Catalyst – solid sorbent – or support therefor: product or process – Catalyst or precursor therefor – Metal – metal oxide or metal hydroxide
Patent
1989-10-10
1991-10-08
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
050554409
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
The invention relates to a catalyst for the elimination of toxic substances in exhaust gases and more particularly for eliminating nitrogen oxides in flue gases. The SCR method (selective catalytic reduction) has theretofore been preferably used for such detoxification.
In this selective catalytic reduction method, the nitrogen 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 eliminating 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 methods.
THE INVENTION
It is therefore the first object of the invention to devise a catalyst that enables the reduction of exhaust nitrogen oxides using and containing carbon monoxide, hydrogen and hydrocarbons as well as ammonia, and secondly to disclose a method with which such a catalyst can be manufactured.
The first object is attained by the catalyst according to claims 1 and 13.
The second object is met by a method for the manufacture of such a catalyst is disclosed in claims 14 and following.
The catalyst according to the invention is distinguished by a long service life, which is dictated by 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.
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, vanadous compounds, 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 of nitrogen oxides is effected in the presence of suitable reducing agents such as carbon monoxide, hydrogen and hydrocarbons and ammonia.
Mixed oxides of spinel structure also contains ions of the transition metals on the crystal surface. The catalytic activity of the pure spinels is lower by comparison with the mixed oxides of pure perovskite structure. By suitable combination of mixed oxides of perovskite struct
REFERENCES:
patent: 3929670 (1975-12-01), Kudo et al.
patent: 4055513 (1977-10-01), Wheelock
patent: 4820678 (1989-04-01), Xu
Chu Wing F.
Reich Andreas
Rohr Franz-Josef
Asea Brown Boveri Aktiengesellschaft
Greenberg Laurence A.
Lerner Herbert L.
Shine W. J.
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