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J. Phys. Chem. B 2002, 106, 4172-4180
NO and NO2 Adsorption on Barium Oxide: Model Study of the Trapping Stage of NOx Conversion via Lean NOx Traps Peter J. Schmitz* and Ronald J. Baird† Ford Research Laboratories, MD3083/SRL, Dearborn, Michigan 48121-2053 ReceiVed: September 5, 2001; In Final Form: January 3, 2002
The use of NOx traps is one strategy being pursed to enable the implementation of more fuel-efficient leanburn gasoline engines. Materials development to enhance NOx storage capacity and sulfur tolerance will be necessary for performance improvement. Progress in these areas will benefit from a more detailed understanding of the base metal oxide-precious metal surface chemistry involved in the trapping, release, and reduction of NOx. In this work, we have focused on the adsorption of NO and NO2 on in-situ evaporated thin films of barium oxide, the primary storage material in lean NOx traps, to accentuate the details of the trapping stage of NOx conversion using these systems. X-ray photoelectron spectroscopy has been used to identify the species formed and their relative abundance following room-temperature adsorption. Annealing experiments were performed to follow changes in adsorbed species with temperature. For NO, our results are consistent with nitrites forming as a result of molecular adsorption. In the case of NO2, nitrates are favored at high exposure and appear to form via a nitrite intermediate. We propose that as coverage increases nitrates form via trimer formation involving two surface nitrites and an additional molecularly adsorbed NO2 resulting in a complex in which all nitrogen centers are nitratelike. In light of results presented, an alternative and more detailed interpretation of the mechanism of NOx trapping is offered, that accounts for the NOx storage capacity benefits resulting from NO oxidation over noble-metal sites.
Introduction Lean-burn gasoline and diesel engine technologies are attractive from the perspective of improved fuel efficiency and the resultant reduction in CO2 emissions. However, the concurrent reduction of NOx emissions in the oxygen-rich exhaust environments, which these technologies create, is a key challenge that must be addressed before auto manufactures will be able to meet impending regulations and make large-scale implementation a reality. A number of strategies involving the selective catalytic reduction of NOx under lean conditions have been extensively studied.1 An alternative after-treatment strategy utilizing NOx trap technology shows promise, especially for lean-burn gasoline applications. Both the automotive engineering2,3,4 and the scientific communities5,6,7,8,9 are actively investigating NOx trap technology. In NOx trap systems, as in all after-treatment systems, surface chemistry plays a key role in molecular conversion processes. NOx traps function by oxidizing and storing NOx during lean operation and releasing and reducing the NOx to N2 during periodic rich excursions. The mechanistic details of this process are still poorly understood. The common belief is that trapping of NOx occurs predominantly through the oxidation of NO to NO2 on noble-metal sites, such as Pt and Rh, with the NO2 either spilling over onto the oxide phase, or being transported to the oxide via the gas phase, where it is trapped primarily as a nitrate.2 Conversion to N2 is also thought to occur over noblemetal sites during rich excursions when the NOx is released and reacts with hydrocarbon, H2, and CO.2 * E-mail:
[email protected]. Fax: (313) 322-7044. † Current address: Department of Chemistry, Wayne State University, Detroit, MI 48202.
A number of challenges exist in NOx trap development. The first is to design systems that are more selective for NOx adsorption. This will be necessary to reduce the poisoning effects of sulfur from the fuel, which with time degrades the efficiency of the trap.10,11 Efforts in this area would benefit from a detailed understanding of the base metal oxide-precious metal surface chemistry involved in the trapping, release, and conversion of NOx and the complimentary competing reactions of SOx. More detailed mechanistic information will provide direction in the search for alternative oxide and mixed oxide phases for study. Understanding the thermodynamics and kinetics of the reactions involved in the trapping and conversion of NOx will help in the development of application strategies for the effective reduction of NOx over a temperature range suitable for leanburn gasoline and diesel applications. The mechanisms involved in NOx trapping and conversion are complex when one considers all the potential reactions involved in adsorption, transport, oxidation, and reduction. For the work presented here, we have focused only on the interaction of NO and NO2 with the primary absorbent used in the trap, barium oxide. The emphasis is to determine differences in adsorption characteristics of NO and NO2 such as sticking coefficients, species formed, and thermal stabilities to provide a more detailed picture of adsorption pathways, relative adsorption rates, site preferences, capacity, and reaction intermediates. Information at this level provides insight into the processes involved in the trapping stage of NOx conversion by NOx traps. To study the contribution from the oxide adsorbent material, independent of the high surface area supports used in commercial formulations, we worked with thin films of barium oxide prepared in-situ in UHV and followed the adsorption process as a function of exposure using X-ray photoelectron spectros-
10.1021/jp0133992 CCC: $22.00 © 2002 American Chemical Society Published on Web 03/20/2002
Study of the Trapping Stage of NOx Conversion copy (XPS). Annealing experiments were performed to measure changes in adsorbed species with temperature. On the basis of the results of these experiments, we propose adsorption pathways that explain observed ad-species, and outline the implications of these results to trapping mechanisms. In light of our results, we offer an alternative and more detailed interpretation of the mechanism of the trapping stage of NOx conversion via lean NOx traps. Experimental Section The experiments were carried out in a UHV system described in detail elsewhere.12 Briefly, the system was equipped with a double-pass cylindrical mirror analyzer and Mg X-ray source for XPS, sputter ion gun, oxide evaporator, prechamber, and gas handling facilities. The substrate used was a ∼1 cm2 piece of thin aluminum sheet with a native aluminum oxide surface film. The substrate was held in contact with a Ta foil strip spot welded to conducting elements to provide resistive heating. The temperature of the substrate was monitored with a chromelalumel thermocouple spot-welded to the Ta foil backing and fed back to an Omega temperature controller for desorption studies. The substrate was selected primarily due to its low background in the N 1s region of the XPS spectrum; however, it is also relevant because alumina is the support used in commercial NOx traps. The barium oxide thin films were prepared in-situ in the UHV system. The barium oxide source was a simple resistively heated filament variety. Technical grade BaO powder (88-94% barium oxide, 6-12% barium carbonate), obtained from Alfa AESAR, was pressed into a Ta wire mesh that served as the charged filament. The charged mesh was spot-welded to Ta wire support rods mechanically fixtured to a standard high current feedthrough. A 6 V AC transformer powered by a variac was used to supply current to the filament. Standard operating conditions were 6 A at 6 V for all depositions. The background pressure rose to 4 × 10-9 Torr during evaporations. The source provided a flux of primarily BaO;13 no evidence of metallic barium or reduction of native aluminum oxide was observed. In addition, no Ta or other metals were observed following exposure of the substrate to the evaporator under these conditions. The evaporator was thoroughly outgassed at temperatures exceeding the decomposition temperature of barium carbonate. Low but measurable levels of carbon were observed for the deposited films; however, the carbon concentration was one-tenth that of barium on a per atom basis (as determined by XPS), and only 10-15% of the carbon observed could be attributed to carbonate. Therefore, the surfaces of the films produced are predominately BaO. The deposited film thickness was on the order of 15-20 Å as assessed by the attenuation of the substrate signal.14 All depositions were carried out with the substrate at room temperature. All films grown are expected to be polycrystalline in nature. Exposures to NO and NO2 were conducted in a prechamber that was accessible by direct transfer from the UHV chamber. The vacuum was maintained in the prechamber using a 25 L/s ion pump or a 170 L/s turbo pump. The prechamber was equipped with a manifold allowing for controlled pressure static dosing. The pressure was measured using a capacitance manometer with a range of 0-10 Torr. NO used for the experiments was a 99% commercial purity grade provided by Matheson. The NO2 used for these experiments was produced on site by mixing O2 and NO at room temperature and pressures of about 100 Torr. The product of the reaction was stored in a glass bottle attached to the prechamber manifold. Assuming
J. Phys. Chem. B, Vol. 106, No. 16, 2002 4173 complete reaction, the estimated composition of the product is 99.985% NO2, 0.010% NO, and 0.005% O2 based on an equilibrium constant of 2.26 ‚ 1012 atm-1 at 25 °C as calculated using values from U.S.N.B.S. tables of molar thermodynamic properties.15 However, the NO and O2 concentrations are likely higher due to the contribution of unreacted components, and the presence of N2O4 is also probable.16 The sample was exposed in the prechamber by backfilling with either NO or NO2, exposing for 2 min, and then evacuating with the turbo pump. After evacuation, the sample was transferred to the UHV chamber for analysis. Initial exposures began at pressures of 40 mTorr (∼5 × 106 L). For higher exposures, samples were transferred back to the prechamber and exposed to higher pressures of the gas of interest. Typical NOx exposures expected in vehicle applications of traps prior to regeneration is approximately 16 × 106 L, consistent with the regime covered by these experiments. Characterization of the relative nitrogen uptake, speciation of the adsorbates, and the adsorbate stability with temperature following NO and NO2 exposures was assessed using XPS. XPS analyses were conducted using a nonmonochromatic Mg source operated at 96 W. All spectra were acquired using a 50 eV pass energy, 100 ms dwell time, and a resolution of 1.0 or 0.1 eV per channel for survey and core level regions, respectively. A survey spectrum and Ba 3d, Ba 3p, O 1s, N 1s, C 1s, and Al 2p core level spectra were acquired following all depositions and exposures. Area sensitivity factors were used to account for differences in photoionization cross sections when required for data reduction.17 The binding energy scale was corrected by referencing the Al 2p line to 74.1 eV. This referencing scheme resulted in the adventitious carbon line shifting to a value of 284.6 eV, as expected. The thermal stabilities of the species formed were assessed by annealing the exposed film to a temperature of interest (100, 200, 300, and 400 °C), holding for 2 min, and cooling to near room temperature, followed by XPS analyses. Results and Discussion NO and NO2 Adsorption on Barium Oxide. A 15-20 Å barium oxide film was deposited on the aluminum substrate and characterized using XPS. The prepared film was then transferred to the prechamber, exposed to NO or NO2, evacuated, and transferred back to the UHV system for analysis. Subsequent exposures were carried out by repeating this cycle. No desorption step was performed between exposures. The resulting N 1s spectra for a series of NO exposures are summarized in Figure 1A. The primary component of the N 1s spectra was observed at a binding energy of 404.0 eV, consistent with a nitritelike ad-species.18,19 A small feature (90%) of both species. The results for the NO2-exposed films, Figure 11B, clearly show that even at 300 °C more than 60% of the adsorbed species remain on the surface. In addition, the relative ratio of nitrate to nitrite has changed from about 6:1 to 1:1. It is clear from the data that this has occurred via transformation of some of the nitrate species to nitrite species. Also evident is the fact that for the NO2 system nitrite species remain following the 300 °C anneal where they were almost completely desorbed at this temperature for the NO-exposed film. These results imply that the nitrite formed as a result of the annealing process is bound differently than that for the NO-exposed system, or is somehow being stabilized by interactions with surface nitrate species. These
observations could be used as further support for our proposal that some of the nitrates come from coordinated complexes with nitrites. The stability of the cluster, rationalized by either thermodynamic or kinetic arguments, could easily exceed that of linear nitrite species as the data suggests. In addition, the breakup of the clusters at higher temperatures could result in a transformation of nitrate to nitrite through the reversal of the pathway shown in Figure 8. Breakup of the cluster would yield two nitritelike nitrogen atoms from 3 original nitratelike adatoms. This picture would be consistent with the data summarized in Figure 11. Since the XPS results presented only provide information on the residual adsorbed species that remain following thermal anneals, further characterization of desorbing species through temperature programmed desorption (TPD) studies may lend more insight into the details of the adsorption, transformation, and desorption pathways. TPD experiments from in-situ prepared barium oxide thin films are currently in progress and should shed more light on the issue of nitrite formation at low NO2 exposures being the result of NO2 dissociative adsorption or NO adsorption from the source gas background. Implications of Results with Regards to NOx Trapping. As noted earlier, it is well accepted that NOx traps function by oxidizing and storing NOx during lean operation and releasing and reducing the NOx to N2 during periodic rich excursions. However, the mechanistic details of this process are poorly understood. It is commonly reported that conversion of NO to NO2 over precious metal sites is required for nitrate formation, and it is assumed that the nitrate forms by direct adsorption/ reaction of NO2 with barium oxide. However, on the basis of the pathway that is most consistent with our data, one could postulate a rather unique adsorption mechanism that has not
Study of the Trapping Stage of NOx Conversion been considered by other researchers. When one considers that “engine-out” NOx is primarily NO, and that the surface area of the oxide far exceeds that of the dispersed noble metal, then one would assume that initial trapping is dominated by the molecular adsorption of NO to form nitritelike ad-species. As NO2 is formed over noble-metal sites, it could either spill-over onto the oxide and dissociatively adsorb to form nitrite species, or if the surface coverage of nitrite is high enough, it could coordinate with two nitrite species to form a complex nitrate as proposed here. Depending on the transport mechanism of NO2 from the noble metal to the oxide surface, surface nitrate formation via this complexation scheme could be limited to areas directly surrounding noble-metal sites if spillover dominates, or could be more far reaching if gas-phase transport is significant. Although our desorption data suggests that NO adspecies are unstable at temperatures less than 300 °C, and NOx traps typically operate between 350 °C and 400 °C, the thermal stability of the NO derived ad-species is influenced by the UHV environment in which our experiments were performed. NO adsorption on barium catalysts (in the absence of Pt) has been observed at temperatures approaching typical operating conditions.5 In light of our results and those presented in the literature, we offer the scenario described above as an alternative mechanism to be considered when explaining why the addition of noble metal increases trapping capacity, and why nitrite formation precedes nitrate formation. It should be noted that at the higher temperatures and oxygen concentrations used in NOx trap operation, oxygen ad-species on the oxide surface derived from O2 or NO2 dissociation could play an important role that would not be covered by the scope of the work presented here. Peroxide species have been reported for the Ba/MgO system, and interesting questions exist as to their potential role in NOx trapping.25,26 Therefore, the influence of surface oxygen species is certainly an area open for further exploration. In addition, the influence of surface carbonates, which are known to exist under the conditions of NOx trap operation, could further influence the adsorption processes of NO and NO2 and would not be addressed by this study.5 Conclusions In this study, we have focused on the adsorption of NO and NO2 on in-situ evaporated thin films of barium oxide, the primary storage material in lean NOx traps, to accentuate the details of the trapping stage of NOx conversion using these systems. The data presented clearly shows the tendency of NO to adsorb predominantly as a nitrite and NO2 to adsorb predominantly as a nitrate on barium oxide following high exposures at room temperature. In addition, results indicate that saturation coverage is approximately 30% higher for NO2 adsorption than for NO adsorption. These observations are consistent with previous reports in the literature. However, this work is the first that attempts to explain the results based on potential adsorption pathways and ad-species transformations. In the case of NO, our results are most consistent with nitrites forming as a result of molecular adsorption. In the case of NO2, nitrates are favored at high exposure and appear to form via a nitrite intermediate. We propose that the high coverage nitrate forms through the combination of two surface nitrites and a molecularly adsorbed NO2 to form a trimer, whereby all nitrogen centers become nitratelike. This pathway is supported by our data and is consistent with other data in the literature. In light of our results, we offer an alternative and more detailed interpretation of the mechanism of the trapping stage of NOx conversion via NOx traps. Conventionally, it is believed
J. Phys. Chem. B, Vol. 106, No. 16, 2002 4179 that oxidation of NO to NO2 over precious metal sites is required for nitrate formation, and it is assumed that the nitrate forms by direct adsorption/reaction of NO2 with barium oxide. However, we offer an alternative mechanism in which initial trapping is likely dominated by the molecular adsorption of NO to form nitritelike ad-species. As NO2 is formed over noblemetal sites, it can either spill-over onto the oxide and dissociatively adsorb to form more nitrite species, or if the surface coverage of nitrite is high enough, it can coordinate with two nitrite species to form a complex nitrate in which all nitrogen atoms are nitratelike. This scenario would explain the benefit of noble-metal addition on trapping efficiency as well as the progression of nitrite to nitrate formation as recently reported.8 This work is an initial attempt at providing a more detailed understanding of the mechanisms involved in the trapping stage of NOx conversion using NOx traps. We believe that materials development to improve NOx storage capacity and sulfur tolerance will benefit the most by first understanding the fundamental mechanistic aspects of NOx traps. Future experimental efforts will expand our knowledge of lower coverage regimes and will also include the influence of higher temperatures as well as the influence of oxygen on NO and NO2 adsorption. Additional computational studies using density functional theory are being pursued to follow up on the energetics of NO and NO2 adsorption and potential adsorbate structures for the barium oxide system. As mentioned, TPD studies are in progress to lend more insight into the details of adsorption, transformation, and desorption pathways. Additional experiments focusing on the vibrational characteristics of adsorbed species formed on the oxide thin films would certainly add insight and potentially corroborating evidence for some of the surface binding configurations proposed in this study. Acknowledgment. The authors gratefully acknowledge George Graham for his contributions to the adsorption experiments, including the use of his UHV system in which they were conducted. In addition, the authors further acknowledge Roscoe Carter, Joe Holubka, George Graham, and Bill Schneider for their suggestions and fruitful discussions during the review of this manuscript. References and Notes (1) Shelef, M. Chem. ReV. 1995, 95, 209. (2) Miyoshi, N.; Matsumoto, S.; Katoh, K.; Tanaka, T.; Harada, J.; Takahashi, N.; Yokota, K.; Sugiura, M.; Kasahara, K. Soc. Auto. Engin. 1995, #19950809. (3) Hepburn, J. S.; Thanasiu, E.; Dobson, D. A.; Watkins, W. L. Soc. Auto. Engin. 1996, #19962051. (4) Fekete, N.; Kemmler, R.; Voigtlander, D.; Krutzsch, B.; Zimmer, E.; Wenninger, G.; Strehlau, W.; van den Tillaart, J. A. A.; Leyrer, J.; Lox, E. S.; Muller, W. Soc. Auto. Engin. 1997, #19970746. (5) Mahzoul, H.; Brilhac, P.; Gilot, P. Appl. Catal. B: EnVir. 1999, 20, 47. (6) Fridell, E.; Skoglundh, M.; Westerberg, B.; Johansson, S.; Smedler, G. J. Catal. 1999, 183, 196. (7) Yawu, C.; Chuang, S. C. J. Phys. Chem. B. 2000, 104, 4673. (8) Westerberg, B.; Fridell, E. J. Mol. Catal. A: Chem. 2001, 165, 249. (9) Huang, H. Y.; Long, R. Q.; Yang, R. T. Energy Fuels. 2001, 15, 205. (10) Dearth, M. A.; Hepburn, J. S.; Thanasiu, E.; McKenzie, J.; Horne, G. S. Soc. Auto. Engin. 1998, #19982595. (11) Engstrom, P.; Amberntsson, A.; Skoglundh, M.; Fridell, E.; Smedler, G. Appl. Catal. B: EnVir. 1999, 22, L241. (12) Graham, G. W. Surf. Sci. 1992, 268, 25. (13) (a) Aldrich, L. T. J. Appl. Phy. 1951, 22, 1168. (b) Inghram, M. G.; Chupka, W. A.; Porter, R. F. J. Chem. Phys. 1955, 23, 2159. (14) Briggs, D.; Seah, M. P. Practical Surface Analysis, 2nd ed.; John Wiley & Sons: New York, 1995; Chapter 5.
4180 J. Phys. Chem. B, Vol. 106, No. 16, 2002 (15) Wagman, D. D.; Evans, W. H.; Parker, V. B.; Schumm, R. H.; Halow, I.; Bailey, S. M.; Churney, K. L.; Nuttall, R. L. J. Phys. Chem. Ref. Data 1982, 11 (Suppl. 2). (16) Graham, G. W.; Logan, A. D.; Shelef, M. J. Phys. Chem. 1994, 98, 1746. (17) Wagner, C. D.; Riggs, W. M.; Davis, L. E.; Moulder, J. F.; Muilenberg, G. E. Handbook of X-ray Photoelectron Spectroscopy; PerkinElmer Corp., 1979. (18) Moulder, J. F.; Stickle, W. F.; Sobol, P. E.; Bomben, K. D. Handbook of X-ray Photoelectron Spectroscopy Physical Electronics; Physical Electronics, Inc., 1995. (19) Schmitz, P. J. Surf. Sci. Spectra, submitted for publication. (20) Xie, S.; Mestl, G.; Rosynek, M. P.; Lunsford, J. H. J. Am. Chem. Soc. 1997, 119, 10186. (21) Mestk, G.; Rosynek, M. P.; Lunsford, J. H. J. Phys. Chem. B 1997, 101, 9321. (22) Xie, S.; Rosynek, M. P.; Lunsford, J. H. J. Catal. 1999, 188, 24.
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