Sol-gel process for obtaining pure and mixed oxide zirconia washcoats, useful as catalysts or catalyst supports

Abstract: A process for obtaining pure or mixed oxide zirconia washcoats consisting of: preparing a zirconium solution in water at a concentration, formally expressed as ZrO2, equal or lower than 400 g/l, starting from a basic zirconium carbonate additioned of glacial acetic acid, and eventually preparing one or more solutions of the other oxides when necessary; preparing a sol of zirconium, eventually mixed to other metals, by a thickening agent; sucking the sol into the honeycomb channels in order to obtain the washcoats, with subsequent gelation with gaseous ammonia; ageing the so obtained gel; drying and calcining.

Description: The invention concerns a process for obtaining pure or mixed oxide zirconia washcoats, useful as catalysts or catalyst supports. Zirconia can be used as a catalyst with specific catalytic action for some reactions like isomerization of 1-butene, dehydration of alcohols, hydrogenation of dienes by H 2 and hydrogen donor molecules like cyclohexadiene, hydrogenation of carboxylic acids, of carbon monoxide, etc. These behaviours are due to the simultaneous presence of weakly acid sites and weakly basic sites on ZrO 2 surface. Furthermore and above all, ZrO 2 is an efficient catalyst support for chromium oxide, copper oxide, Pd, Pt, Rh for processes like the oxidation of CO, the hydrogenation of CO, the removal of SO 2 and NO x from flue gases, propane oxidation, combustion of low-temperature fuel gases and low-temperature steam reforming. We have found a simplified sol-gel technique which can be applied to a wide variety of materials with minor modifications to the procedures and allows to obtain washcoats. The procedure for obtaining pure or mixed oxide zirconia washcoats, matter of the present invention, consists of: preparing a zirconium solution in water at a concentration, formally expressed as ZrO 2 , equal or lower than 400 g/l, starting from a basic zirconium carbonate additioned of glacial acetic acid, and eventually preparing one or more solutions of the other oxides when necessary; preparing a sol of zirconium, eventually mixed to other metals, by a thickening agent; sucking the sol into the honeycomb channels in order to obtain the washcoats, with subsequent gelation; ageing the so obtained gel; drying and calcining. With the claimed procedure we can obtain washcoats with thickness lower than 200µm. In ordinary precipitation zirconium hydroxide has a zirconium polycation structure of α-type or β-type or γ-type depending on the ageing. Usually, 4 zirconium ions are linked together by bridged hydroxyl groups (α-type) or oxygen groups (γ-type). In our case, the situation is more complicated and the gelation is most probably brought about by zirconium polycation structures linked by hydrogen bonds of their terminal hydroxyl groups to -OH, -OCH 3 or -CH 2 OCH 3 groups of methylcellulose chains. So, a commercial zirconium carbonate basic 2ZrO 2 CO 2 H 2 O can more advantageously be used as starting raw material. The typical ZrO 2 content is 40%. The content of impurities which can stongly modify the texture properties of final products is rather low: SO 4 2- < 0.2%, Cl - < 0.3%, Fe < 10 ppm, Na + < 0.2%, Ti 4+ < 25 ppm, SiO 2 ~ 130 ppm. As previously said, the maximum allowed ZrO 2 formal concentration is 400 g/l, after that the risk of spontaneous gelling of the solution is noteworthy. The thickening agent is preferentially chosen among solutions of methylcellulose, hydroxypropyl methylcellulose, hydroxybutyl methylcellulose, polyvinyl alcohol of molecular weight 15,000-186,000, dextrin or dextran. The sol composition in terms of oxides is preferentially selected from the group consisting of: ZrO 2 ; ZrO 2 -Y 2 O 3 , with Y 2 O 3 in the range 1-15%wt.; ZrO 2 -Al 2 O 3 , with Al 2 O 3 in the range 1-30%wt.; ZrO 2 -Al 2 O 3 -CeO 2 , with Al 2 O 3 in the range 1-30%wt. and CeO 2 in the range 5-30%wt and containing also a catalytically effective amount of rhodium and/or platinum and/or palladium. The sol is sucked into the honeycomb channels. In order to assure a better wettability, the sol composition is modified and a solution of zirconium acetate is preferred (example 1). The excess sol is blown out with air. Then the coating is made to gel putting the honeycomb into a cabinet with flowing gaseous ammonia. The honeycomb is finally dried and calcined at 500C at 2K/min. The washcoat shows good adhesion on the walls and high surface area at the temperature of possible utilization (800-900). In comparison to the known art and particularly to the patent application WO-92/05861, this method has the following advantages: the refractory oxides, ZrO 2 , Al 2 O 3 , CeO 2 and the precious metals are mixed on atomic scale, so washcoat composition is much more homogeneous and the single components much more catalytically effective; the washcoat preparation procedure is extremely simplified (single-step) without passing through stages of milling of powders and multiple impregnation steps, as suggested in the examples 1 and 3 of the patent application WO-92/05861; cerium, directly incorporated in the sol composition, helps to prevent catalyst sintering and the redox couple Ce 3+ ⇄Ce 4+ may be stabilized against deactivation. The precious metal can be directly incorporated into the sol composition, as reported in example 1. Herein we show an example. EXAMPLE 1 Zirconia-alumina(8,5%)-ceria(15%)-platinum (1,3%)-rhodium(0,3%) honeycomb washcoat Preparation of zirconium acetate solution in H 2 O (368 g/l as ZrO 2 ): 888 g 2ZrO 2 CO 2 H 2 O 460 ml glacial acetic acid d=1.05 g/ml to 1 l volume with H 2 O Dissolution is carried out under stirring and moderate heating to 50C in order to favour CO 2 evolution. Sol preparation: 518 ml zirconium acetate solution in H 2 O (368 g/l as ZrO 2 ) 213 g waterdispersible alumina hydrate solution in H 2 O (10%wt. as Al 2 O 3 ) 190 ml Ce(NO 3 ) 3 6H 2 O in H 2 O 499 g/l 5 g polyvinyl alcohol (molecular weight 100,000) 3.6 g H 2 PtCl 6 6H 2 O 0.8 g Rh(NO 3 ) 3 2H 2 O to 1 l volume with H 2 O The sol is pneumatically impregnated into a cordierite honeycomb of diameter 117 mm and heigth 76 mm with 64 channels/cm 2 (square cross section). Excess solution is blown away by air. The sol is gelled putting the honeycomb into a cabinet with flowing gaseous ammonia, and then the honeycomb is dried and calcined at 500C for 3 hour.

Claims: A process for obtaining pure or mixed oxide zirconia washcoats consisting of preparing a zirconium solution in water at a concentration, formally expressed as ZrO2, equal or lower than 400 g/l, starting from a basic zirconium carbonate additioned of glacial acetic acid, and eventually preparing one or more solutions of the other oxides when necessary, preparing a sol of zirconium, eventually mixed to other metals, by a thickening agent; sucking the sol into the honeycomb channels in order to obtain the washcoats, with subsequent gelation; ageing the so obtained gel; drying and calcining. A process following claim 1) where the thickening agent is methylcellulose, hydroxypropyl methylcellulose, hydroxybutyl methylcellulose, a polyvinyl alcohol of molecular weight in the range 15,000-186,000, dextrin or dextran. A process following claim 1) where the gelation agent is gaseous ammonia. A process following claim 1) where the sol composition in terms of oxides is selected from the group consisting of ZrO2; ZrO2-Y2O3, with Y2O3 in the range 1-15%wt.; ZrO2-Al2O3, with Al2O3 in the range 1-30%wt.; ZrO2-Al2O3-CeO2, with Al2O3 in the range 1-30%wt. and CeO2 in the range 5-30%wt and containing also a catalitically effective amount of rhodium and/or platinum and/or palladium A process following claim 1) where the washcoat has a thickness < 200 µm.

Inventors: MARELLA MARCELLO, MEREGALLI LETIZIA, TOMASELLI MICHELE, MARELLA, MARCELLO, MEREGALLI, LETIZIA, TOMASELLI, MICHELE
Applicants: VENEZIA TECNOLOGIE S P A, VENEZIA TECNOLOGIE S.P.A.
Application year: 1995
Main classification: C01G25
IPC codes: B01J21, C01G25, B01J37