Alumina Catalysts: Effects of

Nov 22, 2005 - The catalysts containing both barium and cerium demonstrated high NO storage/conversion. The best compositions of the catalysts for hig...
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NOx Storage and Reduction on Pt/Alumina Catalysts: Effects of Alkaline-Earth and Rare-Earth Metal Additives Hsin-Yu Lin, Chia-Jin Wu, and Yu-Wen Chen* Department of Chemical Engineering, Nano-catalysis Research Center, National Central UniVersity, Chung-Li 320 Taiwan

Chiu-Hwang Lee Union Chemical Laboratories, Industrial Technology Research Institute, Hsinchu 300 Taiwan

A series of Pt/Al2O3 catalysts, modified with alkaline-earth or rare-earth metal oxides, were prepared by the incipient-wetness impregnation method. The catalysts were characterized by N2 sorption, scanning electron microscopy, and transmission electron microscopy. The performance and durability of NOx storage of the catalysts were investigated under a lean-burn/rich-burn cycle process. The effects of reaction temperature on the capacity for NOx and duration of the catalysts were also studied. The transient experiments consisted of a storing phase using a lean gas mixture (NO/O2/N2/C3H6/CO) and a regeneration phase during which the O2 flow was switched off. The catalysts consisted of nonuniform aggregates of very small and distinct particles. The barium-promoted catalyst had better storage capability and duration. On the other hand, the ceriumpromoted catalyst had a higher NO storage capability than those without cerium. Comparing the influences of cerium and lanthanum additives, Pt2.5Ce30.5Ba33.4Al100 had a storage capacity of 1020 × 10-6 mol/g at 30 min, but Pt2.5La30.5Ba33.4Al100 had a storage capacity of 341 × 10-6 mol/g. The catalysts containing both barium and cerium demonstrated high NO storage/conversion. The best compositions of the catalysts for high NO storage capacity were Pt2.5Ce30.5Ba33.4Al100 (1020 µmol/g) and Pt2.5Ce22.5Ba41.7Al100 (911 µmol/g). The operating temperature also had a pronounced effect on both NO storage and reduction. The best operating range was 350-400 °C. Introduction The exhaust gases of automobile engines contain carbon monoxide (CO), carbon dioxide (CO2), sulfur dioxide (SO2), nitrogen oxides (NOx), heavy metals, and some hydrocarbons. In most parts of the world, strict environmental laws have been enacted for increasing demands and expectations of environmental protection, human health, and safety. Many research efforts have been carried out to improve fuel efficiency and reduce emissions. A lean-burn engine is one of the effective technologies, offering a higher combustion temperature and enhancing the fuel efficiency of a gasoline engine over stoichiometric operation (air/fuel ) 14.7). However, lean operating conditions are limited, because oxidizing conditions prevent the reduction of emitted NOx on conventional three-way catalysts.1,2 The development of a new automotive catalyst for the reduction of NOx in exhaust gases has become an important subject for science and technology. The selective catalytic reduction (SCR) process has been reported to reduce nitrogen oxides by ammonia (NH3), which is a highly selective reducing agent in the presence of oxygen.3-5 However, ammonia is a harmful substance in internal combustion engines. It is not practical because of the high costs of the required control systems and handling and because of safety problems. Matsumoto6 reported a Cu-ZSM-5 nitrogen oxide reduction catalyst that was prepared by the ion-exchange method. A NOx storage reduction (NSR) catalyst has been developed for the SCR of hydrocarbons under excess oxygen. Epling et al.7 reviewed the past development of NSR catalysts. The concept of NOx reduction under dynamic oxidization * To whom correspondence should be addressed. E mail: ywchen@ cc.ncu.edu.tw. Fax: 886-3-425-2296.

conditions was announced by Toyota.8 NSR catalysts reduce NOx via cyclic operation. When the air/fuel ratio is greater than 14.7, it experiences oxygen-rich (also called lean-burn) conditions. Under these conditions, excess NOx is oxidized and stored in the barium-based catalysts as Ba(NO3)2. When the air/fuel ratio is less than 14.7, the system is under oxygen-poor conditions (also called rich-burn conditions), and the hydrocarbons in the exhaust reduce nitrate (NO3-) species on the catalyst to harmless N2.9-14 In a storage-reduction cycle procedure, NSR catalysts could be promising de-NOx catalysts. Such catalysts have a certain NOx trapping capacity, and once this capacity approaches a level of saturation, unacceptable amounts of NOx will slip through the catalyst. At or before this point, the catalyst is exposed to a rich environment, which induces NOx release from the surface and reduction to N2. It is important to develop a catalyst that can adsorb NOx quickly and has a sufficiently large storage capacity. A suitable catalyst also should have the capability to reduce NO under fuel-rich conditions. Other oxygen storage capacity (OSC) materials such as CeO2 has been used in three-way catalysts for automotive emission control. Ceria has been used to serve as an “oxygen” component under cyclic lean and rich compositions tests. Ceria provides oxygen storage by shifting between Ce2O3 under fuel-rich conditions and CeO2 under fuel-lean conditions.15,16 Huang et al.17 reported NOx storage under lean-burn conditions using noble metals (Pt, Rh, and Pd) supported on alkalineearth metal oxides (BaO and CaO) with a high-surface-area γ-Al2O3 support. The results indicated that NOx adsorption and nitrate formation were enhanced by the loading of noble metals. Fridell et al.18 reported the NOx storage mechanism of Pt-Rh/ BaO/Al2O3 catalyst during lean-burn periods. The NO TPD (temperature-programmed desorption) results showed that the

10.1021/ie050574i CCC: $33.50 © 2006 American Chemical Society Published on Web 11/22/2005

Ind. Eng. Chem. Res., Vol. 45, No. 1, 2006 135 Table 1. Compositions and Surface Areas of the Catalysts catalysta

BaO

composition (wt ratio) SrO CeO2 La2O Al2O3

Pt

SBET (m2/g)

Pt2.5Ce30.5Ba33.4Al100 Pt2.5Ce22.5Ba41.7Al100 Pt2.5Ce30.5Sr33.4Al100 Pt2.5Ce22.5Sr41.7Al100 Pt2.5La30.5Ba33.4Al100 Pt2.5La22.5Ba41.7Al100 Pt2.5La30.5Sr33.4Al100 Pt2.5La22.5Sr41.7Al100

33.4 41.7 33.4 41.7 -

33.4 41.7 33.4 41.7

2.5 2.5 2.5 2.5 2.5 2.5 2.5 2.5

66.5 72.2 30.6 34.3 64.3 39.5 9.5 17.9

a

30.5 22.5 30.5 22.5 -

30.5 22.5 30.5 22.5

100 100 100 100 100 100 100 100

Subscripts on the symbols refer to weight ratio.

maximum of NOx storage occurred at about 395 °C. For CeO2based catalysts, Monte et al.19 reported the study of NO reduction by Pd-loaded Ce0.6Zr0.4O2 supported on Al2O3. Fornasiero et al.20 reported a series of Rh-loaded CexZr1-xO2 (x ) 0.4-0.6) catalysts for NO reduction by CO. The results indicated that the activities were improved after reduction in H2 at 800 °C. However, for the quaternary composite catalysts, the relationship of the contents of these compositions remained unclear. The objective of this study was to develop a catalyst that has a high NOx storage capacity and can reduce NOx effectively with hydrocarbons. A series of Pt-based NOx storage catalysts including cerium (Ce), strontium (Sr), and barium (Ba) and metals including lanthanum (La), platinum (Pt), and titanium (Ti) were synthesized with various compositions by the incipient-wetness impregnation method. The catalysts were characterized by nitrogen sorption, scanning electron microscopy, and transmission electron microscopy. The catalytic reaction was carried out under lean-burn and rich-burn conditions during cycles in a fixed-bed reactor. Experimental Section 2.1. Chemicals. γ-Al2O3 (2-3 µm) obtained from Showa Chemicals was used as the support. The barium and strontium precursors were anhydrous barium(II) acetate [Ba(CH3COO)2], anhydrous strontium(II) nitrate [Sr(NO3)2], respectively, from Showa Chemicals. Cerium(III) nitrate hexahydrate [Ce(NO3)3‚ 6H2O] was purchased from Sigma. Lanthanum(III) nitrate hexahydrate [La(NO3)3‚6H2O, 99.9%] was obtained from Strem Chemicals. The precious metal platinum precursor was platinum dinitrodiammite nitrate (about 5 wt % in solution). All of the synthesized samples are listed in Table 1. 2.2. Catalyst Preparation. The catalysts Pt2.5CexBayAl100 and Pt2.5LaxBayAl100 (x ) 30.5/22.5 and y ) 33.4/41.7) were prepared by incipient-wetness impregnation, essentially similar to the procedure by Iizuka et al.17 γ-Al2O3 was impregnated with an aqueous solution of cerium nitrate or lanthanum nitrate (10 wt % in solution), then dried at 120 °C for 2 h, and calcined at 550 °C for 2 h. The Ce/γ-Al2O3 or La/γ-Al2O3 material was impregnated with a solution of alkaline-earth metals (10 wt % in solution), dried at 120 °C for 2 h, and then calcined at 550 °C for 2 h. It was then impregnated with a solution of platinum dinitrodiammite nitrate (5 wt % in solution), dried at 120 °C, and calcined at 450 °C for 2 h. 2.3. Characterization. 2.3.1. N2 Sorption. N2 sorption isotherms were measured at -197 °C using a Micromeritics ASAP 2010 instrument. Prior to the experiments, the samples were dehydrated at 100 °C until the vacuum pressure was below 5 mPa. The measurement of the surface areas of the samples was achieved by the Brunauerr-Emmett-Teller (BET) method for relative pressures in the range of P/P0 ) 0.05-0.2.

Figure 1. O2 concentration in the inlet gas for cycle test.

2.3.2. SEM. SEM images were obtained with a Hitachi S-800 field-emission microscope using an acceleration voltage of 20 kV. Samples were placed on a stage especially made for SEM. Samples were coated with Au prior to analysis and imaged directly. SEM images were recorded at magnification of 600010 000×. 2.3.3. TEM. The morphologies and particle sizes of the samples were determined by TEM on a JEOL JEM-2000FX Π apparatus operated at 160 kV. Sample grids were prepared via sonication of powdered samples in H2O for 10 min and evaporation of 1 drop of the suspension onto a carbon-coated holey film supported on a 3-mm, 200-300 mesh copper grid. The sample grids were allowed to stand for several days at room temperature. TEM images were recorded at a magnification of 50 000-100 000×. 2.4. Catalytic Activity Measurements. The reaction was carried out in a continuous, vertical quartz tube reactor, with an inner diameter of 22 mm and a length of 65 cm. A sample (2.0 g) was placed in the reactor. The catalyst was reduced under a flow of a 0.1% CO/N2 mixture at 200 °C for 30 min. Nitrogen was used as the carrier gas at a constant flow rate of 1.8 L/min and a GHSV of 12 000 h-1. The inlet gas, containing 1000 ppm NO, 800 ppm C3H6, 0.1% CO, 4.5% O2, 10% H2O, and the remainder N2, was used as a model of a lean-burn exhaust gas. The flow was controlled by a set of mass flow controllers. The experiments were carried out between 200 and 500 °C under atmospheric pressure. The reaction temperatures of 200, 250, 300, 350, 400, 450, and 500 °C were used. A thermocouple was immersed inside the catalyst bed to measure the reaction temperature. The concentration of NO in the outlet of the reactor was analyzed by a Hewlett Packed Series II 5890 gas chromatograph with a thermal conductivity detector. To simulate the NO adsorption-regeneration cycle, the reactions were carried out at a space velocity of 6000 h-1 and a temperature of 400 °C. Lean-burn conditions were represented by a feed stream of 1000 ppm NO, 800 ppm C3H6, 0.1% CO, 4.5% O2, and 0% H2O in a N2 balance. The rich-burn feed was made up of 1000 ppm NO, 800 ppm C3H6, 0.75% CO, no O2, and 10% H2O, in a N2 balance. Every 5 min, the inlet gas was switched to test the catalyst performance and stability during cycles (see Figure 1). 3. Results and Discussion 3.1. Characterization of the Catalysts. The BET specific surface areas (SBET) of the catalysts are listed in Table 1. The

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Figure 2. SEM images of (a) Pt2.5Ce30.5Ba33.4Al100, (b) Pt2.5Ce22.5Ba41.7Al100, (c) Pt2.5Ce30.5Sr33.4Al100, and (d) Pt2.5Ce22.5Sr41.7Al100. Table 2. Storage Capacities (µmol/g) of NO on Pt2.5CexBayAl100 and Pt2.5SrxBayAl100 in Cycle Test cycle

Pt2.5Ce30.5Ba33.4Al100

Pt2.5Ce22.5Ba41.7Al100

Pt2.5Ce30.5Sr33.4Al100

Pt2.5Ce22.5Sr41.7Al100

1 2 3 4 5 6

183.9 183.2 182.8 184.7 182.0 184.5

163.9 155.3 162.0 161.3 158.8 158.0

245.5 236.5 234.8 232.7 229.2 227.6

372.5 309.6 265.1 351.6 326.4 282.9

Table 3. Storage Capacities (µmol/g) of NO on Pt2.5LaxBayAl100 and Pt2.5LaxBayAl100 in Cycle Test cycle

Pt2.5La30.5Ba33.4Al100

Pt2.5La22.5Ba41.7Al100

Pt2.5La30.5Sr33.4Al100

Pt2.5La22.5Sr41.7Al100

1 2 3 4 5 6

112.0 176.1 216.4 192.2 176.4 160.2

192.1 168.3 144.2 201.6 176.4 240.3

391.2 469.6 678.4 601.2 496.0 433.2

280.0 408.0 307.6 340.4 480.8 304.0

surface areas are in the range between 9.5 and 72.2 m2/g. There is no correlation between the surface areas of the samples and the amounts of additives. However, the samples containing Ba had higher surface areas than those containing Sr. Figure 2 shows representative SEM images of the Pt2.5CexBayAl100 and Pt2.5CexSryAl100 samples, where x ) 30.5/22.5 and y ) 33.4/41.7. The morphologies of Pt2.5LaxBayAl100 and Pt2.5LaxSryAl100 (x ) 30.5/22.5 and y ) 22.5/41.7) were similar to

those of Pt2.5CexBayAl100 and Pt2.5SrxBayAl100 (not shown). All catalysts consisted of nonuniform aggregates with very small and distinct particles. The size of the particles was about 0.1-5 µm. As shown in the SEM images, the catalysts had irregularly shaped fundamental particles. These fundamental particles aggregated into larger particles. The catalysts were aggregated dispersively. The different additives exhibited no significant influence on the morphologies of the catalysts. The morphol-

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Figure 3. TEM images of (a) Pt2.5Ce30.5Ba33.4Al100, (b) Pt2.5Ce22.5Ba41.7Al100, (c) Pt2.5Ce30.5Sr33.4Al100, and (d) Pt2.5Ce22.5Sr41.7Al100.

Figure 4. NO storage and conversion on Pt2.5Ce30.5Ba33.4Al100 under leanburn and rich-burn conditions for 30 min at 400 °C.

Figure 5. NO storage for a duration of 30 min on Pt2.5Ce30.5Ba33.4Al100 and Pt2.5Ce30.5Sr33.4Al100.

ogies were cobblestone-like particles aggregating and packing randomly. Furthermore, as shown in the TEM images (Figure 3), the sizes of the rock-like particles were about 1.0-2.0 µm. 3.2. Catalytic Activity. The overall principle of storage/ reduction catalysts during cycles can be described by five

reaction steps: (i) NO oxidation to nitrogen dioxide (NO2), (ii) NO or NO2 soprtion on the surface in the form of nitrites or nitrates, (iii) reductant evolution when the exhaust is switched to the rich conditions, (iv) NOx release from the nitrite or nitrate sites, and (v) NOx reduction to N2. The last step can be described

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The NO conversion is defined by the equation

NO conversion (%) ) (A - B)/A × 100%

Figure 6. (a) NO storage and conversion on Pt2.5Ce30.5Ba33.4Al100 and Pt2.5Ce22.5Ba41.7Al100 samples for a storage/reduction cycle test at 400 °C and (b) NO storage vs reaction temperature on Pt2.5Ce30.5Ba33.4Al100 and Pt2.5Ce22.5Ba41.7Al100.

by three overall reactions occurring under fuel-rich conditions

2CO + O2 f 2CO2

(1)

2C3H6 + 9O2 f 6CO2 + 6H2O

(2)

2CO + 2NO f 2CO2 + N2

(3)

In reaction 2, C3H6 represents a number of different hydrocarbons, and in reaction 3, NO denotes several nitrogen oxides. Composite catalysts incorporating noble metals impregnated onto a high-surface-area, temperature-resistant alumina are normally used. In some cases, transition metal oxides are added.21,22 Reaction 3 between NO and CO is the most important reaction in the catalytic converter because it eliminates two of the major pollutants. Aside from reaction 3, there are two possible reactions between NO and CO:

2NO + CO f N2O + CO2

(4)

N2O + CO f N2 + CO2

(5)

Reaction 4 is not desirable because N2O is also a pollutant; however, this reaction might be an important intermediate step. Reaction 5 shows that N2O can react with CO to produce N2.23

(6)

where A is the NO concentration in the inlet and B is the NO concentration in the outlet of the catalyst bed. To simulate NO storage capacity in a lean-burn engine, the storage/reduction performance test was carried out in lean/rich cycles where the inlet O2 concentration was periodically controlled to 4.5% and 0%, respectively. During lean-burn periods, NO was stored in the catalyst, and during rich-burn periods, NO was released and reduced to N2. The amount of stored NO was obtained by integration of the area related to storage in this type of transient. Figure 4 shows the NO storage and conversion of Pt2.5Ce30.5Ba33.4Al100 catalyst under lean-burn and rich-burn conditions. As shown in the figure, the storage capacity of Pt2.5Ce30.5Ba33.4Al100 decreased with time under leanburn conditions. The storage capacity of NO decreased by about 50% in 30 min. The total NO storage was 1020 µmol/g of catalyst for Pt2.5Ce30.5Ba33.4Al100 and 781 µmol/g of catalyst for Pt2.5Ce22.5Ba41.7Al100. When the gas composition was switched, the stored NO was quickly reduced to N2. As shown in the figure, the significant breakthrough peak under rich-burn conditions indicates that the stored NO was released and reduced to N2. As reported by Bogner et al.,24 rich-burn conditions should be minimized to give the optimum fuel savings through operation under lean-burn conditions. The NO storage capacities on Pt2.5Ce30.5Sr33.4Al100 and Pt2.5Ce30.5Ba33.4Al100 were examined under lean-burn conditions for 30 min. As shown in Figure 5, at 400 °C, Pt2.5Ce30.5Ba33.4Al100 had a storage capacity of 99%, and Pt2.5Ce30.5Sr33.4Al100 had a storage capacity of 89%. The high NO storage capacities of Ba and Sr have been reported by Nunan et al.25 and Iizuka et al.21 It has been reported14 that nitrate is the final form of the products on the barium oxide surface regardless of sequence of NO/O2 adsorption or coadsorption. As shown in Figure 5, barium had a better storage capacity and duration. One can conclude that the alkaline-earth metals oxides have a good NOx adsorption ability under lean-burn conditions. However, the characters of such complexes are not easy to elucidate clearly. Chi and Chuang14 have reported that NO/O2 coadsorbed as a chelating agent bidentate nitrate on La2O3 and as a distinctive bridging bidentrate nitrate on BaO and MgO via the reaction of adsorbed with surface lattice oxygen at 250 °C. The difference in stability between chelating and bridging bidentate nitrates on various metal oxides/r-Al2O3 might provide a wide range of operating temperatures for NOx storage. In addition, the storage capacity of NO within 30 min was 805 µmol/g for Pt2.5Ce30.5Sr33.4Al100 and 911 µmol/g for Pt2.5Ce22.5Sr41.7Al100. In this study, the Pt2.5Ce30.5Sr33.4Al100 sample with a cerium content of 30.5% showed NO storage abilities of 87% at 5 min and 35% at 30 min, which are greater than those reported by Iizuka et al.,21 who reported that the best cerium content was 11-25%. Most of the literature suggests that NO2 is a precursor for sorption in NSR catalysts.7 This does not imply that NO in normal exhaust gas will not ultimately be sorbed onto an NSR catalyst as O2 is consistently present and Pt is an excellent oxidation catalyst. For the Pt2.5LaxBayAl100 and Pt2.5LaxSryAl100 samples, where x ) 30.5/22.5 and y ) 33.4/41.7 by weight ratio, the mechanism is similar to that for the cerium-containing samples. Under leanburn conditions, the NO storage ability decreases with increasing time. The breakthrough peak of NO appears while switching from the storage phase to the regeneration phase. The storage amount of NO in 30 min was 341 µmol/g for Pt2.5La30.5Ba33.4Al100 and 353 µmol/g for Pt2.5La30.5Sr33.4Al100. The results

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Figure 7. (a) NO storage and conversion on Pt2.5Ce30.5Sr33.4Al100 and Pt2.5Ce22.5Sr41.7Al100 samples for a storage/reduction cycle test at 400 °C and (b) NO storage vs reaction temperature on Pt2.5Ce30.5Sr33.4Al100 and Pt2.5Ce22.5Sr41.7Al100.

indicate that cerium-containing catalysts exhibit a higher storage capacity than lanthanum-containing catalysts. Cerium provides oxygen storage by shifting between Ce2O3 under fuel-rich conditions and Ce2O under fuel-lean conditions.15,16 Yao and Kummer26 reported that ceria itself has excellent heat resistance, so that the heat resistance and durability of the catalyst support were improved. It should be noted that lanthanum is a structural promoter most commonly used to stabilize Al2O3 against thermal sintering. Improvement of the hydrothermal stability of Al2O3 has also been observed with MgO and BaO promoters. To investigate NO storage and conversion in a lean-burn engine, the catalysts were tested during lean/rich cycles. Figure 6 shows the NO storage and conversion of Pt2.5Ce30.5Ba33.4Al100 and Pt2.5Ce22.5Ba41.7Al100 catalysts at 400 °C. The curves shown here represent discrete data, not continuous monitory data. As shown in Figure 5a, during lean-burn periods, the storage percentage was more than 80%. The storage amounts of NO for the Pt2.5Ce30.5Ba33.4Al100 and Pt2.5Ce22.5Ba41.7Al100 catalysts during each period of lean-burn conditions are listed in Table 2. During rich-burn periods, the adsorbed NO was released and then reacted with carbon monoxide (CO) and hydrocarbon effectively with excellent selectivity to N2. The reduction of NO under rich-burn conditions was up to 90% (Figure 4a). Furthermore, the Pt2.5Ce30.5Ba33.4Al100 catalyst was better than

Figure 8. (a) NO storage and conversion of Pt2.5La30.5Ba33.4Al100 and Pt2.5La22.5Ba41.7Al100 samples for a storage/reduction cycle test at 400 °C and (b) NO storage vs reaction temperature on Pt2.5La30.5Ba33.4Al100 and Pt2.5La22.5Ba41.7Al100.

the Pt2.5Ce22.5Ba41.7Al100 catalyst in terms of the storage capacity. Figure 4b shows the concentration of NO measured at various temperatures under lean-burn conditions on Pt2.5Ce30.5Ba33.4Al100 and Pt2.5Ce22.5Ba41.7Al100. The results indicate that the maximum NO storage occurred at 400 °C for both catalysts. For strontium-containing catalysts, the capacities and NO conversions of Pt2.5Ce30.5Sr33.4Al100 and Pt2.5Ce22.5Sr41.7Al100 during lean/rich cycling are shown in Figure 7a, where the NO storage in Pt2.5Ce30.5Sr33.4Al100 reached ∼70% and that of Pt2.5Ce22.5Sr41.7Al100 was 47%. The NO storage amounts during each lean/rich cycle are listed in Table 2. During the rich-burn period, all NO was reduced effectively by propylene, as with the standard three-way catalysts. The Pt2.5Ce22.5Sr41.7Al100 catalyst was better than the Pt2.5Ce30.5Sr33.4Al100 catalyst in terms of storage capability. Figure 7b shows the NO storage measured at various temperatures. In the temperature range studied, the maximum in NO storage was at 350 °C on the Pt2.5Ce30.5Sr33.4Al100 and Pt2.5Ce22.5Sr41.7Al100 catalysts. The storage/reduction capabilities of the lanthanum-containing catalysts were studied at 400 °C. The NO storage and conversion of Pt2.5La30.5Ba33.4Al100 and Pt2.5La22.5Ba41.7Al100 during lean/ rich cycling are shown in Figure 8a. The amount of NO storage in each cycle was calculated and is listed in Table 3. The initial

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Al100 had a maximum at the temperature of 400 °C. The outlet concentration of NO did not reach the inlet value of 800 ppm, indicating that some residual adsorption was still present. 4. Conclusion A series of Pt/Al2O3 catalysts, modified with alkaline-earth or rare-earth metal oxides, were prepared by the sequential incipient-wetness impregnation method. The catalysts were characterized by N2 sorption, scanning electron microscopy, and transmission electron microscopy. There is no clear correlation between the surface area and the composition of the catalyst. All catalysts consisted of nonuniform aggregates with very small and distinct particles. The size of the particles was about 0.1-5 µm. As shown in the SEM images, the catalysts had irregularly shaped fundamental particles. These fundamental particles aggregated into larger particles. The morphologies were cobblestone-like particles aggregating and packing randomly. Furthermore, as shown in the TEM images, the size of the rock-like particles was about 1.0-2.0 µm. The NOx storage/reduction performance of the catalyst was investigated under a lean-burn/rich-burn cycle process. The effects of reaction temperature on the NOx storage capacities of the catalysts were also studied. The transient experiments consisted of a storage phase using a lean gas mixture (NO/O2/ N2/C3H6/CO) and a regeneration phase in which O2 was not present. The barium-promoted catalyst had a high storage capacity. On the other hand, the cerium-promoted catalyst had a higher NO storage capability than those without cerium. Comparing the influences of cerium and lanthanum additives, Pt2.5Ce30.5Ba33.4Al100 had a storage capacity of 1020 µmol/g at 30 min, but Pt2.5La30.5Ba33.4Al100 had a storage capacity of 341 µmol/g. The Pt/γ-Al2O3 catalyst containing both barium and cerium demonstrated high NO storage/conversion. The best compositions for a high NO storage capacity were Pt2.5Ce30.5Ba33.4Al100 and Pt2.5Ce22.5Ba41.7Al100. The operating temperature also had a pronounced effect on both NO storage and reduction. The best operating range was 350-400 °C. Figure 9. (a) NO storage and conversion on Pt2.5La30.5Sr33.4Al100 and Pt2.5La22.5Sr41.7Al100 samples for a storage/reduction cycle test at 400 °C and (b) NO storage vs reaction temperature on Pt2.5La30.5Sr33.4Al100 and Pt2.5La22.5Sr41.7Al100.

difference in NO concentrations between inlet and outlet indicates that NO was stored (adsorbed) on the catalyst during the lean-burn periods. Switching to rich-burn conditions, accomplished by turning off the oxygen, immediately led to a steep decrease in NO concentration as a result of the reaction of NO. All NO was reduced to nitrogen by propylene. For NO storage measured at various temperatures (Figure 8b), the maximum storage of Pt2.5La22.5Ba41.7Al100 was 69% at the temperature of 350 °C. A very low storage was observed at the temperatures of 200 and 500 °C. For Pt2.5La30.5Ba33.4Al100, the maximum storage occurred at 400 °C, where the NO storage was about 58%. Figure 9 shows the storage capacity and conversion of NO of Pt2.5La30.5Sr33.4Al100 and Pt2.5La22.5Sr41.7Al100 during lean/rich cycling at 400 °C. It can be seen that the storage capacity was quite low (below 30%) but the conversion was still high. This implies that all NO was effectively reduced by propylene and CO. The storage amount of NO in each lean-burn cycle was calculated and is listed in Table 3. Pt2.5La30.5Sr33.4Al100 had a higher storage ability than Pt2.5La22.5Sr41.7Al100, but both were low. The NO storage capacities measured at different temperatures under lean-burn conditions are shown in Figure 9b. The storage capacities of Pt2.5La30.5Sr33.4Al100 and Pt2.5La22.5Sr41.7-

Acknowledgment This research was supported by the Union Chemical Lab, Industrial Technology Research Institute, Hsinchu, Taiwan. Literature Cited (1) Shelef, M. Selective Catalytic Reduction of NOx with N-Free Reductants. Chem. ReV. 1995, 95, 209. (2) Heck, R. M.; Farrauto, R. J. Catalytic Air Pollution Control: Commercial Technology; J. Wiley & Sons: New York, 2002. (3) Inui, T.; Iwamoto, S.; Matsuba, K.; Tanaka, Y.; Yoshida, T. On the Vital Roles of Zeolitic Matrix in Catalysts for De-NOx Reactions under Conditions Similar to Diesel Engine Exhaust. Catal. Today 1995, 26, 23. (4) Sullivan, J. A.; Cunningham, J.; Morris, M. A.; Keneavey, K. Conditions in which Cu-ZSM-5 Outperforms Supported Vanadia Catalysts in SCR of NOx by NH3. Appl. Catal. B: EnViron. 1995, 7, 137. (5) Forzatti, P. Present Status and Perspectives in De-NOx SCR Catalysis. Appl. Catal. A: Gen. 2001, 222, 221. (6) Matsumoto, S. DeNOx Catalyst for Automotive Lean-burn Engine. Catal. Today 1996, 29, 43. (7) Epling, W. S.; Campbell, L. E.; Yezerets, A.; Currier, N. W.; Parks, J. E., II. Overview of the Fundamental Reactions and Degradation Mechanism Storage/Reduction Catalysts. Catal. ReV. 2004, 46, 163-245. (8) Miyoshi, N.; Matsumoto, S. I.; Katoh, K.; Tanaka, T.; Harada, J.; Takahashi, N.; Yokota, K.; Sugiura, M.; Kasahara, K. Development of New Concept Three-Way Catalyst for Automotive Lean-Burn Engines. SAE Paper 950809; SAE International: Warrendale, PA, 1995. (9) Amberntsson, A.; Persson, H.; Engstrom, P.; Kasemo, B. NOx Release from a Noble Metal/BaO Catalyst: Dependence on Gas Composition. Appl. Catal. B: EnViron. 2001, 31, 27.

Ind. Eng. Chem. Res., Vol. 45, No. 1, 2006 141 (10) Takahashi, N.; Shinjoh, H.; Iijima, T.; Suzuki, T.; Yamazaki, K.; Yokota, K.; Suzuki, H.; Miyoshi, N.; Matsumoto, S.-I.; Tanizawa, T. The New Concept 3-way Catalyst for Automotive Lean-burn Engine: NOx Storage and Reduction Catalyst. Catal. Today 1996, 27, 63. (11) Kabin, K. S.; Muncrief, R. L.; Harold, M. P. NOx Storage and Reduction on a Pt/BaO/Alumina Monolithic Storage Catalyst. Catal. Today 2004, 96, 79. (12) Chi, Y.; Chuang, S. S. C. Infrared Study of NO Adsorption and Reduction with C3H6 in the Presence of O2 over CuO/γ-Al2O3. J. Catal. 2000, 190, 75. (13) Chi, Y.; Chuang, S. S. C. The Effect of Oxygen Concentration on the Reduction of NO with Propylene over CuO/γ-Al2O3. Catal. Today 2000, 62, 303. (14) Chi, Y.; Chuang, S. S. C. Infrared and TPD Studies of Nitrates Adsorbed on Tb4O7, La2O3, BaO, and MgO/γ-Al2O3. J. Phys. Chem. B 2000, 104, 4673. (15) Yao, H. C.; Yao, Y. F. Y. Ceria in Automotive Exhaust Catalysts: I. Oxygen Storage. J. Catal. 1984, 86, 254. (16) Fornasiero, P.; Balducci, G.; Dimonte, R.; Kaspar, J.; Sergo, V.; Gubitosa, G.; Ferrero, A.; Graziani, M. Modification of the Redox Behaviour of CeO2 Induced by Structural Doping with ZrO2. J. Catal. 1996, 164, 173. (17) Huang, H. Y.; Long, R. Q.; Yang, R. T. The Promoting Role of Noble Metals on NOx Storage Catalyst and Mechanistic Study of NOx Storage under Lean-Burn Conditions. Energy Fuels 2001, 15, 205. (18) Fridell, E.; Skoglundh, M.; Johansson, S.; Westerberg, B.; Torncrona, A.; Smedler, G. Investigations of NOx Storage Catalysts. Catal. Autom. Pollut. Control IV 1998, 116, 537. (19) Di Monte, R.; Fornasiero, P.; Kaspar, J.; Ferrero, A.; Gubitosa, G.; Graziani, M. NO Reduction by CO over Pd/CeO2-ZrO2-Al2O3 catalysts. Catal. Autom. Pollut. Control IV 1998, 116, 559.

(20) Fornasiero, P.; Rao, G. R.; Kaspar, J.; L’erario, F. ; Graziani, M. Reduction of NO by CO over Rh/CeO2-ZrO2 CatalystssEvidence for a Support-promoted Catalytic Activity. J. Catal. 1998, 175, 269. (21) Iizuka, H.; Kuroda, O.; Ogawa, T.; Yamashita, H.; Azuhata, S.; Kitahara, Y.; Atago, T.; Tokuda, H.; Ishikawa, T.; Hanaoka, H.; Tobita, H.; Ichikawa, S.; Hiratsuka, T. Exhaust Gas Purifying Method and Catalyst Used Therefore. (Hitachi, Ltd.). U.S. Patent 6,045,764, 2000. (22) Hegedus, L. L.; Summers, J. C.; Schlatter, J. C.; Baron, K. PoisonResistant Catalysts For The Simultaneous Control Of Hydrocarbon, CarbonMonoxide, and Nitrogen-Oxide Emissions. J. Catal. 1979, 56, 321. (23) Schwartz, J. M.; Schmidt, L. D. Reactivity and Microstructure of Rh and Rh-Ce in NO + CO. J. Catal. 1994, 148, 22. (24) Bogner, W.; Kramer, M.; Krutzsch, B.; Pischinger, S.; Voigtlander, D.; Wenninger, G.; Wirbeleit, F.; Brogan, M. S.; Brisley, R. J.; Webster, D. E. Removal of Nitrogen Oxides from the Exhaust of a Lean-tune Gasoline Engine. Appl. Catal. B: EnViron. 1995, 7, 153. (25) Nunan, J. G.; Cohn, M. J.; Donner, J. T. Effect of High-Temperature Lean Aging on the Performance of Pt,Rh/CeO2 and Rare-Earth AlkalineEarth Doped Pt,Rh/CeO2 Catalysts. Catal. Today 1992, 14, 277. (26) Yao, Y. F.; Kummer, J. T. Low-Concentration Supported Precious Metal-Catalysts Prepared by Thermal Transport. J. Catal. 1987, 106, 307.

ReceiVed for reView May 16, 2005 ReVised manuscript receiVed September 15, 2005 Accepted October 21, 2005 IE050574I