Recent advances in automobile exhaust catalysts

Catalysis Today 90 (2004) 183–190

Recent advances in automobile exhaust catalysts Shin’ichi Matsumoto∗ Material Engineering Division 1, Toyota Motor Corporation, 1 Toyota-cho, Toyota, Aichi 471-8572, Japan Available online 7 June 2004

Abstract Catalysts, which were recently developed by Toyota for the control of automobile exhaust, are reviewed. (1) For use in low emission vehicles, a CeO2 -ZrO2 solid solution (CZ) with both high oxygen storage capacity and high heat resistance was developed as a support for a high performance three-way catalyst (TWC). (2) A novel three-way catalyst named the NOx storage-reduction catalyst (NSR) was developed for automotive lean-burn engines. The NSR catalyst can store NOx in an oxidizing atmosphere and then reduce stored NOx at stoichiometric or reducing conditions. Also, it has high tolerance to sulfur poisoning which is the most stringent problem for the NSR catalyst. © 2004 Elsevier B.V. All rights reserved. Keywords: Oxygen storage capacity; Three-way catalyst; NOx storage-reduction catalyst; Sulfur poisoning; Steam reforming; Hexagonal cell

1. Introduction In order to support global environmental protection, automakers endeavor to suppress carbon dioxide emissions and at the same time to clean up automobile exhaust gases. Also, a new generation of engine technology has placed severe demands on the catalyst. Recently, we have faced difficult problems that need to be solved such as the demand for low emission vehicles to reduce more than 99% of emissions, and lean-burn engines to reduce NOx emissions. Three-way catalysts have been developed to improve their performance and durability since 1977 when they were commercialized in the USA and Japan. Their performance is maximized in exhaust gas conditions close to the stoichiometric point. However, the air–fuel (A/F) ratios are occasionally perturbed in actual driving. In order to moderate the perturbed atmosphere on catalysts, materials with oxygen storage capacity (OSC) [1] such as CeO2 have been used in conventional three-way catalysts. The amount of OSC and its durability should be improved to achieve low emission vehicles. Lean-burn engine technology, which has been featured in vehicles since 1984, forces engine combustion to occur at very low air–fuel ratios. The normal 14.5:1 (stoichiometric) ratio produces exhaust gas that contains correct balance of

CO, H2 and HC to reduce NOx and O2 . Lean-burn engines, which operate at air–fuel (A/F) ratios of 25:1 and above, are effectively improve the fuel efficiency of gasoline-engined vehicles, but produce oxygen-rich exhaust gases (Fig. 1). Removal of NOx in an oxygen-rich exhaust is extremely difficult for the conventional three-way catalyst. As a result, the engine has to be operated in a very narrow lean-burn range, and this is the major obstacle for the improvement of fuel efficiency. This situation has prompted the research on the development of a new catalyst technology that is capable of reducing NOx in excess oxygen, that is, the NOx storage-reduction (NSR) catalyst. Recently, catalysts for selective NOx reduction by hydrocarbons under an oxidizing atmosphere have been extensively studied [2–8]. However these catalysts have several serious problems such as low catalytic activities, narrow temperature-windows, insufficient durability etc. [9]. Therefore, they can not be applied for practical use as in automotive catalysts. It is highly desirable to solve the two problems outlined above. In this paper, I describe our recent studies at Toyota to solve these problems.

2. Oxygen storage materials for three-way catalysts 2.1. Requirements for oxygen storage materials

∗

Tel.: +81 565 23 9020; fax: +81 565 23 5781. E-mail address: [email protected] (S. Matsumoto).

0920-5861/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.cattod.2004.04.048

In an actual vehicle driving in the LA# 4 mode test, NOx emission increases during an acceleration when the air–fuel

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S. Matsumoto / Catalysis Today 90 (2004) 183–190 Table 1 The number of nearest neighbor cations (coordination number) of center ion A (Ce or Zr) in the CZ materials measured by XAFS analysis A (center cation)

Ce Ce Zr Zr

B (surrounding cation)

Ce Zr Ce Zr

Coordination number of A M-CZ

S-CZ

R-CZ

11.9 0.0 0.0 12.0

8.0 3.6 4.0 3.0

6.0 6.0 6.0 6.0

2.2. Structures and OSC of oxygen storage materials

Fig. 1. Fuel consumption and three-way performance of a gasoline engine as a function of the air–fuel (A/F) ratio.

ratio fluctuates out of the stoichiometic ratio (Fig. 2). In order to get high conversion of catalysts under such fluctuating conditions, CeO2 has been used in conventional three-way catalysts. CeO2 releases oxygen in an oxygen deficient atmosphere, and stores oxygen in an oxygen excess atmosphere as described by the following reversible reaction: CeO2 ↔ CeO2(1−x) + xO2

(1)

where 0 ≤ x ≤ 0.25. In recent years, the OSC of CeO2 has not been sufficient for low emission vehicles because x was at most 0.005 during typical vehicle driving conditions. The addition of La ions into CeO2 increases its OSC because the substitution of trivalent La ions for quadruvalent Ce ions create oxygen vacancies in the CeO2 -La2 O3 solid solution to compensate for the decreases of positive charge [10,11]. ZrO2 can also make a solid solution with CeO2 . The CeO2 -ZrO2 solid solution with 20 mole% of Zr was developed to improve the thermal stability of CeO2 [12,13]. Now, further improvement of both the amounts of OSC and its durability is necessary to realize low emission vehicles.

Three types of CeO2 -ZrO2 mixed oxide (CZ) were prepared by Suda and co-workers to investigate the relations between the structures of these materials and their OSC properties [14,15]. M-CZ was prepared by the hydrolysis of an aqueous solution of ZrO(NO3 )2 with ammonia on CeO2 powder, followed by the calcination in air at 973 K. The structure of M-CZ was a mixture of CeO2 , ZrO2 and CeO2 -ZrO2 solid solution. S-CZ was prepared by an attrition milling process of CeO2 powder with ZrO2 spheres in ethanol. The structure of S-CZ was a solid solution of CeO2 and ZrO2 . R-CZ was prepared by the calcination of M-CZ with graphite at 1473 K under a reducing atmosphere followed by the re-oxidation in air at 773 K. The structure of R-CZ was a solid solution of CeO2 and ZrO2 when the mole fraction of ZrO2 was below 0.3. The ␬-phase structure of CeZrO4 was appeared when the mole fraction of ZrO2 was above 0.3. The arrangement of atoms in the local structure of CZ with 50 mole% of ZrO2 was studied by Nagai et al. [16,17]. The number of nearest neighbor cations around Ce4+ or Zr4+ are shown in Table 1 which were determined from the data of Ce and Zr K-edge EXAFS. Both a Ce atom and a Zr atom in R-CZ are surrounded by six Ce atoms and six Zr atoms, respectively via Ce–O–Ce, Zr–O–Zr or Ce–O–Zr bonds. The crystal structure of R–CZ was determined by XRD to be a pyrochlore-type structure, that is, Ce and Zr atoms were arranged regularly. On the contrary, the local structure around Ce is different from that around Zr in S-CZ and in M-CZ. Based on these experimental results, the schematic atomic arrangement of each CZ is shown in Fig. 3.

Ce Zr

(a)

Fig. 2. Fluctuation of the air–fuel (A/F) ratio (solid line) and NOx emissions (hatched line) of a vehicle in the LA# 4 driving mode (dotted line).

(b)

(c)

Fig. 3. Schematic figure of the atomic configuration of three types of CeO2 -ZrO2 mixed oxide (CZ). (a) M-CZ was a mixture of CeO2 , ZrO2 and CeO2 -ZrO2 solid solution; (b) S-CZ was a solid solution of CeO2 and ZrO2 ; (c) R-CZ was a solid solution of CeO2 and ZrO2 with a pyrochlore-type structure, that is, Ce and Zr atoms were arranged regularly.

S. Matsumoto / Catalysis Today 90 (2004) 183–190

Table 2 The precious metal particle diameter and the crystal diameter of CZ in two catalysts

Specific OSC

0.3

Samplea

Particle diameter (nm)b

0.2 A B

0.1

0

185

CZ

Pt

17.2 8.7

23.7 19.6

Catalyst A contained 1.5 g dm−3 Pt, 0.35 g dm−3 Rh with CZ, Catalyst B contained the same amount of precious metals with ACZ. a Aged at 950 ◦ C for 100 h. b Measured by XRD.

0

0.5 Mole fraction of ZrO2 in CZs

1.0

Fig. 4. Specific OSC (the amount of OCS per mole of Ce) of CeO2 -ZrO2 mixed oxide (CZ) as a function of ZrO2 content. (䊊) M-CZ; (䊐) S-CZ; (䊏) R-CZ. The amount of OSC was the moles of oxygen released when these materials were treated at 1173 K in air for 15 min, followed by reduction under a stream of 20% hydrogen in nitrogen at 773 K.

The amount of OSC for these oxygen storage materials described above was measured according to the following procedure. Platinum loaded catalysts were prepared by impregnating M-CZ, S-CZ and R-CZ with a solution of Pt(NH3 )2 (NO2 )2 into M-CZ, S-CZ and R-CZ, respectively. One percent of Pt was loaded onto each CZ material. These catalysts were treated at 1173 K in air for 15 min, followed by reduction under a stream of 20% hydrogen in nitrogen at 773 K until the decrease of their mass ceased. Subsequently, they were re-oxidized under a stream of 50% oxygen in nitrogen until the increase of their mass ceased. The amount of mass decrease was almost equal to the increase for each catalyst, thus we defined this quantity as OSC of these materials. Fig. 4 shows the specific OSC of the CZs, i.e. the amount of OCS per mole of Ce. The specific OSC of R-CZ has the maximum value of 0.22 mol(O2 )/mol(Ce), close to the theoretical value of 0.25, at 50 mole% of ZrO2 . The regular arrangement of Ce and Zr atoms in R-CZ with 50 mole% of ZrO2 probably eases the oxygen release from it. In the oxygen release process, the volume of the CZs increases in proportion to the change in the ratio of Ce4+ (smaller cation: 0.094 nm) to Ce3+ (larger cation: 0.114 nm). The stress energy caused by this volume change would suppress further valence change of Ce. The substitution of the smaller ion Zr4+ (0.084 nm) for Ce4+ could compensate for the volume change, which then could ease the valence change of Ce. Eight in 64 oxygen atoms in the unit cell of pyrochlore-type R-CZ crystal are surrounded by four Zr atoms. The other oxygen atoms are surrounded by both Ce and Zr atoms or by four Ce atoms. The eight oxygen atoms would be easier to move than the other 56 atoms because they are completely surrounded by all of smaller Zr ions. The ratio of the mobile eight oxygen atoms (four oxygen molecules) to 16 Ce atoms in the unit cell of R-CZ with 50 mole% of ZrO2 is consistent with the theoretical value of 0.25 mol(O2 )/mol(Ce) for the amount of OSC.

2.3. New materials with the large amount of OSC for a novel three-way catalyst R-CZ with 0.5 mole fraction of ZrO2 has large specific OSC as described above. However, the surface area of R-CZ is smaller than 10 m2 /g because of the high reducing temperature (1473 K) used during the synthesis process. The rate of oxygen release and storage should depend on the surface area of the oxygen storage material as described previously [18–21]. The stoichiometry of real exhaust gas usually fluctuates between oxygen rich and lean at a cycle time in excess of 1 s−1 . Therefore, the surface area of oxygen storage materials should be large enough to compensate for these exhaust gas fluctuations by their OCS. We developed a new material, that we named ACZ, for a three-way catalyst with high activity based on a novel concept, i.e. the diffusion barrier concept as described in Fig. 5 [22]. Diffusion barrier layers of Al2 O3 are built up between CZ particles to inhibit the coagulation or grain growth of CZ. After an aging test at 1273 K for 10 h in air, the surface area of ACZ was 29 m2 /g compared with that of CZ: 2 m2 /g. Table 2 shows the precious metal particle diameter and the crystal diameter of CZ in two catalysts. These catalysts were aged under the exhaust gas stream at 1173 K for 100 h. Catalyst A contained 1.5 g dm−3 Pt, 0.35 g dm−3 Rh with CZ and Catalyst B contained the same amount of precious metals with ACZ. As shown in the table, both the sintering of the precious metals and the CZ particles is inhibited in Catalyst B compared with Catalyst A. Table 3 also shows the light-off temperature of those catalysts. The light-off temperature of Catalyst B is about 15 K lower than

Table 3 Light off temperature of aged catalysts Samplea

A B

Light off temperature (◦ C)b HC

CO

Nox

332 318

322 307

319 302

Catalyst A contained 1.5 g dm−3 Pt, 0.35 g dm−3 Rh with CZ, Catalyst B contained the same amount of precious metals with ACZ. a Aged at 950 ◦ C for 100 h. b Temperature at 50% conversion.

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CeO2-ZeO2 solid solution Al2O3

Thermal aging

(a)

ACZ

Thermal aging

(b)

CZ

Fig. 5. The diffusion barrier concept for ACZ compared with CZ. (a) ACZ: the sintering of CZ is inhibited by Al2 O3 particles dispersed among CZ particles; (b) CZ: sinter easily without any dispersal.

3. NOx storage-reduction catalyst (NSR catalyst) 3.1. An approach to solve an extremely difficult problem for the conventional three-way catalyst: how to remove NOx in an oxygen-rich exhaust? The solution of this problem required novel consideration of the operation of engines themselves. The first step was to explore an engine cycle that featured the combination of two air–fuel ratios, instead of a single fixed ratio. It was discovered, during urban driving tests of vehicles equipped with lean-burn engines, that NOx emissions could be reduced more when the combustion conditions were alternated between the normal 14.5:1 ratio and the lean-burn ratio as compared with the steady lean-burn conditions [23,24]. Fig. 6 shows the NOx storage behavior of the NSR catalyst in an engine bench evaluation. When the air–fuel (A/F) mixture was switched to a lean mixture (A/F = 22) after a rich mixture as shown in Fig. 6, there was a relatively slow increase in the NOx concentration in the catalyst outlet gas. This slow increase means that NOx is stored in the catalyst. Even after 8 min, the NOx concentration in the outlet was lower by 0.018% than in the inlet. The reduction of NOx under lean conditions by the selective reduction with hydrocarbon are known for various catalysts, Cu/ZSM-5, Pt/Al2 O3 , etc. [2,25,26]. Therefore, this removal of about 20% of inlet NOx at the steady state is probably due to the selective reduction to nitrogen or nitrous oxide by hydrocarbon on Pt contained in the NSR catalyst. After the A/F mixture was switched to a rich mixture (A/F = 10), the NOx concentration in the catalyst outlet gas decreased to almost zero again. Based on these findings, we began studies on developing a catalyst that can store NOx under lean air–fuel conditions and reduce NOx during the subsequent phase at the normal air–fuel ratio.

This storage-reduction procedure with the two-step engine cycle proved to be very promising. During the lean-burn stage, excess NOx is oxidized by oxygen and stored in the form of nitrate (NO3 − ) in a barium-based storage material, as described further. Since there is plenty of oxygen present at this stage, HC, H2 , and CO are readly oxidized to water and carbon dioxide. When the engine is switched to operation with the normal air–fuel mixture, the resulting exhaust becomes comparatively oxygen-deficient, then HC, H2 , and CO remains unoxidized. So their components react with the NO3 − stored in the catalyst. These reactions reduce the NO3 − into harmless nitrogen, water, and carbon dioxide (Fig. 7). This is a novel catalysis concept, which we named the NOx storage-reduction (NSR) catalyst. The first NSR catalyst was developed and put into the market by Toyota in 1994 [23,24].

A/F=10(1s) 0.10

NOx concentration / %

that of Catalyst A. In addition, the NOx emission of a vehicle equipped with Catalyst B was about 20% less than that with Catalyst A.

A/F=23.5 0.08 0.06 0.04 0.02 0 12 0 2 4 6 8 10 time after changing to A/F=23.5 from A/F=10 / min

Fig. 6. NOx storage behavior of the NSR catalyst at 673 K in engine bench evaluation and definition of the amount of NOx storage. Catalyst: Pt/Rh/BaO/CeO2 /␥-Al2 O3 . A 1.8 dm3 lean-burn engine was operated at 2000 rpm and 75 kPa. The air–fuel mixture fed was switched to a lean mixture (A/F = 23.5) from a rich mixture (A/F = 10) for 1 s. After 10 min, the air–fuel mixture was switched to a rich mixture for 1 s, and then to a lean mixture again. (䊊) Inlet gas, (䊉) outlet gas.

S. Matsumoto / Catalysis Today 90 (2004) 183–190

187

Fig. 7. Possible mechanism of the NOx storage-reduction on the NSR catalyst.

3.2. Problems of NSR catalyst: sulfur poisoning Although numerous experimental investigations on the NSR catalyst have been conducted and theoretical models for this reaction have been developed [27–34], problems still remain mainly concerning the deactivation of the catalysts. Thermal deterioration is due to the reaction of the NOx storage material with compounds in the wash-coat and to the particle growth of both the precious metals and the NOx storage material [35]. The most difficult problem to be solved for the NSR catalyst is the deactivation caused by sulfur. Although we have improved the durability of the NSR catalyst by developing more sulfur-tolerant components, it can be used only in limited markets such as in Japan where fuels with low sulfur content are available. Sulfur oxides (SOx ) in exhaust gas react on the catalyst in the same way as NOx . The SOx species in automobile exhaust gas is almost exclusively SO2 , which derives from the combustion of sulfur compounds in fuel. Fig. 8 shows the relationship between the NOx conversion and the amount of sulfur deposited on the NSR catalysts after durability tests by using fuel with a certain sulfur content. The NOx conversion decreased with an increase in the sulfur to barium ratio. Sulfate species were detected by FT-IR spectroscopy and BaSO4 was identified by XRD on the catalyst after

100

NOx Conversion / %

80 60 40 20 0 0

1

2

Molar ratio of S / Ba Fig. 8. Relationship between the efficiency of NOx conversion and the amount of sulfur deposited on the catalyst after durability test. Catalyst: Pt/Ba/␥-Al2 O3 .

the durability test. On the basis of these facts together with the thermogravimetric analyses of the catalyst poisoned by sulfur, we propose that there are two mechanisms of sulfur poisoning of NSR catalyst as described further [36]. (1) SO2 in the exhaust gas is oxidized on precious metals, then reacts with alumina (␥-Al2 O3 ) to form aluminum sulfate (Al2 (SO4 )3 ). Al2 (SO4 )3 covers the surface of ␥-Al2 O3 or plugs the micropores of ␥-Al2 O3 . (2) SOx reacts with the NOx storage components to form barium sulfate (BaSO4 ). Since sulfates are more stable than nitrates, once the storage compound has reacted to form BaSO4 , it is no longer available to react with NOx . As time passes, NOx storage capacity gradually drops, and the catalyst loses activity. 3.3. How to overcome sulfur poisoning: new material To cope with the sulfur problem, we needed to develop new catalyst materials that would suppress the amount of sulfate trapped by the catalyst. We thought the geometrical structure of the catalyst would also function to minimize the size of the sulfate particles and make them easier to be removed from the catalyst. In addition, we have tried several other possibilities to counteract the sulfur accumulation and improve the NOx storage capacity. Titanium oxide (TiO2 ) was the most promising candidate. A small amount of TiO2 added to the alumina catalyst coating reduces the amount of sulfate stored and suppresses the size of the sulfate particles formed, without reducing much the NOx storage capacity. Here the size issue was important: the smaller the size of the sulfate particles, the more easily they are removed from the catalyst under reducing conditions (i.e., when the engine is operated at a normal air–fuel ratio). Of the exhaust gas present during reduction, which component is the most effective for removing sulfate? Fig. 9 shows the relationship between the kind of reducing gases and the elimination of H2 S from the aged catalysts (Pt/Rh/Ba/␥-Al2 O3 ) after the sulfur poisoning test under the alternate lean-rich gas conditions. In a hydrogen flow, the elimination of H2 S was prominent when the temperature reached approximately 873 K. In a propene or carbon monoxide flow, on the other hand, the elimination of H2 S

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Rate of H2 synthesis / %

Rate of H2S desorption / a.u.

1.5

950

0 500

1000

600

Inlet gas temperature / K

was observed at temperatures only above 923 K, and the amount of H2 S eliminated was much smaller. This indicates that hydrogen has a much greater reducing power than the other gases. Hence it is expected that the use of hydrogen promotes the sulfur removal according to the following equation: BaSO4 + 4H2 → BaS + 4H2 O

(3)

˙ → BaO + H2 S. BaS + H2 O

(4)

Exhaust gas from a lean-burn engine running in a rich atmosphere (A/F = 14) usually contains about 0.3% hydrogen. However, hydrogen instantly reacts with oxygen adsorbed on the catalyst surface, as well as with NOx stored in the catalyst. Therefore, it may be difficult to promote the decomposition of sulfate by using the hydrogen contained in the exhaust. The most effective way to ensure the reaction of the hydrogen with sulfate is probably to produce hydrogen on the catalyst in the proximity of barium sulfate. We designed a catalyst that promotes the decomposition of sulfate by hydrogen formed in the vicinity of the NOx storage element as described further. Automobile exhaust gas contains about 10% water vapor. We focused on steam reforming reaction: Cn Hm + 2nH2 O → (2n + m)/2H2 + nCO2

(5)

in which hydrogen is formed by the reaction of hydrocarbons, (which is usually found at about 0.5% on a carbon basis in automobile exhaust gas) with water vapor on the catalyst. Rhodium loaded on ZrO2 provided the highest catalytic activity for the steam reforming reaction (Fig. 10). We evaluated the amount of hydrogen formed on the NSR catalyst with Rh/ZrO2 (Rh/ZrO2 -added catalyst), and confirmed that this catalyst has high activity for hydrogen generation. Fig. 11 shows the temperature programmed desorption of H2 S from the catalysts after a sulfur poisoning test. In the

1000

Fig. 10. Formation of hydrogen on several catalysts under a flow of rich gas (A/F = 12). Catalyst: Rh/ZrO2 (䊊), Rh/Al2 O3 (䊉), Rh/TiO2 (䊐), Pt/ZrO2 (䊏). The composition of rich gas was NO (100 ppm), O2 (0.59%), C3 H6 (0.67%), CO (0.59%), H2 O (3%) and CO2 (10%). The gas flow rate was adjusted to 1 dm3 min−1 (standard conditions) with nitrogen as a carrier gas. The concentration of hydrogen was measured by gas chromatography.

flow of hydrogen, both catalysts after the sulfur poisoning test under the continuous lean gas conditions showed similar curves of the H2 S elimination. In the mixture of hydrocarbon and water, however, they showed considerably different behavior. Specifically, while the H2 S elimination of the conventional catalyst dropped substantially, the starting and peak temperatures of the H2 S elimination from the Rh/ZrO2 -added catalyst were almost the same as those in the hydrogen flow, although the profile differed to a certain extent. These results, along with the results shown in Fig. 11, indicate that hydrogen generated in situ by the steam reforming reaction contributes very favorably to the decomposition of the sulfates. With the conventional catalyst and the Rh/ZrO2 -added catalyst, a durability test was conducted simulating a drive in an urban area. The results showed that the Rh/ZrO2 -added catalyst provides approximately 30% improved performance over the conventional catalyst. The same test, conducted with the Rh/ZrO2 -added catalyst wash-coated onto a hexagonal cell substrate, resulted in nearly doubled the performance.

Rate of H2S desorption / a.u.

Fig. 9. Relationship between the kind of reducing gases and the desorption characteristics of H2 S measured by a mass spectrometer. Catalyst: Pt/Ba/␥-Al2 O3 after the sulfur poisoning test under a lean atmosphere at 873 K. The compositions of reducing gases were: 1% hydrogen (䊊), 0.22% propene (䊉), 1% carbon monoxide and 0.22% propene, 3% water (䊐). The gas flow rates were adjusted to 1 dm3 min−1 (standard conditions) with helium as a carrier gas respectively.

700 800 900 Inlet gas temperature / K

B A

500 600 700 800 900 1000 Inlet gas temperature / K

A

B .

900

0.5

Rate of H2S desorption / a.u

850

1.0

500 600 700 800 900 1000 Inlet gas temperature / K

Fig. 11. Temperature-programmed desorption of H2 S measured by a mass spectrometer in the flow of hydrogen (left) and in the mixture of hydrocarbons and water (right). Catalyst A: Pt/Rh/Ba/ZrO2 /␥-Al2 O3 , Catalyst B: Pt/Rh/␥-Al2 O3 (both after the sulfur poisoning test under a lean atmosphere at 873 K).

S. Matsumoto / Catalysis Today 90 (2004) 183–190

189

Fig. 12. Photographs of wash-coat layer on square-cell (left) and hexagonal-cell (right) monolithic substrate.

3.4. How to overcome sulfur poisoning: new structure The removal of sulfur from the catalyst is more difficult in portions where the alumina wash-coat is thick (i.e., thicker than about 100 ␮m). Conventional catalytic converters have square-shaped cells for the structure of catalyst support. When the alumina coating is applied to the square cells, it builds up to a much greater thickness at the corners, making the sulfur removal significantly more difficult. Catalysts currently in use for automobile exhaust treatment employ ceramic monolithic substrate having square cells. When the wash-coat slurry is applied to this substrate, the shape of the cell void becomes round, as shown in Fig. 12 (left), due to the surface tension of the slurry. In other words, the wash-coat is thinner on flat side of the cell, and much thicker at the corner. The wash-coat is approximately 20 ␮m thick at the flat side and 250 ␮m thick at the corner. A decrease in the amount of wash-coat brings about uniform thickness of wash-coat, but the deterioration of catalytic performance by the sulfur poisoning is accelerated [23]. Therefore, for effective use of the catalytic components to promote sulfur elimination, the wash-coat thickness must be uniform to distribute the catalytic components uniformly to the appropriate depth. Fig. 12 (right) is a photograph showing a coated catalyst with a hexagonal cell substrate. The amount of the wash-coat on this substrate is the same as that on the square cell substrate (Fig. 12, left). Clearly, the wash-coat thickness varies much less for the hexagonal cell substrate than for the square cell substrate. The fraction of wash-coat that is less than 100 ␮m thick is about 90% for the hexagonal cell (compared with about 66% for the square cell) the thickness being almost uniform for the former. To ascertain the effect of cell structure on the sulfur desorption, we measured the sulfur desorption under a rich atmosphere for 5 s, after being treated under a lean atmosphere for 60 s. The amount of H2 S desorbed from the catalyst with the hexagonal cell substrate was more than twice that of the square cell substrates. After an engine durability test,

the amount of NOx storage and SOx deposit was measured as well. This revealed that the hexagonal cell substrate had approximately 20% higher NOx storage, and substantially lower SOx deposit than the square cell substrate. 3.5. Evaluation of developed NSR catalyst after on-vehicle durability test A catalyst was designed and developed for practical use based on the technologies described above. We evaluated the newly developed NSR catalyst by an on-vehicle durability test. The catalyst, which was Pt/Rh/Ba/TiO2 /ZrO2 /␥-Al2 O3 with the 2 dm3 hexagonal cell substrate, was installed in a 2 dm3 D-4-engined vehicle and aged by 50,000 km of driving in an urban area with gasoline containing low-concentration sulfur (S = 30 ppm) which is usually available in Japan. In the Japanese 10–15 mode test, NOx emission with the developed catalyst measured after the 50,000 km driving test was about one third that of the conventional catalyst. CO2 emission from a vehicle equipped with the D-4 engine and the NSR catalyst was approximately 30% lower than that of the conventional engine with the conventional three-way catalyst.

4. Conclusions The precise local structure around Ce and Zr atoms of CZ material was investigated to clarify their effects on OSC. The amount of OCS per Ce atom of R-CZ has the maximum value of 0.22 mol(O2 )/mol(Ce). That is close to the theoretical value of 0.25, which means all of the quadruvqlent Ce are reduced into trivalent Ce. The regular arrangement of Ce and Zr atoms in R-CZ with 50 mole% of ZrO2 probably eases the release of oxygen. We developed a new material ACZ for a three-way catalyst with high activity based on a novel concept, i.e. the diffusion barrier concept. Diffusion barrier layers of Al2 O3 are built up between CZ particles to inhibit the coagulation or grain growth of CZ. The sintering

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of precious metals and that of CZ particles were both inhibited in the catalyst containng ACZ. The NOx emission was decreased by ACZ due to its higher OSC. With the aim of enhancing the durability of the NOx storage-reduction catalyst, sulfur poisoning was analyzed in detail, leading to the concept that sulfur poisoning can be suppressed by the acceleration of sulfur desorption. To achieve this, the acidity of the support needs to be optimized. A combination of TiO2 and ␥-Al2 O3 was selected, in order to both maintain the amount of NOx storage and to minimize the amount of SOx deposition for application in the Japanese market. To enhance the removal of sulfate, a hexagonal cell monolithic substrate was developed, which resulted in a uniform catalytic wash-coat thickness, and in situ hydrogen generation on the catalyst was promoted by adding Rh/ZrO2 . The Rh/ZrO2 -added catalyst had a high activity of hydrogen generation via steam reforming. The catalyst developed by integrating these technologies was shown to have improved NOx purification performance on vehicle durability test simulating 50,000 km of driving.

Acknowledgements The author wishes to thank to Dr. Shinjo and his co-workers at Toyota Central Research and Development Laboratories Inc. for their fundamental studies on catalytic materials and discussion through the works. Furthermore, the author also acknowledges Mr. Kanazawa and his co-workers at Toyota Motor Corporation for developing catalysts with the author.

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