Impregnation method, heterogeneous

Navio, J.A., G. Colon, M. Trillas, J. Peral, X. Domenech, J.J. Testa, J. Padron, D. Rodriguez and M.I. Litter (1998). Heterogeneous photocatalytic reactions of nitrite oxidation and Cr(VI) reduction on iron-doped titania prepared by the wet impregnation method. Applied Catalysis B-Environmental, 16(2), 187-196. 

The results presented here demonstrate that well-defined model catalysts with particle sizes in the 10-100-nm regime can be efficiently processed with lithographic methods. These model catalysts possess most of the desirable characteristics that are required in fundamental heterogeneous catalysis research. There are considerable advantages, compared to model catalysts prepared by, e.g., incipient wetness impregnation methods, which are enumerated ... 

In general, activities of chitosan based catalysts prepared by impregnation method in hydrogenation of unsaturated organic compounds were comparable with those of traditional heterogeneous catalyst (as calculated per 1 mole of metal). It should be noted that the chitosan pretreatment influenced very much the catalyst activity. For instance, immediate Pd deposition from alcohol solution on dry chitosan fibers or micro beads led to almost completely inactive catalytic systems, regardless of the metal content. On the other hand, metal deposition on chitosan micro beads or fibers preliminary swollen in water dramatically improved the catalytic activity. 

The complex iron(ll) phthalocyanine, [FePc], supported onto activated CBs [55a] by the impregnation method, was used as a heterogeneous catalyst in the oxidation of alkanes, with TBHP as an oxygen source. Carbon black proved to be an appropriate support for [FePc] complex within the framework of the oxidation of alkanes. The hydrophobicity of the support surface induced the alkane adsorption, leading to a more ideal substrate/oxidant ratio near the active center, which explained the high activity and efficiency reached with the immobilized [FePc],... 

Metallic particles displaying a small particle size, around a few nanometres, and narrow size distribution could easily be obtained via the microemulsion technique. Such particles have proved to be active in several classic heterogeneous catalytic processes, in some cases displaying a higher performance than the catalysts prepared following more traditional impregnation methods. Major... 

The manufacture of heterogeneous catalysts from pre-prepared nanometal colloids as precursors via the so-called precursor concept ll has attracted industrial inter-est.l l An obvious advantage of the new mode of preparation compared with the conventional salt-impregnation method is that both the size and the composition of the colloidal metal precursors can be tailored for special applications independently of the support. In addition, the metal particle surface can be modified by lipophilic or hydrophilic protective shells, and covered with intermediate layers, e.g. of oxide. The addition of dopants to the precursor is also possible. The second step of the manufacture of the catalyst consists in the simple adsorption of the pre-prepared particles by dipping the supports into organic or aqueous precursor solutions at ambient temperature. This has been demonstrated, e.g., for charcoal, various oxidic support materials, even low-surface materials such as quartz, sapphire, and highly oriented pyrolytic graphite. A subsequent calcination step is not required (see Fig. 1). 

So far, we have considered catalytic materials that conform to the side walls of a microreactor. A downside to a functionalized coating at a channel wall is the limited catalytic surface area that can be provided. As an alternative, thin-film technology can be used for depositing catalytic materials on more complex three-dimensional surfaces inside microchannels [49]. Impregnation methods can also be used on porous silicon surfaces [50]. The mass transfer rates described for such structures are sufficient for all but the fastest heterogeneous reactions. 

For example, titania-modified mesoporous silicates was synthesized using titanium tetrabutoxide by an impregnation method with variable Si/Ti ratios and applied to the degradation of a dye. A basic dye such as methylene blue was photo-catalytically degraded by titania-modified mesoporous silica under UV irradiation. And these heterogeneous catalysts were also applied to the degradation of organic pollutants such as phenol and toluene [96]. 

The method of impregnating liquid membranes has become more and more popular. By impregnating fine-pore polymer films with a suitable membrane liquid, relatively stable heterogeneous solid-liquid membranes are obtained. These membranes are shaped as thin, flat barriers or hollow fibers. Usually they are manufactured from oleophilic polymers, wettable by membrane liquid. The two interfaces, F/M and M/R, have equal or close areas which can be made very large by employing modules of spirally wounded flat membrane or bundles of hollow fibers. 

Industrial heterogeneous catalysts and laboratory-scale model catalysts are commonly prepared by first impregnating a support with simple transition metal complexes. Catalytically active metal nanoparticles (NPs) are subsequently prepared through a series of high temperature calcination and / or reduction steps. These methods are relatively inexpensive and can be readily applied to numerous metals and supports however, the NPs are prepared in-situ on the support via processes that are not necessarily well understood. These inherent problems with standard catalyst preparation techniques are considerable drawbacks to studying and understanding complex organic reaction mechanisms over supported catalysts. (4)... 

A more widely used method of heterogenization has also been applied to MTO, by using a metal oxide support onto which the MTO is applied. MTO has been applied onto niobia (Nb2Os) both via impregnation and sublimation [39] and onto silica (Si02) via reaction of a bipyridine-containing siloxane [40]. In a similar manner, MTO has been immobilized on the mesoporous silica MCM-41 [41]. Zeolite NaY has been used as a support material by the in situ immobilization of the MTO catalyst [42]. 

Two applications of ML are heterogeneous catalyst and sorbent synthesis. In these cases it is necessary to distribute in regular intervals very small amounts of substance on a surface. It is usually enough to have from 1 to 4 monolayers. In this case each atom in a monolayer can take part in reaction. This is not obtained using traditional methods of impregnation. 

Methods of catalyst preparation are very diverse and each catalyst may be produced via different routes. Preparation usually involves several successive steps. Many supported metal and oxide catalysts are prepared by the succession of impregnation, drying, calcination, activation zeolite catalysts are prepared by precipitation of gel, crystallisation, washing, ion exchange, drying. The properties of heterogeneous catalysts depend on all their previous history. 

Most of the heterogeneous catalyst which are in practical use consist of one or more catalytically active compounds which are impregnated on supporting carrier materials. This method can be chosen to immobilise acids and bases as well as salts, oxides or complexes. The major drawback is leaching of one or more component which leads to irreversible deactivation of the catalyst. Physisorption can be enhanced by choosing the appropriate porous, chemical and electronical properties. This leads to catalysts with sufficient long term stability due to e.g. ionic linkages. 

For heterogeneous studies. As(III) was adsorbed onto silica gel impregnated with ferric hydroxide, according to the batch method of Yoshida a]. ( ). The initial supernatant solution, and the supernatant solution resulting from an acid rinse (1 M HCl) of the silica gel were analyzed polarographically for As and Fe. 

Microemulsion impregnation of a catalytic support can be a very good method for preparation of heterogeneous catalysts used in reactions with mass transfer limitation, for which high concentration of active metal in the outer shell of catalyst pellets and narrow metal size distribution is important. 

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