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Elements such as lanthanum, zirconium, thorium and cerium are known to raise the thermostability of 7-AI203. Among these elements lanthanum is the most effective. Three procedures have been used to apply lanthanum homogeneously to the surface of pseudo-boehmite and 7-AI203, namely, incipient wetness impregnation, deposition-precipitation from a homogeneous solution, and specific adsorption of a La(EDTA)-complex. The latter procedure results in supports covered homogeneously with lanthanum. The minimum amount of lanthanum required to render the catalyst supports thermostable, depends on the procedure to apply the lanthanum. The specific adsorption procedure offers the possibility to obtain thermostable supports with Ioadings as low as 0.5 wt % La.High-resolution electron microscopy was used to assess the mechanism of the stabilization by application of foreign metal ions in amounts less than a monolayer. It appeared that addition of foreign metal ions brings about crystallization of the amorphous surface layer of 7-AI203.

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