The present invention relates to a passive nitrogen oxide adsorber for the passive storage of nitrogen oxides from the exhaust gas of a combustion engine, which comprises a zeolite of the structure type RHO containing palladium and cesium.

The exhaust gas of motor vehicles that are operated with lean-burn combustion engines, such as diesel engines, contain carbon monoxide (CO) and nitrogen oxides (NOx), as well as components that result from the incomplete combustion of the fuel in the combustion chamber of the cylinder. The latter are in particular residual hydrocarbons (HC) and particle emissions, also referred to as "diesel soot" or "soot particles".

To clean these exhaust gases, the aforementioned components must be converted to harmless compounds as completely as possible. This is only possible with the use of suitable catalysts.

A known method for removing nitrogen oxides from exhaust gases in the presence of oxygen is selective catalytic reduction (SCR.) by means of ammonia on a suitable catalyst. In this method, the nitrogen oxides to be removed from the exhaust gas are converted to nitrogen and water using ammonia. Iron-exchanged and, in particular, copper-exchanged zeolites, for example, may be used as SCR catalysts; see, for example, W02008/106519 Al, WO2008/118434 Al, and WO2008/132452 A2.

The disadvantage of SCR catalysts is that they only work from an exhaust gas temperature of approx. 180 to 200 °C and do not, therefore, convert nitrogen oxides that are formed in the engine's cold-start phase.

In order to remove the nitrogen oxides, so-called nitrogen oxide storage catalysts are also known, for which the term, "Lean NOx Trap," or LNT, is common. Their cleaning action is based upon the fact that in a lean operating phase of the engine, the nitrogen oxides are predominantly stored in the form of nitrates by the storage material (oxides, carbonates, or hydroxides of magnesium, calcium, strontium, barium, alkali metals, rare earth metals, or mixtures thereof) of the storage catalyst, and the nitrates are broken down again in a subsequent rich operating phase of the engine, and the nitrogen oxides which are thereby released are converted with the reducing exhaust gas components in the storage catalyst to nitrogen, carbon dioxide, and water. This operating principle is described in, for example, SAE document SAE 950809. The procedure described in SAE Technical Paper 950809 is also referred to as active nitrogen oxide storage.

In addition, a method known as passive nitrogen oxide storage has also been described. Nitrogen oxides are stored thereby in a first temperature range and released again in a second temperature range, wherein the second temperature range lies at higher temperatures than the first temperature range. Passive nitrogen oxide storage catalysts are used to implement this method, which catalysts are also referred to as PNA (for "passive NOx adsorbers").

By means of passive nitrogen oxide storage catalysts, nitrogen oxides may - particularly at temperatures below 200 °C, at which an SCR. catalyst has not yet reached its operating temperature - be stored and released again as soon as the SCR catalyst is ready for operation. Thus, an increased total nitrogen oxide conversion is realized in the exhaust gas after-treatment system by the interim storage below 200 °C of the nitrogen oxides emitted by the engine, as well as the concerted release of those nitrogen oxides above 200 °C.

Palladium supported on cerium oxide has been described as a passive nitrogen oxide storage catalyst (see, for example, W02008/047170 Al and WO2014/184568 Al). From WO2012/166868 Al, it is known to use a zeolite as a passive nitrogen oxide storage catalyst, which zeolite contains, for example, palladium and a further metal, such as iron, for example.

W02015/085303 Al discloses passive nitrogen oxide storage catalysts which contain a precious metal and a small-pore molecular sieve with a maximum ring size of eight tetrahedral atoms. A passive nitrogen oxide storage catalyst comprising a certain small-pore molecular sieve with a maximum ring size of eight tetrahedral atoms and palladium is also known from EP 3 449 999 Al.

WO2016/135465 Al discloses a passive NOx adsorber which comprises a first noble metal and a molecular sieve having a MAZ Framework Type. The first noble metal can be palladium.

W02020/039015 Al discloses a passive NOx adsorber which comprises a carrier substrate, palladium and a zeolite which belongs to the structure type MOZ or ZSM-10. US2016/279598 discloses a passive NOx adsorber which comprises a first noble metal and a molecular sieve having an OFF Framework Type. The first noble metal can be palladium.

Passive NOx absorbers are also disclosed in US2020/016571.

EP3334912B1 discloses an exhaust system for treating an exhaust gas which comprises (i) a NOx absorber catalyst, (ii) means for introducing hydrocarbons into the exhaust gas and (iii) a lean NOx trap. The NOx absorber catalyst comprises a molecular sieve and a noble metal, wherein the noble metal is preferably palladium whereas the molecular sieve is preferably a small pore molecular sieve. The document discloses a long list of small pore molecular sieves including the Framework Type RHO.

Modern and future diesel engines are becoming more and more efficient, as a result of which exhaust gas temperatures are decreasing. In parallel, the legislation on the conversion of nitrogen oxides is becoming increasingly stringent. The result is that SCR catalysts alone no longer suffice to meet the nitrogen oxide limits. In particular, there continues to be further need for technical solutions that ensure that nitrogen oxides formed during the engine's cold-start phase do not escape into the environment.

It has now been found that zeolites belonging to the structure type RHO which comprise palladium and cesium have excellent passive nitrogen oxide adsorber properties. In particular, it was found that zeolites of the structure type RHO provide an optimum of storage capacity and performance/transience which is believed to be based on the combination of channels of eight rings (8MR), six rings (6R) and double-eight rings (D8R).

The present invention relates, therefore, to a catalyst comprising a carrier substrate of length L, palladium, cesium and a zeolite of the structure type RHO.

Zeolites are two- or three-dimensional structures, the smallest structures of which can be considered to be SiO4 and Al 04 tetra hedra. These tetrahedra come together to form larger structures, wherein two are connected each time via a common oxygen atom. Different-sized rings may be formed thereby - for example, rings of four, six, or even nine tetrahedrally-coordinated silicon or aluminum atoms. The different types of zeolite are often defined via the largest ring size, because this size determines which guest molecules can penetrate the zeolite structure, and which not. It is customary to differentiate between large-pore zeolites with a maximum ring size of 12, mediumpore zeolites with a maximum ring size of 10, and small-pore zeolites with a maximum ring size of 8.

Furthermore, zeolites are grouped by the Structural Commission of the International Zeolite Association into structural types which are each provided with a three-letter code; see, for example, Atlas of Zeolite Framework Types, Elsevier, 5th edition, 2001.

The catalyst according to the invention comprises zeolites whose largest channels are formed by 8 tetrahedrally-coordinated atoms and which belong to the structural type RHO. Given the definition for zeolites according to their biggest pore aperture as described above, RHO is a small pore zeolite

Preferred isotypes of the structure type RHO are Rho (see Adv. Chem. Ser., 121, 106-115 (1973)), LZ-214 (see US 4,503,023) and Pahasapaite (see Jb. Miner.

Mh., 433-440 (1987)).

Preferred zeolites of the structure type RHO have a silica-to-alumina molar ratio (SAR) of 5 to 50.

Zeolites of the structure type RHO can be obtained via known methods, for example via the method described in Microporous Materials, Vol. 4, 231-238 (1995) and in Verified Synthesis Of Zeolitic Materials, edited by Harry Robson and Karl Petter Lillerud, Elsevier Science, 2001, pages 249-250.

The catalyst according to the present invention comprises palladium. The palladium is preferably present thereby as palladium cation in the zeolite structure, i.e., in ion-exchanged form. However, the palladium may also be wholly or partially present as palladium metal and/or as palladium oxide in the zeolite structure and/or on the surface of the zeolite structure.

However, palladium is preferably not present as part of the zeolite structure, i.e. palladium is preferably not part of the crystal lattice of the zeolite.

In particular, the palladium is present

• as palladium cation in the zeolite structure, • as palladium metal in the zeolite structure and/or on the surface on the zeolite structure and/or

• as palladium oxide in the zeolite structure and/or on the surface on the zeolite structure.

The palladium may be present in quantities of 0.01 to 20 wt.% in relation to the sum of the weights of zeolite and palladium and calculated as palladium metal. Palladium is preferably present in quantities of 0.5 to 10 wt.% - particularly preferably, of 0.5 to 4 wt.%, and, very particularly preferably, of 0.5 to 3 wt.% - in relation to the sum of the weights of zeolite and palladium and calculated as palladium metal.

The catalyst according to the present invention comprises cesium. The cesium is preferably present thereby as cesium cation in the zeolite structure, i.e., in ion- exchanged form. However, cesium is preferably not present as part of the zeolite structure, i.e. cesium is preferably not part of the crystal lattice of the zeolite.

The cesium may be present in quantities of 0.01 to 5 wt.% in relation to the sum of the weights of zeolite and cesium and calculated as cesium metal.

Cesium is preferably present in quantities of 0.1 to 4 wt.% - particularly preferably, of 0.3 to 3,5 wt.%, in relation to the sum of the weights of zeolite and cesium and calculated as cesium metal.

A particularly preferred catalyst of the present invention comprises a carrier substrate of length L, palladium in an amount of 0.5 to 4 wt.%, in relation to the sum of the weights of zeolite and palladium and calculated as palladium metal, cesium in an amount of 0.3 to 3,5 wt.%, in relation to the sum of the weights of zeolite and cesium and calculated as cesium metal and a zeolite of the structure type R.HO having a SAR. value of 5 to 50.

The catalyst according to the invention comprises a carrier substrate. This may be a flow-through substrate or a wall-flow filter.

A wall-flow filter is a carrier substrate that comprises channels of length L which extend in parallel between a first and a second end of the wall-flow filter, which are alternatingly sealed either at the first or second end, and which are separated by porous walls. A flow-through substrate differs from a wall-flow filter, in particular, in that the channels of length L are open at its two ends.

In an uncoated state, wall-flow filters have, for example, porosities of 30 to 80% - in particular, 50 to 75%. In the uncoated state, their average pore size is 5 to 30 micrometers, for example.

Generally, the pores of the wall-flow filter are so-called open pores, i.e., they have a connection to the channels. Furthermore, the pores are normally interconnected with one another. This enables easy coating of the inner pore surfaces, on the one hand, and an easy passage of the exhaust gas through the porous walls of the wall-flow filter, on the other hand.

Flow-through substrates are known to the person skilled in the art, as are wall-flow filters, and are commercially available. They consist, for example, of silicon carbide, aluminum titanate, or cordierite.

In one embodiment, the catalyst according to the invention does not comprise any further metal except palladium and cesium - in particular, neither copper, nor iron, nor platinum.

In a further embodiment of the catalyst according to the present invention, the zeolite of the structure type RHO and the palladium and the cesium are present in the form of a coating on the carrier substrate. The coating may thereby extend over the entire length L of the carrier substrate or only over a section thereof.

In the case of a wall-flow filter, the coating may be situated on the surfaces of the input channels, on the surfaces of the output channels, and/or in the porous wall between the input and output channels.

Catalysts according to the present invention, in which the zeolite of the structure type RHO, the palladium and the cesium are present in the form of a coating on the carrier substrate, may be produced according to the methods familiar to the person skilled in the art, such as according to the usual dip coating methods or pump and suck coating methods with subsequent thermal post-treatment (calcination). A person skilled in the art knows that in the case of wall-flow filters, their average pore size and the average particle size of the materials to be coated can be adapted to each other such that they lie on the porous walls that form the channels of the wall-flow filter (on-wall coating). The average particle sizes of the materials to be coated may, however, be selected such that said materials are located in the porous walls that form the channels of the wall-flow filter so that the inner pore surfaces are thus coated (in-wall coating). In this instance, the average particle size of the materials to be coated must be small enough to penetrate the pores of the wall-flow filter.

In one embodiment of the present invention, the zeolite of the structure type RHO, the palladium and the cesium are present coated over the entire length L of the carrier substrate, wherein no further catalytically active coating is found on the carrier substrate.

In other embodiments of the present invention, the carrier substrate may, however, also carry one or more further catalytically active coatings.

For example, the carrier substrate may, in addition to a coating comprising the zeolite of the structure type RHO and the palladium, comprise a further coating which is oxidation-catalytically active.

The oxidation-catalytically-active coating comprises, for example, platinum, palladium, or platinum and palladium on a carrier material. In the latter case, the mass ratio of platinum to palladium is, for example, 1 : 1 to 14: 1.

All materials that are familiar to the person skilled in the art for this purpose are considered as carrier materials. They have a BET surface of 30 to 250 m ² /g - preferably, of 100 to 200 m ² /g (specified according to DIN 66132) - and are, in particular, aluminum oxide, silicon oxide, magnesium oxide, titanium oxide, and mixtures or mixed oxides of at least two of these materials.

Aluminum oxide, magnesium/aluminum mixed oxides, and aluminum/silicon mixed oxides are preferred.

The coating comprising the zeolite of the structure type RHO, the palladium and the cesium (hereinafter referred to as coating A) and the oxidation-catalytically- active coating (hereinafter referred to as coating B) may be arranged on the carrier substrate in various ways.

If the carrier substrate is a flow-through substrate, both coatings may, for example, be present coated over the entire length L or only over a section of the carrier substrate. For example, coating A, starting from an end of the supporting body, may extend over 10 to 80% of its length L, and coating B, starting from another end of the supporting body, over 10 to 80% of its length LA.

In this case, it may be that L = LA + LB applies, wherein LA is the length of the coating A, and LB is the length of the coating B. However, L < LA + LB may also apply. In this case, coatings A and B overlap. Finally, L > LA + LB may also apply if a section of the supporting body remains free of coatings. In the latter case, a gap remains between coatings A and B, which gap is at least 0.5 cm long, i.e., for example, 0.5 to 1 cm.

However, coatings A and B may both also be coated over the entire length L. In this case, coating B, for example, may be present directly on the carrier substrate, and coating A on coating B. Alternatively, coating A may also be present directly on the carrier substrate, and coating B on coating A.

It is further possible that one coating extends over the entire length of the supporting body and the other over only a section thereof.

In a preferred embodiment, a zeolite of the structure type RHO comprises 0.5 to 4 wt.% of palladium, in relation to the sum of the weights of zeolite and palladium and calculated as palladium metal and cesium in an amount of 0.3 to 3,5 wt.%, in relation to the sum of the weights of zeolite and cesium and calculated as cesium metal, lies directly on the carrier substrate over its entire length L, and upon this coating is a coating containing platinum or platinum and palladium in a mass ratio of 1 : 1 to 14: 1, also over the entire length L.

In particular, the lower layer (Pd-Cs-RHO) is present thereby in a quantity of 50 to 250 g/L carrier substrate, and the upper layer (Pt or Pt/Pd) in a quantity of 50 to 150 g/L carrier substrate.

If the carrier substrate is a wall-flow filter, coatings A and B may extend over the entire length L of the wall-flow filter or over only a section thereof in a manner analogous to that described above for flow-through substrates. In addition, the coatings may be on the walls of the input channels, on the walls of the output channels, or in the walls between the input and output channels. In another embodiment of the present invention, the carrier substrate is formed from the zeolite of the structure type RHO, palladium, cesium and a matrix component.

Carrier substrates, flow-through substrates, and wall-flow substrates that do not just consist of inert material, such as cordierite, but additionally contain a catalytically active material are known to the person skilled in the art. To produce them, a mixture consisting of, for example, 10 to 95 wt.% of an inert matrix component and 5 to 90 wt.% of catalytically active material is extruded according to a method known per se. All of the inert materials that are also otherwise used to produce catalyst substrates can be used as matrix components in this case. These are, for example, silicates, oxides, nitrides, or carbides, wherein magnesium aluminum silicates, in particular, are preferred.

In embodiments of the present invention, the extruded carrier substrate, which comprises the zeolite of the structure type RHO, as well as palladium and cesium, may be coated with one or more catalytically-active coatings, e.g., with the oxidation-catalytically-active coatings described above.

In a further embodiment of the present invention a carrier substrate is used which is composed of corrugated sheets of inert material. Such carrier substrates are known to those skilled in the art as corrugated substrates. Suitable inert materials are, for example, fibrous materials having an average fiber diameter of 50 to 250 m and an average fiber length of 2 to 30 mm. Preferably, fibrous materials are heat-resistant and consist of silicon dioxide, in particular of glass fibers.

To produce such carrier substrates, for example, sheets of said fiber materials are corrugated in a known manner and the individual corrugated sheets are formed into a cylindrical monolithically structured body with channels passing through the body. Preferably, by laminating a number of the corrugated sheets to parallel layers having different orientation of corrugation between the layers, a monolithic structured body having a criss-cross corrugation structure is formed. In an embodiment non-corrugated, i.e. flat leaves can be arranged between the corrugated sheets.

Substrates composed of corrugated sheets may be directly coated with the catalyst of the present invention. It is however preferred to coat such substrates with an inert material, for example titania, first and only thereafter with the catalyst of the present invention.

The catalyst according to the invention is especially suitable as a passive nitrogen oxide storage catalyst, i.e., it is able to store nitrogen oxides at temperatures below 200 °C and to release them again at temperatures above 200 °C. It is, therefore, possible, in combination with a downstream SCR. catalyst, to effectively convert nitrogen oxides across the entire temperature range of the exhaust gas, including the cold-start temperatures.

The present invention therefore relates to an exhaust gas system comprising a) a catalyst comprising a carrier substrate of length L, palladium, cesium, and a zeolite of the structure type RHO and b) an SCR catalyst.

In principle, the SCR catalyst in the exhaust gas system according to the invention may be selected from all the active catalysts in the SCR reaction of nitrogen oxides with ammonia - particularly such as are commonly known to the person skilled in the art in the field of automotive exhaust gas catalysis. This includes catalysts of the mixed-oxide type, as well as catalysts based upon zeolites - in particular, upon transition metal-exchanged zeolites.

In embodiments of the present invention, SCR catalysts are used that contain a smallpore zeolite with a maximum ring size of eight tetrahedral atoms and a transition metal. Such SCR catalysts are described in, for example, W02008/106519 Al, WO2008/118434 Al, and WO2008/132452 A2.

In addition, however, large-pore and medium-pore zeolites may also be used, wherein those of the BEA structural type, in particular, come into question. Thus, iron-BEA and copper-BEA are of interest.

Particularly preferred zeolites belong to the BEA, AEI, AFX, CHA, KFI, ERI, LEV, MER, or DDR structure types and are particularly preferably exchanged with cobalt, manganese, iron, copper, or mixtures of two or three of these metals. The term, zeolites, here also includes molecular sieves, which are sometimes also referred to as "zeolite- 1 ike" compounds. Molecular sieves are preferred, if they belong to one of the aforementioned structure types. Examples include silica aluminum phosphate zeolites, which are known by the term, SAPO, and aluminum phosphate zeolites, which are known by the term, AIPO.

These two are particularly preferred, when they are exchanged with cobalt, iron, copper, or mixtures of two or three of these metals.

Preferred zeolites are also those that have an SAR (silica-to-alumina molar ratio) value of 2 to 100, more preferably 5 to 50, and most preferably, the SAR. value is between 10 and 40.

The zeolites or molecular sieves contain transition metal - in particular, in quantities of 1 to 10 wt.%, and especially 2 to 5 wt.% - calculated as metal oxide, i.e., for example, as FezCh or CuO.

Preferred embodiments of the present invention contain beta-type (BEA), chabazite-type (CHA), AEI or Levyne-type (LEV) zeolites or molecular sieves exchanged with copper, iron, or copper and iron as SCR catalysts. Appropriate zeolites or molecular sieves are known, for example, by the names, ZSM-5, Beta, SSZ-13, SSZ-39, SSZ-62, Nu-3, ZK-20, LZ-132, SAPO-34, SAPO-35, AIPO-34, and AIPO-35; see, for example, US 6,709,644 and US 8,617,474.

In an embodiment of the exhaust gas system according to the invention an injection device for reducing agent is located between the catalyst, which a carrier substrate of length L, palladium, and a zeolite whose largest channels are formed by 8 tetrahedrally-coordinated atoms, and the SCR catalyst.

The person skilled in the art may choose the injection device arbitrarily, wherein suitable devices may be found in the literature (see, for example, T. Mayer, Feststoff- SCR-System auf Basis von Ammoniumcarbamat (Solid SCR System Based upon Ammonium Carbamate), Dissertation, TU Kaiserslautern, 2005). The ammonia as such or in the form of a compound may be introduced via the injection device into the exhaust gas flow from which ammonia is formed under the ambient conditions prevailing. As such, aqueous solutions of urea or ammonium formiate, for example, come into consideration, such as solid ammonium carbamate. As a rule, the reducing agent or precursor thereof is held available in an accompanying container which is connected to the injection device.

The SCR. catalyst is preferably present in the form of a coating on a supporting body, which may be a flow-through substrate or a wall-flow filter and may, for example, consist of silicon carbide, aluminum titanate, or cordierite.

Alternatively, however, the supporting body itself may also consist of the SCR catalyst and a matrix component as described above, i.e., be present in extruded form.

The present invention also relates to a method for the cleaning of exhaust gases from motor vehicles that are operated by lean-burn engines, e.g., diesel engines, which method is characterized in that the exhaust gas is channeled through an exhaust gas system according to the invention.

Example 1 a) 1.31 g of 18-crown-6, 1.80 g of CsOH (50%) and 0.44 g NaOH are mixed with 8.15 g of water. Upon obtaining a homogeneous solution, 1.92 g of Na-aluminate are added while stirring. The resulting mixture is stirred for another 15 minutes. Finally, 15.04 g of Ludox HS-40 is added dropwise while stirring. The resulting white gel is left stirring for 24 h before transferring it to a Teflon lined steel autoclave. The autoclave is placed in a preheated oven at 110 °C for 8 days. The product is recovered via filtration and washing until a pH of 10 is reached. b) The structure directing agent (SDA) is removed from the zeolite via a thermal treatment to 600 °C at a rate of 1 °C/min. The zeolite is kept at the maximum temperature for 8 h.

In order to remove at least some of the Cs and Na cations, 1 g of calcined zeolite is mixed in an aqueous 0.5 M NH4CI solution for 4 hours while boiling under reflux conditions. Finally, the powder is recovered by filtration and washing. The cesium content of the product obtained is 3.2 wt.% calculated as Cs. c) The crystallinity and phase purity of the zeolite obtained was verified with X-ray diffraction patterns recorded on a STOE STADI P Combi diffractometer. This equipment has a focusing Ge(lll) monochromator (CuKo radiation, A = 0.154 nm) with high throughput setup in transmission geometry, an image plate position sensitive detector (IP-PSD) and an internal resolution of 0.03°. Figure 1 shows that a phase pure R.HO zeolite was formed. d) The zeolite of the structure type R.HO comprising 3.2 wt.% of cesium as obtained according to the method described above is loaded with 2.4 wt.% of Pd via incipient wetness impregnation as follows: 116 pL of a 16.08 wt.% Pd solution is diluted with 92 pL water and dropwise added to 1 g of zeolite. The zeolite is thoroughly mixed after and dried at 60 °C overnight. The powder thus obtained is subsequently dried at 120 °C and hydrothermally treated at 550 °C. e) In order to avoid pressure build-up during hydrothermal treatment and reactor testing (see below) the zeolite powder is pelletized as follows: the powder is placed between two stainless steel blocks and compressed at a pressure of 15 bar. The obtained pellet is crushed and sieved to obtain granulates sized 125 - 250 pm. These granulates are loaded in a quartz tube and exposed to a 20 mL/min airflow containing 12 vol% water. The temperature is raised with 5 °C/ min up to 750 °C. After 3 hours at 750°C the product is cooled in a 40 mL/min dry air flow. f) The calcined powder containing Cs and Pd is suspended in demineralized water, mixed with 8% of a commercially available boehmite-based binder, and ground by means of a ball mill. Subsequently, according to a conventional method, a commercially available honeycomb ceramic substrate (flow-through substrate) is coated along its entire length with the washcoat thus obtained. The washcoat load is 100 g/L in relation to the zeolite containing Cs and Pd (corresponds to 108 g/L incl. binder), which corresponds to a precious metal load of 68 g/ft ³ Pd. The catalyst thus obtained is calcined at 550 °C.

Comparative Example 1

A comparative example was prepared according to the preparation of Example 1, but using zeolite of type CHA instead of type R.HO.

Adsorption test

The Cs- and Pd-loaded zeolite obtained in step e) of Example 1 is tested as follows: a) 300 mg of product is loaded in a quartz fixed bed tubular continuous flow reactor with online reaction product analysis. NO and NO2 concentrations were analyzed simultaneously on an ABB A02020-LimasllHW UV photometer. CO, N2O and CO2 were analyzed in parallel on an ABB A02020-URAS26 NDIR. photometer. Temperature was measured with a K type thermocouple inserted in the middle of the zeolite bed. b) The test protocol entails four phases (figure 2): a pretreatment (phase (1)), a bypass (phase (2)), an isothermal adsorption (phase (3)) and a temperature programmed desorption (TPD) phase (phase (4)), see figure 2. In the pretreatment phase (1) the gas composition consists of 12 vol.-% O2 and 2.2 vol.-% H2O in N2 as a carrier gas. Temperature is increased with a ramp rate of 5°C/min to 500°C where it remains for 15 minutes. Then the sample is cooled down to 120 °C in the same feed gas. After the pretreatment the gas mixture is switched to bypass (2) and 160 vol. -ppm of NO and 800 vol. -ppm of CO is added to obtain a 1 :5 NO/CO ratio. Once the concentrations in the gas mixture are stabilized, the feed gas is switched from bypass to over the sample to measure the storage capacity in the adsorption phase (3). The isothermal adsorption phase (3) at 120 °C lasts for 60 minutes during which the NOx after catalyst is measured and from which the NOx stored is calculated. Subsequently the temperature programmed desorption phase (4) begins and the temperature is gradually increased with 5 °C/min to 500 °C and remains for 5 minutes at 500 °C. The gas hourly space velocity is fixed at 15,000 h~ ¹ , obtained by a gas flow rate of 167 mL/min over a catalyst bed of ca. 0.67 cm ³ . Figure 3 shows the NOx adsorption data obtained. The solid line shows the temperature adjusted in [°C], values given on the left axis of ordinate, the dashed line the NOx-concentration after the sample of Example 1, and the dotted line the NOx-concentration after the sample of Comparative Example 1 (for both concentrations values are also given on the left axis of ordinate, the unit is [vol.- PPm]).

The following table gives the result obtained for the two catalysts for comparison, stored NOx-amount calculated based on the molecular weight of NO2: