Ethylene hydrogenation, particle size

Except for support effects, structure sensitivity has usually appeared in one of two aspects, variation of rate with svirface crystal face or with particle size. In ICC 1 Gwathmey reported in one of the first experiments with single crystal faces that different faces machined fi om Ni single crystal spheres catalyzed the hydrogenation of ethylene at different rates (ICC 1 paper 5). 

As an alternative approach towards the above requirement, Somorjai introduced the method of electron lithography [119] which represents an advanced HIGHTECH sample preparation technique. The method ensures uniform particle size and spacing e.g. Pt particles of 25 nm size could be placed with 50 nm separation. This array showed a uniform activity similar to those measured on single crystal in ethylene hydrogenation. The only difficulty with the method is that the particle size is so far not small enough. Comprehensive reviews have been lined up for the effect of dispersion and its role in heterogeneous catalysis [23,124,125]. 

The kinetics of ethylene hydrogenation on small Pt crystallites has been studied by a number of researchers. The reaction rate is invariant with the size of the metal nanoparticle, and a structure-sensitive reaction according to the classification proposed by Boudart [39]. Hydrogenation of ethylene is directly proportional to the exposed surface area and is utilized as an additional characterization of Cl and NE catalysts. Ethylene hydrogenation reaction rates and kinetic parameters for the Cl catalyst series are summarized in Table 3. The turnover rate is 0.7 s for all particle sizes these rates are lower in some cases than those measured on other types of supported Pt catalysts [40]. The lower activity per surface... 

Ethylene (C H ) hydrogenation to ethane (C H ) was nsed as a first test reaction (conditions 50 mbar C H, 215 mbar H, 770 mbar He) [43, 51, 55], As expected for a structure-insensitive reaction the steady-state turnover frequency (TOE) at 300 K ( 6 s ) was independent of particle size (1.3-6.1 nm) (and similar to the TOE for Pd(lll)). The reaction orders (ethylene -0.3 hydrogen 1) and the activation energy (about 50-60 kJ mol ) were also very similar to values reported for technical catalysts, demonstrating that Pd-AljO3/NiAl(110) model catalysts closely mimic the properties of impregnated catalysts. 

Some aspects of the particle size, alloying effect, and metal-support interaction in nano-sized supported metal particles are presented for the oxidation of ethylene, the hydrogenolysis of alkanes, and the hydrogenations of unsaturated hydrocarbons and a,j8-unsaturated aldehydes. The influence of these phenomena is highlighted on the... 

Actual metal contents were determined by inductively coupled plasma-atomic emission spectroscopy (ICP-AES). Metal particles were examined by X-ray diffraction, transmission electron microscopy and CO chemisorption. Details about the procedures used can be found elsewhere [9]. In the case of Pd-Ag/C catalysts, the combination of these three techniques enabled us to obtain the metal particles size and their bulk and surface composition [9, 13]. Finally, the Pt/C catalysts were tested for benzene hydrogenation, and the Pd-Ag/C catalysts were used to study mass transfer in the support during a well-known reaction the selective hydrodechlorination of 1,2-diehloroethane into ethylene. 

Figure 5.14 Effect of particle size on ethylene hydrogenation. (Data from Ref.

Very recently, Imamura and Wallace (1981) have used the decomposition tendency of rare earth and transition d metal intermetallics under oxygen atmosphere to prepare highly reactive supported catalysts. X-ray diffraction showed that they consist of mixtures of transition metal and rare earth oxide, the transition metal particle sizes ranging from 90 to 350 A. These catalysts exhibited superior catalytic activity compared to oxide-supported catalysts prepared by the conventional impregnation method for the hydrogenation of ethylene. Although further studies are necessary to elucidate the detailed structure of these catalysts, it appears that the oxidation process of rare earth intermetallics provides a novel means of producing active supported catalysts. 

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