradiated in a silica ampoule in a neutron flux of 1.8 x 1013 neutrons cm-2 s 1 for 1 - 2 h. Two days after irradiation, the y-ray activity at 0.56 MeV is measured with use of a Nal (Tl) spectrometer coupled with a multichannel pulse-height analyser, and compared with that of standards. 

Owing to inadequate detection limits by direct analysis, various workers examined preconcentration procedures, including dithiocarbamate preconcentration [447,732-734], ion exchange preconcentration [735-737], chelation solvent extraction [736], coprecipitation [738], and preconcentration in silica-immobilised 8-hydroxyquinoline [129]. 

A variety of preconcentration procedures has been used, including solvent extraction of metal chelates, coprecipitation, chelating ion exchange, adsorption onto other solids such as silica-bonded organic complexing agents, and liquid-liquid extraction. 

Immobilizing DENs within a sol-gel matrix is another potential method for preparing new supported catalysts. PAMAM and PPI dendrimers can be added to sol-gel preparations of silicas " and zinc arsenates to template mesopores. In one early report, the dendrimer bound Cu + ions were added to sol-gel silica and calcined to yield supported copper oxide nanoparticles. Sol-gel chemistry can also be used to prepare titania supported Pd, Au, and Pd-Au nanoparticle catalysts. Aqueous solutions of Pd and Au DENs were added to titanium isopropoxide to coprecipitate the DENs with Ti02. Activation at 500°C resulted in particles approximately 4 nm in diameter. In this preparation, the PAMAM dendrimers served two roles, templating both nanoparticles and the pores of the titania support. 

Surface acidity and catalytic activity develop only after heat treatment of a coprecipitated mixture of amorphous silicon and aluminum oxides. Similar catalysts can be prepared by acid treatment of clay minerals, e.g., bentonite. The acidity is much stronger with silica-alumina than with either of the pure oxides. Maximum catalytic activity is usually observed after activation at 500-600°. At higher temperatures, the catalytic activity decreases again but can be restored by rehydration, as was shown by Holm et al. (347). The maximum of activity was repeatedly reported for compositions containing 20-40% of alumina. 

Figure 1 shows the IR spectra of TS-1(A), TS-1 (B), TS-1 (C), Ti02-Si02 coprecipitate and pure-silica ZSM-5. The absorption band at 960 cm is characteristic of TS-1 (8). All of TS-1 samples used in this study show this band. The band is not present in Ti02 and ETS-10 (not shown in the figure). However, this band is present in Ti02-Si02 which is the precursor to TS-l(C). The relative intensities of the peak at 960 cm"l are listed in Table n. 

Natural clay catalysts were replaced by amorphous synthetic silica-alumina catalysts5,11 prepared by coprecipitation of orthosilicic acid and aluminum hydroxide. After calcining, the final active catalyst contained 10-15% alumina and 85-90% silica. Alumina content was later increased to 25%. Active catalysts are obtained only from the partially dehydrated mixtures of the hydroxides. Silica-magnesia was applied in industry, too. 

Figure 13 demonstrates the promotional effect of titania on the activity of Cr/silica catalysts. These samples were made by coprecipitation. The chromium was then added and each sample was calcined at 760°C to form surface attached Cr(VI). For comparison, titania concentrations are expressed as Ti atoms per square nanometer of surface, even though a good part of the titania may actually be in the bulk. 

This explains the melt index behavior of coprecipitated silica-titania catalysts which is shown in Fig. 16. With each catalyst, the MI rises with increasing calcining temperature until sintering begins, then it drops. The... 

This sintering is associated with a tendency toward phase separation between silica and the titania. X-ray photoelectron spectroscopy (XPS) indicates that the titania tends to migrate to the surface. This is shown in Table VII, where the XPS intensity ratio Ti/Si is listed for a coprecipitated siiica-titania sample calcined at various temperatures. As the temperature increases, the intensity ratio also increases. Since XPS is a surface technique, this indicates more titania near the surface. 

The removal of sterols, vitamin E vitamers, carotenoids, and other interfering material from the unsaponifiable fraction of food samples has been achieved using one or more of the following techniques coprecipitation of sterols with digitonin (91), precipitation of sterols from a methano-lic solution (195,209), adsorption chromatography on open columns of alumina (70,91,96), thin-layer chromatography on silica plates (209), and solid-phase extraction on silica (68,100) and reversed-phase (210) cartridges. 

Coprecipitates. Silica-alumina coprecipitates are immensely complicated. As a result of structural disorder and substitution of Al(III) for Si (IV) on tetrahedral sites, they exhibit cation exchange behavior (34). The fraction of total Al which occurs on tetrahedral sites and the CEC vary widely in response to the conditions of precipitation and subsequent sample history, especially thermal history. In general, the fraction... 

In Figure 10 the experimental ZPCs of hydrous silica-alumina coprecipitates (64) are compared with those of dried and ignited silica-alumina catalysts (49), some of the previously discussed aluminosilicate minerals, and the composition dependence derived from Equation 18 assuming ... 

High selectivity was also observed on a silica-supported Fe-Cu catalyst prepared by coprecipitation (333 K, 10 atm H2, ethanol)286 and over polymer-protected colloidal Pd-Pt cluster catalysts (303 K, 1 atm H2, ethanol)287,288. In contrast with the above observation, the activity of the bimetallic alloy was 1.4-3 times higher than that of the monometallic Pd cluster reaching the maximum activity at a composition of Pd/Pt = 4 1. 

Catalyst-supporting materials are used to immobilize catalysts and to eliminate separation processes. The reasons to use a catalyst support include (1) to increase the surface area of the catalyst so the reactant can contact the active species easily due to a higher per unit mass of active ingredients (2) to stabilize the catalyst against agglomeration and coalescence (fuse or unite), usually referred to as a thermal stabilization (3) to decrease the density of the catalyst and (4) to eliminate the separation of catalysts from products. Catalyst-supporting materials are frequently porous, which means that most of the active catalysts are located inside the physical boundary of the catalyst particles. These materials include granular, powder, colloidal, coprecipitated, extruded, pelleted, and spherical materials. Three solids widely used as catalyst supports are activated carbon, silica gel, and alumina ... 

Ce in coupon tests with both GGW and brine about 10% of Se was removed for GGW only, and no uCo or 7Cs was removed. In the case of cerium, colloid coprecipitation with amorphous silica may explain these extraction results. The association of selenium with possible hydrated silicates is unknown. Further investigation of these associations will be required before any significance can be attached to these Na CO extraction results. 

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. 

This section shows, for four examples of increasing complexity, how precipitates are formed and how the properties of the precipitates are controlled to produce a material suitable for catalytic applications. The first two examples comprise silica, which is primarily used as support material and is usually formed as an amorphous solid, and alumina, which is also used as a catalytically active material, and which can be formed in various modifications with widely varying properties as pure precipitated compounds. The other examples are the results of coprecipitation processes, namely Ni/ AI2O3 which can be prepared by several pathways and for which the precipitation of a certain phase determines the reduction behavior and the later catalytic properties, and the precipitation of (VOjHPCU 0.5 H2O which is the precursor of the V/P/O catalyst for butane oxidation to maleic anhydride, where even the formation of a specific crystallographic face with high catalytic activity has to be controlled. 

Figure 4. Inhomogeneity of silica-aluminas prepared by various methods. A series of 17 commercial samples of silica-aluminas from seven different producers was submitted to microanalysis. All of them showed considerable fluctuations of composition at the scale of several tens of nanometers to several micrometers. These samples were prepared by coprecipitation or by the sol-gel method. It is not known whether some of these samples were prepared from alkoxides. Smaller but significant fluctuations at the micrometer scale were also observed for two laboratory samples prepared from alkoxides. The samples were dispersed in water with an ultrasonic vibrator. A drop of the resulting suspension was deposited on a thin carbon film supported on a standard copper grid. After drying, the samples were observed and analyzed by transmission electron microscopy (TEM) on a JEOL-JEM 100C TEMSCAN equiped with a KEVEX energy dispersive spectrometer for electron probe microanalysis (EPM A). The accelerating potential used was 100 kV.

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