Precipitation-deposition catalysts prepared

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... 

The incipient wetness catalyst was prepared with an aqueous solution of Ni(N03)2.6 H2O added slowly to the support (Silica Ketjen 77, 270 m g" ). The solution concentration was adjusted to obtain catalysts with ca. 10% nickel content. The precipitation-deposition catalyst was prepared by precipitating nickel from an aqueous solution of nickel nitrate slurried with the support. The precipitation was produced by a slow and homogeneous change in the pH induced by urea thermal decomposition. 

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. 

As the metal particle size decreases the filament diameter should also decrease. It has been shown that the surface energy of thirmer filaments is larger and hence the filaments are less stable (11,17-18). Also the proportion of the Ni(l 11) planes, which readily cause carbon formation, is lower in smaller Ni particles (19). Therefore, even though the reasons are diverse, in practice the carbon filament formation ceases with catalysts containing smaller Ni particles. Consequently, well dispersed Ni catalysts prepared by deposition precipitation of Ni (average metal particle size below 2-3 nm) were stable for 50 hours on stream and exhibited no filamentous coke [16]. 

Figure 13. Specific 2-CP (open symbols) and 2,4-DCP (solid symbols) hydrodechlorination rate constant K) as a function of the average Ni particle diameter ( nO for reaction over Ni catalysts prepared via impregnation with nitrate (0,0), deposition-precipitation (A,A) and impregnation with nickel ethanediamine ( , ) r= 423K reaction data refer to aqueous solutions. (Reprinted from Reference [147], 2003, with permission from Royal Society of Chemistry).

In 1989, Gadalla and Sommer (252) reported that a solid-solution NiO/MgO (1 1.35) catalyst prepared by precipitation can inhibit the carbon deposition in the CO2 reforming of methane however, they obtained a low CO2 conversion (66%), a low H2 selectivity (79%), and a low CO selectivity (77%), even at the very low WHSV of 3714 cm3 (g catalyst)-1 h-1 with a CH4/CO2 (1/1, molar) feed gas and the high temperature of 1200 K. Their relatively high CH4 conversion was partly a consequence of homogeneous gas-phase reactions that occurred under their conditions. Indeed, the authors found extensive carbon deposits plugging the reactor upstream and downstream of the reaction zone. 

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. 

Pd on ceria catalysts prepared by various methods were shown to exhibit varying characteristics depending on the preparation methods.625 Specifically, Pd on ceria samples prepared by the deposition-precipitation method are highly active in comparison with the catalyst prepared by the conventional impregnation method. Cationic palladium species are present in the former samples after reduction with hydrogen at 300°C, suggesting that the active species are produced by strong... 

Figure 3.2 (a) TEM micrograph of an Au/Ti02 catalyst prepared by deposition precipitation (b) schematic model of the interface [31]. 

In this section, some of the most important methods for catalyst preparation including impregnation, grafting, precipitation, and chemical vapor deposition (CVD) are discussed [6-18],... 

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. 

It is clear that for those catalyst preparations in which the mobility of the precursor is very low, the drying step is much less of an issue. This is the case, for example, for ion exchange or deposition precipitation, and these procedures are discussed in the next sections. 

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... 

Since supported gold catalysts prepared by coprecipitation were found to be active for CO oxidation even at temperatures far below room temperature, attempts are increasing to prepare other noble metal catalysts by coprecipitation, deposition-precipitation, and grafting methods, which were used for the preparation of active supported gold catalysts. Although the affinity to CO is markedly different between Pt-group metals and Au supported on selected metal oxides, the contribution of metal-support interactions to the enhancement of low-temperature catalytic activity for CO oxidation appears to be similar, namely, the enhancement of oxygen activation at the perimeter interface. This line of approach may be valid to seek for a new type of catalysts active at lower temperatures for reactions other than CO oxidation [82,83]. 

Shan, W., Feng, Z., Li, Z., Zhang, J., Shen, W., and Li, C. Oxidative steam reforming of methanol on CeogCuo.iOy catalysts prepared by deposition-precipitation, coprecipitation, and complexation-combustion methods. Journal of Catalysis, 2004, 228 (1), 206. 

Sintering of Ni/Si02 Catalysts Prepared by Impregnation and Deposition-Precipitation During CO Hydrogenation... 

---