DETAILED DESCRIPTION OF THE INVENTION

[0018] The catalyst support of the invention comprises a high surface area refractory metal oxide having a monomolecular layer of a second oxide selected from the group consisting of titanium dioxide, cerium dioxide and zirconium dioxide deposited thereon.

[0019] Preferably, the second metal oxide comprises titanium dioxide. In general, the monomolecular layer deposited on the high surface area refractory metal oxide will have a thickness of less-than about 10 angstroms and will be present in an amount of about 5 to about 15 wt. %, based on the weight of the refractory metal oxide.

[0020] High surface area refractory metal oxides are well known in the prior art. Typically, the refractory metal oxide will have a specific surface area of about 60 to about 300 m²/g. Useful refractory metal oxides include alumina, titania, zirconia and mixtures of alumina with one or more of titania, zirconia, ceria, baria and a silicate. Preferably, the refractory metal oxide comprises gamma-alumina. It is further preferred that the gamma-alumina is stabilized with an oxide of a metal selected from the group consisting of barium, silicon, zirconium, a rare earth metal and mixtures thereof. Typically, the stabilizer is utilized in an amount of about 0.01 to about 0.5 g/in³. If desired, an additional layer of alumina, zirconia or a rare earth oxide may be deposited on at least part of the monomolecular layer, thereby creating a “sandwich” type of structure. The additional layer of oxides may be present in the amount of about 3 to about 25 wt. %.

[0021] The vehicular emission control catalysts of the invention will comprise the catalyst support described above and at least one precious metal component deposited on at least part of the monomolecular layer. It is further preferred that the catalyst support containing the precious metal component be deposited on a substrate (the term substrate is synonymous with the term “carrier”).

[0022] The precious metal component may be platinum, palladium, rhodium and mixtures thereof. In general, the precious metal component will be present in a loading of about 10 to about 300 g/ft³.

[0023] Preferably the substrate (or “carrier”) is a ceramic or metal having a honeycomb structure. Any suitable substrate may be employed, such as a monolithic carrier of the type having fine, parallel gas flow passages extending therethrough from an inlet or an outlet face of the carrier, such that passages are open to fluid flow therethrough. The passages, which are essentially straight paths from their fluid inlet to their fluid outlet, are defined by walls on which the catalytic material is coated as a washcoat so that the gases flowing through the passages contact the catalytic material. The flow passages of the monolithic carrier are thin-walled channels, which can be of any suitable cross-sectional shape and size such as trapezoidal, rectangular, square, sinusoidal, hexagonal, oval, circular, etc. Such structures may contain from about 60 to about 900 or more gas inlet openings (i.e., cells) per square inch of cross section.

[0024] The ceramic carrier may be made of any suitable refractory material, e.g., cordierite, cordierite-α-alumina, silicon nitride, zircon mullite, spodumene, alumina-silica-magnesia, zircon silicate, sillimanite, a magnesium silicate, zircon, petalite, α-alumina, an aluminosilicate and the like; cordierite is preferred.

[0025] The substrates useful for the catalysts of the present invention may also be metallic in nature and be composed of one or more metals or metal alloys. The metallic substrates may be employed in various shapes such as corrugated sheet or monolithic form. Preferred metallic supports include the heat resistant metals and metal alloys such as titanium and stainless steel as well as other alloys in which iron is a substantial or major component. Such alloys may contain one or more of nickel, chromium and/or aluminum, and the total amounts of these metals may advantageously comprise at least 15 wt. % of the alloy, e.g., 10-25 wt. % of chromium, 3-8 wt. % of aluminum and up to 20 wt. % of nickel. The alloys may also contain small or trace amounts of one or more other metals such as manganese, copper, vanadium, titanium and the like. The surface or the metal carriers may be oxidized at high temperatures, e.g., 1000° C. and higher, to improve the corrosion resistance of the alloy by forming an oxide layer on the surface the carrier. Such high temperature-induced oxidation may enhance the adherence of the refractory metal oxide support and catalytically promoting metal components to the carrier.

[0026] If the catalyst support employed for the catalysts of the present invention has a “sandwich”type of structure as described above, the layer of alumina is deposited on at least part of the monomolecular layer and is disposed between the monomolecular layer and the precious metal component.

[0027] Catalysts containing the catalyst supports of the invention may also contain at least one NO_x storage component, i.e., a NO_x trap, in the amount of about 0.3 to about 1.5 g/in³, deposited on at least part of the monomolecular layer. A suitable NO_x storage component comprises a basic oxygenated compound of an alkali or alkaline earth metal; the alkali metal may be lithium, sodium, potassium or cesium, and the alkaline earth metal may be magnesium, calcium, strontium or barium. The preferred alkali metal comprises potassium, while the preferred alkaline earth metal comprises barium. If a “sandwich” type of structure is utilized when the NO_x storage component is present, the layer of alumina is deposited on at least part of the monomolecular layer and is disposed between the monomolecular layer- and the precious metal component and the NO_x storage component.

[0028] The catalysts of the invention may be readily prepared by the following steps:

[0029] (a) depositing on a catalytic support a high surface area refractory metal oxide a monomolecular layer of a second oxide selected from the group consisting of titanium dioxide, cerium dioxide and zirconium dioxide;

[0030] (b) impregnating the catalytic support resulting from step (a) with at least one metal support;

[0031] (c) depositing the impregnated catalytic support resulting from step (b) upon a substrate; and

[0032] (d) calcining the catalyst resulting from step (c).

[0033] Typically, step (a) may be carried out using well-known chemical vapor deposition (“CVD”) techniques. For example, titanium tetrachloride is deposited in the gas phase on gamma-aluminum powder using CVD. Thereafter, the resultant material is exposed to a stream of moist nitrogen and it is calcined by heating in air at about 300 to about 500° C. for about 1 to about 6 hours. The calcined material is then washed to a point that the chloride content is <100 ppm and it is again calcined by heating in air at about 300 to about 750° C. for about 1 to about 6 hours.

[0034] If desired, the catalyst resulting from step (d) may be subsequently post-impregnated with at least one precious metal component and/or at least one NO_x storage component and is thereafter again calcined to the extent necessary to convert any post-impregnated NO_x storage component to its oxide form. The calcination in step (d) as well as for any calcination following post-impregnation may take place by heating the material in air at a temperature of about 300 to about 750° C. for about 1 to about 4 hours.

[0035] Typically, the precious metal component is utilized in the form of a compound or complex to achieve dispersion of the component on the refractory metal oxide support, e.g., gamma-alumina. For the purposes of the present invention, the term “precious metal component” means any compound, complex, or the like which, upon calcination or use thereof, decomposes or otherwise converts to a catalytically active form, usually the metal or the metal oxide. Water-soluble compounds or water-dispersible compounds or complexes of the precious metal component may be used as long as the liquid medium used to impregnate or deposit the precious metal component onto the refractory metal oxide support particles does not adversely react with the metal or its compound or its complex or other components which may be present in the catalyst composition and is capable of being removed from the metal component by volatilization or decomposition upon heating and/or application of a vacuum. In some cases, the completion of removal of the liquid may not take place until the catalyst is placed into use and subjected to the high temperatures encountered during operation. Generally, both from the point of view of economics and environmental aspects, aqueous solutions of soluble compounds or complexes of the precious metals are preferred. For example, suitable compounds are chloroplatinic acid, amine-solubilized platinum hydroxide, palladium nitrate or palladium chloride, rhodium chloride, rhodium nitrate, hexamine rhodium chloride, etc. During the calcination step, or at least during the initial phase of use of the composite, such compounds are converted into a catalytically active form of the metal or a compound thereof.

[0036] A preferred method of preparing the catalyst of the invention is to prepare a mixture of a solution of a precious metal compound and the previously prepared catalyst support that is sufficiently dry to absorb substantially all of the solution to form a slurry. Preferably, the slurry is acidic, having a pH of about 2 to less than 7. The pH of the slurry may be lowered by the addition of a minor amount of an inorganic or organic acid such as hydrochloric or nitric acid, preferably acetic acid, to the slurry. Thereafter, if desired, the NO_x storage component, and optional transition metal components, stabilizers and/or promoters may be added to the slurry in the form of soluble salts such as acetates (which will be converted to their oxide forms in the course of the calcination step).

[0037] In a particularly preferred embodiment, the slurry is thereafter comminuted to result in substantially all of the solids having particle sizes of less than 20 microns, i.e., 1-15 microns, in average diameter. The comminution may be accomplished in a ball mill or other similar equipment, and the solids content of the slurry may be, e.g., 20-60 wt. %, preferably 35-45 wt. %.

[0038] The following nonlimiting examples shall serve to illustrate the various embodiments of the present invention.

EXAMPLE 1

[0039] Two catalyst supports of the invention were prepared by depositing titanium dioxide on gamma alumina having a surface area of 150 m²/g using the CVD technique as described above. One catalyst support contained 6 wt. % titanium dioxide, while the other catalyst support contained 10 wt. % titanium dioxide. The x-ray diffraction pattern of the catalyst support calcined at 750° C. for six hours containing 10 wt. % titanium dioxide is shown in FIG. 1. It is significant to note that FIG. 1 does not show any reflexes for any titania phase.

EXAMPLE 2

[0040] Four catalyst formulations were prepared with the ingredients shown in Table I. In each formulation, the alumina was gamma-alumina having a surface area of 150 m²/g. Catalyst “D” was formulated using the catalyst support of the invention as set forth in Example 1 wherein the titania content was 6 wt. %. Zirconyl acetate was also added during the milling process as a binder.

[0041] The procedure for preparing the catalysts was that set forth above. The NO_x storage component was incorporated in each catalyst by post-dipping of the catalyst followed by calcination. 1

[0042] Formulations A, B, C and D were engine-aged at 580° C. for 60 hours in the presence of a fuel containing 150 ppm sulfur and at a space velocity of 27,000 h⁻¹. A comparison was then made of the “RS”(Rich Spike) NO_x capacity measured in grams of NO₂/1.3 liters versus the inlet temperature (° C.) of the catalyst.

[0043] The results of this comparison are graphically shown in FIG. 2. As may be seen from FIG. 2, catalyst D containing the catalyst support of the invention had significantly higher storage capacity than catalysts A, B or C after sulfur aging.

EXAMPLE 3

[0044] Catalysts A and D, formulated with the ingredients set forth in Table I, were evaluated for thermal stability. Each catalyst was aged in steam/air at 750° C. for 12 hours. Thereafter, the percentage of NO_x conversion was measured at temperatures ranging from about 250 to. 550° C.

[0045] The results of this evaluation are shown graphically in FIG. 3. FIG. 3 shows that catalyst D containing the catalyst support of the invention has excellent thermal stability over the entire range of typical catalyst operation temperatures as compared to catalyst A that contains titanium dioxide only.

EXAMPLE 4

[0046] In this example, catalyst E containing the catalyst support of the invention wherein the titanium dioxide content of the monomolecular layer was 10% was compared to catalyst F wherein the catalyst support was solely gamma-alumina.

[0047] Catalyst Preparation for Catalyst E:

[0048] The bottom layer of the catalyst consisted of 75 g/ft³ Pt, 0.42 g/in³ BaO, 1.83 g/in³ TiO₂-mod ified Al₂O₃ support and 0.05 g/in³ ZrO₂, wherein the TiO₂-modified Al₂O₃ support was prepared by the CVD method so as to allow 10% TiO₂ by weight coverage on the surface of the alumina. The TiO₂-modified Al₂O₃ support material was impregnated with a Pt-amine salt solution to achieve the desired loading. The resultant powder was then milled to 90% of the particles below 10 micrometers (d90<10μ). The milling process, which requires the addition of water to the powder, resulted in a high-solids content slurry. Barium acetate was dissolved in water and mixed with the slurry. Zirconyl acetate was also added during the milling process as a binder.

[0049] The top layer of the catalyst consisted of 65 g/ft³ Pt, 10 g/ft³ Rh, 0.08 g/in³ BaO, 1.20 g/in³ of TiO₂-modified Al₂O₃ support (10 wt. % TiO₂) and 0.08 g/in³ ZrO₂. Pt and Rh were sequentially impregnated on the TiO₂-modified Al₂O₃ support, and the resultant powder was then milled to d90<10μ. The barium acetate was dissolved in water and then added to the slurry.

[0050] Catalyst Preparation for Catalyst F:

[0051] Catalyst F was prepared in the same manner as catalyst E, with the exception that the support was Al₂O₃ rather than the TiO₂-modified Al₂O₃ support.

[0052] The two catalysts were subjected to sulfation tests conducted at a constant reaction temperature, i.e., 300° C., with 10 ppm SO₂ present in both lean and rich feed. The space velocity of the gas was 40,000 h⁻¹. The lean cycle lasted 60 seconds with λ=1.50 and the rich cycle lasted 6 seconds with λ=0.86. The sulfation process entailed a space velocity of 40,000 h⁻¹ for six hours at 450° C. The desulfation process entailed heating the gas stream and the catalyst to 650° C. for 15 minutes in a N₂ stream at a space velocity of 50,000 h⁻¹. At the targeted desulfation temperature (650° C.), a 5 second lean/15 second rich alternating stream was turned on. Each catalyst was exposed to the alternating stream for 5 minutes, which completed the desulfation process.

[0053] The data were collected every hour and each collection period lasted for 10 minutes. The averaged NO_x conversions at a given point in time are graphically shown in FIG. 4. FIG. 4 shows four consecutive sulfation and desulfation runs. As may be seen from FIG. 4, the NO_x conversion for catalyst E containing the catalyst support of the invention fully recovered to its original level after each desulfation step. However, catalyst F utilizing only alumina showed a continuous loss in NO_x conversion capacity after each desulfation step.