Rh/CeO> catalyst

The possibility of ethanol formation by such a reaction pathway was tested by different probe molecule experiments. As can be seen in figure 3 the introduction of acetaldehyde in the CO+H flow on a Rh/CeO catalyst enhanced the ethanol formation. The same experiment with Rh/SiO left all the other products including ethanol unchanged. These result are in agreement with those observed recently by Bell... 

The addition of ethylene to a CO-H flow on a Rh-CeO catalyst (fig. 4), which should enhance the surface concentration of C H groups increased the formation of propanol and propionaldehyde and decreased the ethanol and acetaldehyde production. 

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... 

Figure 5.8. NMR data of H2 adsorbed on Rh/CeO catalysts. Left spectra with H2 adsorbed at RT for ex chlohdc and ex nitrate samples af er reduction al increasing temperatures. Middle plot of the intettsity of the shifted line B in the first and second reduction run (filled and open symbols, resp. Circles ex l sample diamonds cx-NO]). Right ibid, for the shift of line B (from ref [93]).

Bernal S, Botana FJ, Calvino JJ, Perez-Omil JA (1995) HREM study of the behaviour of a Rh/CeO catalyst under high temperature reducing and oxidizing conditions. Catal Today 23 219... 

Figure 4.2. TPO-MS trace accounting for the CI2 (nVc 70) evolution from a Rh/CeO catalyst pre-reduced at 773 K. Catalyst prepared ftom RhCls (193). Experimental conditions Pure O2 flow rate 60 cm min Heating rate lOK.min". ...

Rh was loaded by incipient wetness impregnation. SRE reaction over these catalysts revealed that ethanol hydration is favorable over acidic or basic catalysts while dehydrogenation is favorable over redox catalysts. Among the catalysts, a 2%Rh/Ceo.8Zro.202 exhibited the best performance, may be due to strong Rh-support interaction... 

Two types of catalysts, Rh/SiO and Rh-CeO /SiO, were used. The reactivity tests (185°C, 1 atm.) showed that the presence of cerium affected the activity and also the selectivity of Rh/SiO catalysts. The total conversion (calculated as mole per cent of CO converted) after 8 hours on stream was 3.8% on Rh/SiO and 1.9% on the cerium containing catalyst. What is most striking is that the selectivity to oxygenates shifted from 6 to 71% by the addition of cerium. On Rh/SiO, acetaldehyde was obtained as the major oxygenated product whereas on Rh-CeO /SiO catalysts ethanol was the most abundant product. 

To better undertand the role of the cerium in shifting the selectivity to ethanol we studied the Rh-CeO /SiO catalysts by means of F.T.-I.R. spectroscopy and probe molecules. 

Analogous results have been obtained with our Rh-CeO /SiO catalysts (60), suggesting that acetaldehyde is a possible intermediate in ethanol synthesis on rhodium catalysts. 

Fomasiero P., Hickey N., Kaspar J., Montuni T., Graziani M., Redox and chemisorptive properties of ex-chloride and ex-nitrate Rh/Ceo gZro 4O2 catalysts. J. Catal. 189 (2000) pp.339-348. 

In the particular case of the hydrogen chemisorption studies, the interpretation of the experimental data may be heavily disturbed by the occurrence of spillover phenomena. A number of magnetic balance studies carried out on Rh/CeO (193,195,217,218), PdJCeOi (71), and Pd/Ceo.6 o3202 (93) catalysts, an example of which is presented in Table 4.3, have clearly shown that large amounts of hydrogen can be transferred to support, even al room temperature. Consequently, as reported in Table 4.5, conventional chemisorption studies at 298 K may lead to H/M 1. 

Yee A, Morrison SJ, Idriss H (2000) The reactions of ethanol over M/CeO catalysts Evidence of carbon-carbon bond dissociation at low temperatures over Rh/CeO. Catal Today 63 327... 

Figure 5.12. Particle size analysis in supported catalysts through XPS/ion sputtering experiments. Left predicted profiles for several supported particle sizes in monodisperse particle model. Right experimental data for oxidized and reduced ex-chloride and ex-nitrate Rh/CeO samples (from ref. 1701.

Mathematical models to describe the dynamic behaviour of the CO oxidation over noble metals have been developed by various authors [13, 14, 15]. All these studies were performed over Pt/Al Oj catalysts in differential reactors below 200°C. In contrast, this work was carried out over a Pt-Rh/CeO -Al Oj catalyst in an integral reactor at temperatures ranging from 200°C to 600°C, using not only a binary CO/O mixture but also simulated exhaust gas. 

A simple Langmuir-Hinshelwood model, extended by introducing CO and equivalents to mimic the complex exhaust gas, provided a satisfactory description of the dynamic behaviour of a honeycomb type Pt-Rh/CeO -Al Oj catalyst. The suitability of the model was confirmed for steady-state,. -cycling and pulsed-flow operation in the temperature range between 200°C and 600°C. 

However, the most commonly used and smdied noble metal catalysts for carrying out the MSR reaction are Pt and Rh (Figure 2.2). For instance, Kusakabe, Sotowa, Eda, and Iwamoto (2004) compared the performance of Pt, Ru, and Rh supported on Ce-Zr mixed oxides. The highest catalyst activity was obtained for Rh/Ceo.i5Zro.g502, achieving 28% methane conversion at 773 K. A similar catalyst, Rh/Ceo.6Zro.402, was used by Halabi, De Croon, Van Der Schaaf, Cobden, and Schouten (2010), who reached 50% methane conversion at 823 K. Nevertheless, after 25 h, the catalyst was deactivated because of carbon deposition. 

Table 1. Elementary step model for the transient NO rediiction by CO in the presence of O2 over a Pt/Rh/CeO/y-AI2O3 three-way catalyst. (The steps for CO oxidation 1-10 were reported in [24], the steps for NO reduction 11-21 were introduced in [25]). noble metal site, s ceria surfece site, m ceria sub-layer site, y alumina support site...

Parres-Esclapez, S., Illdn-Gdmez, M.J., Salinas-Martlnez de Lecea, C., and Bueno-Ldpez, A. (2012) Preparation and characterization of y-Al203 particles-supported Rh/Ceo,9Pro.i02 catalyst for N2O decomposition in the presence of O2, H2O and NO . Int J. Greenhouse Gas Control, 11, 251-261. 

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