Deposition-precipitation effect

Au/Ti02 systems prepared by deposition-precipitation method. Therefore, it is probable that a more relevant effect is the presence of some residual alkali on the surface due to the preparation method for these Titania nanotubes. 

Interior tube deposits. Coke, salts, and corrosion products can adhere to the inside wall of the tube. The deposits precipitate out of the flowing fluid, due to low tube-side velocity. Once the solids stop moving, they bake onto the tube wall. The flowing fluid can no longer effectively cool that portion of the tube wall, covered with these internal deposits. The minimum acceptable tube-side velocity to retard solids... 

A parameter that determines the performances, as outlined also in patents, is the mean diameter of Pd or doped-Pd particles. This is also one of the claims in Headwaters Nanokinetix Inc. patents. It seems that a maximum in the activity/ selectivity as a function of the particle size is present. Figure 8.11 reports the effect of the Pd-particle size (Pd supported on alumina, prepared by deposition-precipitation method) in the direct synthesis of H202 in water at atmospheric pressure [77]. Low... 

Effecting deposition-precipitation by decreasing the pH level is interesting with metal ions present in the stable state in aqueous solution as anions [35]. With silica no interaction is observed, which has led to the development of the electrochemical reduction procedure. To apply metal ions, such as, molybdenum or vanadium, on alumina, a homogeneous decrease in pH level is interesting. The pH level has been decreased by injection of nitric acid or perchloric acid and electro-chemically. However, the rate of crystallization of the hydrated oxides of vanadium(V) and molybdenum(VI) was observed to be fairly low. To prevent dissolution of the alumina supports the pH cannot be decreased to levels below about 3, at which the crystallization of the hydrated metal oxides does not proceed rapidly. 

Figure 12 Effect of metal particle shape on the TOP at 300 K for CO oxidation over Au/Ti02 and Pt/Ti02- Spherical particles corresponds to the sample prepared by impregnation and hemisperical particles by deposition-precipitation method. (Data are presented in Bamwenda, Catal. Lett. 44 (1997) 83.)...

To prepare supported metal catalysts for low-temperature CO oxidation, coprecipitation, deposition-precipitation, and grafting methods are effective, because they can give strongly interacting metal particles with the support. 

The addition of An to Pd by simultaneous deposition-precipitation on Ce02 increases the catalytic activity for methanol decomposition to CO and H2 at 450K [115], No significant promotional effect was observed by the addition of Rh or Ir to Pd. 

E. E. Stangland, B. Taylor, R. P. Andres, W. N. Delgass, Direct vapor phase propylene epoxidation over deposition-precipitation gold-titania catalysts m the presence of H2/O2 Effects of support, neutralizing agent, and pretreatment,/. Phys. Chem. B 109 (2005) 2321. 

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. 

The deposition-precipitation method is applied to pure anatase titanias with high surface areas and silica mesostructures doped with Ti/Ti02. The effect of the type and density of surface functionalities both on the deposition of gold and on the catalytic properties of the resulting materials in the oxidation of CO in the presence of H2 are discussed. 

Preparation of Pd on Carbon Black by Deposition-precipitation Study of the Effect of the Support Functionalisation... 

The aim of this work is thus to study the role of active phase precursor-support interactions in deposition-precipitation in an attempt to propose a preparation method avoiding the carbon black activation. The effect of carbon activation on deposition and ion exchange of palladium will be compared. A comparison of deposition-precipitation on two supports showing very different affinity towards noble metal, namely a non activated carbon black and TiOa, will be also provided. 

Precipitation or coprecipitation methods are also often used. Suh et al. [40] analyzed the effect of the oxygen surface functionalities of carbon supports on the properties of Pd/C catalysts prepared by the alkali-assisted precipitation of palladium chloride on carbon supports, followed by liquid-phase reduction of the hydrolyzed salt with a saturated solution of formaldehyde. They observed that the metal dispersion increased with increasing amount of oxygen surface groups. Nitta et al. [41] also used a deposition-precipitation method, with sodium carbonate and cobalt chloride or nitrate, to prepare carbon-supported Co catalysts for the selective hydrogenation of acrolein. 

The optimal temperature and atmosphere to treat the catalysts after deposition-precipitation for maximum catalytic activity is another area where there has been no agreement between various researchers. The calcination temperature and atmosphere can affect the oxidation state of the Au catalyst. This will be discussed in detail in the section on the nature of the Au active site. Another effect of heat treatment is the removal of chloride. A reductive treatment in the presence of H2 for catalysts with a high residual chloride content is generally beneficial [9,10]. Since chloride is highly mobile in the presence of moisture, in the selective catalytic CO oxidation in hydrogen (SCO) an inductive period was observed for a Au catalyst with a high chloride content at the low temperature of 373 K [10]. Presumably, during the induction period, the water formed in the reaction removes the chloride from the active site. 

Zhi et al. [41] investigated the effect of synthesis method on the WGS activity of Cu-Ce-La catalysts. They prepared the catalysts using co-precipitation, homogeneous precipitation and deposition-precipitation techniques. Among the various catalysts, the catalysts synthesized by co-precipitation method exhibit better activity. Then, they investigated the effect of precipitator. The sequence of activities are NaOH (61.10%)>NaOH-h(NH2)2CO (60.98%)> (NH2)2C0 (57.4%) NH3 X H2O (5.35%) at 300 °C. 

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