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rich cycling conditions
Catalysis Today 231 (2014) 90–98
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Roles of C3 H6 in NH3 generation and NOx reduction over a Cu-chabazite SCR catalyst under lean/rich cycling conditions夽 Mi-Young Kim a , Jae-Soon Choi a,∗ , Mark Crocker b a b
Fuels, Engines, and Emissions Research Center, Oak Ridge National Laboratory, Knoxville, TN 37932, USA Center for Applied Energy Research, University of Kentucky, Lexington, KY 40511, USA
a r t i c l e
i n f o
Article history: Received 17 September 2013 Received in revised form 16 December 2013 Accepted 17 December 2013 Available online 4 January 2014 Keywords: Coupled LNT–SCR NOx storage and reduction Lean NOx trap Selective catalytic reduction LNT SCR
a b s t r a c t We studied the spatiotemporal profiles of NOx reduction with NH3 , C3 H6 , or NH3 + C3 H6 over a commercial Cu-chabazite SCR catalyst (washcoated honeycomb monolith) to better understand the effects of C3 H6 on NOx reduction under fast lean/rich cycling conditions relevant to coupled LNT–SCR catalysts. NOx reduction by NH3 was very effective with total NH3 consumption within the first quarter of the catalyst at all temperatures. By contrast, NOx reduction by C3 H6 was more gradual along the catalyst length and sensitive to temperature with maximum performance obtained at 300–400 ◦ C. Temperatureprogrammed desorption performed after cycling experiments with C3 H6 evidenced the presence of NH3 and/or NH3 -precursor intermediates on the surface. These surface species were formed as a result of reactions between NOx and C3 H6 during the rich as well as lean phases, but started to be used for NOx reduction only when all the stored hydrocarbons were depleted. When fed together, the contributions of C3 H6 and NH3 to the cycle-averaged NOx conversion were essentially additive. However, temporally resolving N2 formation using isotopically-labeled NO and a mass spectrometer revealed that C3 H6 actually inhibited reactions between NOx and feed NH3 until well into the subsequent lean phase, i.e., until the stored hydrocarbons were depleted. This study highlights the importance of controlling lean/rich cycling time partitioning to alleviate the impact of the inhibiting effect of C3 H6 on NH3 chemistry and achieve the maximum NOx reduction possible over the SCR catalyst in coupled LNT–SCR systems. © 2013 Elsevier B.V. All rights reserved.
1. Introduction The application of lean-burn engines in vehicles has significantly increased owing to their more efficient fuel combustion and lower CO2 emissions compared to engines operating under stoichiometric conditions. However, the reduction of NOx in lean exhausts is more difficult due to the excess oxygen content. Lean NOx trap (LNT) and selective catalytic reduction (SCR) catalysts were developed to overcome the challenge of lean NOx reduction. The chemistry of LNT [1–9] and SCR [10–16] catalysts has been extensively studied, generating fundamental insights needed to enhance the performance
夽 Notice: This submission was sponsored by a contractor of the United States Government under contract DE-AC05-00OR22725 with the United States Department of Energy. The United States Government retains, and the publisher, by accepting this submission for publication, acknowledges that the United States Government retains, a nonexclusive, paid-up, irrevocable, worldwide license to publish or reproduce the published form of this submission, or allow others to do so, for United States Government purposes. ∗ Corresponding author at: Oak Ridge National Laboratory, Fuels, Engines, and Emissions Research Center, 2360 Cherahala Blvd., Knoxville, USA. Tel.: +1 865 946 1368; fax: +1 865 946 1354. E-mail address: [email protected] (J.-S. Choi). 0920-5861/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.cattod.2013.12.021
of these devices. Recently, to further improve the performance and cost of lean NOx controls, so-called coupled LNT–SCR systems have been introduced which combine SCR with LNT catalysts [17–21]. Coupled LNT–SCR catalysts function in cyclic lean/rich modes as do stand-alone LNTs. During normal lean engine operation, the upstream LNT component stores NOx which is released and reduced to N2 , N2 O, or NH3 by intermittent short rich purges. The downstream SCR catalyst can capture NH3 to reduce, in subsequent lean phases, LNT-slipped NOx via well-known NH3 –SCR mechanisms [11]. The benefit of having a downstream SCR catalyst is thus manifested as decreased slips of both NOx (higher conversion) and NH3 . The chemistry of NH3 generation during the regeneration step of LNT catalysts is well documented in the literature [22–27], and the above mentioned NH3 storage and utilization over SCR catalysts (e.g., Cu-exchanged zeolites) is widely accepted as an important NOx reduction pathway [28–34]. Recent vehicle [35] as well as controlled laboratory reactor studies [36], however, revealed that the amount of NH3 entering the downstream SCR catalyst alone cannot always explain the amount of the additional NOx converted over the SCR catalyst. Furthermore, HNCO (a potential “NH3 precursor” molecule) was not detected at the LNT outlet [36]. This discrepancy indicates the involvement of non-NH3 pathways using, as potential reductants, hydrocarbons,
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Table 1 Conditions used for cycling experiments with a Cu-chabazite SCR catalyst. Parameter
Run type 1
Run type 2
Reductant
NH3
C3 H6
Duration NO (ppm) O2 (%) C3 H6 (ppm) NH3 (ppm) CO2 (%) H2 O (%) N2 (%)
Lean 60 s 300 8 0 0 5 5 Balance
Rich 5 s 300 1 0 300 5 5 Balance
Run type 3 NH3 + C3 H6
Lean 60 s 300 8 0 0 5 5 Balance
Rich 5 s 300 1 3333 0 5 5 Balance
Lean 60 s 300 8 0 0 5 5 Balance
Rich 5 s 300 1 3333 300 5 5 Balance
Total flow: 13.7 L/min (STP). Space velocity: 30,000 h−1 . Temperature: 200, 250, 300, 350, 400, and 450 ◦ C.
CO or H2 unused by the upstream LNT. Laboratory experiments indeed showed that SCR catalysts can store hydrocarbons in addition to NH3 in rich phases and use them for NOx reduction in subsequent lean phases [36]. The contributions of C3 H6 and NH3 to cycle-averaged NOx conversion over the SCR component were found to be essentially additive. In the present study, we measured the spatiotemporal distribution of reactions to clarify the chemistry of NOx reduction over a Cu-chabazite SCR catalyst under lean/rich cycling conditions. The results revealed important roles C3 H6 plays in the formation, storage and utilization of NH3 , and/or NH3 -precursor intermediates on the SCR catalyst surface.
employed. First, cycling experiments were performed with the 7.9 cm monolith catalyst (1 L). Then a quarter length of the core was cut out from the back (0.75 L) and the same experiments were carried out. By repeating this procedure, spatiotemporal profiles of gas concentrations were determined at 1 L (outlet), 0.75, 0.5, 0.25, and 0 L (inlet) locations. The data were collected in the temperature range of 200–450 ◦ C with an interval of 50 ◦ C. The cycle-averaged conversion of NO, NH3 , and C3 H6 was calculated by averaging over 5 cycles after the catalyst performance was stabilized at each temperature, which took about 1 h. Prior to the reactor measurements, the catalyst was de-greened at 500 ◦ C for 5 h under the lean/rich cycling conditions corresponding to Run type 2 in Table 1.
2. Experimental
2.3. Temperature-programmed desorption (TPD) experiments
2.1. Catalysts
After lean/rich cycling experiments with C3 H6 as reductant, the catalyst (0.25 L) was purged at 200 ◦ C with a flow of 8% O2 , 5% H2 O, 5% CO2 , and N2 balance until no C3 H6 effluent was observed. Next, the temperature of the reactor was increased to 500 ◦ C at a ramping rate of 20 ◦ C/min under a flow of 8% O2 , 5% H2 O, 5% CO2 , and N2 balance (total flow rate: 13.7 L/min). The gas composition was analyzed using a high-speed FTIR gas analyzer (MKS, MultiGas 2030 HS).
A commercial SCR catalyst based on Cu-chabazite zeolite was used in this study. This catalyst was provided by BASF and prepared using a 400 cpsi/6.5 mil ceramic monolith. For bench reactor evaluation, a core sample of 2 cm in diameter and 7.9 cm in length was drilled out from a monolith brick. More information about catalyst properties can be found in [21]. 2.2. Lean/rich cycling experiments The NOx reduction performance of the catalyst was evaluated in a laboratory bench flow reactor. A 7.9 cm-long core catalyst was wrapped in Zetex insulation tape and inserted into a horizontal quartz reactor tube (2.2 cm inside diameter). The reactor tube was heated by an electric furnace, and synthetic exhaust gas mixtures were prepared using pressurized gas bottles (ultra high purity grade, Air Liquide). The gases were metered with mass flow controllers (Unit Instruments Series 7300, Kinetics Electronics) and pre-heated before entering the quartz reactor. Water was introduced by a high pressure liquid metering pump (Eldex) to a heated zone, vaporized, and added to the simulated exhaust mixture. A rapid switching 4-way valve system was used to alternate between the lean and rich gas mixtures (60/5 s lean/rich cycling). The reaction conditions used for the cycling experiments are listed in Table 1. Briefly, the lean-phase gas mixture contained 300 ppm NO, 8% O2 , 5% H2 O, 5% CO2 , and N2 balance, and the rich-phase gas mixture contained 300 ppm NH3 and/or 3333 ppm C3 H6 , 300 ppm NO, 1% O2 , 5% H2 O, 5% CO2 , and N2 balance. The total gas flow was 13.7 L/min (STP) leading to a gas hourly space velocity (GHSV) of 30,000 h−1 . The gas composition was monitored at the reactor outlet using a high-speed FTIR gas analyzer (MKS, MultiGas 2030 HS). To obtain spatiotemporally resolved profiles of gas concentrations with the FTIR gas analyzer, the following procedure was
2.4. Mass spectrometry analysis with isotopically labeled nitric oxide (15 N18 O) To better understand the chemistry of C3 H6 and its impact on NOx reduction, the timing of N2 evolution was monitored by employing 15 N18 O as the NO feed. The lean-phase gas mixture contained 600 ppm 15 N18 O, 8% O2 , 5% H2 O, 5% CO2 , 0.14% He (as internal standard), and Ar balance, while the rich-phase gas mixture contained 600 ppm NH3 and/or 3333 ppm C3 H6 , 600 ppm 15 N18 O, 1% O2 , 5% H2 O, 5% CO2 , 0.14% He, and Ar balance. The total flow rate was 6.9 L/min (STP) and the front quarter of the core (0.25 L) used in Section 2.2 was used in these experiments. The measurements were done at 350 ◦ C under 60/5 s lean/rich cycling conditions. Note that we chose 600 ppm as the NO and NH3 concentrations as opposed to 300 ppm used in Section 2.2 in order to facilitate the mass spectrometer (MS) analysis by increasing the dynamic range of the MS signals. Even though this NOx level can be higher than what is normally expected at the inlet of the downstream SCR catalyst in a coupled LNT–SCR system, the difference in NOx concentration does not seem to change the nature of chemistry investigated [35,36]. Moreover, the SCR catalyst could encounter a high level of NOx due to inefficient NOx removal over the upstream LNT at low or high temperatures. The gas composition was monitored at the reactor outlet with a quadrupole MS (Pfeiffer, PrismaPlus) using a small capillary probe (185 m OD, 50 m ID) for gas sampling. The m/z ratios monitored
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aldehyde contributions to m/z 29 and 30 subtracted
a.u.
– Time (s)
C3H6 cycling with15 N18O
=
a.u.
C3H 6 cycling without 15 N18 O
a.u.
C3H 6 cycling with15 N18 O
Time (s)
Time (s)
Fig. 1. Schematic representation of the procedure used to calculate the contribution of 15 N14 N and 15 N2 to mass spectrometer (MS) signals at m/z 29 and 30, respectively. 60/5 s lean/rich cycling experiments were performed with and without 15 N18 O (detailed feed gas composition given in Section 2.4). The MS signals obtained during the experiments without NO were subtracted from those obtained with NO in the feed gas to remove the contribution of the aldehyde species formed by oxidation of C3 H6 . The negative values obtained for the rich and early part of lean periods are artifacts due to higher aldehydes formation during the cycling without NO, as more C3 H6 was available for aldehyde formation.
were 29 for 15 N14 N, 30 for 15 N2 , and 31 for 15 N16 O (the major NO species due to very efficient isotopic scrambling between 600 ppm 15 N18 O and 8% 16 O in the feed). The MS signals obtained at m/z 2 29 and 30 could represent both nitrogen and aldehydes. Therefore, the signals obtained from the C3 H6 cycling experiments performed without 15 N18 O in the feed were subtracted from those obtained with 15 N18 O in order to remove the aldehyde-attributable signals. This procedure is represented schematically in Fig. 1. 3. Results and discussion
slipped from an LNT during rich phases is stored over an SCR catalyst placed downstream to be used for additional NOx reduction in subsequent lean phases [28–34]. In contrast, a greater portion of NOx was reduced in the rich phase at higher temperatures, but this reduction regime is likely less relevant to LNT–SCR applications due to the low NH3 yield of LNT regeneration at high temperatures [22]. NH3 adsorption was efficient as shown by the negligible breakthrough even at 0.25 L locations at all temperatures (Fig. 2e). Consistent with previously published studies on Cu-chabazite catalysts such as [37], N2 O formation was minor (i.e., 0–3 ppm peak values, Fig. 2d).
3.1. NOx reduction by NH3 3.2. NOx reduction by C3 H6 Figs. 2a and 2b summarize, respectively, the cycle-averaged conversion of NOx and NH3 along the catalyst length for the experiments of Run type 1 (i.e., “NH3 ” cycling in Table 1). To help understand the temporal development of the reduction processes, the temporal concentration profiles at the first quarter location (0.25 L) are also presented (Fig. 2c–e). The cycle-averaged conversion of NOx reached almost the theoretical maximum value of 7.7% (corresponding to total consumption of NH3 with NH3 :NO stoichiometry of 1:1) within the first quarter length of the catalyst regardless of reaction temperatures. The depletion of NH3 within 0.25 L explains the unchanged NOx conversion over the remaining portion of the catalyst. Even though the axial trends demonstrate efficient NH3 reduction of NOx at all temperatures at the cycle-averaged scale, the temporal NOx profiles in Fig. 2c reveal significant differences in intrinsic catalytic activity depending on reaction temperature. In fact, NOx conversion rate monotonously increased with temperature. Over the entire temperature range studied, one can notice that two temporally distinct reduction regimes were involved which correspond, respectively, to the rich and lean phases of a cycle (Fig. 2c). Note that the “rich” phase of Run type 1 was actually “lean”, since the 300 ppm NH3 present as the sole reductant was well below the amount needed to reduce 300 ppm NO and 1% O2 . For all cases shown in Fig. 2c, NOx reduction occurred in both rich & lean phases. However, the partitioning between rich and lean-phase reduction varied significantly. At low temperatures (i.e., <300 ◦ C), most NOx was converted in the lean phase, which is consistent with a well-accepted “in situ NH3 ” pathway to NOx reduction over SCR catalysts in the coupled LNT–SCR concept. That is, NH3
The spatiotemporally resolved data for NOx reduction with C3 H6 are presented in Fig. 3 and experimental conditions given in Table 1 (Run type 2). The best performance (a maximum cycle-averaged NOx conversion of ca. 18%) was achieved at 300 ◦ C where NOx reduction occurred along the entire catalyst length (1 L, Fig. 3a). Above 300 ◦ C, despite the higher reaction rates due to temperature effects, slightly lower maximum conversion was obtained likely due to the complete depletion of C3 H6 within the front three quarters of the catalyst (0.75 L, Fig. 3b). The effectiveness of C3 H6 for NOx reduction dropped dramatically below 300 ◦ C. The temporal NOx profiles at 0.25 L (Fig. 3c) shows that NOx reduction by C3 H6 occurred both in rich and lean phases as in the case of NH3 , but involved three temporally distinct reduction regimes instead of two: rich, early lean (or lean/rich transition), and mid-late lean periods. A closer inspection of the spatiotemporal profiles reveals that, when C3 H6 was a reductant, a greater portion of NOx was reduced in the lean phase than in the rich phase regardless of temperature. This indicates the importance of reductant storage in the rich phase. When the profiles of reductive species – C3 H6 and three major products of C3 H6 conversion: C2 H4 , HCHO, and CO (Fig. 3f–i) – were compared with those of NOx (Fig. 3c), it becomes clear that a significant portion of the lean NOx reduction took place when these reductive species were absent from the gas phase (compare, for instance, the profiles between 0.8 and 0.9 min at 400 ◦ C). This mismatch implies that some other species derived from C3 H6 transformation were present on the surface for NOx reduction. Indeed, the results presented in Sections 3.4 and 3.5 support this conjecture.
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Fig. 2. Spatiotemporal profiles obtained during lean/rich cycling over Cu-chabazite SCR catalyst using NH3 as reductant: a axial evolution of cycle-averaged NOx conversion, b axial evolution of cycle-averaged NH3 conversion, and temporal profiles of c NOx , d N2 O, and e NH3 concentration at the first quarter of the catalyst length (0.25 L).
As mentioned above, despite the higher reaction rates, the maximum NOx conversion achieved at 350–450 ◦ C was lower than at 300 ◦ C. In addition to the earlier depletion of C3 H6 along the catalyst length (e.g., via oxidation: spatially limited reductant supply),
this could also be due in part to decreased stability of the hydrocarbon species on the surface (temporally limited reductant supply). Decreasing the temperature extended the reductant availability over a longer period of the lean phase. Below 300 ◦ C, however, the
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Fig. 3. Spatiotemporal profiles obtained during lean/rich cycling over Cu-chabazite SCR catalyst using C3 H6 as reductant: a axial evolution of cycle-averaged NOx conversion, b axial evolution of cycle-averaged C3 H6 conversion, and temporal profiles of c NOx , d N2 O, e NH3 , f C3 H6 , g C2 H4 , h HCHO, and i CO concentration at the first quarter of the catalyst length (0.25 L).
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Fig. 4. Spatiotemporal profiles obtained during lean/rich cycling over Cu-chabazite SCR catalyst using NH3 and C3 H6 as reductants: a axial evolution of cycle-averaged NOx conversion, b axial evolution of cycle-averaged NH3 conversion, c axial evolution of cycle-averaged C3 H6 conversion, and temporal profiles of d NOx , e N2 O, f NH3 , g C3 H6 , h C2 H4 , i HCHO, and j CO concentration at the first quarter of the catalyst length (0.25 L).
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Fig. 5. Gas concentration and temperature profiles obtained during temperatureprogrammed desorption performed after a C3 H6 cycling experiment at 450 ◦ C with a Cu-chabazite SCR catalyst. The catalyst was purged at 200 ◦ C until C3 H6 detection became negligible, whereupon the temperature was increased to 500 ◦ C at 20 ◦ C/min.
ability to release and convert stored hydrocarbons became increasingly limited resulting in dramatically lower NOx conversion. These results highlight the importance of balancing spatiotemporal distribution of storage, release and conversion of C3 H6 to achieve maximum NOx reduction. As in the case of NH3 reductant, N2 O formation was limited to 0–3 ppm but occurred at early lean times and not during the rich period (Fig. 3d). This observation as well as the detection of ppm-level NH3 (Fig. 3e) will be further discussed in Section 3.4. 3.3. NOx reduction by NH3 and C3 H6 To get insights into possible interactions among multiple reductants present in real engine exhaust, the cycling experiments were performed with both NH3 and C3 H6 (Run type 3 in Table 1). The axial evolution of cycle-averaged conversion is summarized in Fig. 4a–c. Under these conditions, a maximum NOx conversion of ca. 26% was obtained at 350 ◦ C. The axial profiles of NH3 and C3 H6 conversion were nearly identical to those obtained with single reductants (Figs. 2b and 3b), meaning that the presence of the one reductant did not affect the effectiveness of the other. These results are consistent with recent work by Wang et al. [36] which showed that the contributions of C3 H6 and NH3 to NOx conversion are additive on a cycle-averaged conversion basis. As in the case of the C3 H6 only case, three temporally distinct regimes of NOx reduction were involved (Fig. 4d) and major C3 H6 chemistry included storage, as well as partial oxidation and cracking to form C2 H4 , HCHO, and CO (Fig. 4g–j). N2 O formation was also limited (Fig. 4e), but slightly higher than in the NH3 -only or C3 H6 -only cases. It should be noted that N2 O formation occurred only at early lean times, and that NH3 breakthrough (Fig. 4f) was more significant than the single reductant cases. 3.4. TPD after NOx reduction cycling with C3 H6 as reductant After NOx reduction cycling with C3 H6 , TPD experiments were performed to understand the nature of adsorbed surface species. There were three major desorption peaks consistently observed in repeat experiments: formaldehyde (HCHO), CO, and NH3 were detected at ca. 310, 360, and 390 ◦ C, respectively (Fig. 5). The TPD results were qualitatively similar whether or not O2 was present in the TPD gas mixture. The only major effect of O2 was on the intensity and temperature of desorption of HCHO, CO, and NH3 (results not shown). The TPD results suggest the in situ formation
and storage of NH3 (or NH3 -precursors) over the SCR catalyst from NOx –C3 H6 reactions, the stored NH3 (or NH3 -precursors) acting as a reductant. As discussed in Section 3.5, this NH3 peak could explain the NOx reduction regime III (mid-late lean periods) observed most clearly at 350–450 ◦ C in Figs. 2 and 3. This hypothesis of intermediate NH3 formation by NOx reacting with C3 H6 seems to be further supported by the small amount of NH3 (0–3 ppm) detected during lean/rich cycling with C3 H6 reductant at some “intra”-catalyst locations (e.g., 0.25 L as shown in Fig. 3e). There was some crosssensitivity issue with NH3 measurements via FTIR in the presence of C3 H6 : for instance, we can see 0–2 ppm level NH3 at inlet (0 L) where the concentration of C3 H6 is the highest (Fig. 3e). However, the NH3 detected at early lean times was likely real, as the peaks appeared when C3 H6 was almost depleted in the gas phase (Fig. 3f). The formation of NH3 as an intermediate during HC–SCR has been reported [38,39]. The general HC–SCR steps involve the formation of some type of organonitrogen species (e.g., nitromethane) by reaction between NO2 and partially oxidized hydrocarbons (e.g., aldehydes). The organonitrogen species decompose to form isocyanates which then hydrolyze to form NH3 . In these pathways, surface oxygen plays an important role by oxidizing NO and HC to NO2 and aldehydes, respectively. One could conjecture that similar mechanisms were involved in the lean/rich cycling experiments carried out in this study. The amount of the surface oxygen species stored on the SCR catalyst during the 60 s lean phase appears to have been enough to sustain the transformation of NO and C3 H6 , respectively, to NO2 and oxygenated hydrocarbon species during the 5 s rich phase. For no major differences in NO conversion and profiles were observed when O2 was removed from the rich-phase gas mixture (results not shown). 3.5. Mass spectrometer results from experiments utilizing isotopically labeled 15 N18 O To further understand the C3 H6 and NH3 chemistry involved in NOx reduction and their interaction, we analyzed the evolution of N2 by mass spectrometry during lean/rich cycling with NH3 , C3 H6 , or NH3 + C3 H6 as reductants. 15 N18 O was used to facilitate the interpretation of MS data. Consistent with the observations made so far, 15 N18 O reduction with 14 NH3 (feed reductant) involved two temporally distinct reduction regimes: rich (I) and lean (II), both generating 15 N14 N as the major reduction product (Fig. 6a). 15 N18 O reduction with C3 H6 involved three reduction regimes rich (I), early lean or rich/lean transition (II), and mid-late lean (III) (Fig. 6b and c). During the cycling with C3 H6 only, regime I and the early part of II led to significant NO consumption without any 15 N2 formation. 15 N appeared only in the remaining part of regime II and regime 2 III with additional NO conversion. This means that the NOx reduction in regime I and the early part of II led exclusively to 15 NH3 which was then stored on the surface. The TPD results (Fig. 5) support this argument. As a consequence, when both gas-phase (during regime I) and surface hydrocarbons (during early part of regime II) were depleted, the reaction 15 N18 O + HCs → 15 NH3 + . . . stopped, and NOx reduction via 215 N18 O + 215 NH3 + 0.5O2 → 215 N2 + 3H2 18 O (regime III) started consuming stored 15 NH3 as reductant. When both NH3 and C3 H6 were present, the NO reduction by C3 H6 (i.e., NH3 intermediate pathway) does not seem to have been affected by the presence of NH3 in the feed as indicated by the near identical 15 N2 profiles for both C3 H6 only and C3 H6 + NH3 cases (see Fig. 6b vs. c). In contrast, adsorbed C3 H6 and/or other hydrocarbonderived species completely inhibited the utilization of feed NH3 leading to delayed inception of 15 N14 N generation. As in the case of C3 H6 only, 15 N2 was formed only in the second part of regime II and in regime III. This means that the presence of hydrocarbons impeded the utilization of NH3 regardless of its origin: either as feed or generated “in situ” NH3 . The N2 O formation limited to early
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Fig. 6. Temporal profiles of NO and N2 measured during 60/5 s lean/rich cycling over a Cu-chabazite SCR catalyst at 350 ◦ C using 15 N18 O as NO feed. A mass spectrometer followed NO at m/z of 31 (15 N16 O was the major species due to efficient isotopic scrambling between 600 ppm 15 N18 O and 8% 16 O2 in the feed), and 15 N14 N and 15 N2 to track NO reduction by feed NH3 (15 N18 O + 14 NH3 ) and C3 H6 (15 N18 O + 15 NH3 intermediate), respectively. The Roman numerals indicate the three temporally distinct NO reduction regimes: I: rich, II: early lean or rich/lean transition, III: mid-late lean).
lean times seems to corroborate this conjecture (Fig. 4e). However, the inhibition of NH3 utilization seems to be limited to the midto-high temperature range in which the reaction of C3 H6 with NOx was effective. Indeed, comparing temporal NOx profiles in Figs. 2c, 3c, and 4d, it is evident that feed NH3 was used in the rich phase during cycling at 200–250 ◦ C with both NH3 and C3 H6 in the feed. In addition, it is worth pointing out that despite the inhibiting effect of C3 H6 on NH3 –NO reactions in the mid-to-high temperature range, its impact on NH3 storage capacity was minimal as confirmed by the additive contribution of C3 H6 and NH3 to the cycle-average conversion as discussed in Section 3.3 and [36]. In summary, C3 H6 contributed to in situ formation of NH3 as an intermediate reductant, but inhibited reactions between NO and NH3 . The additive effect of NH3 and C3 H6 on the cycle-averaged NOx conversion was likely due to the cycling conditions (i.e., relatively long lean period compared to rich) which allowed the complete use of both feed and intermediate NH3 despite the inhibiting effect of hydrocarbon species. In other words, a shorter lean time could have made the inhibiting effect of C3 H6 on NH3 reduction of NO discernible even at the cycle-averaged conversion level.
4. Conclusions The spatiotemporal distributions of NOx reduction over a commercial Cu-chabazite SCR catalyst were resolved to better understand the non-NH3 pathways to NOx reduction using C3 H6 during lean/rich cycling conditions relevant to coupled LNT–SCR catalysts. The following conclusions were drawn from the present study: • C3 H6 was an effective reductant for reducing NOx over Cuchabazite SCR catalysts via intermediate NH3 under lean/rich cycling conditions. • C3 H6 and other C3 H6 -derived hydrocarbon species inhibited reactions between NO and NH3 present either in the feed or as a surface intermediate generated in situ from C3 H6 –NO reactions. • Reactions between NO and stored NH3 became effective only when C3 H6 and other C3 H6 -derived species were depleted. • Contributions of C3 H6 and NH3 to cycle-averaged NOx reduction over the SCR catalyst were essentially additive under the conditions used in this study which allowed, at the relatively long lean
period of 60 s, complete consumption of feed as well as intermediate NH3 stored on the surface. • These insights can facilitate the development of optimized coupled LNT–SCR catalysts and operating strategies. • The findings of this study could also be relevant to further understanding Cu-chabazites as stand-alone NH3 –SCR catalysts, such as their resistance to hydrocarbon poisoning [40]. Acknowledgements This project was funded by the U.S. Department of Energy (DOE) under award No. (DE-EE0000205. The authors thank BASF for providing the catalyst used in this study, and Drs. William P. Partridge and Todd J. Toops at Oak Ridge National Laboratory for useful discussions. References [1] P. Forzatti, L. Lietti, I. Nova, E. Tronconi, Catal. Today 151 (2010) 202–211. [2] Y. Ji, T.J. Toops, J.A. Pihl, M. Crocker, Appl. Catal. B: Environ. 91 (2009) 329–338. [3] N.A. Ottinger, T.J. Toops, K. Nguyen, B.G. Bunting, J. Howe, Appl. Catal. B: Environ. 101 (2011) 486–494. [4] R.D. Clayton, M.P. Harold, V. Balakotaiah, C.Z. Wan, Appl. Catal. B: Environ. 90 (2009) 662–676. [5] D.H. Kim, J. Szany, J.H. Kwak, X. Wang, J.C. Hanson, M. Engelhard, C.H.F. Peden, J. Phys. Chem. C 113 (2009) 7336–7341. [6] J.-S. Choi, W.P. Partridge, J.A. Pihl, M.-Y. Kim, P. Koˇcí, C.S. Daw, Catal. Today 184 (2012) 20–26. [7] S.S. Chaugule, V.F. Kispersky, J.L. Ratts, A. Yezerets, N.W. Currier, F.H. Ribeiro, W.N. Delgass, Appl. Catal. B: Environ. 107 (2011) 26–33. [8] V. Easterling, Y. Ji, M. Crocker, M. Dearth, R.W. McCabe, Appl. Catal. B: Environ. 123–124 (2012) 339–350. [9] J.H. Kwak, D. Mei, C.-W. Yi, D.H. Kim, C.H.F. Peden, L.F. Allard, J. Szanyi, J. Catal. 261 (2009) 17–22. [10] Y. Hu, K. Griffiths, P.R. Norton, Surf. Sci. 603 (2009) 1740–1750. [11] M. Colombo, I. Nova, E. Tronconi, Catal. Today 151 (2010) 223–230. [12] J.-Y. Luo, H. Oh, C. Henry, W. Epling, Appl. Catal. B: Environ. 123–124 (2012) 296–305. [13] V.A. Kondratenko, U. Bentrup, M. Richter, T.W. Hansen, E.V. Kondratenko, Appl. Catal. B: Environ. 84 (2008) 497–504. [14] A. Grossale, I. Nova, E. Tronconi, J. Catal. 265 (2009) 141–147. [15] J.-Y. Luo, X. Hou, P. Wijayakoon, S.J. Schmieg, W. Li, W.S. Epling, Appl. Catal. B: Environ. 102 (2011) 110–119. [16] N. Wilken, K. Wijayanti, K. Kamasamudram, N.W. Currier, R. Vedaiyan, A. Yezerets, L. Olsson, Appl. Catal. B: Environ. 111–112 (2012) 58–66. [17] H. Hu, J. Reuter, J. Yan, J. McCarthy Jr., SAE Technical Paper (2006), 2006-013552. [18] J. Theis, E. Gulari, SAE Technical Paper (2006), 2006-01-0210. [19] T. Nakatsuji, M. Matsubara, J. Rouistenmäki, N. Sato, H. Ohno, Appl. Catal. B: Environ. 77 (2007) 190–201.
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Roles of C3 H6 in NH3 generation and NOx reduction over a Cu-chabazite SCR catalyst under lean/rich cycling conditions夽 Mi-Young Kim a , Jae-Soon Choi a,∗ , Mark Crocker b a b
Fuels, Engines, and Emissions Research Center, Oak Ridge National Laboratory, Knoxville, TN 37932, USA Center for Applied Energy Research, University of Kentucky, Lexington, KY 40511, USA
a r t i c l e
i n f o
Article history: Received 17 September 2013 Received in revised form 16 December 2013 Accepted 17 December 2013 Available online 4 January 2014 Keywords: Coupled LNT–SCR NOx storage and reduction Lean NOx trap Selective catalytic reduction LNT SCR
a b s t r a c t We studied the spatiotemporal profiles of NOx reduction with NH3 , C3 H6 , or NH3 + C3 H6 over a commercial Cu-chabazite SCR catalyst (washcoated honeycomb monolith) to better understand the effects of C3 H6 on NOx reduction under fast lean/rich cycling conditions relevant to coupled LNT–SCR catalysts. NOx reduction by NH3 was very effective with total NH3 consumption within the first quarter of the catalyst at all temperatures. By contrast, NOx reduction by C3 H6 was more gradual along the catalyst length and sensitive to temperature with maximum performance obtained at 300–400 ◦ C. Temperatureprogrammed desorption performed after cycling experiments with C3 H6 evidenced the presence of NH3 and/or NH3 -precursor intermediates on the surface. These surface species were formed as a result of reactions between NOx and C3 H6 during the rich as well as lean phases, but started to be used for NOx reduction only when all the stored hydrocarbons were depleted. When fed together, the contributions of C3 H6 and NH3 to the cycle-averaged NOx conversion were essentially additive. However, temporally resolving N2 formation using isotopically-labeled NO and a mass spectrometer revealed that C3 H6 actually inhibited reactions between NOx and feed NH3 until well into the subsequent lean phase, i.e., until the stored hydrocarbons were depleted. This study highlights the importance of controlling lean/rich cycling time partitioning to alleviate the impact of the inhibiting effect of C3 H6 on NH3 chemistry and achieve the maximum NOx reduction possible over the SCR catalyst in coupled LNT–SCR systems. © 2013 Elsevier B.V. All rights reserved.
1. Introduction The application of lean-burn engines in vehicles has significantly increased owing to their more efficient fuel combustion and lower CO2 emissions compared to engines operating under stoichiometric conditions. However, the reduction of NOx in lean exhausts is more difficult due to the excess oxygen content. Lean NOx trap (LNT) and selective catalytic reduction (SCR) catalysts were developed to overcome the challenge of lean NOx reduction. The chemistry of LNT [1–9] and SCR [10–16] catalysts has been extensively studied, generating fundamental insights needed to enhance the performance
夽 Notice: This submission was sponsored by a contractor of the United States Government under contract DE-AC05-00OR22725 with the United States Department of Energy. The United States Government retains, and the publisher, by accepting this submission for publication, acknowledges that the United States Government retains, a nonexclusive, paid-up, irrevocable, worldwide license to publish or reproduce the published form of this submission, or allow others to do so, for United States Government purposes. ∗ Corresponding author at: Oak Ridge National Laboratory, Fuels, Engines, and Emissions Research Center, 2360 Cherahala Blvd., Knoxville, USA. Tel.: +1 865 946 1368; fax: +1 865 946 1354. E-mail address: [email protected] (J.-S. Choi). 0920-5861/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.cattod.2013.12.021
of these devices. Recently, to further improve the performance and cost of lean NOx controls, so-called coupled LNT–SCR systems have been introduced which combine SCR with LNT catalysts [17–21]. Coupled LNT–SCR catalysts function in cyclic lean/rich modes as do stand-alone LNTs. During normal lean engine operation, the upstream LNT component stores NOx which is released and reduced to N2 , N2 O, or NH3 by intermittent short rich purges. The downstream SCR catalyst can capture NH3 to reduce, in subsequent lean phases, LNT-slipped NOx via well-known NH3 –SCR mechanisms [11]. The benefit of having a downstream SCR catalyst is thus manifested as decreased slips of both NOx (higher conversion) and NH3 . The chemistry of NH3 generation during the regeneration step of LNT catalysts is well documented in the literature [22–27], and the above mentioned NH3 storage and utilization over SCR catalysts (e.g., Cu-exchanged zeolites) is widely accepted as an important NOx reduction pathway [28–34]. Recent vehicle [35] as well as controlled laboratory reactor studies [36], however, revealed that the amount of NH3 entering the downstream SCR catalyst alone cannot always explain the amount of the additional NOx converted over the SCR catalyst. Furthermore, HNCO (a potential “NH3 precursor” molecule) was not detected at the LNT outlet [36]. This discrepancy indicates the involvement of non-NH3 pathways using, as potential reductants, hydrocarbons,
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Table 1 Conditions used for cycling experiments with a Cu-chabazite SCR catalyst. Parameter
Run type 1
Run type 2
Reductant
NH3
C3 H6
Duration NO (ppm) O2 (%) C3 H6 (ppm) NH3 (ppm) CO2 (%) H2 O (%) N2 (%)
Lean 60 s 300 8 0 0 5 5 Balance
Rich 5 s 300 1 0 300 5 5 Balance
Run type 3 NH3 + C3 H6
Lean 60 s 300 8 0 0 5 5 Balance
Rich 5 s 300 1 3333 0 5 5 Balance
Lean 60 s 300 8 0 0 5 5 Balance
Rich 5 s 300 1 3333 300 5 5 Balance
Total flow: 13.7 L/min (STP). Space velocity: 30,000 h−1 . Temperature: 200, 250, 300, 350, 400, and 450 ◦ C.
CO or H2 unused by the upstream LNT. Laboratory experiments indeed showed that SCR catalysts can store hydrocarbons in addition to NH3 in rich phases and use them for NOx reduction in subsequent lean phases [36]. The contributions of C3 H6 and NH3 to cycle-averaged NOx conversion over the SCR component were found to be essentially additive. In the present study, we measured the spatiotemporal distribution of reactions to clarify the chemistry of NOx reduction over a Cu-chabazite SCR catalyst under lean/rich cycling conditions. The results revealed important roles C3 H6 plays in the formation, storage and utilization of NH3 , and/or NH3 -precursor intermediates on the SCR catalyst surface.
employed. First, cycling experiments were performed with the 7.9 cm monolith catalyst (1 L). Then a quarter length of the core was cut out from the back (0.75 L) and the same experiments were carried out. By repeating this procedure, spatiotemporal profiles of gas concentrations were determined at 1 L (outlet), 0.75, 0.5, 0.25, and 0 L (inlet) locations. The data were collected in the temperature range of 200–450 ◦ C with an interval of 50 ◦ C. The cycle-averaged conversion of NO, NH3 , and C3 H6 was calculated by averaging over 5 cycles after the catalyst performance was stabilized at each temperature, which took about 1 h. Prior to the reactor measurements, the catalyst was de-greened at 500 ◦ C for 5 h under the lean/rich cycling conditions corresponding to Run type 2 in Table 1.
2. Experimental
2.3. Temperature-programmed desorption (TPD) experiments
2.1. Catalysts
After lean/rich cycling experiments with C3 H6 as reductant, the catalyst (0.25 L) was purged at 200 ◦ C with a flow of 8% O2 , 5% H2 O, 5% CO2 , and N2 balance until no C3 H6 effluent was observed. Next, the temperature of the reactor was increased to 500 ◦ C at a ramping rate of 20 ◦ C/min under a flow of 8% O2 , 5% H2 O, 5% CO2 , and N2 balance (total flow rate: 13.7 L/min). The gas composition was analyzed using a high-speed FTIR gas analyzer (MKS, MultiGas 2030 HS).
A commercial SCR catalyst based on Cu-chabazite zeolite was used in this study. This catalyst was provided by BASF and prepared using a 400 cpsi/6.5 mil ceramic monolith. For bench reactor evaluation, a core sample of 2 cm in diameter and 7.9 cm in length was drilled out from a monolith brick. More information about catalyst properties can be found in [21]. 2.2. Lean/rich cycling experiments The NOx reduction performance of the catalyst was evaluated in a laboratory bench flow reactor. A 7.9 cm-long core catalyst was wrapped in Zetex insulation tape and inserted into a horizontal quartz reactor tube (2.2 cm inside diameter). The reactor tube was heated by an electric furnace, and synthetic exhaust gas mixtures were prepared using pressurized gas bottles (ultra high purity grade, Air Liquide). The gases were metered with mass flow controllers (Unit Instruments Series 7300, Kinetics Electronics) and pre-heated before entering the quartz reactor. Water was introduced by a high pressure liquid metering pump (Eldex) to a heated zone, vaporized, and added to the simulated exhaust mixture. A rapid switching 4-way valve system was used to alternate between the lean and rich gas mixtures (60/5 s lean/rich cycling). The reaction conditions used for the cycling experiments are listed in Table 1. Briefly, the lean-phase gas mixture contained 300 ppm NO, 8% O2 , 5% H2 O, 5% CO2 , and N2 balance, and the rich-phase gas mixture contained 300 ppm NH3 and/or 3333 ppm C3 H6 , 300 ppm NO, 1% O2 , 5% H2 O, 5% CO2 , and N2 balance. The total gas flow was 13.7 L/min (STP) leading to a gas hourly space velocity (GHSV) of 30,000 h−1 . The gas composition was monitored at the reactor outlet using a high-speed FTIR gas analyzer (MKS, MultiGas 2030 HS). To obtain spatiotemporally resolved profiles of gas concentrations with the FTIR gas analyzer, the following procedure was
2.4. Mass spectrometry analysis with isotopically labeled nitric oxide (15 N18 O) To better understand the chemistry of C3 H6 and its impact on NOx reduction, the timing of N2 evolution was monitored by employing 15 N18 O as the NO feed. The lean-phase gas mixture contained 600 ppm 15 N18 O, 8% O2 , 5% H2 O, 5% CO2 , 0.14% He (as internal standard), and Ar balance, while the rich-phase gas mixture contained 600 ppm NH3 and/or 3333 ppm C3 H6 , 600 ppm 15 N18 O, 1% O2 , 5% H2 O, 5% CO2 , 0.14% He, and Ar balance. The total flow rate was 6.9 L/min (STP) and the front quarter of the core (0.25 L) used in Section 2.2 was used in these experiments. The measurements were done at 350 ◦ C under 60/5 s lean/rich cycling conditions. Note that we chose 600 ppm as the NO and NH3 concentrations as opposed to 300 ppm used in Section 2.2 in order to facilitate the mass spectrometer (MS) analysis by increasing the dynamic range of the MS signals. Even though this NOx level can be higher than what is normally expected at the inlet of the downstream SCR catalyst in a coupled LNT–SCR system, the difference in NOx concentration does not seem to change the nature of chemistry investigated [35,36]. Moreover, the SCR catalyst could encounter a high level of NOx due to inefficient NOx removal over the upstream LNT at low or high temperatures. The gas composition was monitored at the reactor outlet with a quadrupole MS (Pfeiffer, PrismaPlus) using a small capillary probe (185 m OD, 50 m ID) for gas sampling. The m/z ratios monitored
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aldehyde contributions to m/z 29 and 30 subtracted
a.u.
– Time (s)
C3H6 cycling with15 N18O
=
a.u.
C3H 6 cycling without 15 N18 O
a.u.
C3H 6 cycling with15 N18 O
Time (s)
Time (s)
Fig. 1. Schematic representation of the procedure used to calculate the contribution of 15 N14 N and 15 N2 to mass spectrometer (MS) signals at m/z 29 and 30, respectively. 60/5 s lean/rich cycling experiments were performed with and without 15 N18 O (detailed feed gas composition given in Section 2.4). The MS signals obtained during the experiments without NO were subtracted from those obtained with NO in the feed gas to remove the contribution of the aldehyde species formed by oxidation of C3 H6 . The negative values obtained for the rich and early part of lean periods are artifacts due to higher aldehydes formation during the cycling without NO, as more C3 H6 was available for aldehyde formation.
were 29 for 15 N14 N, 30 for 15 N2 , and 31 for 15 N16 O (the major NO species due to very efficient isotopic scrambling between 600 ppm 15 N18 O and 8% 16 O in the feed). The MS signals obtained at m/z 2 29 and 30 could represent both nitrogen and aldehydes. Therefore, the signals obtained from the C3 H6 cycling experiments performed without 15 N18 O in the feed were subtracted from those obtained with 15 N18 O in order to remove the aldehyde-attributable signals. This procedure is represented schematically in Fig. 1. 3. Results and discussion
slipped from an LNT during rich phases is stored over an SCR catalyst placed downstream to be used for additional NOx reduction in subsequent lean phases [28–34]. In contrast, a greater portion of NOx was reduced in the rich phase at higher temperatures, but this reduction regime is likely less relevant to LNT–SCR applications due to the low NH3 yield of LNT regeneration at high temperatures [22]. NH3 adsorption was efficient as shown by the negligible breakthrough even at 0.25 L locations at all temperatures (Fig. 2e). Consistent with previously published studies on Cu-chabazite catalysts such as [37], N2 O formation was minor (i.e., 0–3 ppm peak values, Fig. 2d).
3.1. NOx reduction by NH3 3.2. NOx reduction by C3 H6 Figs. 2a and 2b summarize, respectively, the cycle-averaged conversion of NOx and NH3 along the catalyst length for the experiments of Run type 1 (i.e., “NH3 ” cycling in Table 1). To help understand the temporal development of the reduction processes, the temporal concentration profiles at the first quarter location (0.25 L) are also presented (Fig. 2c–e). The cycle-averaged conversion of NOx reached almost the theoretical maximum value of 7.7% (corresponding to total consumption of NH3 with NH3 :NO stoichiometry of 1:1) within the first quarter length of the catalyst regardless of reaction temperatures. The depletion of NH3 within 0.25 L explains the unchanged NOx conversion over the remaining portion of the catalyst. Even though the axial trends demonstrate efficient NH3 reduction of NOx at all temperatures at the cycle-averaged scale, the temporal NOx profiles in Fig. 2c reveal significant differences in intrinsic catalytic activity depending on reaction temperature. In fact, NOx conversion rate monotonously increased with temperature. Over the entire temperature range studied, one can notice that two temporally distinct reduction regimes were involved which correspond, respectively, to the rich and lean phases of a cycle (Fig. 2c). Note that the “rich” phase of Run type 1 was actually “lean”, since the 300 ppm NH3 present as the sole reductant was well below the amount needed to reduce 300 ppm NO and 1% O2 . For all cases shown in Fig. 2c, NOx reduction occurred in both rich & lean phases. However, the partitioning between rich and lean-phase reduction varied significantly. At low temperatures (i.e., <300 ◦ C), most NOx was converted in the lean phase, which is consistent with a well-accepted “in situ NH3 ” pathway to NOx reduction over SCR catalysts in the coupled LNT–SCR concept. That is, NH3
The spatiotemporally resolved data for NOx reduction with C3 H6 are presented in Fig. 3 and experimental conditions given in Table 1 (Run type 2). The best performance (a maximum cycle-averaged NOx conversion of ca. 18%) was achieved at 300 ◦ C where NOx reduction occurred along the entire catalyst length (1 L, Fig. 3a). Above 300 ◦ C, despite the higher reaction rates due to temperature effects, slightly lower maximum conversion was obtained likely due to the complete depletion of C3 H6 within the front three quarters of the catalyst (0.75 L, Fig. 3b). The effectiveness of C3 H6 for NOx reduction dropped dramatically below 300 ◦ C. The temporal NOx profiles at 0.25 L (Fig. 3c) shows that NOx reduction by C3 H6 occurred both in rich and lean phases as in the case of NH3 , but involved three temporally distinct reduction regimes instead of two: rich, early lean (or lean/rich transition), and mid-late lean periods. A closer inspection of the spatiotemporal profiles reveals that, when C3 H6 was a reductant, a greater portion of NOx was reduced in the lean phase than in the rich phase regardless of temperature. This indicates the importance of reductant storage in the rich phase. When the profiles of reductive species – C3 H6 and three major products of C3 H6 conversion: C2 H4 , HCHO, and CO (Fig. 3f–i) – were compared with those of NOx (Fig. 3c), it becomes clear that a significant portion of the lean NOx reduction took place when these reductive species were absent from the gas phase (compare, for instance, the profiles between 0.8 and 0.9 min at 400 ◦ C). This mismatch implies that some other species derived from C3 H6 transformation were present on the surface for NOx reduction. Indeed, the results presented in Sections 3.4 and 3.5 support this conjecture.
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Fig. 2. Spatiotemporal profiles obtained during lean/rich cycling over Cu-chabazite SCR catalyst using NH3 as reductant: a axial evolution of cycle-averaged NOx conversion, b axial evolution of cycle-averaged NH3 conversion, and temporal profiles of c NOx , d N2 O, and e NH3 concentration at the first quarter of the catalyst length (0.25 L).
As mentioned above, despite the higher reaction rates, the maximum NOx conversion achieved at 350–450 ◦ C was lower than at 300 ◦ C. In addition to the earlier depletion of C3 H6 along the catalyst length (e.g., via oxidation: spatially limited reductant supply),
this could also be due in part to decreased stability of the hydrocarbon species on the surface (temporally limited reductant supply). Decreasing the temperature extended the reductant availability over a longer period of the lean phase. Below 300 ◦ C, however, the
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Fig. 3. Spatiotemporal profiles obtained during lean/rich cycling over Cu-chabazite SCR catalyst using C3 H6 as reductant: a axial evolution of cycle-averaged NOx conversion, b axial evolution of cycle-averaged C3 H6 conversion, and temporal profiles of c NOx , d N2 O, e NH3 , f C3 H6 , g C2 H4 , h HCHO, and i CO concentration at the first quarter of the catalyst length (0.25 L).
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Fig. 4. Spatiotemporal profiles obtained during lean/rich cycling over Cu-chabazite SCR catalyst using NH3 and C3 H6 as reductants: a axial evolution of cycle-averaged NOx conversion, b axial evolution of cycle-averaged NH3 conversion, c axial evolution of cycle-averaged C3 H6 conversion, and temporal profiles of d NOx , e N2 O, f NH3 , g C3 H6 , h C2 H4 , i HCHO, and j CO concentration at the first quarter of the catalyst length (0.25 L).
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Fig. 5. Gas concentration and temperature profiles obtained during temperatureprogrammed desorption performed after a C3 H6 cycling experiment at 450 ◦ C with a Cu-chabazite SCR catalyst. The catalyst was purged at 200 ◦ C until C3 H6 detection became negligible, whereupon the temperature was increased to 500 ◦ C at 20 ◦ C/min.
ability to release and convert stored hydrocarbons became increasingly limited resulting in dramatically lower NOx conversion. These results highlight the importance of balancing spatiotemporal distribution of storage, release and conversion of C3 H6 to achieve maximum NOx reduction. As in the case of NH3 reductant, N2 O formation was limited to 0–3 ppm but occurred at early lean times and not during the rich period (Fig. 3d). This observation as well as the detection of ppm-level NH3 (Fig. 3e) will be further discussed in Section 3.4. 3.3. NOx reduction by NH3 and C3 H6 To get insights into possible interactions among multiple reductants present in real engine exhaust, the cycling experiments were performed with both NH3 and C3 H6 (Run type 3 in Table 1). The axial evolution of cycle-averaged conversion is summarized in Fig. 4a–c. Under these conditions, a maximum NOx conversion of ca. 26% was obtained at 350 ◦ C. The axial profiles of NH3 and C3 H6 conversion were nearly identical to those obtained with single reductants (Figs. 2b and 3b), meaning that the presence of the one reductant did not affect the effectiveness of the other. These results are consistent with recent work by Wang et al. [36] which showed that the contributions of C3 H6 and NH3 to NOx conversion are additive on a cycle-averaged conversion basis. As in the case of the C3 H6 only case, three temporally distinct regimes of NOx reduction were involved (Fig. 4d) and major C3 H6 chemistry included storage, as well as partial oxidation and cracking to form C2 H4 , HCHO, and CO (Fig. 4g–j). N2 O formation was also limited (Fig. 4e), but slightly higher than in the NH3 -only or C3 H6 -only cases. It should be noted that N2 O formation occurred only at early lean times, and that NH3 breakthrough (Fig. 4f) was more significant than the single reductant cases. 3.4. TPD after NOx reduction cycling with C3 H6 as reductant After NOx reduction cycling with C3 H6 , TPD experiments were performed to understand the nature of adsorbed surface species. There were three major desorption peaks consistently observed in repeat experiments: formaldehyde (HCHO), CO, and NH3 were detected at ca. 310, 360, and 390 ◦ C, respectively (Fig. 5). The TPD results were qualitatively similar whether or not O2 was present in the TPD gas mixture. The only major effect of O2 was on the intensity and temperature of desorption of HCHO, CO, and NH3 (results not shown). The TPD results suggest the in situ formation
and storage of NH3 (or NH3 -precursors) over the SCR catalyst from NOx –C3 H6 reactions, the stored NH3 (or NH3 -precursors) acting as a reductant. As discussed in Section 3.5, this NH3 peak could explain the NOx reduction regime III (mid-late lean periods) observed most clearly at 350–450 ◦ C in Figs. 2 and 3. This hypothesis of intermediate NH3 formation by NOx reacting with C3 H6 seems to be further supported by the small amount of NH3 (0–3 ppm) detected during lean/rich cycling with C3 H6 reductant at some “intra”-catalyst locations (e.g., 0.25 L as shown in Fig. 3e). There was some crosssensitivity issue with NH3 measurements via FTIR in the presence of C3 H6 : for instance, we can see 0–2 ppm level NH3 at inlet (0 L) where the concentration of C3 H6 is the highest (Fig. 3e). However, the NH3 detected at early lean times was likely real, as the peaks appeared when C3 H6 was almost depleted in the gas phase (Fig. 3f). The formation of NH3 as an intermediate during HC–SCR has been reported [38,39]. The general HC–SCR steps involve the formation of some type of organonitrogen species (e.g., nitromethane) by reaction between NO2 and partially oxidized hydrocarbons (e.g., aldehydes). The organonitrogen species decompose to form isocyanates which then hydrolyze to form NH3 . In these pathways, surface oxygen plays an important role by oxidizing NO and HC to NO2 and aldehydes, respectively. One could conjecture that similar mechanisms were involved in the lean/rich cycling experiments carried out in this study. The amount of the surface oxygen species stored on the SCR catalyst during the 60 s lean phase appears to have been enough to sustain the transformation of NO and C3 H6 , respectively, to NO2 and oxygenated hydrocarbon species during the 5 s rich phase. For no major differences in NO conversion and profiles were observed when O2 was removed from the rich-phase gas mixture (results not shown). 3.5. Mass spectrometer results from experiments utilizing isotopically labeled 15 N18 O To further understand the C3 H6 and NH3 chemistry involved in NOx reduction and their interaction, we analyzed the evolution of N2 by mass spectrometry during lean/rich cycling with NH3 , C3 H6 , or NH3 + C3 H6 as reductants. 15 N18 O was used to facilitate the interpretation of MS data. Consistent with the observations made so far, 15 N18 O reduction with 14 NH3 (feed reductant) involved two temporally distinct reduction regimes: rich (I) and lean (II), both generating 15 N14 N as the major reduction product (Fig. 6a). 15 N18 O reduction with C3 H6 involved three reduction regimes rich (I), early lean or rich/lean transition (II), and mid-late lean (III) (Fig. 6b and c). During the cycling with C3 H6 only, regime I and the early part of II led to significant NO consumption without any 15 N2 formation. 15 N appeared only in the remaining part of regime II and regime 2 III with additional NO conversion. This means that the NOx reduction in regime I and the early part of II led exclusively to 15 NH3 which was then stored on the surface. The TPD results (Fig. 5) support this argument. As a consequence, when both gas-phase (during regime I) and surface hydrocarbons (during early part of regime II) were depleted, the reaction 15 N18 O + HCs → 15 NH3 + . . . stopped, and NOx reduction via 215 N18 O + 215 NH3 + 0.5O2 → 215 N2 + 3H2 18 O (regime III) started consuming stored 15 NH3 as reductant. When both NH3 and C3 H6 were present, the NO reduction by C3 H6 (i.e., NH3 intermediate pathway) does not seem to have been affected by the presence of NH3 in the feed as indicated by the near identical 15 N2 profiles for both C3 H6 only and C3 H6 + NH3 cases (see Fig. 6b vs. c). In contrast, adsorbed C3 H6 and/or other hydrocarbonderived species completely inhibited the utilization of feed NH3 leading to delayed inception of 15 N14 N generation. As in the case of C3 H6 only, 15 N2 was formed only in the second part of regime II and in regime III. This means that the presence of hydrocarbons impeded the utilization of NH3 regardless of its origin: either as feed or generated “in situ” NH3 . The N2 O formation limited to early
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Fig. 6. Temporal profiles of NO and N2 measured during 60/5 s lean/rich cycling over a Cu-chabazite SCR catalyst at 350 ◦ C using 15 N18 O as NO feed. A mass spectrometer followed NO at m/z of 31 (15 N16 O was the major species due to efficient isotopic scrambling between 600 ppm 15 N18 O and 8% 16 O2 in the feed), and 15 N14 N and 15 N2 to track NO reduction by feed NH3 (15 N18 O + 14 NH3 ) and C3 H6 (15 N18 O + 15 NH3 intermediate), respectively. The Roman numerals indicate the three temporally distinct NO reduction regimes: I: rich, II: early lean or rich/lean transition, III: mid-late lean).
lean times seems to corroborate this conjecture (Fig. 4e). However, the inhibition of NH3 utilization seems to be limited to the midto-high temperature range in which the reaction of C3 H6 with NOx was effective. Indeed, comparing temporal NOx profiles in Figs. 2c, 3c, and 4d, it is evident that feed NH3 was used in the rich phase during cycling at 200–250 ◦ C with both NH3 and C3 H6 in the feed. In addition, it is worth pointing out that despite the inhibiting effect of C3 H6 on NH3 –NO reactions in the mid-to-high temperature range, its impact on NH3 storage capacity was minimal as confirmed by the additive contribution of C3 H6 and NH3 to the cycle-average conversion as discussed in Section 3.3 and [36]. In summary, C3 H6 contributed to in situ formation of NH3 as an intermediate reductant, but inhibited reactions between NO and NH3 . The additive effect of NH3 and C3 H6 on the cycle-averaged NOx conversion was likely due to the cycling conditions (i.e., relatively long lean period compared to rich) which allowed the complete use of both feed and intermediate NH3 despite the inhibiting effect of hydrocarbon species. In other words, a shorter lean time could have made the inhibiting effect of C3 H6 on NH3 reduction of NO discernible even at the cycle-averaged conversion level.
4. Conclusions The spatiotemporal distributions of NOx reduction over a commercial Cu-chabazite SCR catalyst were resolved to better understand the non-NH3 pathways to NOx reduction using C3 H6 during lean/rich cycling conditions relevant to coupled LNT–SCR catalysts. The following conclusions were drawn from the present study: • C3 H6 was an effective reductant for reducing NOx over Cuchabazite SCR catalysts via intermediate NH3 under lean/rich cycling conditions. • C3 H6 and other C3 H6 -derived hydrocarbon species inhibited reactions between NO and NH3 present either in the feed or as a surface intermediate generated in situ from C3 H6 –NO reactions. • Reactions between NO and stored NH3 became effective only when C3 H6 and other C3 H6 -derived species were depleted. • Contributions of C3 H6 and NH3 to cycle-averaged NOx reduction over the SCR catalyst were essentially additive under the conditions used in this study which allowed, at the relatively long lean
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