Catalyst synthesis impregnation-reduction

The next step in the synthesis of supported metal catalysts via dendrimers is the immobilization of dendrimer-metal nanocomposites onto a solid support. An array of techniques exists for achieving this task. Wet impregnation and sol-gel incorporation of dendrimer-metal nanocomposites may lead to strongly adhered metal particles. Other techniques, such as functionalization of the support to facilitate dendrimer growth or adhesion, provide a route for deposition of empty dendrimers that can subsequently undergo complexation with metal precursors to form dendrimer-metal complexes and eventually zerovalent nanoparticles. Whereas the complexation and reduction phases of catalyst synthesis via dendrimers can be fairly complicated, most methods of dendrimer deposition are rather straightforward. 

In the development of fuel ceU catalysts, catalyst synthesis plays a critical role in improving catalyst activity and stability. Over the last several decades, many syntheses methods have been developed, including the impregnation-reduction, colloid, sol-gel, and microwave-assisted methods. Experimental results showed all these methods to be effective in synthesizing catalysts for PEM fuel cells. The most important progress in recent years has been in the synthesis of nanostructured catalysts for fuel cell applications [52]. Nanostractured Pt-based catalysts are claimed to be much more active than the commercially available Pt/C catalysts. 

Figure 4. XPS Ru 3d data observed for the Ru/MgO catalysts. The Ru 3d spectra (from bottom to top) were obtained with the precursor after heating in high vacuum to 773 K, after reduction in 1 bar synthesis gas up to 773 K, and after impregnation with aqueous CsNOs solution and subsequent reduction in synthesis gas up to 673 K.

The present paper focuses on the interactions between iron and titania for samples prepared via the thermal decomposition of iron pentacarbonyl. (The results of ammonia synthesis studies over these samples have been reported elsewhere (4).) Since it has been reported that standard impregnation techniques cannot be used to prepare highly dispersed iron on titania (4), the use of iron carbonyl decomposition provides a potentially important catalyst preparation route. Studies of the decomposition process as a function of temperature are pertinent to the genesis of such Fe/Ti02 catalysts. For example, these studies are necessary to determine the state and dispersion of iron after the various activation or pretreatment steps. Moreover, such studies are required to understand the catalytic and adsorptive properties of these materials after partial decomposition, complete decarbonylation or hydrogen reduction. In short, Mossbauer spectroscopy was used in this study to monitor the state of iron in catalysts prepared by the decomposition of iron carbonyl. Complementary information about the amount of carbon monoxide associated with iron was provided by volumetric measurements. 

The tris-neopentyl Mo(VI) nitride, Mo(-CH2- Bu)3(=N) [134], reacts with surface silanols of silica to yield the tris-neopentyl derivative intermediate [(=SiO)Mo (-CH2- Bu)3(=NH)] followed by reductive elimination of neopentane, as indicated by labeling studies from labeled starting organometallic complex, to yield the final imido neopentylideneneopentyl monosiloxy complex [(=SiO)Mo(=CH- Bu)(-CH2 - Bu)(=NH)] [135]. The surface-bound neopentylidene Mo(VI) complex is an active olefin metathesis catalyst [135]. Improved synthesis of the same surface complex with higher catalytic activity by benzene impregnation rather than dichlorometh-ane on silica dehydroxylated at 700 °C has been reported [136],... 

Niobium Products Co., 50 m /g). Many different synthesis methods have been used to prepare supported metal oxide catalysts. In the case of supported vanadium oxide catalysts, the catalysts were prepared by vapor phase grafting with VOCI3, nonaqueous impregnation (vanadium alkoxides), aqueous impregnation (vanadium oxalate), as well as spontaneous dispersion with crystalline V2O5 [4]. No drastic reduction of surface area of the catalysts was observed. 

Impregnation and physically mixed Re catalysts were much less active and much less selective for the phenol synthesis (Table 2.4). The CVD catalyst was almost 18 times more active than the conventional impregnation catalyst. In the physically mixed and impregnated catalysts, the Re7 + precursors partly aggregated as ReOx like Re02 in the presence of the NH3 reductant and such ill-defined Re aggregates decreased both activity and phenol selectivity as shown in Table 2.4. 

In the preparation and activation of a catalyst, it is often the case that the chemical form of the active element used in the synthesis differs from the final active form. For example, in the preparation of supported metal nanoclusters, a solution of a metal salt is often used to impregnate the oxide support. The catalyst is then typically dried, calcined, and finally reduced in H2 to generate the active phase highly dispersed metal clusters on the oxide support. If the catalyst contains two or more metals, then bimetallic clusters may form. The activity of the catalyst may depend on the metal loading, the calcination temperature, and the reduction temperature, among others. 

The preparation of Ru supported catalysts by sol-gel method, indeed, was extended to obtain new formulations by changing the type of support. Alkali-promoted Ru/MgO systems were prepared starting from magnesium ethoxide, Ru3(CO)i2 and a cesium compound [9]. The gels were subjected to an activation/reduction procedure to substantially obtain Ru-CsOH/MgO and then tested as catalysts in the ammonia synthesis at atmospheric pressure. It was evidenced that the sol-gel prepared Cs-promoted Ru/MgO catalysts are much more active, under similar reaction comlitions, than the analogous catalysts prepared by the impregnation procedures reported in literature [10]. 

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