Lanthanum oxide structures

Recent literature data suggest that the use of promoters may be beneficial and lead to a significant improvement in CO conversion and selectivity for preferential selective CO oxidation (under an H2-rich environment).17 Lanthanum oxide (La203) is known to be an effective catalytic textural and structural promoter, increasing the... 

In this paper, we present results of a catalyst prepared by impregnation of a support made from a combination of zirconia and lanthanum oxide, themselves prepared by a sol-gel technique. A systematic study of the parameters influencing the reaction was performed in order that participation in the debate on-going in the literature might be imdertaken and to prove that the structure of the catalyst could ascertain or eliminate certain assumptions relative to the mechanism detailed in recent publications. 

There are a few catalytic applications in which other mixed oxide structures are cited. Among them are pyrochlore compounds such as La2Zr207, spinels such as AB2O4, and lanthanum beta aluminates, all of them with well-defined crystalline structures. There are also a few examples of oxide solid solutions made up of a rare-earth oxide and a transition-metal oxide. 

The thermal transformation fix)m lanthanum hydroxide (La(OH)3) to lanthanum oxide (La203) was studied by Neumann and Walter [188]. They reported two successive endothermic effects, caused by loss of water. Thermal analysis (DTA-TG, DSC), and high-temperature powder XRD were used to characterize this process. Lanthanum hydroxide oxide (LaOOH) was obtained as a temporary product at about 330 C. The structure of lanthanum hydroxide oxide was characterized by powder XRD and subsequent Rietveld refinement. The enthalpies of dehydration were calculated by DSC to be 87 kJ mof for the transformation La(OH)3 to LaOOH, and 54 kJ mof for the transformation LaOOH to La203. The activation energy of the transformation of lanthanum hydroxide to lanthanum hydroxide oxide was estimated by isothermal TG studies. 

Lanthanum Oxide. La203, m.p. approx. 2200 C sp. gr. 6.51 thermal expansion (25-1000°C) 12 X 1(H. A rare earth sometimes used in special optical glasses. Lanthanum Titanates. Two compounds have been reported LaTi203 and La2Ti209 the former has a perovskite (q.v.) structure and can be synthesized by heating a mixture of La203 and Ti203 at 1200°C in a vacuum. 

Sibu C.P., Rajesh Kumar S., Mukundan R, Warrier K.G.K. Structural modifications and associated properties of lanthanum oxide doped sol-gel nanosized titanium oxide. Chem. Mater. 2002 14 2876-2881... 

In the case of the formation of ATLS from lanthanum oxide, the structure of lanthanum hydroxide as an intermediate allows the formation of apatite through the topotactic mechanism. Thus, the coordination number (CN) of lanthanum in lanthanum oxide is 7, while there are 7- and 9-coordinated lanthanum cations in the apatite (Table 6). The lanthanum oxide hydration leads to increase of lanthanum CN from 7 to 9. This is also accompanied by the lattice expansion in the a and b dimensions and its contraction in the c dimension (Table 6). Subsequent acid-base interaction between the lanthanum hydroxide and silica or intermediate amorphous silicate results in the silicon incorporation into the structure to form apatite structure. This leads to the additional lattice expansion in three dimensions and the decrease of CN from 9 to 7 for 64% of lanthanum cations. 

Lanthana and lanthanum-containing composite oxides are the main form for the lanthanum-containg catalytic materials, lanthanum oxides, as active component, additives and supports have been extensively used in catalytic field. The component, structure and morphology of lanthanum oxides have a large influence on their catalytic properties. In this section, the preparation and application in catalysis of lanthanum oxides with different structure and morphology, besides various roles in catalysts will be discussed. 

Reference has been made already to the existence of a set of inner transition elements, following lanthanum, in which the quantum level being filled is neither the outer quantum level nor the penultimate level, but the next inner. These elements, together with yttrium (a transition metal), were called the rare earths , since they occurred in uncommon mixtures of what were believed to be earths or oxides. With the recognition of their special structure, the elements from lanthanum to lutetium were re-named the lanthanons or lanthanides. They resemble one another very closely, so much so that their separation presented a major problem, since all their compounds are very much alike. They exhibit oxidation state -i-3 and show in this state predominantly ionic characteristics—the ions. 

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