Catalysts precipitation deposition

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... 

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. 

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. 

Various studies and some patents have been published on the use of membrane catalysts for the direct synthesis of H202 [73-81]. The redox treatment of the membrane influences the properties both in the synthesis and decomposition of H202. Formation of a hydrophobic layer improves the selectivity, because it limits the consecutive decomposition of hydrogen peroxide, limiting the chemisorption of H2 and re-adsorption of H202 [73]. Either polymeric or ceramic-type membranes could be used, but the latter are preferable to allow more robust operations. The mono- or bi-metallic Pd-based active component could be deposited either in the form of dispersed particles (e.g., by precipitation-deposition) or of a thin film (e.g., by... 

Geus JW, van Dillen AJ. Preparation of supported catalysts by deposition - precipitation. Handbook of Heterogeneous Catalysis. Weinheim Wiley-VCH Verlag GmbH Co. KGaA 2008. p. 428 167. 

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. 

After isolation the supported precipitate is washed, dried and usually calcined to produce a supported oxide which is then reduced, commonly in a hydrogen stream. Reduction of these supported oxides generally proceeds more readily than the mixed oxides produced by coprecipitation since there is only a monolayer in which there is a direct interaction of the active component with the support. This monolayer can be considered to be a silicate or aluminate which is more difficult to reduce than the oxide or hydroxide found in the outer metal-containing layers.33 Precipitation-deposition gives catalysts having compositions similar to those produced by sequential precipitation as shown in Fig. 13.2. 

Precipitation-deposition can be used to produce catalysts with a variety of supports, not only those that are formed from coprecipitated precursors. It has been employed to prepare nickel deposited on silica, alumina, magnesia, titania, thoria, ceria, zinc oxide and chromium oxide.36 It has also been used to make supported precious metal catalysts. For example, palladium hydroxide was precipitated onto carbon by the addition of lithium hydroxide to a suspension of... 

In coprecipitation and deposition, the metal load is determined primarily by the concentration of the metal salt in solution before precipitation or deposition. A coprecipitated catalyst has the active component distributed throughout the resulting catalyst particles. With catalysts prepared by deposition, particularly precipitation-deposition, the active component can be found primarily on the surface of the supporting material. With impregnated catalysts, however, the situation is not as simple with the amount of salt adsorbed and its location on or in the support particles determined by the variables in the adsorption procedure. Thus, the concentration of the precurser salt, the type of salt, solvent, temperature, nature of the support, time of contact with the support and the presence of other materials can all influence both the metal load and the location of the metal in the support particle. 

In Ni/Al203 catalysts prepared by impregnation there is a weaker interaction between the impregnating salt and the support than there is when the catalyst is prepared by deposition-precipitation. Calcination of the former catalysts gave large nickel oxide crystallites and, on reduction, the supported metal was rather easily sintered. Catalysts prepared by precipitation-deposition, on the other hand, were resistant to sintering because of the strong interaction... 

Preparation of iridium catalysts by deposition precipitation room temperature oxidation of CO... 

Two procedures for metal introduction in chitosan base were used impregnation and coprecipitation. According to the first procedure the metal deposition on chitosan micro beads was carried out from aqueous and alcohol solutions of NazPdCU, HzPdCU, RhCb, Rh2(CH3COO)4, ZnS04 and Pb(CH3COO)2. Pd and Pb/Zn in bimetallic catalysts was deposited by subsequent precipitation. Pd-Pb (Zn) atomic ratios were 1/1. Metal contents in the resulting samples were 0.5 - 4%. 

Catalytic properties (at p = 2 MPa, H2/CA = 20) of reduced monometallic Ag catalysts prepared by sol-gel technique, impregnation and precipitation-deposition... 

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)...

A 1.5 % w/w Ni supported on silica (Cab-O-Sil 5M, 194 m g ) catalyst precursor was prepared by homogeneous precipitation/deposition as described in detail elsewhere (24). The hydrated sample, sieved in the 150-200 pm range, was reduced, by heating directly in a 100 cm min stream of dry H2 (99.9%) at 5 K min to 673 1 K which was maintained for 18 h. Under these activation conditions the catalyst supports 1.1 x 10 exposed Ni atoms/g catalyst with a surface weighted average Ni diameter = 1.4 nm these values are based on chemisorption measurements (12). 

Optimized Conditions for the Preparation of Pt Supported Catalysts by Deposition-Precipitation... 

Deposition-precipitation of platinum was initially performed as previously described in literature, using chloroplatinic acid (H2PtCl6) as platinum precursor [1-2]. But, like for the preparation of Au/TiOa catalysts by deposition-precipitation [6], the pH of the suspension was increased up to 7 by adding NaOH. However, only 0.15 wt% of the initial 3 wt% Pt present in solution was... 

Geus, A. J. Van Dillen, Preparation of Supported Catalysts by Deposition Precipitation. In Ertl G, Knoezinger H, Weitkamp J (eds) Preparation of Solid Catalysts. Wiley-VCH, Weinheim, 1999, pp 460-487... 

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. 

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