Catalyst rhodium/ceria

In AI2O3 supported catalysts, both ceria and platinum limit the deactivation of rhodium at high temperatures in an oxidizing medium. Nevertheless, while the rhodium in PtRh/CeA can be regenerated by a reducing treatment, Rh/CeA catalyst cannot be regenerated. 

Karatzas et al. [34] performed autothermal reforming of tet-radecane, low sulfur, and Fischer-Tropsch diesel in a monolithic reformer over rhodium/ceria/lanthana catalyst. The reformer had a thermal power output of 14 kW. It was composed of an inert zirconia-coated alumina foam for feed distribution at the reactor inlet and two 400 cpsi cordierite monoliths coated with the catalyst switched in series. At an O/C ratio of 0.45, a S/C ratio of 2.5 and temperatures exceeding 740°C, full conversion of the low sulfur feed was achieved, while the formation of the byproduct ethylene was between 100 and 200 ppm. As shown in Figure 14.7, an increasing S/C ratio suppresses ethylene formation. The catalyst showed stable performance for 40 h duration. Karatzas et al. [44] determined experimentally as shown in Figure 14.8 that the efficiency of their ATR increased with increasing fuel inlet temperature and O/C ratio. 

Wanat et al. investigated methanol partial oxidation over various rhodium containing catalysts on ceramic monoliths, namely rhodium/alumina, rhodium/ceria, rhodium/ruthenium and rhodium/cobalt catalysts [195]. The rhodium/ceria sample performed best. Full methanol conversion was achieved at reaction temperatures exceeding 550 °C and with O/C ratios of from 0.66 to 1.0. Owing to the high reaction temperature, carbon monoxide selectivity was high, exceeding 70%. No by-products were observed except for methane. 

The isooctane feed was pulsed at frequencies of from 0.001 to 0.5 Hz at a pulse duration of 2 ms corresponding to a gaseous isooctane feed of 4.2 cm per pulse. At the maximum pulse frequency of 0.5 Hz, 480cm min isooctane were therefore fed to the reactor along with a nitrogen flow at 200 cm min. The S/C ratio was set to 1 and the O/C ratio to 0.64, corresponding to autothermal conditions. Up to 98% conversion could be achieved over a rhodium/ceria catalyst. Nickel/tungsten and... 

S. Tagliaferri, R.A. Koeppel, and A. Baiker, Influence of rhodium- and ceria-promotion of automotive palladium catalyst on its catalytic behaviour under steady-state and dynamic operation, Appl. Catal. B 15,159-177 (1998). 

Rhodium-Based Catalysts. Krause et al. showed that Rh and Ni— which are better SR catalysts than Pt—supported on ceria-doped gadolinium... 

Gasolines contain a small amount of sulfur which is emitted with the exhaust gas mainly as sulfur dioxide. On passing through the catalyst, the sulfur dioxide in exhaust gas is partially converted to sulfur trioxide which may react with the water vapor to form sulfuric acid (1,2) or with the support oxide to form aluminum sulfate and cerium sulfate (3-6). However, sulfur storage can also occur by the direct interaction of SO2 with both alumina and ceria (4,7). Studies of the oxidation of SO2 over supported noble metal catalysts indicate that Pt catalytically oxidizes more SO2 to SO3 than Rh (8,9) and that this reaction diminishes with increasing Rh content for Pt-Rh catalysts (10). Moreover, it was shown that heating platinum and rhodium catalysts in a SO2 and O2 mixture produces sulfate on the metals (11). 

Nitrous oxide reacts with carbon monoxide in the presence of a ceria-promoted rhodium catalyst to form dinitrogen and carbon dioxide. One plausible sequence for the reaction is given below ... 

In some other cases, the noble metals were deposited onto ceria from colloidal suspensions. Tlius, a stabilized rhodium hydrosol with an average particles size of 5 nm was used in the preparation of a Rh(i%)/Ce02 catalyst (182). Likewise, a series of Pd/Ce02 catalysts with 0.5, 2.5 and 5 wt.% have been prepared from microemulsion of metallic palladium, further destabilized by addition of tetrahydrofiiran (78). 

Very recently, H-NMR technique has been applied to the investigation of the hydrogen chemisorption on Rh/CeO catalysts prepared from both Rh(N03)3 and RhCla precursors (163). Because of the different chemical shift shown by the hydrogen species adsorbed on the metal and the ceria support, the evolution of the rhodium... 

Figures 4.30(c) and 4.31(c) show HREM images representative of the catalysts reduced at 1173 K and further oxidised in pure O2 at 1173 K. The structure of both catalysts is clearly different from that observed after re-oxidation at 773 K. Notice that in this case both materials seem to be formed by small, crystalline, metal particles dispersed over the ceria surface. Fringe analysis confirms that these crystallites consist of metallic rhodium and platinum, respectively. Thus, the DDPs of the larger particles observed in the image of the Pt catalyst show 0.8 nm Moire-type fringes aligned with the (111 )-Ce02 reflections. These spots arise from double diffraction in the (lll)-Pt and (Ill)-Ce02 planes under a parallel orientation relationship. Therefore this result, in addition to confirm the presence of metallic Pt particles in the sample oxidised at 1173 K, suggest that these particles are epitaxially grown on the support. A detailed inspection also reveals that the exposed surfaces of these particles are clean, i.e. free from support overlayers.

Furthermore, it was clearly shown that noble metals, rhodium in particular, play an active role in promoting the OSC of the support [1,3-6,31]. It was shown for example that only Rh can really promote OSC on alumina catalysts [32,33]. Nevertheless the situation changes when ceria is added to alumina. A comparative study of alumina and ceria-alumina supported bimetallic catalysts [32,33] demonstrated the differences between those systems. Ceria-alumina catalysts were shown to have higher OSC values which do not depend on the composition of the bimetallic (Fig. 7.3). 

There is a limited effect of ceria on Pt and Pd catalysts, rather negative in alkane oxidation. On the contrary, there is a beneficial effect of ceria on rhodium catalysts, particularly marked in CO oxidation. This promoter effect of ceria on Rh in CO oxidation has been the object of several investigations [81-83]. However, most authors found a promoter effect more pronounced around the stoichiometry than in O2 excess. The results obtained by Oh and Eickel [82] clearly illustrate this tendency (Fig. 7.14). 

Finally, the effect of the cerium oxide content on the catalytic activity of rhodium catalysts was analysed for N2O decomposition reaction. The rhodium content was kept constant for all samples. The activity curves are represented in Fig. 4 at 400°C and 430°C. Data were acquired after 2 hours of reaction to ensure steady state conditions. The systems present catalytic activity at temperature above 300 C. Catalysts A prepared without ceria, displayed a good catalytic performance for N2O elimination even at 400 C reaching at 430°C a N2O conversion of 55 %. Nevertheless, a maximum was achieved for catalysts with a Ce02 content of 2 wt% with a conversion of 70%. These behaviour can be related with the result obtained with TiCex powder samples. The proposed new specie (Ti-O-Ce) formed when the cerium oxide content was around 2 wt%, could be responsible for the better performance of... 

Table 4.3. Faraday magnetic balance study of the redox behaviour of two ceria-supported rhodium catalysts prepared from Rh(N03)3 (N) and RhCU (Cl) metal precursors. Metal loading and BET suriace area of the catalysts were 3 wt% and 49 m. g" respectively. Data taken from (195).

For the palladium based catalysts the presence of ceria had a strongly positive effect on CsHg-conversion as well as CO-concentration, independent of the operating mode applied. Moreover, Na-yields increased upon addition of ceria to the catalyst formulation, except for the fully promoted palladium catalyst and mode F. Adding rhodium to the palladium based catalyst had generally a negative impact on CO concentrations as well as on N2-yields for modes E, G and... 

Note the overall good performance of the rhodium free catalyst Pd-Ce, which showed almost the same characteristics as the fully promoted palladium catalyst Pd-Rh-Ce. Ceria addition also increased CgHg conversion of the platinum catalyst, but resulted in a lower N2-yield. Generally, the palladium catalysts Pd-Ce and Pd-Rh-Ce showed similar or even superior catalytic performance compared to Pt-Rh-Ce. 

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