Cerium hydrides

Cerium diiodide, 20 4 Cerium dioxide, 20 67, 69, 73, 74 Cerium hydride, neutron diffraction studies on, 8 238... 

Fig. 13. Structural and activity data for N2/H2 activation at the anomalous CeRu2 sample at 50bar/450°C showing the correlation between the rise of ammonia activity and the growth of the ruthenium and cerium hydride peak intensities (Walker et al. 1989).

In addition to this electronic interaction between finely divided ruthenium metal particles and cerium hydride, a hydrogen spillover was also proposed to explain the high activity for ammonia synthesis of these catalysts. Since hydrogen in the octahedral sites of CeH2+.t is desorbed at 420°C, atomic hydrogen is certainly present at the temperature of ammonia synthesis (450-550°C). Therefore spillover of atomic hydrogen at the hydride/ transition metal interface could occur. 

The data on the of cerium hydrides ( CpjCeH and (C9H7)2CeH [6,7] ) are, evidently, mistaken (see Chapter XI). 

Scheme 7 The tuck-in cerium complex and cerium hydride used in the experiment and models used in the calculations with labelling nomenclature...

The use of eq. (26.5) may be illustrated for the case of cerium hydride. From calorimetric measurements, Sieverts and Gotta (1928) obtained a value of -56.8 kcal/mole hydride for the enthalpy of formation CeH2.69- To calculate a value from dissociation pressure measurements, the partial molal enthalpies of hydrogen in cerium hydride as a function of the H/Ce ratio shown in fig. 26.7 can be used in eq. (26.5). The area under the curve from H/Ce = 1.89 to H/Ce = 2.69 represents the term under the integral sign. One half this value (-8.4 kcal/mole H2) is added to (1.89/2)4/Z,(CeH,.89) [= (1.89/2)(-49.3)], and a... 

The enthalpies of formation of the stoichiometric trihydrides of the first group of rare earths (La through Nd) also may be calculated from eq. (26.5) by integrating the last term up to H/M = 3. Using fig. 26.7 for cerium hydride, the partial molal quantity data of Hardcastle and Warf (1966) and Messer and Hung (1968) for lanthanum hydride, and Messer and Park (1972) for praseodymium and neodymium hydrides along with the data in table 26.3, the values for AHi shown in table 26.5 were calculated. Within the uncertainty of the data and the calculations therefrom, the value for AHi is approximately the same, -58 kcal/mole, for all four trihydrides. 

Partial molal entropies of cerium hydride as a function of r as calculated from the data of Lundin (1966), Streck and Dialer (1%0), and Hardcastle and Warf (1966) are shown in fig. 26.8. The minimum in the curve at about H/Ce = 2 is probably due to a maximum in the configurational entropy at this composition since hydrogen atoms enter the octahedral interstices in the rare earth dihydrides before all the tetrahedral sites are occupied (see section 2.2). Therefore, there is disorder in both the tetrahedral and octahedral sublattices near the stoichiometric dihydride composition. 

For cerium hydride, the last term in eq. (26.6) is obtained from the area under... 

Early studies on polycrystalline samples of lanthanum and cerium hydrides by Stalinski (1957b, 1959) indicated that these materials become semiconductors as the trihydride composition is approached. More recent studies on single crystals of cerium hydride by Libowitz and Pack (1969b) showed a sharp increase in resistivity as the hydride composition approached CeH2 illustrated in fig. 26.10. As mentioned in section 5.1, samples in which H/Ce<2.7 showed metallic behavior. However, samples with H/Ce > 2.8 exhibited typical semiconductor behavior with a linear increase in resistivity with reciprocal temperature. Clearly, there is a metal-to-semiconductor transition in cerium hydride at an... 

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