Urea-resistant catalytic units and methods of using the same

According to at least one aspect of the present invention, a urea-resistant catalytic unit is provided. In at least one embodiment, the catalytic unit includes a catalyst having a catalyst surface, and a urea-resistant coating in contact with at least a portion of the catalyst surface, wherein the urea-resistant coating effectively reduces urea-induced deactivation of the catalyst. In at least another embodiment, the urea-resistant coating includes at least one oxide from the group consisting of titanium oxide, tungsten oxide, zirconium oxide, molybdenum oxide, aluminum oxide, silicon dioxide, sulfur oxide, niobium oxide, molybdenum oxide, yttrium oxide, nickel oxide, cobalt oxide, and combinations thereof.

BACKGROUND

Embodiments of the present invention relate to a urea-resistant catalytic unit for reducing catalyst deactivation due to urea poisoning.

2. Background Art

While offering certain benefits in fuel economy, internal combustion engines such as diesel engines often require special exhaust aftertreatment system to reduce waste species such as nitrogen oxide (NOx) under oxidizing operating conditions.

Urea has been used as a reductant in the exhaust aftertreatment system for reducing NOxemissions in order to meet certain government and industry imposed emission regulations. For instance, aqueous urea has been injected into the exhaust stream within a selective catalytic reduction (SCR) aftertreatment system and hydrolyzed to form ammonia (NH3) which then reduces NOxover a SCR catalyst.

In vehicle applications, and as a result of space restrictions, urea residence time is often short and the liquid urea may not have sufficient time to vaporize and hydrolyze before contacting the SCR catalyst. This situation is more prevalent at lower exhaust temperature, especially below 200 degree Celsius, where liquid urea may accumulate on the SCR catalyst and cause urea byproduct formation and subsequent deactivation of the SCR catalyst.

SUMMARY

According to at least one aspect of the present invention, a urea-resistant catalytic unit is provided for reducing catalyst deactivation due to urea poisoning. In at least one embodiment, the urea-resistant catalytic unit includes a catalyst having a catalytic surface, and a urea-resistant coating in contact with at least a portion of the surface, wherein the urea-resistant coating effectively reduces urea-induced deactivation of the catalyst.

In at least another embodiment, the catalyst is configured as at least one discrete particle, at least a portion of which being covered with the urea-resistant coating.

In at least yet another embodiment, the catalyst is configured as a sheet, at least a portion of which being covered with the urea-resistant coating.

In at least yet another embodiment, the catalyst includes a selective catalytic reduction catalyst.

In at least yet another embodiment, the urea-resistant catalytic unit further includes a substrate having a substrate surface to support the catalyst.

In at least yet another embodiment, the substrate has a porosity selected from the group consisting of a porosity of from 0.5 to 35 volume percent and a porosity of from 35 to 90 volume percent.

In at least yet another embodiment, the urea-resistant coating includes at least one oxide selected from the group consisting of titanium oxide, tungsten oxide, zirconium oxide, molybdenum oxide, aluminum oxide, silicon dioxide, sulfur oxide, niobium oxide, molybdenum oxide, yttrium oxide, nickel oxide, cobalt oxide, and combinations thereof.

In at least yet another embodiment, the urea-resistant coating is present in 0.1 to 30.0 percent by weight of the urea-resistant catalytic unit.

According to at least another aspect of the present invention, an emission control system is provided for reducing waste species from the exhaust of an internal combustion engine. In at least one embodiment, the emission control system includes an exhaust passage for transporting the exhaust from the engine, a reductant disposed within the exhaust passage and downstream of the engine, and a urea-resistant catalytic unit disposed downstream of the reductant. The urea-resistant catalytic unit is defined herein according to embodiments of the present invention.

According to yet another embodiment, a method is provided for reducing waste species from the exhaust of an internal combustion engine. In at least one embodiment, the method include contacting the exhaust with a urea-resistant catalytic unit. The urea-resistant catalytic unit is defined herein according to embodiments of the present invention.

In at least another embodiment, the contact step is conducted in a lean operating condition.

In at least yet another embodiment, the method further includes subjecting the exhaust to an oxidation catalyst prior to the contacting step.

DETAILED DESCRIPTION OF THE EMBODIMENT(S)

Moreover, except where otherwise expressly indicated, all numerical quantities in the description and in the claims are to be understood as modified by the word “about” in describing the broader scope of this invention. Practice within the numerical limits stated is generally preferred. Also, unless expressly stated to the contrary, the description of a group or class of material as suitable or preferred for a given purpose in connection with the invention implies that mixtures of any two or more members of the group or class may be equally suitable or preferred.

As a matter of definition, and when used in this detailed description and in the claims:

“SCR” means selective catalytic reduction and includes a reducing catalyst which speeds or enhances a chemical reduction of NOxthrough the assistance of a reductant during lean operation.

“NOx” means nitrogen oxide and illustratively includes a mixture of compounds of nitric oxide (NO) and nitrogen dioxide (NO2).

“Urea poisoning” means catalyst deactivation due to accumulation of urea molecules on the catalyst and may be manifested by the formation of undesirable urea derived byproducts.

“Catalyst deactivation” means catalytic activity reduction due to urea poisoning. In the event the catalyst is a SCR catalyst, the catalyst deactivation means reduction in NOxconversion as catalyzed by the SCR catalyst due to urea poisoning.

It has been found, according to embodiments of the present invention, that catalyst deactivation due to urea poisoning may be effectively reduced or eliminated when the catalyst is protected with a urea-resistant coating. As will be described in more details below, at least one embodiment relates to a urea-resistant catalytic unit which includes a catalyst having a catalyst surface, and a urea-resistant coating in contact with at least a portion of the catalyst surface, wherein the urea-resistant coating effectively reduces urea-induced deactivation of the catalyst.

It has further been found, according to embodiments of the present invention, that the urea-resistant catalytic unit, according to at least one embodiment, effectively diminishes urea-induced catalyst deactivation, and particularly urea-induced reduction of NO, conversion. The urea-resistant catalytic unit according to embodiments of the present invention is able to diminish urea induced catalyst deactivation in the form of NO, conversion reduction by up to 75 percent, 85 percent, or 95 percent wherein a value of 100 percent decrease in SCR deactivation is theoretically achieved wherein urea poisoning is rendered completely absent.

While not intended to be limited by any theory, one possible mechanism by which the urea-resistant catalytic unit is resistant to urea poisoning may be that the catalyst and in particular the SCR catalyst for NOxconversion is protected through urea hydrolysis function of the urea-resistant coating. In addition, the urea-resistant coating is advantageously chosen and designed to have little or no impairment on the catalytic function of the SCR catalystic unit in converting NOx.

According to at least one aspect of the present invention, an emission control system is provided for reducing waste species from the exhaust of an internal combustion engine. The waste species from the exhaust of an internal combustion engine may include unburned hydrocarbon (HC), carbon monoxide (CO), particulate matters (PM), nitric oxide (NO), and nitrogen dioxide (NO2), with NO and NO2, collectively referred to as nitrogen oxide or NOx. In at least one embodiment, and as depicted inFIG. 1, an emission control system, generally shown at120, includes an exhaust passage114for transporting the exhaust124from the engine112, a reductant118disposed within the exhaust passage114downstream of the engine112, and a urea-resistant catalytic unit100according to various embodiments described hereinafter, wherein the urea-resisting catalytic unit100has a catalyst and a urea-resistant coating which effectively reduces urea-induced deactivation of the catalyst. The reductant118is optionally supplied from a container122. In at least another embodiment, an oxidation catalyst116is disposed in the passage114upstream of the urea-resistant catalytic unit100.

As used herein and unless otherwise indicated, the reductant118may include ammonia, liquid urea, solid urea, or combinations thereof.

The urea-resistant catalytic unit100may include a substrate for support. The substrate is generally a flow-through monolith or any part thereof. A monolith is well known but is generally described as a ceramic block made of a number of substantially parallel flow channels. The monolith may be made of ceramic materials such as cordierite, mullite, and silicon carbide or metallic materials such as iron cromium alloy, stainless steel, and Inconel®. The flow channels of the monolith may be of any suitable size, and in certain embodiments are of a size of 0.5 to 10 millimeters in diameter. Because of the number of the channels, the contact area between an exhaust and a catalyst is enlarged. Further, the channels can be substantially straight, hollow, and parallel to the flow of the exhaust, therefore flow obstruction to the exhaust is effectively minimized.

In at least another embodiment, and as schematically depicted inFIG. 1A, the urea-resistant catalytic unit100includes a substrate configured as a plurality of flow-through channels130arranged substantially parallel from each other in the direction of the flow of the exhaust124. A vertical cross-section of the catalytic unit100, as depicted inFIG. 1A, illustratively shows open ends130aof the flow-through channels130defined by the substrate walls132surrounding the flow-through channels130. An exemplary area “aa” of the vertical section of the substrate wall132will be described in more detail hereinafter, and particularly with relation toFIGS. 2 and 3. In this embodiment, a substantially amount of the exhaust124may pass through the cavity134of the channels130and very little amount of hte exhaust124passes through the walls132. This design of the substrate and the walls thereof is useful where the urea-resistant catalytic unit100functions as a flow-through NOxoccluding catalyst such as a SCR catalyst.

In at least yet another embodiment, and as schematically depicted inFIG. 1B, the urea-resistant catalytic unit100is based on a substrate configured as non flow-through channels140arranged substantially parallel from each other in the direction of the flow of the exhaust124. A vertical cross-section of the catalytic unit100illustratively shows open ends140aand close ends140bof the channels140defined by the substrate walls142surrounding the channels140. An exemplary area “bb” of the vertical section of the substrate wall142will be described in more detail hereinafter, and particularly with relation to FIGS.4and5A-5B. In this embodiment, the exhaust124may pass through both the cavities144and the walls142of the channels140. This design of the substrate and the walls thereof is useful where the urea-resistant catalytic unit100functions as a non flow-through particulate matter filter optionally associated with a NOxoccluding catalyst such as a SCR catalyst.

According to at least one aspect of the present invention, a urea-resistant catalytic unit is provided for reducing catalyst deactivation due to urea poisoning. In at least one embodiment, a urea-resistant catalytic unit, generally shown at200inFIG. 2, includes a catalyst202having a catalyst surface206and a urea-resistant coating204in contact with at least a portion of the catalyst surface206. In certain particular instances, and as shown inFIG. 2, the catalyst202is configured as a sheet covered with the urea-resistant coating204in the form of a layer. It is possible that in certain other instances, the urea-resistant coating204may contact the catalyst surface206in a discontinuous manner, the catalyst202may be configured non-uniform nor flat, or the urea-resistant coating204may be configured non-uniform nor flat, so long as at least 50 percent (%), 60%, 70%, 80%, 90% of the catalyst surface206of the catalyst202is in contact with the urea-resistant coating204.

As a variation to the urea-resistant catalytic unit200, and in correlation to the area “aa” ofFIG. 1Bin an enlarged view, the urea-resistant catalytic unit200is depicted inFIG. 2to further include a substrate208having a substrate surface210, wherein the catalyst202is configured as a sheet intermediate between the urea-resistant coating204and the substrate surface210of the substrate208. Likewise, the catalyst202may be supported on the substrate208through the substrate surface210in a discontinuous fashion. The substrate208ofFIG. 2may be viewed as an equivalent to the portion of the substrate wall132depicted in the area “aa” ofFIG. 1A. Further, the catalyst202may be configured non-uniform nor flat so long as at least 50 percent (%), 60%, 70%, 80%, 90% of the substrate surface210is in contact with the catalyst202. Similarly, the substrate surface210may also be configured non-uniform nor flat so long as at least 50 percent (%), 60%, 70%, 80%, 90% of the catalyst surface206is in contact with the urea-resistant coating204.

In at least another embodiment, a urea-resistant catalytic unit is generally shown at300inFIG. 3A. The urea-resistant catalytic unit300includes a catalyst302having a catalyst surface306, at least a portion of which being in contact with a urea-resistant coating304. In certain particular instances, and as depicted inFIG. 3A, the catalyst is configured as at least one discrete particle having the catalyst surface306covered with the urea-resistant coating304. It is possible that in certain other instances, the urea resistant coating304may cover the particle-shaped catalyst302in a discontinuous fashion, such that the urea-resistant coating304may be configured non-uniform nor smooth, as long as at least 50 percent (%), 60%, 70%, 80%, 90% of the catalyst surface306of the catalyst202is in contact with the urea-resistant coating304.

As a variation to the urea-resistant catalytic unit in accordance withFIG. 3A, and in correlation to the area “aa” ofFIG. 1Bin an enlarged view, the urea-resistant catalytic unit300is depicted inFIG. 3Bto further include a substrate308having a substrate surface310, wherein the catalyst302is configured as the at least one discrete particle in contact with the substrate surface310of the catalyst302. The substrate308ofFIG. 3Bmay be viewed as an equivalent to the portion of the substrate wall132depicted in the area “aa” ofFIG. 1A. It is possible that the catalyst302configured as the discrete particle(s) may be supported on the substrate surface310in a discontinuous fashion, so long as at least 50 percent (%), 60%, 70%, 80%, or 90% of the substrate surface310is being contacted by the catalyst302.

FIG. 4depicts, in an enlarged view, the area “bb” of the substrate wall ofFIG. 1B. Due to the presence of internal pores collectively shown as unshaded area such as various paths420, the substrate thus is defined by an external surface406aand an internal surface406b. In this embodiment, the exhaust passes through both the pores420located within the walls of the channels. At least a portion of the internal surface406bis covered with a catalyst defined herein according toe embodiments of the present invention.

FIG. 5Adepicts an enlarged vertical section of an exemplary substrate wall showing an internal substrate surface at least partially covered with a catalyst502bwhich is configured as a contoured sheet intermediate between the internal substrate surface506band a urea-resistant coating504b. An external substrate surface506ais optionally in contact with a urea-resistant coating504a.

FIG. 5Bdepicts an enlarged vertical section of an exemplary substrate wall showing an internal substrate surface at least partially covered with a catalyst502bwhich is configured as at least one discrete particle supported on the internal substrate surface506band covered with the urea-resistant coating504b. The external substrate surface506ais optionally in contact with a urea-resistant coating504a.

In at least one embodiment, the catalyst202,302,502includes a zeolite based NOxoccluding catalyst or a SCR catalyst. The term “zeolite” generally refers to a framework aluminosilicate containing atoms of oxygen aluminum and/or silicon. An example of a natural zeolite is mordenite or a chabazite. Synthetic zeolites illustratively include type A as synthetic forms of mordenite, type B as ZSM-50 zeolites, and type Y as ultra-stabilized Beta zeolite. The framework structure of the zeolites often acquires an overall negative charge compensated for by exchangeable cations which may readily be replaced by other cations such as metal cations through methods including ion exchange. The NOxoccluding catalyst typically includes an alkaline earth metal exchanged zeolite, precious metal exchanged zeolite such as platinum based and/or a base metal exchanged zeolite such as copper and iron based zeolites. While any type zeolite may be used, some suitable zeolites include X type zeolite, Y type zeolite, and/or ZSM-5 type zeolite.

The alkaline earth metal illustratively include barium, strontium, and calcium. Suitable calcium sources for the alkaline earth metal include calcium succinate, calcium tartrate, calcium citrate, calcium acetate, calcium carbonate, calcium hydroxide, calcium oxylate, calcium oleate, calcium palmitate and calcium oxide. Suitable strontium sources for the alkaline earth metal include strontium citrate, strontium acetate, strontium carbonate, strontium hydroxide, strontium oxylate and strontium oxide. Suitable barium sources for the alkaline earth metal include barium butyrate, barium formate, barium citrate, barium acetate, barium oxylate, barium carbonate, barium hydroxide and barium oxide.

The rare earth metal may illustratively include lanthanum, cerium, and/or neodymium. Suitable neodymium sources for the rare earth metal include neodymium acetate, neodymium citrate, neodymium oxylate, neodymium salicylate, neodymium carbonate, neodymium hydroxide and neodymium oxide. Suitable cerium sources for the rare earth metal include cerium formate, cerium citrate, cerium acetate, cerium salicylate, cerium carbonate, cerium hydroxide and cerium oxide. Suitable lanthanum sources for the rare earth metal include lanthanum acetate, lanthanum citrate, lanthanum salicylate, lanthanum carbonate, lanthanum hydroxide and lanthanum oxide.

The above described zeolite NOxoccluding catalyst or the SCR catalyst contained within the catalyst202,302,502may be prepared by any suitable methods. In the event when the hydrogen-ion-exchanged acid zeolites are used, active ingredients may be incorporated into the zeolites in a manner illustratively shown as follows. A starting material is produced, including the zeolites, by mixing, milling and/or kneading the individual components or their precursor compounds (for example water-soluble salts for the specified metal oxides) and if appropriate with the addition of conventional ceramic fillers and auxiliaries and/or glass fibers. The starting material is then either processed further to form unsupported extrudates or is applied as a coating to a ceramic or metallic support in honeycomb or plate form. The starting material is then dried at a temperature of 20 to 100 degrees Celsius. After the drying operation, the starting material is calcined to form the active material by calcination at temperatures of between 400 and 700 degrees Celsius. In addition, after the calcining process, the calcined active material may be subjected to an optional aging treatment at a temperature that is higher than the calcining temperature. A temperature which is approximately 450 to 850 degrees Celsius may be selected for the optional aging. The optional aging treatment may be carried out for a period of 20 to 80 hours.

In certain instances, a binder may be used to bring together all ingredients to form the catalyst202,302,502and particularly when the catalyst is configured as discrete particles. The binder is used to prevent dissolution and redistribution of the ingredients. Possible binders include acidic aluminum oxide, alkaline aluminum oxide, and ammonium aluminum oxide. In certain particular instances, a soluble alkaline aluminum oxide with a pH of at least 8 is used as the binder. In the event that a binder is used, the binder may be included in an amount of from 1 to 10 weight percent, and particularly 2 to 6 weight percent of the total weight of the catalyst202,302,502.

In at least one embodiment, the urea-resistant coating204,304,504contains at least one oxide illustratively including titanium dioxide, aluminum oxide, silicon dioxide, zirconium oxide, sulfur oxide (SO3), tungsten oxide (WO3), niobium oxide (Nb2O5), molybdenum oxide (MoO3), aluminum oxide, yttrium oxide, nickel oxide, cobalt oxide, or combinations thereof. Without being limited by any theory, the oxide contained within the urea-resistant coating functions at least partially as hydrolyzation molecules that induce the hydrolyzation and hence breakdown of the excess urea and resultant alleviation of the deactivating effects of the excess urea.

In at least one embodiment, the catalyst202,302,502has a loading concentration in percent (%) by dry weight defined as an amount in grams of the catalyst relative to every 100 grams of the total dry weight of the urea-resistant catalytic unit. In at least one particular embodiment, the catalyst202,302,502has a loading concentration in a range independently selected from no less than 2.5% (percent), 5%, 7.5%, 10%, or 12.5%, to no greater than 32.5%, 27.5%, 22.5%, or 17.5%.

In at least one embodiment, the urea-resistant coating204,304,504has a loading concentration in percent (%) by dry weight defined as an amount in grams of the urea-resistant coating relative to every 100 grams of the total dry weight of the urea-resistant catalytic unit. In at least one particular embodiment, the urea-resistant coating has a loading concentration in a range independently selected from no less than 0.5% (percent), 1%, 5%, 7.5%, or 10%, to no greater than 30%, 25%, 20%, or 15%.

In at least one embodiment, the urea-resistant coating204,304,504has a loading concentration in a weight percent (%) defined as an amount in grams of the urea-resistant coating relative to every 100 grams of the total weight of the catalyst202,302,502. In at least one particular embodiment, the loading concentration of the urea-resistant coating in relation to the loading concentration of the catalyst is in a ratio of 1:10 to 10:1, 1:7.5 to 7.5:1, 1:5 to 5:1, or 1:2.5 to 2.5:1.

The urea-resistant coating may be applied to the catalyst through any suitable methods. In at least one particular embodiment, a precursor substance of the urea-resistant coating is powdered, made into an aqueous slurry and then milled. The amount of the precursor substance may be determined by experiment or else be calculated based on the molecular weight and/or solubility of the particular precursor substance used. As a result, the amount of precursor substance present on the urea-resistant coating is such that a high effectiveness of the catalyst is achieved in the reduction of NOxin NOx-containing exhaust gases. The catalyst such as a SCR catalyst, with or without the substrate, is dipped into the coating slurry. After any excess coating slurry is removed, the catalyst is subject to drying and calcination at a temperature of between 450 to 700 degrees Celsius for 2 to 5 hours.

The urea-resistant catalytic unit produced in this way has a considerable long-term hydrothermal stability under the influence of urea poisoning. For example, the SCR activity of the urea-resistant catalyst is not impaired by urea poisoning even after aging for 18 t 36 hours at 800 degrees Celsius or higher.

Suitable zirconium sources of the precursor substance for the urea-resistant coating204,304,504generally include zirconium dioxide, zirconium oxychloride, zirconium tert-butoxide, zirconium ethoxide, zirconium isopropoxide, and colloidal zirconium oxide.

Suitable titanium sources of the precursor substance for the urea-resistant coating204,304,504generally include titanium dioxide, titanium oxychloride, titanium oxynitrate, titanium isobutoxide, titanium n-butoxide, titanium tert-butoxide, titanium ethoxide, titanium isopropoxide, titanium methoxide, titanium n-propoxide, and colloidal titanium oxide.

Suitable aluminum sources of the precursor substance for the urea-resistant coating204,304,504generally include aluminum oxide, aluminum hydroxide, aluminum methoxide, aluminum n-butoxide, aluminum ethoxide, and aluminum isopropoxide.

Suitable silicon sources of the precursor substance for the urea-resistant coating204,304,504generally include silicon oxide and colloidal silicon oxide.

Suitable yttrium sources of the precursor substance for the urea-resistant coating204,304,504generally include yttrium oxide, colloidal yttrium oxide, and yttrium isopropoxide.

Suitable nickel sources of the precursor substance for the urea-resistant coating204,304,504generally include nickel oxide and nickel hydroxide.

Suitable cobalt sources of the precursor substance for the urea-resistant coating204,304,504generally include cobalt oxide and cobalt hydroxide.

According to at least another aspect of the present invention, a method is provided for removing NOxemissions from the exhaust of an internal combustion engine. In at least one embodiment, the method includes contacting the exhaust with a urea-resistant catalytic unit as described in various embodiments herein. In at least another embodiment, the method is applied in a lean operating condition. In at least yet another embodiment, the method further includes directing the exhaust through an oxidation catalyst prior to the contacting step.

EXAMPLES

Employed in the instant example are SCR catalysts of fully formulated monolith Cu/zeolites. One such SCR catalyst used is SCR catalyst “A” having 300 cpsi and of 12 mil wall thickness wherein Cu/zeolite is coated on a high porosity cordierite honeycomb substrate. Another SCR catalyst used is SCR catalyst “B” having 300 cpsi and of 8 mil wall thickness wherein Cu/zeolite is coated on a low porosity cordierite honeycomb substrate.

The SCR catalysts “A” and “B” each having a core dimension of 1″ diameter by 1″ length are aged and examined for NOxconversion efficiency using a laboratory flow reactor system.

The aging process is conducted in three different ways: hydrothermal aging alone (hereinafter “hydrothermal”), hydrothermal aging coupled with pretreatment of wet urea soaking (hereinafter “wet urea+hydrothermal”), and hydrothermal aging coupled with pretreatment of wet urea soaking followed by drying (hereinafter “dry urea+hydrothermal”). Consistent with all the aging process in this and other examples contained herein, the term “hydrothermal aging” means that relevant catalyst is subject to dry heat at a temperature of 860 degrees Celsius for 30 minute.

In the aging process wherein pretreatment with urea is involved, a 32.2 weight percent aqueous urea solution is sprayed at room temperature as a mist onto the surfaces of the SCR catalysts. The amount of urea sprayed is the maximum liquid adsorption amount.

After the aging treatment, the aged SCR catalysts “A” and “B” are each subjected to a simulated exhaust having a composition tabulated in Table I below. Steady state NOxconversion is measured from 170 C to 550 C in 30-50 degree Celsius increments in the flow reactor connected to a FTIR instrument with a heated sample cell for wet gas analysis. The simulated exhaust flows through the SCR catalyst “A” or “B” at a space velocity of 30 Khr−1.

TABLE IComposition of the Simulated ExhaustO2H2OCO2NONH3N2Concentration14%4.5%5%350 ppm350 ppmBalance

FIG. 6depicts effects of differential aging processes on steady state NOxconversion rate exhibited by the SCR catalyst “A”. Within the tested catalyst temperature range of 150 to 550 degree Celsius, the aging process of “wet urea+hydrothermal” is shown to reduce the NOxconversion compared to the hydrothermal aging alone. It is further shown that the aging process of “dry urea+hydrothermal” elicits a greater reduction of the NOxconversion up to a 47%. These results indicate that pretreatment with liquid urea such as urea mist soaking induces SCR catalyst deactivation in various degrees dependent on, for example, whether urea pretreatment is also followed by drying of the urea mist sprayed thereupon.

As a comparison, the SCR catalyst “A” is coated with TiO2prior to being subjected to the three different aging processes as described herein above in this example. The coating of TiO2is conducted as follows. TiO2power is mixed into an aqueous slurry and milled for 3 to 5 hours. Preformed monolithic SCR catalysts are dipped into the TiO2slurry. After the excess slurry is removed, the monolithic SCR catalysts are subjected to drying and calcination at a temperature of 500 to 600 degrees Celsius for 2 to 3 hours. The TiO2 is loaded at an amount of 0.5 to 5 grams per cubic inch of the monolithic SCR catalyst.

As depicted inFIG. 7, the TiO2coating effectively reduces the detrimental effect of urea pretreatment on the NOxconversion efficiency. More particularly, a maximum reduction of NOxconversion between the aging process of hydrothermal alone and the aging process of “wet urea+hydrothermal” is minimized from a 14% as in the situation without TiO2coating to a 6.1% in the situation with TiO2coating. Likewise, the maximum reduction of NOxconversion between the aging process of “hydrothermal alone” and the aging process of “dry urea+hydrothermal” is accordingly minimized from a 47% to a mere 6.1%.

These results indicate that a TiO2coating pretreatment effectively improves the SCR catalyst performance and efficiently reduces urea poisoning otherwise exerted by the urea soaking.

The SCR catalyst “B” is subjected to the different aging treatments as described in Example 1.

As depicted inFIG. 8, the aging process of “dry urea+hydrothermal” elicits a maximum reduction 11% of the NOxconversion rate compared to the aging process of “hydrothermal alone.”

The aging process of “wet urea+hydrothermal” effects a further reduction of the NOxconversion to a maximal amount of 30% compared to the aging process of “hydrothermal alone.”

The SCR catalyst “B” is subjected to one of the three aging treatments of Example 1 following a pretreatment coating with TiO2.

As depicted inFIG. 9, the TiO2pretreatment effectively eliminates the reduction effects of urea on the NOxconversion. The rate of NOxconversion as a function of the catalyst temperature does not significantly differ among the three tested aging processes.

The NOxconversion reduction observed between the “wet urea+hydrothermal” aging and the “hydrothermal alone” aging is decreased to less than 7% as compared to the 11% in the above shown scenario wherein the catalyst is not TiO2-pretreated.

Similarly, the NOxconversion reduction observed between the “dry urea+hydrothermal” aging and the “hydrothermal alone” aging is decreased to 11%, as compared to the 30% in the above shown scenario wherein the catalyst is not TiO2-pretreated.