Soot Combustion Activity and Potassium Mobility in Diesel Particulate

Jun 17, 2019 - (1) It is thought that potassium facilitates catalyst–soot interaction ... (9−14) However, catalytic activity can also be hindered by t...
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Article Cite This: Ind. Eng. Chem. Res. 2019, 58, 11891−11901

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Soot Combustion Activity and Potassium Mobility in Diesel Particulate Filters Coated with a K−Ca−Si−O Glass Catalyst James Zokoe,† Changsheng Su,† and Paul J. McGinn*,‡ †

Cummins Inc., 1900 McKinley Avenue, Columbus, Indiana 47201, United States Department of Chemical and Biomolecular Engineering University of Notre Dame, Notre Dame, Indiana 46556, United States

‡

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S Supporting Information *

ABSTRACT: Potassium volatility and activity in a novel K−Ca−Si−O glass catalyst was investigated over extended periods of soot oxidation. Soot was continuously deposited and oxidized by maintaining a glass catalyst-coated filter at temperatures above the oxidation balance point to provide a soot mass equivalent to 100 000 miles of engine operation. Testing revealed the 52-SiO2, 35-K2O, and 13-CaO wt % glass catalysts show a significant improvement in soot oxidation performance over a Pt catalyst. The glass catalyst retains soot oxidation catalytic activity even after testing equivalent to 100 000 miles of use. Migration of potassium in the catalyzed filter from the inlet to the outlet channels was observed. Although potassium was detected on an uncoated downstream core after 50 h of testing, showing that some volatilization of K occurs, the catalyst retains remarkable activity. Surface analysis characterization shows K enrichment of the catalytic glass surface relative to the bulk, which supports the proposed mechanism that water vapor in the diesel exhaust draws K+ ions to the glass surface. The studies demonstrate that a glass designed to allow the catalyst surface to be replenished with K ions on a continuous basis can overcome previous concerns about the durability of soot catalysts relying on alkali metals. exposed to diesel exhaust, ion exchange of H+ and potassium ions can occur. This results in diffusion of K+ to the glass surface, thereby supplying potassium for soot combustion over extended periods. For such catalysts to be used commercially, they must demonstrate activity over multiple combustion cycles or temperature excursions above 600 °C.7,19 It is known that active catalysts can be released from exhaust treatment systems. For example, vanadium used in selective catalytic reduction (SCR) systems can be released into the atmosphere and pose potential risks to human health.20 Vanadium compounds have also been proposed as soot oxidation catalysts.21,22 Although potassium is not known to pose such health risks, an issue might arise if an alkali-based soot oxidation catalyst is used on a DPF upstream of certain SCR units. Alkali metals (especially K) have been shown to severely degrade the performance of ammonium assisted NOx reduction of the current SCR catalysts such as V2O5, WO3, and CeO2.23,24 Peng et al. measured severe catalytic degradation where only 0.5 wt % loading of K2O on the V2O5−CeO2 SCR catalyst corresponded to a conversion decrease from ∼95% (fresh catalyst) to ∼40% (poisoned catalyst) for catalytic reduction of NOx at 300 °C.23 To a lesser extent, it has been

1. INTRODUCTION Diesel particulate filters (DPF) are used to eliminate soot particles from diesel engine exhaust. Such filters must be regenerated periodically to oxidize the soot and restore filter efficiency. There is interest in developing low cost, nonprecious metal catalysts to enhance soot combustion. It is well established that potassium is among the most active soot oxidation catalysts, as has been recently reviewed.1 It is thought that potassium facilitates catalyst−soot interaction due to its high mobility, resulting in good activity.2 The high mobility is also thought to be responsible for observed activity degradation of potassium-based catalysts during the oxidation process as a result of sublimation losses.3,4 Hence, the conclusion of many researchers is that a lack of durability makes potassium- or other alkali-metal containing soot oxidation catalysts unsuited for extended use in a diesel exhaust environment.5−8 Many researchers have tried to stabilize the K ions through interactions with a support to minimize potassium losses.9−14 However, catalytic activity can also be hindered by the resulting diminished mobility. The challenge in using potassium compounds is the need to balance between activity and stability (i.e., recognizing that enhanced activity through catalytic mobility potentially will increase the K loss rate).15 A novel approach to balance between stability and activity in a Kbased catalyst uses a silica glass with the K+ ions present in the silicate matrix acting as a catalyst.16−18 When the glass is © 2019 American Chemical Society

Received: Revised: Accepted: Published: 11891

April 30, 2019 June 12, 2019 June 17, 2019 June 17, 2019 DOI: 10.1021/acs.iecr.9b02381 Ind. Eng. Chem. Res. 2019, 58, 11891−11901

Article

Industrial & Engineering Chemistry Research

5201 soot generator (Jing Ltd., Switzerland).28 Water was accumulated in the exhaust gas through a humidifier held at elevated temperature. A space velocity of 60 000 reactor volumes was chosen for all pilot reactor testing. This volumetric flow rate equates to 38 600 sccm. Details of the calibration of the pilot soot reactor can be found elsewhere.27 A 0.4 g/h soot loading rate was used as the steady state loading condition for 250 h of continuous soot oxidation on the sample filter core. This provides an EUL equivalent to 100 000 mile of soot mass to be comparable with the average soot emission rate from a Cummins light duty diesel engine. The soot was continuously oxidized on the core by maintaining the sample filter at 500 °C, above the oxidation balance point. The balance point (500 °C) was measured as the temperature at which the pressure drop across the sample filter core was constant for the soot deposition rate of 0.4 g/h. The balance point will decrease if the soot deposition rate decreases. A 1 × 3′′ mullite filter core coated with the KCS-1 was used for this testing. A diesel exhaust analogue gas of 10% O2, 5% CO2, 7% H2O, and N2 balance was used for the continuous soot oxidation of the EUL study. 2.3. Soot Activity Test. Periodic activity testing was performed after intervals of 25, 50, 100, and 250 h of continuous soot oxidation. The activity testing was performed in a separate test cell. A temperature-programmed oxidation (TPO) experiment incorporating heating at 10 °C/min from 125 to 600 °C was used for the activity characterization. Prior to TPO testing, the filter was heated to 650 °C to ensure the filter had no residual soot. Soot loading for the activity characterization was performed in the pilot soot reactor to achieve a core soot loading level of 2−3 g/L. The actual loaded soot amount was determined by weighing the difference in mass of the filter core before and after the deposition of soot. Gas evolution and concentration was measured by an in-line MultiGas 2030 FTIR (MKS Instruments Inc.) An exhaust gas analogue composition of 10% O2, 7% H2O, 5% CO2, and 200 ppm of NOx with a 0.3 NO2/NOx ratio was used for TPO activity characterization. 2.4. Long-Term (250 h) and Hydrothermal Aging. Similar to the soot oxidation studies, a 250 h EUL hydrothermal test was also conducted using the pilot soot reactor. The test was performed with the equivalent experimental parameters as the soot oxidation studies but without the deposition of soot to isolate the effect of hydrothermal exposure on the KCS-1 glass. The hydrothermal exhaust gas was the same as for the EUL soot oxidation experiment with a sustained core temperature of 500 °C. A KCS-1 coated mullite sample from the same coating batch as the EUL soot oxidation and baseline K-mullite filter cores were used for this EUL hydrothermal experiment. 2.5. K Volatilization Studies. An investigation of K volatilization was performed by capturing escaping K on a blank filter core positioned downstream behind a KCS-1 coated cordierite filter core. The experimental set up for this study was equivalent to other testing with only the addition of an uncatalyzed mullite core positioned 5′′ downstream beyond a K-cordierite filter, where it was slightly out of the heating zone (just outside the furnace). Mullite was used instead of cordierite simple because of availability. The substrate plays no role except as a surface to possibly capture volatilized K. An equivalent continuous soot oxidation testing setup as used in the 250 h EUL testing was utilized. Exhaust gases were monitored by an inline FTIR.

shown that Cu-SAPO-34 is also negatively impacted by the presence of potassium.25 K-volatility has been previously reported in K-based coal gasification catalysts3,4,26 and is likely to occur in a K-glass DPF catalyst as well. Thus, it is desirable to investigate the extent of the volatility of a K-glass catalyst due to extensive soot oxidation. In this work, we present the first report of extended durability testing of a K−Ca−Si−O glass carbon oxidation catalyst subjected to continuous deposition and oxidation of soot and also report on the extent of potassium volatility and downstream accumulation.

2. EXPERIMENTAL SECTION 2.1. Core Preparation. One by three inch wall-flow DPF cores of mullite and cordierite, shown in Figure 1b, were used

Figure 1. (a) Schematic layout of the laboratory-scale reactor for soot oxidation and hydrothermal testing in a simulated diesel exhaust environment and (b) example filter core samples of silicon carbide (SiC), mullite (3Al2O3-2SiO2), and cordierite ((MgO)2-(Al2O3)2(SiO2)5) (left to right).

in this study. Filter samples were coated with a glass catalyst, termed KCS-1, described previously.17 This is a K−Ca−Si−O glass (52 wt % SiO2, 35 wt % K2O, and 13 wt % CaO) that is synthesized from nitrate precursors via sol−gel processing. The 1 × 3′′ filters were submerged in the sol and subjected to a vacuum for infiltration. After infiltration, a high pressure air blow-out was used to remove excess sol, leaving evenly coated interior filter surfaces. The process was the same regardless of the filter type. The filter was heated for 10 h at 575 °C followed by 2 h at 600 °C to ensure the removal of nitrates prior to testing. The resultant catalyst loading was ∼5 wt % of the bare filter weight or a 1−2 μm thick coating of all internal surfaces. 2.2. Soot Loading. Testing was performed in a laboratoryscale soot reactor to assess end of useful lifetime (EUL) and hydrothermal durability. The reactor was described in detail in a previous study.27 A schematic of the soot reactor is provided in Figure 1a. For all soot oxidation testing, soot was generated through the quenched combustion of a propane flame in a MiniCAST 11892

DOI: 10.1021/acs.iecr.9b02381 Ind. Eng. Chem. Res. 2019, 58, 11891−11901

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Industrial & Engineering Chemistry Research

(EDAX Orbis micro-XRF), and energy dispersive spectroscopy (EDS: Carl Zeiss LEO EVO-50 scanning electron microscope (SEM) with an Oxford INCA EDS). For XPS, a sampling spot size of 200 μm was used to measure the elemental surface composition at 3 points per axial position per channel. In XRF, a sampling spot size of 1 mm was used to measure total elemental composition at 3 points per axial position per channel. In EDS, a sampling spot size of 500 nm was used at 5 different points per axial position per channel with sampling taken of only the KCS-1 glass film. All error bars indicated were calculated as a standard deviation of the measured values for each position. For data points with zero apparent deviation, the deviation was within the size of the data point. Change in potassium content was characterized by the ratio of measured K to Ca because the X-ray beam penetration depths of XRF and EDS were greater than the catalyst thickness, and the catalyst coverage of the substrate was not 100% (i.e., substrate signal was measured). The baseline radial and axial distribution was determined by performing the equivalent sample preparation and characterization of a separate as-made KCS1 coated mullite filter core from the same sample coating batch.

The 50 h volatilization soot oxidation test was broken into 3 segments: 6, 18, and 26 h. These periods are described as 0−6, 6−24, and 24−50 h. After each segment, both cores were removed from the furnace, and the downstream mullite core was replaced with a fresh core. Both cores, catalyzed and bare, were rewrapped with a fresh glass fiber wrapping for each subsequent test segment. A small amount of soot accumulation was seen in the downstream mullite core after the 6−24 and 24−50 h segments. The bypass of soot was likely due to imperfect filter wrapping and incomplete quartz tube contact of the upstream filter core. Accumulation of this soot mass was unexpected, so the downstream cores were not weighed before and after testing for quantification. Catalytic activity of the Kcoated cordierite core was measured before and after the 50 h continuous soot oxidation test by loading the core with 3.7 and 4.8 g/L of soot, respectively. The total oxidized soot mass was calculated by the sum total of measured CO and CO2 concentration after the background level of each was subtracted from the data. The K-coated cordierite core was held at 525 °C so that the pressure drop across the filter core remained constant, implying that the soot deposition rate was equivalent to the rate of soot oxidation. The downstream mullite core was positioned partially out of the heating zone of the furnace. Four thermocouples were positioned along the filter center axis (R = 0) and filter periphery (R = 1) in the uncoated downstream mullite core to record the temperature profile during continuous soot oxidation. A typical measured temperature profile is shown in Figure S2. A radial temperature gradient was seen due to the core being outside the heating zone of the furnace. Compositional analysis of the uncoated mullite “volatilization” cores was performed after the continuous soot oxidation test. The filter cores were cleaved and analyzed by X-ray fluorescence (XRF) to measure relative K composition axially and radially in the downstream mullite cores. Four channels were analyzed per filter and are termed as follows: R = 0 Inlet and Outlet, R = 1 Inlet and Outlet. These channels correspond to the compositional analysis performed after the EUL testing described below for the catalyzed filters. XRF analysis was performed at 3 points per 1 cm (separated by 1 mm) down each channel length utilizing a 1 mm spot size. The measured XRF compositions were compared against the measured composition of an uncoated fresh mullite core. 2.6. Core Characterization after Aging and K Volatilization Test. After the 250 h EUL experiment was completed, analysis was performed on cores that were cleaved to expose the inner filter channels. Cleaving was accomplished using a method similar to that described in the literature.29 A clean break of the filter in the axial direction leaving undamaged channels was achieved using this method. The core was then cut with a low speed saw in the transverse (radial) direction to produce six 1 cm long samples centered at the normalized axial positions of inlet, 0.2, 0.4, 0.6, 0.8, and outlet. Figure S1 shows an example of a cleaved filter (a) and the methodology for this sample preparation along with the designation of axial and radial channel coordinates (X-axial, Rradial). These 6 samples were then thinned to a thickness of 2 channels (∼3 mm) to accommodate the analytical instrumentation. The axial and radial compositional change in K content was analyzed by X-ray photoelectron spectroscopy (XPS: PHI VersaProbe II X-ray photoelectron spectrometer), XRF

3. RESULTS AND DISCUSSION 3.1. End of Useful Life Activity Characterization of KMullite Core. The end of useful life testing consisted of continuous oxidation of soot under balance point conditions (500 °C) at steady state soot loading rate of 0.4 g/h for 250 h, providing an estimated useful lifetime of the 100 000 mile equivalent soot mass for a light duty Cummins engine. The EUL soot oxidation test results are shown in Figure 2 along with performance data for a cordierite core containing a commercial Pt catalyst.

Figure 2. End of useful life soot oxidation experiment of the K-mullite core. Characterization of activity loss by temperature-programmed soot oxidation at 10 °C/min with 10%O2, 7% H2O, and 200 ppm of NOx with 0.3 NO2/NOx ratio.

The catalytic activity characterizations were performed after 2, 25, 100, and 250 h of continuous soot oxidation. The soot loading for the 2, 25, 100, and 250 h activity characterizations were 1.8, 3.0, 2.4, and 2.7 g/L, respectively. These variations in soot loading do not significantly affect the TPO profiles or their corresponding T50 temperatures. The baseline (2 h) activity test of the K-mullite core measured a Tig of ∼400−415 °C and a T50 temperature of 425 11893

DOI: 10.1021/acs.iecr.9b02381 Ind. Eng. Chem. Res. 2019, 58, 11891−11901

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Industrial & Engineering Chemistry Research

4.62 with K content increasing down the filter length for both the R = 0 and 1 outlet channels. The R = 0 inlet channel measured a range of K/Ca ratios from 2.08 to 5.29 and showed increasing K content from the inlet to outlet axial position. However, the measured K/Ca ratio of the R = 1 inlet channel (along the periphery of the filter) showed no clear trend of change in K/Ca, varying substantially along the axial direction between ratios of 2.94 and 2.11 with an intermediate maximum of 3.76. The increase in surface K/Ca ratio measured down the axial filter length suggests either transport of K or K-enrichment of the surface with the increase occurring to a greater extent at the outlet side of the filter than the inlet. Because the measured Si atomic % values changed minimally across the axial direction of the filter (1−2 atomic %) the increase in measured K at the surface was balanced by a decreased content of Ca. The XRF measured K/Ca values could help determine which of these cases is true. If there is no measured change in K/Ca ratio by XRF down the filter length, then surface enrichment is likely the cause of the XPS measured increase in K content. Conversely, an axial increase in K/Ca ratio measured by XRF would then suggest K transport down the filter. Such large variability of the measured K content could be due to the small sampling area of the XPS beam (200 μm diameter) and minimal number of sample points, but similar sampling was performed on the 250 h EUL hydrothermal core (discussed below), and the axial variation was not erratic. It is possible that variations in local substrate porosity or topography (e.g., small pits or depressions which were seen in SEM imaging, widths of ∼100 μm) may alter the localized soot oxidation characteristics of certain areas which may not capture equivalent amounts of soot over the full EUL testing. Such nonuniform K distribution throughout the filter after prolonged soot oxidation is currently not fully understood. The surface content of the K-glass is thought to be influenced by both the leaching of K to the surface and the volatilization of K from the surface. Figure 4 shows the XRF measured axial and radial distribution of potassium and calcium after the EUL soot

°C. After 25 and 100 h of soot oxidation, there was a measured increase of 20 and 25 °C in T50 temperature to 445 and 450 °C, respectively. At the end of useful life, or 250 h of continuous soot combustion, the measured T50 temperature was 470 °C. Thus, the total activity degradation during the estimated 100 000 mi catalyst lifetime was 45 °C. This EUL activity is still 105 °C lower than a commercial Pt-based catalyst (widely used in diesel particulate filters30) without lifetime testing, which typically has a T50 temperature (see Figure 2) of 575 °C at cost-effective catalyst loadings (∼5 g/ ft3).31 Compositional characterization of the coated filter core was performed after the EUL testing using XPS and XRF. The K/ Ca ratio for both XPS and XRF measurements was used to compare the relative K content because the variation in the measured Si and Ca content is small with respect to the change in K content. The axial change in Si was small for XRF measurements compared to the relative change in K. The beam penetration (∼5 μm) in XRF is deeper than the thickness of the catalytic K-glass film (1−2 μm), so it is assumed that the XRF measurement samples all of the K-glass film that is exposed to the filter channel. XPS measures only the surface concentration of K and Ca in the near ∼5−10 nm of the filter wall exposed to the channel. Figure 3 shows the XPS results of the axial and radial distribution of potassium (normalized as the K/Ca ratio)

Figure 3. K/Ca composition ratio of a K-mullite core analyzed by XPS after the 250 h end of useful life soot oxidation experiment. Kmullite core as-made average K/Ca ratio is also shown.

within the K-mullite core after the 250 h EUL soot oxidation test as well as in the as-made K-mullite core. The four-channel average XPS measured K/Ca ratio for the as-made core is also shown. Minimal radial variation in composition was measured, while the axial K/Ca ratio varied from 2.08 ± 0.0302 to 3.75 ± 0.173 at X = 0 and X = 1 axial positions, respectively. The nominal KCS-1 K/Ca ratio is 3.08. While there is a variation in the axial K/Ca ratios, the values approximate the theoretical values of the KCS-1 glass. Comparing the average as-made values to the 250 h EUL soot oxidation measured K/Ca values seen in Figure 3, the R = 0 inlet, R = 0 outlet, and R = 1 outlet channels measured consistently higher K/Ca ratios than the average as-made Kmullite core values. These values measured between 2.94 and

Figure 4. K/Ca composition ratio analyzed by XRF as a function of distance down the filter channels after the 250 h end of useful life soot oxidation experiment of the K-mullite core. Also included is the Kmullite core as-made average K/Ca ratio. 11894

DOI: 10.1021/acs.iecr.9b02381 Ind. Eng. Chem. Res. 2019, 58, 11891−11901

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Industrial & Engineering Chemistry Research oxidation testing of the K-mullite filter core and the fourchannel average XRF measured K/Ca values for the as-made K-mullite core (i.e. without treatment). The baseline as-made K-mullite core measured average K/ Ca ratios between 1.60 and 1.95. The lowest values of K/Ca were measured in the axial center of the filter. The nominal value of the KCS-1 glass K/Ca ratio is 3.08. The actual measured as-made K/Ca ratio of the K-mullite core was much lower than the nominal glass composition. This is not fully understood but could be caused by incomplete surface coverage by the glass catalyst film. The K/Ca values after 250 h EUL soot oxidation for the R = 0 and 1 inlet channel values ranged between 1.49 and 1.90 with the minimum values occurring at the axial middle of the filter. These inlet channels also measured lower K/Ca values at the X = 0 position (inlet) of 1.49 and 1.56 for the R = 0 and 1 inlet channels, respectively, which were significantly lower than the as-made K/Ca value of 1.90 at the X = 0 position. This difference suggests K migration away from the inlet side of the filter. The R = 0 inlet channel also was found to increase in K/ Ca value between the X = 0.2 and 0.8 axial positions. These K/ Ca values increased in the X = 0.4 and 0.6 axial positions in the filter (1.78 and 1.86, respectively) relative to the as-made values of 1.67 and 1.60 for X = 0.4 and 0.6, respectively. This difference also suggests redistribution of K from the inlet to outlet portions of the filter. The two measured R = 0 and 1 outlet channels show similar trends in axial K/Ca ratio. For the R = 1 outlet channel, the measured K/Ca ranged between 1.67 and 1.95 with a decrease in K content in the axial middle of the filter occurring over a normalized filter length of 0.4 to 0.8. This follows the trend of the as-made K-mullite K/Ca ratio midfilter. However, the R = 0 outlet channel increases from 1.89 to 2.56 with a large increase in K content in the last 40% of the filter (X = 0.6 to 1.0). Both outlet channels show minimal change in the K/Ca ratio at the X = 0 position (immediately behind the inlet side plug) because the gas velocity at this point should be the lowest in the channel. The outlet channels also measured an increase in K/Ca ratio at the axial outlet (2.56 and 1.95 for R = 0 and 1 outlet channels, respectively) relative to the as-made K/Ca value (1.72). The exhaust velocity in a filter is highest at the radial center of the filter,32 which explains the greater K content measured in the outlet side of the R = 0 outlet channel relative to the R = 1 outlet channel, as the higher exhaust velocity would aid the transport of any volatilized K down the filter. There was also a large measured standard deviation for the R = 0 and 1 outlet channels in the final X = 1.0 position compared to all other axial positions. The 3 measurement points used to calculate the K/Ca ratio for that axial position were spaced 1 mm apart at the end of the filter. The K/Ca ratio monotonically increased over the last 3 mm of the filter, suggesting a significant accumulation of potassium at the outlet mouth of the filter (because the Ca content changed minimally). Considering this measurement combined with the axial increase in K in the outlet channels of the filter, there may be some amount of K that escapes the filter during the soot oxidation process. Other groups working with potassium catalysts have reported K volatilization in the presence of a reducing environment such as carbon oxidation.3,4,26,33 With no soot cake present in the outlet channel to either shield the glass or recapture volatilized K, the temperature gradient across the filter may help to limit redeposition of K volatilized from

the inlet channel. During soot oxidation in a DPF, the exhaust gas convectively transfers the energy created from the exothermic oxidation of soot, and an exotherm across the axial direction of the filter is created. This temperature gradient is low in this steady state testing, measured at only 10 °C across the axial length. However, there is minimal filtration occurring to physically adsorb the volatilized K in the outlet channel. Potassium deposition would then be restricted from condensing out of the gas phase and could escape the filter. SEM examination of filter surfaces helped to further elucidate the nature of K movement in the filter. An example of the structure of the as-made KCS-1 coated mullite surface is shown in Figure 5.

Figure 5. Scanning electron image of as-made KCS-1 coated mullite.

Discrete regions of the thin KCS-1 glass film are observed to adhere to the underlying mullite needles in the regions around the vertically protruding needles (indicated by black arrows in Figure 5), and thin KCS-1 coatings can be seen on the protruding needles themselves (indicated by white arrows in Figure 5). The high surface area, large aspect ratio, and variation of protrusion angle of the mullite needles creates a challenging surface to continuously coat with the glass catalyst. The sol−gel coating appears to crack and separate during the drying and aging process (sol condensation). This creation of discrete catalyst regions does not affect physical adhesion, so a change in the coating process parameters (e.g., drying time or binder addition such as polyvinyl butyral17) was not deemed necessary at this stage of testing. Any structural change of the coating resulting from EUL testing, both soot oxidation and hydrothermal, is compared against this KCS-1 baseline structure. Figure 6 shows an example scanning electron image of the KCS-1 coated mullite surface after the EUL soot oxidation testing. The KCS-1 coating on the mullite was macroscopically unchanged in structure after the EUL testing, as seen in Figure 6, as discrete regions of glass adhered to the mullite needles with minimal change in morphology. There was, however, the development of potassium-rich spherical particulates due to the extended soot oxidation. Black arrows in Figure 6 indicate the K-rich particulates. The inset in Figure 6 shows the particulates at higher magnification. These particulates were seen only in the two inlet channels (i.e., the channels performing soot oxidation) but were widespread, present on most discrete KCS-1 glass surfaces. 11895

DOI: 10.1021/acs.iecr.9b02381 Ind. Eng. Chem. Res. 2019, 58, 11891−11901

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Industrial & Engineering Chemistry Research

the filter core was characterized in an equivalent manner as the EUL soot oxidation experiment. The axial and radial compositional changes at the surface of the K-mullite filter in the absence of soot were measured by XPS and are shown in Figure 7.

Figure 6. SEM image of structural change in KCS-1 catalyst after end of useful life soot oxidation experiment. Position: X = 0.4 R = 0 inlet channel.

The size of the particulates varied as did the measured composition. EDS measurement of various particulates revealed them to be composed only of K, Si, and O with an average composition of K0.1Si0.3O0.6 (see Supplementary Table 1 for spherical particle EDS analyses). The spherical shape of the particulates suggests the possibility of melting and solidification. Because the measured K−Si ratio is high for these particulates, the melting point of such a “glass” would be very low. The phase diagram suggests a melting temperature around 775 °C,3435 so melting could occur in localized hot spots of soot oxidation through the excess heat generated from the exothermic oxidation. The continuous leaching of K from the glass due to the suggested K-volatility would create K-rich regions on the KCS-1 glass surface. These regions could agglomerate with continued exposure to high temperature and carbon oxidation to form the observed particulates. An absence of measured carbon suggests the particulates are a glassy Ksilicate compound. X-ray diffraction was not performed on these samples to characterize the particulates because the small fraction of their surface coverage would not yield a discernible X-ray pattern. The results show there is overall transport of potassium in the catalyzed filter from the inlet channels to the outlet channels. The front of the inlet channels shows a decline in the K/Ca ratio, while the rear of the outlet channels shows an increase. Despite this redistribution of K, the active species for soot oxidation, the filters still perform satisfactorily over their expected lifetime. Both XPS and XRF characterization show similar trends of K redistribution. The K/Ca ratio measured by XPS is higher than that measured by XRF. This suggests the glass catalyst surface is K-rich relative to the bulk, which is to be expected if the humidity of the diesel exhaust is drawing K+ ions to the glass surface.36 SEM examination of the channels showed the development of discrete spherical potassium silicate particulates on the glass catalyst surface with extended soot oxidation. These precipitates were only seen in inlet channels where soot oxidation occurred. 3.2. Hydrothermal Testing. The equivalent “hydrothermal” testing of 250 h exposure to diesel exhaust conditions was performed on a K-Mullite filter core to isolate the effect of water vapor and the high flow rate of the exhaust gas by removing soot from the previous testing conditions. Without soot, there should be no reduction of potassium to its metallic state, as can happen during soot oxidation.1,33 After exposure,

Figure 7. K/Ca composition ratio analyzed by XPS after 250 h end of useful life hydrothermal exposure of the K-mullite core.

The comparative XPS measured average K/Ca values of the as-made K-mullite core are provided as the dashed line in Figure 7. The channels all exhibited a K/Ca ratio higher than that of the as-made K-mullite with the exception of the X = 0.2 position. The measured K/Ca values ranged between 3 and 4 in the first 60% of the filter length (axial length). There is an abrupt increase in the surface content of K in the latter 40% of the filter for all channels. The two outlet channels, however, measured the highest increase in K/Ca ratio. At X = 1.0 axial length, the R = 1 and 0 outlet channels measured values of 6.87 and 7.96, respectively. Furthermore, in the last 1 mm of the outlet channels, the respective K/Ca ratios were measured as 8.77 and 13.5 for the R = 1 and 0 outlet channels, respectively. The comparable XRF measurement of redistribution of total K content within the KCS-1 glass catalyst (exposed to the filter channel) after EUL hydrothermal testing is presented in Figure 8. As previously, the resulting K/Ca ratios measured axially and radially are provided. Relative to the average measured catalyst K/Ca ratio of the as-made K-mullite filter core (shown as the dashed line in Figure 8), the potassium in this core has redistributed from the inlet half to the outlet half of the filter. As seen in the XPS measurements (Figure 7), there is a clear increase in K/Ca ratio in the latter 40% of the filter. The front 60% of the filter measured deficient in K relative to the as-made K-mullite core with the back 40% of the filter measuring greater K/Ca ratio than the as-made. The innermost channels (R = 0) were more greatly affected by the hydrothermal treatment than the outer radial channels (R = 1). R = 0 inlet and outlet channels measured K/Ca ratios of between 1.41 and 1.52, respectively, in the first 60% of the filter length compared to the R = 1 inlet and outlet channels, which measured K/Ca ratios between 1.62 and 2.21 in the similar filter length. All 4 analyzed filters measured a significant increase in K content in the final 40% of the filter with the R = 0 channels measuring a maximum K/Ca ratio of 2.54 and 2.67 for the inlet and outlet channels, respectively at the axial outlet of the filter. The R = 1 channels 11896

DOI: 10.1021/acs.iecr.9b02381 Ind. Eng. Chem. Res. 2019, 58, 11891−11901

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Industrial & Engineering Chemistry Research

Figure 9. SEM image of structural change in K-mullite catalyst after end of useful life hydrothermal experiment. Position: X = 0.8 R = 0 inlet channel.

Figure 8. K/Ca composition ratio analyzed by XRF after 250 h end of useful life hydrothermal testing of the K-mullite core.

The underlying glass film structure was minimally affected by the hydrothermal testing, but surface precipitates were in evidence throughout the filter. Arrows in Figure 9 indicate these precipitates. The surface coverage of the precipitates coincides with the increase in K/Ca ratio measured by both XPS and XRF toward the final 40% of the filter length (i.e., precipitates were seen more frequently near the outlet half of the filter). These precipitates differ in morphology compared to those seen after EUL soot oxidation testing, being angular as opposed to the spherical particulates of the EUL soot oxidation. The thin precipitates were too small to accurately determine the composition due to the excess of beam penetration into the underlying KCS-1 glass. However, based on the precipitates formed during hydrothermal testing of glass coupons18 and the increase in measured K content in the outlet portion of the filter, the precipitates are most likely Krich carbonates. When the two 250 h EUL tests were compared, the measured axial migration of K without a soot cake present (Figure 8) was more substantial in this hydrothermal environment than in the soot oxidation EUL test that maintained a constant soot cake (Figure 4). At an axial position of X = 1, the XRF measured average K/Ca ratios (i.e., average of 4 channels at X = 1) were 1.94 and 2.71 for the 250 h soot oxidation EUL (Figure 4) and 250 h hydrothermal EUL tests (Figure 8), respectively. Considering the equivalent testing conditions of these two EUL experiments with the sole difference being the presence of a soot cake, the soot cake appears to protect the K-glass surface from accelerated chemical degradation in the form of leaching and redistribution of K. This agrees with an earlier observation on surfaces of glass catalyst coupons.18 The soot cake may also allow for trapping and redeposition of K volatilized by soot oxidation, limiting K transfer downstream. During actual use, at times when the exhaust temperature is low, soot will accumulate on the catalyst surface. At high temperatures (far above the balance point) the catalyst may not be blanketed by the soot and thus subject to relatively higher K migration rates. Hence, the nature of the driving cycle may affect the rate and extent of K migration. 3.3. K Volatilization Testing. To determine if the axial migration of K measured in the 250 h EUL soot oxidation testing leads to K escaping the filter, a partial catalytic EUL study was used to measure the extent of K transport out of the

in Figure 8 measured a slightly higher K/Ca value at the axial filter outlet compared to the R = 0 channels, but a similar trend of increasing K content was measured. Exhaust flow velocity is highest in the radial center of the filter, which provides a higher mass flux of water vapor across the glass surface.32 The high flow rate in combination with the adsorbing water vapor in the gas cause chemical degradation of the glass and promote leaching of K. The exhaust gas flow then advances the K toward the outlet of the filter. Because this hydrothermal testing is performed in the absence of soot and at the relatively low temperature of 500 °C, sublimation of potassium should not occur in this environment. The gas temperature is low enough that no substantial thermal volatilization of K should occur because the melting point of the glass is ∼1000 °C. Similarly, there is no oxidation of soot to provide a reductive environment to cause the sublimation of K. Thus, the K migration to the outlet of the K-mullite core in a purely hydrothermal treatment is likely a redistributive process rather than a total loss of potassium with potassium interacting with the water vapor present in the stream. In comparing the XPS data (Figure 7) with the XRF data (Figure 8), it is apparent that the R = 1 channels (blue lines) in the XRF data show higher K/Ca ratios than the R = 0 channels (red lines). In the XPS data (Figure 7), the trend is reversed with the center channels (R = 0) exhibiting higher K/Ca ratios than the peripheral (R = 1) channels. This reflects the nature of the measurement difference between XRF, which is sampling the glass bulk and XPS, which measures the surface. The gas velocity is highest at the radial center of the filter, so the R = 0 channels will show greater hydrothermal effects. At the immediate surface, as measured by XPS, this means relatively more K enrichment because the total water vapor volume will be higher. However, transport down the channel is also occurring. Center channels should suffer more Kmovement down the channel, resulting in more K-loss by the glass bulk. This is reflected in the XRF (bulk) data where the center channels are more depleted relative to the peripheral channels. Figure 9 shows an example SEM image of the structural change in the K-mullite catalyst surface after the 250 h EUL hydrothermal testing with the inset showing a small region at a higher magnification. 11897

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Industrial & Engineering Chemistry Research filter during continuous soot oxidation. Fifty hours of total of continuous soot oxidation was performed in the pilot soot reactor in 3 stages of 0−6, 6−24, and 24−50 h. A K-glass coated cordierite core was held at 525 °C with continuous soot deposition, giving conditions of continuous soot oxidation. The catalytic activity before and after the 50 h continuous soot oxidation testing was performed by TPO using the same feed gas composition as previously noted (Section 3.1). Gas composition of the exhaust was monitored to determine the total amount of soot oxidized during these TPOs. The total measured soot mass conversion curves of the K-coated cordierite core before (0 h) and after the 50 h of continuous soot oxidation are shown in Figure 10.

Figure 11. XRF measured K distribution (atom %) in the upstream catalyzed K-cordierite core after 50 h of continuous soot oxidation.

possible variation in KCS-1 film thickness is not the cause of the measured increase in K. The radial difference in these two channels is again a factor of the exhaust velocity profile in which the velocity is highest at the radial center of the filter (R = 0) and lowest at the radius (R = 1).32 The high exhaust velocity at the inner radius causes greater transport of K downstream during soot oxidation, providing more opportunity of migration for any K volatilized into the gas phase. Outlet channels are lower in measured K content relative to the R = 1 inlet channel, likely due to the loss of K downstream. This was investigated by the measurement of the downstream mullite cores. The K-cordierite core used for this testing was the only cordierite sample from this coating batch. Thus, an as-made XRF composition for the K-cordierite core was unavailable for comparative purposes to measure the relative amount of K loss. Qualitatively, the trend in K distribution in the catalyzed Kcordierite core (i.e., from the front to the back of channels as seen in Figure 11) can provide evidence of axial K migration due to soot oxidation, as also seen in Figure 4. The outlet channels experience much less soot oxidation (or perhaps none if all the soot is filtered on the inlet side) but still show significant K migration. This is the result of volatilization from the inlet channels as well as hydrothermal effects on the outlet channels. The composition at the front of the channels in the downstream mullite cores after 6, 18, and 26 h of continuous soot oxidation was measured by XRF. An XRF spectrum of the downstream mullite core after the 24−50 h oxidation segment (Figure S3) showed evidence of potassium. The K signal is sufficient to produce repeatable measurements with suitable uncertainty. XRF detection limits have been shown to be as low as 0.05 wt % (500 ppm) for K.37 The measured composition of K in these cores was measured axially and radially. Cumulative results of the K distribution are shown in Figure 12 (interval 24−50 h of continuous soot oxidation) and in Figure S4 ((a) 0−6 and (b) 6−24 h intervals of continuous soot oxidation) A fresh, uncoated mullite core was used to measure the original composition of the downstream material before testing. The measured composition of the mullite contained 0.16 atom % K and is thought to be an impurity from the substrate production. In all three testing segments, there was a

Figure 10. Soot mass oxidation of K-coated cordierite core before and after 50 h of continuous soot oxidation during temperatureprogrammed oxidation.

The T50 temperatures of the soot mass conversion in Figure 10 measured a small increase in temperature from 0 to 50 h with values of 502 and 510 °C respectively. The two soot oxidation curves were nearly identical for the first 50% of oxidized soot but after 50 h, oxidation temperatures deviate to a higher value in the latter half of the oxidation (i.e., soot which is in poorer contact with the catalyst). The difference in total soot mass loading of the two TPOs (1.1 g/L) should not significantly affect the soot oxidation temperatures. Thus, the degradation in oxidation activity may be attributed to potassium loss. The K-coated cordierite core (upstream, catalyzed core) was analyzed using XRF to measure the K distribution in the core after the 50 h of continuous soot oxidation testing. Figure 11 shows the XRF measured K axial and radial distributions within the upstream K-coated cordierite core. K atom % is plotted instead of K/Ca ratios to compare to the measurements taken of the downstream bare mullite cores which have only trace Ca content. A significant signal from the substrate was detected by the XRF with relatively low measured atom % for K. The K distribution of the upstream, catalyzed K-cordierite core after the 50 h soot oxidation increased in all 4 measured channels from inlet to outlet. Measured values for K range from 1.31 atom % to 2.42 atom % with the R = 0 inlet channel measuring the lowest relative values of K and the R = 1 inlet channel measuring the highest. The measured Mg atom % from the cordierite substrate varied by less than 0.2% across each channel (i.e., it was relatively uniform), indicating that a 11898

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cake (∼125 m2/g27) added filtration surface to capture the volatilized K. The fact that there is no distinction between K content of the R = 0 and 1 channels suggests this deposition mechanism is not due to the radial temperature difference (i.e., 465−475 °C is not sufficiently low to induce the condensation of K from the vapor phase). However, the XRF measurement of the added soot cake increases the sampling volume, but C does not yield a signal in XRF. Thus, the high content of K in the soot is still indicative of K being transported to the downstream core, but the K atom % values cannot be directly compared against the previous samples which had no soot accumulation. Additionally, the increased level of K content throughout all 0−50 h mullite filters relative to the fresh mullite indicates incomplete capture of the total volatilized K. Despite some loss of potassium from the glass catalyst, the catalyst retained significant activity due to continual replenishment of K at the glass surface.16,36 This shows that, despite previous concerns about the durability of potassium-containing soot oxidation catalysts,5−8 a glass matrix can be tuned to facilitate ion exchange with the humid diesel exhaust environment. The glass catalyst surface can be replenished with K ions on a continuous basis, yielding continuous activity for the lifetime of a diesel particulate filter.16,36 The presence of a soot cake, such as during the balance point EUL testing, appears to reduce the rate of K migration. Thus, K migration may vary during operation depending on the operating conditions and relative exhaust temperature. When comparing these measured levels of K deposition in the bare core to the previously reported loss of catalytic SCR activity, it was shown that 1 wt % of K poisoning was sufficient to induce a decrease of ∼50% NOx conversion in vanadia catalysts.23 In the XRF measurements of the mullite cores, ∼0.75 atom % corresponds to ∼1 wt % of K. Hence, after 50 h of continuous soot oxidation of a K-coated DPF filter, a downstream SCR could experience significant activity loss. This measurement suggests that the K-glass catalyst might not be suited for exhaust emission reductions systems, which require a DPF upstream from an SCR. If it is possible to only catalyze the inlet channels (e.g., apply the sol by spraying down the channel walls), the K-losses could be reduced. Soot is captured and oxidized in the inlet channels, so a catalyst is not needed on the outlet side. However, catalyzed outlet channels show significant K migration due to hydrothermal effects on the outlet channels. If they are not catalyzed, one can expect downstream K to be reduced by >1/2 just based on reduced surface area exposure.

Figure 12. XRF measured K content (atom %) down the axial length of the downstream uncoated mullite filters for R = 0 and 1 inlet and outlet channels (axial and radial distribution) from the interval 24−50 h of continuous soot oxidation.

measured increase in K content from the baseline core. In Figure S4a for the 0−6 h segment, the measured K content varied between 0.17 and 0.37 atom % with a slightly lower measured K content at the immediate inlet (X = 0 cm) and outlet (X = 70 cm). Measured K content of the core after the 6−24 h oxidation segment (Figure S4b) ranged from 0.23 to 0.51 atom %. There was also an increased content of K in the inlet channels relative to the outlet channels (∼0.45 and ∼0.3 atom %, respectively). The measured K content also increased from 0.23 to 0.51 atom % in the R = 1 inlet channel. Figure 12 shows the K content after the 24−50 h soot oxidation interval. Values of K ranged from 0.17 to 0.83 atom %. Again, after this prolonged period of soot oxidation, the highest measured K occurred in the inlet channels of the downstream mullite filter. The R = 1 inlet channel also contained slightly more K than the R = 0 inlet channel (∼0.10 atom %). The temperature of the R = 1 inlet channel was much lower for this tested core than the previous 2 segments (400 °C as opposed to the previous 465−475 °C, respectively). Control of the temperature gradient of a core outside the furnace heating zone is much more difficult due to the lack of insulation surrounding the quartz tube. Regardless, the radial difference in K content between the inlet channels of the 24−50 h mullite core was not excessive compared to the total measured K concentration. For the outlet channels, the measured K content was ∼0.34 atom % at the immediate inlet of the filter and tapered off to the baseline fresh mullite K composition of ∼0.16 atom %. The highest amount of K was found in the inlet channels of the downstream cores during the prolonged soot oxidation periods (18 h (6−24 h) and 26 h (24−50 h)). As previously indicated, there was a slip of soot deposition from the upstream DPF core to the downstream core for both of these test periods. This soot accumulation on the mullite cores was not removed via thermal oxidation to avoid any possible losses of the deposited and loosely bound K. The XRF measurements were performed on the thin soot cake layer present in these cores, and a suitable substrate signal was still attained through the soot. Because the method of K deposition on the downstream core is likely physical adsorption, also seen in the deposition of potassium on a downstream carbon bed by Miyazaki et al.,33 the surface area of the highly porous soot

4. CONCLUSIONS Laboratory-scale reactor testing of ceramic filters coated with a novel K−Ca−Si−O glass catalyst (52-SiO2, 35-K2O, and 13CaO wt %) shows a significant improvement in soot oxidation performance over a Pt catalyst after extended periods of soot oxidation. After 250 h of continuous soot combustion, (equivalent to 100 000 miles use) the K-glass retains significant catalytic activity, exhibiting a T50 temperature of 470 °C. The total activity degradation during the estimated 100 000 mi catalyst lifetime was 45 °C. This degraded activity is still 105 °C lower than that of a commercial Pt-based catalyst without lifetime testing. SEM examination of the inlet channels after 250 h of soot combustion showed the development of discrete spherical potassium silicate particulates on the glass catalyst surface. The K/Ca ratio of the glass catalyst after extended use as measured by XPS is higher than that measured by XRF. This 11899

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(7) Lopez-Suarez, F. E.; Bueno-Lopez, A.; Illan-Gomez, M. J.; Ura, B.; Trawczynski, J. Potassium Stability in Soot Combustion Perovskite Catalysts. Top. Catal. 2009, 52, 2097−2100. (8) Neyertz, C. A.; Banus, E. D.; Miro, E. E.; Querini, C. A. Potassium-promoted Ce0.65Zr0.35O2 monolithic catalysts for diesel soot combustion. Chem. Eng. J. 2014, 248, 394−405. (9) Miyazaki, T.; Tokubuchi, N.; Arita, M.; Inoue, M.; Mochida, I. Catalytic combustion of carbon by alkali metal carbonates supported on perovskite-type oxide. Energy Fuels 1997, 11, 832−836. (10) Kimura, R.; Wakabayashi, J.; Elangovan, S. P.; Ogura, M.; Okubo, T. Nepheline from K(2)CO(3)/nanosized sodalite as a prospective candidate for diesel soot combustion. J. Am. Chem. Soc. 2008, 130, 12844. (11) Kimura, R.; Elangovan, S. P.; Ogura, M.; Ushiyama, H.; Okubo, T. Alkali Carbonate Stabilized on Aluminosilicate via Solid Ion Exchange as a Catalyst for Diesel Soot Combustion. J. Phys. Chem. C 2011, 115, 14892−14898. (12) Grzona, C. B.; Lick, I. D.; Castellon, E. R.; Ponzi, M. I.; Ponzi, E. N. Cobalt and KNO3 supported on alumina catalysts for diesel soot combustion. Mater. Chem. Phys. 2010, 123, 557−562. (13) Pecchi, G.; Cabrera, B.; Buljan, A.; Delgado, E. J.; Gordon, A. L.; Jimenez, R. Catalytic oxidation of soot over alkaline niobates. J. Alloys Compd. 2013, 551, 255−261. (14) Lu, C. X.; Liu, T. Z.; Shi, Q. L.; Li, Q.; Xin, Y.; Zheng, L.; Zhang, Z. L. Plausibility of potassium ion-exchanged ZSM-5 as soot combustion catalysts. Sci. Rep 2017, 7, 8. (15) Li, Q.; Wang, X.; Chen, H.; Xin, Y.; Tian, G. K.; Lu, C. X.; Zhang, Z. L.; Zheng, L. R.; Zheng, L. K-supported catalysts for diesel soot combustion: Making a balance between activity and stability. Catal. Today 2016, 264, 171−179. (16) An, H. M.; Su, C. S.; McGinn, P. J. Application of potash glass as a catalyst for diesel soot oxidation. Catal. Commun. 2009, 10, 509− 512. (17) Su, C. S.; McGinn, P. J. Application of glass soot catalysts on metal supports to achieve low soot oxidation temperature. Catal. Commun. 2014, 43, 1−5. (18) Zokoe, J.; McGinn, P. J. Catalytic diesel soot oxidation by hydrothermally stable glass catalysts. Chem. Eng. J. 2015, 262, 68−77. (19) Neyertz, C. A.; Miro, E. E.; Querini, C. A. K/CeO2 catalysts supported on cordierite monoliths: Diesel soot combustion study. Chem. Eng. J. 2012, 181, 93−102. (20) Liu, Z. G.; Ottinger, N. A.; Cremeens, C. M. Vanadium and tungsten release from V-based selective catalytic reduction diesel aftertreatment. Atmos. Environ. 2015, 104, 154−161. (21) van Setten, B.; van Gulijk, C.; Makkee, M.; Moulijn, J. A. Molten salts are promising catalysts. How to apply in practice? Top. Catal. 2001, 16, 275−278. (22) Cousin, R.; Capelle, S.; Abi-Aad, E.; Courcot, D.; Aboukais, A. Copper-vanadium-cerium oxide catalysts for carbon black oxidation. Appl. Catal., B 2007, 70, 247−253. (23) Peng, Y.; Li, J. H.; Huang, X.; Li, X.; Su, W. K.; Sun, X. X.; Wang, D. Z.; Hao, J. M. Deactivation Mechanism of Potassium on the V2O5/CeO2 Catalysts for SCR Reaction: Acidity, Reducibility and Adsorbed-NOx. Environ. Sci. Technol. 2014, 48, 4515−4520. (24) Boxiong, S.; Yan, Y.; Jianhong, C.; Xiaopeng, Z. Alkali metal deactivation of Mn-CeOx/Zr-delaminated-clay for the low-temperature selective catalytic reduction of NOx with NH3. Microporous Mesoporous Mater. 2013, 180, 262−269. (25) Ma, J.; Si, Z. C.; Weng, D.; Wu, X. D.; Ma, Y. Potassium poisoning on Cu-SAPO-34 catalyst for selective catalytic reduction of NOx with ammonia. Chem. Eng. J. 2015, 267, 191−200. (26) Wood, B. J.; Fleming, R. H.; Wise, H. Reactive Intermediate in the Alkali-Carbonate-Catalyzed Gasification of Coal Char. Fuel 1984, 63, 1600−1603. (27) Chen, X.; Kumar, A.; Klippstein, D.; Stafford, R.; Su, C.; Yuan, Y.; Zokoe, J.; McGinn, P. Development and Demonstration of a Soot Generator Integrated Bench Reactor; SAE International, 2014. (28) Moore, R. H.; Ziemba, L. D.; Dutcher, D.; Beyersdorf, A. J.; Chan, K.; Crumeyrolle, S.; Raymond, T. M.; Thornhill, K. L.;

suggests the glass catalyst surface is K-rich relative to the bulk, as expected if the diesel exhaust humidity is drawing K+ ions to the glass surface. The ability of the glass catalyst surface to be replenished with K ions on a continuous basis overcomes previous concerns about the durability of soot catalysts relying on alkali metals. Tests combining both soot oxidation and hydrothermal exposure verified a previous report that the presence of a soot cake slows the chemical degradation of a glass catalyst by shielding it from the exhaust. Migration of potassium from the inlet channels to the outlet channels of the catalyzed filter was observed. An uncoated filter core downstream of a K-glass coated core was used to capture volatilized K present in the gas stream during soot oxidation. A measurable amount of K was detected on the downstream core after 50 h of testing.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.9b02381.

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Sample preparation methodology, axial and radial temperature profiles, XRF spectrum, XRF measured K content, table of spherical precipitates (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Paul J. McGinn: 0000-0001-7340-6528 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was partially supported by the Notre Dame Integrated Imaging Facility (NDIIF) and by the Notre Dame Center for Environmental Science & Technology (CEST). J.Z. acknowledges partial support of this work through a CEST Bayer fellowship. The authors acknowledge the contributions of David Klippstein and Dr. Ashok Kumar for the use and operation of the reactors utilized in this study.

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REFERENCES

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