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Influence of platinum and rhodium composition on the NOx storage and sulphur tolerance of a barium based NOx storage catalyst
Applied Catalysis B: Environmental 46 (2003) 429–439
Influence of platinum and rhodium composition on the NOx storage and sulphur tolerance of a barium based NOx storage catalyst Annika Amberntsson a,b , Erik Fridell a,b,∗ , Magnus Skoglundh a,c a
Competence Centre for Catalysis, Chalmers University of Technology, SE-412 96 Göteborg, Sweden Department of Applied Physics, Chalmers University of Technology, SE-412 96 Göteborg, Sweden c Applied Surface Chemistry, Chalmers University of Technology, SE-412 96 Göteborg, Sweden
b
Received 19 January 2003; received in revised form 21 June 2003; accepted 21 June 2003
Abstract In the present work the influence of the type of noble metals present in barium oxide based NOx storage catalysts was investigated regarding the NOx storage performance, NO oxidation, NO reduction, sulphur deactivation and sulphur regenerability. Monolith samples with combinations of platinum and rhodium, were prepared, tested in a flow-reactor, and characterised by XPS measurements. The flow-reactor experiments simulated NOx storage and reduction cycles at 400 ◦ C in synthetic gas mixtures with oxygen, propene and nitric oxide. For the sulphur deactivation and regenerability investigations 25 ppm (v/v) SO2 was added to the feed gas stream. From the experiments, it was concluded that a combination of platinum and rhodium is required to achieve good NOx storage and reduction performance. The NOx storage capacity was, however, found higher for catalysts containing only platinum compared to catalysts including rhodium. When exposed to SO2 the NOx storage capacity also seemed to deactivate faster for the samples containing rhodium than for samples with platinum as the sole noble metal. Additionally, it was observed that platinum gives high NO oxidation activity during the lean periods both with and without SO2 present in the gas feed. During the rich periods, rhodium showed high activity for NO reduction in sulphur free gas feed as well as in the presence of SO2 . Finally, the results implied that to provide good sulphur regeneration ability of the NOx storage catalyst, a combination of platinum and rhodium is necessary. © 2003 Elsevier B.V. All rights reserved. Keywords: NOx storage catalysts; Platinum; Rhodium; Sulphur deactivation; NO oxidation; NO reduction
1. Introduction To provide good fuel economy of automotive engines lean-burn and diesel technologies are interesting alternatives to the conventional, stoichiometric, Otto ∗ Corresponding author. Tel.: +46-31-772-3372; fax: +46-31-772-3134. E-mail address: [email protected] (E. Fridell).
engine [1]. However, a large surplus of oxygen in the exhausts means that the traditional three-way catalyst (TWC) is not able to efficiently reduce nitrogen oxides (NOx ). Hence, new approaches for treating the NOx emissions from diesel and lean-burn engines are needed [1,2]. One approach is the so-called NOx storage and reduction concept [3] based on storage of NOx in the catalyst for relatively long lean periods, which are interrupted by rich spikes. During
0926-3373/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0926-3373(03)00269-8
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the rich periods, the NOx stored in the lean periods is decomposed and subsequently reduced to nitrogen [4–6]. The catalysts most commonly used for NOx storage applications comprise combinations of noble metals for oxidation and reduction purposes and barium oxide/carbonate as the storage material [3–6]. It has been reported that the nitrogen oxides are trapped as surface nitrates [4–13]. Unfortunately, affinity for nitrate formation also entails affinity for sulphation, which will lead to deactivation of the NOx storage capacity if sulphur is present in the exhaust [14–22]. Since the noble metals seem to play several keyroles in the NOx storage and reduction cycles as well as for sulphur deactivation and for regeneration from sulphur [23], it is of great importance to investigate their role in detail. Salasc et al. investigated the effect of palladium compared to platinum on the performance of NOx storage catalysts. It was found that the palladium containing samples had a slightly higher NOx storage ability at lower temperatures compared to the platinum loaded samples and that the NO oxidation activity is much higher on platinum compared to palladium [24]. The effect of addition of noble metals (palladium, platinum or rhodium) to a calcium oxide based, NOx storage and reduction catalyst was studied by Huang et al. [25]. The authors claim that rhodium provides better NOx storage ability for a CaO/Al2 O3 catalyst, compared to platinum, due to higher formation of NO2 . This statement, however, is contradicted by several studies on alumina supported platinum and rhodium catalysts. For example, Kobajashi et al. reported the same activity for NO oxidation over Rh/Al2 O3 and Pt/Al2 O3 [26]. Efthimiadis et al. [27,28] and Efthimiadis and co-workers [29] repeatedly reported that Rh/Al2 O3 catalyst only may oxidise a maximum of ∼30% of the inlet NO to NO2 in the presence of propene at 400◦ C [27–29]. From thermodynamic considerations, the corresponding NO to NO2 oxidation under such conditions is 50%. The corresponding platinum catalyst in all cases was reported to provide higher amounts of NO2 compared to rhodium. However, the thermodynamic equilibrium concentration of NO2 was never reached due to reduction of NOx with propene [28–30]. Introduction of SO2 in the gas feed has been stated to affect the noble metals in the following way:
(i) The NO oxidation activity during lean conditions was increased for Pt/Al2 O3 [29] but was decreased for Rh/Al2 O3 [28,29]. (ii) The NO reduction activity during rich conditions of Rh/Al2 O3 was increased [27,28] but was not affected for Pt/Al2 O3 [30]. Other authors have reported that NO oxidation up to thermodynamic limited levels are possible over Pt/Al2 O3 in the absence of propene at temperatures around 350 ◦ C. The NO oxidation over this material, however, was not affected by the absence or presence of SO2 [31]. The main function of the noble metals during the rich period of the NOx storage reduction cycle is the ability to reduce NO. This reaction has been extensively studied under stoichiometric or near stoichiometric conditions over precious metals present in TWC [32]. During such conditions, both Pt and Rh possess high activity for NO reduction, however with different product selectivity. The formation of N2 O and NH3 is thus higher over Pt whereas reduction to N2 is favoured over Rh [32]. Further, the interaction with sulphur containing compounds is different for Pt and Rh. The ability to reduce NO has been reported to decline considerably faster for Rh than for Pt when exposed to H2 S. The same study has also shown that Rh is significantly more easy to reduce under hydrogen treatment than Pt [33]. Previously we have shown that sulphur deactivation of NOx storage catalyst proceeds considerably faster under exposure to sulphur under rich conditions compared to lean sulphur exposure implying that poisoning of the noble metals is an important deactivation mechanism for this type of catalyst [23,34,35]. Furthermore, other authors have reported formation of sulphur deposits on the Pt surface [36,37] or even the formation of platinum sulphide [20,38] have been reported upon exposure to sulphur compounds under net reducing conditions. The influence of the noble metal composition (platinum and rhodium) on the performance of model NOx storage catalysts concerning (i) NOx storage capacity in lean/rich cycles, (ii) NO oxidation under lean conditions, (iii) NO reduction activity under rich conditions, (iv) tolerance towards sulphur deactivation and (v) sulphur regenerability are investigated in this study by means of flow-reactor measurements and XPS.
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2. Experimental 2.1. Catalyst preparation The catalysts were prepared starting from cylindrical cordierite monolith substrates (∅ = 20 mm, L = 15 mm, 400 cells per square inch). Washcoats were deposited by immersing the substrates into an aqueous slurry of ␥-alumina (Puralox, Condea) and boehmite (Disperal, Condea) followed by gently blowing air through the channels to remove excess slurry, drying and calcination. The immersing– blowing–drying–calcination procedure was repeated until the desired amount of alumina was deposited. Barium oxide was deposited in a similar manner using an aqueous solution of barium nitrate (Ba(NO3 )2 from Aldrich). The noble metals were deposited by wet impregnation of non-halide noble metal salts of platinum and rhodium (Pt(NO3 )2 and Rh(NO3 )2 supplied by Hereaus and Johnson & Matthey, respectively). The catalyst preparation route is described in detail elsewhere [39]. The washcoat contained 500 mg of Al2 O3 (corresponding to 4.9 mmole) and 75 mg BaO (0.49 mmole) in all cases. Before use, all samples were calcined in air at 600 ◦ C for 90 min and reduced in 2 vol.% H2 at 500 ◦ C for 30 min. The samples prepared are listed in Table 1. 2.2. Flow-reactor experiments The experiments were all performed in a flow-reactor system with separate detection of NO, NO2 , N2 O and SO2 . NO and NO2 were measured with chemiluminescence and N2 O and SO2 were measured with IR instruments. The fully computerised system had the ability to independently supply nine different gases
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and both the inlet gas temperature and the catalyst temperature were monitored. The reactor system is described in detail elsewhere [40]. The initial activity for NOx storage, NO oxidation and NO reduction of each sample was established by performing sulphur free lean/rich cycling in a gas mixture containing 400 ppm (v/v) NO, 500 ppm (v/v) C3 H6 , 8 vol.% (during the lean 5 min periods) or no (during the rich 5 min periods) O2 balanced with Ar to maintain a constant space velocity of 38 000 h−1 . The inlet temperature was 400 ◦ C in all experiments. The equilibrium ratio between NO and NO2 is (about) 50:50 for 8 vol.% O2 at this temperature. The temperature is also close to the temperature previously observed for maximum in NOx storage (380 ◦ C) for similar catalysts [8]. These experimental conditions are considerably far from the conditions in a real, automotive application, i.e. the time scales are longer, the space velocity is lower and the gas mixture only contains a few components. However, to fulfil the scope of this model study the conditions are relevant. The sulphur free measurements were followed by sulphur treatments where 25 ppm (v/v) SO2 was continuously added to the gas mixture described above and 15 lean/rich cycles were performed. The total amount SO2 added during the lean/rich cycling was 0.46 mmole SO2 . Hence, the molar ratio between total amount of sulphur exposed to the sample and the amount of barium in the sample was close to 1 by the end of the experiment. After the SO2 exposure cycles, sulphur regeneration was performed by reducing the sample in 2 vol.% H2 at 750 ◦ C for 30 min followed by oxidation in 10 vol.% O2 at 400 ◦ C for 30 min. To investigate the effect of the regeneration, sulphur free lean/rich cycling was performed after the regeneration and the
Table 1 Summary of catalysts prepared Sample
Pt (mmole)
Rh (mmole)
BaO (mmole)
Al2 O3 (mmole)
4% 2% 2% 1% 1% 3%
0.118 0.059 0.059 0.029 – 0.088
– – 0.056 0.028 0.056 0.084
0.49 0.49 0.49 0.49 0.49 0.49
4.9 4.9 4.9 4.9 4.9 4.9
Pt Pt Pt–1% Rh Pt–0.5% Rh Rh Pt–1.5% Rh
All samples were washcoated with 500 mg Al2 O3 and 75 mg BaO.
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Table 2 Overview of conditions used in the flow-reactor experiments
Lean cycle period Rich cycle period Sulphur treatment Sulphur regeneration Oxidation
Gas composition
Temperature (◦ C)
Exposure time
400 ppm (v/v) NO, 500 ppm (v/v) C3 H6 , 8 vol.% O2 400 ppm (v/v) NO, 500 ppm (v/v) C3 H6 25 ppm (v/v) SO2 added to lean and rich 2 vol.% H2 10 vol.% O2
400 400 400 750 400
5 min 5 min 15 lean/rich cycles 4 × 30 min + 12 h 30 min
All gas mixtures were balanced to 100 vol.% with Ar.
NOx storage capacity, the NO oxidation (lean period) and the NO reduction (rich period) activities were measured. The whole regeneration procedure including reduction, oxidation and sulphur free cycling was repeated five times. The reduction time was 30 min except for the last experiment where the sample was reduced at 750 ◦ C for 12 h. A summary of the gas compositions (balanced with Ar) and experimental conditions for the flow-reactor study is given in Table 2. 2.3. XPS XPS measurements were performed both after the sample had been reduced in 8 vol.% H2 at 450 ◦ C for 30 min and oxidised in 19 vol.% O2 at 400 ◦ C on a monolith sample impregnated with 3% Pt–1.5% Rh on a washcoat of 75 mg BaO/500 mg Al2 O3 . After treatments, described above, in a flow-reactor cell connected to the UHV system (temperature range ∼25–600 ◦ C, Eurotherm PID temperature control unit, maximum gas flow rate of 40 ml/min) the samples were transferred into the XPS UHV chamber without being exposed to air. The XPS-spectrometer used was a Physical Electronics type PHI 5000C with non-monochromatic Al K␣ radiation (1486.6 eV). The detection angle was 45◦ from the surface normal, with excitation normal to the surface (X-ray power 400 W). This system samples an area with an approximate diameter of 0.8 mm. The Pt 4d3/2 and Rh 3p3/2 regions were investigated at a pressure <10−9 mbar with seven overlapped sweeps, with 0.125 eV per step and 100 ms per step. The energy scale was internally calibrated by adjusting the Al 2s peak to 119.3 eV [41].
3. Results and discussion 3.1. NOx storage catalyst performance Fig. 1 shows the NOx outlet concentration during one sulphur free lean/rich cycle for all samples investigated. The figure shows that all samples store a considerable amount of NOx indicated by the slow increase in the traces from t = 0 to ≈200 s. The total amount of NOx stored is calculated as the integrated traces between t = 0 and 300 s subtracted from the concentration at saturation (400 ppm (v/v)) multiplied by the storage time (300 s). The calculation procedure is described in detail elsewhere [8]. At t = 200 s the outlet concentrations of NOx have reached the inlet level which means that the NOx storage capacities of the samples are saturated. The figure also shows that the NOx response from the samples containing rhodium (2% Pt–1% Rh, 1% Pt–0.5% Rh and 1% Rh) give differently shaped curves compared with the platinum-only samples (4% Pt and 2% Pt). The rhodium containing samples all show an initial rapid increase of the NOx signal followed by a slower increase. This is not observed for the platinum-only samples for which the NOx response increases more linearly. We have previously observed a similar behaviour [24] when comparing NOx storage catalysts including Pt or Pd [24]. The catalyst with palladium as the noble metal showed two different slopes of the NOx trace during storage while the platinum one only showed one. The two-step behaviour was explained by oxidation of palladium during the first period and oxidation of NO only during the second step [24]. It is reasonable to expect similar behaviour when comparing rhodium with platinum since rhodium also is more easily oxidised than platinum. It should also be
NOx Outlet Concentrations (ppm)
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800
600
400
200
0 0
100
200
300
400
Time (s) Fig. 1. NOx outlet traces ((䊏) 4% Pt, (䊐) 2% Pt, (䉱) 2% Pt–1% Rh, (䉭) 1% Pt–0.5% Rh, (䊊) 1% Rh) during sulphur free lean/rich cycling in 400 ppm (v/v) NO, 500 ppm (v/v) C3 H6 and 0/8 vol.% O2 at 400 ◦ C over five catalysts with different noble metal compositions loaded onto monoliths coated with 75 mg BaO and 500 mg Al2 O3 .
noted that the two samples containing both platinum and rhodium show similar behaviour regarding NOx storage although they differ with a factor of two in noble metal loading. At t = 300 s the oxygen supply is switched off and the conditions are thus changed from lean to rich. At this point, a NOx break through peak, mainly consisting of NO, is observed for all catalysts. The intensity of the peak is significantly higher for the sample with high Pt-loading (4% Pt) than for the others. The sample containing only rhodium (1% Rh) displays the smallest breakthrough peak. When the break through
peak declines, the NOx outlet traces for all samples reach very low values within 1 min indicating reduction of NOx by propene. The rhodium-only sample shows the slowest decrease in the NOx signal while the combined samples show the fastest decrease in the NOx signal during the rich period implying that more NOx is released unreacted. The amount of NOx released without being reduced is calculated as the integral from t = 300 to 400 s. In Table 3 the NOx storage capacity during one lean period, the total amount of NOx released during one rich period (i.e. released and supplied NOx not being
Table 3 Performance of fresh 75 mg BaO/500 mg Al2 O3 catalyst samples Sample
NOx storage (mole NO/mole BaO)
NOx release (mole)
NO2 formation (%)
NOx reduction (%)
4% 2% 2% 1% 1%
0.116 0.083 0.061 0.061 0.047
21.1 11.4 11.4 11.2 16.7
45 43 29 30 33
92 95 95 96 92
Pt Pt Pt–1% Rh Pt–0.5% Rh Rh
NOx storage and NO2 formation measured in 8 vol.% O2 , 500 ppm (v/v) C3 H6 and 400 ppm (v/v) NO. NOx release (i.e. released and supplied NOx not being reduced) and NO reduction measured in 500 ppm (v/v) C3 H6 and 400 ppm (v/v) NO. All compositions were balanced to 100 vol.% with Ar. Temperature: 400 ◦ C.
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Intensity (a. u.)
reduced), the NO oxidation efficiency at the end of the lean period and the NO reduction activity at the end of the rich period for the fresh samples are listed. From the table it is obvious that the catalysts containing only platinum (4% Pt and 2% Pt) show approximately twice as high NOx storage capacity as the ones containing rhodium (2% Pt–1% Rh, 1% Pt–0.5% Rh and 1% Rh), about 0.1 mole NO/mole BaO compared to about 0.05 mole NO/mole BaO, respectively. The highest value, 0.116 mole NO/mole BaO, was measured for the most highly loaded platinum sample (4% Pt) and the lowest, 0.047 mole NO/mole BaO, for the rhodium-only sample (1% Rh). One may note that the two samples containing both platinum and rhodium show very similar performance in this respect. Hence, increasing the platinum loading increases the NOx storage performance of the catalyst. This could be associated with more platinum being in contact with the barium providing more accessible storage area in the sample. On the other hand, the increased amount of platinum also increases the number of sites available for adsorption of nitrogen oxides and the release of NO when going from lean to rich will increase (as shown in Fig. 1). This is evident from Table 3 where the total amount of released, unreacted, NOx for each sample is shown. It is also obvious that the amount is very high also for the sample containing only rhodium (1% Rh). For 1% Rh the immediate NO peak is very small (see Fig. 1) why this cannot be suggested as the reason for NOx release. Similar results were presented by Nakatsuji et al. for rhodium supported on alumina [42] or beta-zeolites [43] and was then asso-
340 (a)
ciated with the need for reduction of rhodium before NOx reduction is possible. Table 3 also summarises the NO2 formation (i.e. NO oxidation) during the end of the lean period for all the samples. In addition, in this respect, the samples may be divided into the above two groups: samples with rhodium (2% Pt–1% Rh, 1% Pt–0.5% Rh and 1% Rh) and without rhodium (4% Pt and 2% Pt). For the three samples containing rhodium, only about 30% of the inlet NO is oxidised (independent of loading) while for the samples without rhodium the NO2 level almost reaches the equilibrium value of 50%. The observed NO2 formation of 30% for the rhodium containing samples agrees well with the NO oxidation activity obtained by Efthimiadis et al. over Rh/Al2 O3 catalysts at 400 ◦ C [28]. The last characteristic shown in Table 3 is the NOx reduction activity at steady state in towards the end of the rich periods (i.e. not the overall reduction capacity in lean/rich cycles) of the different samples. In this case, no significant difference in activity was observed between the fresh samples. This observation and that an increase in the Pt-loading from 2 to 4 wt.% does not affect the NO2 formation indicates that the loading in both cases is high enough to reach thermodynamic equilibrium. In Fig. 2 the Pt 4d3/2 and Rh 3p3/2 XPS spectra after either reduction or oxidation of a 3% Pt–1.5% Rh sample (see Table 1) are shown. The figure shows that after reduction both metals are present in significant amounts in their metallic states (corresponding to 331 eV for Pt and 496.5 eV for Rh [44]). After
reduced oxidised
336
332
328
Binding Energy (eV)
504 (b)
500
496
492
Binding Energy (eV)
Fig. 2. XPS (a) Pt 4d3/2 and (b) Rh 3p3/2 spectra of reduced (dashed line) and oxidised (solid line) Pt–Rh/BaO/Al2 O3 .
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oxidation, the states of the metals have shifted towards higher values and the respective peaks are observed at 332.8 and 498.5 eV, which indicates formation of platinum and rhodium oxides, respectively. Pt 4d5/2 shifts from 314.6 to 317.3 eV when Pt0 is oxidised to PtO and Rh 3d5/2 shifts from 307 eV (Rh0 ) to 309 eV (Rh2 O3 ) [45]. If the integrated areas in Fig. 2 are compared it is clear that after oxidation the amount of rhodium at the surface has increased with some 35% compared to after reduction. Platinum on the other hand has decreased with about 70% from reducing to oxidising pre-treatment. This observation may at least in part explain the observation that the NO oxidation rate in the lean condition is lower for the catalysts containing both platinum and rhodium (2% Pt–1% Rh and 1% Pt–0.5% Rh) compared to the platinum-only samples (4% Pt and 2% Pt). During lean conditions the surface of the mixed samples may be dominated by Rh, which is a less effective catalyst for NO oxidation. Rhodium enrichment on the surface of three-way catalysts has been reported earlier [46].
sample (see Table 3). From the figure it is obvious that all samples are deactivated by sulphur as the amount of NOx stored decreases with increasing sulphur exposure time. (None of the samples deactivated during a similar experimental series without sulphur present.) Further, it can be observed that for the samples containing rhodium (1% Rh, 2% Pt–1% Rh and 1% Pt–0.5% Rh) the loss of NOx storage capacity is more rapid than for the samples containing only platinum (4% Pt and 2% Pt). The platinum-only samples retain 40% of the initial NOx storage capacity after 60 min of sulphur exposure while for the rhodium containing samples this point is reached after about 30 min. After 60 min the latter samples have lost all of the NOx storage capacity. The samples without rhodium do not become deactivated completely in less than 120 min. Additionally, Fig. 3 shows that for all platinumcontaining samples (4% Pt, 2% Pt, 2% Pt–1% Rh and 1% Pt–0.5% Rh), a small increase in the NOx storage capacity in the very beginning of the SO2 exposure is obtained. This increase is not observed for the sample containing rhodium alone (1% Rh). Fig. 4 displays the effect of SO2 exposure on (a) the NO2 formation during the lean period and (b) the NO reduction during the rich period for all five samples. The values are normalised to the maximum activity of each sample. Fig. 4a shows that the ability to oxidise NO over the rhodium-only sample (1% Rh) is
3.2. Sulphur deactivation Fig. 3 shows the measured NOx storage capacities for the different samples during sulphur exposure, normalised to the initial value obtained for each Relative NOx Storage Capacity (%)
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120 100 80 60 40 20 0 0
20
40
60
80
100
120
140
Sulphur exposure time (min) Fig. 3. Deactivation of NOx storage capacities for the different 75 mg/500 mg Al2 O3 catalysts ((䊐) 2% Pt, (䉫) 4% Pt, (䉭) 2% Pt–1% Rh, ( ) 1% Pt–0.5% Rh, (䊊) 1% Rh) under exposure to 25 ppm (v/v) SO2 at 400 ◦ C during 15 lean/rich cycles. Lean gas mixture: 8 vol.% O2 , 500 ppm (v/v) C3 H6 and 400 ppm (v/v) NO. Rich gas mixture: 500 ppm (v/v) C3 H6 and 400 ppm (v/v) NO. All gas mixtures are balanced to 100 vol.% with Ar.
Relative NO2 formation (%)
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120 100 80 60 40 20 0 0
Relative NOx reduction (%)
(a)
40
60
80
100
120
140
120
140
Sulphur exposure time (min) 120 100 80 60 40 20 0 0
(b)
20
20
40
60
80
100
Sulphur exposure time (min)
Fig. 4. (a) NO2 formation (NO oxidation) and (b) NOx reduction for the different 75 mg/500 mg Al2 O3 catalysts ((䊐) 2% Pt, (䉫) 4% Pt, (䉭) 2% Pt–1% Rh, ( ) 1% Pt–0.5% Rh, (䊊) 1% Rh) normalised to the fresh catalyst activity as function of sulphur exposure time. The lean gas mixture contained 8 vol.% O2 , 500 ppm (v/v) C3 H6 , 400 ppm (v/v) NO and 25 ppm (v/v) SO2 . The rich gas mixture contained 500 ppm (v/v) C3 H6 , 400 ppm (v/v) NO and 25 ppm (v/v) SO2 . All gas mixtures are balanced to 100 vol.% with Ar and the temperature was 400 ◦ C.
severely affected by SO2 exposure and over 75% of the initial NO oxidation activity is lost after 90 min. The deactivation of NO oxidation over Rh/Al2 O3 by SO2 has been observed by others under similar, lean, conditions [28,30] and was then correlated to sulphation of catalytic sites at the rhodium/support interface. This effect is not observed for any of the samples containing platinum (2% Pt–1% Rh, 1% Pt–0.5% Rh, 4% Pt and 2% Pt). Instead, for 2% Pt and 2% Pt–1% Rh the NO oxidation seems to increase with SO2 exposure. This has been suggested to be connected with the formation of lower amounts of Pt-oxides when SO2 is present [48,49]. Under lean conditions SO2 may adsorb on platinum and increase the oxidation activity of the metal. Increased oxidation activity over supported
platinum has been observed previously for hydrocarbon oxidation [47]. Similar results for NO oxidation has also been observed [29]. Additionally, SO2 also may help keeping the platinum in a metallic state. It has earlier been stated that SO2 reduces platinum oxides while forming sulphite or sulphate species [48]. Platinum will become oxidised under exposure to NO2 forming platinum oxides, which have been reported to have lower activity for NO oxidation and NO2 reduction (i.e. for establishing the NO/NO2 equilibrium) than metallic platinum [49]. Fig. 4b shows the corresponding activity for NO reduction during the rich period as a function of SO2 exposure time for the samples. The figure implies that this property is only marginally affected by the sulphur exposure for the samples containing rhodium (2% Pt–1% Rh, 1% Pt–0.5% Rh and 1% Rh), while the platinum-only samples (4% Pt and 2% Pt) loose up to 80% of their initial activity for NOx reduction depending on loading. The higher loaded sample is somewhat more resistant to sulphur than the sample with lower amount of platinum. The loss of platinum activity under rich SO2 exposure over these samples may be due to the formation of reduced sulphur species under net reducing conditions. Atomic sulphur residues or even formation of platinum sulphide have been detected in several studies on similar materials [20,36,37]. For example, formation of PtS was observed by Chang et al. after H2 S exposure of Pt/Al2 O3 catalysts [38] and more recently Sedlmair et al. presented EXAFS data pointing at PtS formation on Pt/BaO/Al2 O3 catalysts in the presence of SO2 and propene [20]. In oxygen deficit SO2 will be reduced to H2 S or COS depending on the gas feed [50]. Both these compounds have been reported to cause severe deactivation of the NOx reduction function of NOx storage catalysts [34] and it was suggested that the deactivation caused by these compounds could be the result of formation of reduced sulphur species on the noble metals. Deactivation of rhodium based catalysts under exposure to H2 S has been reported several times, for example by Nasri et al. However, these studies also indicate that rhodium is more easily regenerated than platinum under hydrogen treatment [33]. The combined samples (2% Pt–1% Rh and 1% Pt–0.5% Rh) show both platinum and rhodium features, i.e. they do not suffer from deactivation of either the oxidation or the reduction properties. However,
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they show deactivation of the NOx storage capacity in the same range as the rhodium-only sample. It is possible that this is connected to the relative enrichment of Rh at the surface as observed by XPS for the catalyst that had been pre-treated in a lean mixture (see Fig. 2). 3.3. Sulphur regeneration Fig. 5 shows (a) the NOx storage capacities in lean/rich cycles, (b) NO oxidation capacity in the lean periods and (c) NO reduction activity in the rich periods after each sulphur regeneration procedure com-
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pared to the values obtained for the fresh samples and the values obtained after sulphur treatment. It can be seen that all samples, regardless of noble metal content, have completely lost the NOx storage capacity after 15 storage/reduction cycles in SO2 containing environment (compare to Fig. 4). This is connected to a combination of blocking of NOx storage sites and deactivation of the noble metals as has been discussed previously [23]. After 30 min under reduction in 2 vol.% H2 at 750 ◦ C however, all samples have recovered some activity. The sulphates will during this treatment be reduced to H2 S or COS depending on the reductant [51]. The recovered activity is, in all cases,
Fig. 5. Normalised (a) NOx storage capacity, (b) NO oxidation during the lean periods and (c) NOx reduction during the rich periods for the different 75 mg BaO/500 mg Al2 O3 catalysts ((䉭) 2% Pt, (䉫) 4% Pt, (䊐) 2% Pt–1% Rh, (䊊) 1% Pt–0.5% Rh, (×) 1% Rh) after sulphur treatment and regeneration procedures compared to the fresh samples. (Sulphur treatment: 15 lean/rich cycles at 400 ◦ C with a lean gas mixture of 8 vol.% O2 , 500 ppm (v/v) C3 H6 , 400 ppm (v/v) NO and 25 ppm (v/v) SO2 and a rich gas mixture of 500 ppm (v/v) C3 H6 , 400 ppm (v/v) NO and 25 ppm (v/v) SO2 . Reduction procedure: 30, 60, 90 min and additional 12 h at 2 vol.% H2 at 750 ◦ C. All gas mixtures are balanced to 100 vol.% with Ar.)
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more or less the maximum recoverable NOx storage capacity since no significant difference is observed after further reduction. For all samples, except 2% Pt–1% Rh, this maximum recoverable NOx storage capacity is about 50% of the initial value. The corresponding value for the 2% Pt–1% Rh sample is about 80%. This is in line with recent results showing that complete regeneration of the NOx storage capacity of a sulphated NOx storage catalyst with only platinum present is not possible [52]. The lean NO oxidation activities for the different samples at fresh, sulphur-treated and regenerated states are shown in Fig. 5b. The figure shows that all the samples containing platinum (1% Pt–0.5% Rh, 4% Pt and 2% Pt) are more or less unaffected by the sulphur exposure (compare to Fig. 4a) and hence no effect of hydrogen treatment on this property is observed. The sample containing rhodium-only (1% Rh) shows a completely different behaviour and looses most of its NO oxidation activity during the sulphur treatment (compare to Fig. 4a). However, after 30 min of reduction the sample has recovered most of its initial activity and with reduction time a steady increase in the oxidation activity is obtained. As discussed above the deactivation of Rh/Al2 O3 under lean SO2 exposure is in agreement with Efthimiadis et al. [28] and the regenerablility in hydrogen was also verified by Nasri et al. [33]. Keeping these results in mind, the opposite performances of the samples are observed when considering the NOx reduction activity under net reducing conditions as shown in Fig. 5c. This property is strongly deactivated by sulphur for the samples containing only platinum (4% Pt and 2% Pt) while the rhodium containing ones (2% Pt–1% Rh, 1% Pt–0.5% Rh and 1% Rh) are only marginally effected by the sulphur treatment. After 30 min of reduction all samples are completely regenerated and are not affected by further reduction. The reduction conditions used in this study are so severe that it is realistic to suppose sintering of the noble metal particles. As shown in the data, no obvious loss of either NO oxidation or NO reduction activity is observed even after more than 12 h under 2 vol.% H2 at 750 ◦ C suggesting that the possible sintering has only marginal effect on the NO oxidation and NO reduction ability. It has previously been stated that the activity for NO2 decomposition to NO over Pt/Al2 O3 and Pt/BaO/Al2 O3
catalysts increase with the Pt-particle size [49]. It is reasonable to assume that this is also the case for the reverse direction of the reaction, i.e. oxidation of NO to NO2 .
4. Conclusions From the results presented, it can be concluded that a combination of Pt and Rh is essential to provide good performance of NOx storage catalysts. It was also shown that the two different noble metals were differently affected by exposure to SO2 . Platinum was identified as important for the NO oxidation activity during the lean periods; more active during sulphur free conditions and more sulphur tolerant in the presence of SO2 . Rhodium on the other hand was shown important for the NO reduction activity during the rich periods in a similar manner. The results presented also indicate that combining platinum and rhodium increases the overall efficiency of the NOx storage catalyst despite the fact that they stored a lower amount of NOx than the catalyst containing only platinum. However, the combined catalysts were shown more easily sulphur regenerated.
Acknowledgements This work was performed within the Competence Centre for Catalysis hosted by Chalmers University of Technology and financially supported by the Swedish Energy Agency and the member companies AB Volvo, Saab Automobile AB, Johnson Matthey-CSD, Perstorp AB, Eka Chemicals/Akzo Nobel Catalysts BV, AVL-MTC AB and the Swedish Space Agency.
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Influence of platinum and rhodium composition on the NOx storage and sulphur tolerance of a barium based NOx storage catalyst Annika Amberntsson a,b , Erik Fridell a,b,∗ , Magnus Skoglundh a,c a
Competence Centre for Catalysis, Chalmers University of Technology, SE-412 96 Göteborg, Sweden Department of Applied Physics, Chalmers University of Technology, SE-412 96 Göteborg, Sweden c Applied Surface Chemistry, Chalmers University of Technology, SE-412 96 Göteborg, Sweden
b
Received 19 January 2003; received in revised form 21 June 2003; accepted 21 June 2003
Abstract In the present work the influence of the type of noble metals present in barium oxide based NOx storage catalysts was investigated regarding the NOx storage performance, NO oxidation, NO reduction, sulphur deactivation and sulphur regenerability. Monolith samples with combinations of platinum and rhodium, were prepared, tested in a flow-reactor, and characterised by XPS measurements. The flow-reactor experiments simulated NOx storage and reduction cycles at 400 ◦ C in synthetic gas mixtures with oxygen, propene and nitric oxide. For the sulphur deactivation and regenerability investigations 25 ppm (v/v) SO2 was added to the feed gas stream. From the experiments, it was concluded that a combination of platinum and rhodium is required to achieve good NOx storage and reduction performance. The NOx storage capacity was, however, found higher for catalysts containing only platinum compared to catalysts including rhodium. When exposed to SO2 the NOx storage capacity also seemed to deactivate faster for the samples containing rhodium than for samples with platinum as the sole noble metal. Additionally, it was observed that platinum gives high NO oxidation activity during the lean periods both with and without SO2 present in the gas feed. During the rich periods, rhodium showed high activity for NO reduction in sulphur free gas feed as well as in the presence of SO2 . Finally, the results implied that to provide good sulphur regeneration ability of the NOx storage catalyst, a combination of platinum and rhodium is necessary. © 2003 Elsevier B.V. All rights reserved. Keywords: NOx storage catalysts; Platinum; Rhodium; Sulphur deactivation; NO oxidation; NO reduction
1. Introduction To provide good fuel economy of automotive engines lean-burn and diesel technologies are interesting alternatives to the conventional, stoichiometric, Otto ∗ Corresponding author. Tel.: +46-31-772-3372; fax: +46-31-772-3134. E-mail address: [email protected] (E. Fridell).
engine [1]. However, a large surplus of oxygen in the exhausts means that the traditional three-way catalyst (TWC) is not able to efficiently reduce nitrogen oxides (NOx ). Hence, new approaches for treating the NOx emissions from diesel and lean-burn engines are needed [1,2]. One approach is the so-called NOx storage and reduction concept [3] based on storage of NOx in the catalyst for relatively long lean periods, which are interrupted by rich spikes. During
0926-3373/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0926-3373(03)00269-8
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the rich periods, the NOx stored in the lean periods is decomposed and subsequently reduced to nitrogen [4–6]. The catalysts most commonly used for NOx storage applications comprise combinations of noble metals for oxidation and reduction purposes and barium oxide/carbonate as the storage material [3–6]. It has been reported that the nitrogen oxides are trapped as surface nitrates [4–13]. Unfortunately, affinity for nitrate formation also entails affinity for sulphation, which will lead to deactivation of the NOx storage capacity if sulphur is present in the exhaust [14–22]. Since the noble metals seem to play several keyroles in the NOx storage and reduction cycles as well as for sulphur deactivation and for regeneration from sulphur [23], it is of great importance to investigate their role in detail. Salasc et al. investigated the effect of palladium compared to platinum on the performance of NOx storage catalysts. It was found that the palladium containing samples had a slightly higher NOx storage ability at lower temperatures compared to the platinum loaded samples and that the NO oxidation activity is much higher on platinum compared to palladium [24]. The effect of addition of noble metals (palladium, platinum or rhodium) to a calcium oxide based, NOx storage and reduction catalyst was studied by Huang et al. [25]. The authors claim that rhodium provides better NOx storage ability for a CaO/Al2 O3 catalyst, compared to platinum, due to higher formation of NO2 . This statement, however, is contradicted by several studies on alumina supported platinum and rhodium catalysts. For example, Kobajashi et al. reported the same activity for NO oxidation over Rh/Al2 O3 and Pt/Al2 O3 [26]. Efthimiadis et al. [27,28] and Efthimiadis and co-workers [29] repeatedly reported that Rh/Al2 O3 catalyst only may oxidise a maximum of ∼30% of the inlet NO to NO2 in the presence of propene at 400◦ C [27–29]. From thermodynamic considerations, the corresponding NO to NO2 oxidation under such conditions is 50%. The corresponding platinum catalyst in all cases was reported to provide higher amounts of NO2 compared to rhodium. However, the thermodynamic equilibrium concentration of NO2 was never reached due to reduction of NOx with propene [28–30]. Introduction of SO2 in the gas feed has been stated to affect the noble metals in the following way:
(i) The NO oxidation activity during lean conditions was increased for Pt/Al2 O3 [29] but was decreased for Rh/Al2 O3 [28,29]. (ii) The NO reduction activity during rich conditions of Rh/Al2 O3 was increased [27,28] but was not affected for Pt/Al2 O3 [30]. Other authors have reported that NO oxidation up to thermodynamic limited levels are possible over Pt/Al2 O3 in the absence of propene at temperatures around 350 ◦ C. The NO oxidation over this material, however, was not affected by the absence or presence of SO2 [31]. The main function of the noble metals during the rich period of the NOx storage reduction cycle is the ability to reduce NO. This reaction has been extensively studied under stoichiometric or near stoichiometric conditions over precious metals present in TWC [32]. During such conditions, both Pt and Rh possess high activity for NO reduction, however with different product selectivity. The formation of N2 O and NH3 is thus higher over Pt whereas reduction to N2 is favoured over Rh [32]. Further, the interaction with sulphur containing compounds is different for Pt and Rh. The ability to reduce NO has been reported to decline considerably faster for Rh than for Pt when exposed to H2 S. The same study has also shown that Rh is significantly more easy to reduce under hydrogen treatment than Pt [33]. Previously we have shown that sulphur deactivation of NOx storage catalyst proceeds considerably faster under exposure to sulphur under rich conditions compared to lean sulphur exposure implying that poisoning of the noble metals is an important deactivation mechanism for this type of catalyst [23,34,35]. Furthermore, other authors have reported formation of sulphur deposits on the Pt surface [36,37] or even the formation of platinum sulphide [20,38] have been reported upon exposure to sulphur compounds under net reducing conditions. The influence of the noble metal composition (platinum and rhodium) on the performance of model NOx storage catalysts concerning (i) NOx storage capacity in lean/rich cycles, (ii) NO oxidation under lean conditions, (iii) NO reduction activity under rich conditions, (iv) tolerance towards sulphur deactivation and (v) sulphur regenerability are investigated in this study by means of flow-reactor measurements and XPS.
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2. Experimental 2.1. Catalyst preparation The catalysts were prepared starting from cylindrical cordierite monolith substrates (∅ = 20 mm, L = 15 mm, 400 cells per square inch). Washcoats were deposited by immersing the substrates into an aqueous slurry of ␥-alumina (Puralox, Condea) and boehmite (Disperal, Condea) followed by gently blowing air through the channels to remove excess slurry, drying and calcination. The immersing– blowing–drying–calcination procedure was repeated until the desired amount of alumina was deposited. Barium oxide was deposited in a similar manner using an aqueous solution of barium nitrate (Ba(NO3 )2 from Aldrich). The noble metals were deposited by wet impregnation of non-halide noble metal salts of platinum and rhodium (Pt(NO3 )2 and Rh(NO3 )2 supplied by Hereaus and Johnson & Matthey, respectively). The catalyst preparation route is described in detail elsewhere [39]. The washcoat contained 500 mg of Al2 O3 (corresponding to 4.9 mmole) and 75 mg BaO (0.49 mmole) in all cases. Before use, all samples were calcined in air at 600 ◦ C for 90 min and reduced in 2 vol.% H2 at 500 ◦ C for 30 min. The samples prepared are listed in Table 1. 2.2. Flow-reactor experiments The experiments were all performed in a flow-reactor system with separate detection of NO, NO2 , N2 O and SO2 . NO and NO2 were measured with chemiluminescence and N2 O and SO2 were measured with IR instruments. The fully computerised system had the ability to independently supply nine different gases
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and both the inlet gas temperature and the catalyst temperature were monitored. The reactor system is described in detail elsewhere [40]. The initial activity for NOx storage, NO oxidation and NO reduction of each sample was established by performing sulphur free lean/rich cycling in a gas mixture containing 400 ppm (v/v) NO, 500 ppm (v/v) C3 H6 , 8 vol.% (during the lean 5 min periods) or no (during the rich 5 min periods) O2 balanced with Ar to maintain a constant space velocity of 38 000 h−1 . The inlet temperature was 400 ◦ C in all experiments. The equilibrium ratio between NO and NO2 is (about) 50:50 for 8 vol.% O2 at this temperature. The temperature is also close to the temperature previously observed for maximum in NOx storage (380 ◦ C) for similar catalysts [8]. These experimental conditions are considerably far from the conditions in a real, automotive application, i.e. the time scales are longer, the space velocity is lower and the gas mixture only contains a few components. However, to fulfil the scope of this model study the conditions are relevant. The sulphur free measurements were followed by sulphur treatments where 25 ppm (v/v) SO2 was continuously added to the gas mixture described above and 15 lean/rich cycles were performed. The total amount SO2 added during the lean/rich cycling was 0.46 mmole SO2 . Hence, the molar ratio between total amount of sulphur exposed to the sample and the amount of barium in the sample was close to 1 by the end of the experiment. After the SO2 exposure cycles, sulphur regeneration was performed by reducing the sample in 2 vol.% H2 at 750 ◦ C for 30 min followed by oxidation in 10 vol.% O2 at 400 ◦ C for 30 min. To investigate the effect of the regeneration, sulphur free lean/rich cycling was performed after the regeneration and the
Table 1 Summary of catalysts prepared Sample
Pt (mmole)
Rh (mmole)
BaO (mmole)
Al2 O3 (mmole)
4% 2% 2% 1% 1% 3%
0.118 0.059 0.059 0.029 – 0.088
– – 0.056 0.028 0.056 0.084
0.49 0.49 0.49 0.49 0.49 0.49
4.9 4.9 4.9 4.9 4.9 4.9
Pt Pt Pt–1% Rh Pt–0.5% Rh Rh Pt–1.5% Rh
All samples were washcoated with 500 mg Al2 O3 and 75 mg BaO.
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Table 2 Overview of conditions used in the flow-reactor experiments
Lean cycle period Rich cycle period Sulphur treatment Sulphur regeneration Oxidation
Gas composition
Temperature (◦ C)
Exposure time
400 ppm (v/v) NO, 500 ppm (v/v) C3 H6 , 8 vol.% O2 400 ppm (v/v) NO, 500 ppm (v/v) C3 H6 25 ppm (v/v) SO2 added to lean and rich 2 vol.% H2 10 vol.% O2
400 400 400 750 400
5 min 5 min 15 lean/rich cycles 4 × 30 min + 12 h 30 min
All gas mixtures were balanced to 100 vol.% with Ar.
NOx storage capacity, the NO oxidation (lean period) and the NO reduction (rich period) activities were measured. The whole regeneration procedure including reduction, oxidation and sulphur free cycling was repeated five times. The reduction time was 30 min except for the last experiment where the sample was reduced at 750 ◦ C for 12 h. A summary of the gas compositions (balanced with Ar) and experimental conditions for the flow-reactor study is given in Table 2. 2.3. XPS XPS measurements were performed both after the sample had been reduced in 8 vol.% H2 at 450 ◦ C for 30 min and oxidised in 19 vol.% O2 at 400 ◦ C on a monolith sample impregnated with 3% Pt–1.5% Rh on a washcoat of 75 mg BaO/500 mg Al2 O3 . After treatments, described above, in a flow-reactor cell connected to the UHV system (temperature range ∼25–600 ◦ C, Eurotherm PID temperature control unit, maximum gas flow rate of 40 ml/min) the samples were transferred into the XPS UHV chamber without being exposed to air. The XPS-spectrometer used was a Physical Electronics type PHI 5000C with non-monochromatic Al K␣ radiation (1486.6 eV). The detection angle was 45◦ from the surface normal, with excitation normal to the surface (X-ray power 400 W). This system samples an area with an approximate diameter of 0.8 mm. The Pt 4d3/2 and Rh 3p3/2 regions were investigated at a pressure <10−9 mbar with seven overlapped sweeps, with 0.125 eV per step and 100 ms per step. The energy scale was internally calibrated by adjusting the Al 2s peak to 119.3 eV [41].
3. Results and discussion 3.1. NOx storage catalyst performance Fig. 1 shows the NOx outlet concentration during one sulphur free lean/rich cycle for all samples investigated. The figure shows that all samples store a considerable amount of NOx indicated by the slow increase in the traces from t = 0 to ≈200 s. The total amount of NOx stored is calculated as the integrated traces between t = 0 and 300 s subtracted from the concentration at saturation (400 ppm (v/v)) multiplied by the storage time (300 s). The calculation procedure is described in detail elsewhere [8]. At t = 200 s the outlet concentrations of NOx have reached the inlet level which means that the NOx storage capacities of the samples are saturated. The figure also shows that the NOx response from the samples containing rhodium (2% Pt–1% Rh, 1% Pt–0.5% Rh and 1% Rh) give differently shaped curves compared with the platinum-only samples (4% Pt and 2% Pt). The rhodium containing samples all show an initial rapid increase of the NOx signal followed by a slower increase. This is not observed for the platinum-only samples for which the NOx response increases more linearly. We have previously observed a similar behaviour [24] when comparing NOx storage catalysts including Pt or Pd [24]. The catalyst with palladium as the noble metal showed two different slopes of the NOx trace during storage while the platinum one only showed one. The two-step behaviour was explained by oxidation of palladium during the first period and oxidation of NO only during the second step [24]. It is reasonable to expect similar behaviour when comparing rhodium with platinum since rhodium also is more easily oxidised than platinum. It should also be
NOx Outlet Concentrations (ppm)
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800
600
400
200
0 0
100
200
300
400
Time (s) Fig. 1. NOx outlet traces ((䊏) 4% Pt, (䊐) 2% Pt, (䉱) 2% Pt–1% Rh, (䉭) 1% Pt–0.5% Rh, (䊊) 1% Rh) during sulphur free lean/rich cycling in 400 ppm (v/v) NO, 500 ppm (v/v) C3 H6 and 0/8 vol.% O2 at 400 ◦ C over five catalysts with different noble metal compositions loaded onto monoliths coated with 75 mg BaO and 500 mg Al2 O3 .
noted that the two samples containing both platinum and rhodium show similar behaviour regarding NOx storage although they differ with a factor of two in noble metal loading. At t = 300 s the oxygen supply is switched off and the conditions are thus changed from lean to rich. At this point, a NOx break through peak, mainly consisting of NO, is observed for all catalysts. The intensity of the peak is significantly higher for the sample with high Pt-loading (4% Pt) than for the others. The sample containing only rhodium (1% Rh) displays the smallest breakthrough peak. When the break through
peak declines, the NOx outlet traces for all samples reach very low values within 1 min indicating reduction of NOx by propene. The rhodium-only sample shows the slowest decrease in the NOx signal while the combined samples show the fastest decrease in the NOx signal during the rich period implying that more NOx is released unreacted. The amount of NOx released without being reduced is calculated as the integral from t = 300 to 400 s. In Table 3 the NOx storage capacity during one lean period, the total amount of NOx released during one rich period (i.e. released and supplied NOx not being
Table 3 Performance of fresh 75 mg BaO/500 mg Al2 O3 catalyst samples Sample
NOx storage (mole NO/mole BaO)
NOx release (mole)
NO2 formation (%)
NOx reduction (%)
4% 2% 2% 1% 1%
0.116 0.083 0.061 0.061 0.047
21.1 11.4 11.4 11.2 16.7
45 43 29 30 33
92 95 95 96 92
Pt Pt Pt–1% Rh Pt–0.5% Rh Rh
NOx storage and NO2 formation measured in 8 vol.% O2 , 500 ppm (v/v) C3 H6 and 400 ppm (v/v) NO. NOx release (i.e. released and supplied NOx not being reduced) and NO reduction measured in 500 ppm (v/v) C3 H6 and 400 ppm (v/v) NO. All compositions were balanced to 100 vol.% with Ar. Temperature: 400 ◦ C.
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Intensity (a. u.)
reduced), the NO oxidation efficiency at the end of the lean period and the NO reduction activity at the end of the rich period for the fresh samples are listed. From the table it is obvious that the catalysts containing only platinum (4% Pt and 2% Pt) show approximately twice as high NOx storage capacity as the ones containing rhodium (2% Pt–1% Rh, 1% Pt–0.5% Rh and 1% Rh), about 0.1 mole NO/mole BaO compared to about 0.05 mole NO/mole BaO, respectively. The highest value, 0.116 mole NO/mole BaO, was measured for the most highly loaded platinum sample (4% Pt) and the lowest, 0.047 mole NO/mole BaO, for the rhodium-only sample (1% Rh). One may note that the two samples containing both platinum and rhodium show very similar performance in this respect. Hence, increasing the platinum loading increases the NOx storage performance of the catalyst. This could be associated with more platinum being in contact with the barium providing more accessible storage area in the sample. On the other hand, the increased amount of platinum also increases the number of sites available for adsorption of nitrogen oxides and the release of NO when going from lean to rich will increase (as shown in Fig. 1). This is evident from Table 3 where the total amount of released, unreacted, NOx for each sample is shown. It is also obvious that the amount is very high also for the sample containing only rhodium (1% Rh). For 1% Rh the immediate NO peak is very small (see Fig. 1) why this cannot be suggested as the reason for NOx release. Similar results were presented by Nakatsuji et al. for rhodium supported on alumina [42] or beta-zeolites [43] and was then asso-
340 (a)
ciated with the need for reduction of rhodium before NOx reduction is possible. Table 3 also summarises the NO2 formation (i.e. NO oxidation) during the end of the lean period for all the samples. In addition, in this respect, the samples may be divided into the above two groups: samples with rhodium (2% Pt–1% Rh, 1% Pt–0.5% Rh and 1% Rh) and without rhodium (4% Pt and 2% Pt). For the three samples containing rhodium, only about 30% of the inlet NO is oxidised (independent of loading) while for the samples without rhodium the NO2 level almost reaches the equilibrium value of 50%. The observed NO2 formation of 30% for the rhodium containing samples agrees well with the NO oxidation activity obtained by Efthimiadis et al. over Rh/Al2 O3 catalysts at 400 ◦ C [28]. The last characteristic shown in Table 3 is the NOx reduction activity at steady state in towards the end of the rich periods (i.e. not the overall reduction capacity in lean/rich cycles) of the different samples. In this case, no significant difference in activity was observed between the fresh samples. This observation and that an increase in the Pt-loading from 2 to 4 wt.% does not affect the NO2 formation indicates that the loading in both cases is high enough to reach thermodynamic equilibrium. In Fig. 2 the Pt 4d3/2 and Rh 3p3/2 XPS spectra after either reduction or oxidation of a 3% Pt–1.5% Rh sample (see Table 1) are shown. The figure shows that after reduction both metals are present in significant amounts in their metallic states (corresponding to 331 eV for Pt and 496.5 eV for Rh [44]). After
reduced oxidised
336
332
328
Binding Energy (eV)
504 (b)
500
496
492
Binding Energy (eV)
Fig. 2. XPS (a) Pt 4d3/2 and (b) Rh 3p3/2 spectra of reduced (dashed line) and oxidised (solid line) Pt–Rh/BaO/Al2 O3 .
A. Amberntsson et al. / Applied Catalysis B: Environmental 46 (2003) 429–439
oxidation, the states of the metals have shifted towards higher values and the respective peaks are observed at 332.8 and 498.5 eV, which indicates formation of platinum and rhodium oxides, respectively. Pt 4d5/2 shifts from 314.6 to 317.3 eV when Pt0 is oxidised to PtO and Rh 3d5/2 shifts from 307 eV (Rh0 ) to 309 eV (Rh2 O3 ) [45]. If the integrated areas in Fig. 2 are compared it is clear that after oxidation the amount of rhodium at the surface has increased with some 35% compared to after reduction. Platinum on the other hand has decreased with about 70% from reducing to oxidising pre-treatment. This observation may at least in part explain the observation that the NO oxidation rate in the lean condition is lower for the catalysts containing both platinum and rhodium (2% Pt–1% Rh and 1% Pt–0.5% Rh) compared to the platinum-only samples (4% Pt and 2% Pt). During lean conditions the surface of the mixed samples may be dominated by Rh, which is a less effective catalyst for NO oxidation. Rhodium enrichment on the surface of three-way catalysts has been reported earlier [46].
sample (see Table 3). From the figure it is obvious that all samples are deactivated by sulphur as the amount of NOx stored decreases with increasing sulphur exposure time. (None of the samples deactivated during a similar experimental series without sulphur present.) Further, it can be observed that for the samples containing rhodium (1% Rh, 2% Pt–1% Rh and 1% Pt–0.5% Rh) the loss of NOx storage capacity is more rapid than for the samples containing only platinum (4% Pt and 2% Pt). The platinum-only samples retain 40% of the initial NOx storage capacity after 60 min of sulphur exposure while for the rhodium containing samples this point is reached after about 30 min. After 60 min the latter samples have lost all of the NOx storage capacity. The samples without rhodium do not become deactivated completely in less than 120 min. Additionally, Fig. 3 shows that for all platinumcontaining samples (4% Pt, 2% Pt, 2% Pt–1% Rh and 1% Pt–0.5% Rh), a small increase in the NOx storage capacity in the very beginning of the SO2 exposure is obtained. This increase is not observed for the sample containing rhodium alone (1% Rh). Fig. 4 displays the effect of SO2 exposure on (a) the NO2 formation during the lean period and (b) the NO reduction during the rich period for all five samples. The values are normalised to the maximum activity of each sample. Fig. 4a shows that the ability to oxidise NO over the rhodium-only sample (1% Rh) is
3.2. Sulphur deactivation Fig. 3 shows the measured NOx storage capacities for the different samples during sulphur exposure, normalised to the initial value obtained for each Relative NOx Storage Capacity (%)
435
120 100 80 60 40 20 0 0
20
40
60
80
100
120
140
Sulphur exposure time (min) Fig. 3. Deactivation of NOx storage capacities for the different 75 mg/500 mg Al2 O3 catalysts ((䊐) 2% Pt, (䉫) 4% Pt, (䉭) 2% Pt–1% Rh, ( ) 1% Pt–0.5% Rh, (䊊) 1% Rh) under exposure to 25 ppm (v/v) SO2 at 400 ◦ C during 15 lean/rich cycles. Lean gas mixture: 8 vol.% O2 , 500 ppm (v/v) C3 H6 and 400 ppm (v/v) NO. Rich gas mixture: 500 ppm (v/v) C3 H6 and 400 ppm (v/v) NO. All gas mixtures are balanced to 100 vol.% with Ar.
Relative NO2 formation (%)
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A. Amberntsson et al. / Applied Catalysis B: Environmental 46 (2003) 429–439
120 100 80 60 40 20 0 0
Relative NOx reduction (%)
(a)
40
60
80
100
120
140
120
140
Sulphur exposure time (min) 120 100 80 60 40 20 0 0
(b)
20
20
40
60
80
100
Sulphur exposure time (min)
Fig. 4. (a) NO2 formation (NO oxidation) and (b) NOx reduction for the different 75 mg/500 mg Al2 O3 catalysts ((䊐) 2% Pt, (䉫) 4% Pt, (䉭) 2% Pt–1% Rh, ( ) 1% Pt–0.5% Rh, (䊊) 1% Rh) normalised to the fresh catalyst activity as function of sulphur exposure time. The lean gas mixture contained 8 vol.% O2 , 500 ppm (v/v) C3 H6 , 400 ppm (v/v) NO and 25 ppm (v/v) SO2 . The rich gas mixture contained 500 ppm (v/v) C3 H6 , 400 ppm (v/v) NO and 25 ppm (v/v) SO2 . All gas mixtures are balanced to 100 vol.% with Ar and the temperature was 400 ◦ C.
severely affected by SO2 exposure and over 75% of the initial NO oxidation activity is lost after 90 min. The deactivation of NO oxidation over Rh/Al2 O3 by SO2 has been observed by others under similar, lean, conditions [28,30] and was then correlated to sulphation of catalytic sites at the rhodium/support interface. This effect is not observed for any of the samples containing platinum (2% Pt–1% Rh, 1% Pt–0.5% Rh, 4% Pt and 2% Pt). Instead, for 2% Pt and 2% Pt–1% Rh the NO oxidation seems to increase with SO2 exposure. This has been suggested to be connected with the formation of lower amounts of Pt-oxides when SO2 is present [48,49]. Under lean conditions SO2 may adsorb on platinum and increase the oxidation activity of the metal. Increased oxidation activity over supported
platinum has been observed previously for hydrocarbon oxidation [47]. Similar results for NO oxidation has also been observed [29]. Additionally, SO2 also may help keeping the platinum in a metallic state. It has earlier been stated that SO2 reduces platinum oxides while forming sulphite or sulphate species [48]. Platinum will become oxidised under exposure to NO2 forming platinum oxides, which have been reported to have lower activity for NO oxidation and NO2 reduction (i.e. for establishing the NO/NO2 equilibrium) than metallic platinum [49]. Fig. 4b shows the corresponding activity for NO reduction during the rich period as a function of SO2 exposure time for the samples. The figure implies that this property is only marginally affected by the sulphur exposure for the samples containing rhodium (2% Pt–1% Rh, 1% Pt–0.5% Rh and 1% Rh), while the platinum-only samples (4% Pt and 2% Pt) loose up to 80% of their initial activity for NOx reduction depending on loading. The higher loaded sample is somewhat more resistant to sulphur than the sample with lower amount of platinum. The loss of platinum activity under rich SO2 exposure over these samples may be due to the formation of reduced sulphur species under net reducing conditions. Atomic sulphur residues or even formation of platinum sulphide have been detected in several studies on similar materials [20,36,37]. For example, formation of PtS was observed by Chang et al. after H2 S exposure of Pt/Al2 O3 catalysts [38] and more recently Sedlmair et al. presented EXAFS data pointing at PtS formation on Pt/BaO/Al2 O3 catalysts in the presence of SO2 and propene [20]. In oxygen deficit SO2 will be reduced to H2 S or COS depending on the gas feed [50]. Both these compounds have been reported to cause severe deactivation of the NOx reduction function of NOx storage catalysts [34] and it was suggested that the deactivation caused by these compounds could be the result of formation of reduced sulphur species on the noble metals. Deactivation of rhodium based catalysts under exposure to H2 S has been reported several times, for example by Nasri et al. However, these studies also indicate that rhodium is more easily regenerated than platinum under hydrogen treatment [33]. The combined samples (2% Pt–1% Rh and 1% Pt–0.5% Rh) show both platinum and rhodium features, i.e. they do not suffer from deactivation of either the oxidation or the reduction properties. However,
A. Amberntsson et al. / Applied Catalysis B: Environmental 46 (2003) 429–439
they show deactivation of the NOx storage capacity in the same range as the rhodium-only sample. It is possible that this is connected to the relative enrichment of Rh at the surface as observed by XPS for the catalyst that had been pre-treated in a lean mixture (see Fig. 2). 3.3. Sulphur regeneration Fig. 5 shows (a) the NOx storage capacities in lean/rich cycles, (b) NO oxidation capacity in the lean periods and (c) NO reduction activity in the rich periods after each sulphur regeneration procedure com-
437
pared to the values obtained for the fresh samples and the values obtained after sulphur treatment. It can be seen that all samples, regardless of noble metal content, have completely lost the NOx storage capacity after 15 storage/reduction cycles in SO2 containing environment (compare to Fig. 4). This is connected to a combination of blocking of NOx storage sites and deactivation of the noble metals as has been discussed previously [23]. After 30 min under reduction in 2 vol.% H2 at 750 ◦ C however, all samples have recovered some activity. The sulphates will during this treatment be reduced to H2 S or COS depending on the reductant [51]. The recovered activity is, in all cases,
Fig. 5. Normalised (a) NOx storage capacity, (b) NO oxidation during the lean periods and (c) NOx reduction during the rich periods for the different 75 mg BaO/500 mg Al2 O3 catalysts ((䉭) 2% Pt, (䉫) 4% Pt, (䊐) 2% Pt–1% Rh, (䊊) 1% Pt–0.5% Rh, (×) 1% Rh) after sulphur treatment and regeneration procedures compared to the fresh samples. (Sulphur treatment: 15 lean/rich cycles at 400 ◦ C with a lean gas mixture of 8 vol.% O2 , 500 ppm (v/v) C3 H6 , 400 ppm (v/v) NO and 25 ppm (v/v) SO2 and a rich gas mixture of 500 ppm (v/v) C3 H6 , 400 ppm (v/v) NO and 25 ppm (v/v) SO2 . Reduction procedure: 30, 60, 90 min and additional 12 h at 2 vol.% H2 at 750 ◦ C. All gas mixtures are balanced to 100 vol.% with Ar.)
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more or less the maximum recoverable NOx storage capacity since no significant difference is observed after further reduction. For all samples, except 2% Pt–1% Rh, this maximum recoverable NOx storage capacity is about 50% of the initial value. The corresponding value for the 2% Pt–1% Rh sample is about 80%. This is in line with recent results showing that complete regeneration of the NOx storage capacity of a sulphated NOx storage catalyst with only platinum present is not possible [52]. The lean NO oxidation activities for the different samples at fresh, sulphur-treated and regenerated states are shown in Fig. 5b. The figure shows that all the samples containing platinum (1% Pt–0.5% Rh, 4% Pt and 2% Pt) are more or less unaffected by the sulphur exposure (compare to Fig. 4a) and hence no effect of hydrogen treatment on this property is observed. The sample containing rhodium-only (1% Rh) shows a completely different behaviour and looses most of its NO oxidation activity during the sulphur treatment (compare to Fig. 4a). However, after 30 min of reduction the sample has recovered most of its initial activity and with reduction time a steady increase in the oxidation activity is obtained. As discussed above the deactivation of Rh/Al2 O3 under lean SO2 exposure is in agreement with Efthimiadis et al. [28] and the regenerablility in hydrogen was also verified by Nasri et al. [33]. Keeping these results in mind, the opposite performances of the samples are observed when considering the NOx reduction activity under net reducing conditions as shown in Fig. 5c. This property is strongly deactivated by sulphur for the samples containing only platinum (4% Pt and 2% Pt) while the rhodium containing ones (2% Pt–1% Rh, 1% Pt–0.5% Rh and 1% Rh) are only marginally effected by the sulphur treatment. After 30 min of reduction all samples are completely regenerated and are not affected by further reduction. The reduction conditions used in this study are so severe that it is realistic to suppose sintering of the noble metal particles. As shown in the data, no obvious loss of either NO oxidation or NO reduction activity is observed even after more than 12 h under 2 vol.% H2 at 750 ◦ C suggesting that the possible sintering has only marginal effect on the NO oxidation and NO reduction ability. It has previously been stated that the activity for NO2 decomposition to NO over Pt/Al2 O3 and Pt/BaO/Al2 O3
catalysts increase with the Pt-particle size [49]. It is reasonable to assume that this is also the case for the reverse direction of the reaction, i.e. oxidation of NO to NO2 .
4. Conclusions From the results presented, it can be concluded that a combination of Pt and Rh is essential to provide good performance of NOx storage catalysts. It was also shown that the two different noble metals were differently affected by exposure to SO2 . Platinum was identified as important for the NO oxidation activity during the lean periods; more active during sulphur free conditions and more sulphur tolerant in the presence of SO2 . Rhodium on the other hand was shown important for the NO reduction activity during the rich periods in a similar manner. The results presented also indicate that combining platinum and rhodium increases the overall efficiency of the NOx storage catalyst despite the fact that they stored a lower amount of NOx than the catalyst containing only platinum. However, the combined catalysts were shown more easily sulphur regenerated.
Acknowledgements This work was performed within the Competence Centre for Catalysis hosted by Chalmers University of Technology and financially supported by the Swedish Energy Agency and the member companies AB Volvo, Saab Automobile AB, Johnson Matthey-CSD, Perstorp AB, Eka Chemicals/Akzo Nobel Catalysts BV, AVL-MTC AB and the Swedish Space Agency.
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