J. Phys. Chem. C 2009, 113, 21123–21129
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Characteristics of Desulfation Behavior for Presulfated Pt-BaO/CeO2 Lean NOx Trap Catalyst: The Role of the CeO2 Support Do Heui Kim,*,† Ja Hun Kwak,† Janos Szanyi,† Xianqin Wang,†,§ Guosheng Li,† Jonathan C. Hanson,‡ and Charles H. F. Peden† Institute for Interfacial Catalysis, Pacific Northwest National Laboratory, Richland, Washington 99354, and Chemistry Department, BrookhaVen National Laboratory, Upton, New York 11973 ReceiVed: July 2, 2009; ReVised Manuscript ReceiVed: October 12, 2009
The desulfation of presulfated Pt-BaO/CeO2 lean NOx trap catalyst was investigated by H2 TPRX (temperature programmed reaction), in situ TR-XRD (time-resolved X-ray diffraction), and in situ S K-edge XANES (Xray absorption near edge spectroscopy) techniques. Compared with Pt-BaO/Al2O3 materials, a reductive treatment in H2 for the CeO2-supported sample up to 1073 K removes, at most, only a very small amount of sulfur species. However, the results of in situ TR-XRD measurements demonstrate that the quantity of a BaS phase formed on Pt-BaO/CeO2 is much smaller than that on Pt-BaO/Al2O3, implying that the formation of BaS crystallites, which occurs during the reduction from sulfate (SO42-) to sulfide (S2-), is significantly suppressed in the CeO2-supported catalyst. As the desulfation temperature increases under reducing conditions (in H2), in situ S XANES spectra show that, compared with alumina-supported samples, the reduction temperature for sulfates (S6+) decreases by about 150 K. Concomitantly, the formation of sulfur species with lower oxidation states (S2--S4+) is enhanced. The absolute intensities of S XANES spectra before and after desulfation are very similar, implying that the amount of sulfur-containing species removed during the reductive treatment is negligible, in agreement with the results of H2 TPRX. These results suggest that H2S produced by the reduction of BaSO4 is readily readsorbed on the ceria support to form ceria-sulfur complexes (e.g., Ce2O2S). The high affinity of ceria for H2S, combined with the ease of reducibility of the ceria support material gives rise to various oxidation states of sulfur after high-temperature H2 treatments. Thus, the results of this study clearly show that the ceria support strongly affects the overall desulfation mechanism. The intrinsic role of the ceria support during desulfation and its effect on the overall NOx storage processes are discussed on the basis of the characterization results obtained here. 1. Introduction Combustion engines operating under highly oxidizing conditions, such as diesel or lean burn gasoline engines, have been receiving increased attention due to their excellent fuel efficiency and low greenhouse gas emissions. However, in order to meet more stringent emission regulations in the near future, the reduction of nitrogen oxides (NOx) emitted from these engines remains a considerable challenge for the catalysis field. Among the candidate technologies, lean NOx traps (LNTs) have shown promising activities for the removal of NOx in the presence of excess oxygen.1,2 LNT catalysts, which consist primarily of precious metals (platinum and rhodium), storage components (barium and/or potassium), and a support material (alumina), have critical issues associated with their durability due to degradation arising from sulfur poisoning and thermal aging.3 Especially, sulfur in the engine exhaust plays a major role in gradually eliminating NOx storage sites, thus leading to the deactivation of the catalyst.4-6 Therefore, in order to develop a practically viable LNT catalyst, the effective regeneration through desulfation steps without severe thermal deactivation must be achieved. * To whom correspondence should be addressed. E-mail:
[email protected]. † Pacific Northwest National Laboratory. ‡ Brookhaven National Laboratory. § Present address: Department of Chemical Engineering, New Jersey Institute of Technology, Newark, NJ 07102.
Cerium oxide is commonly used as an important component of widely used 3-way catalysts due to its special oxygen storage and release properties, as well as its role in maintaining precious metal dispersion. It has also been used as a support7,8 or a promoter9 in LNT catalysts. A known advantage of using ceria in LNT catalysts stems from its role in the generation of hydrogen, through water-gas shift and/or hydrocarbon steam reforming reactions, for the removal of sulfur.10 Even bariumfree Pt/CeO2 catalysts have been reported to show a significant amount of NOx uptake due to the intrinsic ability of ceria to adsorb nitrogen oxides.11 In addition, Baiker and co-workers12 have reported that the thermal aging of Pt is significantly different in ceria- and alumina-supported Pt/BaO catalysts. Recently, we reported13 superior intrinsic NOx uptake of PtBaO/CeO2 samples compared to Pt-BaO/Al2O3 over the entire temperature range studied, and that the ceria-supported sample showed superior sulfur resistance after equivalent exposures to SO2. Indeed, the ceria-supported catalyst exhibited a remarkably high resistance to Pt sintering during high-temperature reductive desulfation, which can be explained by a strong Pt-ceria interaction as suggested by a Toyota group.14 A central question from these recent studies is to account for these improved properties, especially those observed during sulfur uptake and removal processes. Previously, we have investigated the desulfation mechanism of presulfated Pt-BaO/Al2O3 samples by using in situ TR-XRD (time-resolved X-ray diffraction), S XANES (sulfur K-edge
10.1021/jp9062548 CCC: $40.75 2009 American Chemical Society Published on Web 11/19/2009
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X-ray near edge spectroscopy), and H2 TPRX (hydrogen temperature programmed reaction) techniques.15,16 We found that with increasing reduction temperature, sulfate species, which were initially formed on barium sites via SO2 oxidation, are removed as H2S from the sample, while residual sulfur species are transformed to a highly crystalline BaS phase.15 In this contribution, we explore the characteristics of a Pt-BaO/CeO2 sample by focusing on the role of ceria in the desulfation, compared with similarly prepared alumina-supported catalysts. For this purpose, we again used H2 TPRX, in situ S XANES, and synchrotron TR-XRD. Because the experimental conditions in the in situ S XANES and TR-XRD experiments are similar to those of H2 TPRX, the combination of these techniques allows us to obtain comprehensive information about the transformations and nature of residual sulfur-containing species and their morphologies, as a function of reduction temperature, that can ultimately lead to the elucidation of the role of ceria in sulfur uptake and removal processes. 2. Experimental Section High surface area CeO2 was synthesized by using a previously reported protocol.17 After calcination in air at 350 °C for 4 h, the sample had a BET surface area of 110 m2/g, with crystallites that exhibited a rod-like morphology with average diameter of ∼5-10 nm and length of ∼30-80 nm (estimated from TEM images). The 2 wt % Pt-10 wt % BaO/CeO2 and 2 wt % Pt-20 wt % BaO/Al2O3 samples were prepared by conventional impregnation methods, similarly to the procedures described in detail elsewhere.13,18 It is important to note that the ceriasupported 10 wt % BaO catalyst (surface area ∼110 m2/g) has essentially the same amount of BaO per unit surface area as the alumina-supported 20 wt % BaO catalyst (surface area ∼200 m2/g). Prior to sulfation, a small amount of sample (∼0.15 g) was placed into a quartz reactor and treated with 10% O2 in He at 773 K for an additional 2 h. Typical sulfations were performed at 575 K in a gas mixture of 50 ppm of SO2 and 10% O2, balanced with helium (total flow rate: 150 cm3/min). To obtain samples with the same extent of sulfation, the exposure time for both samples (alumina- and ceria-supported) to the SO2containing gas mixture was kept constant (6 h). Note that the sulfation levels given in this paper are indicated by the time of SO2 exposure since no SO2 breakthrough was observed with an MKS Minilab mass spectrometer (MS) for either sample during sulfation. H2 TPRX (temperature programmed reaction) experiments were performed on the sulfated samples in the same quartz reactor by raising the temperature to 1073 K at a rate of 8 deg/ min under flowing 20% H2/He. The desorbed gases were detected with a mass spectrometer. Raman spectroscopy was applied to the as-calcined and sulfated Pt-BaO/CeO2 samples. The Raman scattered light from the catalyst surfaces was collected by an attached Raman probe constructed to work in a backward scattering mode. The excitation laser beam (514.5 nm) generated by an argon ion laser (Coherent, Innova 400) was transmitted to the sample chamber via an optical fiber. The laser power was about 10 mW at the catalyst surface, and a typical accumulation time used in these experiments was 60 s. A spectrometer (Princeton Instruments, Spectrapro 2500i) with an attached back illuminated charge-coupled detector (CCD) (Princeton Instruments, Spec 10, 1340 × 400 array) was used to record spectra in the 0 to 310 cm-1 range. The instrumental resolution was about 2 cm-1 at 514.5 nm wavelength. The instrument has been described in detail previously.19
Kim et al. The TR-XRD and sulfur K-edge XANES experiments were carried out with use of beamlines X7B and X19A, respectively, at the National Synchrotron Light Source (NSLS) at Brookhaven National Laboratory. The detailed experimental setup of the TRXRD has been described elsewhere.20 Briefly, a small amount of sulfated sample was placed in a sapphire capillary tube and heated at 10 deg/min from 300 to 1073 K while continuously flowing a 5% H2 in He gas mixture. XRD patterns were collected in situ every 2 min during the temperature ramping. The X19A beamline used to collect the S K-edge XANES data is equipped with a double-crystal Si(111) monochromator. Fluorescence measurements were performed in an in situ quartz reactor with a Lytle detector. The design of the reactor was adapted from Bare et al.21 A pellet of the sample (diameter 12 mm) was placed in the homemade sample holder, which is located inside the quartz reactor. The sample was heated at 10 deg/min from 300 to 573 K, followed by heating at 2 deg/min from 573 to 973 K while continuously flowing a 5% H2 in He gas mixture. Sulfur K-edge XANES spectra were collected in situ every 9.5 min (thus, every 19 K) during the temperature ramping. The X-ray energy for the K-edge of elemental sulfur (Aldrich) was used for calibrating the energy scale. Its peak position was assigned an energy value of 2472 eV, and scans ranging from 20 eV below to 50 eV above the absorption edge were collected with a step size of 0.08 eV. The statistical noise at X19A is better than 0.001%. The white line intensity of the sulfate peak in the XANES spectra was obtained by the difference in the absorption intensities between the baseline at 2465 eV and the sulfate peak at ∼2482 eV. 3. Results As described in the Experimental Section, although the amount of barium is two times larger in the alumina-supported sample, both alumina- and ceria-supported catalysts contain similar amounts of sulfur, based on the MS results that indicated that there was no evolution of SO2 during sulfation. For the Pt-BaO(10)/CeO2 sample, the SO2 uptake (0.121 mmol) exceeds that necessary for full conversion of BaO to BaSO4 (0.098 mmol). This result can be understood by the participation of the cerium oxide in additional adsorption of SO2 to form sulfate species, consistent with prior literature.22,23 To differentiate between Ba- and Ce-sulfates in the ceria-supported catalyst, we applied Raman spectroscopy. Figure 1 shows the Raman spectra for the as-calcined (a) and sulfated (b) Pt-BaO/CeO2 samples. A strong Raman peak at 458 cm-1 originates from the F2g Raman-active mode characteristic for the fluorite structure of CeO2.24 A smaller peak at 1054 cm-1 for the as-calcined sample can be assigned to the formation of barium carbonate by CO2 adsorption on the BaO surface.25 Characteristic Raman features of BaSO4 and CeSO4 have been reported at 987 and 1030 cm-1, respectively.25,26 These latter results were also confirmed by us using standard reference compounds (spectra not shown). The fact that the Raman spectrum of the sulfated sample (b) contains primarily a peak at 987 cm-1 with, at most, only a small peak at 1030 cm-1 implies that the majority of sulfate species in this sample are preferentially bound to barium rather than cerium sites under the sulfation conditions applied. Previous studies27,28 have reported the use of H2 TPRX methods to characterize desulfation processes of sulfated LNT catalysts by following the amount of H2S desorbed as a function of reduction temperature. We compare two H2 TPRX spectra between Al2O3- and CeO2-supported Pt-BaO catalysts in Figure 2. These two samples showed completely different H2S evolution behavior: the Al2O3-supported one gave rise to a large and
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Figure 3. TR-XRD patterns for sulfated Pt-BaO/CeO2 measured every 40 deg during reduction in H2 from 813 to 1053 K. Figure 1. Raman spectra of Pt-BaO/CeO2 samples: as-calcined (a), sulfated (b), and after H2 TPRX for the sulfated sample followed by oxidation up to 973 K (c).
Figure 2. H2 TPRX of Pt-BaO/CeO2 and Pt-BaO/Al2O3 catalysts. The number in parentheses indicates the integrated area of the H2S/He ratio over the entire temperature range.
broad peak with a maximum near 900 K, while the CeO2supported one exhibited only a very small peak at 725 K. The amount of H2S desorbed from the sulfated alumina-supported catalyst is more than 40 times larger than that from the ceriasupported one, even though the latter begins to desorb H2S at lower temperatures (700 K). Considering that the initial amounts of sulfur in these two samples were practically the same, sulfurcontaining species formed on the ceria-supported sample must be more resistant to removal as H2S during reduction with hydrogen even up to 1073 K than the alumina-supported catalyst. TR-XRD can provide information about phase changes occurring during reduction with H2 at elevated temperature.20 Using this technique, we have previously reported the formation of a crystalline BaS phase during desulfation of presulfated PtBaO/Al2O3 with hydrogen.15 During the desulfation process, we found that the BaS phase begins to form around 823 K, followed by significant growth of BaS crystallites to average sizes of 15 nm after TPRX to temperatures up to 1073 K. To follow the
Figure 4. XRD patterns for Pt-BaO/CeO2 and Pt-BaO/Al2O3 after reduction in H2 up to 1073 K.
phases formed during reductive desulfation of the Pt-BaO/CeO2 sample and compare them with those we have found for PtBaO/Al2O3 catalysts, TR-XRD experiments were carried out for both samples with similar sulfation levels. Figure 3 displays a series of XRD patterns collected from the presulfated Pt-BaO/CeO2 sample during temperatureprogrammed reduction from 813 to 1073 K. (Note that there is no substantial change in XRD patterns below 813 K.) The main peaks at 16.7° and 19.3° are assigned to crystalline CeO2, while two small peaks at 14.0° and 23.3° are due to a newly formed BaS phase, with the latter two peaks increasing in size with reduction temperature. (Note that the main peak of BaS at 16.6° overlaps with the XRD peaks due to CeO2 and, thus, was unsuitable for identifying BaS formation.) The data obtained reveal that the BaS phase appears at temperatures above 850 K and keeps growing up to 1053 K. It is noteworthy that no peaks assignable to a crystalline Ce2O2S phase were observed after treatment with H2 up to 1073 K, as discussed in more detail later. Figure 4 compares the XRD patterns of the sulfated alumina- and ceria-supported Pt-BaO samples after reduction up to 1073 K. It is evident that the diffraction peak intensities
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Figure 5. Sulfur XANES spectra obtained every 19 deg during reduction in H2 from 298 to 827 K for sulfated Pt-BaO/CeO2.
of the BaS phase in Pt-BaO/CeO2 are much lower than those in the Pt-BaO/Al2O3 sample, implying that the tendency to form large BaS crystallites is significantly suppressed in the ceriasupported sample. Since TR-XRD can only detect crystallites that are larger than about 5 nm, we applied sulfur K-edge XANES to investigate in detail the amount and chemical state of residual sulfur species formed during reduction. The advantage of the S-edge XANES technique is that the absorption energy is monotonically dependent on the sulfur oxidation state (-2 for sulfide species to +6 in sulfate).29 In particular, the absorption peak of BaS (S2-) is at 2472 eV, while that of BaSO4 (S6+) is at 2482 eV. To obtain in-depth information about residual sulfur species at increasing reduction temperature, an in situ S K-edge XANES experiment was performed. This setup allows us to heat the sample to 973 K in a continuous H2 flow, thereby providing conditions essentially identical with those applied in both the H2 TPRX and TR-XRD experiments. A series of S K-edge XANES spectra were obtained in the 297-973 K temperature range for sulfated PtBaO/CeO2 during reduction with H2. The spectra taken from room temperature to 573 K (not shown) have a single peak at 2482 eV that can be assigned to sulfate species, and do not show any changes. As the temperature increased above 573 K, a gradual decrease in this sulfate peak (2482 eV) and a coincident appearance of a broad peak centered around 2474 eV were observed,29 with the latter feature assignable to S1+ or S2+ species, as displayed in Figure 5. These changes accelerate above 650 K, as evidenced by the dramatic decrease in the intensity of the sulfate peak. It is interesting to point out a few isosbestic points at 2480, 2485, and 2492 eV (marked V in Figure 5), which indicate that sulfate is directly converting into sulfur species with lower oxidation states without desorbing from the catalyst. In other words, the amount of sulfur removed in this temperature range is negligible, consistent with H2 TPRX results that gaseous sulfur containing species such as H2S were hardly detected during desulfation. The intensities of the two peaks at 2474 and 2482 eV, as a function of reduction temperature, are shown in Figure 6. For comparison, the 2482 eV (sulfate) intensity for the aluminasupported Pt-BaO catalyst is displayed as well. For the ceriasupported catalyst, the intensity of the sulfate peak begins to decrease around 650 K, dropping to about 20% of its initial intensity as the sample temperature reaches 820 K. Concomi-
Kim et al.
Figure 6. Intensities of the peaks at 2482 eV (solid squares) and 2474 eV (solid circles) for sulfated Pt-BaO/CeO2 during reduction in H2 from 298 to 827 K. For comparison, the intensities of the peak at 2482 eV for sulfated Pt-BaO/Al2O3 are displayed with the open square symbols.
Figure 7. Sulfur XANES spectra obtained every 19 K during reduction in H2 from 827 to 973 K for the sulfated Pt-BaO/CeO2 catalyst.
tantly, the broad peak at 2474 eV increases, apparently at the expense of the sulfate peak, as evidenced by the multiple isosbestic points in Figure 5. This higher temperature range (650-820 K) corresponds to the regime where a small amount of H2S was observed in the H2 TPRX experiment (Figure 2). Furthermore, comparing the trends in the intensities of the 2482 eV peak for CeO2- and Al2O3-supported Pt-BaO samples unambiguously demonstrates that the barium sulfate species of the ceria-supported sample are much easier to reduce to lower oxidation states than the alumina-supported catalyst. Besides the above-described changes in intensities of the peaks representing sulfur in different oxidation states for the Pt-BaO/CeO2 catalyst, additional changes occur in peak positions above 820 K reduction temperature, as shown in Figure 7. Between 820 and 839 K, the peak at 2482 eV completely disappears, while a feature centered at 2480 eV appears and continues to grow in intensity with increasing temperature. The 2 eV downshift in the S K-edge XANES is consistent with a
Presulfated Pt-BaO/CeO2 Lean NOx Trap Catalysts
Figure 8. Sulfur XANES spectra for sulfated Pt-BaO/CeO2 and PtBaO/Al2O3 after reduction in H2 up to 973 K.
reduction of S6+ to S5+.29 The intensity of this new peak at 2480 eV continues to grow slightly, then becomes saturated as the temperature increases from 884 to 973 K. At the same time, the broad peak at 2474 eV appears to split into three peaks at 2472, 2474, and 2476 eV at temperatures above 875 K. These three peaks are distinguished more readily as the temperature exceeds 900 K. The increasingly intense peak at 2472 eV (S2-) above 875 K is consistent with TR-XRD results that identified the formation of a crystalline BaS phase above 893 K. Figure 8 compares the S XANES spectra for sulfated PtBaO/CeO2 and Pt-BaO/Al2O3 after reduction in H2 up to 973 K. Although considerable amounts of sulfates are removed from the alumina-supported catalyst as H2S, it still contains a large amount of residual sulfur-containing species mostly as sulfates and smaller amounts of sulfides and sulfites, implying that the reduction from sulfates to sulfites and sulfides is not complete. On the other hand, reduction of the ceria-supported sample under identical conditions results in the complete disappearance of barium sulfate, but produces large amounts of sulfur-containing species in oxidation states ranging from S2- (2472 eV) to S5+ (2480 eV). For the Pt-BaO/CeO2 sample after reduction in H2 up to 973 K in the in situ reactor, followed by cooling in He flow, reoxidization with O2 up to 973 K was performed and S XANES were again measured (not shown). All of the residual sulfur species were oxidized to form primarily sulfates; moreover, the intensities of the sulfate-related peaks were essentially the same as those obtained for the initially sulfated sample (i.e., prior to reduction in H2). In addition, a Raman spectrum of the reoxidized sample (Figure 1c) clearly shows the reformation of barium sulfate rather than cerium sulfate. These results imply that the sulfur-containing species were not removed from the catalyst during reduction by H2. Rather, the oxidation state of the sulfur species changed from S6+ to S2- and S0 during reduction, and then back to S6+ in the subsequent oxidation while they remain on the catalyst. 4. Discussion In a previous publication,13 we have demonstrated that PtBaO/CeO2 LNT catalysts, prepared with a high surface area CeO2 (110 m2/g), exhibited longer complete NOx uptake times and higher total NOx uptake than an alumina-supported one
J. Phys. Chem. C, Vol. 113, No. 50, 2009 21127 over the entire temperature range studied. In fact, the ceriasupported catalyst contained only half the amount of barium implying that the ceria support plays an important role in improving the efficiency of barium oxide for storing NOx. In addition, these ceria-supported catalysts exhibit significantly lower sensitivity to sulfur poisoning for the same amount of sulfur exposed and a higher resistance to Pt sintering during high-temperature desulfation in comparison to the aluminasupported one,13 results that were in good agreement with prior studies.12 To our knowledge, however, no one has reported results on the details of desulfation of ceria-supported samples. Understanding the desulfation behavior of LNT catalysts is of considerable practical importance since the ease of sulfur removal can impact the required frequency of regeneration processes. In particular, if sulfur can be removed easily with less reductant, the frequency of regeneration can be reduced and, in this way, the overall fuel economy of the LNT system can be improved. To understand the chemical nature of the sulfates (Ba or Ce) formed during sulfation, we used Raman spectroscopy. Previous reports suggested that both barium and cerium sulfates can be formed during SO2 oxidation.16,22 Unlike other characterization techniques, such as XPS and S XANES, Raman spectra can differentiate between Ba and Ce sulfates formed in the sulfation process. As shown in Figure 1, Raman spectra clearly demonstrate that barium sulfates are the majority species in the sulfated samples. This result suggests that ceria becomes sulfated only after the baria sites are fully occupied. That ceria will sulfate as well is supported by the observation that the SO2 uptake measured during sulfation exceeds the expected amount calculated from the amount of BaO in the catalyst. All of the results obtained from H2 TPRX and S XANES experiments unambiguously demonstrate that the removal of sulfur is minimal for the ceria-supported catalysts during reduction with H2 up to 1073 K. In other words, the presence of ceria almost completely suppresses the removal of sulfur from these catalysts. To understand the behavior of the sulfated ceriasupported sample during the reductive treatment, we briefly discuss the results of prior studies regarding the interaction between sulfur-containing compounds and ceria.30-33 Ceria has been proposed as a potential absorbent for removing H2S and SO2 from dilute gaseous streams. It has been reported that unlike other support materials (TiO2, Al2O3, and MgO), CeO2 supports have a tendency to strongly adsorb H2S, resulting in more sulfidation than other support materials, a result that was attributed to the labile character of oxygen ions in ceria.32 In addition, H2S, produced by hydrogen reduction of sulfates, is adsorbed in large amounts in a dissociated form on ceria in Pt/CeO2 catalysts.33 These results are very useful for understanding the difficulty of sulfur removal from Pt-BaO/CeO2 catalysts. In particular, it appears that as soon as H2S is formed during reduction of Ba-sulfates with H2, it preferentially adsorbs on ceria rather than desorbing from the catalyst as H2S. The difference in the affinity of ceria and alumina for H2S determines the overall desulfation behavior, in other words, how readily sulfur-containing species are removed from these catalysts. Several groups30,31 have reported the formation of Ce2O2S by XRD, resulting from a reaction between CeO2 and H2S. In the present studies, however, we could not find any evidence for the presence of crystalline Ce2O2S based on TR-XRD results as shown in Figure 3. The possibility of the presence of Ce2O2S with crystallite sizes below the detection limit of the synchrotron XRD (
