Integrated Selective Catalytic Reduction–Diesel Particulate Filter

Oct 20, 2014 - ... durability, and ability to deliver power efficiently under high load conditions. ... The wall-flow diesel particulate filter is a h...
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Integrated Selective Catalytic Reduction−Diesel Particulate Filter Aftertreatment: Insights into Pressure Drop, NOx Conversion, and Passive Soot Oxidation Behavior Kenneth G. Rappé* Pacific Northwest National Laboratory, P.O. Box 999, Richland, Washington 99354, United States ABSTRACT: Integrating urea-selective catalytic reduction (SCR) and diesel particulate filter (DPF) technologies into a single device has the potential to reduce the complexity of current diesel aftertreatment strategies. Fundamental studies were performed to shed light on the pressure drop and reaction behavior of integrated SCR and DPF systems. Details of SCR washcoat amount and location were investigated for effect on pressure drop during soot filtration. The SCR catalyst primarily impacted depth filtration of soot, promoted by increased catalyst located within the upstream portion of the porous filter wall. This effect is believed to be related to the nature of the porous filter substrate and pore network and changing of the rate at which pores plug in the presence of catalyst. SCR catalyst on the wall of the inlet filter channel also had an effect on the pressure rise during cake filtration of soot. NOx reduction efficiency measurements were performed to determine the nature and magnitude of the effect of soot on SCR performance. The effect of soot on the SCR performance is primarily attributed to the contribution of passive soot oxidation, and the propensity for soot oxidation to shift the NO2/NOx fraction relative to 0.5. SCR performance at NO2/NOx < 0.5 is adversely affected by the presence of soot oxidation by increasing the SCR dependency on standard (NO only) SCR reactions; conversely, at NO2/NOx > 0.5, the SCR performance is positively impacted by a decreased dependency on NO2-only SCR reactions. Temperature-programmed oxidation studies were performed to evaluate the impact of SCR on passive soot oxidation. SCR adversely impacts soot oxidation performance via NO2 diffusive effects, decreasing NO2 concentration in the inlet channel. This impact can be minimized or recovered at higher NO2 concentration and NO2/NOx fractions >0.5.

1. INTRODUCTION

Several existing emission control technologies have proven effective at controlling emissions individually. A diesel oxidation catalyst (DOC) effectively oxidizes NO, CO, and hydrocarbon (HC) emissions, and the diesel particulate filter effectively removes particulate matter from the exhaust stream. The wallflow diesel particulate filter is a honeycomb monolithic structure made from porous ceramic that has channels alternately plugged at the ends so that exhaust gas is forced through the porous channel wall. As engine exhaust is passed through a DPF, particulates are retained on the upstream portion of the filter wall and accumulate over time. Soot filtration first occurs by depth filtration, which is the collection of soot within the porous microstructure of the filter wall. This is typically accompanied by a sharp increase in the pressure drop across the filter. However, filtration will usually quickly transition to cake filtration, which is when soot is no longer accumulating within the wall microstructure, but rather, on the upstream surface of the filter wall. In this instance, the soot cake present on the upstream filter wall serves as the filter, and this is typically accompanied by a smaller increase in pressure drop across the filter. Together, these lead to an increase in pressure drop across DPF and, thus, an increase in the back pressure on the engine that adversely affects the engine operation and fuel consumption.

The diesel engine is currently the primary engine employed in the long- and short-haul commercial trucking industry because of its attractive wear characteristics, increased engine durability, and ability to deliver power efficiently under high load conditions. Manufacturers are planning much more widespread usage than historically employed, motivated by reduced fossil fuel consumption, reduced CO2 emissions, and the ability to operate on biologically derived fuels. However, for such a paradigm shift to take place, there are hurdles to overcome. Diesel close-coupled injection, compression, and ignition result in a heterogeneous two-phase burn that takes place largely at fuel droplet edges where the fuel and the air are mixing. The result of this is a near-stoichiometric burn that is very hot and comparatively long in duration, leading to elevated levels of NOx and PM formation. Engine controls and advanced fuel injection and combustion strategies have made significant strides in lowering diesel engine emission levels. However, persistently high particulate matter (PM) and nitrogen oxides (NOx) emissions dictate that exhaust aftertreatment will be a necessity. Exhaust aftertreatment reduces pollutants from the engine exhaust by treating the exhaust gas in a thermally and chemically controlled environment. Widespread introduction of advanced diesel exhaust aftertreatment is necessary for simultaneous compliance with NOx and particulate matter emission standards. Aftertreatment may lessen the trade-offs involved in controlling both NOx and PM emissions while minimizing losses in fuel efficiency. © 2014 American Chemical Society

Received: Revised: Accepted: Published: 17547

July 16, 2014 October 9, 2014 October 20, 2014 October 20, 2014 dx.doi.org/10.1021/ie502832f | Ind. Eng. Chem. Res. 2014, 53, 17547−17557

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Selective catalytic reduction (SCR) has been developed as an effective technology at reducing NOx emissions under lean burn conditions. SCR catalysts selectively reduce NOx to N2 and H2O through the use of NH3 as the reductant. NH3 is supplied via a urea injection system and subsequent thermal decomposition to isocyanic acid and 1 equiv of NH3, as shown in eq 1. A second equivalent of NH3 is produced from the surface-catalyzed decomposition of isocyanic acid, as shown in eq 2. NH 2−CO−NH 2 → NH3 + HNCO

(1)

HNCO + H 2O → NH3 + CO2

(2)

The SCR process begins with reversible ammonia chemisorption on catalytic Brønsted acid sites, where it becomes available for SCR reaction. The main NH3−SCR reactions promoted by Fe- and Cu-exchanged zeolites are shown in eqs 3−5 and consist of the standard SCR reaction (NO only), the fast SCR reaction (equimolar NO and NO2), and the NO2-only SCR reaction, respectively. Zeolite-based catalysts (such as vanadium systems) exhibit maximum NOx reduction activity at NO2/NOx = 0.5 as a result of the prevalence of the fast SCR reaction. The standard SCR reaction participates at NO2/NOx < 0.5 because of the relative abundance of NO; similarly, the NO2-only SCR reaction participates at NO2/NOx > 0.5 because of the relative abundance of NO2. 4NH3 + 4NO + O2 → 4N2 + 6H 2O

(3)

2NH3 + NO + NO2 → 2N2 + 3H 2O

(4)

4NH3 + 3NO2 → 3.5N2 + 6H 2O

(5)

Figure 1. Schematic comparison of a wall-flow DPF, flow-through SCR and integrated wall-flow 2-way DPF/SCR. Reproduced with permission from ref 12. Copyright 2011 Sage Publications.

An integrated SCR/DPF device combines soot filtration with an SCR catalyst, achieving simultaneous soot filtration and NOx reduction within a single device.13 For current conventional aftertreatment strategies, placement of SCR and DPF functionalities in the exhaust stream is governed by SCR activity and soot management. Regeneration of filtered soot requires both heat and oxidant; major oxidants in a diesel application are O2 and NO2, with NO2 oxidizing soot at lower temperature. When exhaust gas temperatures do not reach levels necessary for soot oxidation, active regeneration is necessary; this is facilitated by the injection of fuel in front of the DOC and subsequent oxidation, heating the downstream DPF via the ensuing exotherm to temperature necessary for soot oxidation. For HDD applications, exhaust temperature is comparatively high, and oxidation of soot with NO2 (i.e., passive soot oxidation) is highly desirable. This motivates placement of the SCR catalyst downstream of the DPF to avoid depletion of NO2 prior to the DPF; however, SCR catalyst performance is adversely affected by the upstream DPF because of thermal inertia; Koltsakis and co-workers predicted as high as 10% reduced NOx reduction efficiency due to an upstream DPF.14 For LDD applications, in which the exhaust gas temperature is comparatively low, resulting in low passive soot oxidation potential, the SCR catalyst is placed upstream of the DPF for improved activity and cold-start performance. Assuming adequate soot filtration efficiency is achievable with the wall-flow substrate employed, the requirements for successful SCR/DPF integration are maximizing NOx reduction efficiency while demonstrating acceptable pressure drop across the filter. Maximizing NOx reduction efficiency will be a function of total SCR catalyst washcoat volume and optimum catalyst coating techniques for maximum SCR efficiency. Demonstrating acceptable pressure drop across the filter will be a function of filter characteristics (e.g., total porosity), catalyst washcoat, and soot management. Thus, high porosity filter materials development is a technology facilitator for integrated SCR/DPF systems for integration of larger catalyst volumes while retaining an acceptable pressure drop.15−18 The washcoating technique is also a technology facilitator for similar reasons and, thus, is an area of continued optimization by catalyst manufacturers. This leaves soot management as an area of focus for successful deployment of integrated SCR/DPF technology. For LDD applications that must rely predominantly on active soot

Current focus in the U.S. has been on development of Feand Cu-based zeolite formulations as effective SCR catalysts for meeting NOx emission regulations.1,2 Early zeolite catalysts favored Fe-exchanged versions (Fe−ZSM-5) with mediumsized pores (∼5.5 Å) for heavy-duty diesel (HDD) applications for their improved activity above 350 °C and superior thermal stability as compared with Cu-exchanged versions, which had previously been confined to light-duty diesel (LDD) applications. Fe zeolites exhibit less sensitivity to NO2/NOx > 0.5 because of the efficiency of the NO2-only SCR reaction on these catalysts;3,4 however, recent advancements in Cuexchanged small pore zeolites have made them more much attractive for both HDD and LDD applications.5 Specifically, the small pore Cu-exchanged chabazite-type zeolites Cu-SSZ13 and Cu-SAPO-34 have recently exhibited good activity over a broad temperature range, good thermal stability, and good resistance to hydrocarbon deactivation.6−10 These Cu zeolites are shown to exhibit less sensitivity to NO2/NOx < 0.511 and have also demonstrated improved SCR selectivity to N2 versus undesirable products, including N2O.7 Integration of catalytic SCR and DPF technologies is a relatively new area in exhaust aftertreatment systems. Integrated SCR/DPF technology combines NOx reduction and soot filtration in a single 2-way device. Its development is motivated by emission compliance in a manner that reduces aftertreatment system volume and cost and increases packaging flexibility. As such, it is currently being pursued for both LDD and HDD diesel applications. Figure 1 shows a comparison of wall-flow and flow-through substrates as well as a schematic of an integrated wall-flow 2way device.12 The technology consists of coating an NH3−SCR catalyst within the porous wall of a wall-flow filter (i.e., DPF). 17548

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work will report on the impact of soot on NOx conversion performance to develop a more fundamental understanding of the nature of the interaction. In addition, as previously mentioned, the adverse effect of SCR reaction on passive soot oxidation (with NO2) is a recognized barrier to successful SCR/DPF integration for HDD. However, detailed understanding of the nature of this interaction and the primary reaction mechanisms that govern system performance are lacking. This work will report on the impact of SCR reaction on passive soot oxidation in an attempt to develop a more fundamental understanding of the primary reactive drivers that govern soot oxidation feasibility in the integrated system.

regeneration, soot loading and time between regenerations must be closely controlled because of the exotherm from soot burn and possible adverse effects on SCR catalyst deactivation. In contrast, for HDD applications that rely on significant passive soot oxidation, SCR/DPF integration is more complex. Passive soot oxidation is NO2-dependent, and current emission control strategies employ oxidation functionality in DPFs for enhanced NO2 production and improved passive soot oxidation capacity. Integrated SCR/DPF systems will likely not possess oxidation functionality because of its adverse impact on reductant usage (i.e., NH3 oxidation). In addition, SCR processes will consume NO2 competitively with soot oxidation. Thus, the incorporation of SCR/DPF for HDD applications must be developed and controlled in such a manner that optimizes NO2 availability for passive soot oxidation to avoid (or minimize) the fuel penalty associated with the increased active regeneration frequency. The motivation for integration of DPF and SCR functionalities into a single device resides in both performance and reduced aftertreatment volume. The integration of DPF and SCR functionalities into a single device has the potential to reduce the number of aftertreatment “bricks” required, leading to reduced volume, reduced cost, and increased simplicity. With regard to performance, development of advanced wall-flow DPF substrates have allowed engine manufacturers to focus their attention on reducing the amount of NOx emitted from tailpipes. Integration of DPF and SCR provides a pathway for NOx emission reduction with greater flexibility. By integrating SCR and DPF functionalities, the adverse effect of upstream DPF thermal inertia on SCR catalyst performance can be largely mitigated, thereby improving the cold-start NOx performance.14 Increased SCR catalyst washcoat volumes are feasible through different configurations, which would provide increased NOx conversion capability that could facilitate fuel savings by allowing engines to be calibrated to higher engineout NOx levels; however, there is uncertainty surrounding the effect of SCR catalyst on passive soot oxidation in integrated SCR/DPF solutions. The adverse impact of upstream SCR on DPF passive soot oxidation is well documented; the extent to which this can be minimized is a topic of significant current interest which this paper discusses. A number of early works have been performed to evaluate the feasibility of simultaneous NOx and PM aftertreatment with integrated SCR/DPF systems.19−23 These early works successfully demonstrated the feasibility of combined NOx and PM reduction and identified key barriers to its effective deployment. Some of these key technology hurdles include

2. EXPERIMENTAL METHODS High porosity cordierite wall-flow filters (i.e., DPFs; 25 mm o.d. × 40 mm long) were coated with SCR catalyst for the purpose of this investigation. The SCR catalyst employed was a Cuchabazite catalyst loaded by an unnamed industrial partner to varying catalyst density. Upon receipt, samples were degreened under lean conditions at 10% O2, 5% H2O, balance N2. Soot loading and reactive interrogation of the SCR/DPF samples occurred separately in iterative fashion. The samples were loaded with soot employing the exhaust from a 2003 VW Jetta TDI following a DOC placed upstream of the filters. During soot loading, the exhaust was continually measured with a smoke meter to determine post-DOC particulate density. A slip stream of the exhaust was pulled through the SCR/DPF sample with the use of a vacuum pump, with the slip stream subsequently conditioned and measured for flow rate. Flow rate was electronically controlled to target 55 000 GHSV during loading; measured particulate density and flow rate were combined to target a desired total soot loading, typically 4 g/L. Pressure drop was measured at ports just upstream and downstream of the SCR/DPF sample holding apparatus. Subsequent to soot loading, the samples were removed and weighed at elevated temperature for redundant measurement of soot mass uptake. Estimated soot loading from smoke meter and mass measurements were in good agreement with typically less than 10% variance. Reactive measurements were performed on a dedicated test bench designed to accommodate the soot-loaded filter and housing. The simulated exhaust composition, consisting of 9% O2, 9% CO2, 8% H2O, 500−1500 ppm of NOx (NO and NO2), and balance N2, was blended together to simulate post-DOC heavy-duty lean burn diesel exhaust. For measurements including SCR reaction, NH3 was included at 500−1500 ppm. Dry gases were supplied by electronically controlled MKS mass flow controllers. Air, N2, and CO2 were combined as dry gases and preheated to the desired temperature with an electronically controlled tube furnace. Simultaneous to preheating, the feed stream was humidified to the desired level with an electronically controlled water vaporizer employing a N2 sweep flow. Following preheating, NO, NO2, and NH3 were supplied separately as dry gas feeds to the exhaust flow in desired quantities. The full simulated exhaust was then fed to the reactor assembly consisting of a quartz tube containing the SCR/DPF sample held in place by high temperature alumina paper. An electronically controlled tube furnace controlled the reactor temperature to the desired level. Temperature measurements were made employing Watlow type K thermocouples placed just upstream and downstream of the DPF/SCR sample. Pressure drop measurement was made via ports located just upstream and downstream of the sample holder. Gas analysis

• Increased back pressure resulting from high SCR washcoat volumes • Necessity of improved washcoating technology for maximizing SCR catalyst volume and gas contact • Soot oxidation challenges Washcoat optimization is a topic to which many SCR/DPF investigators have alluded;24 however, little has been mentioned of the details of the SCR catalyst integration within the wallflow filter. This work attempts to provide insight into the impact of SCR catalyst location on soot-loading characteristics of the filter. The intent is to improve the level of understanding for maximizing SCR catalyst washcoat volumes while minimizing the back pressure that results from soot collection. The impact of soot on SCR reactor performance has been reported by many with conflicting results in some cases. This 17549

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alternating ends on the filter, channels in opposing corners of each subfigure in Figure 2 are common to a respective end of the filter; direction of exhaust flow through the samples is reversed by simply rotating the filter axially with respect to flow. With the SCR/DPF sample loaded to 60 g/L catalyst density (B), SCR catalyst is deposited solely within the filter wall microstructure and not on the channel walls. Catalyst is located heavily to one side of the filter wall and penetrating, on average, 40−60% across the width of the wall. In the SCR/DPF sample loaded to 90 g/L catalyst density (C), SCR catalyst is similarly deposited predominantly within the filter microstructure and also heavily to one side of the filter wall. Periodic occurrences of the catalyst penetrating the full width of the porous filter wall are evident, and with little or no catalyst present on either set of the channel walls. Comparatively in the SCR/DPF sample loaded to 150 g/L catalyst density (D), there are notable differences to the previous two samples. First, catalyst of varying amounts is present on one set of channel walls, outside of the wall microstructure. And second, the SCR catalyst consistently penetrates across the full width of the filter wall; thus, catalyst is in intimate contact with both filter walls. It is worth noting that the SEM imaging was conducted at four locations within the coated samples, with the images shown above selected randomly from the samples analyzed. The results discussed in the text with regard to catalyst location were consistently reflected in all samples analyzed. Figure 3 shows the corresponding results of Hg porosimetry analysis of the DPF and SCR/DPF samples, presented as

was accomplished with a Nicolet 6700 FTIR employing a metal gas cell heated to 190 °C. Sample gas transfer lines to the FTIR were heated to 200 °C to avoid NH4NO3 formation. FTIR gas analysis was conducted at 100 Torr through the use of a vacuum pump located downstream from the FTIR gas cell and an electronically controlled MKS pressure controller. Redundant NOx measurement was made via a heated California Analytics chemiluminescent NOx meter. Control of the entire assembly, including mass flow controllers, all heated zones, and data acquisition, was facilitated by a custom designed Visual Basic control system. Bench-scale reactive measurements were performed in pseudosteady state fashion by employing slow thermal ramping, ∼2 °C/min, from ∼200 to 550 °C. Reactive measurements included SCR performance for NOx conversion efficiency and temperature-programmed oxidation (TPO) for soot oxidation characterization. Scanning electron microscopy (SEM) was performed with a JEOL 5900 scanning electron microscope. The sample was cut perpendicular to the cordierite channels with a wet diamond wafer saw, potted in LR white resin, and spun at 2000 rpm for 10 min in a standard centrifuge and subsequently evacuated to remove gases from the filter and resin. Following polymerization and hardening of the resin, the sample was crosssectioned perpendicular to the channel axis, ground and polished with a series of diamond grits (25, 9, 3, and 1 μm), and final polished with 0.05 μm γ-alumina. Pore characteristics (porosity, pore size distribution) were measured via Hg intrusion employing a Micromeritics AutoPore IV 9500 series mercury porosimeter.

3. RESULTS AND DISCUSSION SEM imaging and porosity characteristics were compared with soot collection characteristics in an attempt to determine the combined effect of catalyst loading and location on filtration behavior. Figure 2 shows the SEM imaging of the uncoated DPF substrate (A) and SCR/DPF samples coated with ∼60 (B), ∼90 (C), and ∼150 g/L (D) SCR catalyst loading density. In each of the subfigures, using A as a reference, black is background or void space, (off-)white is cordierite substrate, and gray is SCR catalyst. Since channels are plugged at

Figure 3. Hg porosimetry analysis of DPF (no catalyst) and SCR/DPF samples loaded to 60, 90, and 150 g/L SCR catalyst.

incremental pore volume (mL/g) versus pore diameter (mm). The uncoated DPF measured 63% porosity. The addition of 60 g/L of SCR catalyst decreased the total porosity to 56%, with similar peak pore diameters for both the DPF and 60 g/L SCR/ DPF sample. This, coupled with SEM imaging, provides good indication that the 60 g/L of SCR catalyst was depositing solely within the filter wall microstructure. Similar peak pore diameters support the observation that a large portion of the porous filter wall remained largely void of catalyst. With an additional 30 g/L catalyst (i.e., 90 g/L total SCR catalyst), the total porosity decreased to 52% with a slight shift in the peak pore diameter to a smaller maximum. The shift in the peak pore diameter suggests that the less porous filter wall is void of catalyst versus the 60 g/L sample. These results

Figure 2. SEM imaging of clean DPF (A) and SCR/DPF samples coated with 60 (B), 90 (C), and 150 (D) g/L SCR catalyst loading. 17550

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Figure 4. Soot-loading characteristics of 90 (left) and 150 g/L (right) SCR/DPF samples configured such that catalyst was present predominantly on the upstream portion of the filter microstructure and on the inlet channel wall (for the 150 g/L sample).

Figure 5. Soot-loading characteristics of 90 g/L (left) and 150 g/L (right) SCR/DPF samples configured such that catalyst was present predominantly on the downstream portion of the filter microstructure and on the outlet channel wall (150 g/L sample).

portion of the filter (in close proximity to the collected soot). Figure 5 shows analogous backpressure measurements with the samples configured in the opposite direction (i.e., catalyst on the downstream portion of the filter). The data presented here were very reproducible, with subsequent pressure drop (versus collected soot) traces in close agreement. The pressure drop of the clean SCR/DPF samples was comparatively low: 0.35 and 0.44 kPa for 90 and 150 g/L SCR catalyst loading, respectively, under the conditions tested. Thus, collected soot affected filter permeability much more significantly than the catalyst washcoat; however, both the catalyst concentration and location impacted the magnitude of pressure drop resulting from the collection of soot. The most prominent effect occurred during depth filtration of soot; this is not surprising, given the knowledge that the catalyst is largely located within the wall microstructure. With the catalyst on the upstream portion of the filter (Figure 4), the system demonstrated 11.5 and 20.5 kPa back pressure for the 90 and 150 g/L SCR catalyst densities, respectively, under the conditions tested when it transitioned from depth to cake filtration mode. Comparatively, with the catalyst on the downstream portion of the filter (Figure 5), the system demonstrated 3.2 and 7.4 kPa back pressure for the 90 and 150 g/L SCR catalyst densities, respectively, under the conditions tested when it transitioned from depth to cake filtration mode. The pressure drop during depth filtration is affected by the amount of SCR catalyst located on the upstream portion of the filter wall microstructure. The magnitude of pressure drop does

coupled with SEM indicate that 90 g/L of SCR catalyst continued to deposit predominantly within the filter wall microstructure. With an additional 60 g/L catalyst (i.e., 150 g/L total SCR catalyst), the total porosity did not decrease significantly but, rather, remained fairly constant at 51%, with no change in the peak pore diameter (versus 90 g/L). The SCR catalyst filled a small number of very large pores >20 mm, leading to a slight asymmetric narrowing of the pore distribution. One difference from the previous two samples is that a significant amount of catalyst did not penetrate the wall microstructure but, rather, remained coated on the filter channel. In summary, total porosity and peak pore characteristics, coupled with SEM imaging, support the notion that up to 90 g/ L SCR catalyst was loaded into the filter wall microstructure, penetrating the width of the porous wall to varying degrees but loaded predominantly heavy to one side of the filter wall. Comparatively, the results suggest that >90 g/L SCR catalyst was largely deposited as a coating on the filter channel, with the catalyst deposited typically across the full width of the porous filter wall. 3.1. Soot-Loading Behavior. The 90 and 150 g/L SCR/ DPF samples were loaded with soot to determine the effect of catalyst location on the soot loading characteristics of the samples. Figures 4 and 5 show on-engine backpressure (i.e., pressure drop) resulting from the 90 g/L sample (left) and the 150 g/L sample (right) as they are loaded with soot. In Figure 4, samples are configured with the catalyst on the upstream 17551

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Figure 6. SCR/DPF NOx reduction efficiency at 35 000 GHSV, NH3/NOx = 1, NO2 = 250 ppm, and varying NO2/NOx fraction; NO2/NOx = 0.5 and less shown on the left, NO2/NOx = 0.5 and greater shown on the right.

not correlate solely with the mass of soot collected, but appears to also be a function of SCR catalyst amount and location. Two feasible contributing factors to this would be (1) relative density, and thus permeability, of soot deposits, or (2) relative impact on the flow field in the porous network. Evaluating the first in more detail, if back pressure is governed (solely or predominantly) by the density of soot deposits collected during depth filtration, then one would expect a closer quantitative relationship between the total mass of soot collected, the amount of SCR catalyst present on the upstream portion of the filter wall, and the magnitude of resulting back pressure. Although there appears to be a very general trend, it is not consistent; neither is it quantitative. For example, in both figures, the 90 and 150 g/L catalyst samples filtered similar masses of soot at the transition from depth to cake filtration but exhibited distinctively different back pressure as a result. These results suggest that SCR catalyst on the upstream portion of the filter wall directly affects the impact of soot on the porous network flow field. This brings to light the concept of pore throats. Pore throats are constriction points in the three-dimensional channels of wall-flow monolith pores, and they play a significant role in permeability.25 They are physically manifested a number of ways in the wall microstructure, ranging from simple constriction points along a pore channel to a directional change in the flow field passing through the porous network. Soot collected in depth filtration primarily impedes exhaust flow at pore throats26 and results in increased pressure drop during depth filtration; the extent is governed by the nature and density of those pore throats. Because the difference in clean pressure drop between the samples (0.35 versus 0.44 kPa) was small compared with the effect of soot, the density of pore throats is not expected to be significantly different. Thus, it appears that the nature of a large fraction of pore throats (and their surrounding volumes) is affected by the presence of SCR catalyst. The result is an increased extent of plugging as a result depth filtration of soot in regions of elevated SCR catalyst concentration. Comparing the 150 g/L catalyst sample with the 90 g/L sample, the samples collected similar masses of soot during depth filtration in both configurations; however, the difference in back pressure between the samples was significantly different, depending on flow configuration. With the catalyst on the downstream portion of the filter wall, the 150 g/L sample

measured 4.2 kPa greater back pressure versus the 90 g/L sample. This can be attributed to more SCR catalyst located across the full width of the filter wall for the 150 g/L sample and in close contact with collected soot near the upstream filter-channel wall. However, with the catalyst on the upstream portion of the filter wall, the 150 g/L sample exhibited 9.0 kPa greater back pressure compared with the 90 g/L sample. This can largely be attributed to significantly more catalyst present on the inlet channel wall for the 150 g/L sample versus the 90 g/L sample. This suggests that catalyst on the inlet channel wall has further impact on the extent of plugging during depth filtration and is hypothesized to be a magnification of the pore throat effect in the proximity of the upstream channel-wall interface. Once the system transitioned to cake filtration, additional discrepancies are apparent. Without catalyst on the inlet filter channel walls (Figure 4, left, and Figure 5, left and right), cake filtration behavior was similar in all three samples, exhibiting a pressure-drop rise in the range of 1.5−1.7 kPa/(g/L soot) under the conditions tested. However, with significant SCR catalyst on the inlet filter channel walls (Figure 4, right), cake filtration exhibited a pressure-drop increase of 3.4 kPa/(g/L soot). This is double the mass-specific effect of cake-filtered soot on pressure drop relative to the case with little or no catalyst present on the inlet channel. It is evident that permeability of the filter during cake filtration is adversely affected by the presence of catalyst on the inlet channel wall. Contributing to this effect is believed to be a combination of reduced open volume in the inlet channel and effective filtration area on the upstream channel wall surface, combined with the effect of the catalyst on flow dynamics at the wall and the subsequent effect on soot collection and location. The results above lead to a number of observations that suggest improvement in SCR/DPF catalyst integration is possible. The low-pressure drop of soot-free filters suggests that increased SCR catalyst concentrations are possible with minimal impact on pressure drop behavior if optimally located within the filter. The most significant contributor to pressure drop occurs during depth filtration, and it is largely governed by SCR catalyst on the upstream portion of the filter. This suggests that greater catalyst concentrations are possible if preferentially confined to the downstream portion of the porous filter wall. The presence of significant SCR catalyst on 17552

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the filter channel walls minimally impacted the soot-free pressure drop across the samples; however, the presence of catalyst on the inlet channel walls of the filter was shown to be detrimental to both depth and cake soot loading pressure drop behavior. Comparatively, catalyst on the outlet channel walls of the filter affected soot loading pressure drop characteristics significantly less. This suggests that greater catalyst deposition on the outlet channel walls may be feasible with a minimal effect on pressure drop behavior. 3.2. NOx Reduction Efficiency. The SCR/DPF samples were investigated for NOx reduction efficiency and passive soot oxidation performance to evaluate the nature of interaction of SCR and DPF reactive processes. In the results that follow, the samples were configured with SCR catalyst located predominantly on the downstream portion of the filter microstructure. In an attempt to develop a fundamental understanding of the impact of soot and soot oxidation on NOx reduction performance, the 90 g/L SCR/DPF samples were tested for NOx reduction efficiency in the presence and absence of soot. Since passive soot oxidation is a NO2-driven process, the samples were tested at constant NO2 concentration and by varying the NO2 fraction (by varying total NOx). The samples were tested at 35 000 GHSV and a constant NH3/NOx ratio of 1. Figure 6 shows the NOx reduction efficiency in the absence of soot; the figure on the left shows NO2/NOx fractions of 0.5 and less, whereas the figure on the right shows NO2/NOx fractions of 0.5 and greater. As expected, optimum NOx reduction is achieved at NO2/NOx = 0.5 through ∼400 °C. Significant adverse impact of NO2/NOx > 0.5 on NOx reduction performance is observed across the temperature range examined, as expected by increased dependency on NO2only SCR reaction. Comparatively, less severe adverse impact of NO2/NOx < 0.5 is observed through 400 °C, also as expected from the Cu-zeolite catalyst employed. At >400 °C, NH3 oxidation is significant, as reported by others,27 and its effect becomes evident, leading to decreased NOx reduction efficiency at these temperatures. Figures 7 and 8 show the NOx reduction efficiency of the samples in the presence of 4 g/L initial soot loading (lines) compared with the data from Figure 6 in the absence of soot (symbols) at NO2/NOx = 0.5 and less. Figure 7 compares NO2/NOx = 0.5 and less, whereas Figure 8 compares NO2/

Figure 8. SCR/DPF NOx reduction efficiency at 35 000 GHSV; NH3/ NOx = 1; NO2 = 250 ppm; and NO2/NOx = 0.55, 0.6, and 0.65: effect of 4 g/L initial soot loading on NOx reduction efficiency.

NOx > 0.5. At NO2/NOx ≤ 0.5, soot adversely impacts NOx reduction beginning at ∼240 °C. This effect is observed by soot shifting the NOx reduction efficiency to analogous performance at lower NO2/NOx ratios without soot. As an example, at NO2/ NOx = 0.5, NOx reduction performance above 280 °C with soot is decreased, tracking similarly to NO2/NOx = 0.4 in the absence of soot. Then above 330 °C, NOx reduction is further decreased with soot, tracking similarly to NO2/NOx = 0.35 in the absence of soot. Conversely, at NO2/NOx > 0.5, soot positively impacts NOx reduction due to a similar effect. As an example, at NO2/NOx = 0.6, NOx reduction is significantly improved in the presence of soot, demonstrating comparable performance to NO2/NOx = 0.5 without soot. The effect of soot on NH3 storage was not interrogated, but results from Tan et al.28 suggest it could have impact at low temperature ( 0.5, fast SCR again dominates, but conversely depleting NO until an abundance of NO2 remains, at which point NO2-only SCR reaction proceeds more slowly versus other SCR reaction pathways. It is important to note that for NO2/NOx > 0.5, the peak activity is limited by reductant availability because of the stoichiometry of the NO2-only SCR reaction. Figure 9 (right) shows the NO and NO2 concentrations in the catalyst effluent with and

Figure 7. SCR/DPF NOx reduction efficiency at 35 000 GHSV, NH3/ NOx = 1, NO2 = 250 ppm, and NO2/NOx = 0.5 and less: effect of 4 g/ L initial soot loading on NOx reduction efficiency. 17553

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Figure 9. Effect of 4 g/L initial soot loading on NO and NO2 concentration in the catalyst effluent at NO2/NOx = 0.45 (left) and 0.6 (right).

large balance of CO2 employed in the tests. However, the pressure drop becomes less reliable to gage the magnitude of soot oxidation as the soot becomes significantly oxidized. Thus, discussion specific to the soot oxidation rate are necessarily confined to only the light-off portion of soot oxidation. The retardation of passive soot oxidation in the presence of SCR reactive processes is apparent from the shift of pressure drop profiles to higher temperature with the inclusion of reductant (NH3). Passive soot oxidation performance in the samples is governed by soot reactivity, which is temperaturedependent; reactant availability, which is governed by NO2 concentration; the nature of the SCR reactions present; and subsequent diffusion effects. These results demonstrate the significant effect that NO2 diffusion, driven by the SCR reaction process, has on passive soot oxidation feasibility exhibited in the system. The onset of significant oxidation at 430 °C for NO2/ NOx = 0.35 is believed to be significantly assisted by O2, as evident by supporting TGA/DSC measurements in our laboratory and as reported elsewhere.29 The high temperature required for the onset of soot oxidation indicates significantly compromised reactant (i.e., NO2) availability due to diffusion, even at high temperature. In the absence of the SCR reaction, soot oxidation lights off at ∼300 °C with significant oxidation at 350 °C. Thus, at NO2/NOx = 0.65, soot oxidation is likely affected by low soot reactivity in addition to reactant availability; however, diffusion effects still dominate, as observed by the ∼40 °C shift in light-off behavior. The effect of SCR reactions on passive soot oxidation performance at various NO2/NOx fractions and constant total NOx is expected to be governed by the combined effect of the NO2/NOx fraction (dictating the nature of the SCR process) and NO2 concentration. Thus, separation of these effects is pursued. Figure 11 shows soot oxidation results on SCR/DPF and the effect of varying the NO2/NOx ratio between 0.4 and 0.65 while keeping the NO2 constant at 250 ppm; the data are presented and calculated in analogous fashion, as described above. This technique of keeping the NO2 concentration constant and varying the NO2/NOx ratio independently (by varying balance NO and thus total NOx) provides insight into the primary reactive drivers that are governing the passive soot oxidation activity of the system. At NO2/NOx ≤ 0.5, soot oxidation is relatively unaffected by the NO2/NOx fraction, with light-off occurring at ∼400 °C. The results, in fact, appear almost identical to one another.

without soot at NO2/NOx = 0.6. NO is minimally affected by soot, as expected for this case; however, the presence of soot oxidation converts NO2 to NO, decreasing the dependency on NO2-only SCR; this is evident by decreased NO2 in the catalyst effluent. The result is significantly improved SCR performance in the presence of soot. 3.3. Passive Soot Oxidation. The impact of the SCR reaction on passive soot oxidation for the SCR/DPF samples was investigated for the configurations described above and at analogous flow and SCR reaction conditions. TPO studies were performed to characterize the passive soot oxidation in the presence of SCR. Figure 10 shows the effects on the 150 g/L

Figure 10. TPO of SCR/DPF with 4 g/L initial soot loading displayed as the relative pressure drop versus temperature with the SCR reaction with 500 ppm of NOx at NO2/NOx = 0.35, 0.5, 0.65 and NH3/NOx = 1, compared with NO2/NOx = 0.5 without SCR (NH3/NOx = 0).

SCR/DPF sample of varying the NO2/NOx ratio (0.35, 0.5, 0.65) while keeping the total NOx constant at 500 ppm; the data are presented as the relative (normalized) pressure drop across the SCR/DPF sample as a function of temperature, with a decrease in the pressure drop employed as a tool to characterize the oxidation of soot. The data are compared with NO2/NOx = 0.5 in the absence of the SCR reaction (i.e., ANR = 0). The correlation of the pressure drop to soot oxidation versus CO + CO2 generation was employed because of the 17554

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3.4. Impact of Diffusion. Even though the SCR catalyst is predominantly located downstream from soot, the comparatively large rate of the fast-SCR reaction acts as a NO2 sink, facilitating diffusion in the direction of flow. Evaluation of the Peclet number in the wall allows for comparison of the convective and diffusive transport drivers. The wall Peclet number (Pew) is equal to w·u w Pe w = Dw,NO2 (6) where w is the wall thickness (0.012 in.), uw is the velocity in the wall, and Dw,NO2 is the NO2 wall diffusivity. Dw is calculated in a fashion analogous to that of Park et al.,30 employing porosity and pore diameter values from Figure 3 and bulk diffusivity as determined from Massman.31 Evaluating the Peclet number at 300 °C results in a value of ∼0.6, indicating strong diffusive influence and the potential for the diffusionaldriven NO2 flux facilitated by the fast-SCR reaction to be significant. Considering a simple one-dimensional comparison of diffusive and convective fluxes, under the conditions tested assuming an inlet concentration of 250 ppm of NO2, the convective flux in the wall is 0.046 μmol/cm2-s. Comparatively, assuming 90% NOx reduction as a simplistic step function on the outlet channel wall and employing an NO2 diffusivity (Dw) as described above, the diffusional-driven flux is ∼0.069 μmol/ cm2-s. The ratio of these two agrees well with the Peclet number calculated above. This demonstrates the potentially significant effect of diffusive drivers facilitated by fast SCR decreasing the bulk inlet channel NO2 concentration. The result of comparatively smaller soot oxidation rates (versus SCR rates) means comparatively greater dependency on convective NO2 transport for supporting soot oxidation activity. Thus, the result is decreased soot oxidation in the presence of an active fast-SCR reaction due to decreased NO2 concentration in the inlet channel. In a dynamic flowing reactor, the magnitude and impact of diffusion will be affected by the physical location and proximity of soot and SCR reactive components. Because the SCR reaction acts as a sink facilitating NO2 diffusion in the direction of flow, a final comparison was made evaluating the location of the SCR catalyst and its subsequent effect of NOx conversion and passive soot oxidation. Figure 12 shows NOx reduction efficiency (left) and passive soot oxidation (right) at NO2/NOx

Figure 11. TPO of SCR/DPF with 4 g/L initial soot loading displayed as relative pressure drop versus temperature with the SCR reaction with 250 ppm of NO2 (varying total NOx) at NO2/NOx = 0.4 through 0.65 and NH3/NOx = 1.

Comparatively, at NO2/NOx > 0.5, an increased rate of soot oxidation is observed compared with NO2/NOx ≤ 0.5, the magnitude of which is governed by the NO2/NOx fraction. These results suggest that at an analogous NO2 concentration, passive soot oxidation behavior is dictated by the nature of the dominating SCR reaction mechanisms that prevail. At NO2/ NOx ≤ 0.5, fast-SCR reaction mechanisms dominate NO2 availability in the reactor via diffusion. Only at NO2/NOx > 0.5 is a divergence from this observed. In this case, fast-SCR consumes NO equimolarly with NO2 until NO is depleted and a relative abundance of NO2 remains. The ensuing NO2-only SCR reaction rates are comparatively small on the Cu-zeolite catalyst employed, leading to decreased diffusion of the remaining NO2 and increased availability for passive soot oxidation. These results are supported by the NO2 traces in the catalyst effluent in Figure 10. This indicates that the SCR reaction, particularly the fast-SCR reaction, significantly affects NO2 concentration in front of the filter wall and collected soot; this further highlights the impact of NO2 diffusion on soot oxidation performance in the integrated SCR/DPF system and how that rate of diffusion is governed by the nature of the SCR reaction network.

Figure 12. Effect of SCR catalyst location on NOx reduction efficiency (left) and passive soot oxidation (right) at NO2/NOx = 0.6. 17555

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= 0.6 for the SCR/DPF system comparing performance as described above (SCR catalyst located on the downstream portion of the filter wall) with the sample arranged such that the SCR catalyst was located on the upstream portion of the filter wall. Decreased NOx reduction efficiency and decreased passive soot oxidation activity is observed with the catalyst located upstream versus downstream under the conditions tested. Both of these results indicate an increased rate of diffusion and NO2 depletion in the inlet channel and demonstrate the potential for impact of spatial separation of soot and SCR reaction in the integrated SCR/DPF system.

Article

AUTHOR INFORMATION

Corresponding Author

*Tel: (509) 372-4413. Fax: (509) 372-4252. E-mail: ken. [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was performed at the Applied Process Engineering Laboratory at Pacific Northwest National Laboratory (PNNL). The author wishes to thank Gary Maupin for assistance in operating the soot loading apparatus and Jarrod Crum and Brian Riley for assistance in acquiring SEM images of the SCR/ DPF samples. The author gratefully acknowledges funding provided for the research from the U.S. Department of Energy (DOE), Office of Energy Efficiency and Renewable Energy, Vehicle Technologies Program. PNNL is a multiprogram national laboratory operated for the U.S. Department of Energy by Battelle Memorial Institute under Contract DE-AC0676RLO 1830. Figure 1 is reused with permission from SAGE Publications under license number 3466720963654.

4. CONCLUSIONS Integrated SCR/DPF samples were investigated for their soot loading, NOx conversion, and passive soot oxidation performance. Key observations and conclusions from this study are as follows: • The effect of the SCR catalyst on the SCR/DPF pressure drop is minimal compared with the effect of deposited soot on pressure drop. • The primary effect of the SCR catalyst on the pressure drop characteristics is in depth filtration, which was dependent upon the concentration of the catalyst on the upstream part of the filter wall. The depth filtration behavior is believed to be caused by the SCR catalyst’s affect on pore throats, resulting in the propensity for those pore throats to become plugged (or partially plugged), leading to diversion of exhaust flow through a comparatively smaller area. • Cake filtration behavior was affected only by the presence of SCR catalyst on the inlet channel wall. • The above observations suggest increased SCR catalyst concentrations can be accommodated without adverse impact to the pressure drop behavior by confining the SCR catalyst to the downstream portion of the porous filter wall and on the outlet channel. • The impact of soot on the NOx conversion performance is primarily due to the passive soot oxidation subsequent effect on the NO/NO2 ratio. At NO2/NOx fractions ≤0.5, passive soot oxidation facilitates greater dependency on standard (NO only) SCR reactions, resulting in inferior SCR performance in the presence of soot. At NO 2 /NO x fractions >0.5, passive soot oxidation facilitates decreased dependency on NO2-only SCR reactions, resulting in improved SCR performance in the presence of soot. • The impact of NOx conversion on passive soot oxidation is primarily due to NO2 SCR reactions facilitating NO2 diffusion through the wall. The result is decreased bulk NO2 concentration in the inlet channel, leading to decreased soot oxidation performance. At NO2/NOx ≤ 0.5, NO2 consumption and subsequent diffusion are dominated by the fast SCR reaction, resulting in significantly reduced passive soot oxidation. In comparison, NO2/NOx > 0.5 decreases this effect due to greater dependency of SCR on slower NO2-only SCR reactions, which has the effect of decreasing the rate of depletion of NO2 in the inlet filter channel.

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NOMENCLATURE ANR = NH3 to NOx ratio dC/dt = soot oxidation rate DOC = diesel oxidation catalyst DPF = diesel particulate filter Dw,NO2 = diffusivity of NO2 in the filter wall FTIR = Fourier transform infrared spectroscopy GHSV = gas hourly space velocity HC = hydrocarbon HDD = heavy duty diesel LDD = light duty diesel NRE = NOxreduction efficiency Pew = Peclet number in the filter wall PM = particulate matter SEM = scanning electron microscopy uw = exhaust velocity in the filter wall w = filter wall thickness ΔP = pressure drop REFERENCES

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