Ceria-supported noble-metal catalysts

Of the noble metals, palladium exhibits the highest activity for CO oxidation.As already mentioned for Pd-promoted CeCoO catalysts, the reaction over a noble metal supported on ceria involves a cooperative effect between the metals and the oxide, the so-called dual site mechanism. A recent study by temporal analysis of products (TAP) experiments for Pt supported over ceria showed two independent sites with different activities. The high activity site was associated with the metal/support interface and the low activity one was located on the support. The different sites were characterized by two different activation energies. Moreover, at variance with the stable number of high activity sites, the number of the low active sites increased with reaction temperature. 

The size of the metal particle plays an important role in the structure-sensitive reaction when site coordination (edge, kink, or terrace sites) or crystal orientation affect the activity and selectivity. In recent work, transient curves of gas-phase COg produced with CO-pulse injection on Pd/CeOg at 500-700 °C, established the structure sensitivity of the CO oxidation reaction on supported 

In order to discriminate between the activity of the different gold species on ceria, our group prepared ceria-supported gold catalysts by classical deposition-precipitation (DP) followed by drying at 393 K, and by the solvated metal atom dispersion (SMAD) technique 

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Conventional Ni-based catalysts still dominate in SR applications however, ceria-supported noble metal catalysts have also attracted interest reeently. The study of Rh for both POX and ATR has increased sinee Rh is in general more active for reforming and is less prone to form carbon. H2 and CO selectivities in Rh-based catalysts have been shown to be affected by catalyst geometry. This indicates that feed mixing and mass transfer can play an important role. 

Some of the very first studies dealing with the CO interaction with ceria supported noble metal catalysts consisted of TPD-MS experiments carried out on powdered Pt/Ce02 catalysts (149,330). By combining TPD studies of isotopically labelled O2 and CO species with XPS and IR spectroscopy, the authors were able to show that, as a function of the pre-reduction conditions, CO and CO2 might be inter-converted on their catalysts. 

To summarise the results concerning the study of reversibility of metal-support interaction states, we could first state that the classic reoxidation treatment at 773 K does not allow the recovery of the NM/Ce02 catalysts from the decorated or alloyed states. The noble metal/ceria phase separation may only be achieved upon reoxidation at temperatures well above 773 K. This observation represents an additional major difference between titania and ceria supported noble metal catalysts. Moreover, the likely regeneration of NM/CcOi catalysts reduced at 773 K by reoxidation at 773 K would actually prove, in good agreement with earlier HREM studies on the reduced catalysts (117,194), that the observed deactivation effects are not due to decoration or alloying phenomena, rather consisting of purely electronic effects (105). 

The effect of some anionic modifiers has also been investigated. It is well known the structural and chemical changes induced on ceria by the incorporation of chloride ions into its lattice. This incorporation has been observed during the reduction step of the preparation of some ceria-supported noble metal catalysts, specifically, when chloride-containing noble metal precursors were used [272,289,339,400-403]. As a result, the chemisorptive [272,403] and redox [272,289,339] properties of ceria are drastically modified. Likewise, as revealed by XRD and HREM characterization studies [272,400,404], the incorporation of Cl ions into the ceria lattice leads to the formation of the corresponding oxychloride phase, CeOCl, with inherent stabilization of the Ce oxidation state. According to the TPO study reported in [289], the elimination of the trapped chloride species,... 

Barbier, J., Oliviero, L., Renard, B., etal. (2005). Role of Ceria-supported Noble Metal Catalysts (Ru, Pd, Pt) in WetAir Oxidation of Nitrogen and Oxygen Containing Compounds, Topics Catal., 33, pp. 77-86. 

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). 

Active heterogeneous catalysts have been obtained. Examples include titania-, vanadia-, silica-, and ceria-based catalysts. A survey of catalytic materials prepared in flames can be found in [20]. Recent advances include nanocrystalline Ti02 [24], one-step synthesis of noble metal Ti02 [25], Ru-doped cobalt-zirconia [26], vanadia-titania [27], Rh-Al203 for chemoselective hydrogenations [28], and alumina-supported noble metal particles via high-throughput experimentation [29]. 

Catalysts. - Group VIII metals, conventional base metal catalysts (Ni, Co, and Fe) as well as noble metal catalysts (Pt, Ru, Rh, Pd) are active for the SR reaction. These are usually dispersed on various oxide supports. y-Alumina is widely used but a-alumina, magnesium aluminate, calcium aluminate, ceria, magnesia, pervoskites, and zirconia are also used as support materials. The following sections discuss the base metal and noble metal catalysts in detail, focusing on liquid hydrocarbon SR for fuel cell applications. 

Hardacre el al. (7 75, 174) investigated the properties, structure, and composition of cerium oxide films prepared by cerium deposition on Pt(lll), finding that the activity for CO oxidation is enhanced on Pt(lll) that is partially covered by ceria. It was suggested that new sites at the Pt-oxide interface become available for reaction. A remarkable observation is the high activity for CO oxidation when the Pt(lll) sample is fully encapsulated by ceria (Pt was undetectable by XPS and AES). It was proposed that an ultrathin, disordered ceria film becomes the active catalyst. It was also demonstrated by XPS and AES that Pt dramatically increases the reducibility of cerium oxide that is in intimate contact with Pt. This result suggests that intimate contact between the noble metal and oxide phases is indeed crucial to facile oxygen release from ceria. High-resolution electron microscopy demonstrated the presence of direct contact between ceria and noble metal for supported Pt-Rh catalysts (775). Hardacre et al. (173,174) related the catalytic activity of the ceria phase to partially reduced cerium oxide. 

In the case of the ceria supported non-noble metal catalysts, both conventional impregnation techniques (16,32,264,292) and precipitation of the metal precursor onto the ceria support have been used (43). Some P CeO catalysts have also been prepared in the latter way (284,293). Co-precipitation from a mixed solutions containing both... 

TPR is a routinaiy technique in the redox characterization of M/Ce02 and related catalysts. It has been very extensively used in comparative studies aimed at establishing the influence of variables like the chemical composition (281,341,343,344,350-353), or the high temperature ageing treatments (187,288,337,347,354), on the reducibility of ceria-based mixed oxides, both in the presence and in the absence of a supported noble metal. 

High Resolution Electron Microscopy (HREM) has proven as a very useful technique in the structural characterisation of supported metal catalysts (383-386) in general and, in particular, of noble metal catalysts supported on ceria-based oxides (52,70,72,97,105,109,117,124,135,137,139,144,147,155,171,182-184.194.203,209, 210,218,226,234,235,387) ... 

When considering metal-support interaction effects, the whole set of Electron Microscopy data presented in the previous section point out some important differences between the behaviour of noble metal catalysts supported on ceria and that of titania-supported catalysts. Much higher reduction temperatures are required in the case of ceria-type supports to observe nanostructural features similar to those described for the so called SMS I efTect. 

The reversibility is a major characteristic feature of the SMSI effect (300-302). In the case of NM/TiOj, reoxidation at about 773 K, followed by a reduction at low temperature, 473 K, is known to be effective for recovering the catalysts from the SMSI state (300-302,323). Probably by analogy with these earlier studies on titania-supported noble metal systems, similar reoxidation temperatures (773 K) have also been applied to NM/Ce02 catalysts for recovering their chemisorptive and/or catalytic properties from the deactivated state (133,144,221). Data commented below, in which the nanostructural changes of Rh and Pt catalysts in a redox cycle have been followed, prove, nevertheless, that drastic differences are also observed in the reversibility behaviour of ceria based systems, and also that more severe treatments are required to recover this family of catalysts from their corresponding interaction states. 

Additional measurements on a full series of ceria and cerium-zirconium mixed oxides supported noble metals showed that Ru was at least 10000 times more active than Pd and about 20 times more active than Rh for the activation of oxygen [70]. Up to now, most results were obtained with Rh catalysts but Ru could be a good candidate for surface diffusion measurements. 

Cerium-based catalysts have been successfully used in several processes. For example, ceria (Ce02) is used as an additive [ 1,2] in modem automotive exhaust catalysts. Ceria acts as an excellent oxygen store [3-5] in the catalyst, which is thus rendered a very effective catalyst for combustion [6]. Moreover, addition of ceria to the automotive exhaust catalysts minimises the thermally induced sintering of the alumina support and stabilises the noble metal dispersion [7]. Ceria also enhances nitric oxide dissociation when added to various supported metal catalysts [8], which is another important function of the automotive exhaust catalyst. Recent investigations by Harrison et al have shown that ceria doped with certain lanthanides and promoted with copper and chromium have catalytic activities comparable to that of the noble metal catalysts [9]... 

It was observed that, when supported R catalysts reach temperatures as high as -1273 K, the phase transition occurring in the major support component (y- -> 8,0-Al2Oj) has lethal effects on the adsorptive properties of the supported noble metal. It was also observed that, when a supported R catalyst reaches temperatures higher than those at which the catalyst was first fired and/or reduced, but still lower than those needed for the y—> 8,0-Al2O3 phase transition, the R° adsorptive capacity of pure-alumina-supported catalysts is somewhat increased, whereas the capacity of ceria-containing catalysts is appreciably decreased. This effect was ascribed to an increased strong interaction between R particles and the ceria component of the support. 

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