Ce-based oxides

In principle, these systems constitute commercially interesting catalytic alternatives to classical Cr- and Ce-based oxidants. From a practical viewpoint, however, it is essential that the catalyst retain its activity over long periods of time. In one experiment with 1-phenylethanol and AAnafk and TBHP the catalyst was recovered, dried and reused without loss of Cr A... 

Finally, signihcant ammonia emissions are possible at the NSR catalyst outlet. Two pathways are described, the direct route with H2 and the isocyanate route with CO. However, the ammonia formation is especially favored in the presence of hydrogen, which can be produced in situ or upstream the NSR catalyst. Whatever the ammonia production pathway, ammonia emission strongly depends on the balance between the ammonia formation rate and the ammonia reactivity (with NOx or oxygen from the support). Ammonia emission is particularly linked to the presence of unconsumed hydrogen. In addition, Ce-based oxides are proved to enhance the ammonia reactivity in the NSR catalyst. 

The stabilization of Ca " " and F ions into the Ce-based oxides adopting a fluorite-type structure with nanosized particles can then be explained. In many cases, alkaUne or alkaline earth as well as fluorine are excellent sintering agents, which contribute to the particle growth. In the Cei xCax02 x and Cei /Ca/02-x y/2 y series, the particle size does not increase and remains around 10 nm. The electronegativity, formal charge and ionic radius are quite different for Ce " and Ca + ions as those of and F anions. Ca ... 

The Ce(IV) oxidation of the leuco-base of Malachite Green yields a dye, viz. 

Electrochemistry. The redox processes for porphyrazines 21, 25, 28, 29, the heteroleptic Zr (pz/porphyrin) 30 and 31 have been measured by cyclic voltammetry and the formal potentials are given in Table VII. The potentials are compared to the available data for the analogous porphyrin and pc complexes. In general, the electrochemical behavior of the pz sandwiches more closely mirror that observed for the phthalocyanines than the porphyrins. In particular, all of the porphyrazines have at least one ring-based oxidation, attributable to the formation of the bis Jt-radical cation for Lu(III) sandwiches and the formation of the 7T-radical cation for the Zr(IV) and Ce(IV) sandwiches. Additionally, all of the porphyrazines exhibit at least one ring-based reduction. 

The present authors have been studying water oxidation catalysis by both chemical (Scheme 19.1, using Ce(IV) oxidant) and electrochemical (Scheme 19.2, using polymer-coated electrode) methods, and established that trinuclear, dinuclear and mononuclear ammine ligand-based Ru complexes show high activity as catalysts for water oxidation. 

Primary screening can be done on wafer-based ternary mixed metal oxide libraries. For the oxidative dehydrogenation of ethane, two interesting libraries consist of Ni-Ce-Nb and Ni-Ce-Ta oxides. The maximum amount of ethylene produced is 1800 ppm at 400 °C in nickel-rich regions of the catalyst mixture (Figure 3.35b) compared with inactive Mo-V-Nb oxide catalysts (Figure 3.35a) [7]. 

In the case of the Pt/Ceo.8TbojjQ2-x catalyst, the formation of particles of a LnPts (Ln = Ce, Tb) phase, isostructural with CePts, has been confirmed (155). Figure 4.26(c) shows a HREM in which a particle of this intermetallic is present. The details of the DDP, Figure 4.26(d), can be interpreted as due to a [011] orientation of the alloy phase. HREM thus provides evidence about the incorporation of the lanthanides present in the support to the metal particles but, in the case of the catalysts based on the mixed Ce/Tb oxide, it fails to reveal the extent to which each of them come into the alloyed state. From the analysis of the contrasts in the HREM images of the intermetallic particles it is not possible to precise this point. 

Yoon et al. tried to improve Mn/Ce based catalyst. They used various support such as SiOj, TiOj, ZrSiO, ZrOj, and ZrO -TiOj and found that TiO, is the best support. They also investigated the effect of the additives on Mn/Ce composite oxide supported on TiO,. As Table 14. 2 shows, the best result is obtained with the catalyst modified with a small amount of Co i.e. Mn (2.7 wt%)-Ce(6.8 wt%)-Co(0.5wt%) on TiO, [36]. 

There is not much work on CeO based catalysts containing elements other than Mn or precious metals. Imamura et al. reported that Co/Ce composite oxide with 13 mol% of Ce [Co/Ce(87/l3)j is active for the oxidation of ammonia, although details are not known [25]. 

Recently, in academic circles and in industry, a renewed interest can be observed in the development of other metal catalysts, e.g., Cu, Ag, and Ce-based catalysts. If lean-burning systems were to become available, oxidation would be the only reaction to be catalyzed and catalyst composition will certainly be changed accordingly. Catalytic converters are still in the development stage for diesel engines. 

F. Minisci, F. Recupero, C. Putna, C. Gambarotti, F. Antonietti, F. Fontana, G. F. Pedulli, A novel, selective free-radical carbamoylation of heteroaromatic bases by Ce(lV) oxidation of formamide, catalysed by N-hydroxyphthalimide, Chem. Comm. (2002) 2496. 

The catalysts were tested in isoprene, benzene and naphthalene hydrogenation reactions. The catalysts prepared by oxidation of the intermetallic at 703 K were found to be more active than those oxidized at room temperature. In isoprene hydrogenation the catalyst based on mischmetal has a larger activity than the other alloys and is comparable to the catalysts prepared by other routes. In benzene hydrogenation the Ni and Ce based catalysts prepared by coprecipitation were found the most active. In naphthalene hydrogenation the fully oxidized intermetallics are four times more active than the partially oxidized ones. In addition they are independent of hydrogen pressure and selective into decaline formation. 

Some new rare-earth based oxide catalysts are used to partially hydrogenated the rapeseed oil. The binary oxide Ce-Ni-O presents a good selectivity in the partial hydrogenation but a large extent of Z/E isomerization. The ratio of the iodine value (IV) variation over the pourpoint (PP) variation is lower than one. The introduction of aluminium in the catalyst formulae leads to a signiHcant decrease in the pourpoint variation due to the quasi-elimination of the Z/E isomerization. The oils obtained are more resistant to oxidation. 

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