Cerium mixed valence

Hydroxide. Freshly precipitated cerous hydroxide [15785-09-8] Ce(OH)2, is readily oxidized by air or oxygenated water, through poorly defined violet-tinged mixed valence intermediates, to the tetravalent buff colored ceric hydroxide [12014-56-17, Ce(OH)4. The precipitate, which can prove difficult to filter, is amorphous and on drying converts to hydrated ceric oxide, Ce02 2H20. This commercial material, cerium hydrate [23322-64-7] behaves essentially as a reactive cerium oxide. 

The rare earth oxides have a number of distinguishing properties important in catalytic applications. The oxides are basic O) compared to alumina, lanthanum oxide (La203) being the most basic. The oxides also have good thermal stability, a valuable characteristic in most industrial applications. Some rare earths including cerium, praseodymium, and terbium form non-stoichiomet-ric oxides ( ), an important property shared by many good oxidation catalysts. These mixed valence state compounds are typically polymorphic. 

Hofrnaim first formulated the concept of mixed oxidation states for Prussian blue and other iron-containing mixed valence species. The electronic origin of the color, of course, remained obscure until the development of quantum mechanics. Another concept, described as valence oscillation , was used by Hofinaim and Hoschele following the realization that organic dyes were often found to be unsaturated species with valence resonance an example is cerium-uranium blue (3). 

However, there is much that remains unknown and in particular it is not known how the above phenomenon (Eq. 14.12) reflects the catalyst performance. Fig. 14.10 shows the change in the rate constant k (mol s ) of the oxidation of phenol with the change in the Cu content. The rate constant (k) linearly increases with the Cu content regardless of the calcination temperature of the catalysts. Thus, the active component in CCi Cup g catalysts is Cu. In the subsequent work, they reported that sol-gel technique is better than co-precipitation method, and high dispersion of copper oxide phase on the cerium oxide causes high catalytic activity. They emphasized the importance of the combination of the mixed valence state of Ce and Ce caused by an incorporation of Cu, which causes the reversible addition and removal of oxygen in this inherently defect structure. Here, the defect structure in CeOj is also emphasized. However, their discussion is rather sophisticated and their claim on the inherent function of this catalyst is difficult to be understood. The clear point is that copper in this catalyst is considerably active and durable in the wet-oxidation condition due to the effect of CeO,. 

In Kondo lattices the resonance absorption of the f ions is usually not observable and the systems have to be studied using a further ESR probe, mostly gadolinium. a-Ce is the most prominent mixed-valence system. However, it has been demonstrated that in this compound the 4f electrons do not contribute significantly to the linewidth and to the g-value of the ESR probe (Rettori et al. 1978). Hence, a-cerium was discussed in detail in sect. 4.1. Valence transitions generally evolve slowly as a function of temperature. Besides the well-defined transition in a-cerium another sharp, temperature-induced valence transition has been detected in Ybo.4Ino.6Cu2 (Felner and Nowik 1986). 

Fig. 14. Coulomb term l/(4f") calculated for the rare earth metals and experimental values for (i) mixed valence systems also included in hg. 13, (ii) cerium inter-metallic compounds (Hillebrecht et aL 1984), and (iii) rare earth insulators (Hiifner and Werthdm 1973X...

The brown colour is attributable to the presence of both Ce " " and Ce" ions that have different energy levels in the glass. The energy transition that takes place between Ce " and Ce" causes the brown colour. This mixed-valence state of cerium ion resulted from hydrolysis and oxidation at high temperature. It was first found as a result of this study that... 

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