Deposition methods precipitation

VOx supported on TiOi showed good catalytic activity in the selective oxidation of H2S to ammonium thiosulfate and elemental sulfur. V0x/Ti02 catalysts prepared by the precipitation-deposition method can achieve higher vanadium dispersions, and higher H2S conversions compared to those prepared by the impregnation method. 

V-Sb-oxide samples with Sb V ratios of 1.0 and 3.0 were prepared by the following precipitation-deposition method VCI3 is dissolved in a O.IN aqueous HCl solution (A) and SbCls is dissolved in a 3-5N HCl aqueous solution (B). The dropwise addition of A to B leads to the formation of a white precipitate (mainly Sb-hydroxide) and a dark blue solution due to VO ions. VO is then precipitated over the Sb-hydroxide by adding dropwise a concentred aqueous solution of... 

Moon, S. W., Lee, G. D., Park, S. S. and Hong, S. S. (2004). Catalytic combustion of chlorobenzene over V205/Ti02 catalysts prepared by the precipitation-deposition method. React. Kinet. Catal. Lett. 82(2), 303-310. 

For the preparation of gold nanopartides supported on insoluble solids, the most widely used procedure is the precipitation-deposition method [32-36]. Starting from an aqueous solution of HAuCh, addition of a base leads to precipitation of a mixture of Au(OH)3 and related oxy/hydroxides that adsorbs into the solid and is then reduced to metallic gold by boiling the adsorbed species in methanol or any other alcohol. In this procedure, it has been established that the pH of the precipitation and the other experimental conditions (nature of the alcohol, temperature and time of the reduction, calcination procedure, etc.) can provide a certain control of the particle size of the resulting nanoparticles [3j. Figure 12.2 illustrates the steps required in the formation of supported gold nanoparticles. 

In this work three Ni-based catalysts were synthesized and their catalytic performance in methane steam reforming was investigated. Catalyst denoted as Ni/Si02 was prepared using precipitation deposition method while catalyst denoted as Ni/SA was prepared via deposition precipitation method using alumina modified with silica. Similarly catalyst denoted as Ni.NPs in the first step Ni nano-particles were synthesized and decorated on AI2O3. 

Figure 3.2 Schematic representation of deposition-precipitation (DP) method.

The preparation method of titania support was described in the previous paper [6]. Titanium tetraisopropoxide (TTIP 97%, Aldrich) was used as a precursor of titania. Supported V0x/Ti02 catalysts were prepared by two different methods. The precipitation-deposition catalysts (P-V0x/Ti02) were prepared following the method described by Van Dillen et al. [7], in which the thermal decomposition of urea was used to raise homogeneously the pH of a... 

Generally, the experimental results on electrodeposition of CdS in acidic solutions of thiosulfate have implied that CdS growth does not involve underpotential deposition of the less noble element (Cd), as would be required by the theoretical treatments of compound semiconductor electrodeposition. Hence, a fundamental difference exists between CdS and the other two cadmium chalcogenides, CdSe and CdTe, for which the UPD model has been fairly successful. Besides, in the present case, colloidal sulfur is generated in the bulk of solution, giving rise to homogeneous precipitation of CdS in the vessel, so that it is quite difficult to obtain a film with an ordered structure. The same is true for the common chemical bath CdS deposition methods. 

One solution-based approach that works for gold catalysts, in that it produces highly active catalysts, is the deposition-precipitation (DP) method [8]. The DP method entails adjusting the pH, temperature, and gold concentration of an HAUCI4 solution to form a gold hydroxide species which is then deposited onto the support material [8]. This catalyst precursor is washed, dried, and annealed to form small (<5nm) catalyst particles [9]. The DP method has a number of limitations for example, DP cannot produce Au particles with diameters less than 5 nm on support materials with low-isoelectric points (lEPs) like SiOz and WO3 [5,10,11]. 

The utilization periods for AP01D given by the direct deposition of the excess calcite method has a minimum of 1 month and maximum of 6 months as shown in Fig. 2. It should be noted that the calcite precipitation rate equation at a saturation ratio above 1.72 provided shorter utilization time than the direct deposition method, indicating that the rate law overestimated the amount of calcite deposited. At a saturation ratio below 1.72, however, the rate law indicated a longer utilization period, which was expected since the calcite deposition is kinetically controlled rather than instantaneous deposition of the excess calcite. 

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.

Supported Au catalysts have been extensively studied because of their unique activities for the low temperature oxidation of CO and epoxidation of propylene (1-5). The activity and selectivity of Au catalysts have been found to be very sensitive to the methods of catalyst preparation (i.e., choice of precursors and support materials, impregnation versus precipitation, calcination temperature, and reduction conditions) as well as reaction conditions (temperature, reactant concentration, pressure). (6-8) High CO oxidation activity was observed on Au crystallites with 2-4 nm in diameter supported on oxides prepared from precipitation-deposition. (9) A number of studies have revealed that Au° and Au" play an important role in the low temperature CO oxidation. (3,10) While Au° is essential for the catalyst activity, the Au° alone is not active for the reaction. The mechanism of CO oxidation on supported Au continues to be a subject of extensive interest to the catalysis community. 

As with so many other gold-catalysed reactions, the particle size is important it has been claimed9,20 that only hemispherical particles between 2 and 10 nm in size are suitable and early work recommended 2.3 nm particles as the best.16 The deposition-precipitation (DP) method is favoured because of the intimate contact between metal and support that it gives. Particles smaller than 2 nm appear to shift the reaction mode towards formation of propane,7,9,10,15 which is clearly a waste of both propene and... 

For the deposition of active phase(s), impregnation, adsorption and ion exchange, (co)precipitation, deposition precipitation, and in situ crystallization methods can be used. Moreover, it is possible to mix the active phase in the mixture for extrusion or to deposit the active phase by using a mixed sol containing both the precursors of the oxidic species and the active phase or a slurry with the precursor powder of the active phase. Other coating techniques, e.g., CVD or CVI techniques, can also be used. The dispersion of the active phase depends strongly on the method and conditions used, its precursors form as well as on the history of the active phase. 

Monolayer measurements and film deposition A computer-controlled Langmuir type film balance (LT-102, Belarus) was used for 7t-A isotherm recording and LB film deposition by horizontal precipitation (HP) method. Silicon with native silicon dioxide layer and atomically flat mica surfaces were used for monolayer film deposition. 

Vapor-phase epoxidation of propylene using H2 and O2 was carried out over gold catalysts supported on mesoporous ordered (MCM-41) and disordered titanosilicates prepared hydrothermally or by modified sol-gel method. Gold nanoparticles were homogeneously dispersed on the titanosilicate supports by deposition-precipitation (DP) method. The catalysts and support materials were characterized by XRD, UV-Vis, surface area measurements (N2 adsorption) and TEM. NaOH was found to be the best precipitant to prepare Au catalysts with optimum propylene oxide yields and H2 efficiency. The extent of catalysts washing during preparation was found to affect the activity of the catalyst. The activity and hydrogen efficiency was found to depend on the type of mesoporous support used. 

The method of preparation has a considerable effect on the catalytic properties of supported gold. Coprecipitation, deposition-precipitation, chemical vapor deposition methods are especially effective for depositing gold as nanoparticles with diameters smaller than 5 nm and with strong interaction with the supports. 

With a view to optimizing the activity and selectivity of the Au/C catalyst, a short screening on deposition and activation methods was carried out. Comparing, in Table 3, three different deposition methods, namely alkaline precipitation (entry 1), absorption from diluted solution of HAUCI4 (entry 2) and incipient wetness impregnation (entry 3), followed by reduction of gold to metal, we observed that the first method performed the best. Although all the methods show... 

Figure 2. Ag particle size distribution determined by XRD from Ag(l 11) reflection in Ag catalysts prepared by sol-gel method (SG), precipitation-deposition (P) and impregnation (IMP)...

With the solvent deposition method, the drug and matrix polymer/macromolecule are dissolved in one organic solvent phase that is miscible with water. This solution is dispersed in an aqueous phase, and the organic solvent diffuses into the continuous water phase this leads to the precipitation of polymer and drug (deposition). 

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