Pressure-temperature cerium

Figure 5 shows the isobar data of CeOx-C>2 system. At one atmosphere with a 150 torr oxygen partial pressure, the cerium oxide almost is dioxide, CeC>2, and there is no phase transition until the temperature increases up 1200 °C. However, when the oxygen partial pressure is about 10-9 torr the oxygen content of the... 

In preparing fine particles of inorganic metal oxides, the hydrothermal method consists of three types of processes hydrothermal synthesis, hydrothermal oxidation, and hydrothermal crystallization. Hydrothermal synthesis is used to synthesize mixed oxides from their component oxides or hydroxides. The particles obtained are small, uniform crystallites of 0.3-200 jim in size and dispersed each other. Pressures, temperatures, and mineralizer concentrations control the size and morphology of the particles. In the hydrothermal oxidation method, fme oxide particles can be prepared from metals, alloys, and intermciallic compounds by oxidation with high temperature and pressure solvent, that is, the starting metals are changed into fine oxide powders directly. For example, the solvothermal oxidation of cerium metal in 2-mcthoxycthanol at 473-523 K yields ultrafine ceria particles (ca 2 nm). 

Fig. 11. The pressure-temperature diagrams (schematic representation) of (a) cerium and (b) ytterbium. In the cerium diagram (a) the ietters C.P. means critical point, and tbc means tetragonal body-centered.

There are five established allotrdpic forms of cerium and some evidence for two or three others which actually may be metastable or impurity stabilized. The pressure-temperature relationships of the stable phases, their crystal structures and valences are discussed first under equilibrium conditions. Then non-equilibrium and hysteresis effects relative to the five established phases are reviewed. Finally the known information on the less reliably established phases is examined. 

Fig. 4.1. Pseudo-equilibrium pressure-temperature phase diagram of cerium. The phase boundaries involving 5-Ce and liquid are true equilibrium boundaries. The letters C.P. mean critical point. The question mark for the ala phase boundary indicates that there is considerable doubt about the slope of this boundary, see text for further discussion.

Important points in the cerium pressure-temperature diagram... 

For example, a volume change of about 10 percent occurs when cerium is subjected to high pressures or low temperatures. Cesium s valence appears to change from about 3 to 4 when it is cooled or compressed. The low temperature behavior of cerium is complex. 

Complete dissolution of plutonium residues, especially high temperature calcined plutonium dioxide contained in residues such as incinerator ash, continues to cause problems, despite continued research since the Manhattan Project (9). Methods to improve the Rocky Flats system include the use of additives (e.g., cerium) and electrochemistry, other solvents (HCl-SnCl2) as well as high-temperature fusion methods (10). High pressure dissolution, HF preleaching, fluorination, and other methods are being investigated. 

A predominant feature of the atomic structure of the lanthanide group is the sequential addition of 14 electrons to the 4f subshell (Table 1). The /"electrons do not participate in bond formation and in ordinary aqueous solutions all of the lanthanides exhibit a principal (III) state. The common (III) state confers a similarity in chemical properties to all lanthanide elements. Some of the lanthanides can also exist in the (II) state (Nd, Sm, Eu, Tm, Yh) or in the (IV) state (Ce, Pr, Nd, Tb, Dy). Except for Ce(IV), Eu(II), and Yb(II), these unusual lanthanide oxidation states can only be prepared under drastic redox pressure and temperature conditions, and they are not stable in aqueous solutions. Cerium (IV) is a strong oxidizing agent... 

Mediated electrochemical oxidation (MEO) is an ex situ treatment technology that uses electricity, acid, and a metal catalyst to destroy organic wastes at low temperatures and pressures. The proprietary CerOx Corporation MEO configuration uses cerium metal as a catalyst to oxidize organic waste into carbon dioxide and water. The process occurs in an acidic solution, typically nitric acid. The first step involves the generation of an oxidant at the anode, followed by the reduction of water or another chemical species at the cathode. This technology serves as a nonthermal alternative to incineration. 

Figure 11 The effect of cerium amount on the isopropylation of naphthalene over ceria modified H-mordenite. Reaction conditions catalyst, 1 g naphthalene, 200 mmol propylene pressure, 0.8 MPa temperature, 300 °C period, 4 h.

Methadone hydrochloride (600.0 mg,1.74 mmol) was dissolved in ethanol (10 ml) and the solution was stirred whilst sodium borohydride (3.47 mmol) was added portion-wise over a period of 5 min. When the addition was complete a spatula end of cerium (III) chloride heptahydrate was added. The resultant solution was allowed to stir at room temperature for 30 min then the ethanol was removed under reduced pressure. The residue was portioned between diethyl ether (40 ml) and water (40 ml). The aqueous layer was extracted with more diethyl ether (2 times 20 ml) and then the combined organics were washed with brine (40 ml) and dried (MgS04). The ether was removed under reduced pressure to leave 435.0 mg, (80%) of 6-dimethylamino-4,4-diphenyl-3-heptanol. 

Careful analyses of the thermodynamic data of the praseodymium and terbium oxides led to the construction of their RCL-O2 phase diagrams (Hyde et al., 1966 Hyde and Eyring, 1965). The cerium oxides were studied by means of X-ray powder diffraction (Bevan, 1955) and the CeOx-02 phase diagram was constructed from equilibrium reaction studies at oxygen pressures down to 10-24 atm and temperatures up to 1200 °C (Bevan and Kordis, 1964 Ricken et al., 1984). It is worth to notice that the phase diagrams of Ce0x-02, PrCb-Cb, and Tb0x-02 systems... 

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