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Oscillation of the NOx concentration in its selective catalytic reduction on platinum containing zeolite catalysts
Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S. Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All fights reserved.
1457
Oscillation of the NOx Concentration in Its Selective C a t a l y t i c Reduction on Platinum Containing Zeolite Catalysts Y. Traa, B. Burger and J. Weitkamp Institute of Chemical Technology I, University of Stuttgart, D-70550 Stuttgart, Germany Oscillations of the NOx concentration were observed in the selective catalytic reduction of NOx with propene on Pt-V/H-zeolites. It is assumed that the oscillations have their origin in a periodic transition of Pt between a metallic and an oxidic phase, which is catalyzed by V. 1. INTRODUCTION The selective catalytic reduction of nitrogen oxides by hydrocarbons is a promising process for the purification of exhaust gases from lean-burn gasoline- and diesel-powered engines. Platinum containing catalysts have proven to be efficient catalysts for this process because of their high hydrothermal stability and low lightoff temperature. In order to tune the activity and selectivity of the platinum containing catalysts we used a second metal as promoter. On Pt-V/H-zeolites we observed oscillations of the NOx concentration at high NOx conversions. 2. EXPERIMENTAL
SECTION
The zeolites were transformed into their ammonium forms and loaded with about 3 wt.-% Pt and 1 wt.-% V. The catalytic experiments were performed in a flow-type apparatus with a fixed-bed reactor at atmospheric pressure. 200 mg of the hydrated zeolite were activated in a flow of synthetic air for 1 h at 500 ~ and overnight at 450 ~ The gaseous feed components were premixed and afterwards saturated with water at 45 ~ Typically, the feed contained ca. 0.02 vol.-% NOx, 0.06 vol.-% C3H6, 0.03 vol.-% CO, 4 vol.-% CO2, 9 vol.-% O2 and 10 vol.-% 1-120 in He at a flow rate of 150 cma/min. The product stream was analyzed for nitrogen oxides with a chemiluminescence detector; all other components were analyzed by capillary gas chromatography. Prior to the temperature-programmed reduction with H2 (10 vol.-%) in Ar, the catalyst samples (400 mg) were oxidized in a gas stream containing O2 (10 vol.-%) in He. 3. RESULTS
AND DISCUSSION
Figure 1 shows the oscillation of the NOx conversion observed in the selective catalytic reduction of NOx with propene on 2.6Pt-l.0V/H-ZSM-35 zeolite. (The numbers before the metals indicate the metal content in wt.-% in the dry catalyst.) The oscillations could only be observed at high NOx conversion and within a temperature region of about 20 K. With
1458 100
,
,
,
,
,
90
0-.9.
80
70
OO o
oooc,. ,,
a =
Tea =
0
i
97 ~
Tea t
100
200
300
T I M E / min
Fig. 1. NOx conversion on 2.6Pt-l.0V/H-ZSM-35 (nsJn~ = 8) for different catalyst temperatures. increasing temperature, the amplitude of the oscillations decreases, whereas their frequency increases. The oscillations were fully reproducible, i.e., they occurred as well on other freshly prepared Pt-V/H-ZSM-3 5 batches and at slightly changed experimental conditions. Only minor changes occurred in the amplitude and the frequency of the oscillation, even over a time period of 17 h. While oscillations in the NOx reduction with hydrocarbons have so far only been reported in systems using feed gas streams without water [ 1-5], the presence of relatively large amounts of water seems to be crucial in our systems: We observed that the amplitude and the frequency of the oscillations decrease with decreasing water concentration (cf. Fig. 2).
100 t 90
'
i
'
9.9 vol.-% H20
I
'
i
'
I
3.3 vol.-% H20
6.6 vol.-% H20
i --- J x~
!
~-~t 1
7o
~176 f 50
0
,
I 100
,
I 200
,
I 300
,
I 400
500
T I M E / min
Fig. 2. NOx conversion on 2.6Pt-l.0V/H-ZSM-35 (nsi/nAj = 8) for different partial pressures of water.
1459
N%-ZSM-35insteadof NH4-ZSM-35 2[PtCId insteadof [Pt(NHz)4]CI2/ VOSO4instea~o f ~
NborTainstead ofV ~H2O~ /
~
3Pt-1bV/H-ZS. l -35
n~nv
mcat"~,
,-------
Pd instead of Pt AI203instead of
/
NH4-ZSM-35 decreasing O'H20 increasing Tcat.
Fig. 3. Effect of different parameters on the oscillations (o -- volume concentration). Figure 3 shows the effect of different parameters on the oscillations. The system where oscillations occurred with maximal amplitude at medium frequency and low catalyst temperatures, viz. 3Pt-1V/H-ZSM-35, is arranged in the center of the figure. With decreasing water concentration (cf. Fig. 2) or use of Nb or Ta instead of V, the frequency and the amplitude of the oscillations decrease. If the water concentration approaches zero, a steady NOx conversion is observed. By contrast, with increasing catalyst temperature, decreasing catalyst weight, decreasing npt/nv ratio, use of VOSO4 instead of VCl3 as V source, use of H2[PtCI6] instead of [Pt(NH3)4]C12 as Pt source and use of Na-ZSM-35 instead of NHn-ZSM-35, the frequency of the oscillation increases, whereas at the same time the amplitude decreases in most cases. If the catalyst temperature is further increased, the oscillations stop. No oscillations can be observed, if the catalyst contains only Pt or only V, Pd instead of Pt or A603 instead of NH4-ZSM-35 as cartier. The oscillations are not limited to zeolite ZSM-35 They could also be observed on zeolites with other pore systems, for example on Pt-V/H-Beta and Pt-V/H-ZK-5. On the contrary, on Pt-V/7-A1203 no oscillations occurred. This can be due to its lack of microporosity or rather due to the lower strength of its acid sites. The latter shall be illustrated with the following example: The amplitude of the oscillations decreases from 2.6Pt-l.0V/H-ZSM-35 (nsi/n~a = 8) over 2.0Pt-l.0V/H-ZSM-35 (nsi/nAl = 9) to 2.7Pt-1.1V/Na-ZSM-35 (nsi/n~a = 8). In this order, the number of acid sites decreases. Switching from Pt-V/Na-ZSM-35 to Pt-V/y-A1203, the oscillations stop. One reason for this could be that Pt-V/Na-ZSM-35 still has a significant number of strong acid sites, which could be verified by 1H-MAS-NMR spectroscopy, whereas Pt-V/y-AI203 only disposes of weak acid sites, which might be too weak to initiate the oscillations. However, at this stage of the investigations, it cannot be decided whether the
1460 microporosity or the strength of the acid sites is the key parameter for the occurrence of the oscillations. Another item which should be addressed is the fact that on Pd-V/H-ZSM-35 no oscillations occur (cf. Fig. 3). On Pt-V/H-ZSM-35, oscillations could be observed at high NOx conversion, and on Pd-V/H-ZSM-35, the maximal NOx conversion amounted only to 35 % at 239 ~ One explanation for the absence of oscillations on Pd-V/H-ZSM-35 could therefore be that the NOx conversion is not high enough and is reached at high temperatures only. In other words, a prerequisite for oscillations to be observed is a high catabtic activity at relatively low temperatures. 100
o" >
70
60
,
,
,
1
In further experiments, the Pt and V concentrations were varied. On zeolites containing Pt or V only, no regular oscillations of the NOx concentration were noticed, but irregular fluctuations on Pt/I-I-ZSM-35, which occurred exclusively just under the temperature of the maximum NOx conversion (cf. Fig. 4), i.e., the simultaneous presence of Pt and V is indispensable for the occurrence of regular oscillations with large amplitudes.
In an attempt to elucidate the reasons for this synergism, temperature-programmed reduction with H2 [ ! 50 was applied. This method showed that, on H-ZSM-35 0 50 100 loaded with Pt only or V only, no significant hydrogen T I M E / min consumption occurred at temperatures below 350 ~ Fig. 4. Fluctuation in the NOx con- presumably because the metals are located well version on 2.9Pt/H-ZSM-35 stabilized on cation exchange sites of the zeolite (cf. (nsi/n~a = 8), Teat. = 165 ~ Fig. 5). By contrast, on Pt-V/H-ZSM-35, large amounts of hydrogen were consumed already at 150 to 200 ~ i.e., in the temperature range in which the oscillations occur. (The small peak at 70 ~ is due to desorption of Ar which was previously adsorbed on the zeolite: the resulting smaller 1-12 concentration is recorded as apparent H2 consumption.) This means that Pt catalyzes the reduction of V and vice versa, which can be explained by the formation of bimetallic oxide clusters, in which the metals can be reduced more easily. Another possibility is that H2 dissociates on the first formed Pt and reduces via a spillover mechanism the V compound located elsewhere. In TPR experiments with further samples, it was noticed that on all catalysts on which oscillations with large amplitudes occurred the 1-12consumption was relatively large in the temperature range with high NOx conversions (< 350 ~ By contrast, on samples with small oscillation amplitudes, significantly less 1-12 was consumed in this temperature range compared to 2.6Pt-l.0V/H-ZSM-35, i.e., a certain amount of easily reducible metal has to be present so that oscillations with relatively large amplitudes can be initiated.
1461 For comparison, in Figure 6 the corresponding TPR results on the alumina system are depicted. On Pt/AI203, the Pt is already reduced at a lower temperature compared to the zeolitic system, because the metal is not stabilized at ion exchange sites. Besides, the artifact at 70 ~ is absent because of the much lower Ar adsorption capacity of alumina compared to the zeolite. However, the essential effect, the synergism of Pt and V, is the same with alumina and ZSM-35, i.e., Pt and V mutually catalyze their reduction, and the HE consumption on Pt-V/AI203 is significantly higher than o n P t / A l 2 0 3 and V/AI203. '
I
'
I
'
'
I
"~
I
'
Z
Z
O
0
a..
I'13..
1.0V/H-ZSM-35///~
O9 Z
0t) z
0 o
0 ro
2.7Pt/H-ZSM-35
3.0Pt/AI203
Z iii
z Ill (9 0
(.9 0n," a
a
>--F
>-r" ,
I
,
I
200 400 TEMPERATURE / ~
0
,
600
0
200 400 TEMPERATURE / ~
600
Fig 5. Results of the temperature-programmed Fig. 6. Results of the temperature-programmed reduction on different metal containing reduction on different metal containing H-ZSM-3 5 zeolites, aluminas. This comparison shows that the presence of a certain amount of easily reducible metal is only a necessary, but not a sufficient condition for the occurrence of the oscillations. 4. CONCLUSIONS The whole body of experimental results suggests that oscillations with large amplitudes will occur only if four conditions are fulfilled, viz., 1)
the partial pressure of water in the feed is high,
2)
the catalyst possesses acid sites with a certain strength and concentration and/or disposes of microporosity,
3) 4)
the catalyst displays a high activity already at relatively low temperatures and a large portion of the metal can be easily reduced.
Probably, requirements 2) and 3) have to be met, because the oscillations can only be observed in the range of high NOx conversions, i.e., on a well working catalyst. By contrast,
1462 items 1) and 4) allow to draw conclusions on the reaction mechanism: At this stage of the investigations we believe that the observed oscillations have their origin in a periodic transition of Pt between a metallic and an oxidic phase. This phase transition is possible because of the simultaneous presence of propene, carbon monoxide and oxygen in the feed. In the literature, oscillations on Pt catalysts are thought to be due to a periodic formation and decomposition of surface Pt oxides as well, e.g. [6,7], and the formation of these surface oxides was proven under comparatively mild conditions (160 ~ 5 vol.-% 02) [8]. In our system, the phase transition should be even easier because of the simultaneous presence of Pt and V: Both metals can change their oxidation states, i.e., a combined redox equilibrium could exist, e.g., Pt + 4
V O 2+ + 4 H §
<
> PtO2 + 4 V 3+ + 2
H20
Water could play a role in a slow reversible adsorption step as kind of a buffer step so that the oscillations could possibly be explained with a model comparable to Eigenberger's buffer step model [9]. This buffer step serves to block active sites after the ignition of the reaction and makes them available again after the extinction of the reaction. This could explain the necessity of large amounts of water in the feed (water is formed as reaction product, but probably not in sufficient amounts). Further attempts to elucidate the reasons for the occurrence of the oscillations are under way, but due to the complexity and corrosiveness of the feed in situ characterization and modelling of the system is quite demanding. ACKNOWLEDGMENTS The authors gratefully acknowledge financial support by Deutsche Forschungsgemeinschafi, Fonds der Chemischen Industrie and Max Buchner-Forschungsstifiung. REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9.
B.K. Cho, J.E. Yie and K.M. RahmoeUer, J. Catal. 157 (1995) 14. I. Halasz, A. Brenner, M. Shelefand K.Y.S. Ng, J. Phys. Chem. 99 (1995) 17186. A. Obuchi, M. Nakamura, A. Ogata, K. Mizuno, A. Ohi and H. Ohuchi, J. Chem. Soc., Chem. Commun. (1992) 1150. U.S. Ozkan, M.W. Kumthekar and G. Karakas, J. Catal. 171 (1997) 67. B.K. Cho, J. Catal. 178 (1998) 395. C.G. Vayenas, C. Georgakis, J. Michaels and J. Tormo, J. Catal. 67 (1981) 348. I.V. Yentekakis, S. Neophytides and C.G. Vayenas, J. Catal. 111 (1988) 152. J.J. Ostermaier, J.R. Katzer and W.H. Manogue, J. Catal. 41 (1976) 277. G. Eigenberger, Chem. Eng. Sci. 33 (1978) 1263.
1457
Oscillation of the NOx Concentration in Its Selective C a t a l y t i c Reduction on Platinum Containing Zeolite Catalysts Y. Traa, B. Burger and J. Weitkamp Institute of Chemical Technology I, University of Stuttgart, D-70550 Stuttgart, Germany Oscillations of the NOx concentration were observed in the selective catalytic reduction of NOx with propene on Pt-V/H-zeolites. It is assumed that the oscillations have their origin in a periodic transition of Pt between a metallic and an oxidic phase, which is catalyzed by V. 1. INTRODUCTION The selective catalytic reduction of nitrogen oxides by hydrocarbons is a promising process for the purification of exhaust gases from lean-burn gasoline- and diesel-powered engines. Platinum containing catalysts have proven to be efficient catalysts for this process because of their high hydrothermal stability and low lightoff temperature. In order to tune the activity and selectivity of the platinum containing catalysts we used a second metal as promoter. On Pt-V/H-zeolites we observed oscillations of the NOx concentration at high NOx conversions. 2. EXPERIMENTAL
SECTION
The zeolites were transformed into their ammonium forms and loaded with about 3 wt.-% Pt and 1 wt.-% V. The catalytic experiments were performed in a flow-type apparatus with a fixed-bed reactor at atmospheric pressure. 200 mg of the hydrated zeolite were activated in a flow of synthetic air for 1 h at 500 ~ and overnight at 450 ~ The gaseous feed components were premixed and afterwards saturated with water at 45 ~ Typically, the feed contained ca. 0.02 vol.-% NOx, 0.06 vol.-% C3H6, 0.03 vol.-% CO, 4 vol.-% CO2, 9 vol.-% O2 and 10 vol.-% 1-120 in He at a flow rate of 150 cma/min. The product stream was analyzed for nitrogen oxides with a chemiluminescence detector; all other components were analyzed by capillary gas chromatography. Prior to the temperature-programmed reduction with H2 (10 vol.-%) in Ar, the catalyst samples (400 mg) were oxidized in a gas stream containing O2 (10 vol.-%) in He. 3. RESULTS
AND DISCUSSION
Figure 1 shows the oscillation of the NOx conversion observed in the selective catalytic reduction of NOx with propene on 2.6Pt-l.0V/H-ZSM-35 zeolite. (The numbers before the metals indicate the metal content in wt.-% in the dry catalyst.) The oscillations could only be observed at high NOx conversion and within a temperature region of about 20 K. With
1458 100
,
,
,
,
,
90
0-.9.
80
70
OO o
oooc,. ,,
a =
Tea =
0
i
97 ~
Tea t
100
200
300
T I M E / min
Fig. 1. NOx conversion on 2.6Pt-l.0V/H-ZSM-35 (nsJn~ = 8) for different catalyst temperatures. increasing temperature, the amplitude of the oscillations decreases, whereas their frequency increases. The oscillations were fully reproducible, i.e., they occurred as well on other freshly prepared Pt-V/H-ZSM-3 5 batches and at slightly changed experimental conditions. Only minor changes occurred in the amplitude and the frequency of the oscillation, even over a time period of 17 h. While oscillations in the NOx reduction with hydrocarbons have so far only been reported in systems using feed gas streams without water [ 1-5], the presence of relatively large amounts of water seems to be crucial in our systems: We observed that the amplitude and the frequency of the oscillations decrease with decreasing water concentration (cf. Fig. 2).
100 t 90
'
i
'
9.9 vol.-% H20
I
'
i
'
I
3.3 vol.-% H20
6.6 vol.-% H20
i --- J x~
!
~-~t 1
7o
~176 f 50
0
,
I 100
,
I 200
,
I 300
,
I 400
500
T I M E / min
Fig. 2. NOx conversion on 2.6Pt-l.0V/H-ZSM-35 (nsi/nAj = 8) for different partial pressures of water.
1459
N%-ZSM-35insteadof NH4-ZSM-35 2[PtCId insteadof [Pt(NHz)4]CI2/ VOSO4instea~o f ~
NborTainstead ofV ~H2O~ /
~
3Pt-1bV/H-ZS. l -35
n~nv
mcat"~,
,-------
Pd instead of Pt AI203instead of
/
NH4-ZSM-35 decreasing O'H20 increasing Tcat.
Fig. 3. Effect of different parameters on the oscillations (o -- volume concentration). Figure 3 shows the effect of different parameters on the oscillations. The system where oscillations occurred with maximal amplitude at medium frequency and low catalyst temperatures, viz. 3Pt-1V/H-ZSM-35, is arranged in the center of the figure. With decreasing water concentration (cf. Fig. 2) or use of Nb or Ta instead of V, the frequency and the amplitude of the oscillations decrease. If the water concentration approaches zero, a steady NOx conversion is observed. By contrast, with increasing catalyst temperature, decreasing catalyst weight, decreasing npt/nv ratio, use of VOSO4 instead of VCl3 as V source, use of H2[PtCI6] instead of [Pt(NH3)4]C12 as Pt source and use of Na-ZSM-35 instead of NHn-ZSM-35, the frequency of the oscillation increases, whereas at the same time the amplitude decreases in most cases. If the catalyst temperature is further increased, the oscillations stop. No oscillations can be observed, if the catalyst contains only Pt or only V, Pd instead of Pt or A603 instead of NH4-ZSM-35 as cartier. The oscillations are not limited to zeolite ZSM-35 They could also be observed on zeolites with other pore systems, for example on Pt-V/H-Beta and Pt-V/H-ZK-5. On the contrary, on Pt-V/7-A1203 no oscillations occurred. This can be due to its lack of microporosity or rather due to the lower strength of its acid sites. The latter shall be illustrated with the following example: The amplitude of the oscillations decreases from 2.6Pt-l.0V/H-ZSM-35 (nsi/n~a = 8) over 2.0Pt-l.0V/H-ZSM-35 (nsi/nAl = 9) to 2.7Pt-1.1V/Na-ZSM-35 (nsi/n~a = 8). In this order, the number of acid sites decreases. Switching from Pt-V/Na-ZSM-35 to Pt-V/y-A1203, the oscillations stop. One reason for this could be that Pt-V/Na-ZSM-35 still has a significant number of strong acid sites, which could be verified by 1H-MAS-NMR spectroscopy, whereas Pt-V/y-AI203 only disposes of weak acid sites, which might be too weak to initiate the oscillations. However, at this stage of the investigations, it cannot be decided whether the
1460 microporosity or the strength of the acid sites is the key parameter for the occurrence of the oscillations. Another item which should be addressed is the fact that on Pd-V/H-ZSM-35 no oscillations occur (cf. Fig. 3). On Pt-V/H-ZSM-35, oscillations could be observed at high NOx conversion, and on Pd-V/H-ZSM-35, the maximal NOx conversion amounted only to 35 % at 239 ~ One explanation for the absence of oscillations on Pd-V/H-ZSM-35 could therefore be that the NOx conversion is not high enough and is reached at high temperatures only. In other words, a prerequisite for oscillations to be observed is a high catabtic activity at relatively low temperatures. 100
o" >
70
60
,
,
,
1
In further experiments, the Pt and V concentrations were varied. On zeolites containing Pt or V only, no regular oscillations of the NOx concentration were noticed, but irregular fluctuations on Pt/I-I-ZSM-35, which occurred exclusively just under the temperature of the maximum NOx conversion (cf. Fig. 4), i.e., the simultaneous presence of Pt and V is indispensable for the occurrence of regular oscillations with large amplitudes.
In an attempt to elucidate the reasons for this synergism, temperature-programmed reduction with H2 [ ! 50 was applied. This method showed that, on H-ZSM-35 0 50 100 loaded with Pt only or V only, no significant hydrogen T I M E / min consumption occurred at temperatures below 350 ~ Fig. 4. Fluctuation in the NOx con- presumably because the metals are located well version on 2.9Pt/H-ZSM-35 stabilized on cation exchange sites of the zeolite (cf. (nsi/n~a = 8), Teat. = 165 ~ Fig. 5). By contrast, on Pt-V/H-ZSM-35, large amounts of hydrogen were consumed already at 150 to 200 ~ i.e., in the temperature range in which the oscillations occur. (The small peak at 70 ~ is due to desorption of Ar which was previously adsorbed on the zeolite: the resulting smaller 1-12 concentration is recorded as apparent H2 consumption.) This means that Pt catalyzes the reduction of V and vice versa, which can be explained by the formation of bimetallic oxide clusters, in which the metals can be reduced more easily. Another possibility is that H2 dissociates on the first formed Pt and reduces via a spillover mechanism the V compound located elsewhere. In TPR experiments with further samples, it was noticed that on all catalysts on which oscillations with large amplitudes occurred the 1-12consumption was relatively large in the temperature range with high NOx conversions (< 350 ~ By contrast, on samples with small oscillation amplitudes, significantly less 1-12 was consumed in this temperature range compared to 2.6Pt-l.0V/H-ZSM-35, i.e., a certain amount of easily reducible metal has to be present so that oscillations with relatively large amplitudes can be initiated.
1461 For comparison, in Figure 6 the corresponding TPR results on the alumina system are depicted. On Pt/AI203, the Pt is already reduced at a lower temperature compared to the zeolitic system, because the metal is not stabilized at ion exchange sites. Besides, the artifact at 70 ~ is absent because of the much lower Ar adsorption capacity of alumina compared to the zeolite. However, the essential effect, the synergism of Pt and V, is the same with alumina and ZSM-35, i.e., Pt and V mutually catalyze their reduction, and the HE consumption on Pt-V/AI203 is significantly higher than o n P t / A l 2 0 3 and V/AI203. '
I
'
I
'
'
I
"~
I
'
Z
Z
O
0
a..
I'13..
1.0V/H-ZSM-35///~
O9 Z
0t) z
0 o
0 ro
2.7Pt/H-ZSM-35
3.0Pt/AI203
Z iii
z Ill (9 0
(.9 0n," a
a
>--F
>-r" ,
I
,
I
200 400 TEMPERATURE / ~
0
,
600
0
200 400 TEMPERATURE / ~
600
Fig 5. Results of the temperature-programmed Fig. 6. Results of the temperature-programmed reduction on different metal containing reduction on different metal containing H-ZSM-3 5 zeolites, aluminas. This comparison shows that the presence of a certain amount of easily reducible metal is only a necessary, but not a sufficient condition for the occurrence of the oscillations. 4. CONCLUSIONS The whole body of experimental results suggests that oscillations with large amplitudes will occur only if four conditions are fulfilled, viz., 1)
the partial pressure of water in the feed is high,
2)
the catalyst possesses acid sites with a certain strength and concentration and/or disposes of microporosity,
3) 4)
the catalyst displays a high activity already at relatively low temperatures and a large portion of the metal can be easily reduced.
Probably, requirements 2) and 3) have to be met, because the oscillations can only be observed in the range of high NOx conversions, i.e., on a well working catalyst. By contrast,
1462 items 1) and 4) allow to draw conclusions on the reaction mechanism: At this stage of the investigations we believe that the observed oscillations have their origin in a periodic transition of Pt between a metallic and an oxidic phase. This phase transition is possible because of the simultaneous presence of propene, carbon monoxide and oxygen in the feed. In the literature, oscillations on Pt catalysts are thought to be due to a periodic formation and decomposition of surface Pt oxides as well, e.g. [6,7], and the formation of these surface oxides was proven under comparatively mild conditions (160 ~ 5 vol.-% 02) [8]. In our system, the phase transition should be even easier because of the simultaneous presence of Pt and V: Both metals can change their oxidation states, i.e., a combined redox equilibrium could exist, e.g., Pt + 4
V O 2+ + 4 H §
<
> PtO2 + 4 V 3+ + 2
H20
Water could play a role in a slow reversible adsorption step as kind of a buffer step so that the oscillations could possibly be explained with a model comparable to Eigenberger's buffer step model [9]. This buffer step serves to block active sites after the ignition of the reaction and makes them available again after the extinction of the reaction. This could explain the necessity of large amounts of water in the feed (water is formed as reaction product, but probably not in sufficient amounts). Further attempts to elucidate the reasons for the occurrence of the oscillations are under way, but due to the complexity and corrosiveness of the feed in situ characterization and modelling of the system is quite demanding. ACKNOWLEDGMENTS The authors gratefully acknowledge financial support by Deutsche Forschungsgemeinschafi, Fonds der Chemischen Industrie and Max Buchner-Forschungsstifiung. REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9.
B.K. Cho, J.E. Yie and K.M. RahmoeUer, J. Catal. 157 (1995) 14. I. Halasz, A. Brenner, M. Shelefand K.Y.S. Ng, J. Phys. Chem. 99 (1995) 17186. A. Obuchi, M. Nakamura, A. Ogata, K. Mizuno, A. Ohi and H. Ohuchi, J. Chem. Soc., Chem. Commun. (1992) 1150. U.S. Ozkan, M.W. Kumthekar and G. Karakas, J. Catal. 171 (1997) 67. B.K. Cho, J. Catal. 178 (1998) 395. C.G. Vayenas, C. Georgakis, J. Michaels and J. Tormo, J. Catal. 67 (1981) 348. I.V. Yentekakis, S. Neophytides and C.G. Vayenas, J. Catal. 111 (1988) 152. J.J. Ostermaier, J.R. Katzer and W.H. Manogue, J. Catal. 41 (1976) 277. G. Eigenberger, Chem. Eng. Sci. 33 (1978) 1263.