WO2015101766A1 - Exhaust gas treatment catalysts - Google Patents

Abstract

A catalyst for treating an exhaust gas from a diesel engine is described. The catalyst comprises: (a) 0.1 to 10 wt.% of a Group 8 to 11 transition metal; and (b) 90 to 99.9 wt.% of a support; wherein the support comprises at least 90 wt.% of ceria and 0.1 to 10 wt.% of a dopant doped on the ceria, wherein the dopant comprises niobium (Nb) or tantalum (Ta).

Description

EXHAUST GAS TREATMENT CATALYSTS

FIELD OF THE INVENTION

The invention relates to catalysts that utilize doped ceria supports. The catalysts are useful for exhaust gas treatment.

BACKGROUND OF THE INVENTION

Hydrocarbon combustion in diesel engines, stationary gas turbines, and other systems generates exhaust gas that must be treated to remove nitrogen oxides (NO_x), carbon monoxide (CO), hydrocarbons (HCs), and particulate matter (PM). The exhaust generated in lean-burn engines is generally oxidative, and the NO_x can be reduced selectively with a heterogeneous catalyst and a reductant, which is typically ammonia or a short-chain hydrocarbon. This process is known as selective catalytic reduction (SCR). As regulations are becoming increasingly strict, especially for motor vehicles, treatment of diesel exhaust must hit applicable targets for all of the above-identified pollutants. Modern after-treatment systems for vehicles include a diesel oxidation catalyst (DOC) for converting hydrocarbons and carbon monoxide to carbon dioxide and water, an SCR catalyst for converting NO_x to N₂, and a catalyzed soot filter or diesel particulate filter (see, for example, US 2007/0028604, US 201 1/0138776 and WO 2012/166833). After- treatment systems may also include a NO_x adsorber catalyst which chemically binds nitrogen oxides during lean engine operation. After the adsorber capacity is saturated, the system is regenerated during a period of rich engine operation, and released NO_x is catalytically reduced to nitrogen.

Diesel oxidation catalysts normally include a platinum group metal (e.g. Pd, Pt, Rh, Ru, Ir) and a support. The support can be an inorganic oxide, mixed oxide, zeolite (see US 8,263,032 and US 2012/0308439), clay, or the like. It is also common to coat the metal, with or without a support, onto a ceramic monolith, honeycomb, or other high-surface- area, flow-through substrate or "brick".

Ceria, alumina, or combinations of ceria and alumina have been used to support diesel oxidation catalysts (see US 2012/030843, US 2013/0089481 , US 5,462,907 and EP 0960649). Ceria has also been combined with Group 5 metals in the absence of a platinum group metal for use as SCR catalysts and in other catalytic processes. For examples, see Le Gal et al., J. Phvs. Chem. C 116 (2012) 13516 (tantalum-doped ceria used as a catalyst for water splitting during solar thermochemical hydrogen generation), S. Zhao et al., Appl. Catal. A 248 (2003) 9 (Nb- or Ta-doped cerias as catalysts for butane oxidation), K. Yashiro et al., Solid State Ionics 175 (2004) 341 (ceria prepared in the presence of pre-formed niobia and effect on electrical conductivity), and E. Ramirez- Cabrera et al., Solid State Ionics 136-137 (2000) 825 (niobia-doped cerias and their use to convert methane to synthesis gas). M. Casapu et al. (Appl. Catal. B 103 (201 1) 79) mentions diesel oxidation catalysts as part of a complete after-treatment system that includes a DOC, an SCR catalyst, and a DPF. Doped cerias are taught for use for SCR and soot oxidation. The catalysts tested contained 10 wt.% Nb₂0₅ on a mixed ceria- zirconia support or 30 wt.% Nb₂0₅ on ceria.

Combinations of niobia and ceria are also discussed in patents and published patent applications, again in the absence of a platinum group metal and not for diesel oxidation catalysis. See, for example, EP 2368628 (catalysts comprising at least 10 wt.% ceria and at least 10 wt.% of niobia and their use for an SCR process) and US 2013/0121902 (mixed oxides of ceria, zirconia, niobia, and a rare earth sesquioxide as catalysts for an SCR process).

SCR catalysts comprising niobia and ceria are disclosed in WO 2012/041921 , WO 2012/004263, and WO 2013/037507. Catalysts in WO 2012/004263 comprise 2 to 20 wt.% of niobium oxide. Catalysts having <50% Zr are shown to have greater capability for reducing hydrogen compared with a similar catalyst made with 77.6% Zr. In two examples (Exs. 9 and 10), 3.2 wt.% or 8.6 wt.% of Nb₂0₅ is present, zirconia is omitted, and the balance is ceria. However, these catalysts are essentially ceria-encapsulated niobias rather than niobia "doped on" ceria. As shown in the examples, the catalysts are made by forming ceria in the presence of a small proportion of pre-formed niobia. Our own work (described herein) demonstrates that at identical proportions of niobia and ceria, these catalysts are less effective for NO_x reduction than compositions in which the niobia is doped on ceria. Moreover, as shown in WO 2012/041921 (Table 5, Exs. 9 and 10), these catalysts also appear to deactivate upon hydrothermal aging (750°C, 16 h).

WO 2012/004263 teaches that ceria-encapsulated niobias can be used as a support for a precious metal such as platinum, rhodium, palladium, silver, gold, or iridium (see pp. 13-14), and potential applications for the supported precious metals include their use as oxidation catalysts. However, no specific examples showing diesel oxidation catalysts are provided. As noted earlier, we determined that the ceria-encapsulated niobias disclosed in WO 2012/004263 differ from niobia-doped cerias described herein. EP 0960649 teaches an exhaust clean-up catalyst comprising a platinum group metal supported on 50:50 ceria/alumina, where the ceria/alumina mixture is coated with 2.3% niobium (see Example 2). The catalysts demonstrate high clean-up capability for saturated hydrocarbons, particularly propane.

Known diesel oxidation catalysts supported on ceria are susceptible to sulfur poisoning. ΝΟχ adsorbers are also susceptible to sulfur poising. Because diesel fuels can have high sulfur content, this is an important limitation. Catalysts that have too great a tendency to adsorb sulfur dioxide quickly lose their effectiveness for oxidation.

Improved catalysts for diesel oxidation are needed. In particular, the industry would benefit from the availability of catalysts with improved ability to convert carbon monoxide and hydrocarbons to carbon dioxide and water at relatively low temperatures (e.g. < 200°C and preferably < 150°C). Also of interest are diesel oxidation catalysts having an improved ability to oxidize NO and a reduced tendency to oxidize ammonia at temperatures below 300°C. Ideally, the catalysts would be able to withstand prolonged exposure to sulfur-containing gases such as S0₂ without excessive poisoning.

SUMMARY OF THE INVENTION

It was surprisingly found that niobium (Nb) and tantalum (Ta) doped cerias having low levels of dopant (1-10 wt.%) are excellent supports for Group 8-1 1 transition metal- containing catalysts, such as diesel oxidation catalysts. In particular, the catalysts have improved ability to convert carbon monoxide and hydrocarbons to carbon dioxide and water at relatively low temperatures compared with similar catalysts in which the Group 8-1 1 transition metal is supported on undoped ceria. Additionally, the catalysts have an improved ability to oxidize NO and a reduced tendency to oxidize ammonia, so the catalyst can work more effectively in conjunction with an SCR catalyst. Catalysts of the invention also demonstrate improved resistance to sulfur poisoning. The invention provides a catalyst comprising a Group 8-1 1 transition metal and a support. In particular, the catalyst comprises 0.1 to 10 wt.% of the Group 8-1 1 transition metal and 90 to 99.9 wt.% of a support. The support comprises at least 90 wt.% of ceria and 0.1 to 10 wt.% of a dopant doped on the ceria, wherein the dopant comprises niobium (Nb) or tantalum (Ta). The reference to a "dopant doped on the ceria" refers to ceria doped with the dopant. Typically, the dopant is niobia or tantala. The support may be a calcined support, such as a calcined support that is obtainable by calcining the support at a temperature within the range of 600°C to 1000°C. The catalyst may be a calcined catalyst, such as a calcined catalyst obtainable by calcining the catalyst at a temperature within the range of 600°C to 1000°C.

The catalyst of the invention is typically suitable for treating an exhaust gas from a diesel engine or is a diesel oxidation catalyst. A further aspect of the invention relates to a process for preparing the catalyst. The process comprises: (a) impregnating ceria with an aqueous solution comprising a water- soluble niobium (Nb) or tantalum (Ta) salt [e.g. to obtain an impregnated ceria], (b) calcining the impregnated ceria to obtain or give a support [e.g. a support comprising at least 90 wt.% of ceria and 0.1 to 10 wt.% of a dopant doped on the ceria, wherein the dopant comprises niobium (Nb) or tantalum (Ta)]; (c) impregnating the support with a solution comprising a Group 8-11 transition metal.

Typically, the process of the invention further comprises: (d) calcining the product [e.g. the catalyst] from step (c) at a temperature within the range of 600°C to 1000°C [e.g. to produce a calcined catalyst, wherein the calcined catalyst comprises 0.1 to 10 wt.% of the Group 8-1 1 transition metal].

The invention also relates to a catalyst for treating an exhaust gas from a diesel engine. The catalyst is obtained or obtainable by the process of the invention.

The invention further provides an exhaust gas after-treatment system. The exhaust gas after-treatment system comprises a catalyst of the invention and optionally an SCR catalyst, a diesel particulate filter, a catalyzed soot filter, an ammonia slip catalyst, or a combination thereof.

A further aspect of the invention relates to a process which comprises oxidizing a gaseous diesel exhaust stream comprising carbon monoxide, unsaturated hydrocarbons, or both, in the presence of a catalyst of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 plots lattice parameter versus wt.% Nb₂0₅ for niobia-doped cerias used to make the inventive diesel oxidation catalysts and reference compositions made according to

WO 2012/004263. Fig. 2 shows NOx conversion versus temperature for niobia-doped cerias used to make the inventive diesel oxidation catalysts and reference compositions made according to WO 2012/004263.

Fig. 3 plots sulfur dioxide uptake for a 2% palladium on niobia-doped ceria catalyst (Cat. B) and comparative Pd/Ce0₂ catalysts (Cats. A and C).

Fig. 4 plots sulfur dioxide release versus temperature for a 2% palladium on niobia- doped ceria catalyst (Cat. B) and comparative Pd/Ce0₂ catalysts (Cats. A and C) where the catalysts were previously exposed to sulfur dioxide.

Fig. 5. provides carbon monoxide conversion versus temperature for a 2% palladium on niobia-doped ceria catalyst (Cat. B) and comparative Pd/Ce0₂ catalysts (Cats. A and C) where the catalysts are either aged at 750°C or aged at 750°C and subjected to cyclic sulfation/desulfation.

Fig. 6 provides propylene conversion versus temperature for a 2% palladium on niobia- doped ceria catalyst (Cat. B) and comparative Pd/Ce0₂ catalysts (Cats. A and C) where the catalysts are either aged at 750°C or aged at 750°C and subjected to cyclic sulfation/desulfation.

Fig. 7 plots % conversion of propylene or ammonia versus temperature for 2% palladium on tantala-doped cerias and a comparative Pd/Ce0₂ catalyst.

Fig. 8 plots % conversion of carbon monoxide versus temperature and shows the impact of tantala content for 2% palladium on tantala-doped cerias and a comparative Pd/Ce0₂ catalyst.

Fig. 9 plots % conversion of propylene versus temperature and shows the impact of tantala content for 2% palladium on tantala-doped cerias and a comparative Pd/Ce0₂ catalyst.

Fig. 10 illustrates the effect of tantala on NO oxidation temperature requirements for 2% palladium on tantala-doped cerias and a comparative Pd/Ce0₂ catalyst.

Fig. 1 1 plots sulfur dioxide uptake for a 2% palladium on tantala-doped ceria catalyst and a comparative Pd/Ce0₂ catalyst.

Fig. 12 plots sulfur dioxide release versus temperature for a 2% palladium on tantala- doped ceria and a comparative Pd/Ce0₂ catalyst where the catalysts were previously exposed to sulfur dioxide.

DETAILED DESCRIPTION OF THE INVENTION

The catalyst of the invention is typically a diesel oxidation catalyst or a NO_x absorber. The catalyst typically comprises a substrate (e.g. a macroscopic substrate), such as a metal plate or a substrate monolith (e.g. cordierite honeycomb). In general, the catalyst composition (e.g. the Group 8 to 11 transition metal and the support) are affixed to the substrate. Such an arrangement is particularly advantageous in heterogeneous catalysis.

Generally, the Group 8 to 11 transition metal is dispersed and/or distributed over the surface of the support.

Transition metals suitable for use are in Groups 8-1 1 of the Periodic Table. Suitable metals include, for example, Fe, Ru, Os, Co, Rh, Ir, Ni, Pd, Pt, Cu, Ag, and Au.

Preferred Group 8 to 11 metals include Pt, Ru, Rh, and Ag, particularly Pt. The catalyst may comprise a mixture or combination of any of the Group 8 to 1 1 transition metals. Particularly preferred are combinations of Pd with one or more of Cu, Ag, Au, Pt, Co, Rh, and Ir, and particularly Pd with Pt.

It may be preferable that the Group 8 to 11 transition metal is a transition metal, more preferably the Group 8 to 11 metal is copper (Cu).

The Group 8 to 1 1 transition metal is preferably a noble metal. More preferably the Group 8 to 1 1 transition metal is a platinum group metal. Even more preferably the Group 8 to 11 transition metal is palladium. The catalyst of the invention comprises 0.1 to 10 wt.%, preferably 1 to 5 wt.%, most preferably 1.5 to 3 wt.%, of the Group 8 to 1 1 transition metal, based on the total weight of the Group 8 to 1 1 transition metal plus support.

The Group 8 to 1 1 transition metal can be applied to the support by any method. For example, an aqueous solution containing a soluble salt of Group 8 to 1 1 transition metal (e.g. palladium(ll) nitrate, palladium(ll) chloride, platinum(ll) chloride, palladium(ll) acetate, rhodium(ll) acetate dimer, or the like) can be applied to give an aqueous suspension that is subsequently dried and calcined. An incipient wetness method can also be used in which the solution is added slowly to the support in an amount similar to the pore volume of the support, followed by drying and calcination. The catalyst of the invention comprises 90 to 99.9 wt.%, preferably 95 to 99 wt.%, of a support, based on the total weight of the Group 8 to 1 1 transition metal plus support.

The support may comprise ceria doped with niobia (Nb₂0₅) or ceria doped with tantala (Ta₂0₅). Preferably, the support and/or catalyst is free or essentially free of vanadium or any oxide thereof. Thus, the support and/or catalyst does not comprise vanadium.

Typically, the support comprises at least 90 wt.% of ceria and 0.1 to 10 wt.% of niobia or tantala doped on the ceria. Preferably, the support comprises 90 to 99.5 wt.% of ceria and 0.5 to 10 wt.% of niobia or tantala. More preferably the support comprises 92 to 99 wt.% of ceria and 1 to 8 wt.% of niobia or tantala. Most preferably the support comprises at least 95 wt.% of ceria, preferably 95 to 99 wt.% of ceria and 1 to 5 wt.% of niobia or tantala. The abovementioned weight percent ranges of the support composition are based on the total weight of the support.

A support comprising niobia doped on ceria is particularly preferred.

The ceria typically has a surface area greater than 50 m²/g, more preferably greater than 100 m²/g (as measured using the BET method). Such suitable high-surface-area cerias are commercially available. Examples include HSA20 ceria from Rhodia, high-surface- area cerium oxides available from MolyCorp, HEFA Rare Earth, NanoOxides or other suppliers, and the like. Suitable high-surface-area cerias can also be synthesized, as taught, for instance, in US 7,094,383; US 5,063, 193; US 4,859,432; and US 4,661 ,330, the teachings of which are incorporated herein by reference, as well as WO 2001/036332 and EP 0444470.

A doped ceria may be identified as having a reduced lattice parameter when compared with non-doped ceria, as the dopant (e.g. Nb or Ta) will usually have a smaller ionic radius when compared with cerium. The niobium or tantalum may be either uniformly distributed within the lattice or be in a higher concentration in a layer at or near the surface of the ceria lattice. Here, the ceria is either formed prior to introduction of the niobium or the ceria and niobia are generated essentially simultaneously, as in a co- precipitation process. For example, the niobium or tantalum is applied to ceria particles. Thus, "doped on the ceria" is distinct from physical mixtures of the oxide and excludes compositions in which the ceria is formed in the presence of a pre-made niobia or tantala particles. Such pre-formed compositions are, for example, described in WO

2012/004263 at Examples 9 and 10. The catalyst of the invention may be free or essentially free of a niobia or tantala physical coating on ceria and/or the catalyst may be free or essentially free of a ceria physical coating on niobia or tantala. In preferred niobia- or tantala-doped cerias, the lattice parameter is reduced significantly compared with that of undoped ceria. Such lattice contraction may indicate that the niobia or tantala has become part of the lattice framework. Preferably, the lattice parameter of the niobia- or tantala-doped ceria is at least 0.02% less, more preferably at least 0.04% less, than that of undoped ceria. It has surprisingly found that such lattice contraction can be observed even when the support is calcined under relatively mild (e.g. 500°C) conditions. As shown in Fig. 1 , lattice contraction is evident with increasing levels of niobia when a niobia-doped ceria is prepared by the impregnation method described in Examples 1-3 below. In contrast, the same degree of lattice contraction is not observed when ceria is formed in the presence of niobia as described in WO 2012/004263 and replicated in Comparative Examples 4-6 below.

The support may be prepared by impregnating ceria with a soluble niobium or tantalum salt, followed by calcination. In one suitable synthetic approach, the doped ceria is made by impregnating ceria with an aqueous solution containing a soluble salt of niobium or tantalum, usually an acetate, nitrate, halide, oxalate, or the like such as niobium(V) chloride, niobium(lll) nitrate, ammonium niobate(V) oxalate, or tantalum(V) chloride. If desired, enough water can be used to form a slurry. Alternatively, the amount of water might be minimized, as in an incipient wetness method. Wet mechanical mixing can also be used (see EP 2368628). In any event, the water is then usually removed by drying, and the product is calcined to give niobia- or tantala-doped ceria. For an example of the impregnation approach, see US 2013/0121902, the teachings of which are incorporated herein by reference. The ceria doped with a dopant can also be made by co-precipitation of ceria and niobia or tantala from aqueous media. In this case, an aqueous solution containing dissolved salts of cerium and niobium or tantalum is combined with aqueous ammonia, ammonium carbonate, or another basic compound. Hydrogen peroxide can be added to further encourage precipitation. The product is isolated, washed, dried, and calcined to give the niobia- or tantala-doped ceria. Because certain niobium sources hydrolyze more rapidly than the corresponding cerium compounds, co-precipitation may produce a support having pockets of niobia interspersed with ceria. For examples of the co-precipitation approach, see US 6,605,264, the teachings of which are incorporated herein by reference, and EP 2368628.

The support and/or catalyst may typically be calcined at a temperature within the range of 600°C to 1000°C, preferably 700°C to 950°C, more preferably 750°C to 900°C.

Calcination can be performed briefly (e.g. less than an hour), or it can be more prolonged (e.g. up to 24 hours).

The support and/or catalyst is preferably calcined in air. If desired, however, the support and/or catalyst can be treated hydrothermally. That is, the support and/or catalyst can be aged in the presence of both heat and moisture. Typical conditions for hydrothermal aging include 500°C to 900°C in the presence of added steam.

Calcination (and/or hydrothermal aging) may encourage migration of a portion of the niobia or tantala to the support surface. The degree of migration can be measured using x-ray photoelectron spectroscopy or other suitable techniques.

Typically, the ceria doped with a dopant has, at its surface (e.g. as measured by x-ray photoelectron spectroscopy), a molar ratio of niobium or tantalum to cerium that is at least doubled by the calcination.

The ceria doped with a dopant generally has, at its surface (e.g. as measured by x-ray photoelectron spectroscopy), a molar ratio of niobium or tantalum to cerium of greater than 0.2, preferably greater than 0.3.

The proportion of niobia or tantala at the surface of the support (e.g. through migration) can be expressed in terms of a distribution quotient, Q, which is given by:

Q^— (E_Surface/Ce_surface) / (Ebulk/Cebulk)- In this expression, E_Surface Ce_Surface is the molar ratio of niobium or tantalum to cerium measured at the surface of the support by x-ray photoelectron spectroscopy, and E uik Ce uik is the molar ratio of niobium or tantalum to cerium in a bulk sample of the support. For the ceria doped with a dopant in the catalyst of the invention, Q is preferably greater than 1.5 and more preferably has a value within the range of 2 to 10.

The catalyst may further comprise an additional support material. The additional support may be a zeolite, a clay, alumina, silica-alumina, zirconia, titania, or a mixture or combination of any two or more thereof. More preferably, the additional support is a zeolite, alumina, silica-alumina, zirconia, titania, or a mixture or combination of any two or more thereof. If the catalyst comprises an additional support material, then the support comprises at least 90 wt.% of ceria and 0.1 to 10 wt.% of a dopant doped on the ceria.

It may be preferable that the support or the catalyst is free or essentially free of zirconia. It may be preferable that the support or the catalyst is free or essentially free of alumina and/or silica-alumina. It may be preferable that the support or the catalyst is free or essentially free of titania. It may be preferable that the support or the catalyst is free or essentially free of a zeolite. It may be preferable that the support or the catalyst is free or essentially free of a clay.

It was surprisingly found that niobia- and tantala-doped cerias having low levels of dopant (1-10 wt.%) are excellent supports for Group 8 tol l transition metal-containing oxidation catalysts, particularly diesel oxidation catalysts (DOCs) and mercury oxidation catalysts. For such embodiments, platinum group metals are particularly preferred, for example Pd and/or Pt. Compared with similar catalysts in which the Group 8 to 1 1 transition metal is supported on undoped ceria, the catalyst of the invention has an improved ability to convert carbon monoxide and hydrocarbons at low temperatures, a reduced tendency to oxidize ammonia, a greater tendency to oxidize NO, and improved resistance to sulfur poisoning.

Preferably, the catalyst of the invention (e.g. a diesel oxidation catalyst [DOC]) absorbs, under diesel oxidation conditions in the presence of sulfur dioxide, at least 10%, more preferably at least 20%, and most preferably at least 50%, less sulfur than a similar catalyst (e.g. supported on undoped ceria). By "diesel oxidation conditions," we mean conditions under which CO or hydrocarbons are combusted in an exhaust system under normal operating conditions, i.e., in the presence of an air mixture comprising NO_x, carbon dioxide, water, and oxygen at temperatures in the range of 100°C to 400°C, preferably 100°C to 300°C. Figs. 3-6, 11 , and 12 illustrate the ability of the catalysts of the invention to absorb less sulfur than a comparable catalyst utilizing undoped ceria as a support. Preferably, the catalyst of the invention (e.g. a DOC) has enhanced ability, under diesel oxidation conditions, to convert carbon monoxide, unsaturated hydrocarbons, or both at temperatures below 250°C, preferably below 200°C, relative to a similar catalyst (e.g. supported on undoped ceria). Preferably, the relatively enhanced ability is retained upon prolonged exposure of the catalyst to sulfur. Figs. 5-9 illustrate these attributes of catalysts of the invention. Preferably, the catalyst of the invention (e.g. a DOC) can convert, under diesel oxidation conditions at temperatures less than 300°C, at least 10% less ammonia, more preferably at least 20% less ammonia, and most preferably at least 50% less ammonia, than a similar catalyst (e.g. supported on undoped ceria). Fig. 7 illustrates this attribute. Preferably, the catalyst of the invention (e.g. a DOC) can convert, under diesel oxidation conditions at temperatures less than 400°C, at least 10% more NO, more preferably at least 20% more NO, most preferably at least 50% more NO, than a similar catalyst (e.g. supported on undoped ceria). Fig. 10 illustrates this attribute. The reference to a "similar catalyst" hereinabove refers to a catalyst that has the same composition except that the ceria is not doped (i.e. undoped ceria).

The invention also provides a catalytic washcoat. The catalytic washcoat is preferably a solution, suspension, or slurry. The catalyst of the invention may be prepared, or is obtainable by, applying the catalytic washcoat onto a substrate.

The catalytic washcoat may include a non-catalytic component, such as a filler, a binder, a stabilizer, a rheology modifier, and/or another additive, including one or more of alumina, silica, non-zeolite silica alumina, titania, zirconia, ceria.

The catalytic washcoat may comprise a pore-forming agent, such as graphite, cellulose, starch, polyacrylate, and/or polyethylene, and the like.

The non-catalytic component and/or pore-forming agent do not necessarily catalyze the desired reaction, but instead improve the catalytic material's effectiveness, for example, by increasing its operating temperature range, increasing contact surface area of the catalyst, increasing adherence of the catalyst to a substrate, etc.

Typically, the catalytic washcoat loading is >0.3 g/in³, such as >1.2 g/in³, >1.5 g/in³, >1.7 g/in³ or >2.00 g/in³.

Generally, the catalytic washcoat loading is < 3.5 g/in³, such as < 2.5 g/in³. It is preferred that the catalytic washcoat is applied to a substrate in a loading of about 0.8 to 1.0 g/in³, such as 1.0 to 1.5 g/in³, or more preferably 1.5 to 2.5 g/in³.

Suitable catalytic washcoats may be a surface coating, a coating that penetrates a portion of a substrate, a coating that permeates a substrate, or some combination thereof. Two of the most common substrate designs are plate and honeycomb.

It is preferred that the substrate (i.e. especially for mobile applications) is a flow-through monolith. The flow-through monolith may have a honeycomb geometry that comprises a plurality of (e.g. multiple adjacent) parallel channels, wherein each channel is open at both ends (i.e. open at an inlet face and open at an outlet face) and each channel extends from the inlet face to the outlet face of the substrate. Such an arrangement results in a high-surface area-to-volume ratio. For certain applications, the honeycomb flow-through monolith preferably has a high cell density, for example about 600 to 800 cells per square inch, and/or an average internal wall thickness of about 0.18 - 0.35 mm, preferably about 0.20 - 0.25 mm.

For certain other applications, the honeycomb flow-through monolith preferably has a low cell density of about 150 - 600 cells per square inch, more preferably about 200 - 400 cells per square inch.

Preferably, the honeycomb monolith is porous. Typically, the substrate is made of cordierite, silicon carbide, silicon nitride, a ceramic, a metal, aluminum nitride, silicon nitride, aluminum titanate, a-alumina, mullite, e.g., acicular mullite, pollucite, a thermet such as AI₂OsZFe, AI2O₃/N 1 or B₄CZFe, or composites comprising segments of any two or more thereof. Preferred materials include cordierite, silicon carbide, and alumina titanate.

Plate-type catalysts have lower pressure drops and are less susceptible to plugging and fouling than the honeycomb types, which is advantageous in high efficiency stationary applications, but plate configurations can be much larger and more expensive. A honeycomb configuration is typically smaller than a plate type, which is an advantage in mobile applications, but has higher pressure drops and plug more easily. In certain embodiments the plate substrate is constructed of metal, preferably corrugated metal. The catalytic washcoat can be disposed on the substrate, such as a flow-through monolith, in a zone. The catalytic washcoat zone may be disposed on the substrate in a front zone, such as a front zone extending from the inlet end (i.e. inlet face) of the substrate to 5 to 60 % of the length of the substrate. Alternatively, the catalytic washcoat zone may be disposed on the substrate in a rear zone, such as a rear zone extending from the outlet end (i.e. outlet face) of the substrate to 5 to 60 % of the length of the substrate.

The substrate may be a wall-flow filter. When the substrate is a wall-flow filter, the catalytic washcoat can be disposed on an inlet side or an outlet side of the wall-flow filter.

Typically, the catalyst is produced or is obtainable by a process comprising the step of: (i) applying a catalytic composition, preferably a catalytic washcoat, to a substrate as a layer (e.g. either before or after applying at least one additional layer of another composition for treating exhaust gas has been applied to the substrate). When there is a plurality of layers on the substrate, including a layer of the catalytic composition or washcoat, the layers are arranged in consecutive layers.

The catalyst of the invention may comprise the catalytic composition or washcoat disposed on the substrate as a first layer and another composition, such as an SCR catalyst, is disposed on the substrate as a second layer. Alternatively, the catalytic composition or washcoat may be disposed on the substrate as a second layer and another composition, such as such as an SCR catalyst, is disposed on the substrate as a first layer.

Typically, the second layer is applied to the substrate (e.g. the inert substrate) as a bottom layer and the first layer is a top layer that is applied over the second layer (i.e. as a consecutive series of layers). In such an arrangement, the exhaust gas penetrates (and hence contacts) the first layer, before contacting the second layer, and

subsequently returns through the first layer to exit the catalyst component.

The first layer may be a first zone disposed on (e.g. an upstream portion of) the substrate and the second layer may be a second zone disposed on the substrate, wherein the second zone is downstream of the first zone.

The catalyst may be obtained or produced by a process that comprises the step of: (i) applying a catalytic composition, preferably as a catalytic washcoat, to a substrate as a first zone, and (ii) applying (e.g. subsequently) at least one additional composition or washcoat for treating an exhaust gas to the substrate as a second zone, wherein at least a portion of the first zone is downstream of the second zone. Alternatively, the catalytic composition or catalytic washcoat can be applied to the substrate in a second zone that is downstream of a first zone containing the additional composition or washcoat.

Examples of additional compositions include SCR catalysts and scavenging components (e.g., for sulfur, water, etc.).

To reduce the amount of space required for an exhaust system, individual exhaust components in certain embodiments are designed to perform more than one function. For example, applying a catalyst to a wall-flow filter substrate instead of a flow-through substrate serves to reduce the overall size of an exhaust treatment system by allowing one substrate to serve two functions, namely catalytically treating undesirable

components in the exhaust gas and mechanically removing soot from the exhaust gas.

Accordingly, the substrate may be a wall-flow filter or a partial filter. Wall-flow filters are similar to flow-through substrates in that they contain a plurality of adjacent, parallel channels. However, the channels of flow-through honeycomb substrates are open at both ends, whereas the channels of wall-flow substrates have one end capped, wherein the capping occurs on opposite ends of adjacent channels in an alternating pattern. Capping alternating ends of channels prevents the gas entering the inlet face of the substrate from flowing straight through the channel and existing. Instead, the exhaust gas enters the front of the substrate and travels into about half of the channels where it is forced through the channel walls prior to entering the second half of the channels and exiting the back face of the substrate.

The substrate wall has a porosity and pore size that is gas permeable, but traps a major portion of the particulate matter, such as soot, from the gas as the gas passes through the wall. Preferred wall-flow substrates are high efficiency filters. Wall flow filters for use with the present invention preferably have an efficiency of least 70%, at least about 75%, at least about 80%, or at least about 90%. In certain embodiments, the efficiency will be from about 75 to about 99%, about 75 to about 90%, about 80 to about 90%, or about 85 to about 95%. Here, efficiency is relative to soot and other similarly sized particles and to particulate concentrations typically found in conventional diesel exhaust gas. For example, particulates in diesel exhaust can range in size from 0.05 microns to 2.5 microns. Thus, the efficiency can be based on this range or a sub-range, such as 0.1 to 0.25 microns, 0.25 to 1.25 microns, or 1.25 to 2.5 microns. Porosity is a measure of the percentage of void space in a porous substrate and is related to backpressure in an exhaust system: generally, the lower the porosity, the higher the backpressure. Preferably, the porous substrate has a porosity of about 30 to about 80%, for example about 40 to about 75%, about 40 to about 65%, or from about 50 to about 60%.

The pore interconnectivity, measured as a percentage of the substrate's total void volume, is the degree to which pores, void, and/or channels, are joined to form continuous paths through a porous substrate, i.e., from the inlet face to the outlet face. In contrast to pore interconnectivity is the sum of closed pore volume and the volume of pores that have a conduit to only one of the surfaces of the substrate. Preferably, the porous substrate has a pore interconnectivity volume of at least about 30%, more preferably at least about 40%. The mean pore size of the porous substrate is also important for filtration. Mean pore size can be determined by any acceptable means, including by mercury porosimetry. The mean pore size of the porous substrate should be of a high enough value to promote low backpressure, while providing an adequate efficiency by either the substrate per se, by promotion of a soot cake layer on the surface of the substrate, or combination of both. Preferred porous substrates have a mean pore size of about 10 to about 40 μηι, for example about 20 to about 30 μηι, about 10 to about 25 μηι, about 10 to about 20 μηι, about 20 to about 25 μηι, about 10 to about 15 μηι, and about 15 to about 20 μηι.

In general, the production of an extruded solid body containing the catalyst involves blending the catalyst, a binder, an optional organic viscosity-enhancing compound into an homogeneous paste which is then added to a binder/matrix component or a precursor thereof and optionally one or more of stabilized ceria, and inorganic fibers. The blend is compacted in a mixing or kneading apparatus or an extruder. The mixtures have organic additives such as binders, pore formers, plasticizers, surfactants, lubricants, dispersants as processing aids to enhance wetting and therefore produce a uniform batch. The resulting plastic material is then molded, in particular using an extrusion press or an extruder including an extrusion die, and the resulting moldings are dried and calcined. The organic additives are "burnt out" during calcinations of the extruded solid body. A separate catalyst may also be washcoated or otherwise applied to the extruded solid body as one or more sub-layers that reside on the surface or penetrate wholly or partly into the extruded solid catalytic body. Extruded solid bodies containing catalysts according to the present invention generally comprise a unitary structure in the form of a honeycomb having uniform-sized and parallel channels extending from a first end to a second end thereof. Channel walls defining the channels are porous. Typically, an external "skin" surrounds a plurality of the channels of the extruded solid body. The extruded solid body can be formed from any desired cross section, such as circular, square or oval. Individual channels in the plurality of channels can be square, triangular, hexagonal, circular etc. Channels at a first, upstream end can be blocked, e.g. with a suitable ceramic cement, and channels not blocked at the first, upstream end can also be blocked at a second, downstream end to form a wall-flow filter. Typically, the arrangement of the blocked channels at the first, upstream end resembles a checker-board with a similar arrangement of blocked and open downstream channel ends.

The binder/matrix component is preferably selected from the group consisting of cordierite, nitrides, carbides, borides, intermetallics, lithium aluminosilicate, a spinel, an optionally doped alumina, a silica source, titania, zirconia, titania-zirconia, zircon and mixtures of any two or more thereof. The paste can optionally contain reinforcing inorganic fibers selected from the group consisting of carbon fibers, glass fibers, metal fibers, boron fibers, alumina fibers, silica fibers, silica-alumina fibers, silicon carbide fibers, potassium titanate fibers, aluminum borate fibers and ceramic fibers.

The alumina binder/matrix component is preferably gamma alumina, but can be any other transition alumina, i.e., alpha alumina, beta alumina, chi alumina, eta alumina, rho alumina, kappa alumina, theta alumina, delta alumina, lanthanum beta alumina and mixtures of any two or more such transition aluminas. It is preferred that the alumina is doped with at least one non-aluminum element to increase the thermal stability of the alumina. Suitable alumina dopants include silicon, zirconium, barium, lanthanides and mixtures of any two or more thereof. Suitable lanthanide dopants include La, Ce, Nd, Pr, Gd and mixtures of any two or more thereof.

Sources of silica can include a silica sol, quartz, fused or amorphous silica, sodium silicate, an amorphous aluminosilicate, an alkoxysilane, a silicone resin binder such as methylphenyl silicone resin, a clay, talc or a mixture of any two or more thereof. Of this list, the silica can be Si0₂ as such, feldspar, mullite, silica-alumina, silica-magnesia, silica-zirconia, silica-thoria, silica-berylia, silica-titania, ternary silica-alumina-zirconia, ternary silica-alumina-magnesia, ternary-silica-magnesia-zirconia, ternary silica-alumina- thoria and mixtures of any two or more thereof. Preferably, the catalyst is dispersed throughout, and preferably evenly throughout, the entire extruded catalyst body.

Where any of the above extruded solid bodies are made into a wall-flow filter, the porosity of the wall-flow filter can be from 30-80%, such as from 40-70%. Porosity and pore volume and pore radius can be measured e.g. using mercury intrusion porosimetry.

The catalyst of the invention can also promote the oxidation of ammonia. Thus, the catalyst can be formulated to favor the oxidation of ammonia with oxygen, particularly at concentrations of ammonia typically encountered downstream of an SCR catalyst (e.g., ammonia oxidation (AMOX) catalyst, such as an ammonia slip catalyst (ASC)).

The catalyst may be disposed as a bottom layer under an SCR over-layer. Preferably, the SCR catalyst is a metal promoted zeolite (e.g., Cu or Fe promoted aluminosilicate zeolite having a framework selected from BEA, CHA, AEI, AFX, ZSM-5, ZSM-34, MFI, KFI, LEV, or the like) or metal promoted vanadia (e.g., Fe or W promoted vanadia).

The catalyst may be an ammonia slip catalyst, which is disposed downstream of a selective catalytic reduction (SCR) catalyst. The ammonia slip catalyst oxidizes at least a portion of any nitrogenous reductant that is not consumed by the selective catalytic reduction process. For example, the ammonia slip catalyst may be disposed on the outlet side of a wall flow filter and an SCR catalyst may be disposed on the upstream side of a filter. Alternatively, the ammonia slip catalyst may be disposed on the downstream end of a flow-through substrate and an SCR catalyst may be disposed on the upstream end of the flow-through substrate. In another embodiment, the ammonia slip catalyst and the SCR catalyst each have a separate substrate (i.e. they are each disposed on a separate bricks) within the exhaust system. These separate substrates or bricks can be adjacent to, and in contact with, each other or separated by a specific distance, provided that they are in fluid communication with each other and provided that the SCR catalyst substrate or brick is disposed upstream of the ammonia slip catalyst substrate or brick.

The catalytic oxidation process is typically performed at a temperature of at least 100 °C, such as from about 150 °C to about 750 °C, particularly from about 175 to about 550 °C (e.g. 175 to 400 °C). The catalytic oxidation process may be performed at a temperature of 450 to 900 °C, preferably 500 to 750 °C, 500 to 650 °C, 450 to 550 °C, or 650 to 850 °C. Embodiments utilizing temperatures greater than 450 °C are particularly useful for treating exhaust gases from a heavy and light duty diesel engine that is equipped with an exhaust system comprising (optionally catalyzed) diesel particulate filters which are regenerated actively, e.g. by injecting hydrocarbon into the exhaust system upstream of the filter, wherein the zeolite catalyst for use in the present invention is located downstream of the filter.

Typically, all or at least a portion of the nitrogen-based reductant, particularly NH₃, for consumption in the SCR process can be supplied by a catalyst of the present invention functioning as a NO_x adsorber catalyst (NAC), a lean NO_x trap (LNT), or a NO_x storage/reduction catalyst (NSRC), disposed upstream of the SCR catalyst, e.g., a SCR catalyst of the present invention disposed on a wall-flow filter. NAC components useful in the present invention include a combination of a basic material (such as alkali metal, alkaline earth metal or a rare earth metal, including oxides of alkali metals, oxides of alkaline earth metals, and combinations thereof), and a precious metal (such as platinum), and optionally a reduction catalyst component, such as rhodium. Specific types of basic material useful in the NAC include cesium oxide, potassium oxide, magnesium oxide, sodium oxide, calcium oxide, strontium oxide, barium oxide, and combinations thereof. The precious metal is preferably present at about 10 to about 200 g/ft³, such as 20 to 60 g/ft³. Alternatively, the precious metal of the catalyst is

characterized by the average concentration which may be from about 40 to about 100 grams/ft³.

Under certain conditions, during the periodically rich regeneration events, NH₃ may be generated over a NO_x adsorber catalyst. The SCR catalyst downstream of the NO_x adsorber catalyst may improve the overall system NO_x reduction efficiency. In the combined system, the SCR catalyst is capable of storing the released NH₃ from the NAC catalyst during rich regeneration events and utilizes the stored NH₃ to selectively reduce some or all of the NO_x that slips through the NAC catalyst during the normal lean operation conditions.

The method for treating exhaust gas as described herein can be performed on an exhaust gas derived from a combustion process, such as from an internal combustion engine (whether mobile or stationary), a gas turbine and coal or oil fired power plants. The method may also be used to treat gas from industrial processes such as refining, from refinery heaters and boilers, furnaces, the chemical processing industry, coke ovens, municipal waste plants and incinerators, etc. In a particular embodiment, the method is used for treating exhaust gas from a vehicular lean burn internal combustion engine, such as a diesel engine, a lean-burn gasoline engine or an engine powered by liquid petroleum gas or natural gas.

In certain aspects, the invention is a system for treating exhaust gas generated by combustion process, such as from an internal combustion engine (whether mobile or stationary), a gas turbine, coal or oil fired power plants, and the like. Such systems include a catalytic article comprising the catalyst described herein and at least one additional component for treating the exhaust gas, wherein the catalytic article and at least one additional component are designed to function as a coherent unit. The system can comprise a diesel oxidation catalyst, NO_x adsorber, and/or a mercury oxidation catalyst as described herein and, optionally, one or more of an SCR catalyst, a diesel particulate filter, a catalyzed soot filter, an ammonia slip catalyst, or a combination thereof. Such systems optionally include one or more section of conduit for channeling the exhaust gas from the engine to the end of the after-treatment system, and/or recirculating at least a portion of partially treated exhaust gas within the after-treatment system.

The system may comprise an oxidation catalyst (e.g. , a diesel oxidation catalyst (DOC)) for oxidizing nitrogen monoxide in the exhaust gas to nitrogen dioxide can be located upstream of a point of metering the nitrogenous reductant into the exhaust gas. The oxidation catalyst may be adapted to yield a gas stream entering the SCR zeolite catalyst having a ratio of NO to N0₂ of from about 4: 1 to about 1 :3 by volume, e.g. at an exhaust gas temperature at oxidation catalyst inlet of 250 °C to 450 °C. The invention includes an oxidation process. The process may comprise oxidizing a gaseous diesel exhaust stream comprising carbon monoxide, unsaturated hydrocarbons, or both, in the presence of a diesel oxidation catalyst as described herein. The process is preferably performed under "diesel oxidation conditions" as described earlier, i.e. , in an exhaust system under normal operating conditions, where normal operating conditions include the presence of an air mixture comprising NOx, carbon dioxide, water, and oxygen at temperatures in the range of 100°C to 400°C, preferably 100°C to 300°C.

The invention may include a mercury oxidation process. The invention may include a ΝΟχ adsorption process, particularly when used in combination with a downstream SCR process. The invention further relates to a catalyst prepared by a particular process. The process comprises four steps. First, ceria is impregnated with an aqueous solution comprising a water-soluble niobium or tantalum salt. The water-soluble niobium or tantalum salt is typically an acetate, nitrate, halide, oxalate, or the like such as niobium(V) chloride, niobium(lll) nitrate, ammonium niobate(V) oxalate, or tantalum(V) chloride. Second, the impregnated ceria is calcined at a temperature within the range of 600°C to 1000°C, preferably 700°C to 950°C, more preferably 750°C to 900°C, to give a support. The support comprises at least 90 wt.% of ceria and 0.1 to 10 wt.% of niobia or tantala doped on the ceria. Preferably, the support comprises 91 to 99.5 wt.% of ceria and 0.5 to 9 wt.% of niobia or tantala. Third, the niobia- or tantala-doped ceria support is

impregnated with a solution comprising a Group 8-1 1 transition metal. Finally, the product is calcined at a temperature within the range of 600°C to 1000°C to produce the catalyst. The catalyst comprises 0.1 to 10 wt.% of the Group 8-1 1 transition metal.

DEFINITIONS

As used herein, the term "catalyst" means any substance that modifies the rate of a chemical reaction without being consumed by the reaction, including but not limited to adsorbents.

As used herein, the term "support" with respect to a Group 8-11 metal catalyst, means a solid material, typically having a high surface area, to which the catalyst is affixed (e.g., by impregnation).

By "doped," we mean that that niobium or tantalum is integrated into the ceria lattice structure, typically as a mixed oxide (e.g., niobia and ceria or tantala and ceria).

As used herein, the term "essentially free" of a component, typically used with reference to the support or the catalyst, refers to less than 0.1 weight percent, or even less than 0.01 weight percent of the component. More preferably, the support or the catalyst does not comprise the component.

As used herein, the term "consecutive" with respect to catalyst layers on a substrate means that each layer is contact with its adjacent layer(s) and that the catalyst layers as a whole are arranged one on top of another on the substrate.

As used herein the terms "first layer" and "second layer" are used to describe the relative positions of catalyst layers in the catalyst article with respect to the normal direction of exhaust gas flow-through, past, and/or over the catalyst article. Under normal exhaust gas flow conditions, exhaust gas contacts the first layer prior to contacting the second layer. EXAMPLES

The following examples merely illustrate the invention; the skilled person will recognize many variations that are within the spirit of the invention and scope of the claims.

EXAMPLE 1

Preparation of Niobia-Doped Ceria (3.2 wt.% Nb₂0₅)

Ammonium niobate(V) oxalate (21 % Nb, 1.06 g, 2.4 mmol Nb, equivalent to 0.32 g Nb₂0₅) is dissolved in water (6 mL) with stirring and gentle warming. High-surface-area ceria (9.68 g) is added, and the mixture is stirred. The pore volume of the ceria is slightly exceeded so the sample is stirred and warmed to dryness on a hotplate. The sample is further dried in an oven at 105°C. Portions of the sample are calcined ("fired") at 500°C for 2 h or at 800°C for 4 h (10°C/min ramp rate).

EXAMPLE 2

Preparation of Niobia-Doped Ceria (5.0 wt.% Nb₂0₅)

The procedure of Example 1 is generally followed using ammonium niobate(V) oxalate (3.33 g, 7.52 mmol Nb, equivalent to 1.0 g of Nb₂0₅), ceria (19 g), and water (12 mL).

EXAMPLE 3

Preparation of Niobia-Doped Ceria (8.6 wt.% Nb₂0₅)

The procedure of Example 1 is generally followed using ammonium niobate(V) oxalate (2.86 g, 6.47 mmol Nb, equivalent to 0.86 g of Nb₂0₅), ceria (9.14 g), and water (6 mL).

In Comparative Examples 4-6, the procedure of WO 2012/004263 is generally followed to produce catalysts in which ceria is formed in the presence of niobia. Based on the preparation method, the catalysts should have a core of niobia surrounded by a ceria shell.

COMPARATIVE EXAMPLE 4

Preparation of a niobia sol

Ethanol (100 mL) is dried for 20 h over 3A molecular sieves (18 g). The sieves are pre- fired at 400°C for 1 h to remove water. A round-bottom flask is charged with anhydrous ethanol (40 mL) and a small magnetic stir bar. The ethanol is stirred, and niobium(V) chloride (10 g, 0.037 mol) is added using a plastic spatula. When the addition is complete, a pale yellow solution results. More ethanol (21 mL) is added, stirring is discontinued, and the flask is stoppered and allowed to stand for 2 h. The stopper is replaced with a reflux condenser, and the solution is heated to about 70°C for 1 h. After heating, the solution loses color, and it is allowed to stand overnight at room

temperature.

Concentrated (35%) ammonia (25.1 g) is magnetically stirred in a beaker. The niobium(V) chloride/ethanol solution prepared above is poured into the stirred ammonia simultaneously with water (76 ml_). A white precipitate occurs immediately, and the mixture exotherms. After stirring for 0.5 h, the mixture is filtered, and the solids are washed with water until the thermal conductivity of the filtrate is about 1 mS. The mixture is slow to filter and wash.

The precipitate is stirred in nitric acid (81 ml_ of 1 M aq. HN0₃) for 4 days. A white suspension of Nb₂0₅ is obtained (pH: 0.55; yield of suspension: 122.6 g). Assuming no losses, the niobia sol contains 0.037 mol Nb (0.0001508 mol Nb₂/g). Preparation of ceria-encapsulated niobia

A beaker is charged with water (50 ml_), a magnetic stir bar, and a pH probe. The pH is adjusted to about 9.0 with a couple of drops of ammonia solution (prepared by diluting 24 ml_ of concentrated (35%) ammonia to 100 ml_). Separately, cerium(lll) nitrate hexahydrate (24.4 g, 0.0562 mol, equivalent to 9.68 g Ce0₂) is dissolved in water (180 ml_). A portion of the niobia sol (7.95 g, equivalent to 0.32 g Nb₂0₅, 0.0012 mol Nb₂) is added, followed by 30% hydrogen peroxide (6.37 g, 0.0562 mol). Upon addition of the peroxide, the suspension turns yellow. The suspension is pumped (16 mL/min) into the well-stirred ammonia solution prepared earlier, and additional ammonia solution is added dropwise by hand to maintain the pH at about 9.0. When the addition is complete, the mixture is stirred for 0.5 h and is then filtered. The resulting yellow/orange precipitate is washed with water (3 x 500 ml_) and then dried (105°C) to give a solid product (10.4 g). Portions of the sample are calcined at 500°C for 2 h or at 800°C for 4 h (10°C/min ramp rate). The finished product contains 3.2 wt.% Nb₂0₅.

COMPARATIVE EXAMPLE 5

The procedure of Comparative Example 4 is generally followed using niobia sol (12.47 g, equivalent to 0.5 g Nb₂0₅, 0.00188 mol Nb₂), cerium(lll) nitrate hexahydrate (23.97 g, 0.0552 mol, equivalent to 9.5 g Ce0₂), the ammonia solution, and 30% hydrogen peroxide (6.26 g, 0.0552 mol). Yield: 10.47 g. Portions of the sample are calcined at 500°C for 2 h or at 800°C for 4 h (10°C/min ramp rate). The finished product contains 5.0 wt.% Nb₂0₅.

COMPARATIVE EXAMPLE 6

The procedure of Comparative Example 4 is generally followed using niobia sol (21.44 g, equivalent to 0.86 g Nb₂0₅, 0.00324 mol Nb₂), cerium(lll) nitrate hexahydrate (23.06 g, 0.0531 mol, equivalent to 9.14 g Ce0₂), the ammonia solution, and 30% hydrogen peroxide (6.02 g, 0.0531 mol). Yield: 10.34 g. Portions of the sample are calcined at 500°C for 2 h or at 800°C for 4 h (10°C/min ramp rate). The finished product contains 8.6 wt.% Nb₂0₅.

Determination of Lattice Parameter

A Bruker AXS D8 Advance™ X-ray diffractometer with a 90 position sample changer is used. Lattice parameter (in Angstroms) is measured by Reitveld analysis (L Vol-IB method) using a complete-powder, diffraction-pattern fitting technique.

IMH3-SCR Activity Test Conditions

Powder samples of catalysts are obtained by pelletizing the original samples, crushing the pellets, and then passing the resulting powder through a 255-350 μηι sieve. The sieved powders are loaded into a synthetic catalyst activity test (SCAT) reactor and tested using the following synthetic diesel exhaust gas mixture (at inlet) including ammonia as the reductant: 500 ppm NO, 500 ppm NH₃, 9% 0₂, 5% C0₂, 5% H₂0, 300 ppm CO, balance N₂ at a space velocity of 30,000 h^"1.

Samples are heated gradually from 150°C to 550°C at 5°C/min, and the composition of the off-gases is analyzed using FTIR spectroscopy to determine the % conversion of NOx gases. Results of testing the catalysts from Examples 2 and 3 and Comparative Examples 5 and 6 appear in Fig. 2.

Catalyst B: 2% Pd on Niobia-Doped Ceria

A solution is prepared by dissolving ammonium niobium oxalate (0.66 g, 21 wt.% Nb, 1.5 mmol) in water 912 mL). This solution is used to impregnate high-surface-area ceria (19.8 g). The mixture is dried, calcined at 500°C, and further calcined at 750°C for 2 h to give a product that contains 1 wt.% Nb₂0₅. Thereafter, the niobia-doped ceria is combined with aqueous palladium nitrate (2.6 g of 15% aq. solution, 3.7 mmol), and the mixture is dried at 100°C and calcined at 500°C. The catalyst, which contains 2.0 wt.% of Pd, is further aged at 750°C for 10 h prior to use. Comparative Catalyst A: 2% Pd on Ceria

Ceria (20 g) is combined with aqueous palladium nitrate (2.6 g of 15 % aq. solution, 3.7 mmol), and the mixture is dried at 100°C and calcined at 500°C. The catalyst, which contains 2.0 wt.% of Pd, is further aged at 750°C for 10 h prior to use.

Comparative Catalyst C: 2% Pd on Ceria

Ceria (20 g) is calcined at 750°C, then combined with aqueous palladium nitrate, dried, and calcined as described in the preparation of Comparative Catalyst A. The catalyst, which contains 2.0 wt.% of Pd, is further aged at 750°C for 10 h prior to use.

Sulfur Dioxide Adsorption and Temperature-Programmed Oxidation

For the sulfur dioxide adsorption step, Catalyst B and Comparative Catalysts A and C are exposed to sulfur dioxide (60 ppm, 9 mg S/g) for 30 minutes at 300°C under 10% oxygen and the S0₂ uptake (in ppm) is measured. Results appear in Figs. 3 and 1 1. For temperature-programmed oxidation, the sulfur-treated catalysts are heated from room temperature to 1000°C at 10°C/minute under 10% oxygen, and the concentration of sulfur dioxide liberated is measured as a function of temperature. Results appear in Figs. 4 and 12.

Cyclic Sulfation-Desulfation Procedure for Catalyst Conditioning

Catalyst B and comparative Catalysts A and C are subjected to six cycles of sulfation and desulfation to investigate the impact of sulfur poisoning on a catalyst's ability to oxidize hydrocarbons and carbon monoxide.

Sulfation is performed by exposing the catalyst to a mixture of propylene (300 ppm), NO (200 ppm), 0₂ (10%), C0₂ (4.5%), water (4.5%), and S0₂ (60 ppm) for 20 minutes at 300°C (a total of about 9 mg S/g catalyst). The catalyst is then desulfated by introducing propylene (7500 ppm) and CO (5000) ppm into the gaseous mixture. After 4 min, an exotherm of about 518°C (to 818°C) is calculated.

The sulfation-desulfation cycle is then repeated five more times (total of about 54 mg S/g catalyst). The ability of the treated catalyst to oxidize carbon monoxide or propylene is then tested as described below. Diesel Oxidation Test Conditions

Powder samples of catalysts A-C that were either aged at 750°C or aged and then subjected to cyclic sulfation-desulfation are obtained by pelletizing the original samples, crushing the pellets, and then passing the resulting powder through a 255-350 μηι sieve. The sieved powders are loaded into a synthetic catalyst activity test (SCAT) reactor and tested for their ability to oxidize CO or propylene using the following synthetic diesel exhaust gas mixtures (at inlet) at a space velocity of 30,000 h^"1 : 1500 ppm CO, 150 ppm NOx, 40 ppm propylene, 16 ppm toluene, 30 ppm decane, 40 ppm methane, 4.5% C0₂, 4.5% H₂0, 12% 0₂, balance N₂.

Samples are heated gradually from 100°C to 300°C at 5°C/min, and the composition of the off-gases is analyzed using FTIR spectroscopy to determine the % conversion of carbon monoxide or propylene. Results appear in Figs. 5 and 6. Preparation of Pd on Tantala-Doped Ceria Catalysts

Preparation of tantala-doped ceria supports (1%, 5%, and 10% Ta₂0₅)

Ta Oxalate Colloid

Tantalum(V) chloride (5.0 g, 0.014 mol) is dissolved in concentrated hydrochloric acid (25 ml_, about 0.3 mol), and then water (10 ml_) is added. The solution is cooled to below 10°C in an ice bath. Concentrated ammonia (25 ml_) is diluted to 50 ml_ and added dropwise cautiously to the acid solution not allowing temperature to go above 20°C until the pH reaches 7.5. Ammonia addition is then stopped. The resulting precipitate is recovered by filtration and washed well with water.

Separately, oxalic acid dihydrate (3.53 g, 0.028 mol) is dissolved in water (15 ml_) with gentle warming. The temperature is kept below 70°C, and the precipitated washed tantalum oxide is added slowly. The mixture is stirred with gentle heating for about 1 h, and then stirred overnight without further heating. A stable colloid is obtained.

Weight of prepared sol: 38.2 g. Assuming no losses, the sol should contain about 3 g of Ta₂0₅.

1 wt.% Ta_?Os on Ceria:

A portion of the Ta oxalate colloid prepared as described above (2.55 g, equiv. to 0.2 g Ta₂0₅) is weighed into a small beaker and water is added to give 12 g total. High- surface-area ceria (19.8 g) is added and the mixture is thoroughly stirred. The sample is oven dried at 105°C, then calcined at 500°C in air for 2 h (ramp rate 10°C/min). 5 wt.% Ta?Os on Ceria:

A portion of the Ta oxalate colloid (12.73 g, equiv. to 1.0 g Ta₂0₅) is weighed into a small beaker and water is added to give 12 g total. Ceria (19.0 g) is added and the mixture is thoroughly stirred. The sample is oven dried at 105°C, then calcined at 500°C in air for 2 h.

10 wt.% Ta?Os on Ceria:

A portion of the Ta oxalate colloid (12.73 g, equiv. to 1.0 g Ta₂0₅) is weighed into a small beaker and water is added to give 12 g total. Ceria (9.0 g) is added and the mixture is thoroughly stirred. The sample is oven dried at 105°C, then calcined at 500°C in air for 2 h.

Impregnation of tantala-doped ceria with 2% Pd

Palladium(ll) nitrate solution (1.98 g, 15.14% Pd, equiv. to 0.3 g Pd) is weighed into a small beaker and diluted with water (7.0 mL). A sample of the tantala-doped ceria prepared as described above (1.0 wt% Ta₂0₅/ceria, 14.7 g) is added, and the mixture is thoroughly stirred before drying at 105°C. The sample is calcined at 500°C in air for 2 h (ramp rate 10°C/min). An identical procedure is used to prepare 2% palladium on the cerias doped with 5 and 10 wt.% Ta₂0₅/ceria.

The catalysts are calcined at 750°C prior to testing as diesel oxidation catalysts. Results appear in Figs. 7-10.

The palladium on ceria and palladium on tantala-doped ceria catalysts are investigated for propylene and ammonia oxidation in the SCAT reactor under the following conditions: 335 ppm propylene, 50 ppm NH₃, 8% C0₂, 5% H₂0. Temperature: 150°C to 400°C (5°C/min ramp rate). NO oxidation is measured as described earlier in the diesel oxidation test.

RESULTS

Lattice parameter (in Angstroms) for each of the materials prepared in Examples 1-3 and Comparative Examples 4-6 is determined as described above. Fig. 1 shows that niobia- doped cerias useful as supports for the inventive diesel oxidation catalysts (prepared in Examples 1-3) undergo substantial lattice contraction with increasing levels of niobia.

This is evidenced by the large negative slope in the best-fit line. The results suggest that these materials incorporate niobium into the lattice framework. In contrast, when the procedure of WO 2012/004263 is used to prepare catalysts having the same Nb₂0₅ content (Comparative Examples 4-6), little or no lattice contraction is evident. This demonstrates that the preparation procedure of the '263 publication gives a different product from the ones prepared in Examples 1-3, even if the Nb₂0₅ content is the same. The result is sensible because the procedure of the '263 publication first prepares niobia, then forms ceria in the presence of the niobia. In contrast, niobia is doped to pre-formed ceria in Examples 1-3. Further evidence that the niobia-doped cerias of Examples 1-3 differ from the products of Comparative Examples 4-6 appears in Fig. 2. Fig. 2 plots NOx conversion versus temperature for niobia-doped cerias and for the reference compositions made according to WO 2012/004263. As shown in the figure, the niobia-doped cerias convert a much higher percentage of NO_x at temperatures in the range of 150°C to 550°C when the niobia content is 5.0 wt.% or 8.6 wt.%.

Because the compositions of Examples 1-3 differ from those made in Comparative Examples 4-6 (as evidenced by both the lattice contraction measurements and by the NOx reduction performance of these materials), any diesel oxidation catalyst made by supporting a Group 8-1 1 transition metal on the niobia-doped cerias must also differ.

Fig. 3 plots sulfur dioxide uptake for a 2% palladium on niobia-doped ceria catalyst (Cat. B) and comparative Pd/Ce0₂ catalysts (Cats. A and C). Procedures for making

Catalysts A-C are provided above. Each catalyst is sulfated at 300°C using 60 ppm S0₂ and 10% 0₂ in N₂ to 42 mg S per g catalyst. As shown in the figure, comparative Catalysts A and C adsorb S0₂ almost quantitatively for a much longer time period compared with Catalyst B. Calcination of the support aggravates sulfur uptake (A v. C), while niobia doping dramatically reduces it. Catalyst B is much less inclined to adsorb S0₂ and retreats from the maximum level within minutes in this experiment.

After the sulfur dioxide uptake experiment (Fig. 3), the same catalysts are subjected to temperature-programmed oxidation (TPO). The sulfur dioxide feed is discontinued, and the temperature is gradually increased. The concentration of S0₂ released from the catalyst is monitored. As shown in Fig. 4, all of the catalysts release S0₂ at about the same temperature, but Catalyst B releases much less S0₂. This confirms that the Pd on niobia-doped ceria (Cat. B) adsorbed less S0₂ than the comparative Pd/Ce0₂ catalysts during the uptake phase. Fig. 5 provides carbon monoxide conversion versus temperature for a 2% palladium on niobia-doped ceria catalyst (Cat. B) and comparative Pd/Ce0₂ catalysts (Cats. A and C) where the catalysts are either aged at 750°C or aged at 750°C and subjected to cyclic sulfation/desulfation. As the results with aged catalysts show, doping with niobia allows the catalyst to oxidize CO more efficiently at temperatures below 200°C and reduces the temperature at which conversion becomes nearly quantitative. Calcination of ceria alone (C versus A) improves the oxidation efficiency, but not as much as niobia doping.

Repeated exposure to sulfur poisoning (by cyclic sulfation-desulfation) reduces the oxidation efficiency of all three catalysts. However, the catalyst supported on niobia- doped ceria retains more of its efficacy. Compare the results at 210°C, where CO conversion with Cat. B is about 60% versus about 30% or less with Cat. A or C.

Moreover, any advantage of Cat. C versus Cat. A gained by calcining the ceria appears to be lost upon sulfur treatment. In Fig. 6 propylene conversion versus temperature is compared for Catalysts A-C in the same diesel oxidation experiment used to generate Fig. 5. The trends are generally the same as those observed for CO oxidation. Pd supported on niobia-doped ceria (Cat. B) oxidizes propylene more efficiently at temperatures below 200°C and achieves propylene "light off" (defined as the temperature at which 50% conversion is achieved) at a lower temperature. Upon sulfur exposure, Cat. B demonstrates better propylene conversion versus Cat. A or C at temperatures below about 250°C.

Tantala-doped cerias are effective supports for Group 8-1 1 transition metal catalysts for diesel oxidation. A valuable catalyst will have the ability to oxidize hydrocarbons and NO while also inhibiting ammonia oxidation. This allows the oxidation and SCR catalysts to work concurrently. Fig. 7 plots % conversion of propylene or ammonia versus temperature for 2% palladium on tantala-doped cerias and a comparative Pd/Ce0₂ catalyst. As shown in the figure, doping the ceria with as little as 1 wt.% Ta₂0₅ reduces the propylene light off temperature and inhibits ammonia conversion. Although propylene light off appears best below 5 wt.% Ta₂0₅, more tantala is better for pushing ammonia conversion to higher temperatures.

Fig. 8 plots % conversion of carbon monoxide versus temperature and shows the impact of tantala content for 2% palladium on tantala-doped cerias and a comparative Pd/Ce0₂ catalyst. The figure shows the significant improvement in CO oxidation activity for the tatala-doped catalyst, particularly at 10 wt.% Ta₂0₅. The same trend seen in Fig. 8 is evident in Fig. 9, which shows propylene conversion results with the same catalysts. Propylene light off temperature is reduced dramatically by doping the ceria support with 10% tantala. Fig. 10 shows the effect of tantala on NO oxidation temperature requirements for 2% palladium on tantala-doped cerias and a comparative Pd/Ce0₂ catalyst. The shift of the curves to the left (lower temperature) upon inclusion of as little as 1 wt.% tantala indicates more facile NO oxidation when Ta-doped cerias are used to support the Group 8-1 1 transition metal.

Oxidation catalysts utilizing tantala-doped ceria supports have a reduced tendency to store sulfur compared with Pd/Ce0₂. As shown in Fig. 1 1 , sulfur dioxide uptake for a 2% palladium on tantala-doped ceria catalyst stops adsorbing added S0₂ more quickly than the comparative catalyst without tantala doping of the support. The results in Fig. 12 confirm that the Pd on Ta₂0₅-doped ceria is less inclined to store sulfur. Temperature- programmed oxidation results in a reduced amount of sulfur dioxide release from the catalyst supported on tantala-doped ceria following sulfur dioxide exposure.

The preceding examples are intended only as illustrations; the following claims define the scope of the invention.

Claims

1. A catalyst for treating an exhaust gas from a diesel engine comprising:

(a) 0.1 to 10 wt.% of a Group 8 to 11 transition metal; and

(b) 90 to 99.9 wt.% of a support;

wherein the support comprises at least 90 wt.% of ceria and 0.1 to 10 wt.% of a dopant doped on the ceria, wherein the dopant comprises niobium (Nb) or tantalum (Ta).

2. The catalyst of claim 1 , wherein the dopant is niobia or tantala.

3. The catalyst of claim 1 or claim 2 wherein the Group 8-11 transition metal is

palladium.

4. The catalyst of any one of the preceding claims comprising 1 to 5 wt.% of the Group 8-1 1 transition metal.

5. The catalyst of any one of the preceding claims wherein the support comprises at least 95 wt.% of ceria.

6. The catalyst of any one of the preceding claims wherein the support comprises ceria having a surface area greater than 100 m²/g.

7. The catalyst of any one of the preceding claims wherein the support comprises 1 to 5 wt.% of the dopant.

8. The catalyst of any one of the preceding claims wherein the support is a calcined support and optionally wherein the calcined support is obtainable by calcining at a temperature within the range of 600°C to 1000°C.

9. The catalyst of any one of the preceding claims, which is a calcined catalyst and optionally wherein the calcined catalyst is obtainable by calcining at a

temperature within the range of 600°C to 1000°C.

10. A process for preparing a catalyst for treating an exhaust gas from a diesel

engine, which process comprises:

(a) impregnating ceria with an aqueous solution comprising a water-soluble niobium (Nb) or tantalum (Ta) salt; (b) calcining the impregnated ceria at a temperature within the range of 600°C to 1000°C to obtain a support;

(c) impregnating the support with a solution comprising a Group 8-11

transition metal.

1 1. The process according to claim 10 further comprising:

(d) calcining the product from step (c) at a temperature within the range of 600°C to 1000°C to produce a calcined catalyst, wherein the calcined catalyst comprises 0.1 to 10 wt.% of the Group 8-1 1 transition metal.

12. A catalyst for treating an exhaust gas from a diesel engine obtainable by the process of claim 10 or claim 1 1 , and optionally wherein the catalyst is as defined in any one of claims 1 to 9.

13. An exhaust gas after-treatment system comprising the catalyst of any one of claims 1 to 9 or 12 and optionally an SCR catalyst, a diesel particulate filter, a catalyzed soot filter, an ammonia slip catalyst, or a combination thereof.

14. The catalyst of any one of claims 1 to 9 or 12 or the exhaust gas after-treatment system of claim 13, wherein the catalyst is a diesel oxidation catalyst.

15. The catalyst of any one of claims 1 to 9 or 12 or the exhaust gas after-treatment system of claim 13, wherein the catalyst is a NO_x adsorber.

16. The catalyst of any one of claims 1 to 9 or 12 or the exhaust gas after-treatment system of claim 13, wherein the catalyst is a mercury oxidation catalyst.

17. A process which comprises oxidizing a gaseous diesel exhaust stream

comprising carbon monoxide, unsaturated hydrocarbons, or both, in the presence of the catalyst of any one of claims 1 to 9 , 12 or 14 to 16.