Catalyst – solid sorbent – or support therefor: product or process – Catalyst or precursor therefor – Metal – metal oxide or metal hydroxide
Patent
1989-10-10
1992-03-03
Shine, W. J.
Catalyst, solid sorbent, or support therefor: product or process
Catalyst or precursor therefor
Metal, metal oxide or metal hydroxide
502524, 502525, B01J 2310, B01J 2372, B01J 2378
Patent
active
050933019
DESCRIPTION:
BRIEF SUMMARY
The invention relates to a catalyst for the elimination of toxic substances in exhaust gases from combustion systems and internal combustion engines, having at least one layer of catalyst material.
Such catalysts are preferably used to eliminate toxic substances in exhaust gases. For eliminating nitrogen oxides in flue gases, the SCR method (selective catalytic reduction) is preferably used.
In this method, the nitrigen oxides (NO.sub.x) are reduced by means of the ammonia (NH.sub.3) admixed with the flue gas in a catalyst of titanium oxide and vanadium oxide (TiO.sub.2 /V.sub.2 O.sub.3) at 300.degree. to 400.degree. C., forming harmless nitrogen and water. A disadvantage of the catalysts used in the SCR method is that they are unsuitable for reducing agents contained in fuel or exhaust gas, such as carbon monoxide, hydrogen or hydrocarbons, and in practical use have only a very short service life. These disadvantages, as well as the necessity of using ammonia as a reducing agent, limit the technical practicability of the SCR method considerably. Additional difficulties arise in the storage of the ammonia, as well as in dispensing it accurately. Moreover, the formation of solid ammonium sulfide residues limits the practicability of this method.
It is therefore the object of the invention to devise a catalyst that enables the reduction of nitrogen oxides using carbon monoxide, hydrogen and hydrocarbons as well as ammonia, and to disclose a method with which such a catalyst can be manufactured.
The first part of this object is attained by a catalyst material formed by at least one mixed oxide system, which has unsaturated ions on its surface that effect charge transfer processes.
A method for the manufacture of such a catalyst comprises grinding dry carbonates or oxides of lanthanum, manganese, iron, cooper and strontium in a defined ratio by weight for at least one hour, and sintering the powder mixture thus formed at 1300.degree. C. for forming the perovskite structure or the perovskite and spinel structure.
The catalyst according to the invention is distinguished by a long service life, which can be ascribed to its thermal, chemical and mechanical stability. With it, nitrogen oxides can be reduced with the aid of carbon monoxide, hydrogen and hydrocarbons, which are contained in every exhaust gas from internal combustion engines and combustion systems. Reduction by the additional use of ammonia is also possible with this catalyst. The pore size of the catalyst material is selected such that overly large pores do not reduce the active surface area. Nor are the pores too small, so the diffusion of the chemical reactants into the catalyst and the diffusion of the reaction products out of it is not hindered.
Further characteristics essential to the invention are defined by the dependent claims.
The mixed oxides of perovskite structure, or perovskite spinel structure, used for manufacturing the catalyst are distinguished, over a wide temperature range, between room temperature and a temperature of 1200.degree. C., by high thermal stability and chemical resistance to hot gases, which entrain with them components in the form of corrosive substances, such as oxygen, sulfur, sulfur oxides, sulfates, vanates, and various alkali salts. The catalytic activity of these mixed oxides, especially in the reduction of nitrogen oxides by means of carbon monoxide, hydrogen, hydrocarbons or ammonia, is based on the action of the d-electron orbitals of the transition metal ions that are contained in each of these mixed oxides and are preferably located on the surface of the completed catalyst. In the cubic perovskite lattice of the aforementioned mixed oxides, the ions of the transition metals and the oxygen ions are located on the 100 face of the lattice. On the surface of these crystallites, the ions of the transition metals of these mixed oxides are unsaturated, because of the lack of oxygen ions. These unsaturated ions effect charge transfer processes with adsorbed molecules from the gas phase. As a result, the reduction o
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patent: 4820678 (1989-04-01), Xu
Chu Wing F.
Pfeifer Norbert
Rohr Franz-Josef
Asea Brown Boveri Aktiengesellschaft
Greenberg Laurence A.
Lerner Herbert L.
Shine W. J.
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