Activation of Rh

The hydrogenation of arenes also has attracted interest, but until recently only a few catalytic systems have been found effective.56-60 The catalytic activity of [Rh(MeOH)2(diphos)]X and of [RuH2(H2)(PPh3)3] in the hydrogenation of 9-trifiuoroacetylanthracene and 9-methylanthracene has been explored.61,62... 

The synthesis, aggregation behavior, and catalytic activity of Rh complexes of Xantphos derivatives (129) with surface-active pendant groups have been described.416 The complex [HRh(CO)(TPPTS)3] was used as a catalyst precursor in the hydroformylation of 1-butene, 1-octene, and styrene under biphasic reaction conditions 417 The two-phase hydroformylation of buta-1,3-diene with [HRh(CO)(TPPTS)3], with excess TPPPS, gives high yields of C5-monoaldehydes.418 The coordination behavior of the catalytic species HRh(130)(CO)2] was studied by HP NMR spectroscopy which showed the desired bis-equatorial coordination of the ligand to the rhodium center.419... 

Fig. 3. Relative activity of Rh catalyst activated by RC1. Initial catalyst composition = h mil (EttRhCl). + lOOmAf RC1.

Fig. 5 Activity of Rh-3-SILP catalyst at different syngas compositions (pH2 Pco ratios) and temperatures (Reaction conditions T = 100°C, ptotai = 10bar) [32]...

At 24 °C and 15-60 bar ethylene, [Rh(Me)(0H)(H20)Cn] catalyzed the slow polymerization of ethylene [4], Propylene, methyl acrylate and methyl methacrylate did not react. After 90 days under 60 bar CH2=CH2 (the pressure was held constant throughout) the product was low molecular weight polyethylene with Mw =5100 and a polydispersity index of 1.6. This is certainly not a practical catalyst for ethylene polymerization (TOP 1 in a day), nevertheless the formation and further reactions of the various intermediates can be followed conveniently which may provide ideas for further catalyst design. For example, during such investigations it was established, that only the monohydroxo-monoaqua complex was a catalyst for this reaction, both [Rh(Me)3Cn] and [Rh(Me)(H20)2Cn] were found completely ineffective. The lack of catalytic activity of [Rh(Me)3Cn] is understandable since there is no free coordination site for ethylene. Such a coordination site can be provided by water dissociation from [Rh(Me)(OH)(H20)Cn] and [Rh(Me)(H20)2Cn] and the rate of this exchange is probably the lowest step of the overall reaction.The hydroxy ligand facilitates the dissociation of H2O and this leads to a slow catalysis of ethene polymerization. 

The high activity of Rh compared to conventional Ni-based catalysts may also lead to a lower operating temperature of the reformer, eliminating high-and low-temperature shift reactors and minimizing the O/C. At 550°C (O/C = 1, S/C = 3.0, and GHSV = 179,290 h ), Newson et al. obtained a H2 yield of... 

Dispersion, particle size and activity of Rh catalysts for carvone hydrogenation. 

TABLE 8. Catalytic activity of Rh and Pt catalysts in the hydrogenation of alkenes (303 K, 1 atm H2, ethanol-water)162,165... 

Photochemical electron-transfer can be effected by irradiation of the charge-transfer absorption band of the electron donor-acceptor complex.15 Alternatively, photochemical electron-transfer may proceed by actinic activation of RH followed by quenching with A, or by the reverse sequence involving activation of A and quenching with RH. 

Another level of surface chemical complexity results from catalytic metal-catalyst support surface interactions. Table 3, taken from Bell [7], shows the surface-specific activity of Rh for CO hydrogenation as a function of... 

Table 3. Surface-Specific Activity of Rh for CO Hydrogenation as a Function of the Catalyst Support...

Turning full circle, Duprez and Miloudi studied the activation of Rh/Ti02 by magnetic measurements. They proposed that hydrogen radicals created on the metal were able to spill over as hydrogen protons, while electrons diffused in parallel within the conduction band of the Ti02 support 122). 

The effects of the overlayer on the hydrogenation of carbon monoxide and the hydrogenolysis of ethane were examined. With increasing titania coverage, the activity of Rh for carbon monoxide hydrogenation passes through a maximum, whereas the activity for ethane hydrogenolysis decreases monotonically. 

The enhancement in CO hydrogenation activity of Rh is ascribed to the presence of Ti + sites at the perimeter of TiOx islands. It is proposed that these sites interact with the oxygen in CO chemisorbed on nearby Rh atoms and assist in the disociation of CO. Since the disociation of CO is believed to be the rate-limiting step in this reaction, the participation of Ti + in this step leads to a higher activity. The dependence of the methanation rate on TiOx coverage, seen in Fig. 5, is attributable to the variation in the concentration of Ti + centers with TiOx coverage. 

Conclusions. Submonolayer deposits of titania grow on the surface of Rh in the form of two-dimensional islands until a coverage of nearly a monolayer is achieved, at which point some three-dimensional growth of the islands is observed. The titania islands exclude CO chemisorption on Rh sites covered by the titania. The Ti + ions in the overlayer are readily reduced to TP+. This process begins at the perimeter of the islands and extends inwards as reduction proceeds. Titania promotion of Rh enhances the rate of CO hydrogenation by up to a factor of three and increases the selectivity to C2+ hydrocarbons. By contrast, the activity of Rh for the hydrogenolysis of ethane decreases monotonically with increasing titania promotion. 

It should also be noted that the formation of CO2 (by the shift reaction) is accelerated by Rh-Mo/Al203 but not by Rh/Al203. The presence of CO2 could provide an explanation for increased activity of Rh-Mo/Al203 if methanol formation over Rh catalysts is via CO2 rather than CO. Methanol formation is via CO2 over Cu/ZnO/Al203 catalysts under usual industrial conditions (14). 

The activity of rhodium siloxide complexes is not suppressed in the hydiosilylation process, which means that the same catalyst can be used a few times. Table 2 shows typical activities of [Rh(cod)(PCy3)(OSiMe3)] (2) three times reused in the reaction examined. 

Figure 4 shows the effect of sulfiding for the hydrocracking of benzene over noble metal/HZS M-5 catalysts. It was found that the catalytic activity of noble metal/HZSM-5 was remarkably lowered by the introduction of hydrogen sulfide. The catalytic activity of Rh/HZSM-5, Pd/HZSM-5 and Ru/HZSM-5 did not recover with the time on stream, but, R/HZSM-5 regained almost its original activity. 

Bando KK, Asakura K, Arakawa H, Isobe K, Iwasawa Y (1996) Surface structures and catalytic hydroformylation activities of rh dimers attached on various inorganic oxide supports. J Phys Chem 100 13636... 

In contrast, significant increases in the specific activity of Rh in the CO/H2 and CO2/H2 reactions were observed when the metal was dispersed on W -doped Ti02 carriers [106-108]. The kinetic results are summarized in Figure 8(a), which shows the ratio of the turnover frequency (TOF) of the doped catalysts over that of the undoped catalysts as a function of dopant content in the Ti02 carrier. Doped catalysts were found to be up to 20 times more active in CO2 methanation. The... 

Bruno, T., Beretta, A., Groppi, G., Roderi, M., and Forzatti, P. A study of methane partial oxidation in annular reactor Activity of Rh/a-Al203 and Rh/Zr02 catalysts. Catalysis Today, 2005,... 

Thus, the deactivation observed on the used catalyst may be explained as follows. First, Pt is sintered and Rh is segregated on the surface of noble metal particles by the thermal effect [8, 9], and so, on only thermally deaetivated catalysts, the reactions due to Rh sueh as NO reduction are not slowed, although methane oxidation due to Pt is considerably slowed [3, 4]. However, when Pb is adsorbed on Rh, the catalytic activity of Rh is suppressed and the reaction rate of NO reduction is decreased to the same order as the rate of CH4 oxidation. Further, the surfaee of the wash eoat layer is covered with compounds consisting of Ca, P, Zn and Fe, and the effeetive surface area of the catalyst which the exhaust gas ean reach decreases, causing a considerable decrease in NO conversion and the disappearance of the window, accompanied wdth decreases in CH4 conversion and CO conversion on the rich side. 

A Pt-Rh three way catalyst used in natural gas-fueled engine systems for 21,000 h showed specific deactivation characteristics, including a decrease in the selectivity of NO reduction, which can neither be reproduced by heat treatment nor explained by physical poisoning such as the blockage of micropores. Through chemieal analyses, EPMA, and activity tests of the used catalyst and model-poisoned catalysts, it was found that the activities of Rh on the used eatalyst were decreased by chemical poisoning due to Pb, causing a decrease in the NO reduction selectivity, and that the absolute rates of NO reduction and other reactions are considerably reduced by a decrease in the effective surface area of the catalyst due to accumulated compounds on the wash coat surface, in addition to thermal effects. 

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