Oxidants Ce

The detailed mechanism for the formation of reduced Cu+ species under the hydrothermal synthesis conditions in the presence of CTAB without any additional reducing reagent is not clear at present, but the degree of reduction of the Cu- and oxide-precursors may depend on the oxophilicity of metal oxides Cu oxide (most reducible) < Mo oxide < Zn oxide < Si oxide < A1 oxide Zr oxide Ce oxide (hard to reduce). Further, chemical interaction of the Cu + clusters with the Ce02 surface may also be the key to stabilizing the Cu + clusters on the support. 

Table 2.2 shows the catalytic performances of various Cu and Ce catalysts for CO PROX reactions in excess H2 at 90 °C. Ce oxides, Ce-CTAB and Cu-CTAB were completely inactive for CO oxidation at 90 °C. In contrast, the hydrothermally-prepared Cu/Ce-CTAB catalyst (7.5 wt%Cu) exhibited good catalytic performance for the CO PROX, with 91.9-96.1% CO conversion and 99.4—99.8% 02 selectivity at 90°C in a feed of C0/02/H2= 1 1 50 (Table 2.2). Table 2.3 summarizes the performance of the Cu/Ce-CTAB catalyst under various reaction conditions, different W/F, reaction temperatures and feed compositions. Notably, high CO conversions and 02 selectivities were also achieved in reactant feeds containing substantial amounts of H20 and C02. The CO conversions and 02 selectivities at W/F = 2.24 gcath mol-1 and 90 °C were 85.7% and 98.7%, respectively when H20 (10%) existed, and 81.4% and 98.2%, respectively when H20 (10%) and C02 (20%) co-existed. 

Metal decoration is also a general characteristic feature. As shown in section 4.3.3.2, it has been observed in Rh, Pd and Pt catalysts supponed on ceria and all the investigated mixed oxides (Ce/Tb and Ce/Zr). It is important to stress, however, that the HREM studies have only provided unequivocal proofs of covering phenomena on catalysts reduced at temperatures well above 773 K, typically 973 K. 

The existence of the H02 radical has been confirmed by spectroscopic means and isotopic exchange. The reaction resulting in the formation of the H02 radical is reversible that is, H02 can oxidize Ce(III). Samuni and Czapski confirmed this reversibility and suggested the formation of a Ce(III)-H02 complex. They modified (18-10) and (18-11) to... 

Preparation In the early work with Ce(IV), standard solutions were prepared from relatively crude preparations of Ce(rV) oxide, Ce(TV) sulfate, or Ce(IV) ammonium sulfate. Contamination from other lanthanides and phosphate led to the gradual precipitation of lanthanide phosphates from a sulfuric add solution. 

We have recently identified and characterized several new ternary oxides (Ce,Mo,Te)0( 3), after studying over 100 different compositions calcined in air at temperatures between 400 and 600 C. Combined with the knowledge of the (Te,Mo)0, (Ce,Mo)0, and (Ce,Te)0 chemistry, which was developed in the last decade, it is now possible to describe the complex solid-state relations of the ternary (Ce,Mo,Te)0 system. The results culminated in the identification of the active phase composition of a typical (Ce,Mo,Te)0 acrylonitrile catalyst. 

The cerium-zirconium mixed oxide (Ce/Zr = 64/36, atomic ratio) as well as the pure ceria samples (both without and with Pt) were obtained by a Rhone-Poulenc proprietary process. The purity of the cerium used was higher than 99.5%. For the study of the interaction between Pt and cerium-zirconium mixed oxides, a hexachloroplatinic solution was used as a source of Pt. The OH groups on the surface of the mixed oxides were exchanged by PtCle ions in water. The material obtained was dried in an oven at 120°C and calcined in a muffle furnace at 500°C. For aged catalysts, the calcination was performed at 900°C for 6 hours. The Pt loading was 0.5% (weight %). 

Fig. 15.8 FIA configurations for the off-line speciation of chromium, (a) With a splitting point (A) and a dual-beam detector (a.1) with two cells aligned with the light path (a.2) with a merging point prior to a single flow-cell (a.3). (b) Sequential determination with the aid of a selecting valve. DPC diphenylcarbazide Ox oxidant [Ce(IV)] q flow-rate Vi injection valve L and 0 reactor length and inner diameter V2 selecting valve W waste. (Reproduced from [37] with permission of Elsevier).

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. 

Because of the presence of highly unsaturated fatty acids (as indicated by a high iodine value IV = 117) the rapeseed oil shows poor stability. It must be partially hydrogenated to increase this stability. The rapeseed oil conversion and the product distribution both depend on the Ni/Ce ratio of the mixed Ni-Ce-oxides and on the temperature. These catalwts appear more selective than Ni alone but lead to an important Z/E isomerization which gives rise to a drastic increase in the pourpoint value, llie use of a temaiy oxide (Ce-Ni-Al) allows a decrease in the extent of the Z/E isomerization. The results depend on the relative amount of each metal and the isomerization can be almost totally eliminated. Moreover, from some DSC experiments under an oxidative atmosphere, it appears that the resistance to oxidation can be improved, even at relatively high temperatures. 

Because of the similarity of the biaryl ratios for the oxidants Ce, Fe +, and DDQ, it is reasonable to conclude that they all operate as electron-transfer agents, as has been well documented for Ce +. Preferential formation of the bitolyl from either salt clearly implies that the tolyl group undergoes preferred attack, for a random oxidation would yield a 1 1 biaryl mixture. It is understandable that the tolyl group should be more prone than the phenyl to electron loss. If aryl migration from boron to carbon (60 61) were completely random, then a... 

Generation of a-Acyl Radicals. As a one-electron oxidant, Ce can promote the formation of radicals from carbonyl compounds. In the presence of interceptors such as butadiene and alkenyl acetates, the a-acyl radicals undergo addition. The carbonyl compounds may be introduced as enol silyl ethers, and the oxidative coupling of two such ethers may be accomplished. Some differences in the efficiency for oxidative cyclization of, s-, and ,f-unsaturated enol silyl ethers using CAN and other oxidants have been noted (eq 14). ... 

Oxidative Matrix Polymerization The polymerization of NVK in a matrix of poly(ethylene glycol) (PEG) has been described. As an oxidant, Ce" + has been used. PEG proved to be a more suitable matrix in order to obtain a stable homogeneous ternary complex solution in comparison to poly(acrylic acid) and poly(V-vinyl-2-pyrrolidone). 

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