Cryptomelane as High-Capacity Sulfur Dioxide Absorbent for Diesel

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Cryptomelane as High-Capacity Sulfur Dioxide Absorbent for Diesel Emission Control: A Stability Study Liyu Li and David L. King* Materials Division, Pacific Northwest National Laboratory, Post Office Box 999, Richland, Washington 99354

A high-capacity sulfur oxide absorbent, cryptomelane, has been described previously. Its SO2 capacity can be as high as 70 wt %, which is more than 10 times as high as the capacities of standard metal oxide-based SO2 absorbents. In this study, the stability of cryptomelane under oxidizing, inert, reducing, and lean-rich cycling conditions was determined. Cryptomelane is stable in oxidizing and inert atmospheres. However, it is unstable under reducing atmospheres, converting to lower valent manganese oxide compounds. These low-valent oxides have very low capacity toward SO2. Upon reexposing the reduced cryptomelane products to an oxidizing atmosphere, cryptomelane may re-form. Cryptomelane exposed to the types of lean-rich cycles that have been proposed for NOx traps for diesel emission control, over the temperature range 250-550 °C, remains stable and maintains its very high SO2 capacity. It appears possible to use cryptomelane to protect the NOx traps from sulfur oxide degradation during cyclic leanrich operation. Introduction Diesel engines are becoming increasingly popular because of their fuel efficiency relative to gasoline sparkignited engines. However, the combustion process in a diesel engine results in production and emission of carbon-based particulates and nitrogen oxides (NOx). Allowed levels of these polluting species are being continually decreased through regulation, with very challenging standards having been set for the years 2006 and 2010.1 Engine manufacturers are seeking to meet these standards through introduction of sophisticated emission control technologies based on combining particulate filters with NOx reduction catalysts or NOx absorbers. One promising approach, based on NOx traps, stores NOx as an alkali or alkaline earth nitrate during normal operation (lean conditions) and releases nitrogen as N2 during a brief fuel-rich reduction step (rich conditions).2 Sulfur oxides contribute significantly and deleteriously to the overall performance of the NOx trap system, mostly by reacting with alkali or alkaline earth oxides to form sulfates during lean conditions, which cannot easily be removed during the rich cycles.2-4 To protect the entire emission post-treatment system, one possible approach is to include a separate dedicated sulfur trap upstream of the NOx trap. For operation in a realistic system, this may require the sulfur absorbents to be stable under both lean (oxidizing) and brief rich (reducing) conditions. Cryptomelane, KxMn8O16, has been identified as a high-capacity sulfur oxide absorbent under oxidizing and inert conditions.5 Over a temperature range from 250 to 475 °C, cryptomelane’s SO2 capacity can be as high as 70 wt %, which is almost 10 times as high as that of conventional metal oxide-based SO2 absorbents. The dominant mechanism for SO2 absorption by cryptomelane is through the oxidation of SO2 to SO3 by Mn4+ and Mn3+ followed by SO3 reaction with Mn2+ to form MnSO4. * To whom correspondence should be addressed. Tel: 509375-3908. Fax: 509 375-2186. E-mail: [email protected].

In this paper, we describe a study of the stability of cryptomelane under reducing conditions and lean-rich cycling conditions that simulate the proposed NOx trap post-treatment system. It has been found that although the cryptomelane materials are not stable under most reducing conditions, the reduced form can be oxidized back to the original cryptomelane structure. Cryptomelane that has experienced lean-rich cycles up to 550 °C still has very high SO2 capacity. Experimental Section Cryptomelane was prepared using the reflux methods developed by DeGuzman et al.6 A typical synthesis was as follows: 11.78 g (74 mmol) of KMnO4 in 200 mL of water was added to a solution of 23.2 g (104 mmol) of MnSO4‚4H2O in 60 mL of water and 6 mL of concentrated HNO3. The solution was refluxed at 100 °C for 24 h, and the product was washed and dried at 120 °C. The yield was 18.3 g. An alternative synthesis method (MnSO4 oxidation method) for cryptomelane involved bubbling O2 through a solution of MnSO4 and KOH, followed by calcining the recovered solid at 600 °C.6 In a typical preparation, a solution of 15.7 g (280 mmol) of KOH in 100 mL of water was added to a solution of 14.9 g (88 mmol) of MnSO4‚ H2O in 100 mL of water. Oxygen gas was bubbled (about 10 L/min) through the solution for 4 h. The product was washed with water and calcined in air at 600 °C for 20 h. The yield was 4.9 g. Cryptomelane prepared by the reflux method was used in this work unless otherwise specified. The absorbents were evaluated mostly as 40-80 mesh granules, formed by pressing a tablet at 20 000 psi for 5 min followed by crushing and screening. A large portion of the stability study was carried out using a Netzsch STA 409 thermogravimetric analysis (TGA)-differential scanning calorimetry (DSC)-mass spectroscopy (MS) system. Different gases, including air, 2% H2 in Ar, 2% C3H6 (propylene) in Ar, and He were used for the TGA-DSC-MS analysis. To get a large amount of treated samples for other characterizations,

10.1021/ie050590f CCC: $30.25 © 2005 American Chemical Society Published on Web 08/10/2005

Ind. Eng. Chem. Res., Vol. 44, No. 19, 2005 7389 Table 1. Composition of Simulated Diesel Engine Exhausts Used in the Studya simulated exhaust

carbon soot (g/L)

CO

CO2 (%)

C3H6 (ppm)

H2 (%)

lean conditionsb rich conditionsb lean-rich cycle Ac lean rich lean-rich cycle Bd lean rich

5 × 10-5 2 × 10-5

3250 ppm 2000 ppm

7.1 10

360 1000

0 0

0 10

0 0

0 4%

10 10

0 4000

0 1.3

10 10

0 0

0 2%

5 12.5

0 333

0 2

H2O (%)

0 0

SO2 (ppm)

O2 (%)

NO2 (ppm)

NO (ppm)

N2

0 0

10.2 0-1

0 0

230 500

balance balance

0 0

12 1.5

0 0

500 500

balance balance

5.15 0

12 0

20 0

180 0

balance balance

a Recommendations for all exhaust compositions were provided by Caterpillar, Inc. b Gas compositions used in preparing samples at Caterpillar, Inc. c Treatment conditions under lean-rich cycle A: 475 °C for 6.5 h, cycling at 6 min lean and 30 s rich; lean and rich gas flows at 26 000 h-1 GHSV. d Treatment conditions under lean-rich cycle B: 250, 400, and 550 °C for 6 h, respectively, cycling at 4 min lean and 20 s rich; lean exhaust gas flow at 50 000 h-1 GHSV and rich exhaust gas flow at 10 000 h-1 GHSV.

Figure 1. SO2 absorption capacity of 600 °C humidified-air-treated cryptomelane. Treatment conditions: 600 °C for 3 h in 10% H2O, 90% Air at 30 000 h-1 GHSV. Test conditions: 325 °C, 250 ppm SO2 in air at 8000 h-1 GHSV. Cryptomelane was prepared by oxidation of MnSO4 by O2.

cryptomelane was also treated in a tube furnace with flowing air, 2% C3H6 in Ar, simulated rich condition diesel engine exhaust, and simulated lean condition diesel engine exhaust. Lean-rich cycling treatments were carried out with an AMI-200R-HP unit (Altamira Instruments, Pittsburgh, PA), which can automatically switch the feed to a heated reactor between lean and rich exhaust gases at given time intervals. The composition of these simulated exhausts is given in Table 1. Two samples were prepared by Caterpillar Inc. for subsequent SO2 capacity testing and analysis in our laboratory. The exhaust compositions of those treatments are identified as “lean conditions” and “rich conditions” in Table 1. The two different feed compositions for leanrich cycling experiments carried out in our laboratory are identified as “lean-rich cycle A” and “lean-rich cycle B” in Table 1. Powder X-ray diffraction (XRD), BET surface area (SA), and scanning electron microscopy (SEM) images were collected on some of the tested materials. Powder XRD experiments were conducted with a Philips PW3050 diffractometer using Cu KR radiation and JADE, a commercial software. Sample powders were mounted in a front-loading, shallow-cavity zero-background quartz holder, and the data were collected from 5° to 75° 2θ in step-scan mode using steps of 0.02°. The BET surface area was determined by nitrogen adsorption/desorption using a QUANTACHROME AUTOSORB 6-B gas sorption. Scanning electron microscopy (SEM) analysis was

carried out on a LEO 982 ultrahigh-performance fieldemission scanning electron microscope. The SO2-absorption tests were performed in a small fixed bed quartz tube reactor, which was heated by a small clam-shell furnace. Reactant gases were metered using mass flow controllers. The SO2 analytical system comprised a HP6890 gas chromatograph equipped with a Sulfur Chemiluminescent Detector (SCD), which has been described in detail previously.7 During the experimental run, the analytical system operated continuously, sampling the effluent every 3 min. The minimum detection limit of the system to SO2 is approximately 50 ppb. Typical measurements employed a 0.2 g 40-80 mesh particle sample. The standard absorption tests were performed at 325 °C, using a feed gas of 250 ppm SO2 in air, at a flow of 8000 h-1 gas hourly space velocity (GHSV). Results and Discussion Stability under Oxidizing Conditions. Cryptomelane synthesized by the reflux method has a BET surface area of 74 m2/g, a bulk density of 0.66 g/cm3, and crystallite dimensions of approximately 30 nm × 30 nm × 200 nm, while cryptomelane synthesized by the MnSO4 oxidation method has a BET surface area of 32 m2/g, a bulk density of 0.99 g/cm3, and the same crystallite dimensions.5 Slight agglomeration of the needlelike crystals was observed in the cryptomelane

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Figure 2. SO2 absorption capacity of lean exhaust-treated cryptomelane. Treatment conditions: 500 °C 1 h in simulated lean exhaust (see Table 1 “lean condition”). Test conditions: 325 °C, 250 ppm SO2 in air, 8000 h-1 GHSV. Cryptomelane was prepared by the reflux method.

Figure 3. TG-DSC traces of cryptomelane prepared by the reflux method. Test conditions: 10 °C/min heating rate, He at 40 mL/min flow.

sample synthesized by the MnSO4 oxidation method. Reflux-prepared cryptomelane is stable in air up to 600 °C, whereas cryptomelane synthesized by the MnSO4 oxidation method is stable in air to 850 °C, as determined by XRD and TGA-DSC analysis. Two additional tests were performed to further check the stability in an oxidizing atmosphere. In the first test, cryptomelane prepared by the MnSO4 oxidation method was treated at 600 °C for 3 h in air containing 10% H2O, to test for steam/air stability. In the second test, cryptomelane prepared by the reflux method was treated at 500 °C for 1 h in simulated lean exhaust gas (“lean conditions” in Table 1). Following these treatments, no structural changes were observed on the basis of XRD analysis. The SO2 capacities were measured following these treatments, and the results are given in Figures 1 and 2, respectively. As can be seen, even under these extreme conditions, cryptomelane maintains a high SO2 capacity. Stability under Inert Conditions. Cryptomelane shows high SO2 capacity at 325 °C under an inert atmosphere.5 Its stability under ultra-high-purity helium was studied using TGA-DSC-MS. Figures 3 and 4 show the TGA-DSC analysis results. Each endother-

mic peak in the DSC curves, i.e., at 480, 550, and 770 °C for cryptomelane from the reflux method, and at 640 and 810 °C for cryptomelane from the MnSO4 oxidation method, corresponds to some O2 release, which was detected by mass spectrometer. Under this inert atmosphere, cryptomelane from the reflux method is stable up to 450 °C, and cryptomelane from the MnSO4 oxidation method is stable up to 580 °C. At higher temperatures, cryptomelane decomposes. Cryptomelane is less stable under an inert atmosphere than under air. Stability under Reducing Conditions. As a highvalence manganese oxide, cryptomelane is unstable under reducing conditions. Figure 5a shows the DSC analysis results of cryptomelane prepared by the reflux method under 2% C3H6 in argon. Cryptomelane is reduced as low as 300 °C. After reduction, Mn3O4 and MnO form. The Mn3O4 is further reduced to MnO when the reduction temperature exceeds 550 °C. A similar result was also obtained with 2% H2 in argon as the reductant. Cryptomelane prepared by the MnSO4 oxidation synthesis method shows similar results. In a separate experiment, a sample of cryptomelane was reduced in 2% C3H6 in He at 550 °C for 1 h to produce MnO and this sample was then analyzed in the

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Figure 4. TG-DSC traces of cryptomelane from the MnSO4 oxidation method. Test conditions: 10 °C/min heating rate, He at 40 mL/ min flow.

Figure 5. (a) DSC trace of cryptomelane in 2% C3H6 in Ar. Test conditions: 10 °C/min heating rate, 2% C3H6 in Ar at 40 mL/min flow; (b) DSC trace of 550 °C, 1 h, 2% C3H6 in He treatedcryptomelane heated in air. Test conditions: 10 °C/min, air at 40 mL/min.

DSC apparatus. The MnO readily oxidizes upon heating in an oxidizing atmosphere. Figure 5b gives the DSC trace showing reoxidation of the MnO beginning as low as 250 °C. In a separate experiment, a sample of cryptomelane was treated sequentially by reduction at 550 °C for 1 h in 2% C3H6 in He followed by oxidation at 500 °C for 1 h in air. Most of the reduced cryptomelane recovers the original cryptomelane crystal structure, as shown in Figure 6. The minority product of reoxidation is Mn3O4. Although the crystal structure can be mostly recovered, the morphology and surface area of cryptomelane change dramatically. BET surface area decreases from 74 to 4.9 m2/g, and the needlelike crystal shape is lost (Figure 7). After this 2-h reduction-oxidation cycle, cryptomelane loses much of its SO2 absorption capacity,

as shown in Figure 8. After the SO2 absorption uptake is complete, a significant amount of cryptomelane still remains unconverted. Thus, an extended period of exposure of cryptomelane to reducing atmospheres at elevated temperature results in significant deterioration of physical properties and SO2 absorption performance of the material. Not all reduced cryptomelane can recover its original crystal structure after reoxidation. Similar tests were performed using 2% H2 in He and simulated rich exhaust gas as reductants. Table 2 summarizes the phases found after the reduction-oxidation tests. For all of these tests, the most abundant phase formed after reoxidation is either Mn3O4 or Mn2O3 rather than cryptomelane. As a result, SO2 capacity decreases significantly. Figure 9 shows SO2 absorption of rich-exhausttreated cryptomelane. Stability under Lean-Rich Cycling Conditions. The above results have shown that extended periods of exposure of cryptomelane to reducing conditions followed by subsequent oxidation restore the crystal structure but not surface area or SO2 capacity. However, we wished to investigate the possibility that if the cryptomelane were exposed to only brief periods of reducing atmosphere and longer periods of oxidizing conditions then the loss of surface area and SO2 capacity might be much less. We therefore sought to determine whether cryptomelane could maintain its crystal structure and much of its SO2 absorption capacity through extended periods of reduction-oxidation cycling. We employed lean-rich cycling protocols that have been proposed for NOx traps (see Table 1). A cryptomelane sample was treated under simulated lean-rich cycles at 475 °C for 6.5 h (cycling at 6 min lean and 30 s rich) using lean-rich cycle A. Both lean and rich exhaust gas flow rates were at 26 000 h-1 GHSV. Figure 10 shows the XRD patterns of the cryptomelane sample before and after the lean-rich treatment. Almost identical XRD patterns were obtained. Some morphology changes were observed under SEM (Figure 11), and the BET surface area decreased from 74 to 20 m2/g after treatment. Compared to the result shown in the previous section with samples treated in 2% C3H6 in He at 550 °C for 1 h and then in air at 500 °C for another 1 h, the short duration lean-rich cycling causes much less deterioration of the cryptomelane absorbent. Figure 12 shows the SO2 absorption curve of the lean-rich-treated cryptomelane. Both breakthrough and total capacities are close to that of the fresh

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Figure 6. XRD patterns of reduced (top) and reoxidized (bottom) cryptomelane: reduction in 2% C3H6 in He at 550 °C for 1 h; reoxidation in air at 500 °C for 1 h.

Figure 7. SEM image of (a) as-synthesized cryptomelane and (b) reduced-reoxidized cryptomelane: reduction in 2% C3H6 in He at 550 °C for 1 h; reoxidation in air at 500 °C for 1 h.

absorbent. This result confirms that cryptomelane is more stable under realistic short lean/rich cycling than under an extended period of exposure to rich gas followed by a subsequent oxidation.

If one were to operate a SOx trap in conjunction with a NOx trap, it would need to operate stably over the temperature range from 250 to 550 °C that the NOx trap would experience. We therefore exposed cryptomelane to lean-rich cycling at 250, 400, and 550 °C. Each treatment lasted 6 h, with the cycling protocol 4 min lean (50 000 h-1 GHSV) and 20 s rich (10 000 h-1 GHSV). The composition of the lean and rich gases is given in Table 1 (lean-rich cycle B). No structural changes were observed with cryptomelane from the reflux method after 250 and 400 °C lean-rich treatment. However, after 550 °C treatment, cryptomelane from the reflux method was mostly reduced to Mn2O3. On the other hand, cryptomelane from the MnSO4 oxidation method only showed a slight structural change following the same lean-rich treatment. The XRD patterns of these samples after lean-rich treatment at 550 °C are shown in Figure 13. Figure 14 shows the SO2 absorption performance of cryptomelane, prepared by the reflux method, after 250 and 400 °C lean-rich treatment, and cryptomelane, from the MnSO4 oxidation method, after 550 °C lean-rich treatment. After 6 h of lean-rich cycling all three samples give very high SO2 capacity. Stability of SO2-Loaded Absorbent. After SO2 absorption on cryptomelane sufficient to generate a fully spent sample, MnSO4 and K2Mn2(SO4)3 form. These phases are stable up to 700 °C under an oxidizing atmosphere. The stability of SO2-saturated cryptomelane under reducing conditions was studied using TGA-MS (Figure 15). In terms of sulfur retention, spent cryptomelane is stable up to 600°C in 2% H2 in He. Above 600 °C, SO2 and H2S are released. Potential Application of Cryptomelane as SOx Trap for Diesel Emission Control. The lean-rich cycling protocols proposed for the NOx traps consist of cycles between relatively long periods of lean exhaust (several minutes) and short periods of rich exhaust (several seconds). Since there is only a small amount of reductant in each rich cycle, only the leading edge portion of the cryptomelane bed is reduced by the


Figure 8. SO2 absorption of reduced-reoxidized cryptomelane: reduction in 2% C3H6 in He at 550 °C for 1 h; reoxidation in air at 500 °C for 1 h. Test conditions: 325 °C, in 250 ppm SO2 in air at 8000 h-1 GHSV. Table 2. Phases Formed after Reduction-Oxidation Tests of Cryptomelanea phases after treatment

a

reduction conditions

reduction

500 °C for 1 h in air

600 °C for 1 h in air

2% H2 in He, 250 °C, 1 h 2% H2 in He, 300 °C, 1 h 2% H2 in He, 350 °C, 1 h 2% H2 in He, 550 °C, 1 h rich exhaust,b 550 °C 1 h

KMn8O16 Mn3O4, MnO, KMn8O16 MnO, (Mn3O4) MnO Mn3O4 K2Mn4O8 (MnO)

KMn8O16 Mn2O3 KMn8O16 (Mn3O4) Mn3O4 (KMn8O16 MnO) Mn3O4 (MnO KMn8O16) Mn3O4 KMn8O16 (MnO)

KMn8O16 Mn2O3 KMn8O16 (Mn3O4) Mn2O3 KMn8O16 (Mn3O4) Mn2O3 KMn8O16 (Mn3O4) Mn2O3 KMn8O16 (Mn3O4)

Major phase listed first; minor phase listed in parentheses. b Rich exhaust composition given in Table 1.

Figure 9. SO2 capacity of rich-exhaust-treated cryptomelane. Treatment conditions: 550 °C for 1 h in simulated rich exhaust (see Table 1 for rich gas composition). Test conditions: 325 °C, 250 ppm SO2 in air at 8000 h-1 GHSV.

reductant in the rich exhaust and then reoxidized during the following lean cycle. The rest of the bed actually cycles between “inert exhaust” and lean exhaust, not between rich exhaust and lean exhaust as described in the previous sections. Since cryptomelane is significantly more stable in an inert atmosphere than

in a reducing atmosphere, the majority of the absorbent remains nearly unaffected. Hence, the SO2 absorption capacity of samples treated under realistic lean-rich cycling conditions is almost the same as that of the fresh absorbent. The amount of cryptomelane that is reduced is determined by the amount of the reductant available

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Figure 10. XRD patterns of lean-rich-treated (top) and untreated (bottom) cryptomelane. Treatment conditions: 475 °C for 6.5 h, cycling at 6 min lean and 30 s rich, lean and rich exhaust flow at 26 000 h-1 GHSV. Lean and rich gas composition A is given Table 1.

Figure 11. SEM image of cryptomelane after lean-rich cycling. Treatment conditions: 475 °C for 6.5 h, cycling at 6 min lean and 30 s rich, lean and rich exhaust flow at 26 000 h-1 GHSV. Lean and rich gas composition A is given Table 1.

in the rich exhaust for each rich cycle. For the test conditions given in Table 1, less than 8% of the cryptomelane is reduced during the rich cycle A period and less than 2% during the rich cycle B period. As shown in previous sections, under reducing conditions, there is little difference in the stability of cryptomelane prepared by the MnSO4 oxidation method and by the reflux method. However, under inert conditions, cryptomelane prepared by the MnSO4 oxidation method is much more stable than that prepared by the reflux method (580 °C vs 450 °C). No structural changes were observed with cryptomelane prepared by the reflux method after 250 and 400 °C lean-rich treatment. However, after 550 °C treatment, this material was mostly decomposed to Mn2O3. On the other hand, cryptomelane prepared by the MnSO4 oxidation method showed only a slight structure change after the same lean-rich treatment. The stability difference of the two cryptomelane samples after lean-rich cycling at 550 °C further confirms that the absorbent is mostly cycled

between “inert exhaust” and lean exhaust (as described above), not between rich exhaust and lean exhaust. With the consumption of all the reductant in the rich exhaust by cryptomelane, it is impossible to regenerate the downstream NOx trap during the rich cycle. A system that can provide reversed rich exhaust flow, such as the one proposed by Parks, et al.,8 should be considered. During the lean cycles, the cryptomelane SOx trap can protect the downstream NOx trap from sulfur poisoning. During the reversed-flow rich cycles, the NOx trap will consume much of the reductant in the exhaust and thus the downstream cryptomelane SOx trap will undergo less reduction. In this study, a fixed bed loaded with cryptomelane granulated powder was used for the SO2 absorption measurements. For automotive diesel emission control, it will be necessary to place the absorbent on a monolith support. Formulations enabling the material to be stabilized on monoliths are being studied, and tests with monolith-supported samples will be done subsequently. On the basis of the capacities we have measured in this work and in our previous work,5 to protect the NOx trap for a typical diesel engine, an acceptable volume and weight of cryptomelane absorbent in a monolith configuration could be realized with periodic replacement (at approximately 3 month intervals). Summary and Conclusions Cryptomelane, a manganese-based octahedral molecular sieve material, has demonstrated high absorption capacity toward sulfur oxides. Among the potential uses for this material could be its implementation on a truck for diesel oxide emissions control by placing it upstream of a NOx absorber. A dedicated SOx trap could be replaced during normal service intervals of the engine. Its implementation would require this material to meet several technical requirements, including maintaining capacity in a lean-rich cycling environment and being capable of operating on a monolith. This paper focuses on the first consideration, the stability of this material


Figure 12. SO2 absorption of lean-rich-treated cryptomelane. Treatment conditions: 475 °C for 6.5 h, cycling at 6 min lean and 30 s rich, lean and rich gas flows at 26 000 h-1 GHSV. Lean and rich gas composition A is given Table 1. Test conditions: 325 °C, 250 ppm SO2 in air at 8,000 h-1 GHSV.

Figure 13. XRD patterns of 550 °C lean-rich-treated cryptomelane prepared by the reflux method (upper) and by the MnSO4 oxidation method (lower). Treatment conditions: 550 °C for 6 h, cycling at 4 min lean and 20 s rich; lean gas flow at 50 000 h-1 GHSV, rich gas flow at 10 000 h-1 GHSV. Lean and rich gas composition B is given Table 1.

in an emissions environment. This includes lean-rich cycling under a variety of conditions, using gas compositions proposed to be similar to those encountered during actual engine operation. The second consideration, implementation on a monolith, is the subject of ongoing work. Cryptomelane is very stable under oxidizing atmospheres. However, it is unstable and readily reduces to MnO and Mn3O4 in the presence of reducing gases such as H2, CO, and C3H6, which would be encountered during a rich regeneration cycle of a NOx trap. These lower valent manganese oxides have low SO2 absorption capacity. However, we have found that when reexposed to an oxidizing atmosphere, the cryptomelane structure reappears, as demonstrated by X-ray diffraction. The extent to which cryptomelane can recover its original crystal structure and its SO2 absorption performance after exposure to oxidizing and reducing treatments largely depends on the reductant composition, the period

of time of exposure to rich gases, the temperature, and the method of preparation of the material. Exposure of cryptomelane to 2% C3H6 in He at 550 °C for 1 h followed by a reoxidation treatment results in only partial recovery of the cryptomelane structure. Substantial change in the textural properties of this material is observed, and this reoxidized material has low surface area and absorption capacity. However, exposure of cryptomelane to more realistic lean-rich cycle conditions (4 min lean, 20 s rich) for several hours results in a material that shows much less structural degradation and maintains a high SO2 absorption capacity. There are two methods of cryptomelane synthesis that we have employed in our study, the first a reflux method and the second an oxidation method. We have found that the synthesis procedure affects its surface area and stability. Cryptomelane prepared by the MnSO4 oxidation method has a moderately lower surface area and

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Figure 14. SO2 absorption curves of lean-rich-treated cryptomelane: (top) cryptomelane from the reflux method lean-rich treated at 250 °C; (middle) cryptomelane from the reflux method lean-rich treated at 400 °C; (bottom) cryptomelane from the MnSO4 oxidation method lean-rich treated at 550 °C. Treatment conditions: 250, 400, and 550 °C for 6 h respectively, cycling at 4 min lean and 20 s rich, lean exhaust gas flow at 50 000 h-1 GHSV and rich exhaust gas flow at 10 000 h-1 GHSV. Lean and rich gas composition B is given Table 1. Test conditions: 325 °C, 250 ppm SO2 in air at 8000 h-1 GHSV.


Figure 15. TGA and mass spectral analysis of SO2-loaded cryptomelane under reducing gas flow. Test conditions: 10 °C/min heating rate, 2% H2 in He at 40 mL/min flow.

slightly lower SO2 capacity. Cryptomelane prepared by the reflux method can be exposed to realistic lean-rich cycles up to 475 °C, whereas cryptomelane prepared by the MnSO4 oxidation method maintains its structure and high SO2 capacity following the same treatment up to 550 °C. In actual operation, the amount of reductant that is employed during a rich cycle would be small compared to the total capacity of the cryptomelane absorption bed. Thus only a small portion of the material at the leading edge of the bed would be exposed to reducing conditions during the rich cycle (and reoxidized during the lean cycle), and the remainder of the bed would experience an “inert exhaust” condition under which it would maintain high SOx capacity. This would continue as long as the leading edge of the bed could cycle and maintain capacity. The effect of the SOx trap operation on the NOx trap must also be considered. Since cryptomelane consumes virtually all the reductant during the rich cycle, insufficient reductant would make its way to the downstream NOx trap during this same rich cycle to allow its regeneration. The best method to operate a SOx-NOx trap system would therefore be to pass lean cycle gases first through the SOx trap, and pass the rich cycle gases first through the NOx trap, i.e., by reversing flow. By this method cryptomelane could utilize its high capacity toward SOx during lean conditions, and only a small portion at the opposite end of the bed would be exposed to the rich conditions. The practicality of such an approach would need to be determined. Acknowledgment Support of this work by the U.S. Department of Energy, Office of FreedomCAR and Vehicle Technologies, is gratefully acknowledged. This research was performed in part using the facility in the William R. Wiley Environmental Molecular Sciences Laboratory, a national scientific user facility sponsored by the U.S. Department of Energy’s Office of Biological and Environmental Research and located at the Pacific North-

west National Laboratory. Pacific Northwest National Laboratory is operated by Battelle for the U.S. Department of Energy. We also thank Dr. Alexander Panov of Caterpillar, Inc. for providing lean-rich exhaust cycling conditions, for providing samples that were treated under rich and lean conditions, and for helpful discussions. Literature Cited (1) Heavy-Duty Engine and Vehicle Standards and Highway Diesel Fuel Sulfur Control Regulations; Regulatory Announcement EPA 420-F-00-057; United States Environmental Protection Agency: Washington, DC, Dec 2000 (available at http://www. epa.gov/otaq/regs/hd2007/frm/f00057.pdf, Jun 2004). (2) Takahashi, N.; Shinjoh, H.; Iijima, T.; Suzuki, T.; Yamazaki, K.; Yokota, K.; Suzuki, H.; Miyoshi, N.; Matsumoto, S. I.; Tanizawa, T.; Tanaka, T.; Tateishi, S. S.; Kasahara, K. The New Concept 3-Way Catalyst for Automotive Lean-Burn Engine: NOx Storage and Reduction Catalyst. Catal. Today 1996, 27, 63. (3) Corro, G. Sulfur Impact on Diesel Emission ControlsA Review. React. Kinet. Catal. Lett. 2002, 75(1), 89. (4) Catalyst-Based Diesel Particulate Filters And NOx Adsorbers: A Summary of the Technologies and the Effects of Fuel Sulfur; MECA (Manufacturers of Emission Controls Association): Washington, DC, Aug 2000. (available at http://www.autoenv.org/tech/ aftertreatment/cbdpf-noxadwp. PDF, Jun 2004). (5) Li, L.; King, D. L. High-Capacity Sulfur Dioxide Absorbents for Diesel Emission Control. Ind. Eng. Chem. Res. 2005, 44, 168. (6) DeGuzman, R. N.; Shen, Y. F.; Neth, E, J,; Suib, S. L.; O’Young, C. K.; Levine, S.; Newsam, J. M. Synthesis and Characterization of Octahedral Molecular Sieves (OMS-2) Having the Hollandite Structure. Chem. Mater. 1994, 6, 815. (7) Li, L.; King, D. L. Method for Determining Performance of Sulfur Oxide Adsorbents for Diesel Emission Control Using Online Measurements of SO2 and SO3 in the Effluent. Ind. Eng. Chem. Res. 2004, 43, 4452. (8) Parks, J.; Watson, A.; Campbell, G.; Wagner, G.; Cunningham, M.; Currier, N.; Gallant, T.; Muntean, G. Sulfur Control for NOx Sorbate Catalysts: Sulfur Sorbate Catalysts and Desulfation. SAE Techn. Paper Ser. 2001-01-2001.

Received for review May 18, 2005 Revised manuscript received July 6, 2005 Accepted July 12, 2005 IE050590F

Cryptomelane as High-Capacity Sulfur Dioxide Absorbent for Diesel

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