Abstract
According to the literature and earlier studies in this laboratory, lanthanum oxide appears to be the best of the known additives for inhibiting the sintering of high-surface-area aluminas. The substrates of the present studies were largely γ-alumina, with minor components of transition aluminas, σ-alumina, and possibly θ-alumina. Alumina samples with different lanthanum concentrations, produced by impregnation with aqueous lanthanum nitrate, followed by calcination at various temperatures, were studied by chemisorption of carbon dioxide, Auger electron spectroscopy, Raman spectroscopy, and by X-ray powder diffraction (XRD). All results are consistent with the following description: Up to a concentration of 8.5 μmol La m 2 , the lanthana is in the form of a two-dimensional overlayer, invisible by XRD or Raman spectroscopy. For greater lanthana concentrations, the excess lanthana forms crystalline oxides, detectable by XRD. In samples calcined to 650 °C, this crystalline phase is cubic lanthanum oxide. After calcination at 800 °C, the lanthana reacts to form the lanthanum aluminate LaAlO 3.
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