Ceria-promoted metal-supported catalysts

Cheekatamarla and Lane [62, 63] studied the effect of the presence of Ni or Pd in addition to Pt in the formulation of catalysts for the ATR of synthetic diesel. For both metals, a promotional effect with respect to catalytic activity and sulfur poisoning resistance was found when either alumina or ceria was used as the support. Surface analysis of these formulations suggests that the enhanced stability is due to strong metal-metal and metal-support interactions in the catalyst. 

Automobile exhaust catalysts typically contain noble metals such as Pt, Pd and Rh with a ceria promoter supported on alumina. Traditionally, the principal function of the Rh is to control emissions of nitrogen oxides (NO ) by reaction with carbon monoxide, although the increasing use of Pd has been proposed. For example, recent X-ray absorption spectroscopy studies of Holies and Davis show that the average oxidation state of Pd was affected by gaseous environment with an average oxidation slate between 0 and +2 for a stoichiometric mixture of NO and CO. Exposure of Pd particles to NO resulted in the formation of chemisorbed oxygen and/or a surface oxide layer. 

Even though OSC is an inherently transient phenomenon, it appears that there is a relationship between steady-state reaction rates and OSC [3,17J. For the CO-oxidation, WGS, and steam-reforming reactions, it has been shown that rates can be enhanced by contact between the precious metals and ceria. Furthermore, high-lemperature treatments, which are known to deactivate the OSC properties of pure ceria, also remove the promotional effects associated with ceria [3,20,221, Given that SO2 affects OSC, one should expect SOt to influence the steady-state behavior of ceria-supported catalysts, if OSC is related to these reactions. 

Another approach to preparing model catalysts is the preparation of inverse supported catalysts . In this approach, the catalytically active metal (usually single crystal) is used as a substrate upon which an oxide is deposited, presumably leaving patches of exposed metal. This approach has been used to study reduction of ceria, and methanation kinetics on Rh as promoted by deposited ceria, and chemisorption of various molecules. As stated above, it is generally assumed that thick enough ceria layers will continuously cover the metal substrate, placing a limit on the thickness of the ceria islands that can be achieved for an inverse supported catalyst. The different procedures used for the inverse and metal particle on bulk oxide model catalysts is expected to produce differences in thermal stability, morphology and surface structure which may have consequences for the reactivity of the model catalyst. 

A vaguely defined metal-support interaction has frequently been used to describe modifications of metal properties observed when oxide-supported catalysts are thermally treated. After the original report by Tauster et al. (13) on the SMSI effect in Ti02-supported catalysts, the same authors (32) extended their operational definition to other reducible oxides. Consequently, several investigations were conducted using reducible oxide supports such as vanadia (87, 106, 154-156), niobia (157-162), or ceria (95). In general, the same characteristic features of Ti02 were obtained for these oxides, i.e., suppression in H2 and CO chemisorption capacity, suppression of catalytic activity for several reactions, promotion of the CO/H2 reaction, and reversibility by oxidation. 

In the absence of SO2, Pt supported on silica and alumina show about the same activity for CO oxidation (blue squares) Upon addition of 20 ppm of SOj in the gas feed, the ignition temperature increases only 5°C during the first run in the alumina-supported sample, compared to 3UC in the case of the silica-supported catalyst. Because both samples have similar metal dispersion, the differences in LOT can only be attributed to the nature of the supports. In the literature, it has been proposed that the presence of SOj leads to the formation of sulfates on the alumina surface, which thus becomes a sulfur trap. This would allow the Pt surface to appear not to be affected as strongly as in the case of the silica support in this short time on stream (T-O-S) run. The increase in LOT is relatively small for the ceria-promoted alumina catalyst, probably for the same reason as for unpromoted alumina. Addition of cerium oxide to Pt/silica significantly decreases the LOT (by 26°C compared to Pt(0.63)/SiO2). In the presence of S, however, the activity is the same as that of the Pt without Ce, indicating poisoning of the Ce promotional effect. 

Many authors have shown that the support could play a role, not only in changing particle size but also in modifying adsorption properties of the metals. Ceria could stabilize ionic species of platinum leading to a strong metal-support interaction. Bera et al. have compared the behavior of Pt/Ce02 and Pt/Al203 in TW catalysis." The enhanced activity observed in several reachons (CO-I-O2, CO - - NO and HC -f O2, Table 1.10) has been attributed to the formation of new sites (-0 Ce" +-0 Pt"+-0 with = 2 or 4). Ceria-supported catalysts are more active than alumina ones for all the reactions. NO as an oxidant is more sensitive in nature to support than O2. Moreover, ceria is a better promoter for oxidation of CO and propane than that of methane. Whatever the oxidant (NO or O2), methane oxidation remains difficult with a modest promotion by ceria. 

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. 

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