Patent Application
20080075646
In preparing the preferred embodiments of the present invention, various alternatives may be used to facilitate the objectives of the invention. These embodiments are presented to aid in an understanding of the invention and are not intended to, and should not be construed to, limit the invention in any way. All alternatives, modifications and equivalents that may become obvious to those of ordinary skill upon a reading of the present disclosure are included within the spirit and scope of the present invention.
In the present invention a diffusion barrier is arranged between the catalytically active layers in a structured automotive exhaust gas catalyst which contains a plurality of different, superposed, catalytically active layers applied to a carrier. Preference is given to using diffusion barriers comprising oxidic materials. This ensures, firstly, that the adhesion between the, likewise oxidic, catalytically active layers is not irreversibly destroyed. Secondly, such a choice ensures that the diffusion of the exhaust gas to be purified from one catalytically active coating into the other is not significantly hindered by the barrier layer located in between.
The effect of the diffusion barrier can, depending on the material used and the purification task of the structured automotive catalyst, be based on a mechanical impact or a chemical impact. Preference is given to diffusion barriers having a chemical barrier impact.
As a preferred embodiment of a diffusion barrier having a chemical barrier impact, use is made of unexchanged zeolites, known as H-zeolites. The effect of these H-zeolites is based thereon, that transition metal atoms which are thermally induced to migrate from a catalytically active coating into the zeolitic barrier layer are chemically fixed faster-moving, smaller protons are liberated. The transition metal atoms are fixed chemically at the adsorption sites of the protons. Their migration is initially stopped. Only when all proton adsorption sites in the zeolitic barrier layer have been occupied by transition metal atoms and when accumulation occurs at the interface with the neighbouring catalytically active layer due to further migration into the barrier layer, so that the threshold concentration is exceeded, the diffusion barrier lose its effectiveness. It is possible to prevent this point from being reached by a dimensioning of the diffusion barrier matched to the concentration of the transition metal atoms in the catalytically active coating.
Zeolites suitable for use as diffusion barriers in structured automotive catalysts for the removal of nitrogen oxides from the exhaust gases of predominantly lean operated internal combustion engines include not only H-zeolites but also, in particular and preferred, ammonium-exchanged zeolites. When transition metal atoms migrate from the adjacent catalytically active layers into the barrier layer comprising ammonium-exchanged zeolites, both protons and ammonia are liberated. Ammonia is temporarily stored in the cage structure of the zeolite and can be used for the reduction of nitrogen oxides in a selective catalytic reduction. Furthermore, hydrocarbon molecules from the exhaust gas can be temporarily stored both in the cage structure of H-zeolites and in the cage structure of ammonium-exchanged zeolites. These hydrocarbon molecules are then likewise available as reducing agents when nitrogen oxides are to be removed from exhaust gases from internal combustion engines operated predominantly under lean conditions.
In another embodiment of the invention, diffusion barriers having a mechanical barrier impact are used. Catalytically inert oxides having small particles, preferably ones whose particle size distribution has a median value d50 of not more than 2 microns and an upper value d90 of not more than 10 microns, where the specified parameters of the particle size distribution are determined by means of laser-optical methods, are suitable for this purpose. The specific surface area of the catalytically inert oxides should be less than 100 square metres per gram of oxide, preferably less than 50 square metres per gram of oxide, exceptionally preferably less than 20 square metres per gram of oxide (BET; determined in accordance with DIN 66132). The barrier impact of such diffusion barriers is based on their porosity being too low for unhindered passage of transition metal atoms through the interface into the barrier layer to be able to take place. The positioning of such a barrier layer between two catalytically active coatings accordingly leads to a significant increase in the threshold concentration. Dense layers of fine milled alumina, silica, titanium dioxide or titanates are well suited for use as mechanical diffusion barriers. They preferably have layer thicknesses of less than 50 microns, exceptionally preferably less than 25 microns.
The invention is illustrated below with the aid of a comparative example, an example and two figures. The figures show:
In this comparative example, the thermal aging behaviour of a structured catalyst for the selective catalytic reduction (SCR) of nitrogen oxides comprising two different, superposed, catalytically active layers on an inert ceramic honeycomb carrier with ammonia as reductant was examined. To produce the catalyst, hereinafter designated as CC, a coating comprising 6.4 grams of a copper-exchanged zeolite was applied to an inert ceramic honeycomb carrier having a volume of 0.04 litre, 62 cells per square centimetre and a cell wall thickness of 0.17 millimeter. The coated piece was calcined at 500° C. in air for 2 hours to ensure good adhesion of the coating. A further coating comprising 2 grams of an iron-exchanged zeolite was subsequently applied to the catalyst.
The catalyst CC was measured in the freshly prepared state on a stationary model gas test bench. The following gas concentrations were selected for the test:
In studies on the SCR activity, the molar ratio of ammonia to nitrogen oxides is usually designated by alpha:
From the gas concentrations shown in the table results an alpha value of α=0.85. The gas hourly space velocity in the model gas tests carried out was 30 000 h−1.
Subsequent to the initial measurement, the catalyst CC was subjected to a synthetic, hydrothermal aging procedure. For this purpose, the catalyst CC was exposed to an atmosphere composed of 10% by volume of oxygen and 10% by volume of water vapour in nitrogen for 48 hours in a furnace heated to 700° C. The aged catalyst was then once again subjected to the above-described examination on the model gas test bench.
Since SCR catalysts are used for the reduction of nitrogen oxides in predominantly lean, i.e. oxygen-containing, exhaust gases, there is a risk that the reducing agent ammonia will be converted into dinitrogen oxide (nitrous oxide) in an undesirable secondary reaction, especially at temperatures above 300° C. Such a secondary reaction leads to a reduction in the amount of reducing agent available for the selective catalytic reduction of the primary nitrogen oxides and thereby possibly to a deterioration in the conversion. The nitrous oxide concentration measured downstream of the catalyst is therefore an important magnitude for assessing the selectivity of a SCR catalyst.
In the fresh prepared state, the concentration of nitrous oxide downstream of the catalyst remains constant at 11±3 ppm over the entire temperature range. The reaction sites over copper which are primarily responsible for the oxidation of ammonia are located exclusively in the lower layer. If nitrous oxide is formed there by over oxidation of ammonia, this has to diffuse through the upper layer containing iron-exchanged zeolite prior to desorption. The nitrous oxide can react there with stored residual ammonia to form nitrogen and water so that there is no increase in the dinitrogen oxide concentration downstream of the catalyst even at high temperatures. The residual 11±3 ppm of nitrous oxide emitted corresponds to the proportion of N2O secondary emission resulting from oxidation of ammonia at the less oxidation-active iron sites.
During the hydrothermal aging of the catalyst CC, copper atoms are thermally detached from their adsorption sites in the lower layer and migrate into the upper layer, which contains iron-exchanged zeolite. This increases the oxidation activity of the upper layer and therefore an increased over oxidation of the reducing agent ammonia is observed, in particular above 300° C. Dinitrogen oxide is formed and desorbs immediately. The nitrous oxide concentration downstream of the catalyst accordingly increases linearly at temperatures of 350° C. and above and finally reaches values of 25 ppm at 500° C.
In a manner corresponding to the production of the catalyst CC in the comparative example, a ceramic honeycomb carrier having a volume of 0.04 litre, 62 cells per square centimetre and a cell wall thickness of 0.017 millimetres is provided with a coating comprising copper-exchanged zeolite and calcined at 500° C. in air for 2 hours. Before application of the second catalytically active layer comprising iron-exchanged zeolite, an intermediate coating comprising unexchanged zeolite and ammonium-exchanged zeolite is applied and renewed calcination is carried out at 500° C. in air for 2 hours.
Therefore the nitrous oxide concentration downstream of the aged catalyst can be reduced to concentrations below 15 ppm over the entire temperature range, due to the chemical barrier impact of the intermediate layer inserted as diffusion barrier, which prevents the thermally induced migration of copper atoms into the upper layer containing iron-exchanged zeolite.
| Number | Date | Country | Kind |
|---|---|---|---|
| 06019975 | Sep 2006 | EP | regional |
