Catalytic soot oxidation by Ag/BaCeO 3 having tolerance to SO 2 poisoning

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1 FP F Journal of the Ceramic Society of Japan 117 [11] Paper Catalytic soot oxidation by Ag/BaCeO 3 having tolerance to SO 2 poisoning Keita IKEUE, Shintaro KOBAYASHI and Masato MACHIDA Department of Nano Science and Technology, Graduate School of Science and Technology, Kumamoto University, Kurokami, Kumamoto The effect of SO 2 and NO on the catalytic oxidation of diesel particulate (soot) was studied over two Ce oxides, CeO 2 and BaCeO 3. Silver catalysts loaded on both Ce oxides exhibited the high activity, which was accelerated by the pretreatment in a flow of NO/O 2 mixtures. The adsorption species in the form of NO 2/NO 3 should play a role of oxidizing agents for soot. However, the pretreatment of Ag/CeO 2 in a flow of SO 2/O 2 mixtures caused significant deactivation. Strong adsorption of SO 2 onto active site at the three-phase boundary between soot, Ag/CeO 2 and the gas phase would be a reason for the deactivation. Ag/ BaCeO 3 was found to be a catalyst having much improved tolerance to SO 2. The tolerance may be explained by the SO 2 adsorption site residing apart from the catalytically active oxygen site The Ceramic Society of Japan. All rights reserved. Key-words : Diesel soot oxidation, Catalyst, BaCeO 3, CeO 2, Ag [Received August 10, 2009; Accepted September 11, 2009] 2009 The Ceramic Society of Japan 1. Introduction With an increase of diesel automobiles enabling lower fuel consumptions and lower CO 2 emissions, strong demands for the diesel particulate (soot) abatement have arisen in the last two decade. Present diesel particulate filters (DPF) consisting mainly of a honeycomb shape porous ceramics require periodically regeneration in order to prevent the accumulation of soot. By contrast, continuous regeneration assisted by the soot oxidation catalyst is considered as an advanced system. A great number of studies have therefore been devoted to develop the active soot oxidation catalysts. 1) 13) Among these metal oxide catalysts reported so far, CeO 2 is well-known as one of the most active catalyst components. 11) 19) Porous CeO 2 catalyzes soot combustion at low temperatures even in the absence of precious metal catalysts. The role of CeO 2 has been reported to cause spillover of active oxygen onto the soot surface and its subsequent adsorption at the active carbon site is an intermediate step in the soot oxidation mechanism. 18),19) CeO 2 has also been widely used as an oxygen-storage component (CeO 2 ZrO 2) in three-way catalysts. 12),19) The oxygen storage capacity (OSC) due to redox between Ce 4+ and Ce 3+ is able to store oxygen under oxidizing atmosphere and release it under reducing atmosphere. Such an OSC is pointed out to be correlated with the high activity for soot oxidation. 17) From practical viewpoints of soot oxidation catalysts, the interaction with exhaust gases, NOx and SOx, originating from fuel is very important. These acidic gases should compete for the same adsorption sites, but their effect on soot oxidation activity is quite different. It is pointed out that NO oxidized to NO 2 can promote soot oxidation, whereas SO 2 strongly adsorbed onto the active site often causes significant deactivation. 20) 23) The level of sulfur in diesel fuel will further be decreased in the near future. Nevertheless, the development of SO 2-tolerant catalysts is Corresponding author: M. Machida; machida@chem. kumamoto-u.ac.jp strongly requested for a long-term durability. In the present work, the catalytic soot oxidation over CeO 2 and BaCeO 3 has been studied with a view to elucidate the effect of possible coexisting gases, NO and SO 2, on their activity. Because BaCeO 3 was found to show higher tolerance to SO 2 poisoning than CeO 2, possible reasons have been proposed and discussed with regard to the reactive oxygen species responsible for soot oxidation. 2. Experimental section 2.1 Preparation and characterization High purity CeO 2 (Rhodia Ltd., 99.99%, S BET = 157 m 2 g 1 ) was used without further treatment. BaCeO 3 (0.9 m 2 g 1 ) was synthesized by air calcination of the stoichiometric mixtures of Ba(NO 3) 2 (Wako Pure Chemical Industries, Ltd., 99.9%) and Ce(NO 3) 3 (Kojundo Chemical Laboratory Co., Ltd., 99.9%) at 800 C for 2 h and then at 1000 C for 6 h. Supported Ag catalysts (10 wt% loading as Ag metal) were prepared by a simple impregnation of aqueous solutions of AgNO 3 (Wako Pure Chemical Industries, Ltd., 99.9%) onto the oxide powders, which were evaporated to dryness and subsequently calcined at 450 C for 2 h in air. Ag loading was also applied to a low surface area CeO 2 (0.8 m 2 g 1 ), which was obtained by air calcination of as purchased CeO 2. The crystal structure was identified by use of a powder X-ray diffractometer (XRD, Rigaku Co., Multiflex) with monochromated Cu K α radiation (30 kv, 40 ma). Energy dispersive X-ray fluorescence analysis (XRF, Horiba, Ltd., MESA 500W) was used to determine the chemical composition. The BET surface area (S BET) was calculated from N 2 adsorption isotherms measured at 196 C (Bel Japan, Belsorp Mini). The microstructure of the catalysts was observed by a scanning electron microscope (SEM, JEOL Ltd., JSM 6060LV, 20 kv) and a transmission electron microscope (TEM, FEI TECNAI F20, 200 kv). 2.2 Catalytic reaction and SO 2 adsorption The catalytic activity for soot oxidation was determined using 1153

2 JCS-Japan Ikeue et al.: Catalytic soot oxidation by Ag/BaCeO 3 having tolerance to SO 2 poisoning commercially available carbon black powders (Mitsubishi Chemical Corp., MA7; surface area 115 m 2 g 1, average particle size 24 nm) as model diesel soot. The soot and catalyst with weight ratio of 1/20 were ground in an agate mortar for 4 min to obtain so-called tight-contact mixtures. The catalytic test was carried out in a conventional flow reactor. The sample (0.1 g) of the soot/catalyst mixtures with weight ratio of 1/20 was placed in a quartz container. During the heating at the rate of 10 Cmin 1, gaseous mixtures of 10% O 2 and N 2 balance were fed to the sample (200 cm 3 min 1 ). The effluent gas was analyzed by nondispersive infrared (NDIR) CO/CO 2 analyzers (Horiba, Ltd., VIA510). The soot oxidation test was also carried out after the treatments by dilute SO 2 or NO in a flow reactor. The treatments were done in streams of 200 ppm SO 2, 10% O 2 balanced with He or 1000 ppm NO, 10% O 2 balanced with He at 200 C for 1 h. The effluent gas was analyzed by NDIR (Horiba, Ltd., VA3000) apparatus to obtain breakthrough curves of SO 2/NO adsorption. in situ FT IR spectroscopy was conducted on a Jasco 610 spectrometer and a temperature controllable diffuse reflectance reaction cell (Jasco, Inc., DR600A) with a KBr window which was connected to a gas supply line. The sample was first preheated in flowing He at 200 C for 10 min, followed by admission of gas mixtures of 200 ppm SO 2, 10% O 2 and He balance for 1 h. The call was then purged by He for 10 min at 200 C, where a spectrum was recorded at a resolution of 4 cm 1. All spectra thus obtained were transformed into absorption spectra by the use of Kubelka-Munk function and referenced to those taken just before SO 2/O 2 admission. 3. Results and discussion 3.1 Structure and microstructure of catalysts X-ray diffraction patterns of as prepared and 10 wt% Agloaded oxides are shown in Fig. 1. A weak and broad diffraction peaks due to CeO 2 suggests a very small crystallite size, which is determined to be 12 nm by a line-broadening analysis using Scherrer s equation. This is close to the values obtained from S BET (6 nm) and TEM observation (5 10 nm). As prepared BaCeO 3 showed sharp and strong diffraction peaks suggesting the formation of a highly crystallized single phase of the orthorhombic perovskite-type structure. The high crystallinity is consistent with a low S BET less than 1 m 2 g 1. After impregnation of Ag and subsequent air calcination at 450 C, trace amounts of BaCO 3 and Ba(NO 3) 3 were formed probably because of the contamination from atmospheric CO 2 and AgNO 3, respectively. However, these two phases disappeared after calcinations at 600 C. Silver on each catalyst was detected as a metallic phase. Figure 2 shows SEM photographs of Ag/CeO 2 and Ag/BaCeO 3. Ag/CeO 2 consists of fine particles less than 100 nm, which are agglomerated into secondary grains more than 1 μ m. As evident from TEM observation (not shown), the size of primary particles of CeO 2 was in the range of several nanometers. On the other hand, Ag/BaCeO 3 consists of dense and large crystallites (several micrometers) of BaCeO 3 and Ag particles ( 100 nm) which are dispersed on the surface of BaCeO Soot oxidation activity The catalytic activities of CeO 2 and BaCeO 3 for soot oxidation with and without Ag loading are compared. Figure 3 shows the CO 2 evolution profiles when the tight contacting soot/catalyst mixtures were heated at a constant rate (10 C min 1 ) in a stream of 10% O 2 in N 2. Heating soot alone in 10% O 2/N 2 at constant rate (10 Cmin 1 ) yielded a CO 2/CO peak with a ratio of CO 2:CO = 6:4 at about 660 C. The combustion temperature and CO yield could considerably be reduced by mixing with the catalysts. The catalytic activity can therefore be evaluated by using the temperature of CO 2 peak, i.e., CeO 2 (397 C) is more active than BaCeO 3 (506 C). The evolution of CO by soot combustion was negligible in both case. It is well-known that CeO 2 is among the most active oxide catalysts for soot oxidation. 11) 19) The high catalytic activity would be associated with the redox property and the reactivity of solid oxygen species. The reactive oxygen is formed not only from gaseous O 2 molecules, but also from the lattice oxygen at the CeO 2/soot interface. 19) The lower catalytic activity of BaCeO 3 is in accordance with its low surface area (< 1 m 2 g 1 ) and the lack of reduction-oxidation ability of Ce, the oxidation state of which must be stabilized in 4+ in the perovskite structure. Figure 3 also shows the effluent gas profiles for 10 wt% Agloaded and unloaded catalysts tightly mixed with soot. Clearly, the temperature of CO 2 peak for CeO 2 could significantly be lowered from 397 C to 351 C by loading Ag. Similarly, Ag loading on BaCeO 3 could lower the temperature of CO 2 peak from 506 C to 418 C. We have previously reported that the oxidation of soot by O 2 cannot be promoted by the precious metals such as Pt, Pd and Rh, but Ag becomes a promising cocatalyst. 19) Villani et al. 24) have also pointed out the potential of silver as an active catalyst for soot oxidation. It is well-known that metallic silver surface can promote the formation of superoxide ions (O 2 ), 25),26) which is expected to enhance further the soot oxidation activity. Fig. 1. XRD pattern of a) CeO 2, b) 10 wt% Ag/CeO 2, c) BaCeO 3 and d) 10 wt% Ag/BaCeO 3. Fig. 2. SEM image of 10 wt% Ag/CeO 2 and 10 wt% Ag/BaCeO

3 Journal of the Ceramic Society of Japan 117 [11] JCS-Japan Fig. 3. Temperature programmed soot oxidation over CeO 2, 10 wt% Ag/CeO 2, BaCeO 3 and 10 wt% Ag/BaCeO C min 1, catalyst : soot = 20:1, 10% O 2, N 2 balance, W/F = g min cm 3. Fig. 4. Temperature programmed soot oxidation over 10 wt% Ag/CeO 2 and 10 wt% Ag/BaCeO 3 before and after treatment in 200 ppm SO 2, 10% O 2, He balance or 1000 ppm NO, 10% O 2, He balance (200 C, 1 h). 10 C min 1, catalyst : soot = 20:1, 10% O 2, N 2 balance, W/F = g min cm Effect of SO 2/NO on soot oxidation activity It is well-known that the catalytic soot oxidation is influenced by the presence of NOx and/or SOx in the diesel exhaust. Soot oxidation over Ag/CeO 2 and Ag/BaCeO 3 was next carried out after the pretreatment by SO 2/O 2 or NO/O 2. As shown as dashed lines in Fig. 4, the treatment of the catalysts in NO/O 2 at 200 C shifted the CO 2 peak slightly toward low temperatures. In particular, it renders the Ag/CeO 2 catalyst able to complete soot oxidation at a low temperature of about 380 C and thus the peak at around 340 C is intensified. Such a promotion effect on soot oxidation can be rationalized by the role of adsorbed NOx species as an oxidizing agent for soot. 20),21) The amount of NOx uptake during the pretreatment at 200 C is compared in Table 1. The larger amount of NOx uptake was observed for Ag/CeO 2 having a larger surface area. However, the surface density of adsorbed NOx species calculated from apparent NO uptake divided by S BET is much higher for Ag/BaCeO 3. In the presence of O 2, NO adsorbed onto these oxides in the form of nitrate (NO 3 ) or nitrite (NO 2 ). 27) The reactivity of these species was studied by temperature programmed thermal desorption in a flowing He. The NOx desorption from Ag/CeO 2 started on heating above 300 C, whereas Ag/BaCeO 3 did not eliminate NOx up to 600 C, which was much higher than the soot ignition temperature. These results suggested the reactivity rather than the quantity of adsorbed NOx is an important factor to promote soot oxidation. Stable NOx species bound to Ba should not be suitable for the promotion of low-temperature soot oxidation. This is a reason why the effect of NO was rather small for the Ag/BaCeO 3 catalyst. In complete contrast to the effect of NO, Ag/CeO 2 was found to be significantly deactivated on exposure to SO 2 as was evident from the shift of the CO 2 peak from 351 to 483 C (dotted line Sample Table 1. Adsorption of NO and SO 2 Onto Catalysts a S BET NO uptake b /m 2 g 1 /mmol g 1 NO density c SO 2 uptake d /μ mol m 2 /mmol g 1 SO 2 density e /μ mol m 2 Ag/CeO Ag/CeO 2(LSA) Ag/BaCeO a BET surface area. b 1000 ppm NO, 10% O 2, He balance, 200 C, 1 h. c NO uptake divided by S BET. d 200 ppm SO 2, 10% O 2, He balance, 200 C, 1 h. e SO 2 uptake divided by S BET. in Fig. 4). This is quite different from Ag/BaCeO 3, the CO 2 peak of which showed very little shift from 418 to 429 C. As a result, Ag/BaCeO 3 exhibits a higher activity than Ag/CeO 2 after exposure to SO 2. Because the deactivation should be associated with the adsorption of SO 2, the breakthrough curves of SO 2 adsorption was measured in a flowing gas mixtures of 200 ppm SO 2, 10% O 2 and He balance during the pretreatment at 200 C for 1 h. As summarized in Table 1, Ag/CeO 2 shows the much larger amount of SO 2 uptake (0.218 mmol g 1 ) than Ag/BaCeO 3 (0.037 mmol g 1 ). These SO 2 species were tightly bound to the surface so that thermal desorption of SO 2 could not be observed on the heating up to 600 C in the stream of He. As in the case of NO adsorption, the surface density of adsorbed SO 2 species is calculated in Table 1. BaCeO 3 exhibited the higher surface SO 2 density than Ag/CeO 2 in accordance with the high basicity of Ba. Consequently, the SO 2 tolerance and SO 2 density of the two catalysts are not consistent. In order to cancel the surface area effect of CeO 2 on the NOx promotion as well as SO 2 poisoning, the soot oxidation activity was next measured for the Ag catalyst loaded on a low surface area (LSA) CeO 2 (0.8 m 2 g 1 ), which was prepared by air calci- 1155

4 JCS-Japan Ikeue et al.: Catalytic soot oxidation by Ag/BaCeO 3 having tolerance to SO 2 poisoning Fig. 5. Temperature programmed soot oxidation over 10 wt% Ag/CeO 2 (S BET = 0.9 m 2 g 1 ) before and after treatment in 200 ppm SO 2, 10% O 2, He balance (200 C, 1 h). 10 C min 1, catalyst : soot = 20:1, 10% O 2, N 2 balance, W/F = g min cm 3. nation of CeO 2 at 1000 C. Figure 5 shows the soot oxidation profiles of Ag/CeO 2(LSA) before and after treatment by NO/O 2 or SO 2/O 2 at 200 C. Owing to its significant loss of surface area by sintering, the temperature of the CO 2 peak was increased by 100 C. The amounts of NOx and SO 2 adsorption onto Ag/ CeO 2(LSA), and mmol g 1, respectively, were about 10 times less than those of Ag/CeO 2 (Table 1). By contrast, the calculated surface densities of these adsorbed species were 10 times larger than those of Ag/CeO 2. After the treatments, the shifts of a CO 2 peak occurred as in the case of large surface area Ag/CeO 2 (Fig. 4), indicating the same promotion/poisoning effects. These results clearly demonstrate that the extent of activation/deactivation is not correlated with the apparent amount of adsorption and/or surface density of adsorbed species. The nature of adsorption site rather than the amount of adsorption is considered to be a key factor. From a simple comparison between Ag/ BaCeO 3 (Fig. 4) and Ag/CeO 2(LSA) (Fig. 5), the former is found to be much more insensitive to NO/SO 2. Figure 6 shows in situ FT IR spectra of adsorbed SO 2 on both catalysts measured at 200 C. The Ag/CeO 2 catalyst treated with SO 2 displayed several broad bands in the cm 1 region, A strong band appeared at 1125 cm 1 is characteristic for asymmetric stretching mode of SO 4 due to bulk sulfate. 20),22),28) Other strong bands centered at about 1010 and 925 cm 1 can be assigned to asymmetric and symmetric stretching modes of sulfite (SO 3). 29) Another band exhibiting maximum frequency in the range from 1250 to 1310 cm 1 may be assigned to molecular SO 2 species having bridging and chelating coordination, which were reported to form when CeO 2 was exposed to SO 2 below 200 C. 22),29) Similar bands observed for Ag/CeO 2(LSA) were much less intense, but the presence of molecular SO 2 (~1300 cm 1 ) is more obvious. By contrast, a weak band at 1135 cm 1 for Ag/ BaCeO 3 is likely due to the formation of small amounts of bulk sulfate. 25) Because the total bulk structure preserved its original fluorite-type CeO 2 and/or perovskite-type BaCeO 3 and sulfates of Ag and Ce were not detected by XRD, the sulfate should be formed near the surface only. We have previously proposed possible reaction pathways in catalytic soot oxidation, which involve adsorbed oxygen and lattice oxygen as active species, on the basis of experiment using isotopic oxygen. 17) A larger amount of adsorbed oxygen should be present on a large-surface area CeO 2. As is evident from Figs. 4 and 5, however, two CeO 2 catalysts with different specific surface areas showed a similar SO 2 poisoning behavior. This is indicative of the deactivation mechanism that is associated with SO 2 adsorption onto surface and lattice oxygen sites around the Fig. 6. in situ FT IR spectra of SO 2 adsorbed on (a) 10 wt% Ag/CeO 2 and (b) 10 wt% Ag/CeO 2(LSA) and (c) 10 wt% Ag/BaCeO 3 measured at 200 C. Pretreatment in 200 ppm SO 2, 10% O 2, balanced by He at 200 C for 1 h. three-phase boundary between soot, Ag/CeO 2 and the gas phase. These adsorbed species can be observed as molecular and bulklike SO 4 as shown in the in situ FT IR spectra (Fig. 6). With an increase of SO 2 adsorption, the active lattice oxygen residing around the three-phase boundary is consumed to form stable SO 4 species and thus activity for soot oxidation would be lost. This is not the case for Ag/BaCeO 3; SO 2 adsorption onto Ag/BeCeO 3 yielded SO 4 species too, but they are bulk-like phases, possibly Ba SO 4, forming near the surface. The higher basicity of Ba compared to Ce would be effective in the oxidative SO 2 adsorption especially in the presence of O 2. Thank to this local and strong interaction, part of the surface active oxygen site (Ce O) efficient for soot oxidation would be able to survive. 4. Conclusion The present work demonstrates that the effects of SO 2 and NO on the soot oxidation activity of Ag-loaded CeO 2 and BaCeO 3. Two CeO 2 catalysts having large and small specific surface area were both deactivated significantly by strong SO 2 adsorption onto the active oxygen site. Ag/BaCeO 3 exhibits soot oxidation activity and tolerance to SO 2 poisoning in spite of high basicity of Ba. On contrary to CeO 2, BaCeO 3 is considered to have SO 2 adsorption sites (Ba) apart from the active oxygen sites (Ce O). This idea can broadly be applied to the design of practical catalytic materials efficient for soot oxidation at lowest possible temperatures. 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