Dispersed Supported Catalysts

For the overall performance of potential catalysts in practical application additional factors, such as number of active sites, physical form, and porosity must also be taken into account. The classical commercial iron catalyst is an unsupported catalyst. First of all iron is a cheap material and secondly by the incorporation of alumina a surface area similar to that attained in highly dispersed supported catalysts can be obtained. Of course, for an expensive material such as the platinum group metals, the use of a support material is the only viable option. The properties of the supported catalyst will be influenced by several factors [172]... 

HIGH-DISPERSED SUPPORTED CATALYSTS ON BASIS OF MONODISPERSE PT-SOLES IN PROCESSES REDUCTIVE OF TRANSFORMATION OF HYDROCARBONS... 

Copolymers can be obtained from a single feed (e.g., ethylene) by concurrent tandem catalysis, using two catalysts. The first makes ethylene oligomers, while the second copolymerizes the oligomers with ethylene to give linear, low-density polyethylene (LLDPE). This is illustrated in Scheme 5.8. While the need for mutual chemical compatibility limits possible catalyst combinations in solution, dispersed supported catalysts may be unable to interact as long as they remain immobilized, and may therefore tolerate each other better. However, it is necessary to match the catalytic activity of the two catalysts (via the catalyst ratio, as well as the reaction conditions) so that both contribute significantly to the overall reaction. 

Preparation of highly dispersed supported catalysts by ultrasound... 

Particle size effects of highly dispersed supported catalysts (Pt) on the hydrogen oxidation reaction were evaluated by the same authors using this technique (97). Moreover, electrocatalysts for oxidation of methanol were screened using a technique called scanning differential electrochemical mass spectrometry (98, 99). This method uses a capillary probe scanned over the array that allows the intake and detection by mass spectrometry of products generated locally on each electrode. 

Alyea et al. combined chemical vapor deposition and impregnation in a preparation procedure which they denoted as metal oxide chemical vapor synthesis in order to obtain uniform, well-dispersed supported catalysts [265,266]. In the rotary reaction vessel sketched in Fig. 11, vapors of transition metal oxides... 

Because x-rays are particularly penetrating, they are very usefiil in probing solids, but are not as well suited for the analysis of surfaces. X-ray diffraction (XRD) methods are nevertheless used routinely in the characterization of powders and of supported catalysts to extract infomration about the degree of crystallinity and the nature and crystallographic phases of oxides, nitrides and carbides [, ]. Particle size and dispersion data are often acquired with XRD as well. 

In the case of metal particles distributed on a support material (e.g. supported catalysts), XPS yields infomiation on the dispersion. A higher metal/support intensity ratio (at the same metal content) indicates a better dispersion [3]. 

M0S2 is one of the most active hydroprocessing catalysts, but it is expensive, and the economical way to apply it is as highly dispersed material on a support, y-Al202. The activity of the supported catalyst is increased by the presence of promoter ions, Co " or Ni ". The stmctures of the catalysts are fairly well understood the M0S2 is present in layers only a few atoms thick on the support surface, and the promoter ions are present at the edges of the M0S2 layers, where the catalytic sites are located (100,101). 

Cost. The catalytically active component(s) in many supported catalysts are expensive metals. By using a catalyst in which the active component is but a very small fraction of the weight of the total catalyst, lower costs can be achieved. As an example, hydrogenation of an aromatic nucleus requires the use of rhenium, rhodium, or mthenium. This can be accomplished with as fittie as 0.5 wt % of the metal finely dispersed on alumina or activated carbon. Furthermore, it is almost always easier to recover the metal from a spent supported catalyst bed than to attempt to separate a finely divided metal from a liquid product stream. If recovery is efficient, the actual cost of the catalyst is the time value of the cost of the metal less processing expenses, assuming a nondeclining market value for the metal. Precious metals used in catalytic processes are often leased. 

Hydrogenation. Hydrogenation is one of the oldest and most widely used appHcations for supported catalysts, and much has been written in this field (55—57). Metals useflil in hydrogenation include cobalt, copper, nickel, palladium, platinum, rhenium, rhodium, mthenium, and silver, and there are numerous catalysts available for various specific appHcations. Most hydrogenation catalysts rely on extremely fine dispersions of the active metal on activated carbon, alumina, siHca-alumina, 2eoHtes, kieselguhr, or inert salts, such as barium sulfate. 

As was found in Ref. [13], the method of catalytic decomposition of acetylene on graphite-supported catalysts provides the formation of very long (50 fim) tubes. We also observed the formation of filaments up to 60 fim length on Fe- and Co-graphite. In all cases these long tubules were rather thick. The thickness varied from 40 to 100 nm. Note that the dispersion of metal particles varied in the same range. Some metal aggregates of around 500 nm in diameter were also found after the procedure of catalyst pretreatment (Fig. 2). Only a very small amount of thin (20-40 nm diameter) tubules was observed. 

It will also be shown that the absolute electrode potential is not a property of the electrode but is a property of the electrolyte, aqueous or solid, and of the gaseous composition. It expresses the energy of solvation of an electron at the Fermi level of the electrolyte. As such it is a very important property of the electrolyte or mixed conductor. Since several solid electrolytes or mixed conductors based on ZrC>2, CeC>2 or TiC>2 are used as conventional catalyst supports in commercial dispersed catalysts, it follows that the concept of absolute potential is a very important one not only for further enhancing and quantifying our understanding of electrochemical promotion (NEMCA) but also for understanding the effect of metal-support interaction on commercial supported catalysts. 

Consequently the absolute potential is a material property which can be used to characterize solid electrolyte materials, several of which, as discussed in Chapter 11, are used increasingly in recent years as high surface area catalyst supports. This in turn implies that the Fermi level of dispersed metal catalysts supported on such carriers will be pinned to the Fermi level (or absolute potential) of the carrier (support). As discussed in Chapter 11 this is intimately related to the effect of metal-support interactions, which is of central importance in heterogeneous catalysis. 

Figure 11.3. Schematic of the experimental setup used (a) to induce electrochemical promotion (via YSZ) on Ir02 and Ir02-Ti02 porous catalyst films (b) to compare the electrochemical promotion induced on Pt via YSZ and via Ti02 and (c) to compare the electrochemical promotion behaviour induced by varying UWR on a Rh porous catalyst film (left) and on a fully dispersed Rh catalyst supported on porous (80 m2/g) YSZ support.22...

In dispersed metal-support systems (Fig. 11.2 right), one can vary pe(M) - M-e(S) by varying the support or by doping the support with aliovalent cations. This is known in the literature as dopant-induced metal-support interactions (DIMSI).8,11,41,42 Thus one can again vary the electrochemical potential and thus the coverage of backspillover O2 on the supported catalyst surface. 

Hi ly dispersed supported bimetallic catalysts with bimetallic contributions have been prepared from molecular cluster precursors containing preformed bimetallic bond [1-2]. For examples, extremely high dispersion Pt-Ru/y-AUOa could be prepared successfully by adsorption of Pt2Ru4(CO)ison alumina [2]. By similar method, Pt-Ru cluster with carbonyl and hydride ligands, Pt3Ru6(CO)2i(p3-H)(p-H)3 (A) was used in this work to adsorb on MgO support. The ligands were expectedly removable from the metal framework at mild conditions without breaking the cluster metal core. 

Fig. 4 shows the current density over the supported catalysts measured in 1 M methanol containing 0.5 M sulfuric acid. During forward sweep, the methanol electro-oxidation started to occur at 0.35 V for all catalysts, which is typical feature for monometallic Pt catalyst in methanol electro-oxidation [8]. The maximum current density was decreased in the order of Pt/CMK-1 > Pt/CMK-3 > Pt/Vulcan. It should be noted that the trend of maximum current density was identical to that of metal dispersion (Fig. 2 and Fig. 3). Therefore, it is concluded that the metal dispersion is a critical factor determining the catalytic performance in the methanol electro-oxidation.

Because XPS is a surface sensitive technique, it recognizes how well particles are dispersed over a support. Figure 4.9 schematically shows two catalysts with the same quantity of supported particles but with different dispersions. When the particles are small, almost all atoms are at the surface, and the support is largely covered. In this case, XPS measures a high intensity Ip from the particles, but a relatively low intensity Is for the support. Consequently, the ratio Ip/Is is high. For poorly dispersed particles, Ip/Is is low. Thus, the XPS intensity ratio Ip/Is reflects the dispersion of a catalyst on the support. Several models have been reported that derive particle dispersions from XPS intensity ratios, frequently with success. Hence, XPS offers an alternative determination of dispersion for catalysts that are not accessible to investigation by the usual techniques used for particle size determination, such as electron microscopy and hydrogen chemisorption. 

Transmission electron microscopy is one of the techniques most often used for the characterization of catalysts. In general, detection of supported particles is possible, provided that there is sufficient contrast between particles and support - a limitation that may impede applications of TEM on well-dispersed supported oxides. The determination of particle sizes or of distributions therein is now a routine matter, although it rests on the assumption that the size of the imaged particle is truly proportional to the size of the actual particle and that the detection probability is the same for all particles, independent of their dimensions. 

Metals Dispersion from LEISS. Since He scattering Is very selective to Che outermost surface layer, one should anticipate that LEISS would be a valuable Cool for studies of metals dispersion for supported catalysts. For low oietal concentrations on high area supports, the (oietal/support) LEISS Intensity ratio should be directly proportional to metals dispersion. Recent stiidles In our laboratory have confirmed that expectation. 

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