Ammonia oxidation catalysts

Abstract

Ammonia oxidation catalyst units comprising a pair of honeycomb-type blocks having interplaced between them a layer of a gas permeable material performing the function of radially mixing the gas flow, said blocks comprising an ammonia oxidation catalysts, and having height of less than 15 cm and the interplaced layer height of 3 to 0.5 cm.

Claims

1. A catalyst for ammonia oxidation formed of one or more catalytic units each of which is formed of a pair of blocks having a honeycomb structure containing tubular passages not interconnected through bores, comprising: an ammonia oxidation catalyst, each of said blocks having a height between 2 cm and 15 cm and interplaced between the pair of blocks a foamed layer having a height between 0.5 cm and 3 cm, formed of foamed material having open, randomly connected cells wherein the ammonia oxidation catalyst is obtained by coating with at least three layers, the first layer comprising alumina, ceria and zirconia, the second layer comprising oxides of cobalt, zirconium and cerium, and the third layer comprising platinum metal.

2. The catalyst according to claim 1, wherein both the ceria and the zirconia of the first layer are in a smaller proportion as compared to the alumina of the first layer.

3. A honeycomb structure formed of one or more units each of which is formed of a pair of blocks having tubular passages not connected through bores, each of said blocks having height of less than 6 cm and more than 5 cm and interplaced between them a layer of foamed material having randomly connected cells, said layer having a height of less than 2 cm and more than 0.5 cm, wherein each of said blocks can be coated with a catalyst suitable to oxidize ammonia.

4. The honeycomb structure according to claim 3, wherein the density of cells of the foamed layer ranges from 3 to 10 cells/cm.

5. The honeycomb structure according to claim 3, wherein the blocks are formed of ceramic or metallic material.

6. The honeycomb structure according to claim 3, wherein the blocks have a number of cells per cm, which can be the same or different, from 3 to 10 and wherein the number of pores per cm of the foamed layer is 4 to 20.

Description

DETAILED DESCRIPTION OF THE INVENTION

(1) The blocks have height of not more than 15 cm, preferably not more than 10 and more preferably not more than 6 cm and more than 2 cm. The height of the interplaced layer is more than 0.5 cm and not more than 3 cm preferably not more than 2 cm.

(2) A foamed material having open, randomly connected cells is usable to form the interplaced layer.

(3) For honeycomb-type structure it is meant a structure formed of tubular not interconnected through bores.

(4) As indicated, in the above structure the interplaced layer performs the function of thoroughly mixing the gas flow exiting the first honeycomb block wherein, due to laminar piston-type flow inside the tubular not interconnected through bores, no radial mixing occurs, thus allowing better conversion inside the second block.

(5) The laminar piston-type flow inside the tubular through bores of the blocks favors the maintenance of constant reagents concentration at contact with the catalyst covering the walls of the through bores.

(6) The material of honeycomb-type block preferably is ceramic or metallic; any other material resistant to the high temperatures of the ammonia oxidation reaction can also be used. For example, the material of the honeycomb type block can be a perovskite of the ABO.sub.3 type wherein A is a rare earth-element or an alkaline-earth element or mixtures thereof and B is a transition metal element or mixtures thereof.

(7) The density of the cells ranges from 3 to 10 cells/cm; that of the pores of the foamed material is of 4 to 20/cm.

(8) Commercial honeycomb-type blocks are available from Emitech-Germany; commercial foams from Hi-Tech CeramicsNY, USA.

(9) Foamed alpha alumina and reticulated foams with open cells randomly connected are preferred.

(10) Usable honeycomb-type alloy blocks can also be obtained from tapes prepared according to U.S. Pat. No. 4,820,678.

(11) Any type of ammonia oxidation catalyst can be used in the honeycomb-type structure unit according to the invention. The final catalyst can be obtained by either a) coating an inert honeycomb monolithic structure with the active element or b) extruding the active element powder to a honeycomb monolithic type structure.

(12) A preferred catalyst comprises mixed oxides of cobalt, manganese and rare earth metals having composition expressed as percentage by weight of Co O, Mn O and rare earth oxide in the lowest state of valence as follows: 20-45% Co O, 50-60% Mn O, 0.5-20% rare earth metal oxide, preferably La.sub.2O.sub.3 and its mixtures with CeO.sub.2. The mixed oxides are supported on porous inorganic metal oxides, preferably gamma alumina. Catalysts of this type containing Cu O in place of Co O, and the preparation thereof are described in WO 2008-090450.

(13) Examples of other usable catalysts are described in U.S. Pat. Nos. 5,217,939 and 5,690,900, and WO 99/25650. Other examples are the perovskite type catalyst (ABO.sub.3) and the spinel type (AB.sub.2O.sub.4).

(14) The catalyst unit allows to obtain high conversion of ammonia to NO.

(15) The oxidation reaction conditions are: temperature from 200? C. to 900? C., pressure 1 to 12 bar abs., GHSV 8.000-140.000 h.sup.?1.

(16) The unit, thanks to its specific structure offers advantages with respect to the back pressure and increases the space-time yield since the throughput of the existing plants can be more heavily loaded.

Example 1

(17) A catalyst unit formed of three units of honeycomb-like structure ceramic blocks each having 5 cm height, 62 cells/cm.sup.2 and including two layers of foamed alfa alumina each 2 cm thick interplaced by alternating one block of monolith and one foam until the five structures are arranged in cascade, was prepared by immersing the blocks in a slurry containing gamma alumina milled to 1 to 10 ?m having supported on it a mixed oxides catalyst of composition expressed in percentage by weight of Co O, Mn O and La.sub.2O.sub.3 of 37.4% Co O, 53.4% Mn O and 9.2% La.sub.2O.sub.3. The supported catalyst comprised also Pt in amount of 0.1-0.2 wt %. Said transition metal oxides are supported on gamma alumina in a global amount equal to 20% by weight. The slurry is further composed of deionized water and made acidic to pH 4 with acetic acid.

(18) The catalyst was prepared by impregnating gamma alumina with an aqueous solution of lanthanum nitrate (La(NO.sub.3).sub.3).

(19) The impregnated support was then dried at 110? C., calcined at 600? C. and thereafter impregnated with an aqueous solution of manganese nitrate (Mn(NO.sub.3).sub.2), cobalt nitrate (Co(NO.sub.3).sub.2) and Pt(NH.sub.3).sub.4 Cl.sub.2, and dried at 120? C.

(20) A volume of solution equal to 100% of the pore volume of alumina was used for impregnation.

(21) The immersed blocks were removed from the slurry and calcinated at 500? C. to obtain reduction of platinum ions to metal.

(22) The thus obtained unit was inserted into a reactor for ammonia oxidation. The reaction conditions were: GHSV=10.000 h.sup.?1, temperature of the gas mixtures taken at the inlet of the catalyst unit 550? C., pressure 1 bar and ammonia concentration equal to 1% v/v in air.

(23) The conversion of ammonia to NO was higher than 96%.

Comparison Example 1

(24) A catalyst unit similar to that used in Example 1, but not comprising the foamed alumina layers, was used in a test of ammonia oxidation carried out under the same conditions as in Example 1.

(25) The conversion of ammonia to NO was 87%.

Comparison Example 2

(26) A catalyst unit formed of a single honeycomb-like structure ceramic block having 15 cm height, 62 cells/cm2 was prepared according to the procedure already described in the EXAMPLE 1 and was used in a test of ammonia oxidation carried out under the same conditions as in Example 1.

(27) The conversion of ammonia to NO was 74%.

(28) The disclosures in European Patent Application No. 08172820 from which this application claims priority are incorporated herein by reference.