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Impact of Sulfur Poisoning on the NOx Uptake of a NOx Storage and Reduction (NSR) Model Catalyst Markus Happel,*,† Aine Desikusumastuti,† Marek Sobota,† Mathias Laurin,† and Jo¨rg Libuda†,‡ Lehrstuhl fu¨r Physikalische Chemie II, Friedrich-Alexander-UniVersita¨t Erlangen-Nu¨rnberg, Egerlandstrasse 3, D-91058 Erlangen, Germany, and Erlangen Catalysis Resource Center, Friedrich-Alexander-UniVersita¨t Erlangen-Nu¨rnberg, Egerlandstrasse 3, D-91058 Erlangen, Germany ReceiVed: NoVember 12, 2009; ReVised Manuscript ReceiVed: February 10, 2010
Toward obtaining a more detailed understanding of the influence of SOx poisoning on the mechanism of NOx uptake on a NOx storage and reduction (NSR) catalyst, we have performed a model study under ultrahigh vacuum (UHV) conditions. A supported model catalyst based on a thin well-ordered alumina film grown on a NiAl(110) single crystal is used onto which BaO and Pd particles are deposited. Time-resolved (TR) infrared reflection absorption spectroscopy (IRAS) is applied to identify the species formed during NO2 uptake on the catalyst surface, which has been pretreated with SO2. These results are compared to the NO2 uptake on the corresponding fresh samples. It is shown that SO2 readily decomposes upon adsorption on both the pristine Al2O3 surface and the BaO and Pd loaded model systems. NO2 reoxidizes the decomposition products to surface sulfites and sulfates. A pronounced influence of the SO2 species on the interaction with NO2 is observed leading to reduced NO2 uptake and to a change of the preferential adsorption geometry from bridging to monodentate. A comparison of the integral intensity of the nitrate bands on the Ba-free and the Ba-containing samples furthermore suggests that SO2 adsorbs preferentially on Ba2+ centers rather than on the Al3+ centers. 1. Introduction A substantial reduction of the fuel consumption from automobiles can nowadays be achieved by operating internal combustion engines in the so-called lean-burn mode. During operation of these engines, the air-to-fuel ratio is higher than for conventional operation. The air-to-fuel ratio is usually described by the lambda (λ)-value.1 In lean-burn engines, this value is usually above 1, whereas in conventional combustion engines, which are operated under stoichiometric conditions, the λ-value is around 1. Despite the beneficial effect of reduced fuel consumption, a major problem, however, arises with respect to the conversion of the hazardous substances in the exhaust gas, in particular, NOx. Whereas under stoichiometric conditions the conventional three-way catalyst (TWC)2,3 can be applied, its performance for NOx conversion dramatically decreases under lean-burn conditions. This is mainly due to the large excess of oxygen in the exhaust gas stream. One promising concept to overcome this problem is NOx storage and reduction (NSR) catalysis. It was first suggested by the Toyota Company in 1996.4 In this concept, the catalyst stores NOx by building up a reservoir of intermediates, predominantly, barium nitrates in this case. The lean-burn periods are followed by NOx release and reduction during short, fuel-rich conditions. Numerous studies have been performed during recent years aiming at a more detailed understanding of the mechanisms of NOx adsorption on NOx storage materials.5-10 However, several practical problems persist, including poisoning of the storage material mainly because of SOx. * To whom correspondence should be addressed. Fax: +49-91318528867. E-mail:
[email protected]. † Lehrstuhl fu¨r Physikalische Chemie II. ‡ Erlangen Catalysis Resource Center.
It is generally accepted that SOx blocks several storage sites during lean-burn conditions because of the formation of barium sulfates and aluminum sulfates.11 These species show high thermal stability making it difficult to recover the storage capacity without severe restructuring and deactivation. When switching to fuel-rich conditions, the adsorbed SOx is reduced and forms sulfides on the noble metal particles. These in turn block the oxidation sites for subsequent NOx storage cycles.11,12 Since oxidation of NO to NO2 is crucial to ensure a high-storage capacity, this process will also affect the NOx storage capacity. Thus, SOx leads to severe deactivation during both lean and rich operation periods. Poisoning mechanisms of NSR catalysts and individual components such as Al2O3, BaO, and noble metal surfaces by sulfur oxides have recently been studied by several groups.11,13-19 The underlying reaction mechanism, however, remains poorly understood from a microscopic point of view. This lack of knowledge is mainly due to the complexity of powder catalyst systems and the multicomponent exhaust gas mixture interacting with the surface of the catalyst. Also, the large number of SO2derived surface species and bonding geometries, originating from its amphiphilic character, obstructs the identification of reaction products.20 Typically, these species were assigned by analogy to inorganic complex compounds, and also identification of vibrational frequencies largely rely on comparisons with these complexes.20 One strategy to simplify this problem is to introduce singlecrystal-based model systems. These model systems allow us to simulate certain features of real NSR catalysts without having to deal with their full complexity. The present model study aims at the identification of reaction pathways and intermediates of SO2 adsorption by means of insitu IR spectroscopy as well as exploring the influence of SO2 on the NOx uptake process. We use a well-ordered Al2O3 film
10.1021/jp910777r 2010 American Chemical Society Published on Web 02/19/2010
Sulfur Poisoning on the NOx Uptake of a Catalyst grown on NiAl(110) onto which BaO nanoparticles are deposited. As has been shown previously by high-resolution photoelectron spectroscopy (HR-PES), BaAl2xO1+3x particles form upon thermal treatment of this system.21 Pd nanoparticles were codeposited as the noble metal component. The NO2 uptake mechanism and the kinetics on this Pd/BaAl2xO1+3x/Al2O3/ NiAl(110) model catalyst have been well investigated in detail in our group using scanning tunneling microscopy (STM),22,23 high-resolution photoelectron spectroscopy (HR-PES),21 and infrared reflection absorption spectroscopy (IRAS). In the present study, we present first data on the influence of SO2 adsorption on the mechanism and the kinetics of NOx uptake. 2. Experimental Section The molecular beam/time-resolved IR reflection absorption spectroscopy (MB/TR-IRAS) experiments were performed in an ultrahigh vacuum (UHV) apparatus that allows us to expose the sample surface to up to four effusive and one supersonic beams. A beam monitor is used which allows exact calibration and alignment of the beams. Additionally, the system is equipped with a Fourier transform infrared (FTIR) spectrometer (Bruker IFS 66/v), a beam monitor that allows alignment and intensity calibration of the beams, two quadruple mass spectrometers, a vacuum transfer system, a high-pressure cell, and all necessary preparation tools. The SO2 beam (Linde AG, 99.0%) was generated from the supersonic source and was modulated by a solenoid valve and a shutter (SO2 expansion from room temperature; the backing pressure was approximately 1 bar). All measurements were performed in remote-controlled sequences exposing the sample to variable pulses of SO2 ranging from 53.2 ms to 383 s at a pressure of 1.25 × 10-5 mbar on the sample surface (equivalent to a beam intensity of 2.8 × 1015 cm-2 · s-1). The NO2 beam (Linde AG, 99.0%) was generated from an effusive source and was modulated by a valve system. All measurements were also performed in remote-controlled sequences exposing the sample to pulses of NO2 at variable beam intensities between 6.7 × 1012 cm-2 · s-1 (equivalent pressure: 3.0 × 10-8 mbar) and 1.6 × 1015 cm-2 · s-1 (equivalent pressure: 7.0 × 10-6 mbar) followed by acquisition of IR spectra at the corresponding reaction temperature (300 K). IR spectra were acquired at a spectral resolution of 2 cm-1 with a typical acquisition time of 186 s. For preparation of the model surfaces, the NiAl(110) surface was cleaned by several cycles of Ar+ sputtering and annealing in UHV up to 1300 K. During preparation, the sample was heated by radiative heating and electron bombardment. After preparation of the NiAl(110) surface, two oxidation cycles in 2 × 10-6 mbar O2 at 550 K and UHV annealing at 1135 K followed to prepare the ordered Al2O3 film. With respect to details of the preparation procedure, we refer to the literature.24 The quality of the film was monitored by low-energy electron diffraction (LEED), and completeness of surface oxidation was proven by the absence of CO adsorption at 100 K. For the preparation of the BaAl2xO1+3x particles, we proceeded as follows: First, the Ba metal (rod, Alfa Aesar, 99%) was manually cleaned under inert atmosphere in a glovebox and was placed into a Mo crucible. To prevent oxidation of the metal, the Ba in the crucible was covered with decane before it was mounted inside the evaporator. For Ba deposition, a home-built Knudsen-type evaporator was used that was resistively heated. Ba was deposited at 300 K in UHV at typical rates ranging from 0.03 to 0.065 Å s-1 as calibrated using a quartz microbalance (an average film thickness of 1 Å Ba corresponds to 1.6
J. Phys. Chem. C, Vol. 114, No. 10, 2010 4569 × 1014 atoms cm-2) and subsequently was oxidized by exposure to 1 × 10-6 mbar O2 for 900 s. For the following study, a film thickness of 20 Å Ba was used (3.2 × 1015 atoms cm-2). The BaAl2xO1+3x particles were stabilized by annealing in O2 (1 × 10-6 mbar) up to 800 K for 900 s before exposure to the corresponding gases. Pd (wire, Alfa Aesar, 99.9%) deposition was performed using a commercial electron beam evaporator (Focus EFM 3). The sample was biased to the same potential as the source to avoid ion damage. Typical deposition rates were around 5.7 × 1012 atoms cm-2 · s-1 (1 Å of Pd corresponds to 6.8 × 1014 atoms cm-2) as calibrated using a quartz microbalance. For the underlying study, 4 Å of Pd were deposited on the sample under UHV conditions. When depositing the Pd on the BaAl2xO1+3x particles, the Pd particles were stabilized according to the procedure described elsewhere.25 3. Results and Discussion 3.1. Structure and Morphology of the Model System. The structure, morphology, and composition of the NiAl(110) based model system have been investigated in previous studies by STM and HR-PES.21-23 The results can be summarized as follows. The Al2O3 film on NiAl(110)26 is atomically flat with large terraces of sizes between 500 and 1000 Å separated by monatomic NiAl steps. The defect structure consists of antiphase domain (APD) boundaries and rotational domain boundaries. In addition, some point defects are apparent.22 During deposition of 20 Å Ba on this system, threedimensional particle growth was observed with a particle density of 7 × 1012 cm-2. The uniform distribution of the particles on the terraces and domain boundaries points toward a strong interaction with the support. During oxidation and annealing of the model system, minor sintering occurs, which leads to a decrease of the particle density to 5 × 1012 cm-2, whereas the particulate structure is preserved.27 The strong interaction with the support is confirmed by HR-PES measurements showing an intermixing of Ba2+ and Al3+ ions upon annealing and oxidation leading to the formation of BaAl2xO1+3x mixed oxide particles.21 For the deposition of smaller Ba particles (1 Å), we refer to the literature.22 During deposition of Pd on the pristine Al2O3 film, threedimensional crystallites, which preferentially nucleate at the rotational domain boundaries, are observed. The APD boundaries seem to be less attractive sites for the nucleation, and even fewer particles are found to nucleate on the terraces themselves. The average particle diameter was calculated to be around 6 nm at a particle density around 1 × 1012 cm-2.22 For deposition of Pd on the pristine support, the particles were stabilized by exposure to O2 and CO at elevated temperature. More details about this procedure can be found in the literature.25 A completely different nucleation behavior was observed during deposition of Pd on the previously formed BaAl2xO1+3x particles. A homogeneous nucleation density distributed over the entire surface was observed leading to the conclusion that the particles preferentially nucleate on the preformed mixed oxide particles.27 The same behavior was observed for lower Ba and Pd coverages.22 NO adsorption experiments suggest that the surface properties of the Pd particles on the pristine alumina support and on the BaAl2xO1+3x particles are comparable.27 For more detailed information on the growth and nucleation of the Ba and Pd particles, we refer to the literature.22,27 3.2. NO2 Uptake Experiments on Fresh and SO2-Pretreated Model Systems. The NOx uptake behavior of a Pd/ BaAl2xO1+3x/Al2O3/NiAl(110) model NSR catalyst and its
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Figure 1. (a) Schematic representation of the experimental setup. (b) Acquisition procedure applied in the present experiments (see text for details).
individual components was probed on SO2-predosed samples and was compared to the corresponding fresh samples. Figure 1 shows the procedure applied for the time-resolved IRAS experiments. First, a sequence of SO2 pulses with increasing duration was dosed by a first MB source on the sample surface. The total amount of SO2 dosed was around 1400 L (1 Langmuir corresponds to 1 × 10-6 Torr · s). Subsequently, a sequence of NO2 pulses with increasing intensity and duration was dosed from a second MB source. The total amount of NO2 dosed was around 6000 L. IR spectra were acquired in between all individual pulses. For the fresh samples, the procedure applied for NO2 uptake experiments was identical. NO2 uptake experiments on the fresh samples were published previously,27,28 and the corresponding time-resolved IRAS spectra are not shown again. Nevertheless, the results of these experiments are briefly summarized as the basis for the following discussion. 3.2.1. NO2 Uptake on Fresh and Poisoned Al2O3/NiAl(110). Four prominent peaks are apparent in the spectra during exposure of the fresh Al2O3 sample to NO2. Two weak features appear at 1255 and 1297 cm-1, and two more intense bands are located at 1595 and 1652 cm-1. According to previous studies, the twofold degenerate asymmetric stretching mode νas of ionic NO3- (1380 cm-1) splits into two bands29 upon interaction with the surface. Accordingly, the bands located at 1297 and 1652 cm-1 represent one pair of bands, whereas the bands located at 1255 and 1595 cm-1 may be attributed to a different adsorption site or adsorption geometry. Typically, the splitting is expected to depend on the adsorption geometry and is assumed to be larger for bridging nitrates and smaller for chelating and monodentate nitrates.28,30,31 In a recent study combining density functional theory (DFT) and IRAS,32 it could be shown that the adsorption geometry of surface nitrates on metal-supported model catalysts could also be identified taking into account the metal surface selection rule (MSSR).33 It was shown that for the bridging adsorption geometry, the high-frequency component of νas is polarized perpendicularly to the surface, whereas the low-frequency component shows parallel polarization. As a consequence, the high-frequency component should dominate the spectra in an IRAS experiment. For the monodentate nitrate, the situation is reversed. Following these arguments, the nitrate species on the pristine alumina were identified as bridging
Figure 2. IRAS spectra taken during exposure of Al2O3/NiAl(110) to a total of 6000 L NO2 after predosing the sample with 1400 L SO2. The sample temperature was 300 K. The symbols 4 and O mark the bands that are depicted in the integral intensity plot in Figure 4a. Reference was taken after SO2 exposure. The little sketch shows nitrates in bridging adsorption geometry.
nitrates and the splitting was explained by the adsorption on different sites tentatively on defects and on terraces.28 Figure 2 shows the spectra taken during NO2 exposure on the SO2-predosed Al2O3/NiAl(110). The spectral background was taken after SO2 exposure so that the features observed correspond to the changes on the surface, which occur upon NO2 exposure. A weak absorption feature is visible around 1056 cm-1 together with two more bands at 1255 and 1297 cm-1. In addition, several weak bands are apparent in the region between 1100 and 1400 cm-1. The two dominant bands are located at 1593 and 1657 cm-1. On the basis of the above discussion, the pairs of bands located at 1255 and 1593 cm-1 and at 1297 and 1657 cm-1 are attributed to nitrate species adsorbed on the Al2O3 surface in bridging adsorption geometry. The absorption band apparent at 1056 cm-1 is located in the region where sulfites and sulfates on alumina have been reported to appear. This is noteworthy as the spectra displayed are referenced to a background taken after SO2 exposure. This implies that sulfur-containing species are present on the surface and are modified during NO2 exposure. This might also explain the broad and weak bands between 1000 and 1200 cm-1 apparent in all spectra discussed in the following study. In his book, K. Nakamoto34 gives some possible binding modes for SO2 coordinated to a metal atom and the various vibrational frequencies for the different binding modes. Most of them lie in the range between 1000 and 1300 cm-1. For sulfato complexes, the frequencies cover a range between 970 and 1170 cm-1. For sulfito complexes, the frequencies lie in the range between 800 and 1120 cm-1. Compared to the free SO32- ion, the SO stretching frequencies should be shifted to lower values if coordination occurs via the
Sulfur Poisoning on the NOx Uptake of a Catalyst O atom and should be shifted to higher values if coordinated through the S atom. It is obvious that the spectral regions of the frequencies of the different binding modes of sulfato and sulfito complexes strongly overlap, which complicates a distinct assignment. The strong overlap might also lead to misassignments of the vibrational bands of the sulfur oxide species on surfaces, and it has been shown in the past that one has to treat these comparisons with utmost care. In fact, such comparisons have led to numerous misassignments, for example, for CO, NO, and even nitrates.32,35-37 Well-accepted assignments have been questioned and eventually have been discounted in several cases on the basis of independent experiments or theoretical calculations (see refs 32 and 35-37). Chang38 assigned a band at 1060 cm-1 to sulfites on alumina on the basis of previous studies on MgO and CaO. It is generally assumed that sulfite species are formed upon interaction of SO2 with Lewis acidic sites, such as O2-.38-42 Sedlmair et al.43 also reported that sulfates on alkaline earth metals give rise to an absorption band at 1060 cm-1 together with a band at 1120 cm-1. The formation of SO42- species was also reported by several other groups to take place in the presence of oxygen or other oxidation components and at higher temperatures.38,44 The same situation was found on other alkaline earth metal oxides like MgO. Schneider et al. observed the formation of sulfate species during adsorption of SO2 at higher temperatures (350 °C).45 A transformation of SO3 to SO4 species on MgO was found to take place at higher temperatures or in the presence of oxygen (see ref 46 and refs therein). Thus, oxidation of SO32to SO42- by NO2 may explain the appearance of this peak. One alternative explanation would imply dissociative SO2 adsorption as already reported by Lana et al.47 In this study, it was found that SO2 undergoes exchange reactions with Al2O3 requiring dissociation of the molecule. In this case, the decomposition products could be reoxidized by NO2 to form sulfite. A hint that would support this assumption is the fact that the feature at 1056 cm-1 appears somewhat earlier than the nitrate features at higher wavenumbers. There are further weak and not clearly resolved features located between 1100 and 1400 cm-1, which could most likely also be assigned to sulfite and sulfate species on alumina in accordance with the literature.13,16,38,44,48 However, the width and low intensity of these features prevent a more distinct assignment so far. One explanation for the low intensity would be a flat-lying adsorption geometry of the sulfites in combination with the MSSR allowing the observation of dynamic dipoles perpendicular to the surface only. Similar experiments have been performed to explain the very low intensity of surface nitrites in IRAS.27,28,32,49,50 Indeed, a computational study by Schneider et al.45 confirms this suggestion. They found the molecule to adsorb parallel to the surface on a MgO(001) surface. A possible strategy to experimentally differentiate between surface SO32- and SO42- species would involve adsorption experiments with molecular beams of SO2 and SO3, which are currently in preparation in our laboratory. In this work, we mainly focus on the effect of SO2 preadsorption on the reactivity toward NO2. A direct comparison between the spectra during NO2 adsorption on the fresh and SO2 poisoned samples is given in Figure 3. Only the spectrum corresponding to 6000 L NO2 exposure is shown for each system. In the next step, we focus on the integral intensity of the two more intense nitrate bands at 1593 and 1659 cm-1. The intensity behavior as a function of NO2 exposure is shown in Figure 4a. The corresponding data for the fresh sample is also shown for comparison. A characteristic stepwise increase in the band
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Figure 3. IRAS spectra at the highest NO2 exposure (6000 L) with and without predosing with 1400 L of SO2 for the following samples: (a) Al2O3/NiAl(110), (b) Pd/Al2O3/NiAl(110), (c) BaAl2xO1+3x/Al2O3/ NiAl(110), and (d) Pd/BaAl2xO1+3x/Al2O3/NiAl(110).
intensity is observed for both samples. The effect was previously associated with restructuring of the film in the initial stages of NO2 exposure and saturation at large exposure. For the SO2 predosed sample, both NO2 features are attenuated indicating some degree of blocking of adsorption sites by surface SO32and SO42-. However, the effect is substantially larger for the peak at 1596 cm-1, which was tentatively assigned to nitrates on defect sites. This would indicate that the interaction with SO2 preferentially occurs at the defect sites of the Al2O3 also leading to preferential blocking of these sites. 3.2.2. NO2 Uptake on Fresh and Poisoned Pd/Al2O3/ NiAl(110). Previously, NO2 adsorption on Pd/Al2O3/NiAl(110) was probed at 300 K applying the experimental procedure described before. For a detailed discussion, we refer to a
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Figure 5. IRAS spectra taken during stepwise exposure of Pd/Al2O3/ NiAl(110) to a total of 6000 L NO2 after predosing the sample with 1400 L SO2 (sample temperature 300 K). The symbols 4 and O mark the bands that are depicted in the integral intensity plot in Figure 4b. Reference was taken after SO2 exposure. The little sketch shows nitrates in bridging adsorption geometry.
Figure 4. Comparison of the integral intensity of the most intense nitrate bands after stepwise dosing of a total of 6000 L NO2 on fresh and SO2 predosed (1400 L) samples: (a) pristine Al2O3/NiAl(110), (b) Pd/Al2O3/NiAl(110), (c) BaAl2xO1+3x/Al2O3/NiAl(110), and (d) Pd/ BaAl2xO1+3x/Al2O3/NiAl(110).
previous publication.27 Briefly summarizing the results, the IRAS data can be divided into two regions at low exposure and high exposure. In the limit of the lowest exposure, bands at 1530 (1) and 1630 cm-1 (2) appear. With increasing NO2 dose, these bands blue shift to 1570 and 1668 cm-1, respectively, and an additional feature appears around 1755 cm-1 (3). The three features were assigned to NO on Pd particle sites originating from NO2 dissociation. More specifically, they are attributed to (1) NO at hollow sites of (111) facets, (2) bridge-bonded NO mainly at (111)/(111) edges, and (3) on-top NO mainly at particle corners, respectively.27 The strong blue shift of these features is the result of coverage-dependent dipole interactions, electronic interaction via the Pd support, and the influence of coadsorbed O. With increasing NO2 exposure, all three NO bands disappear completely. Instead, a new band at 1545 cm-1 appears followed by features at 1255, 1297, 1595, and 1652 cm-1 at exposures exceeding 1000 L. The disappearance of NO is attributed to the increasing oxygen coverage leading to subsequent replacement of NO with the band at 1545 cm-1 tentatively being assigned to a nitrate species on oxidized Pd.27 The two pairs of bands at 1255 and 1596 cm-1 and at 1297 and 1655 cm-1 are straightforwardly attributed to the split asymmetric NO stretch of bridging surface nitrates on alumina (see section 3.2.1).27
On the basis of these assignments, we now turn our attention to SO2 predosed Pd/Al2O3/NiAl(110). The corresponding data is displayed in Figure 5. In comparison to the pristine sample, some remarkable differences are observed. First, it is apparent that in the low-exposure region no absorption bands related to NO on Pd are observable. Instead, a feature appears at 1060 cm-1 together with a band located at 1155 cm-1. Similar to the pristine samples, the dominant features in the limit of large exposure appear at 1596 and 1655 cm-1. The absence of the NO related bands in the low-exposure region is the result of a blocking of the Pd particle sites by adsorption of SO2 and its decomposition products. This is in line with SO2 adsorption studies on different transition-metal single-crystal surfaces such as Pt(111), Pd(100), and Ni(111) in which various decomposition products were observed including atomic sulfur, SO, SO3, and SO4.20,51,52 For supported Pt nanoparticles on Al2O3, it was reported that even in the absence of reducing agents sulfide species (S2-) were found on a Pt/ Al2O3 sample.39 A large degree of decomposition to atomic species and flat-lying SOx species would be in agreement with the very weak IRAS signals observed during SO2 exposure. The appearance of the bands at 1060 and 1155 cm-1 upon NO2 exposure suggests that such species are indeed present and can be oxidized to SO32- or SO42- or both. Again, a distinct assignment is not possible because of the various different bonding modes of the sulfur species and the overlapping regions in the spectra (see section 3.2.1). The frequency, comparable with the experiment on Pd-free Al2O3/NiAl(110), may suggest that these species are located on the alumina support (compare refs 38 and 40). With Pd acting as an oxidation catalyst, the formation of sulfate species may also be considered, although formation of sulfates is typically reported at higher tempera-
Sulfur Poisoning on the NOx Uptake of a Catalyst tures.38,44 The band at 1155 cm-1 lies in the region where sulfites and sulfates on alumina have been reported to appear by several groups.13,16,38,44,48 As this feature is much more intense compared to the feature on the pristine alumina sample, new pathways involving Pd can be considered. S and SO species formed on the Pd particles upon SO2 adsorption are likely to be reoxidized by NO2 and, subsequently, may spillover to the alumina forming SO32- and SO42-. In this context, it should be noted that in previous works a strong influence of noble metals on SO32and SO42- has been observed, including a stabilization of these species.53 The two dominant absorption bands at 1596 and 1655 cm-1 are assigned to nitrates on alumina in agreement with the results presented before. However, the overall intensity of the nitrate bands is found to be somewhat reduced when compared to the fresh sample (see Figure 3b). This loss of intensity explains to some extent the absence of the low-frequency component of the nitrates located at 1255 and 1297 cm-1. More importantly, however, these bands may overlap with broad absorption features arising from sulfite and sulfate formation in this region. Sulfite and sulfate species on alumina have been reported to appear in the spectra between 1100 and 1400 cm-1.13,16,38,44,48 If we finally compare the exposure-dependent intensity of the dominating high-frequency bands of the nitrates (see Figure 4b), we find a behavior that is similar to the one observed above for the pristine alumina sample (see section 3.2.1). Again, the more pronounced decrease in intensity for the band at 1596 cm-1 in comparison to the band at 1655 cm-1 suggests preferential interaction of SO2 with the sites tentatively assigned to defects on the support. 3.2.3. NO2 Uptake on Fresh and Poisoned BaAl2xO1+3x/ Al2O3/NiAl(110). The previous results on the interaction of NO2 with BaAl2xO1+3x particles on Al2O3/NiAl(110) may be summarized as follows.28 During the initial stages of NO2 exposure, a broad band appears at 1250 cm-1. This feature was previously assigned to surface nitrites formed during the initial stages of NO2 adsorption. From DFT calculations and the low intensity of this band, a flat-lying adsorption geometry could be derived.32 At higher exposures, a weak band appears at 1030 cm-1 together with two dominating bands at 1331 and 1468 cm-1. The band at 1030 cm-1 was assigned to the symmetrical NO stretch mode (νs) of surface nitrates, and the two dominant bands at higher frequencies correspond to the split asymmetrical NO stretch of surface nitrates (νas). For the adsorption geometry, the same argument holds as for the pristine alumina film: In the monodentate case, the low-frequency component at 1331 cm-1 should be polarized perpendicularly to the surface and, therefore, should be dominant in IRAS. For the bridging adsorption geometry, the high-frequency component at 1468 cm-1 should be oriented perpendicularly. On the basis of these assumptions, it was concluded that in the first stages of NO2 adsorption the monodentate geometry dominates, whereas at large exposure, adsorption occurs predominately in the bridging geometry. The absence of the alumina-related adsorption bands at 1255 and 1297 cm-1 can be explained by the BaAl2xO1+3x particles covering a large fraction of the surface and by preferred adsorption of NO2 on the Ba2+ centers. Depicted in Figure 6 are the spectra taken during NO2 adsorption on the SO2 predosed BaAl2xO1+3x/Al2O3 sample. In the low-frequency range, a weak band appears at 1030 cm-1 together with bands at 1060 and 1155 cm-1. The dominant adsorption bands are located at 1331 cm-1 and 1468 cm-1. In analogy to the pristine sample, the two dominant bands at 1331 and 1468 cm-1 can be assigned to the asymmetrical NO stretch
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Figure 6. IRAS spectra taken of BaAl2xO1+3x/Al2O3/NiAl(110) during stepwise exposure to a total of 6000 L NO2 after predosing the sample with 1400 L SO2 (sample temperature 300 K). The symbols g and ] mark the bands that are depicted in the integral intensity plot in Figure 4c. Reference was taken after SO2 exposure. The little sketch shows nitrates in monodentate and bridging adsorption geometry.
of surface nitrates on Ba2+. The weak feature at 1030 cm-1 appears in parallel with the bands at 1331 and 1468 cm-1 suggesting that it should be attributed to the symmetrical NO stretch mode of the surface nitrates. Finally, the weak and broad bands at 1060 and 1155 cm-1 are again in the region where sulfur oxide species have been reported to appear.13,16,38,44,48 Again, we cannot make distinct assignments because of the various different species and bonding modes of sulfur oxides (see section 3.2.1). As discussed before, these species are most probably formed during NO2 exposure by reoxidation of decomposition products of SO2. The fact that the 1060 cm-1 band begins to appear somewhat before the nitrate bands supports this hypothesis, which is similar to the Al2O3 sample. A direct comparison of the spectra obtained on the pristine and the SO2 predosed BaAl2xO1+3x/Al2O3/NiAl(110) is shown in Figure 3. Pronounced changes appear in the region of the asymmetric NO stretching mode of the surface nitrates. Not only is the total intensity strongly reduced, but also the intensity ratio between the low-frequency component at 1331 cm-1 and the high-frequency component at 1468 cm-1 is strongly affected. The intensity behavior of both components as a function of NO2 exposure is shown in Figure 4c. In contrast to the pristine sample, the high-frequency component after SO2 predosing is strongly attenuated. This observation suggests that upon SO2 pretreatment the adsorption geometry of the surface nitrates changes in favor of monodentate adsorption. Tentatively, this effect may be rationalized by a dilution of the available adsorption sites on Ba2+ by preadsorbed SO32- or SO42- species or both. Under these conditions, it becomes more difficult to find pairs of Ba2+ sites, which would facilitate adsorption of nitrates in bridging geometry. A second observation concerns the effect of SO2 predosing on the formation of surface nitrates on BaAl2xO1+3x and on Al2O3. The much stronger attenuation in the case of BaAl2xO1+3x suggests a more facile SO32- and
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Happel et al. and 1155 cm-1 indicate the formation of SO32- or SO42- species, or both, through reoxidation of decomposition products of SO2 (compare section 3.2.1). Similar to the Pd-free system, the most important observation concerns the split asymmetric stretching mode of the surface nitrates (see Figures 3 and 4d). Here, we only observe partial blocking of the adsorption sites. With respect to the intensity of the high- and low-frequency component at 1333 and 1468 cm-1, the behavior is very similar to the one found for Pd-free BaAl2xO1+3x/Al2O3/NiAl(110) sample. The high-frequency component is found to be strongly suppressed suggesting a change in adsorption geometry in favor of monodentate nitrates. By comparing the general loss of intensity of the nitrate bands on the poisoned sample to pristine samples, we finally conclude that the Pd sites are most efficiently blocked by SO2 followed by the Ba2+ sites and finally by the Al3+ sites which appear to be most the resistant. 4. Conclusion
Figure 7. IRAS spectra of Pd/BaAl2xO1+3x/Al2O3/NiAl(110) taken during stepwise exposure to a total of 6000 L NO2 after predosing the sample with 1400 L SO2 (sample temperature 300 K). The symbols g and ] mark the bands that are depicted in the integral intensity plot in Figure 4d. Reference was taken after SO2 exposure. The little sketch shows nitrates in monodentate and bridging adsorption geometry.
SO42- formation on the Ba2+ centers in comparison to the Al3+ centers on the surface. 3.2.4. NO2 Uptake on Fresh and SO2 Predosed Pd/ BaAl2xO1+3x/Al2O3/NiAl(110). The results obtained during NO2 adsorption on the fresh Pd/BaAl2xO1+3x/Al2O3/NiAl(110) basically combine the results from the Pd/Al2O3 and the BaAl2xO1+3x/ Al2O3 samples:27 In the low-exposure region, again, three absorption bands appear at 1500, 1615, and 1745 cm-1, which are attributed to NO on the Pd nanoparticles (see section 3.2.2). At larger NO2 exposures, bands at 1333 and 1468 cm-1 together with weaker features at 1030, 1250, 1270, 1550, 1620, and 1660 cm-1 are observed. The weak feature at 1250 cm-1 was assigned to flat-lying nitrites, and the dominating bands at 1333 and 1468 cm-1 together with the weak absorption band at 1030 cm-1 were attributed to the asymmetric and symmetric NO stretches of surface nitrates in line with the experiment on the Pd-free sample (see section 3.2.3). The weaker features at 1270, 1550, and 1620 cm-1 are related to the presence of Pd and were attributed to nitrate species on PdO.27 Finally, the feature at 1660 cm-1 only appears when both Pd and Ba are present leading to the tentative assignment as a nitrate species at the PdO/BaAl2xO1+3x boundary or at a mixed-oxide phase.27 The spectra observed during exposure of the SO2 pretreated Pd/BaAl2xO1+3x/Al2O3/NiAl(110) to NO2 are depicted in Figure 7. The most important observation is that all bands attributed to the Pd-related sites are suppressed. Only those bands remain observable which are also observed on the BaAl2xO1+3x/Al2O3/ NiAl(110) sample. This shows that the Pd surface is completely deactivated by SO2 decomposition, not only at small NO2 exposure, but also in the limit of large exposure, where the nonpoisoned Pd particles would normally become oxidized. A complete deactivation by SO2 is in line with results obtained for Pd/Al2O3 for which even the formation of PdS has been observed39 (compare section 3.2.2). Again, the bands at 1060
Using molecular beam methods in combination with timeresolved IRAS, we have studied the influence of SO2 pretreatment on the adsorption of NO2 on model NOx storage and reduction (NSR) catalysts. Specifically, the systems Al2O3/ NiAl(110), Pd/Al2O3/NiAl(110), BaAl2xO1+3x/Al2O3/NiAl(110), and Pd/BaAl2xO1+3x/Al2O3/NiAl(110) were considered. The results were compared to previous studies on the pristine samples and can be summarized as follows. 1. On the Al2O3/NiAl(110), a partial blocking of adsorption sites for surface nitrates is observed after SO2 predosing. The effect is attributed to decomposition of SO2 during exposure at room temperature. The decomposition products are reoxidized by NO2 to surface SO32- or SO42- species or both. The siteblocking effect is moderate and is mainly restricted to defects. The bridging adsorption geometry of nitrates is preserved even on the prepoisoned sample. 2. For Pd/Al2O3/NiAl(110), decomposition of SO2 on the Pd particles leads to complete blocking of NO2 adsorption on the Pd sites even in the limit of large NO2 doses. Upon NO2 exposure, the decomposition products are reoxidized to surface SO32-or SO42- species or both. The effect of SO2 preexposure on the Al2O3 sites is similar as for the Pd-free sample with SO32or SO42- mostly blocking the sites tentatively assigned to alumina defect sites. 3. For BaAl2xO1+3x/Al2O3/NiAl(110), the effect of SO2 predosing on the formation of surface nitrates is substantially larger than for Al2O3/NiAl(110) suggesting preferential adsorption and decomposition of SO2 on the Ba2+ sites. Upon NO2 exposure, the decomposition products are again reoxidized to SO32- or SO42- species or both. The adsorption geometry of the surface nitrates formed on Ba2+ sites is strongly affected by the coadsorbed SO32- and SO42- species. Whereas at large exposure bridging nitrates are dominant on the pristine BaAl2xO1+3x/Al2O3/ NiAl(110), in the presence of coadsorbed SO32- and SO42-, adsorption in monodentate geometry becomes more favorable. 4. For Pd/BaAl2xO1+3x/Al2O3/NiAl(110), similar effects on the reactivity toward NO2 were observed as for BaAl2xO1+3x/Al2O3/ NiAl(110) with SO2 predosing leading to formation of SO32or SO42- species or both during NO2 exposure and partial blocking of adsorption sites for surface nitrates on Ba2+. The preferred adsorption geometry of nitrates changes in favor of bridging species. Finally, SO2 pretreatment leads to complete blocking of NO2 adsorption on the Pd sites even in the limit of large NO2 doses.
Sulfur Poisoning on the NOx Uptake of a Catalyst Acknowledgment. This project was financially supported by the “Deutsche Forschungsgemeinschaft” (DFG). We also acknowledge additional support of the DFG within the Excellence Cluster “Engineering of Advanced Materials” in the framework of the excellence initiative. The present project was performed in cooperation and was supported by the Umicore AG & Co. KG (Automotive Catalysts). We thank Friedemann Rohr (Umicore AG & Co. KG, Hanau) for helpful discussions. M.S. gratefully acknowledges financial support through a Kekule grant of the “Fonds der Chemischen Industrie”. Furthermore, we acknowledge financial support of the “Fonds der Chemischen Industrie”, the DAAD (PPP, Acciones Integradas Hispano-Alemanas), the European Union (COST D-41). The support of Karsten Meyer/Matthias Moll/Carola Vogel (University Erlangen) is acknowledged with respect to the local glovebox facilities. References and Notes (1) Brettschneider, J. Bosch Tech Ber. 1970, 6, 177. (2) Kaspar, J.; Fornasiero, P.; Hickey, N. Catal. Today 2003, 77, 419. (3) Matsumoto, S. I. Catal. Today 2004, 90, 183. (4) Takahashi, N.; Shinjoh, H.; Iijima, T.; Suzuki, T.; Yamazaki, K.; Yokota, K.; Suzuki, H.; Miyoshi, N.; Matsumoto, S.; Tanizawa, T.; Tanaka, T.; Tateishi, S.; Kasahara, K. Catal. Today 1996, 27, 63. (5) Fridell, E.; Persson, H.; Westerberg, B.; Olsson, L.; Skoglundh, M. Catal. Lett. 2000, 66, 71. (6) Broqvist, P.; Gro¨nbeck, H.; Fridell, E. J. Phys. Chem. B 2004, 108, 3523. (7) Desikusumastuti, A.; Happel, M.; Dumbuya, K.; Staudt, T.; Laurin, M.; Gottfried, J. M.; Steinru¨ck, H. P.; Libuda, J. J. Phys. Chem. C 2008, 112, 6477. (8) Desikusumastuti, A.; Staudt, T.; Qin, Z.; Happel, M.; Laurin, M.; Lykhach, Y.; Shaikhutdinov, S.; Rohr, F.; Libuda, J. ChemPhysChem 2008, 9, 2191. (9) Gro¨nbeck, H.; Broqvist, P.; Panas, I. Surf. Sci. 2006, 600, 403. (10) Sedlmair, C.; Seshan, K.; Jentys, A.; Lercher, J. A. J. Catal. 2003, 214, 308. (11) Engstro¨m, P.; Amberntsson, A.; Skoglundh, M.; Fridell, E.; Smedler, G. Appl. Catal., B 1999, 22, L241. (12) Wilson, K.; Hardacre, C.; Baddeley, C. J.; Lu¨decke, J.; Woodruff, D. P.; Lambert, R. M. Surf. Sci. 1997, 372, 279. (13) Abdulhamid, H.; Fridell, E.; Dawody, J.; Skoglundh, M. J. Catal. 2006, 241, 200. (14) Anderson, J. A.; Liu, Z.; Garcia, M. F. Catal. Today 2006, 113, 25. (15) Dawody, J.; Skoglundh, M.; Olsson, L.; Fridell, E. Appl. Catal., B 2007, 70, 179. (16) Fanson, P. T.; Horton, M. R.; Delgass, W. N.; Lauterbach, J. Appl. Catal., B 2003, 46, 393. (17) Kim, D. H.; Kwak, J. H.; Szanyi, J.; Cho, S. J.; Peden, C. H. F. J. Phys. Chem. C 2008, 112, 2981. (18) Mahzoul, H.; Limousy, L.; Brilhac, J. F.; Gilot, P. J. Anal. Appl. Pyrolysis 2000, 56, 179. (19) Rohr, F.; Peter, S. D.; Lox, E.; Ko¨gel, M.; Sassi, A.; Juste, L.; Rigaudeau, C.; Belot, G.; Ge´lin, P.; Primet, M. Appl. Catal., B 2005, 56, 201. (20) Haase, J. J. Phys.: Condens. Matter 1997, 9, 3647. (21) Staudt, T.; Desikusumastuti, A.; Happel, M.; Vesselli, E.; Baraldi, A.; Gardonio, S.; Lizzit, S.; Rohr, F.; Libuda, J. J. Phys. Chem. C 2008, 112, 9835.
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