Preparation of monolithic catalysts

Similar to normal catalyst synthesis whereby ion exchange methods can result in egg-shell structures, in the preparation of monolith catalysts the majority of the metal can be deposited at the entrance of the monolith. Egg-shell structures can be attractive for catalyst particles, but for monoliths, analogous uneven distributions of the active phase are a disaster. Fortunately, extensive literature is available describing ion-exchange procedures for conventional catalysts that yield homogeneous metal distributions. This literature can be used as a guide for preparing satisfactory monolithic catalysts. 

Despite many applications of monolithic catalysts, articles describing their preparation in some detail are scarce [1-5 and the references therein]. The main reason probably is that most preparation work is done in industry and there is no commercial interest in publishing the information gained. In this chapter, methods and techniques used in the preparation of monolithic catalysts are described, partly based on our own research. It will be shown that conventional preparation methods are successfully applied, though more special precautions are advisable. 

Preparation of monolithic catalysts for space propnlsion applications... 

In principle, deposition of an active phase (metal and/or oxide) on a monolithic catalyst support can be carried out in a manner similar to that used to prepare a t) ical catalyst. However, the large dimension of a monolith can easily enhance problems of nonhomogeneous deposition. For example, if in the preparation of conventional catalyst particles the active phase would be deposited at the external surface of the support, the result would be an egg-shell-t) e catalyst, which for many processes can be advantageous. However, if this pattern of deposition were applied to a monolithic support, it could result in a monolith with only the outer charmels of the structure having a significant catalytic activity, resulting in a dramatically poor catalytic reactor. The critical steps in the s)mthesis process are the deposition and drying steps, which are discussed separately below. Calcination, reduction, etc. for monolith catalysts are not different from those used to manufacture t) ical catalysts, and these steps are therefore not discussed here. 

There are three important outstanding issues that need concerted R D. The first issue is the further development of materials with thermal stability under combustion condition at the high temperatures prevailing in a gas turbine combustion chamber. Promising materials have been developed, but none fulfills the demand of a lifespan of at least 1 year. Besides, the most promising materials, such as the family of the hexa-aluminates, must be available in a honeycomb monolith shape, either as washcoat or directly extruded. Much work still needs to be done to optimize the preparation of monolithic thermostable catalysts. 

MFM Zwinkels SG Jaras and PG Menon Preparation of combustion catalysts by washcoating alumina whiskcrs-covered metal monoliths using a sol-gel method. Catalyst Preparation VI (G Poncelet, J Martens, B Delmon, PA Jacobs, and P Grange, Eds), Elsevier, Amsterdam, 1995, p 85... 

Various methods are possible to incorporate a catalytically active phase to the monolith [48-59,85-95]. Figure 3 shows the general scheme for preparing a monolithic catalyst structure from a washcoated monolith. In fact, no fundamental differences exist between incorporation of an active phase in a conventional support (beads, extrudates, spheres) and in monoliths. In practice, precautions are needed because, besides concentration profile on a particle scale, such profile over the length of the monolith also can easily arise. 

In some preparation steps of monolithic catalysts, microwave heating appears to be promising. This heating is faster and more selective than the conventional ones. Moreover, it may lead to a more homogeneous distribution of the active phase in the catalysts. 

H. Vovk. Preparation and Characterization of Monolithic Catalysts. Masters thesis. Delft University of Technology, 1994, The Netherlands. 

The monolithic substrates were cut lengthwise into quarter sections prior to preparation of the catalyst. Once the four catalytic samples were prepared, they were combined to yield a complete monolith by cementing the quartered sections together with Sauereisen Number 8, a ceramic adhesive, as indicated in Figure 1. 

Zwinkels, M.F.M., Jaras, S.G., and Menon, P.G. (1994) Preparation of combustion catalysts by washcoating alumina-whiskers-covered metal monoliths using a sol-gel method. Proc. 6th Int. Symp. on Scientific Bases for the Preparation of Heterogeneous Catalysts, Louvain-la-Neuve, Belgium, Sep. 5-9, Vol. I, pp. 85-94. 

Preparation of combustion catalysts by washcoating alumina whiskers-covered metal monoliths using a sol-gel method... 

New method of dispersed perovskites synthesis based upon mechanochemical activation of the solid starting compounds is elaborated. The influence of defect structure of these compoimds as well as surface segregation on their catalytic properties is discussed. Basic stages of the monolith perovskite catalysts preparation are optimized. The experimental samples of monolith catalysts of various shapes are obtained, possessing high activity, thermal stability and resistance to catalytic poisons. 

Some of the most active catalysts used in the Selective Catalytic Reduction (SCR) processes to remove nitrogen oxides (NOx) from exhaust gas streams are those based on vanadia supported on titania [1]. In order to avoid the problems associated with pressure drop and diffusional limitations, encountered with conventional peletted catalysts in forms of cylinders and spheres etc., the supports should ideally be configured as honeycomb monoliths for these reactions which normally take place with high space velocities due to the large volumes of gas to be treated [2]. However, the difficulties encountered in the preparation of monoliths based solely on titania makes the inclusion of binders to both improve the rheological properties of the paste prior to extruding and the soundness of the monolith with subsequent thermal treatment a necessity [3]. 

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