Ethylene catalyst particle size

Of the three worldwide manufacturers of poly(ethylene oxide) resins. Union Carbide Corp. offers the broadest range of products. The primary quaUty control measure for these resins is the concentrated aqueous solution viscosity, which is related to molecular weight. Specifications for Polyox are summarized in Table 4. Additional product specifications frequendy include moisture content, particle size distribution, and residual catalyst by-product level. 

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... 

As in previous investigations [63, 64], the oxidative dehydrogenation of ethane to ethylene was chosen as a model reaction for an experimental study on the laboratory scale. A VOx/y-AhCti catalyst with 1.4 % V was used, and prepared using standard techniques described in Ref. [65]. The particle size of the support was 1.8 mm. 

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. 

This structure sensitivity of the respective population between the various states of adsorbed oxygen could explain why it has long been found that ethylene oxidation is structure-sensitive, with higher selectivity, up to 60-70%, for nonpromoted catalysts with sizes of about 40-50 nm [68,69]. A similar result has been recently reported for model Ag/a-A Os catalysts of very narrow size distribution (Table 1) [70]. The particle-size sensitivity has been attributed to a change of the equilibrium Ag crystal shape with cluster size (geometric effect) [71]. This leads to a change in the relative concentration of facets and edges at the metal... 

Table 1 Intrinsic Rate of Ethylene Epoxidation and Selectivity Toward Ethylene Oxide as a Eunction of Mean Silver Particle Size for ETnpromoted Ag/Al203 Supported Catalysts... 

Although such effects are well known for hydrocarbon reactions over platinum and other metals,it is unlikely to apply to the present situation where considerable surface reconstruction, corresponding to the formation of an oxide film, is likely. The properties of such a layer would change with particle size simply because of the increase in surface energy with decreasing size. The smaller particles would tend more towards the composition Ag20 than the larger ones. Since it is known that silver(i) oxide is not a selective catalyst for ethylene oxidation such a model could explain these size effects. 

J. Lu, J. J. Bravo-Suarez, A. Takahashi, M. Haruta, S. T. Oyama, In situ UV-vis studies of the effect of particle size on the epoxidation of ethylene and propylene on supported silver catalysts using molecular oxygen, ]. Catal. 232 (2005) 85. 

Fig. 21. Turnover frequency TOF and selectivity (fraction of ethylene reacted that forms ethylene oxide) for the partial oxidation of ethylene over supported silver catalysts, presented as a function of fraction exposed FE and mean particle size d (see Table XIV for details of the studies).

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