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A comparison between Pt and Pd in NOx storage catalysts
Applied Catalysis B: Environmental 36 (2002) 145–160
A comparison between Pt and Pd in NOx storage catalysts Sophie Salasc a,b , Magnus Skoglundh a,b , Erik Fridell a,c,∗ a
b
Competence Centre for Catalysis, Chalmers University of Technology, SE-412 96 Göteborg, Sweden Department of Applied Surface Chemistry, Chalmers University of Technology, SE-412 96 Göteborg, Sweden c Department of Applied Physics, Chalmers University of Technology, SE-412 96 Göteborg, Sweden Received 18 February 2001; received in revised form 30 September 2001; accepted 13 October 2001
Abstract The importance of Pt and Pd in noble metal-barium oxide type NOx storage catalysts was investigated. Model Pt/BaO/Al2 O3 and Pd/BaO/Al2 O3 catalysts were prepared and evaluated with respect to NOx storage capacity, activity towards NO reduction under lean conditions and NO oxidation capacity using synthetic lean burn exhausts containing NO, O2 , C3 H6 and N2 . The study was carried out by performing static and transient flow reactor experiments and temperature-programmed desorption studies. At 300 ◦ C, the Pd/BaO/Al2 O3 sample shows a higher NOx storage capacity than Pt/BaO/Al2 O3 , i.e. more NOx is stored during the lean periods and almost all NOx is released and reduced during the subsequent rich periods. At this temperature (300 ◦ C), the NO reduction is not complete during the rich phase for the Pt-based catalyst suggesting poisoning of Pt-sites by adsorbed species. At 400 ◦ C, Pt/BaO/Al2 O3 stores slightly more NOx than its Pd-based counterpart. XPS measurements on pre-treated catalysts, show some changes in oxidation state for Pd between the rich and lean phases. The oxidation of NO is much more limited on Pd based samples compared to Pt containing catalysts. The importance of NO2 as an intermediate in the storage of NOx as nitrate under lean conditions is confirmed in this study. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Lean burn; NOx storage-reduction catalyst; NSR concept; Platinum; Palladium; Barium; Alumina
1. Introduction Lean-burn engines are interesting from an environmental and fuel economy point of view. However, reduction of nitrogen oxides, NOx , in the exhaust using conventional catalytic techniques is difficult because of the large surplus of oxygen. Under these net-oxidizing conditions, three-way catalysts are not effective for NOx removal. One concept to solve this problem includes the NOx storage-reduction (NSR) catalyst [1]. This catalyst usually contains noble metals for reduction and oxidation reactions and a ∗ Corresponding author. Tel.: +46-31-772-3372; fax: +46-31-772-3134. E-mail address: [email protected] (E. Fridell).
NOx storage compound (typically an alkaline earth compound) supported on alumina with high surface area. The NSR catalyst operates under lean periods separated by rich spikes. During the lean conditions, NOx is stored in the trap, while hydrocarbons and carbon monoxide are oxidized on the noble metals. To regenerate the catalyst, the engine is turned to rich conditions for a short period during which the stored NOx is released and reduced over noble metal sites. Concerning the mechanisms for NOx storage and reduction over noble metal-barium oxide-based catalysts, it is interesting to focus on the role of the noble metals. Indeed, besides their role to oxidize hydrocarbons and carbon monoxide, they are essential both for oxidizing NO to NO2 and for reducing stored NOx . Among various supported platinum group metal
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catalysts (Pt, Pd, Rh, Ir), Pt is the more active component for continuous reduction of NOx by hydrocarbons in oxidizing atmospheres [2–6]. Numerous studies investigating the activity of Pt-based catalysts for NOx reduction by hydrocarbons have been carried out in recent years including both steady-state operation (continuous reduction) [2–16] and cycling conditions (NSR concept) [1,17–19]. Few research groups have as yet published works on Pd based catalysts and they only concern steady-state experiments [2–6]. No results on transient studies have been published so far. Nevertheless, Pd has distinct catalytic properties which could make it a desirable component in NSR catalysts. First of all, Pd exhibits a high three-way activity, i.e. both oxidation and reduction reactions are catalyzed in stoichiometric gas compositions [20–25]. Pd/Al2 O3 catalysts exhibit higher three-way activity around stoichiometric gas compositions than Pt/Al2 O3 catalysts. In specific, compared to Pt, palladium catalyses the simultaneous oxidation of CO and HC, and reduction of NOx more effectively under dynamic conditions and over a wider stoichiometric window under static conditions [26,27]. This behavior could be interesting in the NSR system, since the concept involves the two kinds of reactions. Moreover, supported Pd are among the most promising catalysts for the complete oxidation of methane [28–31], which has a high potential as future automotive fuel. Pd also shows a good thermal stability, in particular its ability to maintain activity under lean conditions at high temperatures [25,32,33] and excellent light-off characteristics (good activity from low temperatures) [25,34]. Finally, some literature data indicate that surface and bulk properties of the Pd/PdO system are quite variable, resulting in structural and activity changes in response to variations in temperature and gas environment, especially the partial pressure of oxygen [25,32,35–39]. For example, during methane oxidation, Pd catalysts supported on alumina can easily undergo a transition between metallic Pd and oxidized PdO [38,39]. This property may be important in the NOx storage concept since the catalyst works alternatively under lean and rich conditions. Considering the Pd characteristics mentioned above, one can expect activity from Pd with respect to promoting storage and release of NOx in noble metal-based NOx storage catalysts.
The objective of this investigation is to compare the reactivity of Pt and Pd in noble metal-barium based NOx storage catalysts. This was carried out by performing transient reactor studies and heating ramps using synthetic lean burn exhausts containing NO, O2 , C3 H6 and N2 as well as temperature programmed desorption (TPD) of nitrogen oxides and XPS, for Pt/Al2 O3 , Pt/BaO/Al2 O3 , Pd/Al2 O3 and Pd/BaO/Al2 O3 catalysts. 2. Experimental 2.1. Experimental section 2.1.1. Catalyst preparation Two monolith catalysts with alumina, a storage compound (BaO) and either Pt or Pd were prepared. Monolith samples of cordierite (2MgO·2Al2 O3 ·5SiO2 ) with a length of 15 mm and a cross-section consisting of 69 channels with 64 channels/cm2 were sequentially impregnated with alumina, barium oxide and salt solutions of platinum or palladium, resulting in a total washcoat weight of about 270 mg with 77, 20 and 3 wt.% of Al2 O3 , BaO and noble metal, respectively. Alumina was deposited by repeated immersions in an alumina slurry (80 wt.% ␥-Al2 O3 and 20 wt.% boehmite as binder). The excess of slurry was removed by blowing air through the channels and then the alumina washcoat was dried with hot air (95 ◦ C) for 5 min and calcined in air at 550 ◦ C for 5 min in order to transform boehmite into alumina and fixate the alumina to the monolith walls. This procedure was repeated until the desired amount of alumina was deposited. The alumina washcoat was finally calcined in air at 550 ◦ C for 90 min. This dip-coating procedure was also applied for the impregnation of barium using a barium nitrate slurry. The noble metal, Pt or Pd, was deposited by filling the monolith channels with aqueous solutions of tetra-ammineplatinum(II) hydroxide, [Pt(NH3 )4 ][OH]2 , or tetraamminepalladium(II)nitrate, [Pd(NH3 )4 ][NO3 ]2 , followed by evaporation at 90 ◦ C for 2 h and finally oxidation in air at 550 ◦ C for 1 h. Two monoliths without any storage compound were also prepared using the same methods. The compositions and the BET surface areas of the four monolith samples are given in Table 1.
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Table 1 Composition, estimated noble metal dispersion and BET surface area of the samples
Pt/BaO/Al2 O3 Pd/BaO/Al2 O3 Pt/Al2 O3 Pd/Al2 O3
Washcoat mass (mg)
Pt (wt.%)
Pd (wt.%)
BaO (wt.%)
Al2 O3 (wt.%)
Dispersion (%)a
SBET (m2 /g catalyst)
260 274 270 268
2.7 – 2.8 –
– 2.7 – 2.7
24.2 21.9 – –
73.1 75.4 97.2 97.3
9 7 6.5 6
18.7 16.6 27.3 31.5
a Estimated from NO TPD experiments performed on pre-reduced samples assuming one NO adsorption site per surface noble metal atom.
2.2. Reactors and testing procedures Transient reactor studies at constant temperature and heating ramps with constant gas composition were performed in a flow reactor system described elsewhere [24]. The catalyst was placed in a horizontal quartz tube encased in a furnace. The gases were introduced via mass-flow controllers and the outlet gases were analyzed on-line: total hydrocarbon (VE5 flame ionization detector, JUM Engineering), CO2 (UNOR 6N IR detector, Maihak), O2 (OM-14 polarographic oxygen analyzer, Beckman), N2 O (UNOR 610 IR detector, Maihak), NO and NO2 (CLD 700 EL ht chemiluminescence detector Tecan). Prior to the transient reactor studies and heating ramps, the catalysts were reduced in H2 at 450 ◦ C for 15 min and stabilized in the lean gas mixture (see Table 2) for 2 h. The heating ramps from about 50 to 550 ◦ C were performed under constant lean gas composition with a heating rate of 5 ◦ C/min. For the transient experiments, in order to simulate NSR cycles, the catalyst was intermittently exposed to a synthetic lean and rich exhaust gas mixture (see Table 2). Successive series of lean storage (240 s) and rich regeneration (60 s) periods were performed
Table 2 Gas composition during flow reactor studiesa
Lean Rich
NO (ppm)b
C3 H6 (ppm)
O2 (%)
300–600 300–600
900 900
8 0
a The gas mixtures were balanced with nitrogen to maintain a constant flow of 3500 ml/min for the pre-treatments and 2500 ml/ min for the experiments. b 600 ppm NO during the pre-treatments and temperature ramps; 300 ppm NO during the transient experiments.
at 400 ◦ C. Maximum in NOx storage has previously been observed around 380 ◦ C for Pt-Rh/BaO/Al2 O3 [19]. The amount of stored NOx was calculated from the difference between the inlet NOx level and the measured NOx signal, as described in [19]. The TPD measurements were performed in a different flow reactor system [40]. It consists of a vertical quartz tube connected to a quadrupole mass spectrometer (Balzers QMA 120). The samples were either pre-reduced (5% H2 in Ar) or pre-oxidized (5% O2 in Ar) at 500 ◦ C for 15 min. Then, they were exposed to 1000 ppm of either NO (15 min) or NO2 (30 min) in Ar at room temperature with a flow of 100 ml/min. After flushing with Ar at room temperature for 30 min, a 40 ◦ C/min heating ramp up to 550 ◦ C was performed in an Ar flow (50 ml/min). The noble metal dispersions of the samples were estimated from NO-TPD experiments performed on pre-reduced samples (see Table 1). XPS analysis was performed for Pt/BaO/Al2 O3 and Pd/BaO/Al2 O3 monolith samples prepared in the same way as described above. The samples were pretreated in a reactor connected to the XPS chamber (Perkin Elmer PHI 5000 C). They were initially reduced in H2 at 450 ◦ C for 15 min and then exposed to a lean gas mixture (1000 ppm NO, 1000 ppm C3 H6 , 8% O2 ) first at 500 ◦ C and then at 400 ◦ C for 30 min. After evacuation, the samples were cooled down to room temperature under vacuum and transferred to the UHV analysis chamber without being exposed to air. The XPS spectra were recorded using monochromatic Al K␣ radiation and the Pd 3d and Pt 4f levels were studied. Spectra were also taken after exposing the samples to a rich gas mixture (1000 ppm NO, 1000 ppm C3 H6 ) at 400 ◦ C for 20 min. The energy scale was calibrated by setting the Al 2s peak at 119.3 eV [50].
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3. Results 3.1. Heating ramp with constant lean gas composition The outlet NO, NO2 , NOx (NO + NO2 ), N2 O and CO2 concentrations during heating ramps under the lean gas mixture are shown in Fig. 1 for the Pt/BaO/ Al2 O3 and Pd/BaO/Al2 O3 samples. The nitrogen balance, Nbal , which is the difference between the inlet
nitrogen oxide concentration and the concentration of detected nitrogen compounds in the outlet gas mixture, i.e. Nbal = NOinlet −NOoutlet −NO2outlet −2N2 Ooutlet , is calculated. Assuming that no other nitrogen containing species but N2 , N2 O, NO and NO2 are formed, (1/2)Nbal is equal to the amount of N2 formed during the experiment. This calculated N2 concentration is shown as “N2 ” in Fig. 1. The hydrocarbon light-off (rapid increase of the CO2 signal) occurs in the same temperature range, between 220 and 240 ◦ C for the
Fig. 1. Outlet concentrations of NO, NO2 , NOx (NO + NO2 ), N2 O, CO2 and calculated nitrogen, “N2 ”, for a heating ramp (5 ◦ C/min) in a gas mixture of 600 ppm NO, 900 ppm C3 H6 , 8% O2 (balanced with N2 ) for (a) Pt/BaO/Al2 O3 , (b) Pd/BaO/Al2 O3 .
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Fig. 2. Conversion of NO (NOconv.), part of the conversion due to reduction to N2 O and N2 (NOred.) and part due to oxidation to NO2 (NOoxid.) for a heating ramp (5 ◦ C/min) in a gas mixture of 600 ppm NO, 900 ppm C3 H6 , 8% O2 (balanced with N2 ) for Pt/BaO/Al2 O3 and Pd/BaO/Al2 O3 .
two catalysts. For the Pt/BaO/Al2 O3 catalyst, the maximum in NO conversion is observed at 230 ◦ C with about 45% conversion (see Fig. 2). The NO conversion starts with an initial reduction of NO (N2 and N2 O formation) followed by NO oxidation to NO2 (see Figs. 1a and 2). On the other hand, for the Pd/BaO/Al2 O3 catalyst, there is almost no NO oxidation observed and the NO reduction is limited with a maximum of 7% at 245 ◦ C (see Figs. 1b and 2). The storage of NOx will not significantly influence the results from this type of experiment since the time scale for the temperature ramp is slow (5 ◦ C/min) compared to the storage rate. The storage capacity will, thus, become saturated and conditions close to steady-state will prevail throughout the ramp. The resulting NO concentrations are essentially determined by continuous reduction of NO by propene under lean conditions. The storage capacity is best measured in transient experiments at constant temperature (see following sections). The results in NO conversion are quite similar with or without barium oxide (not shown). For Pt/Al2 O3 , the maximum in NO conversion is as high as for the catalyst containing barium oxide (45% at 240 ◦ C). However, Pt/Al2 O3 shows higher NO oxidation activity (and less NO reduction) compared to Pt/BaO/Al2 O3 [48]. For Pd/Al2 O3 , there is almost no NO oxidation and the NO conversion
due to NO reduction reaches a maximum of 9% at 235 ◦ C. 3.2. Transients in gas composition at constant temperature In the concept where NOx storage catalysts are used for NOx reduction in lean exhausts, the normal lean conditions are interrupted by short rich periods in order to regenerate the catalyst from stored NOx . The difference between the lean period (NOx storage) and the rich period (regeneration) in this investigation is that the oxygen is turned on and off, respectively and compensated with inert nitrogen to maintain a constant flow. In the present experiments, a lean period of 240 s is followed by a rich period of 60 s. The concept of NOx storage, the procedure and the calculation of the amount of NOx stored are given in detail elsewhere [18,24]. Four cycles were run in order to check the reproducibility. The gas compositions are given in Table 2. Fig. 3 shows the results of the whole experiment performed using the Pt/BaO/Al2 O3 catalyst at 400 ◦ C. When oxygen is switched on (see for example the second cycle in Fig. 3 at around 305 s), there is a relatively slow increase in the NOx signal (both NO and NO2 signals) which means that some NOx is stored in
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Fig. 3. Outlet NO, NO2 , NOx (NO + NO2 ), N2 O and O2 concentrations during a transient experiment for the Pt/BaO/Al2 O3 catalyst at 400 ◦ C with the gas compositions given in Table 2.
the catalyst. When oxygen is turned off (rich phase), almost all NOx is reduced by C3 H6 . Furthermore, the NOx stored in the previous lean period is also released and reduced. As there is a good reproducibility of the cycles, only the second cycle will be shown in the next figures. In Fig. 4, the outlet NO and NO2 signals resulting from transient experiments at 400 ◦ C for the Pt/BaO/ Al2 O3 , Pd/BaO/Al2 O3 , Pt/Al2 O3 and Pd/Al2 O3 catalysts are shown. First of all one may note that for the catalysts without barium oxide, i.e. Pt/Al2 O3 and Pd/Al2 O3 , no NOx storage can be observed at 400 ◦ C which is manifested by the rapid increase of the outlet NO and NO2 signals (close to the system response) immediately after switching from rich to lean gas composition (see Fig. 4). Moreover, the NO reduction by propene during the rich period is progressively diminished with time, especially for the Pt/Al2 O3 sample. For the catalysts containing the NOx storage compound, i.e. Pt/BaO/Al2 O3 and Pd/BaO/Al2 O3 , there is storage of NOx during the lean phase (see Fig. 4). The amount of stored NOx at 400 ◦ C is 0.033 and 0.031 mole NOx per mole Ba for the Pt/BaO/Al2 O3 and Pd/BaO/Al2 O3 samples, respectively. The amount stored NOx in the Pt-containing catalyst is in agreement with previous results [19]. For the Pd-containing
sample, we can observe a larger NO breakthrough peak when we switch from the regeneration (rich period) to the storage phase (lean period), as can be seen in Fig. 4a at around 305 s. This means that there is low effective NOx storage for a short while at the beginning of the lean period. Contrary to the catalysts without the NOx storage compound, almost all NO is reduced by C3 H6 during the rich phase for the Pd/BaO/Al2 O3 catalyst. Break-through peaks in the NOx signal (mainly NO) can also be observed when switching from the storage phase to the regeneration phase (see, e.g. at around 545 s in Figs. 3 and 4). The break-through peaks observed when switching from lean to rich conditions are related to the decomposition of stored nitrate compounds and the difference in binding properties for NO on reduced and oxidized noble metal [19]. Another point to notice is the production of NO2 during the lean phase which is much lower for the Pd-containing catalysts than for the samples with Pt. The transient experiments were also performed at 300 ◦ C for the Pt/BaO/Al2 O3 and Pd/BaO/Al2 O3 samples. The NOx signals are shown in Fig. 5 for the second cycle. At 300 ◦ C, for the Pd-based catalyst, the NOx breakthrough peak when switching from the rich to the lean period cannot be observed. The amount of stored NOx at 300 ◦ C is 0.016 and 0.029 mol NOx per mole Ba for the Pt/BaO/Al2 O3 and Pd/BaO/Al2 O3
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Fig. 4. NO (a) and NO2 (b) concentrations during a transient experiment over Pt/BaO/Al2 O3 , Pd/BaO/Al2 O3 , Pt/Al2 O3 and Pd/Al2 O3 at 400 ◦ C with the gas compositions given in Table 2.
catalysts, respectively. For the Pt-containing catalyst, the NO reduction is incomplete and continuously decreases with time during the rich phase. 3.3. TPD measurements Temperature-programmed desorption experiments were performed for both pre-reduced and pre-oxidized
Pt/BaO/Al2 O3 and Pd/BaO/Al2 O3 samples and for pre-reduced Pt/Al2 O3 and Pd/Al2 O3 . The samples were exposed to either NO or NO2 at room temperature. Selected experimental results are presented in Fig. 6. Fig. 6a and b show TPD results after exposing pre-oxidized Pt/BaO/Al2 O3 and Pd/BaO/Al2 O3 samples to NO2 . Three NO-desorption peaks can be
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Fig. 5. Outlet NOx (NO + NO2 ) concentration during a transient experiment for the Pt/BaO/Al2 O3 and Pd/BaO/Al2 O3 catalysts at 300 ◦ C with the gas compositions given in Table 2.
observed (m/e: 30). The two first peaks are located around the same temperature for the two catalysts (115 and 310 ◦ C for the Pt-based and 120 and 295 ◦ C for the Pd-containing catalyst). These peaks are assigned as weakly adsorbed NO (tentatively the first one from noble metal sites and the second peak from the support). The third peak observed at higher temperature, corresponds to desorbed NO2 and/or decomposed NO3 species either as NO2 (m/e: 46) or decomposed to NO and O2 (NO m/e: 30 and O2 m/e: 32). There is also desorption of N2 O (m/e: 44) and a small desorption of N2 (m/e: 28). The m/e: 44 could also partly consist of CO2 as the Ba-containing samples will form BaCO3 when exposed to ambient air. The desorbed NO2 may come from both the noble metal and the support. The slow decrease observed after the peak maximum could be assigned as NO2 from the barium oxide [19]. Indeed, in the case of samples without BaO, i.e. Pt/Al2 O3 and Pd/Al2 O3 , the high temperature peak is better defined and is finished before the end of the temperature ramp (see Fig. 6e and f). The maximum of this peak is observed at a lower temperature for the Pt/BaO/Al2 O3 sample (435 ◦ C compared with 505 ◦ C for the Pd/BaO/Al2 O3 sample, see Fig. 7). It is also interesting to compare the desorption spectra in Fig. 7 at 300 and 400 ◦ C (the temperatures at which transient experiments were performed). At 300 ◦ C, there is almost the same amount of desorbed species for the two catalysts whereas at 400 ◦ C the amount is significantly
larger for the Pt/BaO/Al2 O3 catalyst compared with the Pd/BaO/Al2 O3 sample. At 300 ◦ C only NO desorption can be observed whereas at 400 ◦ C also NO2 desorbs. Thus, for the two catalysts, NO desorption occurs in the same range of temperature and almost the same amount of NO desorbs, whereas NO2 desorption at lower temperatures is faster in the case of the Pt catalyst. The TPD curves after exposing pre-reduced samples to NO2 (not shown) are similar to those for pre-oxidized ones. This is most likely because NO2 oxidizes the samples already at room temperature [41]. Similar experiments but with NO instead of NO2 were performed for pre-reduced and pre-oxidized samples (see Fig. 6c and d for pre-oxidized and Fig. 6g and h for pre-reduced samples). A much smaller quantity is adsorbed on the catalysts compared with NO2 exposure. The NO experiments show one main NO-desorption peak at around 110–120 ◦ C. There are also two NO peaks at higher temperatures. The first one at 465 and 495 ◦ C for Pt/BaO/Al2 O3 and Pd/BaO/Al2 O3 , respectively, and the second one emerging from 500 to 530 ◦ C, respectively, from these two samples. The latter peak is connected with corresponding O2 and mass 44 (probably CO2 ) peaks and is not finished yet at 550 ◦ C. For the pre-reduced samples, some N2 O desorbs together with NO with peaks at about the same temperatures as for the pre-oxidized case. At around 250 ◦ C, some N2 also desorbs. Thus,
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Fig. 6. NO and NO2 TPD measurements: (a) NO2 on pre-oxidized Pt/BaO/Al2 O3 ; (b) NO2 on pre-oxidized Pd/BaO/Al2 O3 ; (c) NO on pre-oxidized Pt/BaO/Al2 O3 ; (d) NO on pre-oxidized Pd/BaO/Al2 O3 ; (e) NO2 on pre-reduced Pt/Al2 O3 ; (f) NO2 on pre-reduced Pd/Al2 O3 ; (g) NO on pre-reduced Pt/BaO/Al2 O3 and (h) NO on pre-reduced Pd/BaO/Al2 O3 .
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Fig. 6 (Continued ).
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Fig. 6 (Continued ).
Fig. 7. NO and NO2 concentrations from NO2 -TPD for pre-oxidized Pt/BaO/Al2 O3 and Pd/BaO/Al2 O3 samples.
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NO is reduced on the surface but since no desorption of oxygen is observed this indicates that the samples become oxidized. 3.4. XPS studies The Pd 3d XPS spectra of the Pd/BaO/Al2 O3 sample treated at 400 ◦ C under lean and rich conditions are shown in Fig. 8a. For the two different pre-treatments, XPS data show binding energies of the Pd 3d(5/2) level at 335.2–335.3 and 336.9–337 eV,
corresponding to the oxidation states 0 and +2, respectively [42]. When switching from lean (a) to rich (b) conditions, we can observe a change in the ratio PdO/Pd0 . This ratio is high after lean pre-treatment and becomes lower after rich exposure. This means that Pd is partially reduced when the gas mixture is switched from lean to rich. Similar XPS experiments for the Pt/BaO/Al2 O3 catalyst are shown in Fig. 8b. This spectra is influenced by the presence of the Al 2p peak (at 74 eV for ␥-alumina). From the size of the Al 2s peak (which should be somewhat larger than the
Fig. 8. XPS spectra of the Pd 3d level for Pd/BaO/Al2 O3 (a) and the Pt 4f level for Pt/BaO/Al2 O3 (b) after pretreatment at 400 ◦ C under ‘a’—lean mixture (1000 ppm NO, 1000 ppm C3 H6 , 8% O2 ) and ‘b’—rich mixture (1000 ppm NO, 1000 ppm C3 H6 ).
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Al 2p peak) we note that the Al 2p peak contribute with approximately 20% of the peak area in Fig. 8b. However, the interesting point here is to compare the spectra after the two pre-treatments. From these experiments, we could observe only metallic Pt. No shift could be observed between the lean and rich pre-treatment conditions at 400 ◦ C.
4. Discussion: differences between Pt and Pd in NOx storage The heating ramp experiments show that Pd containing catalysts (i.e. Pd/Al2 O3 and Pd/BaO/Al2 O3 ) are almost inactive towards continuous NO reduction under static lean conditions (Fig. 1b) while activity is observed for Pt (Fig. 1a). This is in agreement with the literature data about the activity of Pd [2–6] and Pt [4,5,10,18,47] for continuous reduction of NOx by hydrocarbons under oxidizing conditions. Nevertheless, Pd was found to be active under transient conditions. Indeed, significant amounts of NOx were found to be stored under lean conditions in both Pt and Pd catalysts containing barium oxide. At relatively low temperature (300 ◦ C), the Pd/BaO/Al2 O3 catalyst is more active to store and reduce NOx than its Pt-based counterpart. On the other hand, at 400 ◦ C, the Pt/BaO/Al2 O3 sample stores slightly more NOx . Some differences between the Pt/BaO/Al2 O3 and Pd/BaO/Al2 O3 catalysts have been pointed out from this study: at 400 ◦ C, lower effective storage of NOx at the beginning of the lean period for the Pd based catalyst; at 300 ◦ C, no complete reduction of NO on the Pt-containing catalyst during the rich period; different behavior towards NO2 . These differences are discussed in the following paragraphs. 4.1. Pt-Pd oxidation state under transient experiments During the transient experiments performed at 400 ◦ C, there is a NO breakthrough peak when switching from rich to lean conditions for all samples. The peak is most significant for the samples without BaO, pronounced for the Pd/BaO/Al2 O3 sample and just an indication for the Pt/BaO/Al2 O3 sample (Fig. 4a). We suggest that this peak originates from desorption of NO adsorbed on Pt or Pd during the rich phase. For a
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Pt/Al2 O3 sample, Olsson et al. [46] have calculated a NO coverage of about 0.1 when the sample is exposed to 600 ppm NO at atmospheric pressure and 400 ◦ C. When oxygen is introduced, the NO-surface bond may be weakened and NO will be replaced by more strongly bonded oxygen. The mechanism for this is not clear but it has been observed that the activation energy for oxygen desorption will decrease with increasing oxygen coverage [43]. It is possible that chemisorbed oxygen has a similar effect on NO. Further, if subsurface Pt oxide is formed, as suggested in several studies of oxygen adsorption on Pt at atmospheric pressure [44,45], the NO–Pt bond may become weaker. A similar peak is not observed for Rh containing samples [19]. In this case no coverage of NO will be built up since the reaction between NO and C3 H6 will be effective, i.e. the NO reduction is complete during the entire rich phase for Rh containing samples. In order to explain the difference between the Pd/BaO/Al2 O3 and the Pt/BaO/Al2 O3 samples in this respect we suggest that during the rich phase there is reduction (or partial reduction) of PdO. This hypothesis is supported by the fact that for the Pd/BaO/Al2 O3 catalyst there is a small production of NO2 during the rich phase (see Fig. 4b between 250 and 310 s) probably originating from a reaction between NO and the palladium oxide forming NO2 and reduced Pd sites. Then, when the conditions are switched from rich to lean, it is difficult (because of the reduced sites) to produce NO2 which is a precursor to stored nitrate [1,19,47] and, therefore, the effective NOx storage is relatively low at the beginning of the lean period. Nevertheless, after a short time under lean conditions, the reduced palladium becomes oxidized and the NOx storage can start. For the Pt/BaO/Al2 O3 catalyst the NO adsorbed on Pt can more rapidly be oxidized into NO2 and subsequently stored leading to only a small NO desorption peak. The XPS data show that Pd is partially reduced under rich conditions at this temperature (400 ◦ C). This strengthens the hypothesis of a change in the oxidation state of Pd when switching from rich to lean atmosphere under the transient experiments. On the other hand, no change was observed between the Pt XPS spectra when switching between lean and rich pre-treatments. In principle, the NO peak observed at the transition from rich to lean conditions for the Pd/BaO/Al2 O3 sample could also be a desorption peak originating in different binding properties
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for NO on Pd and PdO, respectively. However, the TPD measurements indicate no such difference. At 300 ◦ C, the NO peak cannot be observed anymore probably because of the lower desorption rate at this lower temperature. It is also likely that this temperature is too low for reduction of Pd to take place during the rich periods. If palladium is kept in oxidized form, NO oxidation is possible as soon as the conditions are altered to lean and then the NOx storage can occur. The formation of these peaks may also be connected with the oxidation of HC derived species, formed on the noble metal surface during the rich conditions, that may cause a local temperature increase when switching to lean conditions, in turn leading to NO desorption. 4.2. Self-poisoning during the rich phase For the Pt/Al2 O3 and Pd/Al2 O3 catalysts, NO was incompletely reduced by propene at 400 ◦ C during the rich phase of the transient experiments (Fig. 4a). This may be due to self-poisoning of the reaction by adsorption on the noble metal sites by NO or by propene or propene derived species (CO, carbonaceous products). The NO reduction is less effective on Pt/Al2 O3 than on Pd/Al2 O3 . Therefore, the reaction on Pt seems to be more sensitive to self-poisoning than on Pd but this observation may also be influenced by that the Pd catalyst contains roughly twice as many noble metal sites as the Pt catalyst. At this temperature (400 ◦ C), in the presence of barium oxide, i.e. for Pt/BaO/Al2 O3 and Pd/BaO/Al2 O3 , the reduction of NO is almost complete during the rich periods. The barium oxide, thus, seems to prevent the self-poisoning of the reaction. This may be explained by migration of NOx originating from the decomposition of barium nitrate to the noble metal sites via spill-over [48] or by spill-over of hydrocarbons that adsorb on BaO. These migrating species may then react with adsorbed species on the noble metal surface. The importance of reversible NO2 spill-over between Pt and BaO sites for effective NOx storage/release has previously been addressed for the Pt/BaO/Al2 O3 system [48]. For Pt/Al2 O3 or Pd/Al2 O3 , either NO or C3 H6 may not be able to adsorb on the precious metal sites because of the blocking effect of the other components and, thus, have difficulty to react. Another explanation for this promotion of NO-reduction by
barium oxide under rich periods could be some kind of interaction between the noble metal and barium oxide [41]. Indeed, during the deposition of the noble metals while preparing the catalyst, some dissolution and re-precipitation of barium oxide can occur, leading to a partial coverage of the noble metal crystallites by barium containing species. However, no shift in the IR band for chemisorbed CO on Pt0 is observed when Pt-containing samples with or without barium oxide, are exposed to CO [49], i.e. no specific influence of Ba on the CO–Pt bond could be observed. Thus, the former explanation seems more consistent, i.e. migrating species which react with adsorbed species on the noble metal surface. At 300 ◦ C, the barium oxide does not prevent the self-poisoning of the NO reduction over Pt/Al2 O3 (Fig. 5). This confirms that Pt is more sensitive than Pd/PdO with respect to self-poisoning under these conditions. 4.3. Behavior towards NO2 The results reported above indicate that Pt and Pd containing catalysts do not show the same behavior towards NO2 . During NO2 -TPD with pre-oxidized samples (Figs. 6a and b and 7), almost the same amount of species desorbs up to 350 ◦ C, species which are assigned as adsorbed NO. The high temperature peaks corresponding to NO2 desorption are shifted to higher temperatures for the Pd based catalysts. Thus, NO2 is more strongly bound in the case of the Pd containing catalysts. We also pointed out that NO oxidation is more favored on Pt containing catalysts than on the corresponding Pd containing catalyst. Less NO2 is produced on Pd based samples during the lean phase of the transient experiments (Fig. 4b) and no oxidation of NO could be observed for these catalysts during the temperature ramp (Figs 1b and 2). A reason for less NO2 production on Pd-based samples could be that NO2 is more strongly bound to the Pd surface than to the surface of Pt as mentioned above. There is a higher adsorption yield of NO2 at room temperature compared to NO (Fig. 6a and b compared to Fig. 6c and d). We can also notice that at 400 ◦ C for the Pd/BaO/Al2 O3 catalyst, the breakthrough peak at the switch from rich to lean conditions during the transient experiment is only observed for NO and not for NO2 indicating that NO2 is directly stored. These
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results are consistent with NO2 being an intermediate in the storage of NO as nitrates under lean conditions as assumed by several authors in [1,19,47]. 5. Conclusions In this study we have compared Pt and Pd in noble metal-barium type of NOx storage catalysts. This investigation has shown that: 1. at relatively low temperature (300 ◦ C), it turns out that Pd/BaO/Al2 O3 has higher NOx storage capacity than Pt/BaO/Al2 O3 , i.e. more NOx is stored during the lean periods and almost all NOx is reduced during the subsequent rich periods. At higher temperature (400 ◦ C), the storage is slightly better for Pt/BaO/Al2 O3 than for the Pd/BaO/Al2 O3 catalyst; 2. during transient cycles, at 400 ◦ C, there is a NO breakthrough peak when switching from rich to lean conditions in the case of the Pd/BaO/Al2 O3 catalyst, leading to low effective NOx storage at the beginning of the lean phase. This is most likely connected with desorption of NO induced by oxygen and to reoxidation of PdO/Pd-particles that have been partially reduced during the rich phase; 3. for Pt/BaO/Al2 O3 , at 300 ◦ C, there is no complete reduction of NOx during the rich phase. We assume that some adsorbed species block the Pt sites. At 400 ◦ C, the barium oxide prevents the self-poisoning of the noble metal sites. Pt is more sensitive than Pd with respect to this phenomenon; 4. NO oxidation is much more limited on Pd samples compared to Pt samples. NO2 is more strongly bonded in the case of the Pd catalyst; 5. some observations confirm the crucial role of NO2 as a precursor in the storage of NOx as nitrate under lean conditions.
Acknowledgements This work has been performed within the Competence Centre for Catalysis, which is financially supported by the Swedish National Energy Administration and the member companies: AB Volvo, Johnson Matthey-CSD, Perstorp AB, Saab Automobile AB, MTC AB and EKA Chemicals AB.
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A comparison between Pt and Pd in NOx storage catalysts Sophie Salasc a,b , Magnus Skoglundh a,b , Erik Fridell a,c,∗ a
b
Competence Centre for Catalysis, Chalmers University of Technology, SE-412 96 Göteborg, Sweden Department of Applied Surface Chemistry, Chalmers University of Technology, SE-412 96 Göteborg, Sweden c Department of Applied Physics, Chalmers University of Technology, SE-412 96 Göteborg, Sweden Received 18 February 2001; received in revised form 30 September 2001; accepted 13 October 2001
Abstract The importance of Pt and Pd in noble metal-barium oxide type NOx storage catalysts was investigated. Model Pt/BaO/Al2 O3 and Pd/BaO/Al2 O3 catalysts were prepared and evaluated with respect to NOx storage capacity, activity towards NO reduction under lean conditions and NO oxidation capacity using synthetic lean burn exhausts containing NO, O2 , C3 H6 and N2 . The study was carried out by performing static and transient flow reactor experiments and temperature-programmed desorption studies. At 300 ◦ C, the Pd/BaO/Al2 O3 sample shows a higher NOx storage capacity than Pt/BaO/Al2 O3 , i.e. more NOx is stored during the lean periods and almost all NOx is released and reduced during the subsequent rich periods. At this temperature (300 ◦ C), the NO reduction is not complete during the rich phase for the Pt-based catalyst suggesting poisoning of Pt-sites by adsorbed species. At 400 ◦ C, Pt/BaO/Al2 O3 stores slightly more NOx than its Pd-based counterpart. XPS measurements on pre-treated catalysts, show some changes in oxidation state for Pd between the rich and lean phases. The oxidation of NO is much more limited on Pd based samples compared to Pt containing catalysts. The importance of NO2 as an intermediate in the storage of NOx as nitrate under lean conditions is confirmed in this study. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Lean burn; NOx storage-reduction catalyst; NSR concept; Platinum; Palladium; Barium; Alumina
1. Introduction Lean-burn engines are interesting from an environmental and fuel economy point of view. However, reduction of nitrogen oxides, NOx , in the exhaust using conventional catalytic techniques is difficult because of the large surplus of oxygen. Under these net-oxidizing conditions, three-way catalysts are not effective for NOx removal. One concept to solve this problem includes the NOx storage-reduction (NSR) catalyst [1]. This catalyst usually contains noble metals for reduction and oxidation reactions and a ∗ Corresponding author. Tel.: +46-31-772-3372; fax: +46-31-772-3134. E-mail address: [email protected] (E. Fridell).
NOx storage compound (typically an alkaline earth compound) supported on alumina with high surface area. The NSR catalyst operates under lean periods separated by rich spikes. During the lean conditions, NOx is stored in the trap, while hydrocarbons and carbon monoxide are oxidized on the noble metals. To regenerate the catalyst, the engine is turned to rich conditions for a short period during which the stored NOx is released and reduced over noble metal sites. Concerning the mechanisms for NOx storage and reduction over noble metal-barium oxide-based catalysts, it is interesting to focus on the role of the noble metals. Indeed, besides their role to oxidize hydrocarbons and carbon monoxide, they are essential both for oxidizing NO to NO2 and for reducing stored NOx . Among various supported platinum group metal
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catalysts (Pt, Pd, Rh, Ir), Pt is the more active component for continuous reduction of NOx by hydrocarbons in oxidizing atmospheres [2–6]. Numerous studies investigating the activity of Pt-based catalysts for NOx reduction by hydrocarbons have been carried out in recent years including both steady-state operation (continuous reduction) [2–16] and cycling conditions (NSR concept) [1,17–19]. Few research groups have as yet published works on Pd based catalysts and they only concern steady-state experiments [2–6]. No results on transient studies have been published so far. Nevertheless, Pd has distinct catalytic properties which could make it a desirable component in NSR catalysts. First of all, Pd exhibits a high three-way activity, i.e. both oxidation and reduction reactions are catalyzed in stoichiometric gas compositions [20–25]. Pd/Al2 O3 catalysts exhibit higher three-way activity around stoichiometric gas compositions than Pt/Al2 O3 catalysts. In specific, compared to Pt, palladium catalyses the simultaneous oxidation of CO and HC, and reduction of NOx more effectively under dynamic conditions and over a wider stoichiometric window under static conditions [26,27]. This behavior could be interesting in the NSR system, since the concept involves the two kinds of reactions. Moreover, supported Pd are among the most promising catalysts for the complete oxidation of methane [28–31], which has a high potential as future automotive fuel. Pd also shows a good thermal stability, in particular its ability to maintain activity under lean conditions at high temperatures [25,32,33] and excellent light-off characteristics (good activity from low temperatures) [25,34]. Finally, some literature data indicate that surface and bulk properties of the Pd/PdO system are quite variable, resulting in structural and activity changes in response to variations in temperature and gas environment, especially the partial pressure of oxygen [25,32,35–39]. For example, during methane oxidation, Pd catalysts supported on alumina can easily undergo a transition between metallic Pd and oxidized PdO [38,39]. This property may be important in the NOx storage concept since the catalyst works alternatively under lean and rich conditions. Considering the Pd characteristics mentioned above, one can expect activity from Pd with respect to promoting storage and release of NOx in noble metal-based NOx storage catalysts.
The objective of this investigation is to compare the reactivity of Pt and Pd in noble metal-barium based NOx storage catalysts. This was carried out by performing transient reactor studies and heating ramps using synthetic lean burn exhausts containing NO, O2 , C3 H6 and N2 as well as temperature programmed desorption (TPD) of nitrogen oxides and XPS, for Pt/Al2 O3 , Pt/BaO/Al2 O3 , Pd/Al2 O3 and Pd/BaO/Al2 O3 catalysts. 2. Experimental 2.1. Experimental section 2.1.1. Catalyst preparation Two monolith catalysts with alumina, a storage compound (BaO) and either Pt or Pd were prepared. Monolith samples of cordierite (2MgO·2Al2 O3 ·5SiO2 ) with a length of 15 mm and a cross-section consisting of 69 channels with 64 channels/cm2 were sequentially impregnated with alumina, barium oxide and salt solutions of platinum or palladium, resulting in a total washcoat weight of about 270 mg with 77, 20 and 3 wt.% of Al2 O3 , BaO and noble metal, respectively. Alumina was deposited by repeated immersions in an alumina slurry (80 wt.% ␥-Al2 O3 and 20 wt.% boehmite as binder). The excess of slurry was removed by blowing air through the channels and then the alumina washcoat was dried with hot air (95 ◦ C) for 5 min and calcined in air at 550 ◦ C for 5 min in order to transform boehmite into alumina and fixate the alumina to the monolith walls. This procedure was repeated until the desired amount of alumina was deposited. The alumina washcoat was finally calcined in air at 550 ◦ C for 90 min. This dip-coating procedure was also applied for the impregnation of barium using a barium nitrate slurry. The noble metal, Pt or Pd, was deposited by filling the monolith channels with aqueous solutions of tetra-ammineplatinum(II) hydroxide, [Pt(NH3 )4 ][OH]2 , or tetraamminepalladium(II)nitrate, [Pd(NH3 )4 ][NO3 ]2 , followed by evaporation at 90 ◦ C for 2 h and finally oxidation in air at 550 ◦ C for 1 h. Two monoliths without any storage compound were also prepared using the same methods. The compositions and the BET surface areas of the four monolith samples are given in Table 1.
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Table 1 Composition, estimated noble metal dispersion and BET surface area of the samples
Pt/BaO/Al2 O3 Pd/BaO/Al2 O3 Pt/Al2 O3 Pd/Al2 O3
Washcoat mass (mg)
Pt (wt.%)
Pd (wt.%)
BaO (wt.%)
Al2 O3 (wt.%)
Dispersion (%)a
SBET (m2 /g catalyst)
260 274 270 268
2.7 – 2.8 –
– 2.7 – 2.7
24.2 21.9 – –
73.1 75.4 97.2 97.3
9 7 6.5 6
18.7 16.6 27.3 31.5
a Estimated from NO TPD experiments performed on pre-reduced samples assuming one NO adsorption site per surface noble metal atom.
2.2. Reactors and testing procedures Transient reactor studies at constant temperature and heating ramps with constant gas composition were performed in a flow reactor system described elsewhere [24]. The catalyst was placed in a horizontal quartz tube encased in a furnace. The gases were introduced via mass-flow controllers and the outlet gases were analyzed on-line: total hydrocarbon (VE5 flame ionization detector, JUM Engineering), CO2 (UNOR 6N IR detector, Maihak), O2 (OM-14 polarographic oxygen analyzer, Beckman), N2 O (UNOR 610 IR detector, Maihak), NO and NO2 (CLD 700 EL ht chemiluminescence detector Tecan). Prior to the transient reactor studies and heating ramps, the catalysts were reduced in H2 at 450 ◦ C for 15 min and stabilized in the lean gas mixture (see Table 2) for 2 h. The heating ramps from about 50 to 550 ◦ C were performed under constant lean gas composition with a heating rate of 5 ◦ C/min. For the transient experiments, in order to simulate NSR cycles, the catalyst was intermittently exposed to a synthetic lean and rich exhaust gas mixture (see Table 2). Successive series of lean storage (240 s) and rich regeneration (60 s) periods were performed
Table 2 Gas composition during flow reactor studiesa
Lean Rich
NO (ppm)b
C3 H6 (ppm)
O2 (%)
300–600 300–600
900 900
8 0
a The gas mixtures were balanced with nitrogen to maintain a constant flow of 3500 ml/min for the pre-treatments and 2500 ml/ min for the experiments. b 600 ppm NO during the pre-treatments and temperature ramps; 300 ppm NO during the transient experiments.
at 400 ◦ C. Maximum in NOx storage has previously been observed around 380 ◦ C for Pt-Rh/BaO/Al2 O3 [19]. The amount of stored NOx was calculated from the difference between the inlet NOx level and the measured NOx signal, as described in [19]. The TPD measurements were performed in a different flow reactor system [40]. It consists of a vertical quartz tube connected to a quadrupole mass spectrometer (Balzers QMA 120). The samples were either pre-reduced (5% H2 in Ar) or pre-oxidized (5% O2 in Ar) at 500 ◦ C for 15 min. Then, they were exposed to 1000 ppm of either NO (15 min) or NO2 (30 min) in Ar at room temperature with a flow of 100 ml/min. After flushing with Ar at room temperature for 30 min, a 40 ◦ C/min heating ramp up to 550 ◦ C was performed in an Ar flow (50 ml/min). The noble metal dispersions of the samples were estimated from NO-TPD experiments performed on pre-reduced samples (see Table 1). XPS analysis was performed for Pt/BaO/Al2 O3 and Pd/BaO/Al2 O3 monolith samples prepared in the same way as described above. The samples were pretreated in a reactor connected to the XPS chamber (Perkin Elmer PHI 5000 C). They were initially reduced in H2 at 450 ◦ C for 15 min and then exposed to a lean gas mixture (1000 ppm NO, 1000 ppm C3 H6 , 8% O2 ) first at 500 ◦ C and then at 400 ◦ C for 30 min. After evacuation, the samples were cooled down to room temperature under vacuum and transferred to the UHV analysis chamber without being exposed to air. The XPS spectra were recorded using monochromatic Al K␣ radiation and the Pd 3d and Pt 4f levels were studied. Spectra were also taken after exposing the samples to a rich gas mixture (1000 ppm NO, 1000 ppm C3 H6 ) at 400 ◦ C for 20 min. The energy scale was calibrated by setting the Al 2s peak at 119.3 eV [50].
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3. Results 3.1. Heating ramp with constant lean gas composition The outlet NO, NO2 , NOx (NO + NO2 ), N2 O and CO2 concentrations during heating ramps under the lean gas mixture are shown in Fig. 1 for the Pt/BaO/ Al2 O3 and Pd/BaO/Al2 O3 samples. The nitrogen balance, Nbal , which is the difference between the inlet
nitrogen oxide concentration and the concentration of detected nitrogen compounds in the outlet gas mixture, i.e. Nbal = NOinlet −NOoutlet −NO2outlet −2N2 Ooutlet , is calculated. Assuming that no other nitrogen containing species but N2 , N2 O, NO and NO2 are formed, (1/2)Nbal is equal to the amount of N2 formed during the experiment. This calculated N2 concentration is shown as “N2 ” in Fig. 1. The hydrocarbon light-off (rapid increase of the CO2 signal) occurs in the same temperature range, between 220 and 240 ◦ C for the
Fig. 1. Outlet concentrations of NO, NO2 , NOx (NO + NO2 ), N2 O, CO2 and calculated nitrogen, “N2 ”, for a heating ramp (5 ◦ C/min) in a gas mixture of 600 ppm NO, 900 ppm C3 H6 , 8% O2 (balanced with N2 ) for (a) Pt/BaO/Al2 O3 , (b) Pd/BaO/Al2 O3 .
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Fig. 2. Conversion of NO (NOconv.), part of the conversion due to reduction to N2 O and N2 (NOred.) and part due to oxidation to NO2 (NOoxid.) for a heating ramp (5 ◦ C/min) in a gas mixture of 600 ppm NO, 900 ppm C3 H6 , 8% O2 (balanced with N2 ) for Pt/BaO/Al2 O3 and Pd/BaO/Al2 O3 .
two catalysts. For the Pt/BaO/Al2 O3 catalyst, the maximum in NO conversion is observed at 230 ◦ C with about 45% conversion (see Fig. 2). The NO conversion starts with an initial reduction of NO (N2 and N2 O formation) followed by NO oxidation to NO2 (see Figs. 1a and 2). On the other hand, for the Pd/BaO/Al2 O3 catalyst, there is almost no NO oxidation observed and the NO reduction is limited with a maximum of 7% at 245 ◦ C (see Figs. 1b and 2). The storage of NOx will not significantly influence the results from this type of experiment since the time scale for the temperature ramp is slow (5 ◦ C/min) compared to the storage rate. The storage capacity will, thus, become saturated and conditions close to steady-state will prevail throughout the ramp. The resulting NO concentrations are essentially determined by continuous reduction of NO by propene under lean conditions. The storage capacity is best measured in transient experiments at constant temperature (see following sections). The results in NO conversion are quite similar with or without barium oxide (not shown). For Pt/Al2 O3 , the maximum in NO conversion is as high as for the catalyst containing barium oxide (45% at 240 ◦ C). However, Pt/Al2 O3 shows higher NO oxidation activity (and less NO reduction) compared to Pt/BaO/Al2 O3 [48]. For Pd/Al2 O3 , there is almost no NO oxidation and the NO conversion
due to NO reduction reaches a maximum of 9% at 235 ◦ C. 3.2. Transients in gas composition at constant temperature In the concept where NOx storage catalysts are used for NOx reduction in lean exhausts, the normal lean conditions are interrupted by short rich periods in order to regenerate the catalyst from stored NOx . The difference between the lean period (NOx storage) and the rich period (regeneration) in this investigation is that the oxygen is turned on and off, respectively and compensated with inert nitrogen to maintain a constant flow. In the present experiments, a lean period of 240 s is followed by a rich period of 60 s. The concept of NOx storage, the procedure and the calculation of the amount of NOx stored are given in detail elsewhere [18,24]. Four cycles were run in order to check the reproducibility. The gas compositions are given in Table 2. Fig. 3 shows the results of the whole experiment performed using the Pt/BaO/Al2 O3 catalyst at 400 ◦ C. When oxygen is switched on (see for example the second cycle in Fig. 3 at around 305 s), there is a relatively slow increase in the NOx signal (both NO and NO2 signals) which means that some NOx is stored in
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Fig. 3. Outlet NO, NO2 , NOx (NO + NO2 ), N2 O and O2 concentrations during a transient experiment for the Pt/BaO/Al2 O3 catalyst at 400 ◦ C with the gas compositions given in Table 2.
the catalyst. When oxygen is turned off (rich phase), almost all NOx is reduced by C3 H6 . Furthermore, the NOx stored in the previous lean period is also released and reduced. As there is a good reproducibility of the cycles, only the second cycle will be shown in the next figures. In Fig. 4, the outlet NO and NO2 signals resulting from transient experiments at 400 ◦ C for the Pt/BaO/ Al2 O3 , Pd/BaO/Al2 O3 , Pt/Al2 O3 and Pd/Al2 O3 catalysts are shown. First of all one may note that for the catalysts without barium oxide, i.e. Pt/Al2 O3 and Pd/Al2 O3 , no NOx storage can be observed at 400 ◦ C which is manifested by the rapid increase of the outlet NO and NO2 signals (close to the system response) immediately after switching from rich to lean gas composition (see Fig. 4). Moreover, the NO reduction by propene during the rich period is progressively diminished with time, especially for the Pt/Al2 O3 sample. For the catalysts containing the NOx storage compound, i.e. Pt/BaO/Al2 O3 and Pd/BaO/Al2 O3 , there is storage of NOx during the lean phase (see Fig. 4). The amount of stored NOx at 400 ◦ C is 0.033 and 0.031 mole NOx per mole Ba for the Pt/BaO/Al2 O3 and Pd/BaO/Al2 O3 samples, respectively. The amount stored NOx in the Pt-containing catalyst is in agreement with previous results [19]. For the Pd-containing
sample, we can observe a larger NO breakthrough peak when we switch from the regeneration (rich period) to the storage phase (lean period), as can be seen in Fig. 4a at around 305 s. This means that there is low effective NOx storage for a short while at the beginning of the lean period. Contrary to the catalysts without the NOx storage compound, almost all NO is reduced by C3 H6 during the rich phase for the Pd/BaO/Al2 O3 catalyst. Break-through peaks in the NOx signal (mainly NO) can also be observed when switching from the storage phase to the regeneration phase (see, e.g. at around 545 s in Figs. 3 and 4). The break-through peaks observed when switching from lean to rich conditions are related to the decomposition of stored nitrate compounds and the difference in binding properties for NO on reduced and oxidized noble metal [19]. Another point to notice is the production of NO2 during the lean phase which is much lower for the Pd-containing catalysts than for the samples with Pt. The transient experiments were also performed at 300 ◦ C for the Pt/BaO/Al2 O3 and Pd/BaO/Al2 O3 samples. The NOx signals are shown in Fig. 5 for the second cycle. At 300 ◦ C, for the Pd-based catalyst, the NOx breakthrough peak when switching from the rich to the lean period cannot be observed. The amount of stored NOx at 300 ◦ C is 0.016 and 0.029 mol NOx per mole Ba for the Pt/BaO/Al2 O3 and Pd/BaO/Al2 O3
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Fig. 4. NO (a) and NO2 (b) concentrations during a transient experiment over Pt/BaO/Al2 O3 , Pd/BaO/Al2 O3 , Pt/Al2 O3 and Pd/Al2 O3 at 400 ◦ C with the gas compositions given in Table 2.
catalysts, respectively. For the Pt-containing catalyst, the NO reduction is incomplete and continuously decreases with time during the rich phase. 3.3. TPD measurements Temperature-programmed desorption experiments were performed for both pre-reduced and pre-oxidized
Pt/BaO/Al2 O3 and Pd/BaO/Al2 O3 samples and for pre-reduced Pt/Al2 O3 and Pd/Al2 O3 . The samples were exposed to either NO or NO2 at room temperature. Selected experimental results are presented in Fig. 6. Fig. 6a and b show TPD results after exposing pre-oxidized Pt/BaO/Al2 O3 and Pd/BaO/Al2 O3 samples to NO2 . Three NO-desorption peaks can be
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Fig. 5. Outlet NOx (NO + NO2 ) concentration during a transient experiment for the Pt/BaO/Al2 O3 and Pd/BaO/Al2 O3 catalysts at 300 ◦ C with the gas compositions given in Table 2.
observed (m/e: 30). The two first peaks are located around the same temperature for the two catalysts (115 and 310 ◦ C for the Pt-based and 120 and 295 ◦ C for the Pd-containing catalyst). These peaks are assigned as weakly adsorbed NO (tentatively the first one from noble metal sites and the second peak from the support). The third peak observed at higher temperature, corresponds to desorbed NO2 and/or decomposed NO3 species either as NO2 (m/e: 46) or decomposed to NO and O2 (NO m/e: 30 and O2 m/e: 32). There is also desorption of N2 O (m/e: 44) and a small desorption of N2 (m/e: 28). The m/e: 44 could also partly consist of CO2 as the Ba-containing samples will form BaCO3 when exposed to ambient air. The desorbed NO2 may come from both the noble metal and the support. The slow decrease observed after the peak maximum could be assigned as NO2 from the barium oxide [19]. Indeed, in the case of samples without BaO, i.e. Pt/Al2 O3 and Pd/Al2 O3 , the high temperature peak is better defined and is finished before the end of the temperature ramp (see Fig. 6e and f). The maximum of this peak is observed at a lower temperature for the Pt/BaO/Al2 O3 sample (435 ◦ C compared with 505 ◦ C for the Pd/BaO/Al2 O3 sample, see Fig. 7). It is also interesting to compare the desorption spectra in Fig. 7 at 300 and 400 ◦ C (the temperatures at which transient experiments were performed). At 300 ◦ C, there is almost the same amount of desorbed species for the two catalysts whereas at 400 ◦ C the amount is significantly
larger for the Pt/BaO/Al2 O3 catalyst compared with the Pd/BaO/Al2 O3 sample. At 300 ◦ C only NO desorption can be observed whereas at 400 ◦ C also NO2 desorbs. Thus, for the two catalysts, NO desorption occurs in the same range of temperature and almost the same amount of NO desorbs, whereas NO2 desorption at lower temperatures is faster in the case of the Pt catalyst. The TPD curves after exposing pre-reduced samples to NO2 (not shown) are similar to those for pre-oxidized ones. This is most likely because NO2 oxidizes the samples already at room temperature [41]. Similar experiments but with NO instead of NO2 were performed for pre-reduced and pre-oxidized samples (see Fig. 6c and d for pre-oxidized and Fig. 6g and h for pre-reduced samples). A much smaller quantity is adsorbed on the catalysts compared with NO2 exposure. The NO experiments show one main NO-desorption peak at around 110–120 ◦ C. There are also two NO peaks at higher temperatures. The first one at 465 and 495 ◦ C for Pt/BaO/Al2 O3 and Pd/BaO/Al2 O3 , respectively, and the second one emerging from 500 to 530 ◦ C, respectively, from these two samples. The latter peak is connected with corresponding O2 and mass 44 (probably CO2 ) peaks and is not finished yet at 550 ◦ C. For the pre-reduced samples, some N2 O desorbs together with NO with peaks at about the same temperatures as for the pre-oxidized case. At around 250 ◦ C, some N2 also desorbs. Thus,
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Fig. 6. NO and NO2 TPD measurements: (a) NO2 on pre-oxidized Pt/BaO/Al2 O3 ; (b) NO2 on pre-oxidized Pd/BaO/Al2 O3 ; (c) NO on pre-oxidized Pt/BaO/Al2 O3 ; (d) NO on pre-oxidized Pd/BaO/Al2 O3 ; (e) NO2 on pre-reduced Pt/Al2 O3 ; (f) NO2 on pre-reduced Pd/Al2 O3 ; (g) NO on pre-reduced Pt/BaO/Al2 O3 and (h) NO on pre-reduced Pd/BaO/Al2 O3 .
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Fig. 6 (Continued ).
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Fig. 6 (Continued ).
Fig. 7. NO and NO2 concentrations from NO2 -TPD for pre-oxidized Pt/BaO/Al2 O3 and Pd/BaO/Al2 O3 samples.
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NO is reduced on the surface but since no desorption of oxygen is observed this indicates that the samples become oxidized. 3.4. XPS studies The Pd 3d XPS spectra of the Pd/BaO/Al2 O3 sample treated at 400 ◦ C under lean and rich conditions are shown in Fig. 8a. For the two different pre-treatments, XPS data show binding energies of the Pd 3d(5/2) level at 335.2–335.3 and 336.9–337 eV,
corresponding to the oxidation states 0 and +2, respectively [42]. When switching from lean (a) to rich (b) conditions, we can observe a change in the ratio PdO/Pd0 . This ratio is high after lean pre-treatment and becomes lower after rich exposure. This means that Pd is partially reduced when the gas mixture is switched from lean to rich. Similar XPS experiments for the Pt/BaO/Al2 O3 catalyst are shown in Fig. 8b. This spectra is influenced by the presence of the Al 2p peak (at 74 eV for ␥-alumina). From the size of the Al 2s peak (which should be somewhat larger than the
Fig. 8. XPS spectra of the Pd 3d level for Pd/BaO/Al2 O3 (a) and the Pt 4f level for Pt/BaO/Al2 O3 (b) after pretreatment at 400 ◦ C under ‘a’—lean mixture (1000 ppm NO, 1000 ppm C3 H6 , 8% O2 ) and ‘b’—rich mixture (1000 ppm NO, 1000 ppm C3 H6 ).
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Al 2p peak) we note that the Al 2p peak contribute with approximately 20% of the peak area in Fig. 8b. However, the interesting point here is to compare the spectra after the two pre-treatments. From these experiments, we could observe only metallic Pt. No shift could be observed between the lean and rich pre-treatment conditions at 400 ◦ C.
4. Discussion: differences between Pt and Pd in NOx storage The heating ramp experiments show that Pd containing catalysts (i.e. Pd/Al2 O3 and Pd/BaO/Al2 O3 ) are almost inactive towards continuous NO reduction under static lean conditions (Fig. 1b) while activity is observed for Pt (Fig. 1a). This is in agreement with the literature data about the activity of Pd [2–6] and Pt [4,5,10,18,47] for continuous reduction of NOx by hydrocarbons under oxidizing conditions. Nevertheless, Pd was found to be active under transient conditions. Indeed, significant amounts of NOx were found to be stored under lean conditions in both Pt and Pd catalysts containing barium oxide. At relatively low temperature (300 ◦ C), the Pd/BaO/Al2 O3 catalyst is more active to store and reduce NOx than its Pt-based counterpart. On the other hand, at 400 ◦ C, the Pt/BaO/Al2 O3 sample stores slightly more NOx . Some differences between the Pt/BaO/Al2 O3 and Pd/BaO/Al2 O3 catalysts have been pointed out from this study: at 400 ◦ C, lower effective storage of NOx at the beginning of the lean period for the Pd based catalyst; at 300 ◦ C, no complete reduction of NO on the Pt-containing catalyst during the rich period; different behavior towards NO2 . These differences are discussed in the following paragraphs. 4.1. Pt-Pd oxidation state under transient experiments During the transient experiments performed at 400 ◦ C, there is a NO breakthrough peak when switching from rich to lean conditions for all samples. The peak is most significant for the samples without BaO, pronounced for the Pd/BaO/Al2 O3 sample and just an indication for the Pt/BaO/Al2 O3 sample (Fig. 4a). We suggest that this peak originates from desorption of NO adsorbed on Pt or Pd during the rich phase. For a
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Pt/Al2 O3 sample, Olsson et al. [46] have calculated a NO coverage of about 0.1 when the sample is exposed to 600 ppm NO at atmospheric pressure and 400 ◦ C. When oxygen is introduced, the NO-surface bond may be weakened and NO will be replaced by more strongly bonded oxygen. The mechanism for this is not clear but it has been observed that the activation energy for oxygen desorption will decrease with increasing oxygen coverage [43]. It is possible that chemisorbed oxygen has a similar effect on NO. Further, if subsurface Pt oxide is formed, as suggested in several studies of oxygen adsorption on Pt at atmospheric pressure [44,45], the NO–Pt bond may become weaker. A similar peak is not observed for Rh containing samples [19]. In this case no coverage of NO will be built up since the reaction between NO and C3 H6 will be effective, i.e. the NO reduction is complete during the entire rich phase for Rh containing samples. In order to explain the difference between the Pd/BaO/Al2 O3 and the Pt/BaO/Al2 O3 samples in this respect we suggest that during the rich phase there is reduction (or partial reduction) of PdO. This hypothesis is supported by the fact that for the Pd/BaO/Al2 O3 catalyst there is a small production of NO2 during the rich phase (see Fig. 4b between 250 and 310 s) probably originating from a reaction between NO and the palladium oxide forming NO2 and reduced Pd sites. Then, when the conditions are switched from rich to lean, it is difficult (because of the reduced sites) to produce NO2 which is a precursor to stored nitrate [1,19,47] and, therefore, the effective NOx storage is relatively low at the beginning of the lean period. Nevertheless, after a short time under lean conditions, the reduced palladium becomes oxidized and the NOx storage can start. For the Pt/BaO/Al2 O3 catalyst the NO adsorbed on Pt can more rapidly be oxidized into NO2 and subsequently stored leading to only a small NO desorption peak. The XPS data show that Pd is partially reduced under rich conditions at this temperature (400 ◦ C). This strengthens the hypothesis of a change in the oxidation state of Pd when switching from rich to lean atmosphere under the transient experiments. On the other hand, no change was observed between the Pt XPS spectra when switching between lean and rich pre-treatments. In principle, the NO peak observed at the transition from rich to lean conditions for the Pd/BaO/Al2 O3 sample could also be a desorption peak originating in different binding properties
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for NO on Pd and PdO, respectively. However, the TPD measurements indicate no such difference. At 300 ◦ C, the NO peak cannot be observed anymore probably because of the lower desorption rate at this lower temperature. It is also likely that this temperature is too low for reduction of Pd to take place during the rich periods. If palladium is kept in oxidized form, NO oxidation is possible as soon as the conditions are altered to lean and then the NOx storage can occur. The formation of these peaks may also be connected with the oxidation of HC derived species, formed on the noble metal surface during the rich conditions, that may cause a local temperature increase when switching to lean conditions, in turn leading to NO desorption. 4.2. Self-poisoning during the rich phase For the Pt/Al2 O3 and Pd/Al2 O3 catalysts, NO was incompletely reduced by propene at 400 ◦ C during the rich phase of the transient experiments (Fig. 4a). This may be due to self-poisoning of the reaction by adsorption on the noble metal sites by NO or by propene or propene derived species (CO, carbonaceous products). The NO reduction is less effective on Pt/Al2 O3 than on Pd/Al2 O3 . Therefore, the reaction on Pt seems to be more sensitive to self-poisoning than on Pd but this observation may also be influenced by that the Pd catalyst contains roughly twice as many noble metal sites as the Pt catalyst. At this temperature (400 ◦ C), in the presence of barium oxide, i.e. for Pt/BaO/Al2 O3 and Pd/BaO/Al2 O3 , the reduction of NO is almost complete during the rich periods. The barium oxide, thus, seems to prevent the self-poisoning of the reaction. This may be explained by migration of NOx originating from the decomposition of barium nitrate to the noble metal sites via spill-over [48] or by spill-over of hydrocarbons that adsorb on BaO. These migrating species may then react with adsorbed species on the noble metal surface. The importance of reversible NO2 spill-over between Pt and BaO sites for effective NOx storage/release has previously been addressed for the Pt/BaO/Al2 O3 system [48]. For Pt/Al2 O3 or Pd/Al2 O3 , either NO or C3 H6 may not be able to adsorb on the precious metal sites because of the blocking effect of the other components and, thus, have difficulty to react. Another explanation for this promotion of NO-reduction by
barium oxide under rich periods could be some kind of interaction between the noble metal and barium oxide [41]. Indeed, during the deposition of the noble metals while preparing the catalyst, some dissolution and re-precipitation of barium oxide can occur, leading to a partial coverage of the noble metal crystallites by barium containing species. However, no shift in the IR band for chemisorbed CO on Pt0 is observed when Pt-containing samples with or without barium oxide, are exposed to CO [49], i.e. no specific influence of Ba on the CO–Pt bond could be observed. Thus, the former explanation seems more consistent, i.e. migrating species which react with adsorbed species on the noble metal surface. At 300 ◦ C, the barium oxide does not prevent the self-poisoning of the NO reduction over Pt/Al2 O3 (Fig. 5). This confirms that Pt is more sensitive than Pd/PdO with respect to self-poisoning under these conditions. 4.3. Behavior towards NO2 The results reported above indicate that Pt and Pd containing catalysts do not show the same behavior towards NO2 . During NO2 -TPD with pre-oxidized samples (Figs. 6a and b and 7), almost the same amount of species desorbs up to 350 ◦ C, species which are assigned as adsorbed NO. The high temperature peaks corresponding to NO2 desorption are shifted to higher temperatures for the Pd based catalysts. Thus, NO2 is more strongly bound in the case of the Pd containing catalysts. We also pointed out that NO oxidation is more favored on Pt containing catalysts than on the corresponding Pd containing catalyst. Less NO2 is produced on Pd based samples during the lean phase of the transient experiments (Fig. 4b) and no oxidation of NO could be observed for these catalysts during the temperature ramp (Figs 1b and 2). A reason for less NO2 production on Pd-based samples could be that NO2 is more strongly bound to the Pd surface than to the surface of Pt as mentioned above. There is a higher adsorption yield of NO2 at room temperature compared to NO (Fig. 6a and b compared to Fig. 6c and d). We can also notice that at 400 ◦ C for the Pd/BaO/Al2 O3 catalyst, the breakthrough peak at the switch from rich to lean conditions during the transient experiment is only observed for NO and not for NO2 indicating that NO2 is directly stored. These
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results are consistent with NO2 being an intermediate in the storage of NO as nitrates under lean conditions as assumed by several authors in [1,19,47]. 5. Conclusions In this study we have compared Pt and Pd in noble metal-barium type of NOx storage catalysts. This investigation has shown that: 1. at relatively low temperature (300 ◦ C), it turns out that Pd/BaO/Al2 O3 has higher NOx storage capacity than Pt/BaO/Al2 O3 , i.e. more NOx is stored during the lean periods and almost all NOx is reduced during the subsequent rich periods. At higher temperature (400 ◦ C), the storage is slightly better for Pt/BaO/Al2 O3 than for the Pd/BaO/Al2 O3 catalyst; 2. during transient cycles, at 400 ◦ C, there is a NO breakthrough peak when switching from rich to lean conditions in the case of the Pd/BaO/Al2 O3 catalyst, leading to low effective NOx storage at the beginning of the lean phase. This is most likely connected with desorption of NO induced by oxygen and to reoxidation of PdO/Pd-particles that have been partially reduced during the rich phase; 3. for Pt/BaO/Al2 O3 , at 300 ◦ C, there is no complete reduction of NOx during the rich phase. We assume that some adsorbed species block the Pt sites. At 400 ◦ C, the barium oxide prevents the self-poisoning of the noble metal sites. Pt is more sensitive than Pd with respect to this phenomenon; 4. NO oxidation is much more limited on Pd samples compared to Pt samples. NO2 is more strongly bonded in the case of the Pd catalyst; 5. some observations confirm the crucial role of NO2 as a precursor in the storage of NOx as nitrate under lean conditions.
Acknowledgements This work has been performed within the Competence Centre for Catalysis, which is financially supported by the Swedish National Energy Administration and the member companies: AB Volvo, Johnson Matthey-CSD, Perstorp AB, Saab Automobile AB, MTC AB and EKA Chemicals AB.
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