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Inverse hysteresis during the NO oxidation on Pt under lean conditions
Applied Catalysis B: Environmental 93 (2009) 22–29
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Applied Catalysis B: Environmental journal homepage: www.elsevier.com/locate/apcatb
Inverse hysteresis during the NO oxidation on Pt under lean conditions W. Hauptmann a,b, M. Votsmeier a,*, J. Gieshoff a, A. Drochner b, H. Vogel b a b
Umicore AG & Co. KG, D-63403 Hanau-Wolfgang, Germany Ernst-Berl-Institut fu¨r Technische und Makromolekulare Chemie, Technische Universita¨t Darmstadt, D-64287 Darmstadt, Germany
A R T I C L E I N F O
A B S T R A C T
Article history: Received 30 June 2009 Received in revised form 28 August 2009 Accepted 7 September 2009 Available online 16 September 2009
It is shown that a platinum catalyst operated under a transient temperature profile exhibits a hysteresis behavior. The catalytic activity during heat-up phases exceeds the activity of the cool down phases. This effect is the opposite of the well-known hysteresis that can be caused by the existence of multiple steady states or the thermal inertia of the catalyst. It is therefore referred to as ‘inverse hysteresis’. If the catalyst is operated at one temperature for a longer time, a steady state activity is observed that is in-between the higher activity of the transient heat-up and the lower activity of the transient cool down. The most likely explanation for the observed hysteresis behavior is a reversible oxidation of the platinum surface by NO2 and a reduction of the platinum surface by NO. A micro-kinetic surface mechanism including the reversible surface oxidation has been developed and implemented in a monolith reactor model. The observed hysteresis effects can be quantitatively reproduced by the numerical model. If the feed contains CO/NO/O2 an inverse hysteresis is observed for NO and CO oxidation. This means that in the presence of NO the hysteresis behavior of the CO oxidation switches from the typical standard hysteresis to an inverse hysteresis. ß 2009 Elsevier B.V. All rights reserved.
Keywords: NO oxidation Deactivation Diesel exhaust Platinum Monolithic catalyst Inverse hysteresis Micro-kinetic model
1. Introduction The oxidation of NO to NO2 on platinum plays an important role in a number of Diesel exhaust aftertreatment systems. One example is the Diesel particulate filter (DPF), which requires NO2 for passive regeneration. Another example is the selective catalytic reduction (SCR) by NH3 which is much more efficient if the feed contains a mixture of NO/NO2. A third example is the regenerable NOx storage catalyst where under many conditions the oxidation of NO is an essential first step of the storage mechanism. Motivated by the importance of this reaction in exhaust catalysis the kinetics of NO oxidation on platinum has been studied by a number of researchers [1–13]. Global rate equations have been fitted to experimental data in [14–19] and micro-kinetic models of the reaction have also been published [20–26]. One important feature of the reaction is that the reaction rate per surface site seems to be dependent on the size of the platinum particles, even to the extent that the activity of a catalyst increases during thermal aging due to a growth in particle size [1]. It has been observed in a number of studies that the activity of a platinum catalyst for NO oxidation is decreased by oxidative pretreatments (with NO2) and that the catalyst can be reactivated
* Corresponding author. Fax: +49 6181 59 4693. E-mail address: [email protected] (M. Votsmeier). 0926-3373/$ – see front matter ß 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.apcatb.2009.09.008
by a reductive or thermal treatment. For example, Despre´s et al. [8,9] have shown that pretreatment at 250 8C with 500 ppm NO2 causes a strong decrease in activity. The initial activity can be restored by a treatment with NH3 or NO in nitrogen at 250 8C or by thermal regeneration at 650 8C in air. Olsson and Fridell [1] have shown that when a catalyst previously oxidized by NO2 is operated with a feed stream containing 630 ppm NO and 8% O2, the activity increases over time. Thus it is apparent that a catalyst that has been deactivated by NO2 can be regenerated by NO/O2, despite the large O2 excess in this mixture. The commonly accepted explanation for the reversible deactivation by oxidative treatments is an oxide formation on the platinum surface. The formation of such surface oxides has been demonstrated by a number of experimental techniques, see [1] for a literature summary. The nature of the platinum oxides does not seem to be fully understood. Possible candidates are so-called subsurface oxygen [27,28], the formation of a platinum oxide layer [29] or even the bulk oxidation of the platinum particles [3]. It is one characteristic of exhaust aftertreatment that the catalysts are operated under transient temperature conditions, mainly due to changes in engine load. In this paper we study the NO oxidation process during different temperature cycles that consist of upwards and downwards ramps. The main objective of this paper is to demonstrate that even with a constant exhaust composition reversible deactivation phenomena can be observed.
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Furthermore, a micro-kinetic model of NO oxidation that takes into account the reversible oxidation of the platinum surface is set up. The purpose of the modeling work is to show that the observed transient phenomena can be quantitatively explained by the assumption of a reversible surface oxidation. 2. Methodology 2.1. Experimental 2.1.1. Experimental setup The apparatus used in this work consisted of a gas mixing unit, a gas pre-heater with a reactor and an analyzer module. Mass flow controllers were used to control and monitor the flow rate of each gas species. For experiments with water steam was dosed to the feed using an evaporator system. Before passing through the monolith the gas mixture was pre-heated in a tubular furnace. To ensure a uniform temperature profile and to enhance mixing of the gases static mixers were installed in the oven. A monolithic Diesel oxidation model catalyst (Umicore AG & Co. KG, Pt/Al2O3, length: 7.6 cm, diameter: 2.5 cm, 400 cpsi) was used for all experiments. The monolith was wrapped with insulation and placed in a stainless steel tubular reactor taking care to ensure no gas bypass of the monolith occurred. Temperature profiles in the monolith were monitored using four K-type thermocouples placed inside blocked monolith channels at different positions. The impact on the flow field is assumed to be negligible since only 1% of the channels are blocked. For reference purposes the temperature at the catalyst entrance is used. Additionally, the inlet gas temperature was measured 0.3 cm in front of the monolith. A pressure sensor monitored the pressure in front of the monolith which ranged typically between 1.08 and 1.10 bar absolute. NO and NOx at the reactor outlet were analyzed using a chemiluminescence detector. CO and CO2 were measured using NDIR spectroscopy. A paramagnetic method was utilized for O2 monitoring. For further information see ref. [30]. In general, experiments were performed using a total flow rate of 965 L h1 resulting in a gas hourly space velocity (GHSV) of 25 000 h1. N2 was used as balance. 2.1.2. Pretreatment Before each experiment the catalyst was treated with 3 vol.% H2 in N2 for 1 h at 370 8C. After 1 h the hydrogen was turned off and the catalyst was cooled down under N2 to the next desired experimental temperature. 2.1.3. Light-off/light-down-experiments During the light-off/light-down-experiments the temperature was increased from 80 to 370 8C with a heating rate (b) of 5 8C min1. After reaching 370 8C it was cooled to 80 8C with an applied rate of 5 8C min1. This sequence of heating and cooling will further be referenced as run 1. This cycle of heating and cooling was repeated without further pretreatment (run 2). During the heating and cooling phases the feed gas composition was held constant. Additionally, experiments with a more complex temperature variation protocol have been performed. This profile could be described as follows: heating from 80 to 270 8C, cooling from 270 to 120 8C, heating to 370 8C, cooling from 370 to 120 8C, heating from 120 to 270 8C, cooling from 270 to 120 8C (b = 5 8C min1). This temperature cycle was repeated without a pretreatment. 2.1.4. Temperature-step-experiments A temperature-step-experiment in which the temperature was increased from 80 to 370 8C before decreasing to 80 8C was performed under N2. At selected temperatures the heating/cooling was paused. After reaching a constant temperature the feed gas was switched from N2 to the standard feed composition (450 vol.-
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ppm NO, 6 vol.% O2) for 30 min. This measurement was repeated afterwards without a pretreatment between the two runs. 2.2. Simulation 2.2.1. Simulation program The simulation tool applied in this work is 1D. Instead of simulating the whole catalyst one representative channel is modeled. All relevant physical processes (convection in the gas phase; heat and mass transfer between gas phase and washcoat; heat transfer between washcoat and substrate) are incorporated into the model. Chemical reactions can occur between gaseous and adsorbed species. Reaction rates are calculated using Eq. (1) (rj: reaction rate (mol m3 s1); kj: rate constant (s1); cG,i: concentration of gas species (mol m3); uS,i: surface coverage (unitless); cPt: site concentration (mol m3); yi,j (unitless)). Y yi; j Y yi; j cG;i uS;i cPt (1) rj ¼ kj i
i
The rate constants for adsorption reactions are calculated using Eq. (2) (R: universal gas-constant (J mol1 K1); Mi: molecular mass (kg mol1); G: site density (mol m2); s0,i: sticking coefficient (unitless)), whereas the rate constants for all other surface reactions are calculated via Eq. (3) (Aj: pre-exponential factor (s1); Eaj: activation energy (J mol1)): k j;Ads ¼
G
RT pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi s0;i 2 p Mi R T
Ea j : k j ¼ A j exp RT
(2)
(3)
The program solves the conservation equations of mass and energy for gas and solid phase. So the physicochemical model allows a fully transient computation of surface coverage, storage and reaction phenomena, by solving the system of nonlinear partial differential equations. 2.2.2. Optimization strategy To fit the kinetic parameters the response surface method [31– 33] using orthogonal central-composite designs [32,33] and second-order polynomials [32,33] was applied. During the optimization process scaled parameters were utilized. Instead of fitting all parameters only those already optimized in [20] and those added in this work were estimated. 3. Results and discussion 3.1. Inverse hysteresis during oxidation of NO Fig. 1 shows the NO conversion as a function of temperature in an experiment where the inlet temperature was ramped from 80 8C up to 370 8C and back to 80 8C. Obviously, the conversion of NO to NO2 is higher during the positive ramp than during the negative ramp. If the cycle is repeated without a new pretreatment, the conversion during the second ramp closely follows the conversion of the first ramp. Similarly the second negative ramp reproduces the result of the first negative ramp. Moreover, if the cycle is repeated a third time again identical results are obtained (not shown). Many oxidation reactions are known to show a hysteresis behavior with a higher conversion during cool down than during heat-up. A prominent example of this phenomenon is CO oxidation on platinum [34,35]. It is important to note that this well-known hysteresis effect is the opposite of what is observed in this study of NO oxidation, i.e. herein a lower conversion is found during the
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of NO oxidation. For this reason none of these effects can explain the observed inverse hysteresis for NO oxidation. One potential explanation of the inverse hysteresis behavior is a reversible oxidation of the platinum particles. It is thus proposed that at high temperatures platinum is oxidized by NO2 or O2 to form an oxide that is less catalytically active than the metallic platinum particle. At lower temperatures the particle would be reduced back to its original state, most likely by NO. The experiment of Fig. 1 has been repeated for a range of different NO inlet concentrations giving the results summarized in Table 1. In all cases a similar inverse hysteresis is observed. A closer inspection of Fig. 1 reveals that during the second heatup phase the performance of the first heat-up is not fully recovered. Although this effect is small, it has been consistently observed in a large number of experiments. The difference in the light-off-temperature between the first and the second heat-up decreases with increasing NO in the feed (see Table 1). The decreased activity during the second heat-up is attributed to a decreased extent of reduction by NO/O2 vs. that achieved in the reduction pretreatment. Higher NO concentrations will reduce the platinum surface more efficiently than low NO concentrations and therefore show a smaller performance difference between first and second heat-up. Despre´s [8] performed an experiment where the catalyst was pretreated by NO2 and then heated up and cooled down. In this experiment it was found that the catalyst shows a regular hysteresis with a higher activity during the cool down phase. Despite the fact that this is the opposite hysteresis behavior than observed in this paper, their observation is not inconsistent with our results. NO2 alone has a much higher oxidation potential than the NO/NO2 mixtures that occur during the NO oxidation experiments. It can therefore be assumed that after the pretreatment with NO2 the platinum surface is highly oxidized and that the catalyst surface is partially reduced during the NO oxidation experiments. This explains the lower activity during the initial heat-up. Fig. 2 shows results of an experiment with a more complex temperature profile. In this case after the first heat-up to 270 8C the catalyst is only cooled down to an intermediate temperature of 120 8C and then heated up again (to 370 8C). During the second heat-up phase a significantly lower NO conversion is observed compared to the first heat-up. However, the activity during the second heat-up phase is still higher than during the cool down phases. This result is consistent with the explanation of the inverse hysteresis by a reversible catalyst oxidation. It seems that cooling down the catalyst to 120 8C does not reduce the catalyst to the same extent as cooling down to 80 8C. As a consequence after the partial cool down the catalyst only partially recovers its original activity.
Fig. 1. NO conversion for the first and second run of a light-off/light-downexperiment with 450 vol.-ppm NO and 6 vol.% O2 (N2 as balance, b = 5 8C min1).
negative ramp. We therefore use the term ‘inverse hysteresis’ to describe this effect. There are several physical and chemical effects that can cause a regular hysteresis (extinction temperature < ignition temperature): Thermal inertia of the catalyst. During heat-up phase the effective average catalyst temperature will lag behind the inlet temperature. Thus comparably during a cool down phase the inlet temperature will be below the average catalyst temperature. If the conversion is plotted vs. the inlet temperature, one observes a hysteresis with an increased conversion during the cool down phase. Multiple steady states due to the reaction exotherm. In the case of an exothermic reaction the heat of reaction will sustain the reaction even at inlet temperatures below the ignition temperature. This effect also leads to a hysteresis with an increased conversion during the extinction phase. Multiple steady states due to surface inhibition. This effect is, for example, observed for CO oxidation. During the ignition phase surface coverage of the catalyst is dominated by CO. However, once light-off is achieved and all COads is converted, the surface becomes oxygen covered. In this oxygen covered state the catalyst remains active at temperatures below the ignition temperature. Each, or a combination of all, of these effects can cause a decrease of the extinction temperature compared to the ignition temperature, which is the opposite of what is observed in the case
Table 1 Experimentally obtained NO-light-off and light-down-temperatures for different experimental conditions. All experiments were performed using 6 vol.% O2. N2 was used as balance. NO (vol.-ppm)
450 150 300 300 450 450 450 450 450
H2O (vol.%)
0 0 0 0 0 6 0 0 6
CO2 (vol.%)
0 0 0 0 0 0 6 0 6
CO (vol.-ppm)
0 0 0 0 0 0 0 1000 1000
b (8C min1)
5 5 5 5 10 5 5 5 5
GHSV (h1)
25 000 25 000 25 000 50 000 25 000 25 000 25 000 25 000 25 000
Light-off-temperature (8C)
Light-down-temperature (8C)
Run 1
Run 2
Run 1
Run 2
138 113 131 141 137 137 138 149 143
143 127 140 151 144 151 143 153 147
199 176 189 208 200 219 202 214 236
202 179 192 211 203 219 206 208 229
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Table 2 Surface reaction mechanism for the NO oxidation on Pt. Number Reaction
Fig. 2. NO conversion for a light-off/light-down-experiment with 450 vol.-ppm NO and 6 vol.% O2 (N2 as balance, b = 5 8C min1). The temperature program was 80 ! 270 ! 120 ! 370 ! 120 ! 270 ! 80 8C.
To explain the inverse hysteresis behavior it is not sufficient to assume that the catalyst can be reversibly oxidized by NO2 and reduced by NO. It is also necessary that the deactivation process occurs at a time scale comparable to the timescale of the temperature changes. To further illustrate this effect an experiment was performed where the catalyst is heated up and cooled down in steps of 25 8C with 30 min of steady state operation at each temperature. Fig. 3 shows the NO conversion at the end of the steady state phases. Compared to the transient temperature cycle, the temperaturestep-experiment shows a strongly decreased inverse hysteresis. The steady state conversion is always between the conversion of the transient heat-up and the transient cool down. This result is consistent with the proposal of inverse hysteresis by a reversible oxidation of platinum. If the catalyst is operated at steady state for extended periods then its oxidation state will approach the steady state value. This steady state value will be the same for the heat-up and the cool down phase. However, during a transient heat-up the catalyst oxidation will not be fast enough to bring the platinum oxidation state to steady state so that the catalyst shows a comparably higher activity. In a similar way during the cool down phase the reduction of the platinum surface will lag behind its steady state value resulting in a decreased activity, compared to steady state. The fact that a small inverse hysteresis is observed
(R1)
O2 þ 2Pt ! 2Pt-O
5.42 102
0
(R2)
2Pt-O ! O2 þ 2Pt
8.41 1012
207 134uO
(R3)
NO þ Pt ! Pt-NO
8.80 101
0
(R4)
Pt-NO ! NO þ Pt
1.00 1016
138 29uO 33uNO
(R5)
NO2 þ Pt ! Pt-NO2
9.70 101
0
(R6)
Pt-NO2 ! NO2 þ Pt
1.00 1016
119 45uO
(R7)
Pt-NO þ Pt-O ! Pt-NO2 þ Pt 1.00 1011
124 40uO 26uNO
(R8)
Pt-NO2 þ Pt ! Pt-NO þ Pt-O 9.65 1011
58 + 14uO + 7uNO
(R9)
Pt-NO2 ! Pt Ox þ NO
3.80 103
32
(R10)
Pt Ox þ NO ! Pt-NO2
2.70 102*
0
# *
Values below 1 indicate sticking coefficient (so); otherwise pre-exponentials. The pre-exponential has the dimension m3 mol1 s1.
even for the steady state experiment might be attributed to the fact that 30 min is not long enough to fully reach the steady state activity at each temperature. 3.2. Implementation of the reversible surface oxidation in a kinetic model In order to demonstrate that the observed inverse hysteresis behavior can be quantitatively explained by the assumption of a reversible surface oxidation, this effect has been incorporated into a micro-kinetic model of NO oxidation. For this purpose the microkinetic model of Hauptmann et al. [20] has been extended by the two reactions (R9 and R10): Pt-NO2 ! Pt Ox þ NO Pt Ox þ NO ! Pt þ NO2
Fig. 3. NO conversion for the first run of a light-off/light-down-experiment (black) and the first run of a temperature-step-experiment (symbols).
s0 (unitless)# or Ea/(kJ mol1) A/s1
(R9) (R10)
The four unknown parameters for (R9) and (R10) as well as the three parameters already modified in [20] have been fitted to the results of Fig. 1 (see Section 2.2 for details). The parameters obtained by the fit are reported in Table 2. Fig. 4a compares the output of the parameterized model with the experimental data. The inverse hysteresis behavior is well reproduced by the kinetic model. Fig. 4b shows the development of the platinum oxide coverage as computed by the kinetic model. At higher temperatures the platinum surface is oxidized by NO2 while at low temperature the surface is reduced by NO. During transient temperature changes the surface oxide coverage lags behind its steady state value. This causes the inverse hysteresis. After the first temperature cycle the initially completely reduced state of the platinum surface is not
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W. Hauptmann et al. / Applied Catalysis B: Environmental 93 (2009) 22–29
Fig. 5. Comparison between experimental (solid lines) and simulated NO conversion (dashed lines) for the first and second run of a light-off/light-downexperiment with 300 vol.-ppm NO and 6 vol.% O2 (N2 as balance, b = 5 8C min1). This experiment was not considered during the model development.
Fig. 4. (a) Comparison between experimental (solid lines) and simulated NO conversion (dashed lines) for the first and second run of a light-off/light-downexperiment with 450 vol.-ppm NO and 6 vol.% O2 (N2 as balance, b = 5 8C min1). (b) Simulated platinum oxide fraction for the first and second run of a light-off/lightdown-experiment with 450 vol.-ppm NO and 6 vol.% O2 (N2 as balance, b = 5 8C min1).
completely recovered, leading to a small activity decrease between the first and all subsequent heat-up ramps. In a first validation step the parameterized model has been successfully used to predict the NO conversion of an experiment with a different NO inlet concentration (Fig. 5). Fig. 6a and b shows the predicted NO conversion responses for the more complex inlet temperature profile of Fig. 2. The fact that an interruption of the cool down phase leads to an incomplete regeneration of the platinum surface is reproduced by the kinetic model. At this point it should be emphasized that the purpose of the presented simulations is to demonstrate that the inverse hysteresis can be explained assuming a reversible surface oxidation by NO2. It is clear that, despite the excellent agreement between experiment and simulation, this cannot prove the correctness of the applied reaction mechanism. 3.3. Influence of CO2 and H2O on NO oxidation The temperature ramp experiments of Fig. 1 were next reproduced, but with the addition of CO2 or H2O in the feed, giving the results summarized in Table 1. These data indicate that the addition of CO2 has a negligible influence on the NO conversion in accordance to literature [23,24]. Similarly, as may be seen in
Fig. 6. Comparison between experimental (solid lines) and simulated NO conversion (dashed lines) for a light-off/light-down-experiment with 450 vol.ppm NO and 6 vol.% O2 (N2 as balance, b = 5 8C min1). The temperature program was 80 ! 270 ! 120 ! 370 ! 120 ! 270 ! 80 8C. This experiment was not considered during the model development. (a) NO conversion vs. temperature; (b) NO conversion vs. experimental duration.
Fig. 7, the presence of 6 vol.% water has a negligible influence on conversion during the first heat-up. Thus, again an inverse hysteresis is observed. Indeed, during the cool down phase a yet lower activity is observed for the experiments with water than
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Fig. 7. NO conversion for the first and second run of a light-off/light-downexperiment with 450 vol.-ppm NO, 6 vol.% O2 and 6 vol.% H2O (N2 as balance, b = 5 8C min1).
without water. Moreover if water is added to the feed, the light-offtemperature (50% conversion) of the second and all following heatup ramps is increased by 14 8C compared to the first heat-up. This effect is much more pronounced than in the experiment without water (5 8C). The results can be qualitatively rationalized by assuming that the presence of water has an inhibiting effect on the NO induced regeneration at low temperatures. 3.4. Simultaneous oxidation of CO and NO Fig. 8a shows the NO conversion during heat-up and cool down with an inlet mixture that contains a mixture of CO, NO and O2. The same inverse hysteresis is observed that has been found previously for oxidation of NO alone. Compared to the same experiment without CO (Fig. 1) the onset of reaction is shifted to higher temperatures by about 11 8C. The shift in light-offtemperature can be explained by the well-known effect of active site blocking due to preferential CO adsorption on the platinum surface thereby inhibiting the oxidation of NO. Fig. 8b shows the CO conversion in the same experiment. Surprisingly, an inverse hysteresis is also observed for the oxidation of CO. This is an unexpected result as it is known that oxidation of CO alone shows a standard hysteresis, with the conversion in the cool down phase exceeding the conversion in the heat-up phase. Such a standard hysteresis has been shown in [35] for a similar catalyst and similar reaction conditions. The observation of standard hysteresis for the oxidation of CO without NO has been confirmed in this study (not shown). Therefore we attribute this unexpected result to the presence of NO resulting in an inversion in hysteresis behavior of the CO oxidation. A potential explanation is again that NO2 oxidizes the platinum which results in a reduced activity during the cool down phase. At lower temperatures the surface will be reduced by NO which restores the initial activity. 3.5. Why is NO/O2 more effective for platinum oxidation than O2? The fact that hysteresis behavior of the CO oxidation switches from a standard hysteresis in the absence of NO to an inverse hysteresis in the presence of NO further highlights the important role of NO/NO2 for the reversible deactivation of the platinum catalyst. Obviously, NO2 is more effective in oxidizing the catalyst surface than O2. On the first sight this is not surprising since it has been known that NO2 can oxidize a platinum surface [36–38] and also the deactivation resulting from this oxidation has been shown
Fig. 8. (a) NO conversion for the first and second run of a light-off/light-downexperiment with 450 vol.-ppm NO, 6 vol.% O2 and 1000 vol.-ppm CO (N2 as balance, b = 5 8C min1). (b) CO conversion for the first and second run of a light-off/lightdown-experiment with 450 vol.-ppm NO, 6 vol.% O2 and 1000 vol.-ppm CO (N2 as balance, b = 5 8C min1).
before [1]. However, in all these studies NO2 was fed to the reactor and used as oxidative agent. Pure NO2 has a much higher thermodynamic oxidation potential than O2. In our experiments all NO2 is formed by oxidation of NO. Therefore the thermodynamic oxidation potential of NO2 in the resulting NO/NO2/O2 gas mixture can never exceed the oxidation potential of O2. The higher oxidation capacity of NO2 relative to O2 might be explained by the fact that the formation of surface oxides from oxygen is kinetically more limited than the corresponding reaction with NO2. From a mechanistic point of view such a difference in the surface oxidation kinetics might be linked to the oxygen surface coverage in the different environments. Getman et al. [39] recently showed experimentally and by DFT calculations that the dissociative adsorption of O2 on a platinum surface is kinetically hindered if the oxygen surface coverage exceeds a certain value. This means that O2 uptake will result in a maximum oxygen coverage significantly below the value allowed by thermodynamics. NO2 does not show such a kinetic limitation so that reaction with NO2/NO mixtures leads to a higher oxygen coverage than a treatment with O2 alone, even if the NO/NO2 mixture has the same thermodynamic oxidation potential than O2 [39]. Due to repulsive interactions between adsorbed oxygen atoms the chemical potential of adsorbed oxygen strongly increases with increasing surface coverage [40,41]. For this reason it seems a reasonable assumption that the higher oxygen surface concentrations under NO2/NO treatment result in a more effective formation of platinum oxides.
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W. Hauptmann et al. / Applied Catalysis B: Environmental 93 (2009) 22–29
4. Conclusion 4.1. Contribution of the paper This paper shows that a platinum catalyst operated with a transient temperature cycle exhibits a hysteresis behavior. The catalytic activity of the heat-up phases exceeds the activity of the cool down phases. This effect is the opposite of what is observed for many oxidation reactions and therefore in this paper is referred to as ‘inverse hysteresis’. If the catalyst is operated at one temperature for a longer time, a steady state activity is observed that is in-between the higher activity of the transient heat-up and the lower activity of the transient cool down. The most likely explanation for the observed hysteresis behavior is a reversible oxidation of the platinum surface by NO2 and a reduction of the platinum surface by NO. This effect has been implemented in a surface reaction mechanism for NO oxidation. The observed hysteresis effects can well be explained by this extended reaction mechanism. If the feed contains CO/NO/O2 an inverse hysteresis is observed for NO and CO oxidation. This means that in the presence of NO the hysteresis behavior of the CO oxidation switches from the typical standard hysteresis to an inverse hysteresis. Also this effect can qualitatively be explained by a reversible oxidation of the platinum surface. A reversible deactivation of platinum catalysts has been observed before. However, the previously published studies involved either a treatment in a strongly reducing atmosphere such as oxygen-free H2, NH3 or NO, or an oxidation by NO2, which has a much higher thermodynamic oxidation potential than oxygen. In our experiments all NO2 is formed by oxidation of NO. The thermodynamic oxidation potential of the NO/NO2/O2 mixtures will therefore never exceed the oxidation potential of oxygen alone at the same oxygen concentration. The surprising result of this paper is that reversible deactivation phenomena seem to play a role even in the absence of strongly oxidizing or reducing conditions. This implies that such deactivation effects are relevant for a much wider range of applications than suggested by the previous work. 4.2. Consequences for exhaust aftertreatment 4.2.1. Exhaust system modeling Current computer simulation approaches for catalytic converters are based on the concept of chemical rate equations with reaction rates that are defined by temperature and the exhaust composition. The results presented in this paper show, that – even for constant inlet concentrations – transient variations of the exhaust temperature cause reversible changes in catalyst activity. This means that reaction rates do not only depend on temperature and gas composition but also on catalyst history, an effect not accounted for in current kinetic models. Furthermore, existing kinetic data frequently have been collected under conditions where catalyst activity changes during the test procedure. For example, in the development of the existing kinetic equations for NO oxidation [16,17], most studies have fitted equations to temperature ramp data similar to the data presented herein. However, such data may not represent the true kinetics but will implicitly contain effects due to reversible changes of the catalysts activity. In Section 3.2 it was shown that the reversible activity changes due to NO/NO2 in the feed could in principal be included in a kinetic model, at least for the very simple system NO/O2. In more complex systems several gas species will have an impact on the catalysts oxidation state (see Sections 3.3 and 3.4), e.g. hydrocarbons, CO and H2, etc. will have some reducing
effect on platinum surface. Experimental identification of all these effects is an ambitious task. It is therefore important to be aware of the fundamental limitations of current kinetic modeling approaches. 4.2.2. Catalyst development Much effort has been expended in the development of simple laboratory test procedures that allow an efficient screening of new catalyst formulations for their performance under vehicle driving conditions. Until today, these efforts have only been partially successful. In consequence, the development of industrial automotive exhaust catalysts still heavily relies on expensive engine test procedures that closely mimic the operation during real world vehicle operation. Reversible activity changes during transient catalyst operation are one of the effects not captured by oversimplified laboratory tests. For this reason a better understanding of the transient deactivation phenomena might help to develop more meaningful laboratory screening procedures. 4.2.3. Engine control algorithms Finally, the NO/NO2 ratio is an important parameter for many algorithms implemented in the engine control system. Examples include urea dosing for the SCR catalyst and the estimation of the soot loading of a particulate filter. For this reason engine control systems contain look-up tables that compute the predicted catalytic conversion of NO to NO2 as a function of the catalyst operating parameters. These look-up tables contain steady state conversion values and do not take into account the inverse hysteresis behavior. It is therefore important to be aware of the natural limitations of such tabulation approaches. For control strategies that require a very precise estimation of the NO/NO2 ratio it might become necessary to include reversible deactivation phenomena into the estimation algorithms. References [1] L. Olsson, E. Fridell, J. Catal. 210 (2002) 340. [2] L. Olsson, M. Abul-Milh, H. Karlsson, E. Jabson, P. Thormahlen, A. Hinz, Top. Catal. 30–31 (2004) 85. [3] S.S. Mulla, N. Chen, L. Cumaranatunge, G.E. Blau, D.Y. Zemlyanov, W.N. Delgass, W.S. Epling, F.H. Ribeiro, J. Catal. 241 (2006) 389. [4] P. Bourges, S. Lunati, G. Mabilon, Catalysis and Automotive Pollution Control IV, Studies in Surface Science and Catalysis, vol. 116, 1998, pp. 213–222. [5] B.J. Cooper, J.E. Thoss, SAE Technical Paper 890404, 1989. [6] J. Dawody, M. Skoglundh, E. Fridell, J. Mol. Catal. A: Chem. 209 (2004) 215. [7] J. Dawody, M. Skoglundh, L. Olsson, E. Fridell, J. Catal. 234 (2005) 206. [8] J. Despre´s, Ph.D. Thesis, Diss. ETH No. 15006, ETH Zu¨rich, 2003. [9] J. Despre´s, M. Elsener, M. Koebel, O. Kro¨cher, B. Schnyder, A. Wokaun, Appl. Catal. B 50 (2004) 73. [10] I.V. Yentekakis, V. Tellou, G. Botzolaki, I.A. Rapakousios, Appl. Catal. B 56 (2005) 229. [11] E. Xue, K. Seshan, J.R.H. Ross, Appl. Catal. B 11 (1996) 65. [12] G. Corro, M.P. Elizalde, A. Velasco, React. Kinet. Catal. Lett. 76 (2002) 117. [13] D. Bhatia, R.W. McCabe, M.P. Harold, V. Balakotaiah, J. Catal. 266 (2009) 106. [14] W. Hauptmann, A. Drochner, H. Vogel, M. Votsmeier, J. Gieshoff, Top. Catal. 42–43 (2007) 157. [15] S.S. Mulla, N. Chen, W.N. Delgass, W.S. Epling, F.H. Ribeiro, Catal. Lett. 100 (2005) 267. [16] R. Marques, P. Darcy, P. Da Costa, H. Mellottee, J.-M. Trichard, G. Djega-Mariadassou, J. Mol. Catal. A: Chem. 221 (2004) 127. [17] L. Olsson, R.J. Blint, E. Fridell, Ind. Eng. Chem. Res. 44 (2005) 3021. [18] B.R. Kromer, L. Cao, L. Cumaranatunge, S.S. Mulla, J.L. Ratts, A. Yezerets, N.W. Currier, F.H. Ribeiro, W.N. Delgass, J.M. Caruthers, Catal. Today 136 (2008) 93. [19] A.D. Smeltz, R.B. Getman, W.F. Schneider, F.H. Ribeiro, Catal. Today 136 (2008) 84. [20] W. Hauptmann, M. Votsmeier, J. Gieshoff, D.G. Vlachos, A. Drochner, H. Vogel, Top. Catal. (2009), doi:10.1007/s11244-009-9369-z. [21] L. Olsson, B. Westerberg, H. Persson, E. Fridell, M. Skoglundh, B. Andersson, J. Phys. Chem. B 103 (1999) 10433. [22] L. Olsson, H. Persson, E. Fridell, M. Skoglundh, B. Andersson, J. Phys. Chem. B 105 (2001) 6895. [23] M. Crocoll, Ph.D. Thesis, Universita¨t Karlsruhe, 2003. [24] M. Crocoll, S. Kureti, W. Weisweiler, J. Catal. 229 (2005) 480. [25] J. Koop, Ph.D. Thesis, Universita¨t Karlsruhe, 2008. [26] D. Chatterjee, Ph.D. Thesis, Rubrecht-Karls-Universita¨t Heidelberg, 2001. [27] H. Niehus, G. Comsa, Surf. Sci. 93 (1980) L147. [28] A. von Oertzen, A. Mikhailov, H.H. Rotermund, G. Ertl, Surf. Sci. 350 (1996) 259.
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[35] S. Salomons, R. Hayes, M. Votsmeier, A. Drochner, H. Vogel, S. Malmberg, J. Gieshoff, Appl. Catal. B 70 (2007) 305. [36] D.H. Parker, M.E. Bartram, B.E. Koel, Surf. Sci. 217 (1989) 489. [37] D.H. Parker, B.E. Koel, J. Vac. Sci. Technol. A 8 (1990) 2585. [38] J. Segner, W. Vielhaber, G. Ertl, Isr. J. Chem. 22 (1982) 375. [39] R.B. Getman, W.F. Schneider, A.D. Smeltz, W.N. Delgass, F.H. Ribeiro, Phys. Rev. Lett. 102 (2009), 076101/1. [40] C.T. Campbell, G. Ertl, H. Kuipers, J. Segner, Surf. Sci. 107 (1981) 220. [41] J.F. Weaver, J.-J. Chen, A.L. Gerrard, Surf. Sci. 592 (2005) 83.
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Applied Catalysis B: Environmental journal homepage: www.elsevier.com/locate/apcatb
Inverse hysteresis during the NO oxidation on Pt under lean conditions W. Hauptmann a,b, M. Votsmeier a,*, J. Gieshoff a, A. Drochner b, H. Vogel b a b
Umicore AG & Co. KG, D-63403 Hanau-Wolfgang, Germany Ernst-Berl-Institut fu¨r Technische und Makromolekulare Chemie, Technische Universita¨t Darmstadt, D-64287 Darmstadt, Germany
A R T I C L E I N F O
A B S T R A C T
Article history: Received 30 June 2009 Received in revised form 28 August 2009 Accepted 7 September 2009 Available online 16 September 2009
It is shown that a platinum catalyst operated under a transient temperature profile exhibits a hysteresis behavior. The catalytic activity during heat-up phases exceeds the activity of the cool down phases. This effect is the opposite of the well-known hysteresis that can be caused by the existence of multiple steady states or the thermal inertia of the catalyst. It is therefore referred to as ‘inverse hysteresis’. If the catalyst is operated at one temperature for a longer time, a steady state activity is observed that is in-between the higher activity of the transient heat-up and the lower activity of the transient cool down. The most likely explanation for the observed hysteresis behavior is a reversible oxidation of the platinum surface by NO2 and a reduction of the platinum surface by NO. A micro-kinetic surface mechanism including the reversible surface oxidation has been developed and implemented in a monolith reactor model. The observed hysteresis effects can be quantitatively reproduced by the numerical model. If the feed contains CO/NO/O2 an inverse hysteresis is observed for NO and CO oxidation. This means that in the presence of NO the hysteresis behavior of the CO oxidation switches from the typical standard hysteresis to an inverse hysteresis. ß 2009 Elsevier B.V. All rights reserved.
Keywords: NO oxidation Deactivation Diesel exhaust Platinum Monolithic catalyst Inverse hysteresis Micro-kinetic model
1. Introduction The oxidation of NO to NO2 on platinum plays an important role in a number of Diesel exhaust aftertreatment systems. One example is the Diesel particulate filter (DPF), which requires NO2 for passive regeneration. Another example is the selective catalytic reduction (SCR) by NH3 which is much more efficient if the feed contains a mixture of NO/NO2. A third example is the regenerable NOx storage catalyst where under many conditions the oxidation of NO is an essential first step of the storage mechanism. Motivated by the importance of this reaction in exhaust catalysis the kinetics of NO oxidation on platinum has been studied by a number of researchers [1–13]. Global rate equations have been fitted to experimental data in [14–19] and micro-kinetic models of the reaction have also been published [20–26]. One important feature of the reaction is that the reaction rate per surface site seems to be dependent on the size of the platinum particles, even to the extent that the activity of a catalyst increases during thermal aging due to a growth in particle size [1]. It has been observed in a number of studies that the activity of a platinum catalyst for NO oxidation is decreased by oxidative pretreatments (with NO2) and that the catalyst can be reactivated
* Corresponding author. Fax: +49 6181 59 4693. E-mail address: [email protected] (M. Votsmeier). 0926-3373/$ – see front matter ß 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.apcatb.2009.09.008
by a reductive or thermal treatment. For example, Despre´s et al. [8,9] have shown that pretreatment at 250 8C with 500 ppm NO2 causes a strong decrease in activity. The initial activity can be restored by a treatment with NH3 or NO in nitrogen at 250 8C or by thermal regeneration at 650 8C in air. Olsson and Fridell [1] have shown that when a catalyst previously oxidized by NO2 is operated with a feed stream containing 630 ppm NO and 8% O2, the activity increases over time. Thus it is apparent that a catalyst that has been deactivated by NO2 can be regenerated by NO/O2, despite the large O2 excess in this mixture. The commonly accepted explanation for the reversible deactivation by oxidative treatments is an oxide formation on the platinum surface. The formation of such surface oxides has been demonstrated by a number of experimental techniques, see [1] for a literature summary. The nature of the platinum oxides does not seem to be fully understood. Possible candidates are so-called subsurface oxygen [27,28], the formation of a platinum oxide layer [29] or even the bulk oxidation of the platinum particles [3]. It is one characteristic of exhaust aftertreatment that the catalysts are operated under transient temperature conditions, mainly due to changes in engine load. In this paper we study the NO oxidation process during different temperature cycles that consist of upwards and downwards ramps. The main objective of this paper is to demonstrate that even with a constant exhaust composition reversible deactivation phenomena can be observed.
W. Hauptmann et al. / Applied Catalysis B: Environmental 93 (2009) 22–29
Furthermore, a micro-kinetic model of NO oxidation that takes into account the reversible oxidation of the platinum surface is set up. The purpose of the modeling work is to show that the observed transient phenomena can be quantitatively explained by the assumption of a reversible surface oxidation. 2. Methodology 2.1. Experimental 2.1.1. Experimental setup The apparatus used in this work consisted of a gas mixing unit, a gas pre-heater with a reactor and an analyzer module. Mass flow controllers were used to control and monitor the flow rate of each gas species. For experiments with water steam was dosed to the feed using an evaporator system. Before passing through the monolith the gas mixture was pre-heated in a tubular furnace. To ensure a uniform temperature profile and to enhance mixing of the gases static mixers were installed in the oven. A monolithic Diesel oxidation model catalyst (Umicore AG & Co. KG, Pt/Al2O3, length: 7.6 cm, diameter: 2.5 cm, 400 cpsi) was used for all experiments. The monolith was wrapped with insulation and placed in a stainless steel tubular reactor taking care to ensure no gas bypass of the monolith occurred. Temperature profiles in the monolith were monitored using four K-type thermocouples placed inside blocked monolith channels at different positions. The impact on the flow field is assumed to be negligible since only 1% of the channels are blocked. For reference purposes the temperature at the catalyst entrance is used. Additionally, the inlet gas temperature was measured 0.3 cm in front of the monolith. A pressure sensor monitored the pressure in front of the monolith which ranged typically between 1.08 and 1.10 bar absolute. NO and NOx at the reactor outlet were analyzed using a chemiluminescence detector. CO and CO2 were measured using NDIR spectroscopy. A paramagnetic method was utilized for O2 monitoring. For further information see ref. [30]. In general, experiments were performed using a total flow rate of 965 L h1 resulting in a gas hourly space velocity (GHSV) of 25 000 h1. N2 was used as balance. 2.1.2. Pretreatment Before each experiment the catalyst was treated with 3 vol.% H2 in N2 for 1 h at 370 8C. After 1 h the hydrogen was turned off and the catalyst was cooled down under N2 to the next desired experimental temperature. 2.1.3. Light-off/light-down-experiments During the light-off/light-down-experiments the temperature was increased from 80 to 370 8C with a heating rate (b) of 5 8C min1. After reaching 370 8C it was cooled to 80 8C with an applied rate of 5 8C min1. This sequence of heating and cooling will further be referenced as run 1. This cycle of heating and cooling was repeated without further pretreatment (run 2). During the heating and cooling phases the feed gas composition was held constant. Additionally, experiments with a more complex temperature variation protocol have been performed. This profile could be described as follows: heating from 80 to 270 8C, cooling from 270 to 120 8C, heating to 370 8C, cooling from 370 to 120 8C, heating from 120 to 270 8C, cooling from 270 to 120 8C (b = 5 8C min1). This temperature cycle was repeated without a pretreatment. 2.1.4. Temperature-step-experiments A temperature-step-experiment in which the temperature was increased from 80 to 370 8C before decreasing to 80 8C was performed under N2. At selected temperatures the heating/cooling was paused. After reaching a constant temperature the feed gas was switched from N2 to the standard feed composition (450 vol.-
23
ppm NO, 6 vol.% O2) for 30 min. This measurement was repeated afterwards without a pretreatment between the two runs. 2.2. Simulation 2.2.1. Simulation program The simulation tool applied in this work is 1D. Instead of simulating the whole catalyst one representative channel is modeled. All relevant physical processes (convection in the gas phase; heat and mass transfer between gas phase and washcoat; heat transfer between washcoat and substrate) are incorporated into the model. Chemical reactions can occur between gaseous and adsorbed species. Reaction rates are calculated using Eq. (1) (rj: reaction rate (mol m3 s1); kj: rate constant (s1); cG,i: concentration of gas species (mol m3); uS,i: surface coverage (unitless); cPt: site concentration (mol m3); yi,j (unitless)). Y yi; j Y yi; j cG;i uS;i cPt (1) rj ¼ kj i
i
The rate constants for adsorption reactions are calculated using Eq. (2) (R: universal gas-constant (J mol1 K1); Mi: molecular mass (kg mol1); G: site density (mol m2); s0,i: sticking coefficient (unitless)), whereas the rate constants for all other surface reactions are calculated via Eq. (3) (Aj: pre-exponential factor (s1); Eaj: activation energy (J mol1)): k j;Ads ¼
G
RT pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi s0;i 2 p Mi R T
Ea j : k j ¼ A j exp RT
(2)
(3)
The program solves the conservation equations of mass and energy for gas and solid phase. So the physicochemical model allows a fully transient computation of surface coverage, storage and reaction phenomena, by solving the system of nonlinear partial differential equations. 2.2.2. Optimization strategy To fit the kinetic parameters the response surface method [31– 33] using orthogonal central-composite designs [32,33] and second-order polynomials [32,33] was applied. During the optimization process scaled parameters were utilized. Instead of fitting all parameters only those already optimized in [20] and those added in this work were estimated. 3. Results and discussion 3.1. Inverse hysteresis during oxidation of NO Fig. 1 shows the NO conversion as a function of temperature in an experiment where the inlet temperature was ramped from 80 8C up to 370 8C and back to 80 8C. Obviously, the conversion of NO to NO2 is higher during the positive ramp than during the negative ramp. If the cycle is repeated without a new pretreatment, the conversion during the second ramp closely follows the conversion of the first ramp. Similarly the second negative ramp reproduces the result of the first negative ramp. Moreover, if the cycle is repeated a third time again identical results are obtained (not shown). Many oxidation reactions are known to show a hysteresis behavior with a higher conversion during cool down than during heat-up. A prominent example of this phenomenon is CO oxidation on platinum [34,35]. It is important to note that this well-known hysteresis effect is the opposite of what is observed in this study of NO oxidation, i.e. herein a lower conversion is found during the
W. Hauptmann et al. / Applied Catalysis B: Environmental 93 (2009) 22–29
24
of NO oxidation. For this reason none of these effects can explain the observed inverse hysteresis for NO oxidation. One potential explanation of the inverse hysteresis behavior is a reversible oxidation of the platinum particles. It is thus proposed that at high temperatures platinum is oxidized by NO2 or O2 to form an oxide that is less catalytically active than the metallic platinum particle. At lower temperatures the particle would be reduced back to its original state, most likely by NO. The experiment of Fig. 1 has been repeated for a range of different NO inlet concentrations giving the results summarized in Table 1. In all cases a similar inverse hysteresis is observed. A closer inspection of Fig. 1 reveals that during the second heatup phase the performance of the first heat-up is not fully recovered. Although this effect is small, it has been consistently observed in a large number of experiments. The difference in the light-off-temperature between the first and the second heat-up decreases with increasing NO in the feed (see Table 1). The decreased activity during the second heat-up is attributed to a decreased extent of reduction by NO/O2 vs. that achieved in the reduction pretreatment. Higher NO concentrations will reduce the platinum surface more efficiently than low NO concentrations and therefore show a smaller performance difference between first and second heat-up. Despre´s [8] performed an experiment where the catalyst was pretreated by NO2 and then heated up and cooled down. In this experiment it was found that the catalyst shows a regular hysteresis with a higher activity during the cool down phase. Despite the fact that this is the opposite hysteresis behavior than observed in this paper, their observation is not inconsistent with our results. NO2 alone has a much higher oxidation potential than the NO/NO2 mixtures that occur during the NO oxidation experiments. It can therefore be assumed that after the pretreatment with NO2 the platinum surface is highly oxidized and that the catalyst surface is partially reduced during the NO oxidation experiments. This explains the lower activity during the initial heat-up. Fig. 2 shows results of an experiment with a more complex temperature profile. In this case after the first heat-up to 270 8C the catalyst is only cooled down to an intermediate temperature of 120 8C and then heated up again (to 370 8C). During the second heat-up phase a significantly lower NO conversion is observed compared to the first heat-up. However, the activity during the second heat-up phase is still higher than during the cool down phases. This result is consistent with the explanation of the inverse hysteresis by a reversible catalyst oxidation. It seems that cooling down the catalyst to 120 8C does not reduce the catalyst to the same extent as cooling down to 80 8C. As a consequence after the partial cool down the catalyst only partially recovers its original activity.
Fig. 1. NO conversion for the first and second run of a light-off/light-downexperiment with 450 vol.-ppm NO and 6 vol.% O2 (N2 as balance, b = 5 8C min1).
negative ramp. We therefore use the term ‘inverse hysteresis’ to describe this effect. There are several physical and chemical effects that can cause a regular hysteresis (extinction temperature < ignition temperature): Thermal inertia of the catalyst. During heat-up phase the effective average catalyst temperature will lag behind the inlet temperature. Thus comparably during a cool down phase the inlet temperature will be below the average catalyst temperature. If the conversion is plotted vs. the inlet temperature, one observes a hysteresis with an increased conversion during the cool down phase. Multiple steady states due to the reaction exotherm. In the case of an exothermic reaction the heat of reaction will sustain the reaction even at inlet temperatures below the ignition temperature. This effect also leads to a hysteresis with an increased conversion during the extinction phase. Multiple steady states due to surface inhibition. This effect is, for example, observed for CO oxidation. During the ignition phase surface coverage of the catalyst is dominated by CO. However, once light-off is achieved and all COads is converted, the surface becomes oxygen covered. In this oxygen covered state the catalyst remains active at temperatures below the ignition temperature. Each, or a combination of all, of these effects can cause a decrease of the extinction temperature compared to the ignition temperature, which is the opposite of what is observed in the case
Table 1 Experimentally obtained NO-light-off and light-down-temperatures for different experimental conditions. All experiments were performed using 6 vol.% O2. N2 was used as balance. NO (vol.-ppm)
450 150 300 300 450 450 450 450 450
H2O (vol.%)
0 0 0 0 0 6 0 0 6
CO2 (vol.%)
0 0 0 0 0 0 6 0 6
CO (vol.-ppm)
0 0 0 0 0 0 0 1000 1000
b (8C min1)
5 5 5 5 10 5 5 5 5
GHSV (h1)
25 000 25 000 25 000 50 000 25 000 25 000 25 000 25 000 25 000
Light-off-temperature (8C)
Light-down-temperature (8C)
Run 1
Run 2
Run 1
Run 2
138 113 131 141 137 137 138 149 143
143 127 140 151 144 151 143 153 147
199 176 189 208 200 219 202 214 236
202 179 192 211 203 219 206 208 229
W. Hauptmann et al. / Applied Catalysis B: Environmental 93 (2009) 22–29
25
Table 2 Surface reaction mechanism for the NO oxidation on Pt. Number Reaction
Fig. 2. NO conversion for a light-off/light-down-experiment with 450 vol.-ppm NO and 6 vol.% O2 (N2 as balance, b = 5 8C min1). The temperature program was 80 ! 270 ! 120 ! 370 ! 120 ! 270 ! 80 8C.
To explain the inverse hysteresis behavior it is not sufficient to assume that the catalyst can be reversibly oxidized by NO2 and reduced by NO. It is also necessary that the deactivation process occurs at a time scale comparable to the timescale of the temperature changes. To further illustrate this effect an experiment was performed where the catalyst is heated up and cooled down in steps of 25 8C with 30 min of steady state operation at each temperature. Fig. 3 shows the NO conversion at the end of the steady state phases. Compared to the transient temperature cycle, the temperaturestep-experiment shows a strongly decreased inverse hysteresis. The steady state conversion is always between the conversion of the transient heat-up and the transient cool down. This result is consistent with the proposal of inverse hysteresis by a reversible oxidation of platinum. If the catalyst is operated at steady state for extended periods then its oxidation state will approach the steady state value. This steady state value will be the same for the heat-up and the cool down phase. However, during a transient heat-up the catalyst oxidation will not be fast enough to bring the platinum oxidation state to steady state so that the catalyst shows a comparably higher activity. In a similar way during the cool down phase the reduction of the platinum surface will lag behind its steady state value resulting in a decreased activity, compared to steady state. The fact that a small inverse hysteresis is observed
(R1)
O2 þ 2Pt ! 2Pt-O
5.42 102
0
(R2)
2Pt-O ! O2 þ 2Pt
8.41 1012
207 134uO
(R3)
NO þ Pt ! Pt-NO
8.80 101
0
(R4)
Pt-NO ! NO þ Pt
1.00 1016
138 29uO 33uNO
(R5)
NO2 þ Pt ! Pt-NO2
9.70 101
0
(R6)
Pt-NO2 ! NO2 þ Pt
1.00 1016
119 45uO
(R7)
Pt-NO þ Pt-O ! Pt-NO2 þ Pt 1.00 1011
124 40uO 26uNO
(R8)
Pt-NO2 þ Pt ! Pt-NO þ Pt-O 9.65 1011
58 + 14uO + 7uNO
(R9)
Pt-NO2 ! Pt Ox þ NO
3.80 103
32
(R10)
Pt Ox þ NO ! Pt-NO2
2.70 102*
0
# *
Values below 1 indicate sticking coefficient (so); otherwise pre-exponentials. The pre-exponential has the dimension m3 mol1 s1.
even for the steady state experiment might be attributed to the fact that 30 min is not long enough to fully reach the steady state activity at each temperature. 3.2. Implementation of the reversible surface oxidation in a kinetic model In order to demonstrate that the observed inverse hysteresis behavior can be quantitatively explained by the assumption of a reversible surface oxidation, this effect has been incorporated into a micro-kinetic model of NO oxidation. For this purpose the microkinetic model of Hauptmann et al. [20] has been extended by the two reactions (R9 and R10): Pt-NO2 ! Pt Ox þ NO Pt Ox þ NO ! Pt þ NO2
Fig. 3. NO conversion for the first run of a light-off/light-down-experiment (black) and the first run of a temperature-step-experiment (symbols).
s0 (unitless)# or Ea/(kJ mol1) A/s1
(R9) (R10)
The four unknown parameters for (R9) and (R10) as well as the three parameters already modified in [20] have been fitted to the results of Fig. 1 (see Section 2.2 for details). The parameters obtained by the fit are reported in Table 2. Fig. 4a compares the output of the parameterized model with the experimental data. The inverse hysteresis behavior is well reproduced by the kinetic model. Fig. 4b shows the development of the platinum oxide coverage as computed by the kinetic model. At higher temperatures the platinum surface is oxidized by NO2 while at low temperature the surface is reduced by NO. During transient temperature changes the surface oxide coverage lags behind its steady state value. This causes the inverse hysteresis. After the first temperature cycle the initially completely reduced state of the platinum surface is not
26
W. Hauptmann et al. / Applied Catalysis B: Environmental 93 (2009) 22–29
Fig. 5. Comparison between experimental (solid lines) and simulated NO conversion (dashed lines) for the first and second run of a light-off/light-downexperiment with 300 vol.-ppm NO and 6 vol.% O2 (N2 as balance, b = 5 8C min1). This experiment was not considered during the model development.
Fig. 4. (a) Comparison between experimental (solid lines) and simulated NO conversion (dashed lines) for the first and second run of a light-off/light-downexperiment with 450 vol.-ppm NO and 6 vol.% O2 (N2 as balance, b = 5 8C min1). (b) Simulated platinum oxide fraction for the first and second run of a light-off/lightdown-experiment with 450 vol.-ppm NO and 6 vol.% O2 (N2 as balance, b = 5 8C min1).
completely recovered, leading to a small activity decrease between the first and all subsequent heat-up ramps. In a first validation step the parameterized model has been successfully used to predict the NO conversion of an experiment with a different NO inlet concentration (Fig. 5). Fig. 6a and b shows the predicted NO conversion responses for the more complex inlet temperature profile of Fig. 2. The fact that an interruption of the cool down phase leads to an incomplete regeneration of the platinum surface is reproduced by the kinetic model. At this point it should be emphasized that the purpose of the presented simulations is to demonstrate that the inverse hysteresis can be explained assuming a reversible surface oxidation by NO2. It is clear that, despite the excellent agreement between experiment and simulation, this cannot prove the correctness of the applied reaction mechanism. 3.3. Influence of CO2 and H2O on NO oxidation The temperature ramp experiments of Fig. 1 were next reproduced, but with the addition of CO2 or H2O in the feed, giving the results summarized in Table 1. These data indicate that the addition of CO2 has a negligible influence on the NO conversion in accordance to literature [23,24]. Similarly, as may be seen in
Fig. 6. Comparison between experimental (solid lines) and simulated NO conversion (dashed lines) for a light-off/light-down-experiment with 450 vol.ppm NO and 6 vol.% O2 (N2 as balance, b = 5 8C min1). The temperature program was 80 ! 270 ! 120 ! 370 ! 120 ! 270 ! 80 8C. This experiment was not considered during the model development. (a) NO conversion vs. temperature; (b) NO conversion vs. experimental duration.
Fig. 7, the presence of 6 vol.% water has a negligible influence on conversion during the first heat-up. Thus, again an inverse hysteresis is observed. Indeed, during the cool down phase a yet lower activity is observed for the experiments with water than
W. Hauptmann et al. / Applied Catalysis B: Environmental 93 (2009) 22–29
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Fig. 7. NO conversion for the first and second run of a light-off/light-downexperiment with 450 vol.-ppm NO, 6 vol.% O2 and 6 vol.% H2O (N2 as balance, b = 5 8C min1).
without water. Moreover if water is added to the feed, the light-offtemperature (50% conversion) of the second and all following heatup ramps is increased by 14 8C compared to the first heat-up. This effect is much more pronounced than in the experiment without water (5 8C). The results can be qualitatively rationalized by assuming that the presence of water has an inhibiting effect on the NO induced regeneration at low temperatures. 3.4. Simultaneous oxidation of CO and NO Fig. 8a shows the NO conversion during heat-up and cool down with an inlet mixture that contains a mixture of CO, NO and O2. The same inverse hysteresis is observed that has been found previously for oxidation of NO alone. Compared to the same experiment without CO (Fig. 1) the onset of reaction is shifted to higher temperatures by about 11 8C. The shift in light-offtemperature can be explained by the well-known effect of active site blocking due to preferential CO adsorption on the platinum surface thereby inhibiting the oxidation of NO. Fig. 8b shows the CO conversion in the same experiment. Surprisingly, an inverse hysteresis is also observed for the oxidation of CO. This is an unexpected result as it is known that oxidation of CO alone shows a standard hysteresis, with the conversion in the cool down phase exceeding the conversion in the heat-up phase. Such a standard hysteresis has been shown in [35] for a similar catalyst and similar reaction conditions. The observation of standard hysteresis for the oxidation of CO without NO has been confirmed in this study (not shown). Therefore we attribute this unexpected result to the presence of NO resulting in an inversion in hysteresis behavior of the CO oxidation. A potential explanation is again that NO2 oxidizes the platinum which results in a reduced activity during the cool down phase. At lower temperatures the surface will be reduced by NO which restores the initial activity. 3.5. Why is NO/O2 more effective for platinum oxidation than O2? The fact that hysteresis behavior of the CO oxidation switches from a standard hysteresis in the absence of NO to an inverse hysteresis in the presence of NO further highlights the important role of NO/NO2 for the reversible deactivation of the platinum catalyst. Obviously, NO2 is more effective in oxidizing the catalyst surface than O2. On the first sight this is not surprising since it has been known that NO2 can oxidize a platinum surface [36–38] and also the deactivation resulting from this oxidation has been shown
Fig. 8. (a) NO conversion for the first and second run of a light-off/light-downexperiment with 450 vol.-ppm NO, 6 vol.% O2 and 1000 vol.-ppm CO (N2 as balance, b = 5 8C min1). (b) CO conversion for the first and second run of a light-off/lightdown-experiment with 450 vol.-ppm NO, 6 vol.% O2 and 1000 vol.-ppm CO (N2 as balance, b = 5 8C min1).
before [1]. However, in all these studies NO2 was fed to the reactor and used as oxidative agent. Pure NO2 has a much higher thermodynamic oxidation potential than O2. In our experiments all NO2 is formed by oxidation of NO. Therefore the thermodynamic oxidation potential of NO2 in the resulting NO/NO2/O2 gas mixture can never exceed the oxidation potential of O2. The higher oxidation capacity of NO2 relative to O2 might be explained by the fact that the formation of surface oxides from oxygen is kinetically more limited than the corresponding reaction with NO2. From a mechanistic point of view such a difference in the surface oxidation kinetics might be linked to the oxygen surface coverage in the different environments. Getman et al. [39] recently showed experimentally and by DFT calculations that the dissociative adsorption of O2 on a platinum surface is kinetically hindered if the oxygen surface coverage exceeds a certain value. This means that O2 uptake will result in a maximum oxygen coverage significantly below the value allowed by thermodynamics. NO2 does not show such a kinetic limitation so that reaction with NO2/NO mixtures leads to a higher oxygen coverage than a treatment with O2 alone, even if the NO/NO2 mixture has the same thermodynamic oxidation potential than O2 [39]. Due to repulsive interactions between adsorbed oxygen atoms the chemical potential of adsorbed oxygen strongly increases with increasing surface coverage [40,41]. For this reason it seems a reasonable assumption that the higher oxygen surface concentrations under NO2/NO treatment result in a more effective formation of platinum oxides.
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4. Conclusion 4.1. Contribution of the paper This paper shows that a platinum catalyst operated with a transient temperature cycle exhibits a hysteresis behavior. The catalytic activity of the heat-up phases exceeds the activity of the cool down phases. This effect is the opposite of what is observed for many oxidation reactions and therefore in this paper is referred to as ‘inverse hysteresis’. If the catalyst is operated at one temperature for a longer time, a steady state activity is observed that is in-between the higher activity of the transient heat-up and the lower activity of the transient cool down. The most likely explanation for the observed hysteresis behavior is a reversible oxidation of the platinum surface by NO2 and a reduction of the platinum surface by NO. This effect has been implemented in a surface reaction mechanism for NO oxidation. The observed hysteresis effects can well be explained by this extended reaction mechanism. If the feed contains CO/NO/O2 an inverse hysteresis is observed for NO and CO oxidation. This means that in the presence of NO the hysteresis behavior of the CO oxidation switches from the typical standard hysteresis to an inverse hysteresis. Also this effect can qualitatively be explained by a reversible oxidation of the platinum surface. A reversible deactivation of platinum catalysts has been observed before. However, the previously published studies involved either a treatment in a strongly reducing atmosphere such as oxygen-free H2, NH3 or NO, or an oxidation by NO2, which has a much higher thermodynamic oxidation potential than oxygen. In our experiments all NO2 is formed by oxidation of NO. The thermodynamic oxidation potential of the NO/NO2/O2 mixtures will therefore never exceed the oxidation potential of oxygen alone at the same oxygen concentration. The surprising result of this paper is that reversible deactivation phenomena seem to play a role even in the absence of strongly oxidizing or reducing conditions. This implies that such deactivation effects are relevant for a much wider range of applications than suggested by the previous work. 4.2. Consequences for exhaust aftertreatment 4.2.1. Exhaust system modeling Current computer simulation approaches for catalytic converters are based on the concept of chemical rate equations with reaction rates that are defined by temperature and the exhaust composition. The results presented in this paper show, that – even for constant inlet concentrations – transient variations of the exhaust temperature cause reversible changes in catalyst activity. This means that reaction rates do not only depend on temperature and gas composition but also on catalyst history, an effect not accounted for in current kinetic models. Furthermore, existing kinetic data frequently have been collected under conditions where catalyst activity changes during the test procedure. For example, in the development of the existing kinetic equations for NO oxidation [16,17], most studies have fitted equations to temperature ramp data similar to the data presented herein. However, such data may not represent the true kinetics but will implicitly contain effects due to reversible changes of the catalysts activity. In Section 3.2 it was shown that the reversible activity changes due to NO/NO2 in the feed could in principal be included in a kinetic model, at least for the very simple system NO/O2. In more complex systems several gas species will have an impact on the catalysts oxidation state (see Sections 3.3 and 3.4), e.g. hydrocarbons, CO and H2, etc. will have some reducing
effect on platinum surface. Experimental identification of all these effects is an ambitious task. It is therefore important to be aware of the fundamental limitations of current kinetic modeling approaches. 4.2.2. Catalyst development Much effort has been expended in the development of simple laboratory test procedures that allow an efficient screening of new catalyst formulations for their performance under vehicle driving conditions. Until today, these efforts have only been partially successful. In consequence, the development of industrial automotive exhaust catalysts still heavily relies on expensive engine test procedures that closely mimic the operation during real world vehicle operation. Reversible activity changes during transient catalyst operation are one of the effects not captured by oversimplified laboratory tests. For this reason a better understanding of the transient deactivation phenomena might help to develop more meaningful laboratory screening procedures. 4.2.3. Engine control algorithms Finally, the NO/NO2 ratio is an important parameter for many algorithms implemented in the engine control system. Examples include urea dosing for the SCR catalyst and the estimation of the soot loading of a particulate filter. For this reason engine control systems contain look-up tables that compute the predicted catalytic conversion of NO to NO2 as a function of the catalyst operating parameters. These look-up tables contain steady state conversion values and do not take into account the inverse hysteresis behavior. It is therefore important to be aware of the natural limitations of such tabulation approaches. For control strategies that require a very precise estimation of the NO/NO2 ratio it might become necessary to include reversible deactivation phenomena into the estimation algorithms. References [1] L. Olsson, E. Fridell, J. Catal. 210 (2002) 340. [2] L. Olsson, M. Abul-Milh, H. Karlsson, E. Jabson, P. Thormahlen, A. Hinz, Top. Catal. 30–31 (2004) 85. [3] S.S. Mulla, N. Chen, L. Cumaranatunge, G.E. Blau, D.Y. Zemlyanov, W.N. Delgass, W.S. Epling, F.H. Ribeiro, J. Catal. 241 (2006) 389. [4] P. Bourges, S. Lunati, G. Mabilon, Catalysis and Automotive Pollution Control IV, Studies in Surface Science and Catalysis, vol. 116, 1998, pp. 213–222. [5] B.J. Cooper, J.E. Thoss, SAE Technical Paper 890404, 1989. [6] J. Dawody, M. Skoglundh, E. Fridell, J. Mol. Catal. A: Chem. 209 (2004) 215. [7] J. Dawody, M. Skoglundh, L. Olsson, E. Fridell, J. Catal. 234 (2005) 206. [8] J. Despre´s, Ph.D. Thesis, Diss. ETH No. 15006, ETH Zu¨rich, 2003. [9] J. Despre´s, M. Elsener, M. Koebel, O. Kro¨cher, B. Schnyder, A. Wokaun, Appl. Catal. B 50 (2004) 73. [10] I.V. Yentekakis, V. Tellou, G. Botzolaki, I.A. Rapakousios, Appl. Catal. B 56 (2005) 229. [11] E. Xue, K. Seshan, J.R.H. Ross, Appl. Catal. B 11 (1996) 65. [12] G. Corro, M.P. Elizalde, A. Velasco, React. Kinet. Catal. Lett. 76 (2002) 117. [13] D. Bhatia, R.W. McCabe, M.P. Harold, V. Balakotaiah, J. Catal. 266 (2009) 106. [14] W. Hauptmann, A. Drochner, H. Vogel, M. Votsmeier, J. Gieshoff, Top. Catal. 42–43 (2007) 157. [15] S.S. Mulla, N. Chen, W.N. Delgass, W.S. Epling, F.H. Ribeiro, Catal. Lett. 100 (2005) 267. [16] R. Marques, P. Darcy, P. Da Costa, H. Mellottee, J.-M. Trichard, G. Djega-Mariadassou, J. Mol. Catal. A: Chem. 221 (2004) 127. [17] L. Olsson, R.J. Blint, E. Fridell, Ind. Eng. Chem. Res. 44 (2005) 3021. [18] B.R. Kromer, L. Cao, L. Cumaranatunge, S.S. Mulla, J.L. Ratts, A. Yezerets, N.W. Currier, F.H. Ribeiro, W.N. Delgass, J.M. Caruthers, Catal. Today 136 (2008) 93. [19] A.D. Smeltz, R.B. Getman, W.F. Schneider, F.H. Ribeiro, Catal. Today 136 (2008) 84. [20] W. Hauptmann, M. Votsmeier, J. Gieshoff, D.G. Vlachos, A. Drochner, H. Vogel, Top. Catal. (2009), doi:10.1007/s11244-009-9369-z. [21] L. Olsson, B. Westerberg, H. Persson, E. Fridell, M. Skoglundh, B. Andersson, J. Phys. Chem. B 103 (1999) 10433. [22] L. Olsson, H. Persson, E. Fridell, M. Skoglundh, B. Andersson, J. Phys. Chem. B 105 (2001) 6895. [23] M. Crocoll, Ph.D. Thesis, Universita¨t Karlsruhe, 2003. [24] M. Crocoll, S. Kureti, W. Weisweiler, J. Catal. 229 (2005) 480. [25] J. Koop, Ph.D. Thesis, Universita¨t Karlsruhe, 2008. [26] D. Chatterjee, Ph.D. Thesis, Rubrecht-Karls-Universita¨t Heidelberg, 2001. [27] H. Niehus, G. Comsa, Surf. Sci. 93 (1980) L147. [28] A. von Oertzen, A. Mikhailov, H.H. Rotermund, G. Ertl, Surf. Sci. 350 (1996) 259.
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