On Supported Catalysts

The supported catalysts can be modified either by noble metals or base metals. 

When the modifier is a base metal, the most commonly used redox reaction is catalytic reduction. Nevertheless, the refilling method with preadsorbed hydro- 

In the course of the catalytic reduction, deposition of the second metal (or the third) can occur on both the parent metal and the support, depending on the nature of the support and the operating conditions. Effectively, in the case of bimetallic Rh-Ge [41] and Pt-Sn [38] catalysts, it was observed that the amount of Ge or Sn deposited is higher on alumina than on silica supported catalysts (Fig. 

Furthermore, the same experiments carried out under nitrogen flow do not lead to additive deposition on siUca supported catalysts, while some deposit occurs on alumina supported catalysts. This result was explained by the adsorption of the additive anionic species in an acidic medium on the alumina support compared to a lack of adsorption on the siUca support, which is in accord with their isoelectric points. Moreover, the greater additive deposition under hydrogen flow compared to that observed under nitrogen supported the idea that hydrogen activated on the parent metal particles participates in the deposition reaction. 

In order to decrease the contribution of the support, different parameters can be optimized  

---

A kinetic model which accounts for a multiplicity of active centres on supported catalysts has recently been developed. Computer simulations have been used to mechanistically validate the model and examine the effects on Its parameters by varying the nature of the distrlbultons, the order of deactivation, and the number of site types. The model adequately represents both first and second order deactivating polymerizations. Simulation results have been used to assist the interpretation of experimental results for the MgCl /EB/TlCl /TEA catalyst suggesting that... 

Transmission infrared spectroscopy is very popular for studying the adsorption of gases on supported catalysts and for studying the decomposition of infrared active catalyst precursors during catalyst preparation. Infrared spectroscopy is an in situ technique that is applicable in transmission or diffuse reflection mode on real catalysts. 

Figure 4 shows the rate of ethane hydrogenolysls over a ruthenium catalyst as a function of H2 partial pressure (12). In agreement with studies on supported catalysts ( ), the reaction Is negative order with respect to hydrogen for partial pressures of H2 above 40... 

Recyclability can be achieved by heterogenization of the reaction mixture, by binding the catalyst and products to different phases. This can be achieved by (i) immobilization of the catalyst on a solid inorganic or polymeric support (solid-liquid protocols) or (ii) partitioning the catalyst and reagents/products in different liquid phases (liquid-liquid protocols) (see Chapter 9.9 for more details on supported catalysts). 

Studies on Supported Catalysts at More Realistic Conditions... 

In addition to diffraction, one can perform small-angle scattering, where the scattering centers are the interfaces between particle and support, particle and gas phase, and support and gas phase. Examples can be found in the work of Goodisman et al. [16] on supported catalysts, and of Beelen and colleagues on catalyst supports... 

We illustrate the sensitivity of the C-0 stretching frequency for the bonding configuration with a perhaps somewhat dated but still very instructive study of the adsorption sites of alloy surfaces. Soma-Noto and Sachtler [18] reported an infrared investigation of CO adsorbed on silica-supported Pd-Ag alloys some of their spectra are shown in Fig. 8.5. On pure palladium, CO adsorbs mainly in a twofold position, evidenced by the intense peak around 1980 cm 1, although some CO appears to be present in threefold and linear geometries as well. This is a common feature in adsorption studies on supported catalysts, where particles exhibit a variety of surface... 

Some of the above results were obtained on evaporated films, others on supported catalysts in the case of ethylene hydrogenation, a similar order of activities was observed for both types of catalysts (Schuit and Van Reijen, 9Ifi). 

Werber, F. X. Polymerization of Olefins on Supported Catalysts. VoL 1, pp. 180-191. Wichterle, O., Sebenda, J., and Kralicek, The Anionic Polymerization of Caprolactam. 

The disproportionation activity in the supported species is parallel to the increased activity of ethylene polymerization on supported catalysts. Many of the steps in the reaction may be identical for example, the initial coordination of olefin to the metal center will be common to both systems. Indeed, some of these catalysts are also ethylene polymerization catalysts (see Table IV) although their activities are much less than the corresponding zirconium derivatives. A possible intermediate common to both disproportionation and polymerization could be the hydrocarbyl-olefin species (Structure I). Olefin disproportionation would result if the metal favored /3-hydrogen elimination to give the diolefin intermediate (Structure II) which is thought to be necessary for olefin disproportionation. Thus, the similarity between the mechanism and activation of olefin disproportionation and polymerization is suggested. 

---