Heterogeneous catalysts supported metal particle preparation

The first chronological appearance of clusters in catalysis is their use as models for heterogeneous catalysts. More precisely, it was found that polynuclear metal complexes such as transition-metal clusters can act as soluble models for supported metallic particles, that are much more complicated to study. Clusters can be isolated and characterized by the classical methods of preparative chemistry. They show typical characteristics of metal surfaces, such as polycentric ligand-metal bonds and delocalized metal-metal bonds. The use of metal clusters as models for the surface of catalysts was named by Muetterties the duster-surface analogy [10]. The first development in this area of research was mainly structural, and consisted in investigating the interaction... 

Active heterogeneous catalysts have been obtained. Examples include titania-, vanadia-, silica-, and ceria-based catalysts. A survey of catalytic materials prepared in flames can be found in [20]. Recent advances include nanocrystalline Ti02 [24], one-step synthesis of noble metal Ti02 [25], Ru-doped cobalt-zirconia [26], vanadia-titania [27], Rh-Al203 for chemoselective hydrogenations [28], and alumina-supported noble metal particles via high-throughput experimentation [29]. 

This method is certainly the oldest one described in the literature the first example concerns the ion exchange of [Pt(NH3)4]2+ and the surface of a sulfonated silica.15 Even now, the preparation of many heterogeneous catalysts (i.e., supported metal or oxide particles) involves as the first step the reaction of a coordination complex with the surface of an ionic solid such as alumina,... 

The induction of steric effects by the pore walls was first demonstrated with heterogeneous catalysts, prepared from metal carbonyl clusters such as Rh6(CO)16, Ru3(CO)12, or Ir4(CO)12, which were synthesized in situ after a cation exchange process under CO in the large pores of zeolites such as HY, NaY, or 13X.25,26 The zeolite-entrapped carbonyl clusters are stable towards oxidation-reduction cycles this is in sharp contrast to the behavior of the same clusters supported on non-porous inorganic oxides. At high temperatures these metal carbonyl clusters aggregate to small metal particles, whose size is restricted by the dimensions of the zeolitic framework. Moreover, for a number of reactions, the size of the pores controls the size of the products formed thus a higher selectivity to the lower hydrocarbons has been reported for the Fischer Tropsch reaction. 

In heterogeneous catalysis by metal, the activity and product-selectivity depend on the nature of metal particles (e.g., their size and morphology). Besides monometallic catalysts, the nanoscale preparation of bimetallic materials with controlled composition is attractive and crucial in industrial applications, since such materials show advanced performance in catalytic processes. Many reports suggest that the variation in the catalyst preparation method can yield highly dispersed metal/ alloy clusters and particles by the surface-mediated reactions [7-11]. The problem associated with conventional catalyst preparation is of reproducibility in the preparative process and activity of the catalyst materials. Moreover, the catalytic performances also depend on the chemical and spatial nature of the support due to the metal-support interaction and geometrical constraint at the interface of support and metal particles [7-9]. 

The active components of many commercial supported heterogeneous catalysts are oxides or salts. Even for many metal catalysts, the precursors of metallic particles are also oxides or salts in some dispersed form. Hence the preparation of heterogeneous catalysts is deeply concerned in one way or another about the dispersion of oxides or salts on support surfaces. Furthermore, promoters or additives added to heterogeneous catalyst systems are also oxides or salts. Therefore, the spontaneous monolayer dispersion of oxides or salts on supports with highly specific surfaces as a widespread phenomenon will find extensive application in heterogeneous catalysis. Examples illustrative of this viewpoint are cited in the following sections. 

In this chapter, we will review the reaction dynamics studies which has been performed on supported model catalysts in order to unravel the elementary steps of heterogeneous catalytic reactions. In particular we will focus on the aspects that cannot be studied on extended surfaces like the effect of the size and shape of the metal particles and the role of the substrate in the reaction kinetics. In the first part we will describe the experimental methods and techniques used in these studies. Then we present an overview of the preparation and the structural characterization of the metal particle. Later, we will review the adsorption studies of NO, CO and 02. Finally, we will review the two reactions that have been investigated on the supported model catalysts the CO oxidation and the NO reduction by CO. 

The non-ideality of catalyst surfaces has ever been one the major difficulties in understanding the detailed mechanisms of contact catalysis. The Advances in Catalysis were opened in 1948 by an article of Taylor on the heterogeneity of catalyst surfaces for chemisorption [4] that the matter was not easy to model is understood by observing that 41 years later the role of particle size on the catalytic activity of supported metals was the subject of of another review in the same series [5] moreover, the family of solids of catalytic interest has since Taylor s review been increased by the availability of new techniques for the preparation of highly dispersed solids, like crystalline zeolites and amorphous aerogels. 

The results described in a previous paper [7] and this one indicate that the use of chelated metal precursors for the preparation of heterogeneous catalysts can suitably be extended to mesoporous support materials. The mechanisms underlying the fundamental processes occurring during catalyst preparation appear to be the same for both types of support materials. Therefore no limitations appear to exist to apply a wide variety of other elements into the pores of several types of mesoporous supports, with retention of the unique textural and structural properties of the support materials. Catalysts thus prepared will feature very high dispersions of active phase as well as very small particles with sizes even smaller tlmn on conventional support materials (due to the limiting size of the pores of mesoporous support materials). 

Incipient wetness impregnation is by far the most widely used method for the preparation of heterogeneous catalysts. This method is attractive because of its technical simplicity, low costs and limited amount of waste. Generally, a support is impregnated with a precursor-containing solution and dried. The dry product is then further treated through activation treatments (e.g. calcination and/or reduction) to obtain the desired catalyst. It is important that optimal synthesis parameters are found, since the efficiency of a catalyst is defined by the size of the active metal(oxide) particles, and their accessibility and distribution over the support. It is well-known that the use of metal nitrates as precursor salts for cobalt, iron and nickel catalysts ultimately yields poorly dispersed catalysts [1-3]. This is regrettable, as the use of nitrates as precursor is... 

The first pubhshed studies concerning the use of CNTs or CNFs in the field of heterogeneous catalysis date back to 1994 [18,19], and specific activities and selectivities of CNT or CNF-supported catalysts were reported. As a matter of fact, an industrial interest exists in the area of supported catalysts for fluid-phase reactions [20] or for fuel cell electrodes [21]. Since these first reports, numerous studies have brought better knowledge about these carbon nanostructmes and opened the way to their application in catalysis. Besides the study of the specific properties of these materials, special attention has been paid to the design of CNT/CNF supports, to the preparation of metal particles on CNTs or CNFs [22],... 

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