Effect of H2O on the Morphological Changes of KNO3 Formed on K2O

Feb 6, 2014 - reduction, aerodynamic design, and electrification of the powertrain. Although lean burn engines provide higher fuel efficiency, there r...
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Effect of H2O on the Morphological Changes of KNO3 Formed on K2O/Al2O3 NOx Storage Materials: Fourier Transform Infrared and Time-Resolved X‑ray Diffraction Studies Do Heui Kim,*,† Kumudu Mudiyanselage,‡ János Szanyi,‡ Jonathan C. Hanson,§ and Charles H. F. Peden‡ †

School of Chemical and Biological Engineering, Institute of Chemical Processes, Seoul National University, 1 Gwanak-ro, Gwanak-gu, Seoul 151-742, Republic of Korea ‡ Institute for Integrated Catalysis, Pacific Northwest National Laboratory, Richland, Washington 99354, United States § Chemistry Department, Brookhaven National Laboratory, Upton, New York 11973, United States ABSTRACT: Based on the combined FTIR and XRD studies, we report here that H2O induces a morphological change of KNO3 species formed on model K2O/Al2O3 NOx storage-reduction catalysts. Specifically as evidenced by FTIR, the contact of H2O with NO2 preadsorbed on K2O/Al2O3 promotes the transformation from bidentate (surface-like) KNO3 species to ionic (bulk-like) ones irrespective of K loadings. Once H2O is removed from the sample, a reversible transformation into bidentate KNO3 is observed, demonstrating a significant dependence of H2O on such morphological change. TR-XRD results show the formation of two different types of bulk KNO3 phases (orthorhomobic and rhombohedral) in an as-impregnated sample. Once H2O begins to desorb above 400 K, the former is transformed into the latter, resulting in the existence of rhombohedral KNO3 phase only. On the basis of consistent FTIR and TR-XRD results, we propose a model for the morphological changes of KNO3 species with respect to NO2 adsorption/desorption, H2O and/or heat treatments. Compared with the BaO/Al2O3 system, K2O/Al2O3 shows some similarities with respect to the formation of bulk nitrates upon H2O contact. However, there are significant differences that originate from the lower melting temperature of KNO3 relative to Ba(NO3)2.

1. INTRODUCTION For the purpose of reducing use of fossil fuels and the corresponding reduction of CO2 emissions, automotive manufacturers are focused on the development of technologies for improving fuel efficiency. Lean burn combustion engines are one of the possible solutions, in addition to vehicle mass reduction, aerodynamic design, and electrification of the powertrain. Although lean burn engines provide higher fuel efficiency, there remains a challenging task to remove nitrogen oxides (NOx) emitted from the exhaust due to the difficulty of reducing NOx by using the conventional three-way catalysts under these engine’s highly oxidizing conditions. Furthermore, NOx emissions will be regulated more strictly in the near future. Among technologies that remove NOx from fuel-efficient lean burn engines, NOx storage-reduction catalysts (NSR), also known as lean NOx traps (LNT), are one of the most effective solutions for the removal of NOx under highly oxidizing (lean) conditions.1,2 NSR catalysts include three crucial components; platinum group metals (Pt or Pd), an alkali (K) or alkaline earth oxide (Ba) storage element, and a catalyst support material (Al2O3, MgAl2O4, or CeO2).3 In NSR catalysts, platinum group metals oxidize NO to NO2 during the lean phase of operation. The formed NO2 is subsequently stored as nitrate on the neighboring storage element. Stored nitrates are then reduced © 2014 American Chemical Society

during a short rich period by reacting with pulses of reductants. Thus, a typical NSR system operates in a cycled mode with alternating lean and rich phases. K and Ba have been the most commonly used storage elements in NSR catalyst applications. Ba has been applied for storing NOx at moderate temperatures (523−623 K), while K has been utilized for higher temperature applications (673−773 K). Due to the typically low temperature characteristics of exhaust from diesel engines, Ba has been studied more extensively and commercialized for mobile (vehicle) applications. Meanwhile, because of more recent developments of lean burn gasoline engine technologies including lean burn gasoline direct injection (GDI), with exhaust temperatures considerably higher than those of diesel engines, K is gaining more interest as the storage element. In addition to several reports on the NOx storage performance of Pt−K2O supported NSR catalysts with an emphasis on their high temperature activity,4−7 fundamental studies of nitrate formation and decomposition on these catalysts have also been reported. Toops et al. have used an in situ DRIFT method to follow nitrate formation on Pt−K2O/Al2O3 catalysts Received: November 3, 2013 Revised: February 5, 2014 Published: February 6, 2014 4189

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in the presence of H2O and CO2.8,9 Forzatti and co-workers10,11 observed the formation of two types of nitrate species (bidentate and ionic nitrates), and examined the reactivity of these species with reductants such as CO and H2. Because H2O is always present in the exhaust of internal combustion engines (ICEs), the effects of H2O on Ba species in BaO-based NSR catalysts have been extensively studied. An especially interesting finding is that BaAl2O4, which forms by a solid state reaction between BaO and Al2O3 at high (>1073 K) temperatures, is decomposed to form BaCO3 by contact with CO32‑ ions dissolved in H2O.12,13 Similarly, the contact of H2O with NO2-preadsorbed BaAl2O4 induces the transformation from BaAl2O4 into a highly crystalline Ba(NO3)2 phase, thus providing a way to regenerate NSR catalysts that were thermally deactivated as a result of BaAl2O4 formation.14 Furthermore, on the basis of combined in situ FTIR and TR-XRD studies of BaO/Al2O3 samples, it was revealed that in the presence of H2O surface-type (bidendate) barium nitrates convert to bulk (ionic) barium nitrates, thus facilitating the formation of large Ba(NO3)2 crystallites.15 In other words, H2O affects the morphological properties of barium species in various and significant ways. Recent reports on similar behavior of Ba and K with respect to their nitrate species (two types of nitrates species as a function of loading)16 have motivated us to investigate morphological changes of KNO3 formed on Al2O3 support materials species as a function of H2O exposure. To this end, we prepared several K2O/Al2O3 samples and adsorbed NO2 to promote K-nitrate formation, followed by exposure of H2O and/or thermal heat. In this contribution we report the physicochemical changes of these materials monitored by FTIR, TR-XRD, and NO2 TPD measurements. Our findings allowed us to suggest a model for the morphological changes of K species during these treatments.

Temperature programmed decomposition/desorption (TPD) experiments were performed for a freshly impregnated and dried 20 wt % KNO3/Al2O3 sample in a fixed bed microcatalytic quartz reactor. First, temperature ramping was applied to the 100 mg sample at 10 K/min while flowing He gas (100 cm3/min). During the temperature ramp, the evolution of NO and NO2 were monitored with a chemiluminescence NOx analyzer (42C high level, Thermo Electron Co.). After fully decomposing nitrate species at 973 K, the samples (20 wt % K2O/Al2O3) were cooled down to room temperature and a gas mixture of 0.5% NO2/He (99.999% Purity, Matheson) was passed over the sample until the NO2 level returned to its input value (0.5%). All gas flows were metered by mass flow controllers (Brooks Co.). After saturation, the catalysts were purged with helium for 2 h to remove physisorbed NOx species, and then the temperature was raised to 973 K while measuring the evolution of both NO and NO2 under the same conditions as the first TPD experiment. The TR-XRD experiments were carried out on beamline X7B at the National Synchrotron Light Source (NSLS) at Brookhaven National Laboratory. The detailed experimental setup of the TR-XRD has been described elsewhere,17 and the procedures were essentially the same as those used for the TPD experiments. Briefly, a small amount of 20 wt % KNO3/Al2O3 sample was placed in a sapphire capillary tube and heated at 10 deg/min from 300 to 973 K while continuously flowing He gas. XRD patterns were collected in situ every 3 min during the temperature ramping. After decomposition of nitrate species at 973 K, the sample was cooled down to room temperature. While taking the XRD pattern, 500 ppm of NO2 in N2 was introduced to investigate morphology changes resulting from NO2 exposure. After saturation of NO2 at RT, the temperature was raised at 10 K/min to 1073 K while continuously obtaining in situ XRD patterns. In addition, conventional powder X-ray diffraction (XRD) data were collected on a Philips X’Pert MPD (model PW3040/00) instrument with a vertical 2θ goniometer (190 mm radius). The X-ray source was a long fine-focus and sealed ceramic X-ray tube (Cu anode) operated at 40 kV and 50 mA (2000 W). Detailed experimental and analysis conditions for these measurements are described elsewhere.12

2. EXPERIMENTAL SECTION 2.1. Preparation of Catalysts. The K2O/Al2O3 NOx storage materials studied here were prepared by traditional impregnation methods. KNO3 (Aldrich) was first dissolved in deionized water, and then the KNO3 solutions were applied to the γ-Al2O3 support materials (surface area of 200 m2/g; Condea) using incipient wetness. The resulting materials were dried and then calcined in a 10% O2/He gas flow at high enough temperature (973 K) for 2 h to decompose KNO3 to K2O (KNO3 melts and subsequently decomposes at 900 K). The samples used for the FTIR, XRD, and NO2 TPD experiments contained 2, 10, and 20 wt % of K2O on γ-Al2O3. 2.2. Characterization of Catalysts. The FTIR experiments were carried out in transmission mode, using a Bruker Vertex 80 spectrometer, and IR spectra were collected at 4 cm−1 resolution. Each recorded spectrum was the average of 256 scans and was referenced to a background spectrum obtained from the clean (adsorbate-free) sample. Powder samples were pressed onto a high-transmittance, fine-tungsten mesh that, in turn, was attached to a copper sample holder assembly mounted onto ceramic feed-throughs on a stainless steel rod. A K-type thermocouple was spot-welded to the top center of the W grid to monitor the sample temperature, and the samples were heated resistively. The IR cell was a 6-way stainless steel cube, attached to pumping and gas handling system. Further details of the FTIR experimental apparatus and procedures used here have been previously described in detail elsewhere. A mass spectrometer (MS, UTI 100C) was connected to the IR cell through both leak and gate valves. This setup allows us to monitor the gas-phase composition during adsorption and desorption experiments.

3. RESULTS According to previously published results,11 NO2 adsorption on 5.4 wt % K2O/Al2O3 samples gave rise to two different types of potassium nitrate species: ionic and chelating bidentate nitrates. The former were identified by FTIR spectra with absorption features at 1352 and 1397 cm−1, while the latter were evidenced by peaks at 1310 and 1575 cm−1. The formation of these two species depends primarily on the potassium loading since the ratio of ionic to bidentate nitrates gradually increases with increasing K loadings up to 5 wt %.18 This behavior is quite similar to that of barium, which also forms two types of nitrates upon NO2 adsorption as a function of barium loadings. For barium, the two types of nitrate species have been proposed to result from NO2 adsorption on highly dispersed barium oxide and particulate barium oxide, as evidenced by both density functional theory (DFT) calculations and experiments.19,20 The similarity of the FTIR experimental results after NO2 adsorption on potassium/alumina and barium/alumina materials as a function of loading suggests analogous morphological properties for these two storage elements. In the following, we use aluminasupported K-based NSR catalyst materials with varying Kloadings (K(x)/Al2O3; x = 2, 10, and 20) to study coadsorption effects of water and NO2 on the catalyst morphology. 4190

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Figure 1. FTIR spectra collected during stepwise H2O adsorption at 300 K to an NO2 preadsorbed K2O(2)/Al2O3 sample.

3.1. H2O Adsorption on NO2 Adsorbed K2O/Al2O3. The effect of water on the NOx species formed upon the exposure of K2O(2)/Al2O3 samples to NO2 at 300 K was studied first. In these experiments, the sample was saturated with NO2 and then evacuated for 10 min at 300 K before introducing the first H2O aliquot. Subsequently, water was added to the NO2-saturated sample stepwise, and an IR spectrum was taken after each dosing step. Consistent with prior results,15,21 the low-loading K2O(2)/ Al2O3 catalyst gave rise to two absorption peaks at 1310 and 1575 cm−1 upon NO2 saturation, indicating the formation of bidentate nitrates (black curve in Figure 1). The series of IR spectra collected during subsequent dosing of H2O are also presented in Figure 1. In the nitrate region of the IR spectra, the intensities of the 1575 cm−1 band decreased, while those at 1352 and 1397 cm−1 bands increased. It is interesting to note that the band at 1310 cm−1 still remains with the increment of H2O dose. According to the previous study about the H2O dose on NO2 preadsorbed γ-Al2O3 or BaO/γ-Al2O3, the peak at 1310 cm−1 is assigned to the water-solvated nitrate species formed on γAl2O3.15,22 Hence, the peak at 1572 cm−1 is used as the sole indicator about whether bidentate KNO3 species appear or disappear. Another noticeable change during H2O dose is evident in the spectral region of −OH bands. With increasing doses of H2O, a broad band due to hydroxyl groups at 3246 cm−1 grows significantly. At the final stage of H2O dosing, the bidentate nitrate bands nearly disappear leaving primarily the peaks due to ionic nitrates (1352 and 1397 cm−1). These spectral changes are accompanied by the appearance of two reasonably well-developed isosbestic points at ∼1300 and ∼1500 cm−1. Thus, these results demonstrate that H2O induces a transformation from bidentate to ionic nitrates on NO2 preadsorbed K2O(2)/Al2O3 sample at room temperature. Similar transformation of nitrate species induced by H2O was also proposed for barium-based materials,23 showing again analogous morphological properties of supported Ba- and K-based NSR catalysts. We have also recently reported that the ratio of ionic nitrates to bidentate nitrates increases with increasing amounts of K2O after NO2 is adsorbed on K2O(x)/MgAl2O4.24 As briefly mentioned above,15,20 NO2 adsorption on K2O(10)/Al2O3 gives rise to the

formation of both bidentate and ionic nitrates as demonstrated in Figure 2 (black line). This sample was then exposed to varying

Figure 2. FTIR spectra collected during stepwise H2O adsorption at 300 K to an NO2 preadsorbed K2O(10)/Al2O3 sample.

H2O doses step by step and FTIR spectra obtained sequentially. As shown in Figure 2, the peaks at 1352 and 1390 cm−1 grow gradually and, more noticeably, the peak at 1558 cm−1 decreases quite rapidly with increasing water exposures. The existence of an isosbestic point around at 1500 cm−1 demonstrates that bidentate nitrates are transformed into ionic nitrates on the K2O(10)/Al2O3 catalyst. In other words, regardless of the loading of K2O, H2O can induce the transformation in the morphology of KNO3, resulting in the predominant formation of ionic KNO3 species. Also shown in Figure 2 are growing features in the hydroxyl region of 3200−3500 cm−1. As a follow-up experiment, the temperature was raised to 580 K to desorb H2O from the H2O saturated NO2−K2O(10)/Al2O3 sample. For ease of comparison, FTIR spectra obtained after NO2 adsorption and the subsequent H2O treatment from Figure 4191

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2A are again plotted in Figure 3a,b, respectively. Heating the sample gives rise to the reappearance of the peak at 1563 cm−1

Figure 4. FTIR spectra collected during the stepwise NO2 adsorption at 300 K to a H2O preadsorbed K2O(10)/Al2O3 sample. Figure 3. FTIR spectra collected after NO2 adsorption at 300 K to a K2O(10)/Al2O3 sample (a), followed by H2O treatment at RT (b), and subsequent heating to 580 K (c).

gradual transformation into nitrate species with increasing NO2, as displayed in BaO/Al2O3 sample.15 On the other hand, only nitrate species are formed for the case of H2O dosing on NO2 preadsorbed K2O/Al2O3 since there are already enough nitrate species on the sample. Another interesting thing to note is the complete absence of the peak at 1550 cm−1, indicating that bidentate nitrate species are not formed on H2O preadsorbed K2O(10)/Al2O3. The production of only ionic nitrate species is quite different from NO2-exposed K2O(10)/Al2O3 in the absence of water. After adsorption of NO2 on H2O preadsorbed K2O(10)/ Al2O3 at 300 K, the IR spectrum was taken every 50 K while raising the temperature, as demonstrated in Figure 5A. First of all, the intensities of hydroxyl groups (3200−3800 cm−1) decreased monotonically to 600 K, indicating the gradual desorption of H2O with increasing temperature. There are also several notable changes in the region of 1200−1600 cm−1. The nitrite peak at 1231 cm−1 decreases and is finally gone at 450 K. Meanwhile, the peaks at 1361 and 1393 cm−1, characteristic of ionic nitrates, begin to decrease above 400 K and continue to do so up to 750 K. While the ionic nitrate species are disappearing from the K2O(10)/Al2O3 surface, the two peaks at 1307 and 1542 cm−1 associated with bidentate nitrates begin to appear above 400 K and are maximized at 650 K, followed by a gradual intensity decrease. This result suggests that some of ionic nitrate species are transformed into bidentate nitrate species with increasing temperature. During the temperature ramping mass spectrometer data were also obtained and, as displayed in Figure 5B for H2O (m/e = 18), NO (m/e = 30), NO2 (m/e = 46), and O2 (m/e = 32). Note that m/e = 30 signal can result from desorption of either or both NO and/or NO2 due to the fragmentation of NO2 to NO in the mass spectrometer. Around 370 K, the MS signal can be attributed to the desorption of physisorbed H2O in the sample. Subsequently, the simultaneous desorption of H2O and NO is observed with the peak at 480 K. The monotonic decrease of hydroxyl groups up to 600 K is consistent with the observed H2O (m/e = 18) desorption during TPD. Another NO desorption peak is also found at ∼750 K along with an MS peak due to O2, while H2O desorption is notably absent at these temperatures. Combining the IR and the MS results, it appears that, especially above 400 K

and the simultaneous decrease of the peaks at 1361 and 1391 cm−1 (Figure 3c). Note also the significant reduction of the peaks at 3200 - 3500 cm−1. As a result of the thermal treatment, the IR spectrum of the sample heated to 580 K (Figure 3c) is quite similar to that of the sample before H2O treatment (Figure 3a), suggesting that the transformation of potassium nitrate species between bidentate and ionic nitrates is a reversible process depending on the presence and absence of H2O in the sample. Furthermore, it is clear that H2O plays an essential role in determining the type of nitrate species formed on NO2 preadsorbed K2O/Al2O3 samples. We also performed a similar H2O treatment to an NO2 adsorbed K2O(10)/Al2O3 sample except at an elevated temperature of 475 K (results not shown here). This experiment resulted in very similar transformation behavior as evidenced in the FTIR data. 3.2. NO2 Adsorption on H2O Preadsorbed K2O(10)/ Al2O3 Sample. In this section, the results of experiments that were conducted in a reverse manner, in other words, NO2 dosed after the initial adsorption of H2O onto K2O(10)/Al2O3, are presented. We again use FTIR spectroscopy to monitor the changes in the nature (structure) of the K-nitrate species on this model NSR catalyst, and the results are presented in Figure 4. The IR spectrum at the bottom (black) was obtained after the exposure of the sample to H2O and subsequent evacuation at room temperature. This treatment resulted in a broad absorption feature in the hydroxyl region between 3600 and 3000 cm−1, with also a fairly sharp peak at 3507 cm−1. Upon the introduction of the first NO2 dose, IR features characteristic of bulk nitrates (1361 and 1394 cm−1) and nitrites (1234 cm−1) appear and grow simultaneously with additional NO2 doses. The nitrite peaks become saturated after several NO2 doses, while the bands at 1361 and 1394 cm−1, assigned to the ionic nitrates, grow steadily with increasing NO2 exposure. Compared with FTIR spectra obtained while dosing H2O on NO2 preadsorbed K2O/Al2O3 (Figure 2), nitrite species are observed for the case of the reverse adsorption (Figure 4). This phenomenon can be understood by the formation of nitrite species at low NO2 dose, followed by the 4192

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Figure 5. (A) FTIR spectra collected during the stepwise heating after NO2 adsorption at 300 K to a H2O preadsorbed K2O(10)/Al2O3 sample. (B) Intensity of mass spectrometer signals for several masses (18, 30, 32, and 46) during the thermal treatment.

where H2O begins to be removed from the surface, the ionic nitrates, initially formed at 300 K, have two routes: decomposition to release NOx and the transformation to bidentate nitrates. At 650 K where most of ionic nitrates and hydroxyl groups are gone, only bidentate nitrates remain. The residual bidentate nitrate species are finally decomposed to release NO and O2 with the peak at ∼750 K. 3.3. TR-XRD Studies on the Decomposition of KNO3/ Al2O3. TR-XRD provides especially useful information about phase changes under the simulated reaction conditions studied here. We have previously followed the morphological changes of barium species upon NO2 and/or H2O adsorption at various temperatures by using TR-XRD experiments.23 For example, the phase transformation from nanocrystalline Ba(NO3)2 to bulklike Ba(NO3)2 was clearly observed on NO2 preadsorbed BaO/ Al2O3 after H2O treatments, in good agreement with FTIR results. In order to investigate the phase change of potassium species, we applied various treatments such as temperature ramping and NO2 adsorption to K2O/Al2O3 samples, while obtaining XRD data. Before presenting the TR-XRD results, it should be pointed out that XRD only detects crystalline phases, while IR provides information on the adsorbed species (e.g., nitrates). The starting material for these studies is an as-impregnated and dried KNO3(20)/Al2O3 sample. As shown in Figure 6, this sample primarily contains the rhombohedral (R) KNO3 phase with a minor amount of the orthorhombic (O) KNO3 phase (XRD pattern taken at 307 K). As the temperature is increased linearly during temperature programmed decomposition of this sample, two changes are noticeable: a gradual disappearance of the orthorhombic KNO3 phase and the slight shift of peaks for the rhombohedral KNO3 peak at 5.5° toward lower angles, likely due to the expansion of the rhombohedral KNO3 structure with increasing temperature. At 423 K, the orthorhombic KNO3 phase is completely gone and only the rhombohedral KNO3

Figure 6. TR-XRD patterns collected while heating (307−423 K) an asimpregnated KNO3(20)/Al2O3 sample.

phase remains. As displayed in Figure 7a, further heating of the sample above 423 K results in a gradual decrease of the rhombohedral phase peaks, until they finally disappear at temperatures above ∼550 K. Note that these temperatures (550−650 K) are well below those required to fully decompose K-nitrates as evidenced by the results in Figure 5. Thus, Knitrates are transformed to an X-ray invisible (amorphous or liquid-like) structure before decomposing. The gradual phase transition of KNO3 with increasing temperature (from orthorhombic to rhombohedral to liquid) is consistent with the phase diagram of KNO3 under ambient conditions.25 This conclusion can be further confirmed with separate temperature 4193

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Figure 7. (A) TR-XRD patterns collected during further heating (423−610 K) of as-impregnated KNO3(20)/Al2O3. (B) Temperature programmed decomposition data for a KNO3(20)/Al2O3 sample obtained in a separate experiment.

temperature reaches 973 K and the sample subsequently cooled to RT, a new phase at 2θ = 6.65° is evident and assigned to the formation of a KAlxOy phase that results from a solid state reaction between K2O and Al2O3. A similar solid state reaction between BaO and Al2O3 leads to the formation of BaAl2O4 above 1173 K for BaO/Al2O3 materials.13 Unlike BaO/Al2O3 however, NO2 adsorption at room temperature induces a phase transformation of the KAlxOy phase back into the rhombohedral KNO3 phase, as clearly displayed in Figure 8. The transformation of the KAlxOy phase into KNO3 suggests a higher mobility for potassium ions in an aluminate phase upon NO2 adsorption than that we have seen for Ba. Again, a crystalline Ba(NO3)2 phase is very slowly formed from thermally aged BaAl2O4/Al2O3 after NO2 adsorption at room temperature.14 As shown in Figure 8, only the rhombohedral KNO3 phase is observed for the thermally treated K2O/Al2O3 sample after NO2 adsorption, without any evidence for the orthorhombic KNO3 phase. The fact that the orthorhombic KNO3 phase completely disappears above 423 K where H2O is simultaneously desorbed (Figure 6) suggests to us that the KNO3 phase may be closely related to the presence of H2O. In addition, this hypothesis is supported by the fact that the orthorhombic KNO3 phase is not formed after NO2 adsorption for the thermally treated sample (to 973 K), where also H2O is completely removed. To confirm the effect of treatments related to H2O on the formation of two KNO3 phases on KNO3/Al2O3 sample, we took the sequential XRD patterns after each treatment. As shown in Figure 9a, the as-impregnated and dried KNO3/Al2O3 sample contains two different KNO3 phases: rhombohedral and orthorhombic. The rhombohedral KNO3 is the major phase with relatively small amounts of orthorhombic KNO3, as in the TR-XRD patterns for an as-impregnated sample shown in Figure 6. The as-impregnated sample was then treated in an oven at 623 K for 30 min and cooled to room temperature, which provides an environment for the complete removal of H2O from the sample. In addition, all KNO3 crystalline species are expected to “melt” and then be recrystallized upon cooling. After such a treatment, only the rhombohedral KNO3 phase is observed as shown in the middle XRD pattern, Figure 9b. For the sample with the rhombohedral KNO3 phase only, we then applied a single drop

programmed decomposition experiments for the same KNO3(20)/Al2O3 sample where desorbing NOx species are measured with a NOx chemiluminescence analyzer. The results, shown in Figure 7b, reveal the absence of desorbing NOx up to 573 K. Instead, the desorption peaks of NO2 and NO are maximized at 733 and 1033 K, respectively. Thus, the disappearance of the XRD peaks due to crystalline KNO3 phases around 573 K is not indicating the decomposition of the phase to release NOx, but rather the formation of an amorphous KNO3 phase. Again, this result is in consistent with the known melting temperature of solid KNO3 (607 K). Figure 8 plots XRD data obtained as the sample in Figures 6 and 7a is further heated. Above 623 K where the KNO3 crystalline phase disappears, no significant new phases are observed during the temperature ramping. However, when the

Figure 8. TR-XRD patterns collected at 623 K and above during further heating of an as-impregnated KNO3(20)/Al2O3 sample. Also shown is an XRD diffractogram obtained after heating the sample to 973 K and subsequently adsorbing NO2 at RT. 4194

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Figure 9. XRD patterns for as-impregnated KNO3(20)/Al2O3 (a), after heating the sample to 623 K for 30 min and cooling to RT (b), and after subsequent H2O dosing (c).

Figure 10. Summary of the observed KNO3 morphology changes with respect to temperature, NO2 adsorption/desorption and H2O treatment.

information about the morphology changes of KNO3 species in alumina-supported NSR catalysts. While only crystalline KNO3 phases are visible with XRD, FTIR can observe most nitrate species on the samples. Based first on the results of the XRD data, a summary model for the observed morphological changes of the aluminasupported KNO3 phases as a function of temperature and H2O exposure is presented in Figure 10. In particular, the TR-XRD results show that the as-impregnated KNO3/Al2O3 sample contains a mixture of rhombohedral and orthorhombic KNO3

of H2O so that the sample was wet to the level of incipient wetness. The XRD pattern after this treatment (Figure 9c) clearly demonstrates a complete transformation from rhombohedral to orthorhombic KNO3 phase. The series of XRD patterns confirms our proposal that the presence of H2O can induce the transformation from rhombohedral to orthorhombic KNO3.

4. DISCUSSION Characteristics of the two experimental techniques used here (FTIR and XRD) provide valuable and complementary 4195

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crystalline (ionic) nitrate phases may be leading to a spreading of the KNO3 species on the alumina surface that are characterized by peaks associated with bidentate nitrate species. The enhanced mobility of K-species in various forms, especially as a “melted” amorphous nitrate phase, may be one of the significant drawbacks, although K is regarded as the potential storage element of NSR catalysts for high temperature application. In particular, it was reported that K reaction with the cordierite monolith in production models of this catalyst technology led to degradation of the cordierite material as well as the loss of storage function.26,27

phases. As the temperature increases above 423 K, the orthorhombic KNO3 phase is transformed into the rhombohedral one, coinciding with the desorption of H2O. Subsequently, the intensity of the XRD peaks due to the rhombohedral KNO3 phase decrease upon further increases in temperature above 573 K, and finally disappear altogether. However, as demonstrated by the results shown in Figure 7B, the desorption of NO and NO2 does not occur until 673 K. Thus, as illustrated in Figure 10, the rhombohedral KNO3 phase melts to form an amorphous or liquid-like structure above 573 K. After complete decomposition of nitrate species at 973 K, the TR-XRD results suggest the formation of a potassium aluminate phase, although we cannot rule out that X-ray invisible amorphous and/or nanometer sized K2O species are also present. Unlike barium aluminate which adsorbs very little NO2 without a phase change, NO2 adsorption at room temperature induces a facile transition from KAlxOy to the rhombohedral KNO3 phase. The results given in Figure 9b also show that cooling of the amorphous KNO3 species to room temperature reformed the rhombohedral KNO3 phase. In both cases, the rhombohedral KNO3 phase was formed in the absence of H2O. However, when H2O was dosed at room temperature to a catalyst that only contains the rhombohedral phase, KNO3 is completely transformed into the orthorhombic phase, underlining a close relationship between the orthorhombic KNO3 phase and H2O. With regard to the FTIR results, we and others have described the variations in the amounts of bidentate and ionic nitrates with K-loading for alumina-supported catalysts.15,20 In particular, bidentate nitrates dominate the FTIR spectra for low-loaded catalysts, while ionic nitrates are most abundant at high loadings. From this, and in analogy with the behavior of Ba-nitrates, we have concluded that bidentate nitrates result from NO2 adsorption on monolayer-like K2O structures on the alumina surface.20 At higher K-loadings, we have proposed that ionic (bulk-like) nitrates form on alumina-supported particles of K2O. One of the most significant results provided in this work from the FTIR experiments is the explicit evidence for the transformation from bidentate (or surface-type) to ionic (or bulk-type) potassium nitrates by contact with H2O. Such transformations are reversible as evidenced by the reappearance of bidentate nitrate species upon the removal (desorption) of H2O at elevated (>500 K) temperatures. As in the case for Ba-based NSR catalyst materials,21 mobility of the NOx storage element in the NSR catalysts has important implications for the operation of these materials, and the role of H2O in promoting mobility is considerable. As will be discussed next, mobility of K-nitrate species as a function of temperature may be more of an issue for these NSR catalysts systems relative to the Ba-based materials in part because K-nitrates ‘melt’ before decomposition. As discussed above, the proposed morphology changes as a function of temperature and the presence/absence of H2O described in Figure 10 are based primarily on the TR-XRD results. In particular, FTIR will not distinguish between the two crystalline KNO3 phases (orthorhombic and rhombohedral) with both of these “bulk-like” nitrate phases giving rise to FTIR features due to ionic nitrates. Still, the results provide consistent and complementary information about some of the morphological changes depicted in Figure 10. In particular, around 600 K where the rhombohedral KNO3 phase begins to disappear as evidenced by TR-XRD (Figure 7), most of the ionic nitrate species at 1393 and 1361 cm−1 are almost gone (Figure 5A). Even though the ionic nitrate species disappear, bidentate nitrate species survive to higher temperatures. Thus, the “melting” of

5. CONCLUSION Combined FTIR and XRD results demonstrate that H2O induces significant morphological changes of the KNO3 species formed on model K2O/Al2O3 NSR catalyst materials. Specifically, the presence of H2O leads to the transformation from surface-type bidentate KNO3 species to bulk-type ionic phases irrespective of K loading, as evidenced by FTIR. Upon removal (desorption) of H2O from the sample, this transformation process reverses. NO2 adsorption on H2O preadsorbed samples resulted in the formation of only ionic KNO3, underlining the importance of H2O in the morphology changes. TR-XRD indicates the formation of two different types of bulk-like KNO3 phases, orthorhombic and rhombohedral. The former is directly related to the presence of H2O; notably, H2O treatment transforms the rhombohedral phase to the orthorhombic one. On the basis of consistent FTIR and TR-XRD results, we propose a model for the morphological changes of alumina-supported KNO3 species with respect to NO2 adsorption/desorption and H2O and/or thermal treatments.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: 82 2 880 1633. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support was provided by the U.S. Department of Energy (DOE), Office of Energy Efficiency and Renewable Energy, Vehicle Technologies Program. The research was performed in the Environmental Molecular Sciences Laboratory (EMSL), a national scientific user facility sponsored by the U.S. DOE’s Office of Biological and Environmental Research, and located at Pacific Northwest National Laboratory (PNNL). PNNL is a multiprogram national laboratory operated for the U.S. Department of Energy by Battelle under Contract DEAC05-76RL0 1830. Prof. Do Heui Kim acknowledges the partial support of Research Settlement Fund for the new faculty of Seoul National University.

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REFERENCES

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