Rhodium oxide-supported metal catalysts

This is explained by a possible higher activity of pure rhodium than supported metal catalysts. However, two other reasons are also taken into account to explain the superior performance of the micro reactor boundary-layer mass transfer limitations, which exist for the laboratory-scale monoliths with larger internal dimensions, are less significant for the micro reactor with order-of-magnitude smaller dimensions, and the use of the thermally highly conductive rhodium as construction material facilitates heat transfer from the oxidation to the reforming zone. 

The reduction of palladium oxide, rhodium oxide or ruthenium oxide gives the corresponding metal blacks generated by in situ hydrogenation in the reaction mixture. At present the use of these oxides, as well as Adam s catalyst, is not common because of the cost of the materials and the relatively large amounts which are required. These materials have been replaced by the more reactive and less expensive supported metal catalysts described in Chapter 13. 

Another ion exchange procedure involves the interaction of a metal acetylacetonate (acac) with an oxide support. Virtually all acetylacetonate complexes, except those of rhodium and ruthenium, react with the coordinatively unsaturated surface sites of 7 alumina to produce stable catalyst precursors. On thermal treatment and reduction these give alumina supported metal catalysts having relatively high dispersions. 38 Acetylacetonate complexes which are stable in the presence of acid or base such as Pd(acac)2, Pt(acac)2 and Co(acac)3, react only with the Lewis acid, Al" 3 sites, on the alumina. Complexes which decompose in base but not in acid react not only with the Al 3 sites but also with the surface hydroxy groups. Complexes that are sensitive to acid but not to base react only slightly, if at all, with the hydroxy groups on the surface. It appears that this is the reason the rhodium and ruthenium complexes fail to adsorb on an alumina surface. 38... 

Indeed, there is a unity with the field of heterogeneous catalysis. As evidence of this, similar (or identical) Rh (C0)2 sites can be prepared either by CO chemisorption on preformed metal particles [69] or by decomposition of rhodium carbonyl clusters on the oxide surface [62-66]. Further evidence for this can be seen from the observation of metal carbonyl clusters under operating supported metal catalysts. For example, ruthenium catalysts for the conversion of synthesis gas to polymethylene [122] afford mixtures of cluster species at elevated temperatures (120°C) and pressures (1000 atm) [123]. One of these was Ru3(CO)i2 others appear to be ill-characterised. A similar observation has been recently reported for Ru/MgO and Os/MgO synthesis gas conversion catalysts [124]. On this basic support, two anionic clusters were isolated, viz. [Ru5C(C0)i5] and [OsiQC(CO)24] 7 which may be synthesised in solution by thermolysis in basic or reducing media. It is unclear whether these clusters are actually effecting the catalysis. They may instead, as highly stable species, be formed in a side reaction. 

TPR of supported bimetallic catalysts often reveals whether the two metals are in contact or not. The TPR pattern of the 1 1 FeRh/SiOi catalyst in Fig. 2.4 shows that the bimetallic combination reduces largely in the same temperature range as the rhodium catalyst does, indicating that rhodium catalyzes the reduction of the less noble iron. This forms evidence that rhodium and iron are well mixed in the fresh catalyst. The reduction mechanism is as follows. As soon as rhodium becomes metallic it causes hydrogen to dissociate atomic hydrogen migrates to iron oxide in contact with metallic rhodium and reduces the oxide instantaneously. 

We begin with the structure of a noble metal catalyst. The emphasis is on the preparation of rhodium on aluminum oxide and the nature of the metal-support interaction. Next we focus on a promoted surface in a review of potassium on noble metals. This section illustrates how single crystal techniques have been applied to investigate to what extent promoters perturb the surface of a catalyst. The third study deals with the sulfidic cobalt-molybdenum catalysts used in hydrotreating reactions. Here we are concerned with the composition and structure of the catalytically active... 

Precious Metal Catalysts, Precious metals are deposited throughout the TWC-activated coating layer. Rhodium plays an important role in the reduction of NO and is combined with platinum and/or palladium for the oxidation of HC and CO. Only a small amount of these expensive materials is used (31) (see PLATINUM-GROUP metals). The metals are dispersed on the high surface area particles as precious metal solutions, and then reduced to small metal crystals by various techniques. Catalytic reactions occur on the precious metal surfaces. Whereas metal within the crystal cannot direcdy participate in the catalytic process, it can play a role when surface metal oxides are influenced through strong metal to support reactions (SMSI) (32,33). Some exhaust gas reactions, for instance the oxidation of alkanes, require larger Pt crystals than other reactions, such as the oxidation of CO (34). 

Gasolines contain a small amount of sulfur which is emitted with the exhaust gas mainly as sulfur dioxide. On passing through the catalyst, the sulfur dioxide in exhaust gas is partially converted to sulfur trioxide which may react with the water vapor to form sulfuric acid (1,2) or with the support oxide to form aluminum sulfate and cerium sulfate (3-6). However, sulfur storage can also occur by the direct interaction of SO2 with both alumina and ceria (4,7). Studies of the oxidation of SO2 over supported noble metal catalysts indicate that Pt catalytically oxidizes more SO2 to SO3 than Rh (8,9) and that this reaction diminishes with increasing Rh content for Pt-Rh catalysts (10). Moreover, it was shown that heating platinum and rhodium catalysts in a SO2 and O2 mixture produces sulfate on the metals (11). 

To evaluate the influence of the support on the equilibration reaction on the metal particle a wide number of oxides supported rhodium catalysts have been studied (Table 7.7). 

Saturation of a carbohydrate double bond is almost always carried out by catalytic hydrogenation over a noble metal. The reaction takes place at the surface of the metal catalyst that absorbs both hydrogen and the organic molecule. The metal is often deposited onto a support, typically charcoal. Palladium is by far the most commonly used metal for catalytic hydrogenation of olefins. In special cases, more active (and more expensive) platinum and rhodium catalysts can also be used [154]. All these noble metal catalysts are deactivated by sulfur, except when sulfur is in the highest oxidation state (sulfuric and sulfonic acids/esters). The lower oxidation state sulfur compounds are almost always catalytic poisons for the metal catalyst and even minute traces may inhibit the hydrogenation very strongly [154]. Sometimes Raney nickel can... 

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