DE102009053951A1 - Aging stable Rh zeolite catalyst - Google Patents

Abstract

The present invention relates to a catalytic composition comprising a porous support material and rhodium, the rhodium being essentially in the pores of the porous support material. The invention further relates to a method for producing the rhodium-containing catalytic composition according to the invention, the use of the catalytic composition as an NO reduction catalyst and hydrocarbon reservoir, and a catalyst component which contains the catalytic composition according to the invention.

Description

The present invention relates to a catalytic composition comprising a porous support material and rhodium, wherein the rhodium is substantially in the pores of the porous support material. The invention further relates to a process for the preparation of the catalytic composition of the invention, the use of the catalytic composition as a NO _x reduction catalyst and hydrocarbon storage and a catalyst component containing the catalytic composition according to the invention.

Three-way catalysts have long been used to purify exhaust gas from gasoline engines. They usually contain platinum, rhodium and a metal oxide such as cerium oxide, which has an oxygen storage function. The platinum is responsible for the oxidation reaction, ie the oxidation of carbon monoxide (CO) and hydrocarbons (HC) to CO ₂ and H ₂ O. The rhodium is responsible for the reduction of nitrogen oxides (NO _x ) to nitrogen (N ₂ ).

Three-way catalysts are normally operated with an exhaust gas composition that oscillates around λ = 1. This means that in the ideal case, the same amount of oxidizing (NO _x , O ₂ ) and reducing (CO, HC) gases are present in the exhaust gas. Since this can not always be exactly guaranteed, the ceria serves as an oxygen storage (OSC), which is charged under oxidative conditions with oxygen (formation of CeO ₂ ) and can release oxygen again under reducing conditions (formation of Ce ₂ O ₃ ).

The supported with noble metal supported catalysts used for such applications are usually prepared by a multi-step process. In this case, for example, in a first step, a carrier material is impregnated with a noble metal salt solution of the desired noble metal. After removal of the solvent from the support material in a subsequent step, the support material is then calcined in a further step, wherein the noble metal can be converted by the thermal treatment in an oxide form. In many cases, the oxide form is already the catalytically active species, so that the catalyst can be used in this form. It is also possible to convert the oxide form in a further step, for example by means of hydrogen, carbon monoxide or wet chemical reducing agent in the highly disperse noble metal of oxidation state 0, which can also act as a catalytically active species.

The known supported noble metal catalysts significantly lose activity when exposed to high thermal stress. The activity of a catalyst is directly related to the metal dispersion. As a result of the thermal stress, metal particles sinter on the surface of the catalyst support and larger, less active clusters form. The activity of supported metal catalysts is usually dependent on the size of the metal particles. The speed of this so-called thermal aging process is dependent on the height of the temperature at which the catalyst is used. Namely, as the use temperature increases, the speed of the aging process increases, for which an increased mobility of the metal particles on the substrate surface at higher temperatures and a concomitant increased sintering tendency is the cause. This is not wanted.

The object of the present invention is therefore to provide a metal catalyst, in particular a rhodium catalyst for use in high-temperature applications, which has a relatively low tendency to thermal aging and, correspondingly, maintains its catalytic activity virtually unchanged over a long service life.

The object is achieved by a catalytic composition comprising a porous support material and rhodium, characterized in that the rhodium is substantially in the pores of the porous support material.

Surprisingly, it has been found that a catalytic composition comprising a porous support material and rhodium, and in which the rhodium is substantially in the pores of the porous support material, maintains its catalytic activity after use at high temperatures almost unchanged, whereas corresponding from the Prior art known supported metal catalysts usually have lost significantly due to a sintering of the metal particles already in activity. The catalytic composition according to the invention has significantly increased thermal stability compared to the supported metal catalysts known in the prior art.

The catalytic composition according to the invention exhibits the advantage mentioned in particular in high-temperature applications in which corresponding conventional supported metal catalysts tend to undergo rapid thermal aging due to the high mobility of the metal particles caused by the prevailing high temperatures and a concomitant fusion of the particles into larger units. This is the case, for example, in the exhaust gas treatment, inter alia in a three-way catalyst (TWC).

In one embodiment of the invention it is preferred that the above-mentioned porous carrier material is an open-pore carrier material. The open-pored carrier material is preferably an inorganic, open-pore carrier material.

Furthermore, it is preferred that the open-pore carrier material is a carrier material with monomodal or polymodal pore distribution.

According to a further embodiment of the invention, the abovementioned support material preferably comprises a material selected from the group consisting of titanium oxide; γ-, θ- or Δ-alumina; cerium oxide; silicon oxide; Zinc oxide; magnesium oxide; Aluminum-silicon oxide; Silicon carbide and magnesium silicate or a mixture of two or more of the aforementioned materials. Furthermore, it may be preferred that the carrier material consists of one of the aforementioned materials or mixtures thereof.

According to another embodiment of the invention, it is preferred that the above mentioned carrier material is a zeolite material.

A zeolite material is used in the context of the present invention as defined by the International Mineralogical Association ( DS Coombs et al., Can. Mineralogist, 35, 1997, 1571 ) understood a crystalline substance with a characterized by a skeleton of interconnected tetrahedra structure. Each tetrahedron consists of four oxygen atoms surrounding a central atom, the framework containing open cavities in the form of channels and cages, which are normally occupied by water molecules and extra cationic cations, which can be exchanged frequently. The channels of the material are large enough to allow guest connections access. In the case of the hydrated materials, dehydration usually occurs at temperatures below about 400 ° C and is for the most part reversible.

The zeolite material contained in the catalytic composition may be both a zeolite and a zeolite-like material, for example a silicate, an aluminum silicate, an aluminum phosphate, a silicon aluminum phosphate, a metal aluminum phosphate, a metal aluminum phosphosilicate, a gallium aluminum silicate, a gallium silicate, a boroaluminosilicate, a borosilicate or a titanosilicate, with aluminum silicates and titanium silicates being particularly preferred.

The term "aluminum silicate" is used as defined by the International Mineralogical Association ( DS Coombs et al., Can. Mineralogist, 35, 1997, 1571 ) understood a crystalline substance with spatial network structure of the general formula M ^{n +} [(AlO ₂ ) _x (SiO ₂ ) _y ] · H ₂ O, which is composed of SiO _4/2 and AlO _4/2 tetrahedra, by common oxygen atoms are linked to a regular three-dimensional network. The atomic ratio of Si / Al = y / x is always greater than or equal to 1 according to the so-called "Löwenstein rule", which prohibits the adjacent occurrence of two adjacent negatively charged AlO _4/2 tetrahedra. Although there are more exchange sites for metals available at a low Si / Al atomic ratio, the zeolite is becoming increasingly thermally unstable.

The abovementioned zeolite materials can be used in the context of the present invention both in the alkali form, for example in the Na and / or K form, and in the alkaline earth form, ammonium form or in the H form. In addition, it is also possible that the zeolite material is a mixed form, for example, an alkali / alkaline earth mixed form.

Preferably, the zeolite has a SiO ₂ / Al ₂ O ₃ modulus of from 5 to 300, more preferably from 10 to 200, most preferably from 15 to 100.

The zeolite material may preferably correspond to one of the following structural types: ABW, ACO, AEI, AEL, AEN, AET, AFG, AFI, AFN, AFO, AFR, AFS, AFT, AFX, AFY, AHT, ANA, APC, APD, AST, ASV, ATN, ATO, ATS, ATT, ATV, AWO, AWW, BCT, BEA, BEC, BIK, BOG, BPH, BRE, CAN, CAS, CDO, CFI, CGF, CGS, CHA, CRI, CLO, CON, CZP, DAC, GDR, DFO, DFT, DOH, DON, EAB, EDI, EMT, EON, EPI, ERI, ESV, ETR, EUO, EZT, FAR, FAU, FER, FRA, GIS, GIU, GME, GON, GOO, HEU, IFR, IHW, ISV, ITE, ITH, ITW, IWR, IWV, IWW, JBW, KFI, LAU, LEV, LIO, LIT, LOT, LOV, LTA, LTL, LTN, MAR, MAZ, MEI, MEL, MEP, MER, MFI, MFS, MON, MOR, MOZ, MSE, MSO, MTF, MTN, MTT, MTW, MWW, NAB, NAT, NES, NON, NPO, NSI, OBW, OFF, OSI, OSO, OWE, PAR, PAU, PHI, PON, RHO, RON, RRO, RSN, RTE, RTH, ROD, RWR, RWY, SAO, SAS, SAT, SAV, SBE, SBS, SBT, SFE, SFF, SFG, SFH, SFN, SFO, SGT, SIV, SOD, SOS, SSY, STF, STI, STT, SZR, TER, THO, TON, TSC, TUN, UEI, UFI, UOZ, USI, UTL, VET, VFI, VNI, VSV, WEI, WEN, YUG and ZON, with structural type zeolite materials (BEA) being particularly preferred. The above three-letter code nomenclature corresponds to the "IUPAC Commission of Zeolite Nomenclature".

Also preferred according to the invention are those zeolite materials which are prepared using amphiphilic compounds. Preferred examples of such materials are in US 5,250,282 and are incorporated by reference into the present invention. Further suitable and inventively preferred zeolite materials of this type are further in "Review of Ordered Mesoporous Materials", O. Ciesla and F. Schoth, Microporous and Mesoporous Materials, 27, (1999), 131-49 and are incorporated by reference into the present invention.

Zeolite materials which are likewise preferred according to the invention are mesoporous zeolite materials of silicates or aluminosilicates, which are summarized in the literature under the name M41S, for example MCM-41, MCM-48 and MCM-50. These zeolite materials are detailed in the US 5,098,684 and in the US 5,102,643 and are incorporated by means of references in the present invention. A preferred subclass of this family according to the invention are mesoporous silicates which bear the designation MCM-41 and MCM-48. MCM-41 is particularly preferred and has a hexagonal array of mesopores of uniform size. The MCM-41 zeolite material has a SiO ₂ / Al ₂ O ₃ molar ratio of preferably greater than 100, more preferably greater than 200, and most preferably greater than 300. Other preferred zeolite materials used in the present invention may be those which have the designation MCM-1, MCM-2, MCM-3, MCM-4, MCM-5, MCM-9, MCM-10, MCM-14, MCM-22, MCM-35, MCM- 37, MCM-49, MCM-58, MCM-61, MCM-65 or MCM-68.

Another suitable zeolite material is the zeolite material designated ITQ-33.

The rhodium is according to the invention essentially in the pores of the zeolite material, ie the inner surface. "Substantially" in the sense of the invention preferably means that at least 90% of the rhodium is in the pores (the inner surface) of the zeolite, more preferably at least 95%, even more preferably at least 99% and especially at least 99.9%. The percentages are wt .-%, based on the total weight of the rhodium in the catalytic composition. Rhodium, which is not in the inner surface of the porous support material, is usually present in an amount which is either below the detection limit or has no influence on the preferred properties of the composition according to the invention.

The zeolite or the zeolite-like material according to the invention is free of Rh reflections in the X-ray diffractogram (XRD). This shows that the rhodium sits in the pores of the zeolite and there are no larger clusters (> 5 nm) on the outer surface. This can also be demonstrated by CO adsorption (after selective poisoning of Rh clusters on the surface) in FTIR. The catalytic composition of the invention has a Rh-C = O stretching vibration. The Rh-C = O stretching vibration is present even after poisoning with adamantanecarbonitrile. Adamantancarbonitrile is a bulky molecule that, due to its size, can not penetrate into the pore system of the porous support. Adsorption of adamantanecarbonitrile, therefore, causes only Rh clusters on the outer surface of the porous support to be poisoned, if any. If CO is adsorbed following this poisoning, it can only bind to the non-poisoned Rh clusters inside the porous support material. The presence of the Rh-C = O stretching vibration after adamantanecarbonitrile poisoning proves that the Rh sits in the pores of the porous support material. The remaining intensity after poisoning is much higher than for a comparative catalyst.

According to the invention, "internal surface" is understood to mean the internal pore system of the zeolite, by which on the one hand the pores / channels and on the other hand the cavities are understood.

In one embodiment, the above-mentioned carrier material has a polymodal pore distribution, ie that the zeolite material comprises micropores, mesopores and macropores. In this regard, and in the context of the present invention under the terms micropores, mesopores and Macropore pores having a diameter of less than 1 nm, a diameter of 1 to 50 nm and a diameter greater than 50 nm.

The zeolite according to the invention preferably contains at least 1% by weight of rhodium, preferably at least 1.5% by weight, more preferably 2% by weight, most preferably 2.5% or more by weight of rhodium, in particular at least 3% by weight. -% based on the total weight of rhodium and support material.

In order to avoid an overcharging of the zeolite material with rhodium, which could lead to a closure of the pore and channel system of the zeolite material and concomitantly to a reduced accessibility of the reactants to be converted to the catalytically active rhodium particles is provided according to a further preferred embodiment of the catalytic composition of the invention in that the proportion of rhodium in the loaded zeolite material is preferably ≤ 10% by weight, more preferably ≤ 8% by weight, more preferably ≤ 7% by weight and most preferably ≤ 5% by weight.

In principle, it is advantageous if the rhodium present in the zeolite material in the form of rhodium particles (also referred to as rhodium dispersion) has a particle size which is as small as possible, since the rhodium particles then have a very high degree of dispersion. In this case, the ratio of the number of rhodium atoms which form the surface of the rhodium particles to the total number of rhodium atoms of the rhodium particles is understood as the degree of dispersion. However, a favorable mean rhodium particle diameter also depends on the application in which the catalytic composition is to be used, as well as on the pore distribution and in particular the pore radii and channel radii of the zeolite material. According to a preferred embodiment of the catalytic composition according to the invention, the rhodium particles have an average diameter which is smaller than the pore diameter and which is greater than the channel diameter of the carrier material. As a result, the rhodium particles are mechanically trapped in the support material, which leads to a high thermal aging resistance of the catalyst according to the invention. According to a preferred embodiment of the catalytic composition according to the invention, the rhodium particles have an average particle diameter of 0.5 to 5 nm, preferably from 0.5 to 4 nm, preferably from 0.5 to 3 nm and particularly preferably one from 0.5 to 2 nm. The mean particle diameter is preferably determined by digestion of the zeolite material and measurement of the remaining Rh particles by means of transmission electron microscopy (TEM).

In general, it is preferred if the dispersion value is as high as possible, since in this case as many metal atoms as possible are freely accessible for a catalytic reaction. That is, with a high dispersion value of a supported metal catalyst, a certain catalytic activity thereof with a relatively small amount of metal used can be achieved.

According to a further embodiment of the catalytic composition according to the invention, the dispersion of the rhodium particles is 1 to 10%, preferably more than 20%, more preferably more than 30%, preferably more than 35%, more preferably more than 40%, more preferably more than 45% and more preferably more than 50%. The values of the dispersion by means of hydrogen are according to DIN 66136-2 certainly.

Furthermore, according to a further embodiment of the catalytic composition according to the invention, it may be provided that the BET surface area of said porous support material and in particular of the zeolite material is preferably 100 m ² / g to 1500 m ² / g, more preferably 150 m ² / g to 1000 m ² / g, and most preferably 200 m ² / g to 600 m ² / g. The BET surface area is reduced by adsorption of nitrogen DIN 66132 to determine.

• a) Imprägnieren eines porösen Trägermaterials mit einer Rhodiumsulfitlösung,
• b) Kalzinieren des imprägnierten porösen Trägermaterials.

Another object of the invention is a process for the preparation of a catalytic composition as described above comprising the steps

• a) impregnating a porous support material with a rhodium sulfite solution,
• b) calcining the impregnated porous support material.

According to the invention, the porous support material is preferably a zeolite or a zeolite-like material, as described above. The same preferences thus also apply to the method according to the invention.

According to a further embodiment of the method according to the invention, it can be provided that a drying step takes place between step a) and step b) of the method according to the invention.

The drying step is carried out between the impregnation and the calcination. The drying temperature is preferably between 25 ° C and 250 ° C, more preferably between 50 ° C and 200 ° C, more preferably between 100 ° C and 180 ° C and most preferably at 120 ° C.

It is preferred to dry for a period of more than 1 minute, more preferably over a period of more than 1 hour, more preferably over a period of more than 5 hours, and even more preferably over a period of more than 12 hours, with a drying time of 10 h may be particularly preferred. In this context, it may also be advantageous if the duration of the drying step does not exceed a period of 48 hours, preferably does not exceed a period of 24 hours.

The calcining is preferably carried out in air, ie in an oxygen-containing atmosphere. The term "calcination" is generally understood to mean heating to high temperatures with the aim of, for example, materially or structurally changing the treated material or a component thereof. By calcination, for example, a thermal degradation, a phase transition or the removal of volatile substances can be achieved.

The calcination of the impregnated porous support material is preferably carried out at a temperature of 300 ° C to 1200 ° C, more preferably in a temperature range of 400 ° C to 950 ° C, even more preferably in the range of 500 to 900 ° C, even more preferably 550 up to 850 ° C, more preferably 650 to 830 ° C, in particular 750 to 780 ° C.

Moreover, it is particularly preferable that the calcination is carried out at a temperature of at least 770 ° C, preferably 800 ° C or more. When calcined at a temperature of at least 770 ° C catalytic compositions can be obtained by the method according to the invention, despite high rhodium loading of, for example, 3 wt .-%, based on the weight of the rhodium and the porous support material, substantially free of sulfur. Thus, by means of the process according to the invention, for example, catalytic compositions can be prepared which contain 1 to 5% by weight of rhodium, based on the weight of the rhodium and the porous support material, and a sulfur content of less than 0.004% by weight, preferably <0.003% by weight. -%, based on the weight of the rhodium and the support material. A low content of sulfur is particularly advantageous since sulfur acts as a catalyst poison, in particular with respect to noble metals.

The heating rate in the calcination is preferably 0.5 ° C / min to 5 ° C / min, more preferably 1 ° C / min to 4 ° C / min, and most preferably 2 ° C / min.

The duration of calcination at maximum temperature is preferably in a range of 1 minute to 48 hours, more preferably in a range of 30 minutes to 12 hours, and particularly preferably in a range of 1 hour and 7 hours, with a calcination time of 5 hours or 6 h is particularly preferred.

Calcination produces a rhodium (precursor) compound which, if necessary, can preferably be reduced following calcination. In principle, however, a reduction can take place even during the calcination, in which case, however, a reducing atmosphere instead of air would have to be used.

According to a further embodiment of the method according to the invention is preferably at least one further metal selected from Ni, Sn, Ag, Mn, Pb, Co, Fe, Cr, Ce and Cu, before, during or after step a), preferably in the form of an oxide introduced into the porous carrier material. Preferably, the metal is selected from Ni, Mn, Co, Fe, Cr, Ce or Cu, most preferably Ni, Fe, Ce and / or Cu.

The introduction of the abovementioned metals as oxide in the process according to the invention is preferred since such an oxide contributes to a further thermal stabilization of the catalytically active Rh species in the catalyst obtainable according to the invention.

The proportion of the metal oxide in the resulting catalytic composition is in a range of 0.1-10 wt .-%, preferably 1-5 wt .-%, most preferably from 1.5 to 3 wt .-%, based on the weight of the catalytic composition obtained after calcination.

The introduction of the metal oxide into the catalytic composition obtained according to the invention can be carried out before, after or simultaneously with the impregnation of the porous support material with the rhodium sulfite solution. If the introduction is to be carried out before or after the impregnation, it is preferably carried out in a conventional manner by means of ion exchange (liquid or solid exchange). When incorporation is to be performed prior to rhodium impregnation, it is preferred that the porous metal substrate exchanged with the metal be calcined prior to impregnation with rhodium to convert the metal to the oxidic form. If the introduction is to be carried out simultaneously with the impregnation, a corresponding mixed solution of the desired components (rhodium sulfite, optionally metal salt) is prepared and the porous support is impregnated with this mixed solution and then calcined.

A reduction which can be carried out after calcination is preferably carried out from a mixture of a reducing gas (hydrogen, hydrocarbon, carbon monoxide, ethene, methanol, ethanol, etc.) and an inert gas. Preferred inert gases are, for example, argon, helium, neon and the like. The inert gas in the reduction step is to be understood as a carrier gas, hydrogen or another reductive gas preferably being in a concentration of 1 to 10% by volume, more preferably 3 to 7% by volume, particularly preferably about 5% by volume, based on the total volume of reducing gas and inert gas.

The reduction is usually carried out until complete or near completion (preferably greater than 95%) of the rhodium precursor compound has occurred.

The reduction is preferably carried out over a period of 3 to 7 hours, more preferably 4 to 6 hours, particularly preferably about 5 hours.

The reduction is preferably carried out at elevated temperatures. Preferably, the reduction is carried out at a temperature of 200 to 500 ° C, more preferably 250 to 350 ° C, most preferably about 300 ° C. For the reduction, the catalyst is usually placed in a catalyst bed while flowing through the reducing agent. Likewise, the catalyst can be covered with the reducing gas and advantageously brought to an elevated temperature. The increase in the temperature can be effected, for example, by heating the catalyst bed. It is also possible that the Reduktionsgsgas is already heated in advance, for example by the gas inlet is heated, in which case the heated reducing gas is passed over the catalyst to be reduced.

The impregnation of the zeolite with the Rhodiumsulfitlösung can be done via a dip impregnation, spray impregnation or Incipient Wetness method. The impregnation preferably takes place via an incipient wetness method.

Surprisingly, it has been found that incipient wetness impregnation of the porous support material, preferably a zeolite powder, with rhodium sulfite solution and subsequent calcination in air at high temperatures can produce a catalytic composition in which even after this high temperature load most of the rhodium in the pores of the porous support material or in the zeolite pores remains. This can be demonstrated by X-ray diffraction (XRD) and CO adsorption in FTIR after selective poisoning with adamantanecarbonitrile. XRD and FTIR are standard analytical methods in chemistry. It is believed that the XRD spectrum of the catalytic composite is free of Rh signals because the outer surface of the porous support material is substantially free or completely free of Rh particles having a size corresponding to X-ray radiation in accordance with the diffraction pattern of Rhodium, typically about 5 nm.

Surprisingly, it has also been found that the catalytic composition produced in this way has an increased sulfur resistance compared to previously known systems. The catalytic activity of the thermally aged composition according to the invention is not altered by sulfur poisoning and subsequent high-temperature desulfurization.

The catalytic composition prepared according to the invention also shows an improved aging behavior.

A further advantage of the catalytic composition according to the invention over the prior art is further that the noble metal is applied only to one component, the porous support or the zeolite, which at the same time acts as a storage.

The composition of the invention is thus at the same time catalyst and hydrocarbon storage.

Catalysts according to the prior art consist of a catalytically active component (usually noble metal on an oxidic support) and a pure storage component (usually zeolite). As a result, manufacturing steps and thus costs can be saved.

A further advantage of the catalytic composition according to the invention over the prior art is further that the noble metal is applied only to one component, the porous support or the zeolite, which at the same time acts as a storage. Catalysts according to the prior art consist of a catalytically active component (usually noble metal on an oxidic support) and a pure storage component (usually zeolite).

The composition of the invention is thus at the same time catalyst and hydrocarbon storage. The increased zeolite content also significantly increases the storage capacity for hydrocarbons (see, for example EP 691 883 B1 . US 5,804,155 ). The storage capacity is of great importance if, for example, a three-way catalytic converter has not yet reached the required operating temperature and can not yet oxidise or reduce the resulting exhaust gases. In addition, manufacturing steps and thus costs can be saved.

The invention thus also relates to a catalytic composition which is produced in particular by the process according to the invention.

Another object of the invention is the use of the catalytic composition as described above or obtainable by the process described above as a reduction catalyst and hydrocarbon storage. Zeolites are known as hydrocarbon reservoirs. In conjunction with the high rhodium dispersion in the pores of the zeolite, however, it is also outstandingly suitable as a reduction catalyst with a corresponding cumulative hydrocarbon storage function.

The fact that rhodium is applied only to a zeolite and not, as known in the art, to other metal oxides, results in a simple catalyst system that can be produced inexpensively.

Advantageously, the catalytic composition is used as a component of a three-way catalyst. There, it should primarily serve the reduction of nitrogen oxides NO _x and as a hydrocarbon reservoir. In addition, due to the inclusion of the active component Rh in the catalytic composition of the invention, the selectivity to a desired reaction product can be increased. The invention thus also relates to the use of the catalytic composition according to the invention in chemical catalysis.

The present invention further relates to the use of a catalytic composition according to the invention in catalytic processes in which at least partial temperatures of more than 700 ° C may occur.

According to one embodiment of the use according to the invention, the abovementioned catalytic process is preferably a purification of exhaust gases which are produced during the combustion of biological or fossil fuels, eg. As industrial or vehicle exhaust gases, such as preferably car, ship, locomotive exhaust etc.

The zeolite according to the invention can advantageously be processed into a washcoat and applied correspondingly to a shaped catalyst body. How such a washcoat can be made is known to the person skilled in the art. The necessary coating techniques for coating a catalyst article are also known to the person skilled in the art. So z. B. processed the impregnated and dried zeolite to an aqueous coating dispersion. This dispersion may contain a binder, e.g. As silica sol, are added. The viscosity of the dispersion can be adjusted by its own additives, so that it is possible, the required coating amount in a single operation on the walls of the Apply flow channels. If this is not possible, then the coating can be repeated several times, wherein the freshly applied coating is fixed in each case by an intermediate drying and calcined as needed. For the drying and calcination temperatures, the above-mentioned preferences apply here.

Another object of the invention is thus a washcoat containing the catalytic composition of the invention.

According to a further embodiment it can be provided that the catalytic composition according to the invention is preferably formed as a powder, as a shaped body or as a monolith. Preferred shaped bodies are, for example, spheres, rings, cylinders, perforated cylinders, trilobes or cones, and a preferred monolith is, for example, a honeycomb body.

Another object of the invention is a catalyst component containing a catalytic composition as described above.

The catalyst component comprises a catalyst support, wherein the catalytic composition is present as a coating on the catalyst support.

As the catalyst former, a metallic or ceramic monolith, a non-woven or a metal foam may be used. Other known in the art catalyst bodies or catalyst supports are suitable according to the invention. Particularly preferred is a metallic or ceramic monolith having a plurality of parallel passage openings, which are provided with the Washcoatbeschichtung. The catalyst support preferably has passage openings with a round, triangular, quadrangular or polygonal cross section.

Metallic honeycomb bodies are often formed from metal sheets or metal foils. In this case, the honeycomb bodies are produced for example by alternating arrangement of layers of structured sheets or films. Preferably, these arrangements consist of a position of a smooth sheet alternating with a corrugated sheet, wherein the corrugation may be formed, for example, sinusoidal, trapezoidal, omega-shaped or zigzag. Corresponding metallic honeycomb bodies and processes for their preparation are described, for example, in US Pat EP 0 049 489 A1 or the DE 28 56 030 A1 described.

In the area of the catalyst supports, metallic honeycomb bodies have the advantage that they heat up more quickly and thus, as a rule, catalyst supports based on metallic substrates show a better response at cold start conditions.

The honeycomb body preferably has a cell density of 30 to 1500 cpsi, particularly preferably 200 to 600 cpsi, in particular about 400 cpsi.

The shaped catalyst body, to which the catalytic composition of the invention may be applied, may be formed of any metal or metal alloy, e.g. B. by extrusion or by winding or stacking or folding of metal foils. Known in the field of emission control are temperature-resistant alloys with the main components iron, chromium and aluminum. Preferred for the catalytic composition of the invention are free-flowing monolithic catalyst support with or without inner leading edges for Abgasverwirbelung or metal foams having a large inner surface and where the catalyst of the invention adheres very well. However, catalyst supports with slots, perforations, perforations and embossments in the metal foil can also be used.

In the same way, catalyst supports made of ceramic material can be used. Preferably, the ceramic material is an inert, low surface area material such as cordierite, mullite, aluminum titanate or α-alumina. However, the catalyst support used may also consist of high surface area support material such as γ-alumina.

Likewise, a metal foam, for example a metallic open-pore foam material, can be used as catalyst support. In the context of the present invention, the term "metallic open-pore foam material" is to be understood as meaning a foam material made of any desired metal or alloy, which may optionally also contain additives and which has a multiplicity of pores which are conductively connected to one another such that For example, a gas can be passed through the foam material.

Metallic open cell foam materials have a very low density due to the pores and voids, but have considerable rigidity and strength. The production of metal foams, for example, by means of a metal powder and a metal hydride. Both powders are usually mixed together and then compacted by hot pressing or extrusion to a molding material. The molding material is then heated to a temperature above the melting point of the metals. The metal hydride releases hydrogen gas and foams the mixture.

However, there are other ways to make metal foams, for example, by blowing gas into a molten metal that has been previously made foamable by adding solid ingredients. For aluminum alloys, for example, 10 to 20% by volume of silicon carbide or aluminum oxide is added for stabilization. In addition, open-pore metallic foam structures with a pore diameter of 10 ppi to about 50 ppi can be produced by special precision casting techniques.

The catalyst support can in principle also be extruded and injection-molded. Again, metallic and / or ceramic materials are possible, in the case of ceramic materials, for example, form aids are added and, for example, binders and other additives. Extruded supports can take any geometry, preferably those mentioned above.

The catalyst component according to the invention preferably comprises at least two layers, the first layer assigned to the catalyst support comprising an oxidation-catalytically active component and an oxygen storage component, and the subsequent second layer comprising the catalytic composition as described above.

The first layer preferably comprises platinum and / or palladium on an oxidic support material, which is particularly preferably Al ₂ O ₃ . The oxygen storage component used is preferably cerium oxide or cerium / zirconium oxide. The molar ratio of Al ₂ O ₃ to cerium oxide or cerium / zirconium oxide can be varied depending on the application, and is preferably in the range of from 70:30 to 30:70. Further, the Ce / Zr ratio may also be varied depending on the application, and is preferably in the range of 0.1: 9.9 to 9.9: 0.1.

The concentration of platinum and / or palladium is advantageously chosen so that the concentration of Pt and / or Pd on the coated carrier body (for example a honeycomb) 0.5 to 5 g / l, preferably 0.6 to 3 g / l, on most preferably 1 g / l carrier body volume. Platinum can also be completely replaced by palladium.

The upper, ie the second layer contains the catalytic composition according to the invention. The amount used depends on the desired Rh concentration. Particularly preferred is a Rh concentration of 0.05 to 1 g / l, more preferably about 0.1 g / l of carrier body volume (eg honeycomb volume). Since the Rh concentration is very high in the catalytic composition according to the invention with 3 wt .-%, a significantly lower amount of catalytic composition is required than in the previously known catalysts. As a result, a resource saving is possible and the layer thicknesses on the support can be reduced and thus also the pressure drop in a catalyst component according to the invention.

The amount of catalytic composition saved can theoretically be balanced by other components, for example by an additional amount of a zeolite component as a hydrocarbon reservoir or by an increase in the amount of an oxygen storage component.

The invention will now be explained in more detail with reference to some not to be understood as limiting the scope of the invention embodiments. In this case, reference is additionally made to the following figures.

1 shows an XRD spectrum of two loaded with Rh H-BEA zeolites after calcination in air or under argon.

2 shows the CO conversion of a three-way catalyst according to the invention compared to conventional catalysts.

3 shows the propene conversion of a three-way catalyst according to the invention in comparison with conventional catalysts.

4 shows the NO conversion of a three-way catalyst according to the invention compared to conventional catalysts.

embodiments

example 1

A dried carrier (H-BEA-35) was impregnated with 3% by weight rhodium from rhodium sulfite solution and dried at 120 ° C overnight (8 hours). Subsequently, the impregnated zeolite was calcined in air at at least 770 ° C (≈ 800 ° C). The heating rate was 2 ° C / min with a gas volume of 2 l / min.

Example 2

For comparison, an identical sample was prepared, but calcined under argon.

The XRD spectrum (see 1 ) shows no Rh reflections when calcined under air, whereas significant calcination under argon shows distinct Rh reflections. The absence of the Rh signals indicates that, despite the high calcination temperature, no larger clusters have formed on the surface and fumed rhodium is in the channels / pores of the zeolite. In addition, after calcination, the catalyst contains almost no sulfur (<0.003 wt% after calcination in air, 0.014 wt% after calcination under argon).

Example 3: Coating and Performance Comparison

Example 3.1: Comparative experiment

For comparison with the composition according to the invention zirconium oxide was impregnated with Rh (NO ₃ ) ₃ according to a standard method for the preparation of rhodium catalysts and calcined at 550 ° C in air. The rhodium concentration was also 3% by weight.

Example 3.2: Coating

16.6 g of the Rh zeolite from Examples 1 and 2 and Rh-ZrO ₂ from Example 3.1 were each mixed with 5.9 g SiO ₂ binder and 37.3 g zirconium oxide with the aid of an Ultra-Turrax stirrer in 150 ml Water suspended. The suspension was milled with a planetary ball mill (Retsch PM 100) with 10 mm balls of yttrium-stabilized Zr oxide to a particle size of d ₅₀ of about 2 μm. A cordierite honeycomb (400 cpsi) was coated with this washcoat and calcined so that in the end 0.3 g Rh / l honeycomb volume was contained on the honeycomb.

Example 3.3: Performance comparison

The catalyst honeycombs prepared with the washcoats of Examples 1, 2 and 3.1 were tested in a reactor under the following conditions for the oxidation of CO and propene and the simultaneous reduction of NO: space velocity 70000 h ^-1 CO 1.5 vol.% C ₃ H ₆ 110 ppm NO _x 2000 ppm O ₂ 1 vol.% CO ₂ 10 vol.% H ₂ O 10 vol.% T var 5 K / min Test conditions close to λ = 1 [actually ~ 1.3]

Following this test, the honeycombs were aged at 900 ° C for 1 h and tested again under the conditions indicated.

Based on the results can be clearly seen (see 2 to 4 ) that in the fresh state rhodium on the zeolite (curve with the diamonds) has the highest activity and rhodium on ZrO ₂ (curve with triangles) the lowest activity. The catalyst according to the invention (squares) is in each case in the middle. However, the advantage of the catalyst according to the invention becomes evident after aging (curves with dashed lines). The catalyst according to the invention is characterized by a far higher thermal stability than the two comparative examples whose catalytic activity is much more influenced by the thermal stress.

QUOTES INCLUDE IN THE DESCRIPTION

This list of the documents listed by the applicant has been generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Cited patent literature

• US 5250282 [0020]
• US 5098684 [0021]
• US 5102643 [0021]
• EP 691883 B1 [0060]
• US 5804155 [0060]
• EP 0049489 A1 [0073]
• DE 2856030 A1 [0073]

Cited non-patent literature

• DS Coombs et al., Can. Mineralogist, 35, 1997, 1571 [0014]
• DS Coombs et al., Can. Mineralogist, 35, 1997, 1571 [0016]
• "Review of Ordered Mesoporous Materials", O. Ciesla and F. Schoth, Microporous and Mesoporous Materials, 27, (1999), 131-49 [0020]
• DIN 66136-2 [0031]
• DIN 66132 [0032]

Claims (15)

A catalytic composition comprising a porous support material and rhodium, characterized in that the rhodium is substantially in the pores of the porous support material. Catalytic composition according to claim 1, characterized in that the porous carrier material is a zeolite material or a zeolite-like material. Catalytic composition according to claim 2, characterized in that the zeolite material is a zeolite structure-type zeolite. A catalytic composition according to claim 2, characterized in that the zeolite material is a zeolite material of the MCM structural family. Catalytic composition according to one of claims 1 to 4, characterized in that at least 1.5 wt .-%, preferably at least 3 wt .-% rhodium is contained. Catalytic composition according to one of claims 1 to 5, characterized in that the XRD spectrum of the catalytic composition is free from rhodium signals. A process for the preparation of a catalytic composition according to any one of claims 1 to 6 comprising the steps a) impregnating a porous support with a rhodium sulfite solution, b) calcining the impregnated porous support. Process according to claim 7, wherein the calcination is carried out in air. A method according to claim 7 or 8, wherein the calcination is carried out at a temperature of 500 to 900 ° C. Method according to one of claims 7 to 9, wherein after the calcination, a reduction takes place. Use of a catalytic composition according to any one of claims 1 to 6 or obtainable by a process according to any one of claims 7 to 10 as a reduction catalyst and hydrocarbon storage. Use according to claim 11, characterized in that the catalytic composition is a component of a three-way catalyst. Catalyst component containing a catalytic composition according to one of claims 1 to 6 or prepared according to one of claims 7 to 10. A catalyst component according to claim 13 comprising a support wherein the catalytic composition is present as a coating on the support. Catalyst component according to claim 13 or 14 comprising at least two layers, wherein the first, the carrier associated layer comprises an oxidation catalytically active component and an oxygen storage component, and the subsequent second layer comprises the catalytic composition according to any one of claims 1 to 6.

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