release of a fully formulated lean NOx trap

Applied Catalysis B: Environmental 101 (2011) 486–494

Contents lists available at ScienceDirect

Applied Catalysis B: Environmental journal homepage: www.elsevier.com/locate/apcatb

Effect of lean/rich high temperature aging on NO oxidation and NOx storage/release of a fully formulated lean NOx trap Nathan A. Ottinger a , Todd J. Toops a,∗ , Ke Nguyen b , Bruce G. Bunting a , Jane Howe c a b c

Fuels, Engines and Emissions Research Center, Oak Ridge National Laboratory, 2360 Cherahala Blvd., Knoxville, TN 37932, USA Mechanical, Aerospace, and Biomedical Engineering Department, University of Tennessee, Knoxville, TN 37996, USA High Temperature Materials Laboratory, Oak Ridge National Laboratory, 1 Bethel Valley Rd., Oak Ridge, TN 37831, USA

a r t i c l e

i n f o

Article history: Received 15 January 2010 Received in revised form 13 October 2010 Accepted 18 October 2010 Available online 23 October 2010 Keywords: DRIFTS Lean NOx trap Thermal aging Fully formulated PGM dispersion NOx storage capacity NOx release STEM XRD

a b s t r a c t Commercial-intent lean NOx traps (LNTs) containing Pt, Pd, Rh, Ba, Ce, Zr, and other proprietary additives were thermally aged at 750, 880, 930, and 1070 ◦ C using lean/rich cycling and then investigated for effects of aging on NOx storage capacity, NO oxidation, NOx reduction, and materials properties. Additionally, DRIFTS analysis was used to determine the effects of high temperature aging on surface chemistry and NOx storage. As platinum group metal (PGM) dispersion decreases with aging, the NO turnover frequency (TOF) for NO oxidation at 200, 300, and 400 ◦ C is shown to increase. The fraction of stored NOx that is successfully reduced also increases with aging, and it is suggested that this is accounted for by a slower release of more stable NOx species resulting from thermal aging. NOx storage and NOx release experiments performed with DRIFTS at 200, 300, and 400 ◦ C indicate that a substantial amount of NOx is stored on Al2 O3 as nitrates at 200 and 300 ◦ C before aging. However, almost no nitrates are seen on alumina after aging at 900 and 1000 ◦ C, resulting in a significant reduction in NOx storage capacity. This is most likely due to a 45% reduction in total surface area and a high temperature redispersion of Ba over remaining alumina sites. No evidence of BaAl2 O4 was observed with XRD. © 2010 Elsevier B.V. All rights reserved.

1. Introduction High fuel cost, environmental concerns, and new EPA and European regulations are driving the design and production of cleaner combustion technologies. At the forefront of this recent movement toward “green” fuels and vehicles is clean diesel. Diesel engines have a higher fuel efficiency than other combustion engines, and with recent developments in diesel particulate filter and NOx reduction technologies, diesels have shown the ability to meet the stringent emissions regulations [1–3]. Several NOx reduction techniques are currently being implemented in order to reduce tail-pipe NOx levels, and lean NOx traps (LNTs), also known as NOx adsorber catalysts or NOx storage and reduction (NSR) catalysts, offer one pathway to reduce NOx emissions. LNTs function by oxidizing and storing NO during a long lean phase and later releasing and reducing NOx during a short rich phase. They typically consist of an alkali or alkaline earth metal NOx storage material such as Ba, a precious metal such as Pt for NO oxidation and NOx reduction, and a high surface area Al2 O3 support. Since the short rich phase requires periodic introduction

∗ Corresponding author. Tel.: +1 8659461207; fax: +1 8659461354. E-mail address: [email protected] (T.J. Toops). 0926-3373/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.apcatb.2010.10.020

of supplemental fuel, a better understanding of the NOx storage and reduction processes, especially on fully formulated aged catalysts, is necessary to improve catalyst performance and minimize the associated fuel penalty. The majority of the research previously done on LNTs has focused on simple model Pt/Ba/Al2 O3 or Pt/Al2 O3 catalysts in order to identify the contributions of individual catalyst components. However, because these studies ignore the effects of commonly used additives, more studies based on fully formulated LNTs are needed [4–6], especially since these additives are typically employed to improve stability and durability of the catalyst components. In this study, the effects of high temperature cyclic aging on NO oxidation as well as NOx storage and stability are the focus. PGM sintering due to high temperature aging is a well reported phenomena [4–8], and while a number of studies have shown sintering to be a primary deactivation mechanism of LNTs [9–11], others have shown PGM growth to benefit either NO oxidation or NOx reduction [12–17]. The present study helps clarify the effect that reduction in PGM dispersion has on LNT performance by comparing PGM dispersion varying from 3% to 36% to steady-state NO oxidation experiments. A number of studies have investigated NOx storage on the Al2 O3 component of Pt/Ba/Al2 O3 LNTs [18–25]. These studies have shown that Al2 O3 can store a significant amount of NOx both in the pres-

N.A. Ottinger et al. / Applied Catalysis B: Environmental 101 (2011) 486–494

487

Table 1 Properties of fresh and aged LNTs studied. Aging temperature (◦ C)

BET surface area (m2 /gcat )

Fresh

38

2.5

750

34

7.6

880

31

13

930

21

18

1070

21

26

TEM PGM size (nm)

Evaluation temperature (◦ C)

SS NOx conversion (%)

200 300 400 200 300 400 200 300 400 200 300 400 200 300 400

64 99 96 64 97 92 49 97 86 57 82 75 42 69 71

ence and absence of Ba, at temperatures up to 400 ◦ C. However, the catalysts used in these studies were all model LNTs without stabilizing additives and only one study analyzed the effect of high temperature on Al2 O3 -based NOx storage. Pazé et al. have looked at the effect of Ba addition on Al2 O3 NOx storage after aging at 800 ◦ C and found that increasing levels of Ba only modestly improves NOx storage on aged catalysts [24]. Clearly a more thorough investigation is still needed to determine the effect of high temperature aging on the distribution of NOx storage sites in commercial-intent LNTs. This study analyzes NOx storage and stability on both the Ba and the Al2 O3 components of LNTs at 200, 300 and 400 ◦ C before and after aging. A variety of analytical techniques have been used to characterize the LNTs examined in this study. Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS) is used to identify the types of NOx stored on the surface of the LNTs, as well as to determine the stability of the different stored NOx species. NO oxidation, NOx storage, and BET surface area experiments were performed in a microreactor. Scanning transmission electron microscopy/energy dispersive spectroscopy (STEM/EDS) measurements and powder Xray diffraction (XRD) were used to determine average PGM sizes, and results from XRD experiments were also used to identify phase changes. 2. Experimental The LNT catalysts analyzed in this study contained Pt, Pd, Rh, Ba, Ce, and Zr, in addition to a number of proprietary additives. The lean/rich thermal aging experiments performed on the LNTs with a bench-core reactor are detailed elsewhere [26,27], but briefly they involved the periodic introduction of known quantities of H2 , CO, and O2 to obtain intra-catalyst nominal aging temperatures of 700, 800, 900, and 1000 ◦ C. After a prescribed number of lean/rich aging cycles, the steady-state NOx conversion of the LNT was evaluated at 200, 300, and 400 ◦ C with a gas hourly space velocity (GHSV) of 30,000 h−1 while switching between short lean/rich cycles of 60 s/5 s durations. The 5 s rich-phase consisted of 1.13% CO and 0.68% H2 , while 5% H2 O, 5% CO2 and 300 ppm NO were continuously flowed; during the 60 s lean-phase, 10% O2 was included. The steady-state NOx conversion was defined when there was no cycle-to-cycle variation in NOx slip for at least 5 consecutive cycles. During aging and evaluation experiments, the axial temperature gradient across the catalyst was monitored with Type K thermocouples at four axial locations. Because aging exotherms were obtained via periodic introduction of reactants, typical temperature gradients were approximately 100 ◦ C with the front of the catalyst the most severely aged. After aging, the LNTs were sectioned at each

Storage capacity (␮mol NO/gcat ) 137 183 93 129 145 97 113 138 89 76 78 76 44 64 56

NO to NO2 oxidation (%) 20 58 44 20 58 46 20 51 35 17 39 40 9 33 43

thermocouple location for precise characterization. In a previous study, all four sections of the LNTs have been analyzed [26], but in this study, only the front section – the most severely aged section – of each of the four aged LNTs has been further analyzed. At nominal aging temperatures of 700 and 800 ◦ C, the front section was actually aged for 200 cycles at 750 and 880 ◦ C, respectively, while at nominal aging temperatures of 900 and 1000 ◦ C the front sections were aged for 300 cycles at 930 ◦ C and for 350 cycles at 1070 ◦ C, respectively. Details on the BET, STEM/EDS, and XRD experiments are also described elsewhere [26,27], but steady-state NOx conversion, surface area, and PGM size measurements obtained with these techniques are summarized in Table 1 along with results from NOx storage and NO oxidation experiments. The microreactor used in this study for NOx storage and NO oxidation measurements has been previously described [3,28]. Briefly, it consists of lean and rich gas lines controlled by a 4-way valve, a bypass loop for flow equilibration and analyzer calibration, a mass spectrometer (SRS RGA100), and two NOx analyzers (CAI 400HCLD) for measuring NO and NOx . The mass spectrometer is used for BET surface area measurements [29], while the NOx analyzers are used for NOx storage and NO oxidation experiments. The microreactor was loaded with approximately 300 mg of catalyst sample positioned between two plugs of quartz wool. Prior to each experiment, samples were pretreated with a flow of 2% H2 and balance Ar for 1 h at 450 ◦ C and 400 cm3 (STP)/min (sccm). For NOx storage capacity measurements, the LNT was cooled to the desired temperature in Ar, and once the temperature stabilized, a flow of 1000 ppm NO, 10% O2 , and balance Ar was introduced and continued until the NOx concentration at the outlet was the same as that at the inlet. The microreactor outlet gases were diluted with an air flow rate of 5 LPM before being routed into the NOx analyzers. NO oxidation at 200, 300, and 400 ◦ C was measured at the end of the NOx storage profiles since the use of two NOx analyzers made possible the simultaneous measurement of NO and NOx concentrations. The previously described DRIFTS apparatus consists of a Midac M2500 spectrometer coupled with a Harrick barrel ellipse diffuse reflectance attachment and an integrated heated stainless steel reaction cell [18,19]. The reaction cell is capable of achieving sample temperatures of 550 ◦ C and is operated at a pressure of 500 Torr in order to prevent gas stagnation and maintain a seal between the reaction cell and the hemispherical zinc selenide (ZnSe) dome. Water is introduced via a bubbler system immersed in a recirculating constant temperature bath at 40 ◦ C, while mass flow controllers meter the flows of inlet gases. All spectra were collected at a resolution of 2 cm−1 , and are presented in absorbance units. Background spectra were recorded prior to the introduction of reacting gases.

N.A. Ottinger et al. / Applied Catalysis B: Environmental 101 (2011) 486–494

Temperature (ºC)

700

Clean 1%H2+Ar

500

100

Heat to 500C

NOx Storage

Cool to T

600

Ar

NOx Conversion (%)

488

Ar

400 NO+O2+Ar

300 200

a

100

0

20

40

60

80

100

700°C 800°C

90

900°C 1000°C

80 70 60

120

0

100

Temperature (ºC)

Time (min) 700

Clean

600

1%H2 Ar

500

NOx Storage

Cool to T

400

Ar

200

300

400

Number of Aging Cycles NOx Release

Fig. 2. NOx conversion at 300 ◦ C after aging at the indicated nominal aging temperatures.

3. Results

Ar

300 200

NO+O2+Ar

b

3.1. NO oxidation, NOx storage, and lean/rich switching experiments

100

0

50

100

150

200

250

Time (min) Fig. 1. DRIFTS protocols for (a) NOx storage and (b) NOx release experiments. Background spectra were taken after cooling to the desired evaluation temperature.

Results from lean/rich NOx conversion experiments, NO storage, NO oxidation, BET surface area, and STEM average PGM size measurements are presented in Table 1. To determine the NOx conversion Eq. (1) was employed for the steady state cycle: NOx conversion =

The gases used in DRIFTS experiments were from Air and had the following purities: Ar (UHP), 10% H2 in Ar (UHP), 1% NO in Ar (UHP), and O2 (Research). CO2 and H2 O traps removed any remaining impurities from Ar lines. Two types of experiments were performed with the DRIFTS reactor: NOx storage and NOx release. For each experiment a wafer of washcoated monolith wall was obtained from the appropriate catalyst sample and positioned on a substrate. The wafer and substrate were then placed on the sample stage, and a Type K thermocouple was used to measure the catalyst temperature and hold the sample in place. Before each experiment, the catalyst was pretreated for 12 h at 350 ◦ C with alternating 10 min exposures to 300 ppm NO with 10% O2 and 1% H2 in order to minimize features associated with a carbonate species. Pretreatments and initial background spectra were all performed with an Ar balance in both lean and rich lines. NOx storage experiments were performed in the DRIFTS reactor at 200, 300, and 400 ◦ C with a flow of 300 ppm NO, 10% O2 , and balance Ar. A graphical representation of the experimental protocol for NOx storage experiments is shown in Fig. 1a, and as seen in the figure, the catalyst was first heated to 500 ◦ C in 1% H2 and balance Ar for 30 min. After cooling to the desired evaluation temperature, a background spectrum was taken in 10% O2 and balance Ar, and then the NOx storage experiment was performed for 1 h. In between each evaluation temperature, the catalyst was again heated to 500 ◦ C in a flow of 1% H2 and balance Ar for 30 min to remove all stored NOx and provide a clean background scan for the next experiment. The protocol for NOx release experiments is shown in Fig. 1b, and similar to NOx storage experiments, the catalyst was first pretreated at 500 ◦ C in 1% H2 and balance Ar. Background scans were then taken in Ar from 500 to 200 ◦ C in 50 ◦ C increments while cooling. A 1 h NOx storage experiment was performed with 300 ppm NO, 10% O2 and balance Ar, and after the NOx storage experiment, a NOx release experiment was performed to 500 ◦ C. During the release experiment, spectra were taken in 50 ◦ C increments, and a dwell time of 10 min was used at each temperature to give the catalyst surface time to stabilize. The gas concentrations used are in accordance with the guidelines set forth by the Cross-cut Lean Exhaust Emissions Reduction Simulations (CLEERS) protocol for LNT catalysts [30].

NOx fed to reactor

(1)

The NOx stored during the lean phase is the difference between the blank reactor NOx profile and the experimental NOx profile during the lean phase. The unconverted NOx released is the NOx measured during the rich phase. Fig. 2 shows the high steady-state NOx conversions at 300 ◦ C are maintained when aging at 700 and 800 ◦ C, but there is a significant decrease when the samples are heated to 900 and 1000 ◦ C. NOx conversion at 200 ◦ C is initially much lower, but is similarly affected by high temperature aging. BET surface area measurements show a similar trend. Aging at 700 and 800 ◦ C results in a maximum surface area loss of only 18%, whereas aging at 900 and 1000 ◦ C results in a much larger, 44%, reduction in surface area – reported surface areas include both washcoat and cordierite substrate. On the other hand, average PGM size increases from 2.5 nm in fresh LNT to 26 nm in LNT aged at 1070 ◦ C. NO storage capacity and NO oxidation measurements were also measured at 200, 300, and 400 ◦ C. As an example, NO storage capacity and NO oxidation of a fresh LNT at 400 ◦ C are shown in Fig. 3. Breakthrough begins at approximately 10 s after the introduction of NO, and the concentration of NO gradually increases until inlet NOx levels are achieved. The NO to NO2 conversion is constant at 44% after only approximately 400 s of exposure to NO. As seen in Table 1, the highest NOx storage capacity is at 300 ◦ C with the lowest storage at 400 ◦ C, except in the case of the sam1500

NO NO2

1200

NO(x) (ppm)

Liquide®

 NO stored − unconverted NO released  x x

NOx Storage

NO Oxidation

NOx 900 600 300 0 0

200

400

600

800

1000

Time (s) Fig. 3. Representative NOx storage and NO oxidation experiment performed on a fresh LNT at 400 ◦ C (1000 ppm NO, 10% O2 and Ar bal.).

N.A. Ottinger et al. / Applied Catalysis B: Environmental 101 (2011) 486–494

Fresh

120

750ºC 880ºC

NO Conversion (%)

100

930ºC 1070ºC

80

Equilibrium

60 40 20 0 150

200

250

300

350

400

450

Evaluation Temperature (C) Fig. 4. Results from NO oxidation experiments performed before and after aging (1000 ppm NO, 10% O2 and Ar bal.). The equilibrium concentration over the temperature range is also plotted.

489

experiments. Equilibrium concentrations shown in this figure were obtained with Gaseq, a chemical equilibrium program. Fig. 5 shows NOx outlet profiles at an evaluation temperature of 400 ◦ C carried out in a bench-core reactor for LNTs aged at nominal aging temperatures of 900 and 1000 ◦ C. These profiles are taken from the final steady-state portion of experiments performed after aging for 0, 5, 150, and either 300 or 350 cycles. Lean and rich phases are demarcated on the plots, and 300 ppm NO was flowing throughout both phases as previously mentioned. At both 900 and 1000 ◦ C, the amount of NOx slip during the lean phase monotonically increases with increasing number of aging cycles as the storage capacity is reduced. However, the rich phase excursion unexpectedly decreases after an initial, sharp increase following 5 cycles of aging. In Fig. 5a, at an aging temperature of 900 ◦ C the peak NOx excursion during the rich phase rich phase decreases from 172 to 133 ppm as the number of aging cycles increases from 5 to 300. This seems to indicate that aging improves the LNTs ability to reduce stored NOx and will be discussed further in Section 4. 3.2. DRIFTS

ple aged at a nominal aging temperature of 1000 ◦ C where the lowest NO storage capacity is at 200 ◦ C. The range of storage capacities obtained in this study is similar to that reported by Fridell et al. [31] on a Pt-Rh/BaO/Al2 O3 catalyst with a similar surface area (∼30 m2 /g). The authors of that study found that NOx storage varied from 43 to 77 mol NO/m2 from 300 to 400 ◦ C, with a maximum storage capacity at 300 ◦ C. Furthermore, although the capacities obtained in this study are much lower than those presented by Lietti et al. [32] due to a large difference in surface area (30 vs. 160 m2 /g) the authors of that study also found that 300 ◦ C is the optimum temperature for the storage of NOx on a Pt/Ba/Al2 O3 LNT. Even though storage capacity was not significantly affected by aging at 750 or 880 ◦ C, aging at higher temperatures did have a significant impact. At 300 ◦ C, the storage capacity decreases 65% from 183 ␮mol NO/gcat in the fresh sample to only 64 ␮mol NO/gcat after aging at 1070 ◦ C, whereas the reductions in NOx storage capacity at 200 and 400 ◦ C were 68% and 41%, respectively. NO to NO2 oxidation measurements obtained at the end of NOx storage experiments show that NO oxidation is similarly most efficient at 300 ◦ C if aging below 900 ◦ C, however, the highest oxidation rate is achieved at 400 ◦ C when aging is carried out at 900 or 1000 ◦ C. From Fig. 4 it is clear that the effect of aging on NO oxidation is dependent on the evaluation temperature. At 200 ◦ C there is little effect until after aging at 1000 ◦ C, similar to the NO storage results previously presented for this temperature. However, at 300 ◦ C NO oxidation monotonically decreases with aging temperatures above 880 ◦ C, and aging at 1070 ◦ C leads to a 43% reduction as compared to the fresh sample. NO oxidation at 400 ◦ C is equilibrium-limited for all

3.2.1. NOx storage experiments NOx storage experiments were performed on fresh and aged LNTs at 200, 300, and 400 ◦ C while flowing 300 ppm NO, 10% O2 , and balance Ar at a total flow rate of 50 sccm. Experiments performed on a fresh LNT are shown in Fig. 6. At 200 ◦ C (Fig. 6a), Ba nitrites first form at 1215 cm−1 [33–35] and are followed by the simultaneous formation of Ba nitrates at 1430 and 1320 cm−1 [21,22,33–35] and aluminum nitrates at 1550, 1465, 1412, and 1250 cm−1 [18,21,35–38]. The barium nitrate peak at 1430 cm−1 is obscured by the aluminum nitrate peaks at 1465 and 1412 cm−1 . The highest intensity peak is an aluminum nitrate peak at 1550 cm−1 , and this peak accounts for approximately 25% of the total peak area with the Ba nitrate peaks at 1320 and 1430 cm−1 responsible for another 60%. At 300 ◦ C (Fig. 6b), Ba nitrite formation is still substantial in the first minute, but the Ba nitrate peak at 1320 cm−1 is also beginning to form because of the faster NO to NO2 oxidation kinetics at 300 ◦ C. After 1 h of 300 ◦ C NO storage, there is very little difference compared to the 200 ◦ C spectra, indicating that NOx storage mechanisms are similar at these temperatures. On the other hand, at 400 ◦ C (Fig. 6c) the predominant peaks after 1 min of storage are Ba nitrate peaks, indicating that at this temperature NO2 storage is occurring at a rate faster than the time-resolution of the DRIFTS instrument. There is also no evidence of aluminum nitrate formation at 400 ◦ C, suggesting that nitrates on the Al2 O3 -phase are unstable at 400 ◦ C [22,33]. In Fig. 7, the results of the 200 and 300 ◦ C NOx storage experiments performed in the DRIFTS reactor are shown for LNTs aged

Aged at 1000°C

NOx Outlet Concentration (ppm)

NOx Outlet Concentration (ppm)

Aged at 900°C

(a)

180

5 cycle

160

150 cycle

140

300 cycle

120 300 cycle

100

150 cycle

80 60 5 cycle

40

Rich

Lean

20

Lean

Fresh

0 0

20

40

60 Time (s)

80

100

(b)

160

5 cycle 150 cycle 350 cycle

140 120

350 cycle

100

150 cycle

80 60

5 cycle

40 Fresh

20 Lean

Rich

Lean

0 0

20

40

60

80

100

Time (s)

Fig. 5. Final steady-state cycle of 400 ◦ C lean/rich evaluation experiments after aging at (a) 900 ◦ C and (b) 1000 ◦ C for the indicated number of cycles. Lean and rich cycles are labeled.

490

N.A. Ottinger et al. / Applied Catalysis B: Environmental 101 (2011) 486–494

200°C (a)

1550 1465

300°C (b)

1550 1465

400°C (c)

1412 1320

1412

1430

1320 1250

0.02

0.02

0.02

1320

1250 30 m

Absorbance

1246 10 m

1214 1215

1214

30 m

1h

3m

10 m

30 m 10 m

5m

5m 3m

3m

2m

2m

2m 1m

1800

5m

1h

1m

1m

1600

1400

1200

1000

1800

1600

1400

1200

1000

1800

1600

1400

1200

1000

Wavenumber (cm-1) Fig. 6. DRIFTS spectra during NOx adsorption on fresh LNT at (a) 200 ◦ C, (b) 300 ◦ C, and (c) 400 ◦ C (300 ppm NO, 10% O2 , Ar bal.). All spectra referenced to background at t = 0.

for 300 cycles at 930 ◦ C and 350 cycles at 1070 ◦ C. Nitrite formation still precedes nitrates on Ba at 200 ◦ C. However, at 300 ◦ C there is no evidence of nitrite formation, but nitrates begin forming after only 2 min on these two samples. After aging at 930 ◦ C, the aluminum nitrate peak at 1547 cm−1 is still prevalent when evaluating at 200 and 300 ◦ C, but the less intense aluminum nitrate peaks at 1465 and 1246 cm−1 are no longer visible. Also, the 1547 cm−1 peak is no longer as intense as the Ba nitrate peak at 1430 cm−1 , indicating that aging has disproportionately affected NOx storage on the Al2 O3 -phase. After aging at 1070 ◦ C, the highest intensity aluminum nitrate peak at 1547 cm−1 is barely visible, but Ba(NO3 )2 peaks at 1430 and 1320 cm−1 are much less affected. However, at lower aging temperatures of 750 and 880 ◦ C the same large reduction in aluminum nitrate peak intensity is not noted. In Fig. 8, peak height ratios comparing the heights of the aluminum nitrate peak at 1547 cm−1 and the barium nitrate peak at 1430 cm−1 during 200 and 300 ◦ C NOx adsorption experiments are plotted for all aging temperatures as a function of adsorption time. The 1547:1430 cm−1 peak height ratios at 750 and 880 ◦ C are approximately the same as those on a fresh LNT for all adsorption times, whereas on the LNTs aged at higher temperatures, the peak height ratios decrease as the aging temperature increases and the amount of aluminum nitrate formed decreases.

3.2.2. NOx release experiments NOx release experiments were performed on fresh and aged LNTs in order to determine the effect that aging has on the stability of the adsorbed NOx species. Before each release experiment, NOx adsorptions were performed at 200 ◦ C in 300 ppm NO, 10% O2 , and balance Ar. For all LNT samples, the NOx release experiments indicate that the triplet of peaks assigned to nitrates on Al2 O3 are the least stable. As seen in Fig. 9a, aluminum nitrates at 1550 cm−1 are completely desorbed by 350 ◦ C in the fresh sample, and all adsorption sites including Ba are nitrate free by 400 ◦ C. In comparison, nitrates are more stable on the aged samples and are still adsorbed at 400 ◦ C. After aging at 930 ◦ C, as shown in Fig. 9b, nitrates desorb from Al2 O3 above 350 ◦ C and from Ba above 400 ◦ C, 50 ◦ C higher than on the fresh sample. In Fig. 9c this trend is less noticeable at 1070 ◦ C because the total amount of NOx stored is only 33% of the fresh uptake; however, a portion of the nitrates are still present at 400 ◦ C.

4. Discussion The results presented above are a follow-up study to a multiyear rapid aging protocol development project. The overall goal of this project was the development of a protocol capable of simulating in-vehicle aging by subjecting LNT catalysts to similar conditions within bench-core and microreactors. Several publications have been presented detailing the work to date [14,26–28], and this study follows up on the most recent of those [26] by further exploring the effects of material changes on NOx conversion after aging at 930 and 1070 ◦ C. Several possible high temperature deactivation mechanisms have just been separately presented. These include reductions in NO storage capacity, NO oxidation, and surface area as well as a loss of Al2 O3 NOx storage sites and an increase in NO3 − stability. It has also been shown that the amount of NOx released during the rich phase actually decreases with increasing number of high temperature aging cycles. It is suspected that many of these component mechanisms are related, and these relationships will be further discussed. NO storage capacity is normalized to surface area (m2 /gcat ) in Fig. 10. The results of this normalization indicate three distinct relationships between NO storage capacity and surface area. At 200 ◦ C the storage capacity is relatively constant on a surface area basis – within 4% of the average uptake – until aging at 1000 ◦ C when the storage capacity decreases to 2.1 ␮mol NO/m2 , a reduction of 42%. Comparing this to NOx storage experiments with DRIFTS at the same temperature, this large reduction at 1070 ◦ C is accounted for by an almost complete loss of Al2 O3 NOx storage sites at this temperature. At 300 ◦ C, the capacity decreases from 4.8 to 3.0 ␮mol NO/m2 , a reduction of 37%, and this again can be accounted for by the large reduction in aluminum nitrates at this temperature. Finally, at 400 ◦ C the NO storage capacity per surface area actually increases slightly until aging at 1070 ◦ C. This result seems to indicate that surface area reduction has little effect on NOx storage capacity of Ba, since from DRIFTS experiments it is clear that no nitrates are stored on Al2 O3 at this temperature. However, something is clearly affecting the NOx storage capacity of Ba because the reduction in NOx storage at 400 ◦ C can only be accounted for by Ba. It has been shown in previous work on this LNT that BaCO3 XRD peaks are no longer observed after aging at temperatures above 900 ◦ C, but that elemental Ba – detectable with electron

N.A. Ottinger et al. / Applied Catalysis B: Environmental 101 (2011) 486–494

Aged at 930°C

200°C

1430

200°C 1320

1320 1247

0.02

0.02 Absorbance

Aged at 1070°C

1430 1547

1545

1227 1h 30 m

1h 30 m 10 m 5m 3m

10 m 5m 3m 2m

2m

1m

1m

1800

1600

300°C

1400

1200

1000

1800

1600

300°C

1545 1430

1400

1200

1000

1430

1320

1320

0.02

0.02 Absorbance

491

1247

1545

1h 1h

30 m

30 m 10 m 5m

10 m 5m 3m

3m 2m

2m

1m

1m

1800

1600

1400

1200

1000

1800

1600

1400

1200

1000

Wavenumber (cm-1) Fig. 7. DRIFTS spectra during 200 and 300 ◦ C NOx adsorption of LNTs aged at 930 and 1070 ◦ C (300 ppm NO, 10% O2 , Ar bal.). All spectra referenced to background at t = 0.

(b)

1.6

Fresh

1.4

750°C

1.2

880°C

200°C

930°C

1

1070°C

0.8 0.6 0.4 0.2 0 0s

1min 2min 3min 5min 10min 30min 1 hr

Adsorption Time

Peak Ratio (1550:1430cm-1)

Peak Ratio (1550:1430cm-1)

(a)

1.6

Fresh

1.4

750°C

1.2

880°C

300°C

930°C

1

1070°C

0.8 0.6 0.4 0.2 0 0s

1min 2min 3min 5min 10min 30min 1hr

Adsorption Time

Fig. 8. DRIFTS peak height ratios of 1550 cm−1 Al2 O3 nitrate peak and 1430 cm−1 Ba(NO3 )2 peak after 1 h NOx adsorption experiments at (a) 200 and (b) 300 ◦ C after aging at all temperatures.

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N.A. Ottinger et al. / Applied Catalysis B: Environmental 101 (2011) 486–494

Aged at 930°C

Fresh

(a)

(b)

1465 1420 1547 1320

1440

0.02

0.02

Aged at 1070°C

(c)

1547 1320

0.02

1320

1547

1250

200

1250

1250

Absorbance

1440

250 300 350

200 250

200 250 300 350 400

350

450

400 450

450

500

500

500

1900

400

300

1700

1500

1300

1100 1900

1700

1500

1100 1900

1300

1700

1500

1300

1100

Wavenumber (cm-1)

5 4 3

200°C

2

300°C

1

400°C

0 Fresh

750ºC

880ºC

930ºC

1070ºC

Aging Condition

NO Turnover Frequency (s-1)

Storage (μmol NO/m2)

Fig. 9. DRIFTS spectra of NOx release experiments of (a) fresh LNT and LNTs aged at (b) 930 ◦ C and (c) 1070 ◦ C (NOx storage at 200 ◦ C in 300 ppm NO, 10% O2 , and Ar. bal. and release in Ar). All spectra referenced to background at t = 0, and all temperatures shown in ◦ C.

0.2

Fresh 754°C

0.15

883°C 929°C

0.1

1070°C

0.05

0 200°C

Fig. 10. Normalization of storage capacity to surface area for LNTs aged at 750, 880, 930, and 1070 ◦ C.

400°C

Fig. 11. Normalization of ␮mol NO reacted per second to mol PGMs for all aging conditions.

shown in Table 1 [43]. mol PGMs =

(0.9/sizePGM ) LoadingPGM × MWPt

(2)

The molecular weight of Pt is used because the loading of individual PGM components is unknown, but from STEM/EDS mea100%

Aging T:

900°C

1000°C

98%

ηNOx

probe micro-analysis – is still present at levels equivalent to fresh LNTs [26,27]. Before now, the reason for the Ba peak disappearance was uncertain. However, with the additional characterization performed in this study a conjecture is now possible. There is strong evidence that the Ba is redispersing into a phase that is undetectable with XRD. The higher Ba surface coverage resulting from redispersion explains the near complete loss of Al2 O3 storage sites seen with DRIFTS. Kim et al. have shown that large Ba(NO3 )2 crystallites can be decomposed to BaO nanoparticles by exposure to NO2 after calcining at 1000 ◦ C [39,40]. Furthermore, Piacentini et al. [41,42] have shown that undetectable XRD Ba-phase is less active for NOx storage and forms more stable nitrates than “bulk-like” BaCO3 . With TPD experiments the authors showed a shift from 455 to 517 ◦ C of maximum NOx desorption temperature when going from crystalline to an undetectable Ba-phase. Thus, it is proposed that the Ba phase in this study is redispersing to a lower storage capacity, more stable phase. The effect of aging on NO to NO2 oxidation has also been measured, and even though an excess of NO2 was detected in all NOx conversion experiments regardless of aging condition (i.e., NO to NO2 oxidation is not responsible for decreased NOx conversion) it is still interesting to analyze the effect that aging has on this LNT mechanism. In Fig. 11, the NO turnover frequency (TOF) with respect to NO oxidation is presented. The mol PGMs used to calculate the TOF is calculated with Eq. (2) using average PGM sizes

300°C

Aging Condition

96% 94% 92% 0

5

100

300

350

Number of Aging Cycles Fig. 12. Normalization of rich phase ␮mol NOx released per lean phase ␮mol NOx stored (NOx ) at 400 ◦ C after aging at 930 and 1070 ◦ C (Lean: 300 ppm NO, 10% O2 , 5% CO2 , 5% H2 O, N2 bal. Rich: 1.13% CO, 0.68% H2 , 300 ppm NO, 5% CO2 , 5% H2 O, N2 bal.).

N.A. Ottinger et al. / Applied Catalysis B: Environmental 101 (2011) 486–494

493

Fig. 13. Schematic of how NOx reduction and its products depend on NOx release rate.

surements (not shown) it is clear that the majority of PGM is in the form of Pt. The NO TOF increases at all evaluation temperatures with each step-up in aging temperature. At 400 ◦ C the effect of reduction in PGM dispersion is most pronounced. The NO TOF at this temperature increases from 0.2 to 1.9 s−1 , and this increase corresponds to an increase in average PGM size from 2.6 to 26 nm. Whereas, effects at 200 and 300 ◦ C are more modest, there is still more than a threefold increase in the amount of NO oxidized per mol PGMs after aging at 1070 ◦ C. This increase in NO oxidation per mol PGMs indicates that PGM particle size growth leads to more active PGM sites for the oxidation of NO, a conclusion attributed in the literature to the relative inability of larger PGM particles to form deactivating oxides in comparison to smaller PGM particles which are easily oxidized [13–16]. The inability of larger PGM particles to form oxides has been attributed by Douidah et al. to a decrease in the Pt/Al2 O3 strong metal support interaction (SMSI) resulting from a reduction in PGM dispersion [44]. This increase in NO TOF with increasing aging temperature is in contrast to the NO to NO2 oxidation results presented in Table 1 which show an overall decrease in the integral oxidation performance of the entire catalyst with increasing aging temperature. However, this is to be expected since a reduction in PGM dispersion from 36% to 4% after aging at 1070 ◦ C greatly reduces the overall availability of PGM sites. The fraction of stored NOx that is successfully reduced was also shown to benefit from increased aging time. By integrating the areas under the indicated rich and lean regions of the NOx outlet profiles in Fig. 5, the ␮mol NOx released per ␮mol NOx stored, defined as NOx in Eq. (3), is calculated. NOx = 1 −

mole NOx released mole NOx stored

(3)

NOx released is referenced to NOx stored during the preceding storage cycle in order to account for varying NOx loadings at the time of release. The NOx values for 400 ◦ C evaluations of a fresh LNT and LNTs aged at 900 and 1000 ◦ C are shown in Fig. 12. Before aging, NOx is approximately 100% because there is almost no NOx release. However, after initially decreasing with 5 cycles of aging, NOx increases with increasing aging time for both aging temperatures as a result of a significant decrease in the amount of NOx released. The amount of NOx released decreases with aging time as a result of the stabilization of NOx stored on Ba as observed in DRIFTS NOx release experiments. The increased stability of Ba(NO3 )2 resulting from high temperature Ba redispersion decreases the release rate of NOx and improves NOx reduction at 400 ◦ C. At LNT evaluation temperatures of 200 and 300 ◦ C, reductions in NOx storage capacity with aging are not followed with concomitant reductions in NOx release. At 200 ◦ C, the amount of

rich phase NOx released is constant with increasing aging time and decreasing NOx storage capacity. This indicates a similar, though weaker, relationship between aging time and NOx stability/release. Rich phase NOx releases at 300 ◦ C are minor at all aging temperatures and times, and so no reduction in NOx release with aging time is seen. The effect of increasing NOx stability on NOx release is graphically illustrated in Fig. 13. With a rapid NOx release, the high concentration of NOx in the release-front overwhelms the available reductants, and a significant amount of NOx slips through the LNT unreduced. With a slower NOx release, local reductant concentrations are higher and capable of reducing more NOx . Though not analyzed in this study, higher local H2 concentrations resulting from slower NOx release could lead to NH3 or N2 O formation [3,45–50]. While NH3 emissions from a vehicle would be a potential concern, there is considerable interest in utilizing this byproduct in a dual LNT–SCR system [51–55]. These complex catalyst systems rely primarily on the LNT to store and reduce NOx , but also incorporate a zeolite-based NH3 -SCR catalyst after the LNT that stores NH3 generated during the rich phase. This approach would eliminate the need for urea as an on-board reductant. Therefore, several ongoing studies are focused on how NH3 forms over LNTs and is utilized for subsequent NOx reduction, especially as a function of aging. 5. Conclusions This study provides insight into the effects of high temperature aging on the NO oxidation, NOx storage, and NOx reduction of fully formulated lean NOx traps by comparing results from performance and surface degradation studies. It has been shown that reductions in PGM dispersion resulting from aging lead to a significant increase in NO turnover frequency at 200, 300 and 400 ◦ C, with the largest increase seen at 400 ◦ C. Furthermore, NOx storage site availability and stability has shown that Al2 O3 stores a significant amount of NOx at 200 and 300 ◦ C, but none are present on alumina at 400 ◦ C. Little reduction in NOx storage capacity was seen unless aging at temperatures greater than 900 ◦ C, and the reduced capacity seen after aging at 930 and 1070 ◦ C have been linked to the loss of Al2 O3 storage sites as well as a redispersion of Ba into a phase with a lower capacity for NOx . Finally, the stabilization of nitrate species resulting from Ba redispersion has been shown to improve NOx reduction efficiency by decreasing the release speed of stored nitrates. Acknowledgements This work was funded by the U.S. Department of Energy (DOE), Office of FreedomCar and Vehicle Technologies, and the fully for-

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