Monometallic supported

So far, all the characteristics mentioned for monometallic-supported catalysts can be generalized for bimetallic systems. For instance, Pt-Ir on alumina, studied by Engels et al. 28), exhibited an enhancement of H2 adsorption (more than three times the sum of the individual capacities) for 80 % Pt 20 % Ir (by weight). Sinfelt and Via (29) also observed a rather high ratio [H/(Pt + Ir) = 1.7] for H2 chemisorption at room temperature on a series of Pt-Ir/alumina catalysts with low metal loading. They considered spillover as a possible explanation for the H/(Pt + Ir) ratio that they found. 

X-Ray Absorption Studies of Monometallic Supported Metal Catalysts... 

It is seen from Fig. 21.4 that our nanocluster catalyst, RUj Pt is exceptional in its selectivity to other bimetallic nanocatalysts, and it is also superior to industrially used monometallic supported catalysts such as Pt and Rh. This augurs well for the future use of high-area, thermally stable, nanoparticle catalysts in a wide range of hydrogenations to yield desirable chemical products from plant-crop sources. 

In this work, the sol-gel preparations of monometallic supported systems Ru/Nb205, Rh/Nb205, Ru/AljOs, Pd/Al203 and bimetallic PdRu/Al203 are described. The catalytic behaviour of these systems in the NO removal reaction is preliminarly tested. 

Preparation of monometallic supported systems 5% Ru/Nb20s and 5% Rh/Nb20s... 

Generally supported bimetallic catalysts are being prepared using the same procedures as for the production of monometallic supported catdysts, viz., impregnation, deposition-precipitation, and ion exchange. These procedures, however, usually result in supported catalyst precursors of a non-uniform chemical composition of tiie individual active particles. The variation of the cherrucal composition is mainly due to a lack of interaction between the two metals to be alloyed during the various steps of the preparation procedure. 

The controlled modification of the monometallic supported rhodium phase has a strong influence on the catalytic properties, activity and selectivity ... 

A MgO-supported W—Pt catalyst has been prepared from IWsPttCOIotNCPh) (i -C5H5)2l (Fig. 70), reduced under a Hs stream at 400 C, and characterized by IR, EXAFS, TEM and chemisorption of Hs, CO, and O2. Activity in toluene hydrogenation at 1 atm and 60 C was more than an order of magnitude less for the bimetallic cluster-derived catalyst, than for a catalyst prepared from the two monometallic precursors. 

MgO-supported model Mo—Pd catalysts have been prepared from the bimetallic cluster [Mo2Pd2 /z3-CO)2(/r-CO)4(PPh3)2() -C2H )2 (Fig. 70) and monometallic precursors. Each supported sample was treated in H2 at various temperatures to form metallic palladium, and characterized by chemisorption of H2, CO, and O2, transmission electron microscopy, TPD of adsorbed CO, and EXAFS. The data showed that the presence of molybdenum in the bimetallic precursor helped to maintain the palladium in a highly dispersed form. In contrast, the sample prepared from the monometallie precursors was characterized by larger palladium particles and by weaker Mo—Pd interactions. ... 

These processes are very rapid and allow the preparation of inorganic supports in one step. This technique allows large-scale manufacturing of supports such as titania, fumed silica, and aluminas. Sometimes the properties of the material differ from the conventional preparation routes and make this approach unique. Multicomponent systems can be also prepared, either by multimetallic solutions or by using a two-nozzle system fed with monometallic solutions [22]. The as-prepared powder can be directly deposited onto substrates, and the process is termed combustion chemical vapor deposition [23]. 

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.

Synthesis methods such as those described earlier for monometallics have been applied with metal carbonyls incorporating two metals. The resultant supported species may be small supported metal clusters [41,42], and, as for monometallics, the usual products are supported species that are nonuniform in both composition and structure [42]. There are several examples of well-defined metal carbonyl clusters in this category but hardly any examples of well-defined decarbonylated bimetalhcs on supports. 

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