[Chopin Thierry(Saint-Leu la Foret,FRX), Touret Olivier(la Rochelle,FRX), Vilmin Gabriel(Princeton,NJ)] Colloidal dispersions of finely divided particulates, well suited for the production of catalysts for the catalytic conversion of exhaust gases emanating from internal combustion engines, notably vehicular exhaust gases, comprise at least one rare earth compound and, optionally, at least one compound of at least one other element selected from among those of groups IVa, VIIa, VIII, Ib, IIb, IIIb and IVb of the Periodic Table, said particulates being at least 50% dispersible in water.

[Chopin Thierry(Saint-Leu La Foret,FRX), Touret Olivier(La Rochelle,FRX), Vilmin Gabriel(Princeton,NJ)] Thermally stable/highly reducible catalyst compositions, well suited for the catalytic conversion of exhaust gases emanating from internal combustion engines notably vehicular exhaust gases, comprise alumina, the oxides of cerium praseodymium and zirconium, and, optionally, at least one other oxide of bismuth, of a rare earth, or of an element of group VIII of the Periodic Table, and exhibit a stabilized hydrogen-absorption capacity up to a temperature of at least 1,000.degree. C.

[Chopin Thierry(Saint-Leu-la-Foret,FRX), Dupuis Dominique(Deuil-la-Barre,FRX), Lehaut Corinne(Paris,FRX), Chartier Pascal(Orsay,FRX), Talpaert Xavier(Paris,FRX)] The invention relates to a substrate provided, on at least a portion of one of its faces, with a coating with a photocatalytic property based on titanium dioxide which is at least partially crystalline and which is incorporated in the said coating partly in the form of particles predominantly crystallized in the anatase form. The invention also relates to a process for the preparation of this substrate and organic dispersions of titanium dioxide particles used in the said process for the preparation of the substrate.

[Chopin Thierry(Saint-Leu la Foret,FRX), Dupuis Dominique(Deuil-la-Barre,FRX), Pacaud Bernard(Kobe,JPX)] The invention concerns titanium dioxide particles coated at least partially: with a first layer of at least a cerium and/or iron compound, and a second layer of at least a metal oxide, hydroxide or oxohydroxide, the said particles having a BET specific surface area of at least 70 m.sup.2 /g and a density of 2.5. The invention also concerns a method for preparing these particles and their use as anti-UV agent.

[Chopin Thierry(Saint-Leu la Foret,FRX), Macaudiere Pierre(Asnieres/Seine,FRX), Touret Olivier(La Rochelle,FRX)] Storage-stable colloidal organic sols comprise suspended tetravalent metal oxide particulates in an organic diluent medium, notably cerium dioxide particulates, the tetravalent metal oxide particulates having a particle size d.sub.90 no greater than 200 nanometers and being complexed with at least one amphiphilic organic acid, and either said tetravalent metal oxide particulates comprising agglomerates of crystallites, the d.sub.80 of which being no greater than 5 nanometers and at least 90% of said agglomerates comprising from 1 to 5 crystallites, and/or said at least one amphiphilic organic acid has from 11 to 50 carbon atoms and is branched in at least one of the .alpha.-, .beta.-, .gamma.- or .delta.-positions relative to the atom bearing the acidic hydrogen.

[Chopin Thierry(Saint-Leu la Foret,FRX), Dupuis Dominique(Deuil-la-Barre,FRX), Labarre Dominique(Neuilly sur Seine,FRX), Mur Gilles(Villiers sur Marne,FRX)] Composite particles having a core consisting of an organic polymer and a coating including at least one aluminum, silicon, zirconium and/or transition metal oxide and/or hydroxide, and optionally containing calcium carbonate, as well as methods for preparing same, are disclosed. The use of said composite particles for preparing hollow particles with a shell of the same kind as the above-mentioned coating is also disclosed. Finally, the use of said hollow composite particles as fillers or additives in plastics and elastomers is disclosed.

[Bonneau Lionel(Angliers,FRX), Chopin Thierry(Saint-Leu-la-Foret,FRX), Vilmin Gabriel(Princeton,NJ), Touret Olivier(La Rochelle,FRX)] The invention concerns a procedure for preparation of a precursor of a mixed cerium and zirconium oxide-based composition, characterized by the following steps: a liquid mixture is prepared containing cerium and zirconium compounds; the preceding mixture is placed in contact with carbonate or bicarbonate, thus forming a reactive medium exhibiting a neutral or basic pH during the reaction; the precipitate obtained is collected. A mixed cerium and zirconium oxide-based composition is obtained by calcining this precursor. This composition is characterized by the fact that it has a total pore volume of at least 0.6 cm.sup.3 /g, and that at least 40%, and more particularly at least 50%, of the total pore volume is composed of pores having a diameter of at most 1 .mu.m.

[Chane-Ching Jean-Yves(Eaubonne,FRX), Chopin Thierry(Saint-Leu la Foret,FRX), Persello Jacques(Besancon,FRX)] In a first embodiment of the invention, a colloidal dispersion of a cerium compound is characterised in that it has a pH of at least 4 and a conductivity of at most 5 mS/cm. In a second embodiment, the colloidal dispersion is characterised in that it has a pH of more than 5. In a third embodiment, the colloidal dispersion is characterised in that the colloids have a carbon content such that the carbon/cerium mole ratio is at Least 0.01 and in that the pH is greater than 5. A process for the preparation of these dispersions is characterised in that a first colloidal dispersion is used as a starting material, with a pH which is lower than the pH of the dispersion which is to be prepared, and the pH of the first dispersion is raised while reducing the ionic strength. For the third embodiment, an organic acid is added to the starting dispersion.

[Chane-Ching Jean-Yves(Eaubonne,FRX), Chopin Thierry(Saint-Leu La Foret,FRX)] In a first embodiment, the present invention concerns a colloidal dispersion of a cerium compound, characterised in that it has a CeO.sub.2 concentration of between 500 and 700 g/l and a conductivity of at most 50 mS/cm. In a second embodiment, the colloidal dispersion is characterised in that it has a CeO.sub.2 concentration of more than 700 g/l. A process for the preparation of the dispersion of the invention is characterised in that a starting suspension or dispersion is treated by dialysis or with a cationic resin and an anionic resin. Colloidal dispersions of the invention can be used in automobile post combustion catalysis, in cosmetics, in lubrication, in ceramics or as an anticorrosion agent.

[Chopin Thierry(Saint-Leu la Foret,FRX), Huguenin Denis(Paris,FRX), Mathurin Alain(Ouistreham,FRX)] Colloidal dispersions of solid colloidal particulates in a liquid medium, preferably an aqueous medium, said colloidal particulates comprising a core of cerium oxide at least partially coated with a sheathing layer of titanium oxide, are well suited for the photocatalytic coating of a variety of substrates, e.g., glass or metal, to impart anti-IV, anti-reflecting and/or anti-staining properties thereto.