Rh/AljOj

Fig. 14. Infrared spectra from ethene on Rh near room temperature except where indicated (A) Rh/Si02 (19, 38) (B) Rh/AljOj, 195 K, dotted band from physical adsorption (47) (C) Rh/Al203, reprinted with permission from (50), copyright 1987 American Chemical Society (D) Rh/Al20, reprinted with permission from (58), copyright 1991 American Chemical Society (F) Rh/Si02 (67).

Finally a study on the efTect of temperature on the OSC was performed on Pt/AljOj, Rh/AljOj, Pt/Ce02-Al203 and Rh/CeOj-AljOj catalysts [32 33]. For Pt/Al20a almost no influence of the temperature could be detected (Fig. 7.4). 

H/Rh ratios of 0.8-1.0 are found on the highly dispersed Rh-AljOj and Rh-SiOj species. Consequently, the CO/H ratios on the Rh-rich clusters are markedly higher than those on the Ir-rich clusters. It is known that the H/Rh and CO/Ir stoichiometries are approximately 1 on the conventional Rh and Ir catalysts at high dispersion D = 0.8-1.0) (32), and the values of CO/H are not too sensitive to the particle size of Ir. In this sense, the Rh-rich clusters inside zeolites exhibit unique behavior accessible for CO, possibly owing to the unusually higher electron deficiency in their orbitals compared with those of the bulk metals. 

Rupprechter G, Seeber G, GoUer H, Hayek K (1999) Structme-activity correlations on Rh/ AljOj and Rh/TiO thin film model catalysts after oxidation and reduction. J Catal 186 201 Haerudin H, Bertel S, Kramer R (1998) Surface stoichiometry of titanium suboxide, part I volumetric and FTIR study. J Chem Soc Faraday Trans 94 1481... 

Rhodium (5%) on alumina, Rh-AljOj. Supplier Engelhard Industries. 

In synthetic work in the Senecio alkaloid series, Adams and co-workers ° prepared a 1-hydroxypyrrolizidine of the desired stereochemistry (2) by Rh-AljOj hydrogenation of (1), a reaction involving complete saturation of the dienone system. 

Fig. lfi. A comparison of acetate TPD after dosing acetic acid on oxygen predosed (a) Rh(110), and (b) Rh/AljOj catalyst. From U and Bowkcr (1993a) and Cassidy ct al. (1993). 

The diastereoselective hydrogenation of N-(2-methylbenzoyl)-f5j-proline methyl ester 1 into optically active 2-methylcyclohexane carboxylic acids was studied on rhodium catalysts supported on active carbon, graphite and alumina. The diastereoselectivity was highly dependent upon the nature of the support. Without modification of the catalysts by EDCA, a 40% d.e. was measured over Rh/AljOj, in contrast to Rh/C or Rh/G catalysts which were unselective. The adsorption of the aromatic substrate via a specific face on Rh/ AljOj was interpreted in terms of electronic and/or steric factors on the basis of TEM and XPS studies. EDCA addition had comparatively little effect on the initial rates and d.e. of Rh/AljO, because the amine was preferentially adsorbed on the acidic sites of the alumina support. In contrast, the... 

In conclusion, the TEM and XPS studies indicate that on the Rh/AljOj catalyst, the rhodium particles tend to spread on the alumina platelets and to be positively charged probably because they are interacting with the electron acceptor sites of the oxide support. 

The behaviour of 3.7% Rh/AljOj was quite different (figure 4 e-f). Regardless of the presence of EDCA, the diastereoselectivity was in favour of 3 (38% and 30%, respectively) from the start of the reaction. Then, the d.e. value remained relatively constant during the course of the hydrogenation of 1, but it slightly decreased as the cyclohexenic compound 6, formed in... 

Figure 5. Initial rates of hydrogenation of 1 in ethanol with or without EDCA (a) and final diastereoisomeric execess d.e. (b) for Rh/AljOj samples pretreated in hydrogen at different temperatures.

The partial pressure dependencies of the CO+NO reaction rate on monometallic Pt/AljO, Rh/AljOj and bimetallic Pt-Rh/R, Pt-Rh/D catalysts can be discussed in terms of competitive as well as non-competitive adsorptions of the reactants (NO and CO). In the case of bimetallic Pt-Rh/AljOj catalysts these two kinetic models have been discriminated using graphic and mathematical methods. The comparison between kinetic and thermodynamic constants obtained from these two methods with those previously determined on monometallic Pt/Al203 and Rh/AljOj catalysts allows us to state on the role of RIi incorporation to kinetic behaviour of Pt. 

Kinetic performances of Rh/AljOj and Pt/AljO, catalysts have been examined using the competitive adsorption model. The intrinsic rate constant of NO dissociation k, and the equilibrium adsorption constant of NO X-nq, are considerably higher on RI1/AI2O3 than on Pt/Al203. This result corroborates earlier observations on the adsorption of NO on Rh (111) and Pt (111) surfaces showing that NO dissociates easier on Rh than on Pt [3-5]. 

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