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Catalytic diesel soot oxidation by hydrothermally stable glass catalysts
Accepted Manuscript Catalytic Diesel Soot Oxidation by Hydrothermally Stable Glass Catalysts James Zokoe, Paul J. McGinn PII: DOI: Reference:
S1385-8947(14)01271-6 http://dx.doi.org/10.1016/j.cej.2014.09.075 CEJ 12693
To appear in:
Chemical Engineering Journal
Received Date: Accepted Date:
22 August 2014 20 September 2014
Please cite this article as: J. Zokoe, P.J. McGinn, Catalytic Diesel Soot Oxidation by Hydrothermally Stable Glass Catalysts, Chemical Engineering Journal (2014), doi: http://dx.doi.org/10.1016/j.cej.2014.09.075
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Catalytic Diesel Soot Oxidation by Hydrothermally Stable Glass Catalysts James Zokoe and Paul J. McGinn Department of Chemical and Biomolecular Engineering University of Notre Dame, Notre Dame, IN 46556 email addresses:
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[email protected] Corresponding Author: Paul McGinn: 182 Fitzpatrick, Department of Chemical and Biomolecular Engineering, University of Notre Dame, Notre Dame, IN 46556. [email protected], (574) 631-6151 Keywords: soot oxidation, potassium catalyst, glass catalyst, cordierite filter, diesel particulate filter
Abstract: K-Ca-Si glass catalyst coated cordierite was exposed to extended continuous soot oxidation testing in a bench top reactor utilizing flame soot deposition. These samples maintained a T50 temperature of 500°C at the end of an estimated 100,000 mi equivalent continuous soot oxidation testing in a low humidity environment. High temperature (500-700°C) hydrothermal exposure led to potassium and calcium carbonate formation on the glass catalyst surface which degraded oxidation activity with increasing hydrothermal temperature up to 700°C due to reduced potassium surface mobility. The presence of a soot layer during hydrothermal exposure shielded the glass surface from extensive deactivation, suggesting glasses as promising catalysts for extended DPF use.
Graphical Abstract:
1.
Introduction Increasing statutory limitations on diesel exhaust emissions have forced engine
manufacturers to pursue a range of technologies to comply with the new standards, including reducing emission of carbonaceous soot particles. A diesel particulate filter (DPF) is usually used to remove soot from the exhaust stream.
Over time soot accumulates in the filter and must be
removed to avoid a large pressure drop in the exhaust stream. Soot removal, or regeneration of the filter, can be done actively by raising the temperature sufficiently high for the carbon to oxidize into CO2 (550-600°C). Regeneration can also be accomplished passively by changing the engine operating conditions to raise the amount of nitrogen dioxide (NO2) which aids carbon oxidation and/or by lowering the required temperature (250-500°C) through the use of a catalyst so that oxidation can proceed at normal exhaust temperatures. Catalysts used to lower the soot oxidation temperature often include platinum which substantially increases the price of the emissions abatement system. A platinum replacement is desired to lower the cost of an emissions reduction unit while maintaining low soot oxidation temperatures. Catalysts based on potassium can satisfy both of those criteria. However, alkali based catalysts are not yet commercially viable as a long life catalyst in diesel applications, as they have been unable to maintain activity over multiple combustion cycles or temperature excursions above 600°C [1, 2]. The high activity of K in loose physical contact has been explained by the “tunneling” ability of K as a mobile oxide in which the catalyst bores into a carbon mass by oxidizing along the edges of graphitic basal planes of individual soot particles within aggregates [3]. Many groups have reported on the rapid degradation of potassium based catalysts and have attributed the loss of activity to the same high
mobility of the potassium ions, which can experience sublimation during the oxidation process [3, 4]. One novel approach that has been developed uses a potassium containing glass as a catalyst, in which K+ ions are present within a silicate matrix. With exposure to water vapor, ion exchange of H+ and potassium ions can occur, promoting migration of K+ to the glass catalyst surface and providing a continuous supply of potassium for long term catalytic soot combustion. By optimizing the glass composition, soot combustion temperatures in the 375-400°C range have been achieved [5]. However, a glass catalyst, as with any DPF catalyst, must survive the harsh conditions of the diesel exhaust environment for extended periods of time. This means the glass must have appropriate resistance to elevated temperatures, structural changes caused by hydrothermal exposure, ash buildup, sulfur, etc. Few studies have investigated the longevity of K-based catalysts. Two studies reported multi cycle TGA measurements as a means for quantifying combustion durability but no durability testing was reported for longer than 6 cycles [1, 6] until our recently published work on K-glass coated metal substrates, that showed minimal catalytic degradation over 30 cycles [7]. The need for studies of hydrothermal durability was stated in a recent review, where it was noted that, “despite the attempts to prepare stable alkaline metalcontaining soot combustion catalysts, the hydrothermal stability remains a major problem awaiting solution for a practical use of alkaline metals in soot combustion catalysts formulations.” [8] Here we report on the formation of phases on the surface of a catalytic glass subjected to hydrothermal conditions similar to that found in a diesel exhaust stream. Additionally, the effect
of the presence of soot on the surface is studied, as it may alter the interaction of the glass with the environment by consuming K ions that would otherwise accumulate at the surface [9-11].
2. Experimental methods Cordierite DPF substrates were coated with a potassium-rich catalytic glass film and tested for activity and long term chemical and physical durability. In previous testing glass powder synthesized through traditional melt processes with the composition 52 wt% SiO2, 35 wt% K2O, and 13 wt% CaO (~At%: 47% Si, 40% K, 13% Ca respectively) was shown to be promising as a K-glass catalyst [12]. This composition will be referred to as KCS-1. For sol gel synthesis of this glass composition, tetraethylorthosilicate (TEOS) (Si(OC2H5)4, 98%), calcium nitrate tetrahydrate (Ca(NO3)2.4H2O, ACS reagent) and potassium nitrate (KNO3, ACS reagent) were used as starting materials, following a well-known approach [13]. TEOS was dissolved in ethanol. Calcium nitrate and potassium nitrate were dissolved in DI water and added to the TEOS-ethanol mixture. 10 cP was chosen as the coating viscosity for cordierite filter samples which produced a thin glass film of ~2 µm. The films are thin enough that when coated onto a cordierite filter they do not fill pores and cause a negligible pressure drop along the filter. [14] The drying and coating conditions for the cordierite filter slices are shown schematically in Figure 1a along with images of uncoated (Fig. 1b) and KCS-1 coated (Fig. 1c) cordierite samples. Flow through cordierite filter slices (8 x 8 x 3 mm3 ) with 1 mm channels were dip coated with the KCS-1 sol following the process depicted in Figure 1a. This sample size and mass is compatible with the TGA for catalytic activity characterization. Samples were submerged in the sol and a vacuum was applied to degas and impregnate the inner pore structure of the cordierite.
The excess sol was subsequently drained and blown off the filter slices and the samples were then dried and aged. High temperature burnout of the precursors was conducted in air, with the samples heated to 650°C, held for 10 min hold to ensure complete removal of nitrates from the glass, and then furnace cooled to room temperature. A consistent 5 wt% catalyst loading was achieved through this method. It can be seen from Figure 1 (c) that the KCS-1 applied film is not continuous. The drying conditions were chosen on the basis of maintaining a reasonable timeframe for sample preparation to achieve a workable catalyst film. Crack-free films are possible through the addition of polymer additives and slower heating parameters [7], but are unnecessary for this research focus (i.e. adhesion of the film is not affected). Flat glass slices of KCS-1 composition were used for scanning electron microscopy and energy dispersive X-ray spectroscopy (SEM/EDS) surface characterization. These were made by melting a stoichiometric mixture of carbonate powders as described previously [9, 12]. 8 x 5 x 3 mm3 (L x W x H) rectangular slices were subsequently cut from the cooled bulk glass. The slices were then dry polished without a polishing medium (i.e. water, oil, etc.) with SiC (180, 600 grit) and alumina (5, 0.5 µm) pads to a scratch-free finish. Water was avoided to circumvent alkali leaching from the surface during polishing. Structural and chemical changes near the surface (~1-2 µm) were characterized by SEMEDS and attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR). SEM-EDS analysis was performed on a Carl Zeiss LEO EVO-50 with an Oxford INCA energy dispersive spectrometer. ATR-FTIR was performed on a Bruker Tensor 27 FTIR with a platinum single reflection diamond ATR module.
Catalytic activity was characterized by the soot ignition temperature (Tig) and the 50% soot conversion temperature (T50) measured by a TA Instruments 2950 high resolution thermogravimetric analyzer (HR-TGA). The TGA was programmed to slow the heating ramp rate from 20°C/min to 2°C/min when the onset of weight loss was detected. A mixed diesel exhaust gas analogue of composition 10% O2, 5% CO2, 3% H2O with balance N2 was used. Any activity degradation, whether it is due to hydrothermal surface modification or a loss of active potassium, will be detected as an increase in the soot oxidation temperature. To closely mimic the real conditions experienced by a DPF on a diesel engine, “loose” soot contact achieved by flame soot deposition was utilized for all tested samples [15]. KLEAROL® white mineral oil with ~1 ppm sulfur was used as the fuel oil for soot generation. This fuel was chosen to mitigate any degradation effects that might result from sulfate formation. To test long-term soot oxidation durability, a homemade testing stand was developed to utilize the flame soot deposition method. Figure 2 is a schematic of the extended continuous soot oxidation (ECSO) testing unit. The ECSO unit is composed of a stainless steel cylindrical sample holder with a picture frame type cover plate (Fig. 2a). Flow channels drilled through the sample holder allow for soot deposition through the TGA samples without excessive surface accumulation. The sample holder is positioned 25.4 cm above the flame. The stainless steel holder heats the substrates to the soot deposition-oxidation balance point temperature for sustained soot combustion (400500°C). The samples were open to laboratory atmosphere without additional H2O vapor since a quantitative means for supplying H2O into the flame environment was not feasible. Figure 2b shows the overall set up for the ECSO unit. Samples were exposed to continuous soot deposition and oxidation, slightly above the balance point, for periods of up to 24 hours without significant
fluctuation in the soot deposition rate. Catalytic activity was characterized after each ECSO testing period by TGA. Soot deposition for TGA characterization was performed in the ECSO apparatus at 150°C for 20 minutes to yield a catalyst to soot ratio of 10:1. The soot deposition rate was determined for each ECSO experiment from the mass of soot oxidized during the TGA run after each prolonged oxidation period and from the soot deposition time for that run (i.e. soot oxidized/soot deposition time of the TGA test). Based on estimates from larger scale bench top reactors, the total amount of soot necessary for 100,000 mi of equivalent lifetime on a TGA slice was determined to be ~93 mg for the 8 x 8 x 3 mm3 TGA sample size. This calculation was based on conversations with industrial collaborators concerning soot output for an average light duty diesel engine over this mileage. At an average soot deposition rate of 0.005 mg/min in the ECSO system, 100,000 mi equivalent lifetime testing requires ~300 hours of flame soot exposure. Hydrothermal (chemical) degradation was studied on KCS-1 samples exposed to humidified gas at temperatures between 500-700°C. Flowing air or N2 (120 mL/min) was bubbled through a heated water bath (40°C) to accumulate ~7% H2O vapor before it was fed into a quartz tube furnace which housed the filter samples. The 7% H2O content was chosen to approximate the diesel exhaust environment. The line connecting the water bubbler and furnace was heated above 40°C with heating tape to assure no condensation occurred upstream of the samples. Hydrothermal tests were conducted for 2 hours before the catalytic activity was characterized by TGA. In a real diesel exhaust application, a particulate filter will usually contain some amount of soot before regeneration is performed, unless the exhaust temperature is high enough to allow for passive regeneration. Under urban driving conditions lower exhaust temperatures (200-
350°C) are expected, and thus, an applied catalyst would experience hydrothermal conditions in the presence of soot. The presence of soot on the glass may change the effect of the hydrothermal environment on the glass. To test this condition, filter samples were coated with soot by flame soot deposition and then exposed to the hydrothermal testing previously described. Samples both with and without pre-applied flame soot were used for comparison. Samples with pre-deposited soot did not receive additional soot application after the hydrothermal treatment.
3. Results and Discussion The KCS-1 glass composition, either as a sol-gel glass coating on cordierite filter slices or as bulk melt glass slices, served as the catalyst baseline testing composition. The use of flame soot deposition creates a loose contact condition that mimics the poor contact between an actual DPF and exhaust deposited soot. TGA measured activity of the KCS-1 on cordierite, in these flame soot contact conditions, yields a Tig of 390°C and T50 of 410°C with typical sample batch variability of within ±5°C [5]. KCS-1 catalyst shows a 200°C decrease in oxidation temperature compared to oxidation on uncoated cordierite. At these temperatures, partial passive soot oxidation might occur under extra-urban or highway driving conditions. Oxidation temperatures in the sub 400°C range are typical of alkali and specifically K-based catalysts [10, 11] but maintaining these temperatures has previously been a challenge. Thus prolonged soot oxidation studies were carried out on the KCS-1 coated cordierite filter slices to test the catalyst’s combustion durability.
3.1 Extended Continuous Soot Oxidation Study of KCS-1 Coated Cordierite
The ECSO testing was designed to decrease the overall time needed to assess long term oxidation stability without performing hundreds of TGA cycles. The ECSO testing differs from TGA cycling in that the sample is held at a constant temperature for an extended period of time, but one that is lower than the maximum temperature in a TGA cycle. Figure 3 shows the TGA results measured periodically during ECSO testing of 4 KCS-1 coated cordierite samples with error bars indicating the standard deviation. An overall, fairly linear decrease in activity (i.e. increase in ignition temperature and T50) can be seen throughout the length of testing (Figure 3). The loss of activity was measured at 1.1°C/mg of oxidized soot for Tig and 0.99°C/mg for T50 based on a linear least squares fit. At 50% of estimated lifetime (i.e. 46 mg) the KCS-1 has a measured ignition temperature of 431°C; a rise of 55°C in ignition temperature. At 97 mg of soot combusted, or the end of estimated lifetime, the Tig and T50 temperatures have degraded to 475°C and 499°C respectively. This performance cannot be directly correlated to the real degradation conditions due to the lack of water vapor content. Testing is being performed in conjunction with industry collaborators to assess any differences with increased sample size and with prolonged combustion in the presence of water vapor and NOx. A recent study [16] has shown NOx to decrease the combustion temperatures of K-based soot oxidation by 30°C in tight contact conditions. Due to the absence of elevated H2O vapor, NOx, and high gas flow rate it is expected that the testing reported in this paper is a worst case environment for catalytic soot oxidation. In the absence of additional H2O vapor during oxidation, the loss of activity may be exacerbated due to surface depletion of K. A net loss of potassium at the glass surface could arise if the rate of K loss due to sublimation were greater than the K+ ion diffusion through the bulk glass. In the dry combustion condition of our current ECSO testing, this depletion of active
surface potassium seems likely. To test this hypothesis a 24 hr 60°C hydrothermal treatment with 7% H2O (henceforth termed “humidity treatment”) was performed on the ECSO samples after 97 mg of oxidized soot. The presence of water vapor in the absence of soot should induce the leaching of K+ ions and replenish the surface with K. Subsequent TGA of these humidity treated samples produced a sustained decrease in oxidation temperatures over five consecutive TGA cycles as shown in Figure 4. The humidity treatment recovered a measured 23°C of activity in T50 and a sustained decrease in T50 of 14°C after five consecutive TGA cycles. This increased catalytic activity, however, was temporary. With subsequent ECSO runs (not shown), the combustion temperatures eventually returned to the pre-hydrothermal treatment trend. These data suggest there is ample K remaining in the glass which is available for soot oxidation and the dry combustion environment of the ECSO testing is depleting the glass surface of “active potassium” (i.e. potassium available for direct soot contact and oxidation). Regardless, the result of this extended soot oxidation testing indicates the KCS-1 glass catalyst maintains sufficient potassium content and durability in the combustion temperature range to sustain acceptable catalytic activity. The addition of a relatively small atomic percentage of Ca (13%) provides adequate resistance to the K+ ion exchange from the silicate matrix to withstand soot combustion over this estimated 100,000 mi catalyst lifetime. A polished KCS-1 bulk glass slice was exposed to an equivalent treatment for SEM/EDS study to visualize and qualitatively determine how the low temperature humidity treatment affects the glass to increase activity. The glass sample was polished to a scratch and defect free surface prior to exposure. Figure 5 (a) shows an SEM image of the KCS-1 glass sample surface after 24 hr 60°C humidity treatment in air with 7% H2O vapor.
The glass surface experienced growth of segregated alkali and alkaline precipitates. Access to water vapor causes rapid leaching of K and Ca ions from the glass through the aforementioned silicate hydration (6) and in the presence of CO2 and O2, carbonates are rapidly formed. ATR-FTIR and elemental mapping in SEM-EDS both confirm the formation of calcium and potassium carbonates. Figures 5b -5e show the respective EDS elemental maps (C, K, Ca, Si) of the resultant glass surface. Precipitates were found to be K-rich and Ca-rich carbonates. The actual elemental compositions of these precipitates could not be accurately determined due to signal contribution from the underlying glass surface. This extensive ion-exchange of K seen in the bulk KCS-1 glass is an intrinsic property of the glass and should be comparable in the KCS-1 coated cordierite samples. Figure 5 (f) depicts the resultant IR spectrum in which the singular peak for calcium carbonate present at 1381 cm-1 with obvious sharpening of the peaks at 708 and 870 cm-1 confirms mainly CaCO3 formation at this low temperature [17]. The underlying broad peaks for Si-O-Si and Si-O-X are still evident at 982 and 870 cm-1 indicating the surrounding flat glass surface has not endured extensive structural degradation [18]. This humidity treatment thus increases the activity of the glass catalyst by enriching the glass in K through the leaching of Ca which separates as the carbonate precipitate. This process likely also facilitates faster and more extensive subsequent K leaching due to the loss of the stabilizing element Ca. Microstructural analysis of the KCS-1 coated cordierite glass surface was performed by utilizing SEM-EDS. Figure 6 depicts the SEM imaging of the KCS-1 coated cordierite surface after the full 97 mg soot of oxidized soot in ECSO testing. The KCS-1 glass coating developed small growths of tortuous potassium silicate structures on the edges of discrete glass regions. EDS analysis of these structures measured an average composition (in At%) of 64.4 ± 12.9% Si,
33.4 ± 14.2% K, 2.21 ± 2.04% Ca. The large variability in measured Si and K content suggest potassium is lost or diffuses along the silicate surface during sustained combustion. These silicate growths are infrequent on the sample surface however, as can be seen at lower magnification in Figure 6 (b) in which most of the catalyst surface is structurally unaffected. The presence of water vapor during sustained combustion will decrease the oxidation temperatures by increasing the mobility of K through the surface glass layers. It is unknown, however, whether the addition of water vapor during combustion yields an increased rate of K loss which could counteract the increase in catalytic activity. Testing has been performed by an industry partner on larger KCS-1 coated filters in a bench top reactor capable of testing in a more realistic diesel exhaust environment (i.e. high flow rate and water vapor inclusion). The results indicate that the presence of water vapor decreases the catalytic activity loss in comparison to the ECSO testing. 3.2 Hydrothermal Testing of Bare Glass Catalyst The harsh hydrothermal environment of diesel exhaust provides an additional means for catalytic degradation. To test the chemical durability of the KCS-1 glass composition, samples of cordierite were coated through the aforementioned sol-gel process and subsequently exposed to various hydrothermal environments. Figure 7 shows the results of TGA characterized soot oxidation (T50 temperature) after 2 hour exposures to temperatures ranging from 500-700°C under N2 and air, both with and without water vapor. In each of the 3 gas environments tested, there is a direct relationship between catalytic deactivation (increase in oxidation temperature) and hydrothermal exposure temperature. An increase in hydrothermal exposure temperature increases the extent of measured catalytic deactivation. There are minimal differences measured for the samples exposed to air and N2
with 7% H2O vapor for temperatures up to 600°C but there is a noticeable T50 difference of 17°C due to air exposure at 700°C compared to N2 with H2O vapor. One factor contributing to deactivation at 700oC may be enhanced surface crystallization kinetics at higher temperatures. [19] The enhanced nucleation is also seen in observation of glass surfaces (Fig.8, discussed below). For the samples tested under N2 without water vapor, this is the predominant degradation mechanism. For the case of N2 + H2O, there is the additional contributing deactivation caused by chemical degradation of the glass surface in the hydrothermal environment. Exposure to water vapor will further enhance creation of precipitates containing K+ ions (i.e. alkali carbonates or sulfates) [20, 21] that will further decrease the amount of active potassium at the surface. Finally, it was observed that hydrothermal exposure at 700°C in air resulted in an even larger decrease in activity relative to the similar testing condition in N2. In air, the presence of CO2 will facilitate carbonate formation on the surface, further reducing the total surface potassium available for soot oxidation and thus further decrease catalytic activity. To gain a better understanding of changes to the glass surface during high temperature hydrothermal degradation, samples of flat, polished KCS-1 glass were exposed to a range of temperatures in a hydrothermal environment and then examined in the SEM. The samples were held at temperatures of 300, 500, 600, and 700°C for two hours in flowing humidified air (7% H2O vapor, 120 mL/min). Figure 8 shows SEM images of the KCS-1 glass surfaces after the hydrothermal exposures. Smaller precipitates with increased surface coverage are observed at higher temperatures (Fig. 8c and d). EDS characterization through point and area (~1 mm2) analysis was performed at five or more particulate and flat glass points per sample. The data is tabulated in Table 1. It can be seen that the K/Ca ratio increases from an as-synthesized value of 2.80 to a
value of 3.69 at a treatment temperature of 300°C but decreases as the treatment temperature increases to a value of 2.93 after 700°C exposure. However, all values are higher than the asmade value, i.e. K-enriched. The K/Ca ratio of the as-synthesized glass was lower than the theoretical value of 3.07 likely due to K sublimation at the melt synthesis temperature of 1200°C. Measurements of the particulate growths and the surrounding flat regions reveal an increase in K content of the particulates and a corresponding localized decrease of K in adjacent regions which exhibit K/Ca ratios ranging from 2.43 to 2.75. At temperatures above 300C, carbon was not detected in any appreciable amount in the flat regions near particulates, suggesting the accumulations are a mixture of K and Ca carbonates formed from nearby carbon sources. The Si detected in the point measurements of the particulates is likely from the underlying glass. At elevated temperatures (i.e. exhaust temperature range) the molecular bonding of the non-bridging oxygen atoms within the Si matrix is weaker which allows for an increase in chemical degradation, allowing for an increase in the K content on the surface. In the presence of CO2, the surface potassium readily forms K2CO3 [22-24]. At temperatures near or exceeding the Ts there appears to be a change in the precipitation mechanism. In Figure 8 it can be seen that surface coverage by the precipitates increases while the overall size of the precipitates declines with increasing temperature. The glass surface in this temperature region may soften, with rearrangement of the Si-O-Si structure facilitating easier movement of alkali ions. This seems to aid the nucleation and precipitation of smaller and more numerous particulates. These surface accumulations of K compounds decrease the number of active K sites by concentrating the K within the precipitates and lessening opportunities for soot contact. The potassium rich precipitates develop by drawing surface potassium from the surrounding areas, depleting the catalytic activity of the regions around the precipitates. At elevated temperature
and with access to abundant CO2, the formation of carbonates would greatly increase and thus intensify the segregation of these depleted and enriched zones. The separation of potassium content could give rise to a non-uniform oxidation of soot as seen in HR-TGA. To better understand the impact of precipitate formation, a sample of KCS-1 coated cordierite was exposed to extended 500°C hydrothermal treatments in air for 2, 6, and 72 hours. A hydrothermal temperature of 500°C was chosen for analysis to correspond with the upper range of ECSO testing temperatures. After each hydrothermal exposure the change in activity was characterized by HR-TGA. The soot weight loss results are shown in Figure 9a. During the first 2 and 6 hours of hydrothermal testing the sample undergoes activity degradation as measured by an increase in T50 of 4 and 7°C respectively. As seen in Figure 9 (b), there is an increase in the rate of soot oxidation with these exposures from an initial 2.5 %/°C to 3.8 %/°C after 2 hours of exposure. It is likely the final high temperature burnout of the sol-gel process creates a poor surface for K ion exchange (i.e. coarsening of the outer surface through sintering) which undergoes a surface restructuring and replenishment of catalyst from ion exchange during the hydrothermal treatment. After 72 hours of hydrothermal exposure there is a clear development of a secondary combustion mechanism occurring which can be seen as the large shoulder formation after the initial oxidation peak in the derivative of weight loss curve in Figure 9 (b). The shoulder arises from the K enrichment of carbonate particulates and the associated depletion of the surrounding glass surface. Areas with less available K (lower K/Ca ratios) lead to a shift in combustion to higher temperatures since more energy is required to initiate oxidation. This surface restructuring, both chemical and structural, is the cause of the activity degradation through the aforementioned loss of catalyst and soot contact.
Similar samples of polished KCS-1 glass were exposed to equivalent 500°C hydrothermal conditions and analyzed by ATR-FTIR spectroscopy as seen in Figures 9 (c) and (d). ATR-FTIR spectroscopy of the as-made polished KCS-1 glass (Figure 9 (c)) revealed peaks at 735, 872, and 991 cm-1. The peaks at 735 and 991 cm-1 represent the symmetric and asymmetric Si-O-Si stretching respectively while the peak at 872 cm-1 corresponds to the nonbridging Si-O-X stretching with X representing K or Ca [17, 18]. Figure 9 (d) shows the IR spectra for KCS-1 glass samples exposed to extended hydrothermal conditions for 2, 6, and 72 hours. Characteristic bands for calcium and potassium carbonate seen at 1350-1450 cm-1 develop with increasing exposure time. The development and sharpening of the peaks at 710 and 880 cm-1 are also indicative of carbonate formation. Dual peak formation in the 1350-1450 cm-1 region was not previously seen in the potash glass corrosion study by Vilarigues and da Silva [18]. To clarify these results, a range of varying K-Ca mixtures of ground KCS-1 glass, potassium carbonate, and calcium carbonate powders (10% total carbonates by weight) were prepared and mixed by mortar and pestle and characterized by ATR-FTIR for comparison. The dual peak formation in the carbonate wavenumber region was seen only in samples where K2CO3 was present. The existence of these two peaks in the 500°C hydrothermally treated sample in Figure 9 (d) and the large K/Ca ratio of the precipitates measured by EDS (Table 1) can thus allow characterizing the precipitates seen in Figure 8 (b) as a potassium dominated mixture of K2CO3 and CaCO3. 3.3 Hydrothermal Testing of Soot Covered Catalyst In an operating DPF, glass will not be covered by soot for only brief periods of time, such as immediately after a regeneration cycle. Between regeneration cycles during periods of low engine load it is expected a soot layer will be present and growing. In such a case, a layer of soot
might cover the glass catalyst, eliminating direct exposure of the glass to the diesel exhaust environment. Thus effects of a hydrothermal environment on soot-covered KCS-1 glass were also investigated. Lower level exhaust temperatures of 200°C and 300°C were chosen as subignition temperatures for testing. KCS-1 coated cordierite samples with and without soot before hydrothermal exposure were compared. The samples with no soot applied before the hydrothermal treatment were coated afterwards prior to TGA testing. Figure 10 shows the TGA results for multiple oxidation cycles after a single hydrothermal treatment. For both cases in which soot was present during the hydrothermal treatment there is a significant decrease in oxidation temperature compared with the non-soot covered sample. For the 300°C hydrothermal treatments, ~40% of the pre-applied soot was oxidized during the 2 hr treatment (i.e. a very low oxidation temperature) but no additional soot was applied before performing TGA. Additional applied soot would have likely resulted in more complex TGA oxidation curves due to differences in the contact conditions of the pre-applied and posthydrothermal soot. Both sets of samples showed a reduction in Tig and T50 from the as-prepared state (treatment #0). The increase in activity of the samples without pre-applied soot suggests the glass experienced an enrichment of surface potassium in the mild hydrothermal conditions. Further TGA testing, however, shows that the effect is limited to a single combustion with TGA runs 2 and 3 returning to pre-hydrothermal treatment temperatures (Tig ~ 390°C) in both 200°C (Fig. 10a) and 300°C (Fig. 10b) tests. The 200°C and 300°C samples with pre-applied soot experienced a decrease in both Tig and T50 of 40°C and 30°C respectively from the as-prepared state. This corresponds to 10°C further reduction of Tig and T50 compared to the KCS-1 samples which had no pre-applied soot.
To determine the longevity of the observed decreased soot oxidation temperatures, multiple hydrothermal treatments at 200°C and 300°C with and without pre-applied soot were also performed. A single TGA cycle was used to measure the catalytic activity after each hydrothermal treatment. Soot was reapplied before each subsequent hydrothermal run for the “pre-applied” samples. Figure 11 shows the TGA results of 5 hydrothermal cycles of cordierite samples coated with KCS-1 glass with and without pre-applied soot at both 200°C (Fig. 11a) and 300°C (Fig. 11b). As before, hydrothermal treatment #0 in the plot corresponds to the average measured Tig and T50 for as made KCS-1 coated cordierite samples (i.e. before any treatment). In both the 200°C and 300°C testing with soot present, there is a sustained decrease in oxidation temperatures compared to the samples without applied soot. The initial decrease in oxidation temperatures for the samples without soot is lost after the second hydrothermal treatment for both temperatures tested. The presence of soot, however, yields a sustained increase in catalytic activity (or decrease in activity degradation) with repeated 200°C hydrothermal exposures and only a 10°C increase in combustion temperatures from the first to fifth 300°C hydrothermal treatment (Tig from 358°C to 368°C respectively). The measured oxidation temperatures for the 300°C testing may be slightly elevated due to the soot oxidation that occurs during the hydrothermal test period. Soot which is in tightest physical contact will oxidize at the lowest temperature, with the less optimally contacted soot remaining. The increase in catalytic activity can likely be attributed to increased physical soot-potassium contact through the chemical degradation of the glass catalyst. Since the soot is already in contact with the glass before the introduction of the hydrothermal environment, the potassium which is pulled to the surface through ion exchange can migrate into the near surface soot cake. This increases the total soot-
catalyst contact surface area and thus increases the catalytic activity. These results are encouraging in that they show the soot cake provides a sort of protective layer that shields the surface from direct exposure to the hydrothermal chemical degradation which was seen in the previous section. It is likely that the thicker the soot cake, the lower the near surface gas flow velocity will be and thus less mass flux of water can reach the surface: depressing the chemical degradation. In a real DPF application, this would increase the lifetime of the catalyst and ultimately decrease the necessary regeneration temperature while increasing the amount of passively oxidized soot between regenerations (when the exhaust reaches over 350°C). 6. Conclusions A potassium-rich K-Ca-Si soot oxidation catalyst with long term stability can be realized by implementing a slow release glass catalyst coating on a DPF.
The replenishment of active
potassium counteracts catalytic deactivation that might occur through potassium sublimation. Continuous soot oxidation testing over an estimated 100,000 mi equivalent in a low humidity environment causes a slow rise in the T50 from approximately 400°C to 500°C. Improved activity can be temporarily restored by a low temperature humidification treatment. High temperature (500-700°C) hydrothermal exposure of causes more rapid activity degradation due to carbonate formation on the catalyst surface. The presence of a soot layer during hydrothermal exposure protects the glass surface, thereby reducing activity degradation, making glass catalysts a promising technology for extended DPF use. 7. Acknowledgements This work was partially supported by Notre Dame Integrated Imaging Facility (NDIIF) and the Center for Environmental Science and Technology (CEST) Bayer Fellowship (JZ). We are also
grateful to Professor Joan Brennecke for use of a viscometer. We are grateful to CEST for use of the ATR-FTIR.
References
[1] F.E. Lopez-Suarez, A. Bueno-Lopez, M.J. Illan-Gomez, B. Ura, J. Trawczynski, Potassium Stability in Soot Combustion Perovskite Catalysts, Topics in Catalysis, 52 (2009) 2097-2100. [2] C.A. Neyertz, E.E. Miro, C.A. Querini, K/CeO2 catalysts supported on cordierite monoliths: Diesel soot combustion study, Chem. Eng. J., 181 (2012) 93-102. [3] D.W. McKee, D. Chatterji, Catalytic Behavior Of Alkali-Metal Carbonates And Oxides In Graphite Oxidation Reactions, Carbon, 13 (1975) 381-390. [4] D.W. McKee, Gasification Of Graphite In Carbon-Dioxide And Water-Vapor - The Catalytic Effects Of Alkali-Metal Salts, Carbon, 20 (1982) 59-66. [5] C. Su, P.J. McGinn, The effect of Ca2+ and Al3+ additions on the stability of potassium disilicate glass as a soot oxidation catalyst, Applied Catalysis B: Environmental, 138–139 (2013) 70-78. [6] G. Pecchi, B. Cabrera, A. Buljan, E.J. Delgado, A.L. Gordon, R. Jimenez, Catalytic oxidation of soot over alkaline niobates, J. Alloy. Compd., 551 (2013) 255-261. [7] C.S. Su, P.J. McGinn, Application of glass soot catalysts on metal supports to achieve low soot oxidation temperature, Catal. Commun., 43 (2014) 1-5. [8] A.M. Hernandez-Gimenez, D.L. Castello, A. Bueno-Lopez, Diesel soot combustion catalysts: review of active phases, Chem. Pap., 68 (2014) 1154-1168. [9] H.M. An, C. Kilroy, P.J. McGinn, Combinatorial synthesis and characterization of alkali metal doped oxides for diesel soot combustion, Catal. Today, 98 (2004) 423. [10] T. Miyazaki, N. Tokubuchi, M. Arita, M. Inoue, I. Mochida, Catalytic combustion of carbon by alkali metal carbonates supported on perovskite-type oxide, Energy Fuels, 11 (1997) 832-836.
[11] T. Miyazaki, N. Tokubuchi, M. Inoue, M. Arita, I. Mochida, Catalytic activities of K2CO3 supported on several oxides for carbon combustion, Energy Fuels, 12 (1998) 870-874. [12] H.M. An, C.S. Su, P.J. McGinn, Application of potash glass as a catalyst for diesel soot oxidation, Catal. Commun., 10 (2009) 509-512. [13] P. Saravanapavan, L.L. Hench, Mesoporous calcium silicate glasses. I. Synthesis, J. NonCryst. Solids, 318 (2003) 1-13. [14] C.S. Su, Ph.D. Thesis, Stabilization of potassium in soot oxidation catalysts and their application on diesel particulate filters, in: Chemical & Biomolecular Engineering, University of Notre Dame, Notre Dame, IN, 2011. [15] C.S. Su, P.J. McGinn, The effect of Ca2+ and Al3+ additions on the stability of potassium disilicate glass as a soot oxidation catalyst, Appl. Catal. B-Environ., 138 (2013) 70-78. [16] P. Legutko, P. Stelmachowski, T. Trebala, Z. Sojka, A. Kotarba, Role of Electronic Factor in Soot Oxidation Process Over Tunnelled and Layered Potassium Iron Oxide Catalysts, Topics in Catalysis, 56 (2013) 489-492. [17] H.A. ElBatal, M.A. Azooz, E.M.A. Khalil, A.S. Monem, Y.M. Hamdy, Characterization of some bioglass-ceramics, Mater. Chem. Phys., 80 (2003) 599-609. [18] M. Vilarigues, R.C. da Silva, Characterization of potash-glass corrosion in aqueous solution by ion beam and IR spectroscopy, J. Non-Cryst. Solids, 352 (2006) 5368-5375. [19] R. Muller, E.D. Zanotto, V.M. Fokin, Surface crystallization of silicate glasses: nucleation sites and kinetics, J. Non-Cryst. Solids, 274 (2000) 208-231. [20] A. Paul, Chemical Durability of Glasses - Thermodynamic Approach, J. Mater. Sci., 12 (1977) 2246-2268.
[21] C.M. Jantzen, Thermodynamic Approach to Glass Corrosion, in: D.E. Clark, B.K. Zoitos (Eds.) Corrosion of Glass, Ceramics, and Ceramic Superconductors: principles, testing, characterization and applications, Noyes Publications, Park Ridge, NJ, 1992, pp. 153-217. [22] I.L.C. Freriks, H.M.H. Vanwechem, J.C.M. Stuiver, R. Bouwman, Potassium-Catalyzed Gasification Of Carbon With Steam - A Temperature-Programmed Desorption And FourierTransform Infrared Study, Fuel, 60 (1981) 463-470. [23] J.M. Saber, J.L. Falconer, L.F. Brown, Interaction Of Potassium Carbonate With Surface Oxides Of Carbon, Fuel, 65 (1986) 1356-1359. [24] B.J. Wood, R.H. Fleming, H. Wise, Reactive Intermediate in the Alkali-CarbonateCatalyzed Gasification of Coal Char, Fuel, 63 (1984) 1600-1603.
Figure Captions Figure 1 - (a) Sol gel procedure for the coating of cordierite filter slices. (b) SEM images of bare cordierite filter and (c) applied K-glass thin film on cordierite substrate. Figure 2 - Extended continuous soot oxidation (ECSO) bench top reactor. Figure 3 - HR-TGA Tig and T50 results for extended continuous soot oxidation (ECSO) treatment of KCS-1 coated cordierite samples. Figure 4 - HR-TGA weight loss curves of post-lifetime testing (97 mg soot combusted) KCS-1 coated cordierite showing activity regeneration after low temperature (60°C) humidification in air. Figure 5 - SEM image (a), EDS elemental mapping (b-e), and IR spectrum (f) of polished KCS-1 bulk glass slice after 24 hr exposure to 60°C hydrothermal treatment in air with 7% H2O vapor. Figure 6 - Scanning electron images of KCS-1 coated cordierite surface after extended continuous soot oxidation (ECSO) testing. Figure 7 - TGA measured soot oxidation T50 after 2 hours exposure to various hydrothermal environments. Separate samples of KCS-1 coated cordierite were used in each test. Figure 8 - SEM images of polished KCS-1 melt glass slices after 2 hour hydrothermal exposures under air at (a) 300°C and (b) 500°C (c) 600°C and (d) 700°C. Separate samples were used for each testing condition. Figure 9 - HR-TGA plots of oxidized soot weight loss (a) and derivative of soot weight loss (b) for KCS-1 coated cordierite samples exposed to 500°C hydrothermal treatments in air for periods of 2, 6, and 72 hours. ATR-FTIR spectra of KCS-1 polished glass slices (c) as made and (d) after 2, 6, 18, and 72 hour periods of exposure. Figure 10 - Multiple TGA soot oxidation cycles post single hydrothermal treatment at (a) 200°C and (b) 300°C. Figure 11 - TGA data for multiple hydrothermal treatments with and without pre-applied soot at (a) 200°C and (b) 300°C.
Table 1 - EDS measurements of 300-700°C hydrothermally treated KCS-1 glass under flowing air.
Figure-1
Figure-2
Figure-3
Figure-4
Figure-5
Figure-6
Figure-7
Figure-8
Figure-9
Figure-10
Figure-11
Table 1 - EDS measurements of 300-700°C hydrothermally treated KCS-1 glass under flowing air.
Hydrothermal Treatment Temperature (°C) 2
1 mm EDS Rectangular Spectra
C
O
Si
K
Ca
K/Ca Ratio
25(As Made)
0
67.5
16.4
11.8
4.22
2.80
300
29.7
32.4
14.7
18.3
4.95
3.69
500 600
15.6 11.3
39.3 42.2
18.1 19.1
20.7 20.8
6.33 6.56
3.27 3.18
700
11.7
42.2
18.7
20.5
6.98
2.93
300
31.5 ± 5.06
24.7 ± 1.28
15.3 ± 3.15
23.2 ± 2.44
5.21 ± 1.23
4.46 ± 1.15
500 600
19.7 ± 9.07 15.1 ± 5.84
41.6 ± 7.45 41.8 ± 4.04
7.52 ± 6.42 15.2 ± 2.93
28.9 ± 9.72 21.2 ± 2.40
2.32 ± 2.27 6.65 ± 1.20
12.4 ± 12.9 3.19 ± 0.680
700
12.3 ± 6.36
40.3 ± 3.72
18.9 ± 2.03
21.4 ± 1.78
7.12 ± 0.800
3.00 ± 0.420
EDS Point Spectra Avg. of Particulates
EDS Point Spectra Avg. of Flat Glass 300
10.1 ± 4.36
43.9 ± 2.15
20.5 ± 1.16
18.5 ± 0.691
7.02 ± 0.446
2.63 ± 0.194
500 600
0 1.69 ± 2.89
47.4 ± 0.606 46.1 ± 1.26
23.2 ± 0.257 22.6 ± 0.631
21.3 ± 0.470 21.7 ± 0.810
8.00 ± 0.160 7.88 ± 0.280
2.67 ± 0.080 2.75 ± 0.140
700
0
48.3 ± 0.570
22.7 ± 0.198
20.6 ± 0.390
8.46 ± 0.190
2.43 ± 0.070
Highlights • • • •
A glass catalyst had a T50 of 500°C after oxidizing 105 mi of equivalent soot mass K-deficient catalyst regions were found after extended continuous soot oxidation Hydrothermal exposure created surface precipitates of K2CO3 and CaCO3. Presence of soot during hydrothermal treatment reduced catalytic deactivation.
S1385-8947(14)01271-6 http://dx.doi.org/10.1016/j.cej.2014.09.075 CEJ 12693
To appear in:
Chemical Engineering Journal
Received Date: Accepted Date:
22 August 2014 20 September 2014
Please cite this article as: J. Zokoe, P.J. McGinn, Catalytic Diesel Soot Oxidation by Hydrothermally Stable Glass Catalysts, Chemical Engineering Journal (2014), doi: http://dx.doi.org/10.1016/j.cej.2014.09.075
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Catalytic Diesel Soot Oxidation by Hydrothermally Stable Glass Catalysts James Zokoe and Paul J. McGinn Department of Chemical and Biomolecular Engineering University of Notre Dame, Notre Dame, IN 46556 email addresses:
[email protected]
[email protected] Corresponding Author: Paul McGinn: 182 Fitzpatrick, Department of Chemical and Biomolecular Engineering, University of Notre Dame, Notre Dame, IN 46556. [email protected], (574) 631-6151 Keywords: soot oxidation, potassium catalyst, glass catalyst, cordierite filter, diesel particulate filter
Abstract: K-Ca-Si glass catalyst coated cordierite was exposed to extended continuous soot oxidation testing in a bench top reactor utilizing flame soot deposition. These samples maintained a T50 temperature of 500°C at the end of an estimated 100,000 mi equivalent continuous soot oxidation testing in a low humidity environment. High temperature (500-700°C) hydrothermal exposure led to potassium and calcium carbonate formation on the glass catalyst surface which degraded oxidation activity with increasing hydrothermal temperature up to 700°C due to reduced potassium surface mobility. The presence of a soot layer during hydrothermal exposure shielded the glass surface from extensive deactivation, suggesting glasses as promising catalysts for extended DPF use.
Graphical Abstract:
1.
Introduction Increasing statutory limitations on diesel exhaust emissions have forced engine
manufacturers to pursue a range of technologies to comply with the new standards, including reducing emission of carbonaceous soot particles. A diesel particulate filter (DPF) is usually used to remove soot from the exhaust stream.
Over time soot accumulates in the filter and must be
removed to avoid a large pressure drop in the exhaust stream. Soot removal, or regeneration of the filter, can be done actively by raising the temperature sufficiently high for the carbon to oxidize into CO2 (550-600°C). Regeneration can also be accomplished passively by changing the engine operating conditions to raise the amount of nitrogen dioxide (NO2) which aids carbon oxidation and/or by lowering the required temperature (250-500°C) through the use of a catalyst so that oxidation can proceed at normal exhaust temperatures. Catalysts used to lower the soot oxidation temperature often include platinum which substantially increases the price of the emissions abatement system. A platinum replacement is desired to lower the cost of an emissions reduction unit while maintaining low soot oxidation temperatures. Catalysts based on potassium can satisfy both of those criteria. However, alkali based catalysts are not yet commercially viable as a long life catalyst in diesel applications, as they have been unable to maintain activity over multiple combustion cycles or temperature excursions above 600°C [1, 2]. The high activity of K in loose physical contact has been explained by the “tunneling” ability of K as a mobile oxide in which the catalyst bores into a carbon mass by oxidizing along the edges of graphitic basal planes of individual soot particles within aggregates [3]. Many groups have reported on the rapid degradation of potassium based catalysts and have attributed the loss of activity to the same high
mobility of the potassium ions, which can experience sublimation during the oxidation process [3, 4]. One novel approach that has been developed uses a potassium containing glass as a catalyst, in which K+ ions are present within a silicate matrix. With exposure to water vapor, ion exchange of H+ and potassium ions can occur, promoting migration of K+ to the glass catalyst surface and providing a continuous supply of potassium for long term catalytic soot combustion. By optimizing the glass composition, soot combustion temperatures in the 375-400°C range have been achieved [5]. However, a glass catalyst, as with any DPF catalyst, must survive the harsh conditions of the diesel exhaust environment for extended periods of time. This means the glass must have appropriate resistance to elevated temperatures, structural changes caused by hydrothermal exposure, ash buildup, sulfur, etc. Few studies have investigated the longevity of K-based catalysts. Two studies reported multi cycle TGA measurements as a means for quantifying combustion durability but no durability testing was reported for longer than 6 cycles [1, 6] until our recently published work on K-glass coated metal substrates, that showed minimal catalytic degradation over 30 cycles [7]. The need for studies of hydrothermal durability was stated in a recent review, where it was noted that, “despite the attempts to prepare stable alkaline metalcontaining soot combustion catalysts, the hydrothermal stability remains a major problem awaiting solution for a practical use of alkaline metals in soot combustion catalysts formulations.” [8] Here we report on the formation of phases on the surface of a catalytic glass subjected to hydrothermal conditions similar to that found in a diesel exhaust stream. Additionally, the effect
of the presence of soot on the surface is studied, as it may alter the interaction of the glass with the environment by consuming K ions that would otherwise accumulate at the surface [9-11].
2. Experimental methods Cordierite DPF substrates were coated with a potassium-rich catalytic glass film and tested for activity and long term chemical and physical durability. In previous testing glass powder synthesized through traditional melt processes with the composition 52 wt% SiO2, 35 wt% K2O, and 13 wt% CaO (~At%: 47% Si, 40% K, 13% Ca respectively) was shown to be promising as a K-glass catalyst [12]. This composition will be referred to as KCS-1. For sol gel synthesis of this glass composition, tetraethylorthosilicate (TEOS) (Si(OC2H5)4, 98%), calcium nitrate tetrahydrate (Ca(NO3)2.4H2O, ACS reagent) and potassium nitrate (KNO3, ACS reagent) were used as starting materials, following a well-known approach [13]. TEOS was dissolved in ethanol. Calcium nitrate and potassium nitrate were dissolved in DI water and added to the TEOS-ethanol mixture. 10 cP was chosen as the coating viscosity for cordierite filter samples which produced a thin glass film of ~2 µm. The films are thin enough that when coated onto a cordierite filter they do not fill pores and cause a negligible pressure drop along the filter. [14] The drying and coating conditions for the cordierite filter slices are shown schematically in Figure 1a along with images of uncoated (Fig. 1b) and KCS-1 coated (Fig. 1c) cordierite samples. Flow through cordierite filter slices (8 x 8 x 3 mm3 ) with 1 mm channels were dip coated with the KCS-1 sol following the process depicted in Figure 1a. This sample size and mass is compatible with the TGA for catalytic activity characterization. Samples were submerged in the sol and a vacuum was applied to degas and impregnate the inner pore structure of the cordierite.
The excess sol was subsequently drained and blown off the filter slices and the samples were then dried and aged. High temperature burnout of the precursors was conducted in air, with the samples heated to 650°C, held for 10 min hold to ensure complete removal of nitrates from the glass, and then furnace cooled to room temperature. A consistent 5 wt% catalyst loading was achieved through this method. It can be seen from Figure 1 (c) that the KCS-1 applied film is not continuous. The drying conditions were chosen on the basis of maintaining a reasonable timeframe for sample preparation to achieve a workable catalyst film. Crack-free films are possible through the addition of polymer additives and slower heating parameters [7], but are unnecessary for this research focus (i.e. adhesion of the film is not affected). Flat glass slices of KCS-1 composition were used for scanning electron microscopy and energy dispersive X-ray spectroscopy (SEM/EDS) surface characterization. These were made by melting a stoichiometric mixture of carbonate powders as described previously [9, 12]. 8 x 5 x 3 mm3 (L x W x H) rectangular slices were subsequently cut from the cooled bulk glass. The slices were then dry polished without a polishing medium (i.e. water, oil, etc.) with SiC (180, 600 grit) and alumina (5, 0.5 µm) pads to a scratch-free finish. Water was avoided to circumvent alkali leaching from the surface during polishing. Structural and chemical changes near the surface (~1-2 µm) were characterized by SEMEDS and attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR). SEM-EDS analysis was performed on a Carl Zeiss LEO EVO-50 with an Oxford INCA energy dispersive spectrometer. ATR-FTIR was performed on a Bruker Tensor 27 FTIR with a platinum single reflection diamond ATR module.
Catalytic activity was characterized by the soot ignition temperature (Tig) and the 50% soot conversion temperature (T50) measured by a TA Instruments 2950 high resolution thermogravimetric analyzer (HR-TGA). The TGA was programmed to slow the heating ramp rate from 20°C/min to 2°C/min when the onset of weight loss was detected. A mixed diesel exhaust gas analogue of composition 10% O2, 5% CO2, 3% H2O with balance N2 was used. Any activity degradation, whether it is due to hydrothermal surface modification or a loss of active potassium, will be detected as an increase in the soot oxidation temperature. To closely mimic the real conditions experienced by a DPF on a diesel engine, “loose” soot contact achieved by flame soot deposition was utilized for all tested samples [15]. KLEAROL® white mineral oil with ~1 ppm sulfur was used as the fuel oil for soot generation. This fuel was chosen to mitigate any degradation effects that might result from sulfate formation. To test long-term soot oxidation durability, a homemade testing stand was developed to utilize the flame soot deposition method. Figure 2 is a schematic of the extended continuous soot oxidation (ECSO) testing unit. The ECSO unit is composed of a stainless steel cylindrical sample holder with a picture frame type cover plate (Fig. 2a). Flow channels drilled through the sample holder allow for soot deposition through the TGA samples without excessive surface accumulation. The sample holder is positioned 25.4 cm above the flame. The stainless steel holder heats the substrates to the soot deposition-oxidation balance point temperature for sustained soot combustion (400500°C). The samples were open to laboratory atmosphere without additional H2O vapor since a quantitative means for supplying H2O into the flame environment was not feasible. Figure 2b shows the overall set up for the ECSO unit. Samples were exposed to continuous soot deposition and oxidation, slightly above the balance point, for periods of up to 24 hours without significant
fluctuation in the soot deposition rate. Catalytic activity was characterized after each ECSO testing period by TGA. Soot deposition for TGA characterization was performed in the ECSO apparatus at 150°C for 20 minutes to yield a catalyst to soot ratio of 10:1. The soot deposition rate was determined for each ECSO experiment from the mass of soot oxidized during the TGA run after each prolonged oxidation period and from the soot deposition time for that run (i.e. soot oxidized/soot deposition time of the TGA test). Based on estimates from larger scale bench top reactors, the total amount of soot necessary for 100,000 mi of equivalent lifetime on a TGA slice was determined to be ~93 mg for the 8 x 8 x 3 mm3 TGA sample size. This calculation was based on conversations with industrial collaborators concerning soot output for an average light duty diesel engine over this mileage. At an average soot deposition rate of 0.005 mg/min in the ECSO system, 100,000 mi equivalent lifetime testing requires ~300 hours of flame soot exposure. Hydrothermal (chemical) degradation was studied on KCS-1 samples exposed to humidified gas at temperatures between 500-700°C. Flowing air or N2 (120 mL/min) was bubbled through a heated water bath (40°C) to accumulate ~7% H2O vapor before it was fed into a quartz tube furnace which housed the filter samples. The 7% H2O content was chosen to approximate the diesel exhaust environment. The line connecting the water bubbler and furnace was heated above 40°C with heating tape to assure no condensation occurred upstream of the samples. Hydrothermal tests were conducted for 2 hours before the catalytic activity was characterized by TGA. In a real diesel exhaust application, a particulate filter will usually contain some amount of soot before regeneration is performed, unless the exhaust temperature is high enough to allow for passive regeneration. Under urban driving conditions lower exhaust temperatures (200-
350°C) are expected, and thus, an applied catalyst would experience hydrothermal conditions in the presence of soot. The presence of soot on the glass may change the effect of the hydrothermal environment on the glass. To test this condition, filter samples were coated with soot by flame soot deposition and then exposed to the hydrothermal testing previously described. Samples both with and without pre-applied flame soot were used for comparison. Samples with pre-deposited soot did not receive additional soot application after the hydrothermal treatment.
3. Results and Discussion The KCS-1 glass composition, either as a sol-gel glass coating on cordierite filter slices or as bulk melt glass slices, served as the catalyst baseline testing composition. The use of flame soot deposition creates a loose contact condition that mimics the poor contact between an actual DPF and exhaust deposited soot. TGA measured activity of the KCS-1 on cordierite, in these flame soot contact conditions, yields a Tig of 390°C and T50 of 410°C with typical sample batch variability of within ±5°C [5]. KCS-1 catalyst shows a 200°C decrease in oxidation temperature compared to oxidation on uncoated cordierite. At these temperatures, partial passive soot oxidation might occur under extra-urban or highway driving conditions. Oxidation temperatures in the sub 400°C range are typical of alkali and specifically K-based catalysts [10, 11] but maintaining these temperatures has previously been a challenge. Thus prolonged soot oxidation studies were carried out on the KCS-1 coated cordierite filter slices to test the catalyst’s combustion durability.
3.1 Extended Continuous Soot Oxidation Study of KCS-1 Coated Cordierite
The ECSO testing was designed to decrease the overall time needed to assess long term oxidation stability without performing hundreds of TGA cycles. The ECSO testing differs from TGA cycling in that the sample is held at a constant temperature for an extended period of time, but one that is lower than the maximum temperature in a TGA cycle. Figure 3 shows the TGA results measured periodically during ECSO testing of 4 KCS-1 coated cordierite samples with error bars indicating the standard deviation. An overall, fairly linear decrease in activity (i.e. increase in ignition temperature and T50) can be seen throughout the length of testing (Figure 3). The loss of activity was measured at 1.1°C/mg of oxidized soot for Tig and 0.99°C/mg for T50 based on a linear least squares fit. At 50% of estimated lifetime (i.e. 46 mg) the KCS-1 has a measured ignition temperature of 431°C; a rise of 55°C in ignition temperature. At 97 mg of soot combusted, or the end of estimated lifetime, the Tig and T50 temperatures have degraded to 475°C and 499°C respectively. This performance cannot be directly correlated to the real degradation conditions due to the lack of water vapor content. Testing is being performed in conjunction with industry collaborators to assess any differences with increased sample size and with prolonged combustion in the presence of water vapor and NOx. A recent study [16] has shown NOx to decrease the combustion temperatures of K-based soot oxidation by 30°C in tight contact conditions. Due to the absence of elevated H2O vapor, NOx, and high gas flow rate it is expected that the testing reported in this paper is a worst case environment for catalytic soot oxidation. In the absence of additional H2O vapor during oxidation, the loss of activity may be exacerbated due to surface depletion of K. A net loss of potassium at the glass surface could arise if the rate of K loss due to sublimation were greater than the K+ ion diffusion through the bulk glass. In the dry combustion condition of our current ECSO testing, this depletion of active
surface potassium seems likely. To test this hypothesis a 24 hr 60°C hydrothermal treatment with 7% H2O (henceforth termed “humidity treatment”) was performed on the ECSO samples after 97 mg of oxidized soot. The presence of water vapor in the absence of soot should induce the leaching of K+ ions and replenish the surface with K. Subsequent TGA of these humidity treated samples produced a sustained decrease in oxidation temperatures over five consecutive TGA cycles as shown in Figure 4. The humidity treatment recovered a measured 23°C of activity in T50 and a sustained decrease in T50 of 14°C after five consecutive TGA cycles. This increased catalytic activity, however, was temporary. With subsequent ECSO runs (not shown), the combustion temperatures eventually returned to the pre-hydrothermal treatment trend. These data suggest there is ample K remaining in the glass which is available for soot oxidation and the dry combustion environment of the ECSO testing is depleting the glass surface of “active potassium” (i.e. potassium available for direct soot contact and oxidation). Regardless, the result of this extended soot oxidation testing indicates the KCS-1 glass catalyst maintains sufficient potassium content and durability in the combustion temperature range to sustain acceptable catalytic activity. The addition of a relatively small atomic percentage of Ca (13%) provides adequate resistance to the K+ ion exchange from the silicate matrix to withstand soot combustion over this estimated 100,000 mi catalyst lifetime. A polished KCS-1 bulk glass slice was exposed to an equivalent treatment for SEM/EDS study to visualize and qualitatively determine how the low temperature humidity treatment affects the glass to increase activity. The glass sample was polished to a scratch and defect free surface prior to exposure. Figure 5 (a) shows an SEM image of the KCS-1 glass sample surface after 24 hr 60°C humidity treatment in air with 7% H2O vapor.
The glass surface experienced growth of segregated alkali and alkaline precipitates. Access to water vapor causes rapid leaching of K and Ca ions from the glass through the aforementioned silicate hydration (6) and in the presence of CO2 and O2, carbonates are rapidly formed. ATR-FTIR and elemental mapping in SEM-EDS both confirm the formation of calcium and potassium carbonates. Figures 5b -5e show the respective EDS elemental maps (C, K, Ca, Si) of the resultant glass surface. Precipitates were found to be K-rich and Ca-rich carbonates. The actual elemental compositions of these precipitates could not be accurately determined due to signal contribution from the underlying glass surface. This extensive ion-exchange of K seen in the bulk KCS-1 glass is an intrinsic property of the glass and should be comparable in the KCS-1 coated cordierite samples. Figure 5 (f) depicts the resultant IR spectrum in which the singular peak for calcium carbonate present at 1381 cm-1 with obvious sharpening of the peaks at 708 and 870 cm-1 confirms mainly CaCO3 formation at this low temperature [17]. The underlying broad peaks for Si-O-Si and Si-O-X are still evident at 982 and 870 cm-1 indicating the surrounding flat glass surface has not endured extensive structural degradation [18]. This humidity treatment thus increases the activity of the glass catalyst by enriching the glass in K through the leaching of Ca which separates as the carbonate precipitate. This process likely also facilitates faster and more extensive subsequent K leaching due to the loss of the stabilizing element Ca. Microstructural analysis of the KCS-1 coated cordierite glass surface was performed by utilizing SEM-EDS. Figure 6 depicts the SEM imaging of the KCS-1 coated cordierite surface after the full 97 mg soot of oxidized soot in ECSO testing. The KCS-1 glass coating developed small growths of tortuous potassium silicate structures on the edges of discrete glass regions. EDS analysis of these structures measured an average composition (in At%) of 64.4 ± 12.9% Si,
33.4 ± 14.2% K, 2.21 ± 2.04% Ca. The large variability in measured Si and K content suggest potassium is lost or diffuses along the silicate surface during sustained combustion. These silicate growths are infrequent on the sample surface however, as can be seen at lower magnification in Figure 6 (b) in which most of the catalyst surface is structurally unaffected. The presence of water vapor during sustained combustion will decrease the oxidation temperatures by increasing the mobility of K through the surface glass layers. It is unknown, however, whether the addition of water vapor during combustion yields an increased rate of K loss which could counteract the increase in catalytic activity. Testing has been performed by an industry partner on larger KCS-1 coated filters in a bench top reactor capable of testing in a more realistic diesel exhaust environment (i.e. high flow rate and water vapor inclusion). The results indicate that the presence of water vapor decreases the catalytic activity loss in comparison to the ECSO testing. 3.2 Hydrothermal Testing of Bare Glass Catalyst The harsh hydrothermal environment of diesel exhaust provides an additional means for catalytic degradation. To test the chemical durability of the KCS-1 glass composition, samples of cordierite were coated through the aforementioned sol-gel process and subsequently exposed to various hydrothermal environments. Figure 7 shows the results of TGA characterized soot oxidation (T50 temperature) after 2 hour exposures to temperatures ranging from 500-700°C under N2 and air, both with and without water vapor. In each of the 3 gas environments tested, there is a direct relationship between catalytic deactivation (increase in oxidation temperature) and hydrothermal exposure temperature. An increase in hydrothermal exposure temperature increases the extent of measured catalytic deactivation. There are minimal differences measured for the samples exposed to air and N2
with 7% H2O vapor for temperatures up to 600°C but there is a noticeable T50 difference of 17°C due to air exposure at 700°C compared to N2 with H2O vapor. One factor contributing to deactivation at 700oC may be enhanced surface crystallization kinetics at higher temperatures. [19] The enhanced nucleation is also seen in observation of glass surfaces (Fig.8, discussed below). For the samples tested under N2 without water vapor, this is the predominant degradation mechanism. For the case of N2 + H2O, there is the additional contributing deactivation caused by chemical degradation of the glass surface in the hydrothermal environment. Exposure to water vapor will further enhance creation of precipitates containing K+ ions (i.e. alkali carbonates or sulfates) [20, 21] that will further decrease the amount of active potassium at the surface. Finally, it was observed that hydrothermal exposure at 700°C in air resulted in an even larger decrease in activity relative to the similar testing condition in N2. In air, the presence of CO2 will facilitate carbonate formation on the surface, further reducing the total surface potassium available for soot oxidation and thus further decrease catalytic activity. To gain a better understanding of changes to the glass surface during high temperature hydrothermal degradation, samples of flat, polished KCS-1 glass were exposed to a range of temperatures in a hydrothermal environment and then examined in the SEM. The samples were held at temperatures of 300, 500, 600, and 700°C for two hours in flowing humidified air (7% H2O vapor, 120 mL/min). Figure 8 shows SEM images of the KCS-1 glass surfaces after the hydrothermal exposures. Smaller precipitates with increased surface coverage are observed at higher temperatures (Fig. 8c and d). EDS characterization through point and area (~1 mm2) analysis was performed at five or more particulate and flat glass points per sample. The data is tabulated in Table 1. It can be seen that the K/Ca ratio increases from an as-synthesized value of 2.80 to a
value of 3.69 at a treatment temperature of 300°C but decreases as the treatment temperature increases to a value of 2.93 after 700°C exposure. However, all values are higher than the asmade value, i.e. K-enriched. The K/Ca ratio of the as-synthesized glass was lower than the theoretical value of 3.07 likely due to K sublimation at the melt synthesis temperature of 1200°C. Measurements of the particulate growths and the surrounding flat regions reveal an increase in K content of the particulates and a corresponding localized decrease of K in adjacent regions which exhibit K/Ca ratios ranging from 2.43 to 2.75. At temperatures above 300C, carbon was not detected in any appreciable amount in the flat regions near particulates, suggesting the accumulations are a mixture of K and Ca carbonates formed from nearby carbon sources. The Si detected in the point measurements of the particulates is likely from the underlying glass. At elevated temperatures (i.e. exhaust temperature range) the molecular bonding of the non-bridging oxygen atoms within the Si matrix is weaker which allows for an increase in chemical degradation, allowing for an increase in the K content on the surface. In the presence of CO2, the surface potassium readily forms K2CO3 [22-24]. At temperatures near or exceeding the Ts there appears to be a change in the precipitation mechanism. In Figure 8 it can be seen that surface coverage by the precipitates increases while the overall size of the precipitates declines with increasing temperature. The glass surface in this temperature region may soften, with rearrangement of the Si-O-Si structure facilitating easier movement of alkali ions. This seems to aid the nucleation and precipitation of smaller and more numerous particulates. These surface accumulations of K compounds decrease the number of active K sites by concentrating the K within the precipitates and lessening opportunities for soot contact. The potassium rich precipitates develop by drawing surface potassium from the surrounding areas, depleting the catalytic activity of the regions around the precipitates. At elevated temperature
and with access to abundant CO2, the formation of carbonates would greatly increase and thus intensify the segregation of these depleted and enriched zones. The separation of potassium content could give rise to a non-uniform oxidation of soot as seen in HR-TGA. To better understand the impact of precipitate formation, a sample of KCS-1 coated cordierite was exposed to extended 500°C hydrothermal treatments in air for 2, 6, and 72 hours. A hydrothermal temperature of 500°C was chosen for analysis to correspond with the upper range of ECSO testing temperatures. After each hydrothermal exposure the change in activity was characterized by HR-TGA. The soot weight loss results are shown in Figure 9a. During the first 2 and 6 hours of hydrothermal testing the sample undergoes activity degradation as measured by an increase in T50 of 4 and 7°C respectively. As seen in Figure 9 (b), there is an increase in the rate of soot oxidation with these exposures from an initial 2.5 %/°C to 3.8 %/°C after 2 hours of exposure. It is likely the final high temperature burnout of the sol-gel process creates a poor surface for K ion exchange (i.e. coarsening of the outer surface through sintering) which undergoes a surface restructuring and replenishment of catalyst from ion exchange during the hydrothermal treatment. After 72 hours of hydrothermal exposure there is a clear development of a secondary combustion mechanism occurring which can be seen as the large shoulder formation after the initial oxidation peak in the derivative of weight loss curve in Figure 9 (b). The shoulder arises from the K enrichment of carbonate particulates and the associated depletion of the surrounding glass surface. Areas with less available K (lower K/Ca ratios) lead to a shift in combustion to higher temperatures since more energy is required to initiate oxidation. This surface restructuring, both chemical and structural, is the cause of the activity degradation through the aforementioned loss of catalyst and soot contact.
Similar samples of polished KCS-1 glass were exposed to equivalent 500°C hydrothermal conditions and analyzed by ATR-FTIR spectroscopy as seen in Figures 9 (c) and (d). ATR-FTIR spectroscopy of the as-made polished KCS-1 glass (Figure 9 (c)) revealed peaks at 735, 872, and 991 cm-1. The peaks at 735 and 991 cm-1 represent the symmetric and asymmetric Si-O-Si stretching respectively while the peak at 872 cm-1 corresponds to the nonbridging Si-O-X stretching with X representing K or Ca [17, 18]. Figure 9 (d) shows the IR spectra for KCS-1 glass samples exposed to extended hydrothermal conditions for 2, 6, and 72 hours. Characteristic bands for calcium and potassium carbonate seen at 1350-1450 cm-1 develop with increasing exposure time. The development and sharpening of the peaks at 710 and 880 cm-1 are also indicative of carbonate formation. Dual peak formation in the 1350-1450 cm-1 region was not previously seen in the potash glass corrosion study by Vilarigues and da Silva [18]. To clarify these results, a range of varying K-Ca mixtures of ground KCS-1 glass, potassium carbonate, and calcium carbonate powders (10% total carbonates by weight) were prepared and mixed by mortar and pestle and characterized by ATR-FTIR for comparison. The dual peak formation in the carbonate wavenumber region was seen only in samples where K2CO3 was present. The existence of these two peaks in the 500°C hydrothermally treated sample in Figure 9 (d) and the large K/Ca ratio of the precipitates measured by EDS (Table 1) can thus allow characterizing the precipitates seen in Figure 8 (b) as a potassium dominated mixture of K2CO3 and CaCO3. 3.3 Hydrothermal Testing of Soot Covered Catalyst In an operating DPF, glass will not be covered by soot for only brief periods of time, such as immediately after a regeneration cycle. Between regeneration cycles during periods of low engine load it is expected a soot layer will be present and growing. In such a case, a layer of soot
might cover the glass catalyst, eliminating direct exposure of the glass to the diesel exhaust environment. Thus effects of a hydrothermal environment on soot-covered KCS-1 glass were also investigated. Lower level exhaust temperatures of 200°C and 300°C were chosen as subignition temperatures for testing. KCS-1 coated cordierite samples with and without soot before hydrothermal exposure were compared. The samples with no soot applied before the hydrothermal treatment were coated afterwards prior to TGA testing. Figure 10 shows the TGA results for multiple oxidation cycles after a single hydrothermal treatment. For both cases in which soot was present during the hydrothermal treatment there is a significant decrease in oxidation temperature compared with the non-soot covered sample. For the 300°C hydrothermal treatments, ~40% of the pre-applied soot was oxidized during the 2 hr treatment (i.e. a very low oxidation temperature) but no additional soot was applied before performing TGA. Additional applied soot would have likely resulted in more complex TGA oxidation curves due to differences in the contact conditions of the pre-applied and posthydrothermal soot. Both sets of samples showed a reduction in Tig and T50 from the as-prepared state (treatment #0). The increase in activity of the samples without pre-applied soot suggests the glass experienced an enrichment of surface potassium in the mild hydrothermal conditions. Further TGA testing, however, shows that the effect is limited to a single combustion with TGA runs 2 and 3 returning to pre-hydrothermal treatment temperatures (Tig ~ 390°C) in both 200°C (Fig. 10a) and 300°C (Fig. 10b) tests. The 200°C and 300°C samples with pre-applied soot experienced a decrease in both Tig and T50 of 40°C and 30°C respectively from the as-prepared state. This corresponds to 10°C further reduction of Tig and T50 compared to the KCS-1 samples which had no pre-applied soot.
To determine the longevity of the observed decreased soot oxidation temperatures, multiple hydrothermal treatments at 200°C and 300°C with and without pre-applied soot were also performed. A single TGA cycle was used to measure the catalytic activity after each hydrothermal treatment. Soot was reapplied before each subsequent hydrothermal run for the “pre-applied” samples. Figure 11 shows the TGA results of 5 hydrothermal cycles of cordierite samples coated with KCS-1 glass with and without pre-applied soot at both 200°C (Fig. 11a) and 300°C (Fig. 11b). As before, hydrothermal treatment #0 in the plot corresponds to the average measured Tig and T50 for as made KCS-1 coated cordierite samples (i.e. before any treatment). In both the 200°C and 300°C testing with soot present, there is a sustained decrease in oxidation temperatures compared to the samples without applied soot. The initial decrease in oxidation temperatures for the samples without soot is lost after the second hydrothermal treatment for both temperatures tested. The presence of soot, however, yields a sustained increase in catalytic activity (or decrease in activity degradation) with repeated 200°C hydrothermal exposures and only a 10°C increase in combustion temperatures from the first to fifth 300°C hydrothermal treatment (Tig from 358°C to 368°C respectively). The measured oxidation temperatures for the 300°C testing may be slightly elevated due to the soot oxidation that occurs during the hydrothermal test period. Soot which is in tightest physical contact will oxidize at the lowest temperature, with the less optimally contacted soot remaining. The increase in catalytic activity can likely be attributed to increased physical soot-potassium contact through the chemical degradation of the glass catalyst. Since the soot is already in contact with the glass before the introduction of the hydrothermal environment, the potassium which is pulled to the surface through ion exchange can migrate into the near surface soot cake. This increases the total soot-
catalyst contact surface area and thus increases the catalytic activity. These results are encouraging in that they show the soot cake provides a sort of protective layer that shields the surface from direct exposure to the hydrothermal chemical degradation which was seen in the previous section. It is likely that the thicker the soot cake, the lower the near surface gas flow velocity will be and thus less mass flux of water can reach the surface: depressing the chemical degradation. In a real DPF application, this would increase the lifetime of the catalyst and ultimately decrease the necessary regeneration temperature while increasing the amount of passively oxidized soot between regenerations (when the exhaust reaches over 350°C). 6. Conclusions A potassium-rich K-Ca-Si soot oxidation catalyst with long term stability can be realized by implementing a slow release glass catalyst coating on a DPF.
The replenishment of active
potassium counteracts catalytic deactivation that might occur through potassium sublimation. Continuous soot oxidation testing over an estimated 100,000 mi equivalent in a low humidity environment causes a slow rise in the T50 from approximately 400°C to 500°C. Improved activity can be temporarily restored by a low temperature humidification treatment. High temperature (500-700°C) hydrothermal exposure of causes more rapid activity degradation due to carbonate formation on the catalyst surface. The presence of a soot layer during hydrothermal exposure protects the glass surface, thereby reducing activity degradation, making glass catalysts a promising technology for extended DPF use. 7. Acknowledgements This work was partially supported by Notre Dame Integrated Imaging Facility (NDIIF) and the Center for Environmental Science and Technology (CEST) Bayer Fellowship (JZ). We are also
grateful to Professor Joan Brennecke for use of a viscometer. We are grateful to CEST for use of the ATR-FTIR.
References
[1] F.E. Lopez-Suarez, A. Bueno-Lopez, M.J. Illan-Gomez, B. Ura, J. Trawczynski, Potassium Stability in Soot Combustion Perovskite Catalysts, Topics in Catalysis, 52 (2009) 2097-2100. [2] C.A. Neyertz, E.E. Miro, C.A. Querini, K/CeO2 catalysts supported on cordierite monoliths: Diesel soot combustion study, Chem. Eng. J., 181 (2012) 93-102. [3] D.W. McKee, D. Chatterji, Catalytic Behavior Of Alkali-Metal Carbonates And Oxides In Graphite Oxidation Reactions, Carbon, 13 (1975) 381-390. [4] D.W. McKee, Gasification Of Graphite In Carbon-Dioxide And Water-Vapor - The Catalytic Effects Of Alkali-Metal Salts, Carbon, 20 (1982) 59-66. [5] C. Su, P.J. McGinn, The effect of Ca2+ and Al3+ additions on the stability of potassium disilicate glass as a soot oxidation catalyst, Applied Catalysis B: Environmental, 138–139 (2013) 70-78. [6] G. Pecchi, B. Cabrera, A. Buljan, E.J. Delgado, A.L. Gordon, R. Jimenez, Catalytic oxidation of soot over alkaline niobates, J. Alloy. Compd., 551 (2013) 255-261. [7] C.S. Su, P.J. McGinn, Application of glass soot catalysts on metal supports to achieve low soot oxidation temperature, Catal. Commun., 43 (2014) 1-5. [8] A.M. Hernandez-Gimenez, D.L. Castello, A. Bueno-Lopez, Diesel soot combustion catalysts: review of active phases, Chem. Pap., 68 (2014) 1154-1168. [9] H.M. An, C. Kilroy, P.J. McGinn, Combinatorial synthesis and characterization of alkali metal doped oxides for diesel soot combustion, Catal. Today, 98 (2004) 423. [10] T. Miyazaki, N. Tokubuchi, M. Arita, M. Inoue, I. Mochida, Catalytic combustion of carbon by alkali metal carbonates supported on perovskite-type oxide, Energy Fuels, 11 (1997) 832-836.
[11] T. Miyazaki, N. Tokubuchi, M. Inoue, M. Arita, I. Mochida, Catalytic activities of K2CO3 supported on several oxides for carbon combustion, Energy Fuels, 12 (1998) 870-874. [12] H.M. An, C.S. Su, P.J. McGinn, Application of potash glass as a catalyst for diesel soot oxidation, Catal. Commun., 10 (2009) 509-512. [13] P. Saravanapavan, L.L. Hench, Mesoporous calcium silicate glasses. I. Synthesis, J. NonCryst. Solids, 318 (2003) 1-13. [14] C.S. Su, Ph.D. Thesis, Stabilization of potassium in soot oxidation catalysts and their application on diesel particulate filters, in: Chemical & Biomolecular Engineering, University of Notre Dame, Notre Dame, IN, 2011. [15] C.S. Su, P.J. McGinn, The effect of Ca2+ and Al3+ additions on the stability of potassium disilicate glass as a soot oxidation catalyst, Appl. Catal. B-Environ., 138 (2013) 70-78. [16] P. Legutko, P. Stelmachowski, T. Trebala, Z. Sojka, A. Kotarba, Role of Electronic Factor in Soot Oxidation Process Over Tunnelled and Layered Potassium Iron Oxide Catalysts, Topics in Catalysis, 56 (2013) 489-492. [17] H.A. ElBatal, M.A. Azooz, E.M.A. Khalil, A.S. Monem, Y.M. Hamdy, Characterization of some bioglass-ceramics, Mater. Chem. Phys., 80 (2003) 599-609. [18] M. Vilarigues, R.C. da Silva, Characterization of potash-glass corrosion in aqueous solution by ion beam and IR spectroscopy, J. Non-Cryst. Solids, 352 (2006) 5368-5375. [19] R. Muller, E.D. Zanotto, V.M. Fokin, Surface crystallization of silicate glasses: nucleation sites and kinetics, J. Non-Cryst. Solids, 274 (2000) 208-231. [20] A. Paul, Chemical Durability of Glasses - Thermodynamic Approach, J. Mater. Sci., 12 (1977) 2246-2268.
[21] C.M. Jantzen, Thermodynamic Approach to Glass Corrosion, in: D.E. Clark, B.K. Zoitos (Eds.) Corrosion of Glass, Ceramics, and Ceramic Superconductors: principles, testing, characterization and applications, Noyes Publications, Park Ridge, NJ, 1992, pp. 153-217. [22] I.L.C. Freriks, H.M.H. Vanwechem, J.C.M. Stuiver, R. Bouwman, Potassium-Catalyzed Gasification Of Carbon With Steam - A Temperature-Programmed Desorption And FourierTransform Infrared Study, Fuel, 60 (1981) 463-470. [23] J.M. Saber, J.L. Falconer, L.F. Brown, Interaction Of Potassium Carbonate With Surface Oxides Of Carbon, Fuel, 65 (1986) 1356-1359. [24] B.J. Wood, R.H. Fleming, H. Wise, Reactive Intermediate in the Alkali-CarbonateCatalyzed Gasification of Coal Char, Fuel, 63 (1984) 1600-1603.
Figure Captions Figure 1 - (a) Sol gel procedure for the coating of cordierite filter slices. (b) SEM images of bare cordierite filter and (c) applied K-glass thin film on cordierite substrate. Figure 2 - Extended continuous soot oxidation (ECSO) bench top reactor. Figure 3 - HR-TGA Tig and T50 results for extended continuous soot oxidation (ECSO) treatment of KCS-1 coated cordierite samples. Figure 4 - HR-TGA weight loss curves of post-lifetime testing (97 mg soot combusted) KCS-1 coated cordierite showing activity regeneration after low temperature (60°C) humidification in air. Figure 5 - SEM image (a), EDS elemental mapping (b-e), and IR spectrum (f) of polished KCS-1 bulk glass slice after 24 hr exposure to 60°C hydrothermal treatment in air with 7% H2O vapor. Figure 6 - Scanning electron images of KCS-1 coated cordierite surface after extended continuous soot oxidation (ECSO) testing. Figure 7 - TGA measured soot oxidation T50 after 2 hours exposure to various hydrothermal environments. Separate samples of KCS-1 coated cordierite were used in each test. Figure 8 - SEM images of polished KCS-1 melt glass slices after 2 hour hydrothermal exposures under air at (a) 300°C and (b) 500°C (c) 600°C and (d) 700°C. Separate samples were used for each testing condition. Figure 9 - HR-TGA plots of oxidized soot weight loss (a) and derivative of soot weight loss (b) for KCS-1 coated cordierite samples exposed to 500°C hydrothermal treatments in air for periods of 2, 6, and 72 hours. ATR-FTIR spectra of KCS-1 polished glass slices (c) as made and (d) after 2, 6, 18, and 72 hour periods of exposure. Figure 10 - Multiple TGA soot oxidation cycles post single hydrothermal treatment at (a) 200°C and (b) 300°C. Figure 11 - TGA data for multiple hydrothermal treatments with and without pre-applied soot at (a) 200°C and (b) 300°C.
Table 1 - EDS measurements of 300-700°C hydrothermally treated KCS-1 glass under flowing air.
Figure-1
Figure-2
Figure-3
Figure-4
Figure-5
Figure-6
Figure-7
Figure-8
Figure-9
Figure-10
Figure-11
Table 1 - EDS measurements of 300-700°C hydrothermally treated KCS-1 glass under flowing air.
Hydrothermal Treatment Temperature (°C) 2
1 mm EDS Rectangular Spectra
C
O
Si
K
Ca
K/Ca Ratio
25(As Made)
0
67.5
16.4
11.8
4.22
2.80
300
29.7
32.4
14.7
18.3
4.95
3.69
500 600
15.6 11.3
39.3 42.2
18.1 19.1
20.7 20.8
6.33 6.56
3.27 3.18
700
11.7
42.2
18.7
20.5
6.98
2.93
300
31.5 ± 5.06
24.7 ± 1.28
15.3 ± 3.15
23.2 ± 2.44
5.21 ± 1.23
4.46 ± 1.15
500 600
19.7 ± 9.07 15.1 ± 5.84
41.6 ± 7.45 41.8 ± 4.04
7.52 ± 6.42 15.2 ± 2.93
28.9 ± 9.72 21.2 ± 2.40
2.32 ± 2.27 6.65 ± 1.20
12.4 ± 12.9 3.19 ± 0.680
700
12.3 ± 6.36
40.3 ± 3.72
18.9 ± 2.03
21.4 ± 1.78
7.12 ± 0.800
3.00 ± 0.420
EDS Point Spectra Avg. of Particulates
EDS Point Spectra Avg. of Flat Glass 300
10.1 ± 4.36
43.9 ± 2.15
20.5 ± 1.16
18.5 ± 0.691
7.02 ± 0.446
2.63 ± 0.194
500 600
0 1.69 ± 2.89
47.4 ± 0.606 46.1 ± 1.26
23.2 ± 0.257 22.6 ± 0.631
21.3 ± 0.470 21.7 ± 0.810
8.00 ± 0.160 7.88 ± 0.280
2.67 ± 0.080 2.75 ± 0.140
700
0
48.3 ± 0.570
22.7 ± 0.198
20.6 ± 0.390
8.46 ± 0.190
2.43 ± 0.070
Highlights • • • •
A glass catalyst had a T50 of 500°C after oxidizing 105 mi of equivalent soot mass K-deficient catalyst regions were found after extended continuous soot oxidation Hydrothermal exposure created surface precipitates of K2CO3 and CaCO3. Presence of soot during hydrothermal treatment reduced catalytic deactivation.