Hydrated boehmite with nano-sized crystallites AlO(OH)Á0.8H 2 O are obtained through a very reproducible sol-gel procedure and characterized using powder X-ray diffraction, in-situ IR spectroscopy, thermal analysis, BET surface area and photoelectron spectroscopy. The unit cell parameters, obtained after deleting the first shifted (020) diffraction peak, are a ¼ 3.686 A ˚, b ¼ 12.179 A ˚and c ¼ 2.855 A ˚. They are consistent with well-crystallized boehmite and with the position of the harmonic (080) diffraction peak. For the (020) peak, the correlation between position and peak width is confirmed for nano-crystallites. The average shape of the crystallites, determined from three peak widths, corresponds to slabs with dimensions 7.9 Â 2.7 Â 8.7 nm. Based on this crystallite shape, a model of hydrated boehmite is proposed and the formula corresponding to a full monolayer of chemically adsorbed water molecules is AlO(OH)Á0.55H 2 O.The thermal evolution of hydrated boehmite leads first to hydrated g-alumina Al 2 O 3 Á0.33H 2 O ¼ Al 2 O 2.67 (OH) 0.66 and the number of remaining hydroxyl groups is critical for the porosity. Between Al 2 O 3 Á0.33H 2 O and Al 2 O 3 Á0.2H 2 O the surface area remains approximately constant (300 m 2 g À1 ) and the hydroxyl density decreases deeply. For calcination temperature higher than 800 K, the loss of the remainder of the water leads to a strong decrease of this area. The limiting value corresponds to about 9-10 OH-groups nm À2 and can be related to the hydrogen spinel HAl 5 O 8 . The same type of isolated OH groups are present on hydrated boehmite and hydrated transition alumina (IR bands at 3670 and 3730 cm À1 ). A simple model of partial dehydroxylation of boehmite is proposed, in agreement with the remaining water and unit cell parameters.

Ce-Pr-O mixed oxides prepared by coprecipitation and calcined at 1173 K reveal a non stoichiometric structure with oxygen vacancies in the bulk. In situ XRD and Raman spectroscopy were used to investigate these properties simultaneously and identified a thermal expansion coefficient twice as high as that of pure ceria at room temperature and the presence of a Raman band at 560 cm 21 , characteristic of the oxygen vacancies. These results confirm that praseodymium atoms are able to induce oxygen mobility in the bulk at 298 K. This is due to a structure containing a high number of anionic vacancies pre-existing at ambient temperature.

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The generation of 6 wt. % potassium-modified alumina (K-A1,03) from K,CO,-impregnated alumina (AI , O, / K, CO, ) has been explored by means of surface and bulk analytical techniques, so as to characterize the interfacial synthetic events involved in the various preparative steps, namely: (i) wet impregnation, (ii) the drying process and (iii) high-temperature calcination. K+(aq) adsorption measurements indicated that 50% of the potassium content in the impregnating solution is loaded onto AI , O, via, essentially, a specific adsorption mechanism, involving a type of cation exchange with isolated AI-OH groups and formation of chemically bound AI-OK surface groups. ln situ infrared spectroscopic observations supported the adsorption measurements, revealing that after the impregnated material had been dried at 393 K, most of the isolated AI-OH groups are eliminated and, simultaneously, KAI(CO,)(OH,)-like surface species are formed. Calcination products up to 1273 K were examined using ex situ X-ray diffraction and photoelectron spectroscopy. The high-temperature treatment was found to activate various solid/solid interfacial interactions leading to low-temperature decomposition of surface carbonate and hydroxy groups, and formation of highly dispersed K-AI,O,.The results further indicate that the presence of K+ ions has no significant bearing on the accessible area of the alumina and on the thermal stability and structure of the bulk.

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Zr 0.10 (Ce 12x Pr x ) 0.90 O 2 mixed oxides (x between 0 and 0.75) were prepared by coprecipitation (nitrates) or by the sol-gel route. Zirconium n-propoxide and cerium and/or praseodymium nitrates were used as precursors. ''Sol-gel'' oxides calcined at 900 uC were shown to be cubic with a fluorite-type structure. Coprecipated oxides could not be obtained as solid solutions. The BET surface area of these samples rapidly decreases when xw0.50. A Raman study confirmed that all oxides were cubic and evidenced the presence of oxygen vacancies. The optimum oxygen storage capacity (OSC) was obtained for Zr 0.10 (Ce 0.50 Pr 0.50 ) 0.90 O 2 . It appears that the substitution of cerium by praseodymium in Zr 0.10 Ce 0.90 O 2 mixed oxides leads to a material with improved redox properties. The presence of vacancies, associated with Pr 3z /Pr 4z ions, is thought to be responsible for these enhanced OSCs.

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