Washcoating technique for perovskite catalysts
View Patent ↗Precursor cations of A and B elements of an ABO 3 perovskite in aqueous solution are formed as an ionic complex gel with citric acid or other suitable polybasic carboxylic acid. The aqueous gel is coated onto a desired catalyst substrate and calcined to form, in-situ, particles of the crystalline perovskite as, for example, an oxidation catalyst on the substrate. In one embodiment, a perovskite catalyst such as LaCoO 3 is formed on catalyst supporting cell walls of an extruded ceramic monolith for oxidation of NO in the exhaust gas of a lean burn vehicle engine.
1. A method of forming crystalline particles of a perovskite catalyst material on cell wall surfaces of an extruded ceramic monolithic catalyst support substrate, wherein the perovskite crystalline particles comprise (i) cations of one or more elements, A-group cations, having a first range of ionic radii; (ii) cations of one or more elements, B-group cations, having a range of radii smaller than the first ionic radii range; and oxygen anions, and to be composed in elemental proportions as ABO 3 , the method comprising:
forming an aqueous solution comprising A-group cations with a substantially equivalent number of B-group cations;
adding a polybasic, carboxyl group-containing organic acid to the aqueous solution in an amount to provide an excess of carboxyl groups with respect to the total of A-group cations and B-group cations, and reacting the carboxyl groups and cations to form an aqueous ionic complex of cations and acid;
adjusting the water content of the aqueous ionic complex, if necessary, for coating of the aqueous ionic complex on cell wall surfaces of an extruded ceramic monolithic catalyst support;
applying the aqueous ionic complex to cell wall surfaces of the extruded ceramic monolithic catalyst support to form an adherent coating layer of the ionic complex;
evaporating superficial water from the coating layer; and
calcining the coating layer in air at elevated temperature to form particles of crystalline perovskite on the cell wall surfaces of the extruded ceramic monolithic catalyst support for catalytic treatment of a as flowing through the cells of the extruded ceramic monolithic catalyst support.
2. A method as recited in claim 1 in which the water content of the aqueous ionic complex is reduced for coating of the aqueous ionic complex on the cell wall surfaces of the extruded ceramic monolithic catalyst support.
3. A method as recited in claim 1 in which the aqueous solution is formed using oxygen-containing inorganic acid salts of the A-group cations and B-group cations.
4. A method as recited in claim 1 in which the aqueous solution is formed using nitrate salts of the A-group cations and B-group cations.
5. A method as recited in claim 1 in which citric acid is added as the polybasic, carboxyl group-containing organic acid.
6. A method as recited in claim 1 in which the A-group cations comprise one or more elements selected from the group consisting of rare earth elements, alkaline earth elements, and alkali group elements.
7. A method as recited in claim 1 in which the A-group cations comprise one or more elements selected from the group consisting of lanthanum, strontium, cerium, and barium.
8. A method as recited in claim 1 in which the B-group cations comprise one or more transition elements selected from the group consisting of elements of groups 3d, 4d, and 5d of the periodic table.
9. A method as recited in claim 1 in which the B-group cations comprise one or more of cobalt, manganese and iron.
10. A method as recited in claim 1 in which the concentration of each of the A-group cations and B-group cations in the aqueous ionic complex is in the range of about 0.45 to about 0.65 gram mole per liter of the aqueous ionic complex.
11. A method of forming crystalline particles of a perovskite oxidation catalyst material on cell wall surfaces of an extruded ceramic monolithic catalyst support substrate, wherein the perovskite crystalline particles comprise (i) cations of one or more elements, A-group cations, having a first range of ionic radii; (ii) cations of one or more elements, B-group cations, having a range of radii smaller than the first ionic radii range; and oxygen anions, and to be composed in elemental proportions as ABO 3 , the method comprising:
(a) forming an aqueous solution of A-group cations with a substantially equivalent number of B-group cations, the solution being formed with nitrate salts of the respective cations;
(b) adding citric acid to the aqueous solution in an amount to provide an excess of carboxyl groups with respect to the total of A-group cations and B-group cations and reacting the carboxyl groups and cations to form an ionic complex of the cations and acid;
(c) adjusting the water content of the aqueous ionic complex, if necessary, for coating of the aqueous ionic complex on cell wall surfaces of an extruded ceramic monolithic catalyst support;
(d) applying the aqueous ionic complex to cell wall surfaces of the extruded ceramic monolithic catalyst support to form an adherent coating layer of the ionic complex;
(e) evaporating superficial water from the coating layer;
(f) calcining the coating layer in air at elevated temperature to form particles of crystalline perovskite on the cell wall surfaces of the catalyst support for catalytic treatment of a as flowing through the cells of the extruded ceramic monolithic catalyst support;
(g) determining if a desired amount of crystalline perovskite has been formed on the cell wall surfaces of the catalyst support; and, if not,
repeating steps (d)-(f) until a desired amount has been formed.
12. A method as recited in claim 11 in which the water content of the aqueous ionic complex is reduced for coating of the aqueous ionic complex on the cell wall surfaces of the extruded ceramic monolithic catalyst support.
13. A method as recited in claim 11 in which the cell wall surfaces of the catalyst support include a plurality of open ended cells with cell walls aligned with the axis of extrusion and the aqueous ionic complex is applied to the cell walls by immersing the catalyst support in the aqueous ionic complex.
14. A method as recited in claim 11 wherein superficial water is evaporated from the coating layer by heating the catalyst support and coating layer at a temperature of about 120° C. for about one hour.
15. A method as recited in claim 11 wherein the coating layer is calcined at about 700° C.
16. A method as recited in claim 11 in which the A-group cations comprise one or more elements selected from the group consisting of rare earth elements, alkaline earth elements, and alkali group elements.
17. A method as recited in claim 11 in which the A-group cations comprise one or more elements selected from the group consisting of lanthanum, strontium, cerium, and barium.
18. A method as recited in claim 11 in which the B-group cations comprise one or more transition elements selected from the group consisting of elements of groups 3d, 4d, and 5d of the periodic table.
19. A method as recited in claim 11 in which the B-group cations comprise one or more of cobalt and manganese.
20. A method as recited in claim 11 in which the concentration of each of the A-group cations and B-group cations in the aqueous ionic complex is in the range of about 0.45 to about 0.65 gram mole per liter of the aqueous ionic complex.