Cerium aluminate

Thermal deactivation involves processes such as diffusion and solid-state reaction. In early three-way catalysts where both the active metal and ceria were dispersed onto high-surface-area Y-A1203, loss of contact between them, due to sintering of either one or both, could effectively eliminate oxygen storage. The temperature required for ceria to sinter, somewhat above 800°C, was typically not attained under normal operating conditions, although relatively harsh conditions, with temperatures well in excess of 800°C under rich exhaust gas, did exist in heavy-duty truck operation, and in this case, reaction between ceria and alumina at times produced stable, inert cerium aluminate. 

Similarly, Fig. 75 shows the effect of both the amount and the type of precious metal on the extent of the solid state reaction between aluminum oxide and cerium oxide, leading to the formation of cerium aluminate. The overall effect of these interactions is that the catalytic activity and the durability of the precious metals vary as a function of the type of washcoat oxide they are deposited on [45]. 

Each catalyst was studied by XRD. To improve the intensity of the diffraction lines, the washcoat was scraped off the cordierite before analysis. The XRD spectra show the presence of alumina, ceria and some residual cordierite. The precious metals are never detected, as metal or oxide. For the firesh catalysts, the lines are broad meaning poorly crystallized phases. The resolution is greatly improved after aging. No other phase like cerium aluminate or other transformation product were evidenced on the XRD spectra. 

As an attempt to simulate real operating conditions of automotive converters, a laboratory bench has been designed and ageing procedures determined to reproduce simultaneous chemical and thermal modifications encountered by catalysts in the exhaust line. Characterization of commercial samples after ageing according to different temperature cycles evidences formation of both platinum/rhodium alloys and cubic perovskite-type compound, CeA103. Simultaneously with the formation of cerium aluminate, a thermal stabilization of catalysts is observed, in terms of mean noble metal particles size and concentration of rhodium in alloyed phases. An interpretation based on the crystallographic adaptation of alumina, cerium aluminate and ceria is proposed. 

Similarly to Pt/Rh alloying, modifications of step 1 duration show that cerium aluminate is formed only when reducing conditions are present in the cycle, most probably to ensure reduction of Ce4+ (Ce02) to Ce + (CeAlOs). Moreover, as deduced firom X-ray intensities measurements, the CeA103/Ce02 ratio increases with duration of the reducing step (for these experiments, the number of cycles was kept constant to 50 in four hours by modifying neutral phases durations, steps 2 and 4). 

Numerous characterizations of catalysts after real driving conditions and various mileages on different vehicles, show, among other features, Pt/Rli alloying and cerium aluminate formation [12]. Tlie similarity of these results with oin own observations on commercial samples suggest that the laboratory procedures described herein allow to reproduce significant behaviom of catalysts under real driving conditions. 

The above results indicate that fonnation of cerium aluminate and absence of modification of catalysts, in tenns of noble metal particles sizes and amount of rhodium in alloyed phases, are simultaneous. Consequently, it can be assumed that this stabilization of catalysts in the present conditions must be related to the presence of the complex oxide CeAlOs. 

For both noble metal particles sintering and amomit of allied rhodium, no differences are observed after ageing under cycle 2 and cycle 3 conditions. Moreover, both procedures lead to the formation of cerium aluminate, CeAlOs. 

The lanthanum phosphate phosphor is usually prepared by starting with a highly purified coprecipitated oxide of lanthanum, cerium, and terbium blended with a slight excess of the stoichiometric amount of diammonium acid phosphate. Unlike the case of the aluminate phosphor, firing is carried out in an only slightly reducing or a neutral atmosphere of nitrogen at a temperature 1000° C. Also this phosphor is typically made with the addition of a flux,... 

The tendency for high surface area gamma-alumina to siater and lose that cmcial area duriag high temperature operatioa is retarded by the intimate additioa of several perceat of cerium oxide. The mechanism is stiU under debate but may iavolve a surface LN—aluminate species on the alumina. 

Cerium magnesium aluminate is a highly efficient green phosphor (lmax. = 541 nm), which is used in trichromatic fluorescent lamps. The high quantum yield of 65 % is effected by the energy transfer from the sensitizer, Ce3 +, to the activator, Tb3 +. ... 

Recent studies have demonstrated that cerium containing magnesium aluminate spinels such as MgAl204 and Mg2Al205 are very effective SOx reduction catalysts for FCC units. These magnesium aluminate spinels can be prepared by three different techniques (i) thermal co-condensation... 

Designing a catalyst for effective removal of SOx (S02 + S03) in a fluid catalyst cracking unit regenerator is a challenging problem. One must come up with a particle having physical properties similar to FCC catalysts which will 1) oxidize S02 to S03, 2) chemisorb the S03/ and 3) be able to release it as H2S as it enters the reactor side of the unit. A cerium containing magnesium aluminate spinel was found to be very effective for this purpose (1). The preparation methods and characterization techniques utilized for this spinel catalyst and how the SOx abatement activity of this catalyst is related to the preparative route used are discussed in this paper. 

Implantations of yttrium and cerium in 15 % Cr/4% A1 steel and aluminized coatings on nickel-based alloys did not improve the high-temperature oxidation resistance even though conventional yttrium alloy addition had an effect. The differences for the various substrates are attributed to different mechanisms of oxidation of the materials. The austenitic steel forms a protective oxide film and the oxidation proceeds by cation diffusion. Thus, the yttrium is able to remain in a position at the oxide/metal interface. The other materials exhibit oxides based on aluminum. In their growth anion diffusion is involved which means an oxide formation directly at the oxide/metal interface. The implanted metals may, therefore, be incorporated into the oxide and lost by oxide spalling. 

We previously proposed an interpretation of thermal stabilization of supported catalysts by rare-earth elements [10, 11, 18] by means of interfacial structural coherence between alumina and rare-earth aluminates. This model, developped for lanthanum and neodymium, can be easily extended to cerium since LaAlOs, NdAlOs and CeAlOs are iso-structural mixed oxides with identical lattice parameters (a = 7.6 A). 

Bartram RH, Hamilton DS, Kappers LA, Lempicki A (1997) Electron traps and transfer efficiency of cerium-doped aluminate scintillators. J Lumin 75 183—192... 

Another type of deformation of the perovskite structure is known to occur among the LT modifications of cerium and praseodymium aluminates and solid solutions based on them. In the orthorhombic Imma structures, 12 nearest distances in the RO12 polyhedra split into a set of five imique R-O distances, typically grouped as 1-4-2-4-1. The six Al-O bonds within the octahedra split into two shorter and four longer distances, but this deformation is very weak and octahedra are distorted mainly due to considerable deviations of the O-Al-O angles from 90° (see Table 28). Moreover, the octahedra tilt either about the twofold [110]p axis or about both [010]p and [001]p pseudo-axes of the perovskite cell, which corresponds to the tilt system a% b (Figure 39C). 

The rhombohedral-to-orthorhombic transition R3c-Imtm observed in cerium and praseod)unium aluminates is discontinuous and hence, first-order in character. The same holds true for the orthorhombic-to-tetragonal Imma—Ii/mcm and... 

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