Topics in Catalysis Vols. 16/17, Nos. 1–4, 2001
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Periodic control for improved low-temperature catalytic activity Per-Anders Carlsson a,b,c,∗ , Peter Thormählen a,b,c , Magnus Skoglundh a,d , Hans Persson a,c , Erik Fridell a,c , Edward Jobson a,e and Bengt Andersson a,b a Competence Centre for Catalysis, Chalmers University of Technology, SE-412 96 Göteborg, Sweden
E-mail:
[email protected] b Department of Chemical Reaction Engineering, Chalmers University of Technology, SE-412 96 Göteborg, Sweden c Department of Applied Physics, Chalmers University of Technology, SE-412 96 Göteborg, Sweden d Department of Applied Surface Chemistry, Chalmers University of Technology, SE-412 96 Göteborg, Sweden e Volvo Technological Development, SE-405 08 Göteborg, Sweden
The influence of transient changes in the gas composition on the low-temperature activity of a commercial three-way catalyst and a Pt/Al2 O3 model catalyst has been studied. By introducing well-controlled periodic O2 pulses to simple gas mixtures of CO or C3 H6 (in N2 ), a substantial improvement of the low temperature oxidation activity was observed for both catalysts. The reason for low activity at low temperatures is normally attributed to self-poisoning by CO or hydrocarbons. The improved catalytic performance observed here is suggested to origin from the transients causing a surface reactant composition that is favourable for the reaction rate. KEY WORDS: platinum; carbon monoxide; catalytic oxidation; low temperature activity; self-poisoning; cold start emissions; periodic pulsing; transient changes
1. Introduction The major part of the total emissions from cars with stoichiometric engines equipped with catalytic three-way converters, is released during the first minute of driving [1,2]. This is due to the fact that the catalyst is inactive at low temperatures. It is believed that this inactivity of the catalyst at low temperatures arises from self-poisoning [3] of different compounds on the catalyst surface. It is therefore important to find techniques that can minimise the self-poisoning and convert the emissions at low temperatures immediately after a cold start of the engine. The catalytic exhaust cleaning systems on cars are normally operating under transient conditions since throttle changes lead to variations in the engine exhaust composition. In order to optimise the operation of the catalytic converter and minimise the emissions released during unsteadystate driving conditions, there are sophisticated control systems monitoring several parameters like the ignition and fuel injection. The target for these control systems is to keep the engine exhaust composition near the stoichiometric point, i.e., close to the balance between the oxidising and reducing components in the exhaust [4]. However, these advanced engine control systems can also be used to create optimal conditions for the catalyst during the cold start period, e.g., by creating short pulses of different exhaust compositions. Previously [5] we have shown that transient concentration changes in the composition of the synthetic exhaust gas affects the ignition temperature for both a commercial and a model catalyst. Hence, by introducing well-controlled transient changes in the gas composition, the time to light-off can be shortened. One advantage of using this technique in ∗ To whom correspondence should be addressed.
real exhaust systems compared to the use of constant concentrations is the lower cooling effect. The objective of this study was to investigate and explain the involved underlying mechanisms during lowtemperature catalytic ignition, induced by transient changes in the gas composition. To mimic the behaviour of a cold start we have chosen CO oxidation as a simple and suitable model reaction since it shows a strong self-poisoning effect by CO on noble metals at low temperatures (T 150 ◦C). As a comparison, we have also studied propene oxidation. In specific, different experiments with simple gas mixtures over both a commercial and a model catalyst were performed in order to study the light-off process. For CO + O2 or C3 H6 + O2 gas mixtures, we have compared a stepwise increase of the O2 concentration with short O2 pulses.
2. Experimental 2.1. Preparation of the catalysts The commercial monolith catalyst, 15 mm long and 12 mm in diameter, was cut out from a Pt/Pd/Rh (weight proportions 1/14/1) monolith three-way catalyst (400 cpsi, channels per square inch). The catalyst is of double layer type with Pd on a washcoat with oxygen storage capacity in the top layer. The noble metal content of the catalyst sample is 105 g/ft3 . The Pt/Al2 O3 model monolith catalyst was synthesised by using H2 PtCl6 as a precursor for the active material, see [6,7] for details. The Al2 O3 and Pt content is 200 and 20 mg, respectively, which corresponds to 330 g Pt/ft3 . CO-TPD showed an amount of 7.8 µmol of adsorbed CO corresponding to a Pt dispersion of 11% [7], with the as1022-5528/01/0900-0343$19.50/0 2001 Plenum Publishing Corporation
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Figure 1. CO oxidation processes over the Pt/Al2 O3 monolith catalyst at an inlet gas temperature of 150 ◦ C during a stepwise increase in the O2 concentration (left) and for 16.0 vol% O2 pulses, 4 s long, once every minute (right).
sumption that 0.7 CO molecules adsorb per surface Pt [9]. The BET surface area was measured to be 27.5 m2 .
3. Results 3.1. CO oxidation over the Pt/Al2 O3 model monolith catalyst
2.2. Flow reactor measurements The continuous gas flow reactor, used for the measurements [10], consists of a horizontal quartz tube placed in a divisible tubular furnace. Each catalyst was sealed in the middle of the heated zone with quartz wool and the gases were introduced into the reactor via mass flow controllers. The inlet gas temperature and the catalyst wall temperature were measured with two thermocouples. The tip of the thermocouple, for measuring the inlet gas temperature, was fixed by an inactive monolith block and positioned 11 mm before the catalyst in order to avoid disturbances by heat radiation from the active catalyst. The thermocouple measuring the catalyst temperature was placed in the center channel in the rear half of the catalyst (this channel was plugged by the thermocouple). The outlet gas composition was analysed online with respect to the CO and CO2 concentrations (UNOR 6N infrared analysers, Maihak) and the total hydrocarbon content (VE-5 flame ionisation detector, JUM Engineering). In order to clean the catalyst surface, each measurement series was started with a pre-treatment of the catalyst with an 8.0 vol% O2 in N2 gas mixture at an inlet gas temperature of 500 ◦C for about 5 min. The catalyst was then cooled in N2 to the relevant temperature to be studied. In the CO oxidation experiments the catalyst was treated with 1.0 vol% CO in N2 for 5 min to get a CO saturated catalyst surface. In the C3 H6 case the catalyst was treated with 0.17 vol% C3 H6 in N2 for 5 min. A total gas flow of 800 ml/min (NTP), corresponding to a space velocity of 30000 h−1 , was used during all measurements. A standard concentration of 1.0 vol% CO or 0.17 vol% C3 H6 in N2 was used.
The results for the CO oxidation experiments over the Pt/Al2 O3 model monolith catalyst are shown in figure 1 (the measured CO and CO2 outlet concentrations (top), the measured inlet gas and catalyst temperatures (middle) and the inlet O2 concentration (bottom)). Using an inlet gas temperature of 150 ◦C, the O2 step experiment yields a small CO conversion at low O2 concentrations (left) and the conversion increases stepwise with the O2 concentration before light-off. At the 8.0 vol% O2 concentration level the catalyst ignites after about 120 s and complete CO conversion is rapidly reached with a corresponding production of CO2 . There is a small overshoot in the CO2 response immediately after the ignition. At low O2 concentrations, the inlet gas temperature and the catalyst temperature are almost equal. The catalyst temperature increases then rapidly close to the light-off, showing a similar overshoot as the CO2 curve, and then it increases somewhat more until a steady state temperature of 190 ◦C is reached. The results from the CO oxidation experiment, where short 16.0 vol% O2 pulses (4 s long, once every minute) were introduced to a 1.0 vol% CO and 0.6 vol% O2 in N2 gas mixture at an inlet gas temperature of 150 ◦C, are displayed in figure 1 (right). The average O2 concentration is here 1.6 vol%. An increasing CO conversion after each O2 pulse is clearly seen. Between the pulses the conversion decays, but it remains at a higher level than before the pulse. The overall CO conversion increases gradually for each O2 pulse and after several pulses complete CO conversion is seen. At this point small overshoots in the CO2 production for each O2 pulse are observed. The catalyst temperature shows a more or less pronounced stepwise increase for each
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Figure 2. CO oxidation processes over the commercial monolith catalyst at an inlet gas temperature of 125 ◦ C with a stepwise increase in the O2 concentration (left) and for short O2 pulses, as in figure 1.
O2 pulse. It decays somewhat between the pulses but an overall temperature increase is obvious and a maximum catalyst temperature of 220 ◦C is reached. 3.2. CO oxidation over the commercial monolith catalyst Figure 2 displays the corresponding results for the commercial catalyst. The CO oxidation experiment with stepwise changes in O2 concentration shows about 20% CO conversion even at low O2 concentrations at an inlet gas temperature of 125 ◦C. A stepwise increase in CO conversion due to the stepwise increase in O2 concentration is observed. After each change in O2 concentration the CO and CO2 responses quickly develop distinct levels until light-off is obtained at the 6.0 vol% O2 concentration level where complete CO conversion is achieved. The CO oxidation experiment with O2 pulses, similar to the case described in section 3.1 with an inlet gas temperature of 125 ◦C, shows an increasing CO conversion for each O2 pulse. The average O2 concentration is here 2.1 vol%. The behaviour is very similar to the analogous experiment using the Pt/Al2 O3 catalyst. 3.3. C3 H6 oxidation over the Pt/Al2 O3 model monolith catalyst In figure 3 the results from the C3 H6 (0.17 vol%) oxidation experiments, at an inlet temperature of 150 ◦C, with stepwise increase and pulsed addition of O2 , respectively, over the Pt/Al2 O3 catalyst are displayed. For the stepped O2 concentration, 10% C3 H6 conversion at the 1.0 vol% O2 concentration level is obtained and the C3 H6 conversion increases with the O2 concentration. The response is somewhat slower compared to the previous CO oxidation experiments. For 4.0 vol% O2 concentration, light-off can be seen and complete C3 H6 conversion is rapidly reached. A small
overshoot in the CO2 response immediately after the lightoff can also be observed. The catalyst temperature increases rapidly close to light-off from 160 to 200 ◦C, showing a similar overshoot as in the earlier experiments. In the O2 pulsed experiment, short 16.0 vol% O2 pulses (4 s long, once every minute) were introduced to a 0.17 vol% C3 H6 and 1.0 vol% O2 in N2 gas mixture at an inlet gas temperature of 150 ◦C. An increasing C3 H6 conversion for each O2 pulse is seen. Between the pulses the C3 H6 conversion decays, but it ends at a higher conversion level than before the pulse. The C3 H6 conversion increases gradually for each O2 pulse and after several pulses complete C3 H6 conversion is seen. Large overshoots in the CO2 response for each O2 pulse are observed. The catalyst temperature shows a gradual increase for each O2 pulse. It decays somewhat between the pulses but an overall temperature increase is obvious until the temperature levels out at 200–220 ◦C. The production of CO (due to incomplete oxidation of C3 H6 ) is negligible in the two experiments. 4. Discussion The catalytic ignition of CO on Pt is usually described as a sudden self-acceleration of the surface reaction rate. This occurs in a narrow temperature interval according to a Langmuir–Hinshelwood mechanism, also at atmospheric pressure [11–14]. The self-acceleration is due to several phenomena. At the ignition temperature the heat generated by the exothermic chemical reaction exceeds the heat dissipation of the system and the catalyst temperature increases rapidly. This leads to a higher reaction rate. Before ignition the platinum surface is almost completely covered by CO due to self-poisoning. This CO self-poisoning reflects the competition between CO and O2 adsorption on the platinum sites at low temperature. An increased catalyst temperature leads to an increased desorption rate of CO, leaving
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Figure 3. C3 H6 oxidation over a Pt/Al2 O3 monolith catalyst at an inlet gas temperature of 150 ◦ C during a stepwise increase in the O2 concentration (left) and during short 16.0 vol% O2 pulses, 4 s long, once every minute (right).
a larger number of active sites free for oxygen adsorption, which favours the reaction rate. After the ignition the surface is almost solely covered by atomic oxygen [11–13]. Hence, the catalytic ignition, by itself, carries a transition from a CO covered surface to an O-covered surface. This means however not that the catalyst is self-poisoned by O since O cannot inhibit the adsorption of CO. Furthermore, the relative pressure of O2 and CO affects the light-off temperature. This can be understood from the fact that a relative increase of the O2 partial pressure results in a higher O2 impingement rate and hence an increased surface reaction rate. To understand the differences between the stepwise and the pulsed oxidation experiments in our study we start to analyse the CO oxidation over the Pt/Al2 O3 catalyst (figure 1). The stepwise change in O2 concentration from 4.0 to 8.0 vol% results at first in a moderate increase in the CO oxidation. At the 8.0 vol% O2 concentration level the amount of heat generated by the chemical reaction is large enough to raise the catalyst temperature to the ignition point (Ti ≈ 160 ◦C). The transition from a CO covered to an O-covered surface is recognised in the overshoots both in the CO2 response and in the catalyst temperature during the ignition. The high Pt content and thereby the large active surface area of the catalyst makes this phenomenon clearly visible. The over-production of CO2 during the transition causes the temporary over-production of heat. At the start of the pulsed oxidation experiment, figure 1 (right), the reaction rate is very low due to the CO selfpoisoning. The oxygen excess during a pulse leads to an increase in the oxygen impingement rate and the O coverage. This results in a higher surface reaction rate with the corresponding generation of heat and the system is thus turned
into a regime where the CO oxidation self-accelerates. After the pulse, the system slowly turns back towards the CO self-poisoned regime and the reaction rate decreases. Due to heat transport limitations in the system, continuous pulsing increases the temperature to a final state where CO selfpoisoning is negligible and where complete CO conversion is reached. The average O2 concentration of 1.6 vol% in the pulsed experiment compared to the ignition concentration of 8.0 vol% O2 in the stepwise case indicates a substantially improved low temperature activity due to O2 pulsing. The over-shoots in the CO2 response appear in the pulsed experiment after ignition, but also for the stepwise experiments this effect can be observed. This can be understood from the fact that even at complete CO conversion only part of the active catalyst surface area needs to be active. This means that some parts within the catalyst may be completely covered with CO. The large excess of O2 in the short O2 pulse converts this CO and the gas inlet CO into CO2 , leading to the over production of CO2 and heat. Between the pulses CO is, due to the temperature increase, slowly adsorbed again. The CO oxidation over the commercial catalyst, figure 2, shows, in many aspects, the same behaviour as the CO oxidation over the Pt/Al2 O3 monolith catalyst. The average O2 concentration of 2.1 vol% in the pulsed experiment, figure 2 (right), should be compared to the ignition concentration of 6.0 vol% O2 in the stepwise case, figure 2 (left). However, the gain in CO conversion due to O2 pulsing over the commercial monolith catalyst is not as large as in the case of the Pt/Al2 O3 monolith catalyst. This indicates a lower self-poisoning of the commercial catalyst compared to the Pt/Al2 O3 catalyst. The lower self-poisoning is most prob-
P.-A. Carlsson et al. / Periodic control for improved low-temperature catalytic activity
ably due to the presence of many different compounds in the catalyst, such as noble metals (e.g., Pt, Pd and Rh) and metal oxides (e.g., CeO2 and ZrO), improving the catalytic low-temperature activity. Oxygen spill over from the metal oxides to the noble metals is one possible reason to the rather high catalytic activity before light-off. An improved low temperature activity, due to O2 pulsing, on the C3 H6 oxidation on the Pt/Al2 O3 monolith catalyst is seen in figure 3. C3 H6 shows lower self-poisoning effect compared to CO but otherwise the characteristics are very similar the Pt/Al2 O3 catalyst. 5. Conclusions The experimental results show that transient changes in the gas composition, i.e., O2 pulsing, can improve the lowtemperature catalytic activity for CO oxidation for both a Pt/Al2 O3 model catalyst and a commercial three-way catalyst. The transient gas composition changes are believed to cause disturbances in the adsorbate layer on the active sites, lowering the CO self-poisoning effect. By continuous pulsing a, more or less, stepwise increase in conversion and catalyst temperature is achieved, which finally gives a temperature high enough to achieve complete conversion. The positive pulse effect is also seen for C3 H6 oxidation. Acknowledgement This work has been performed within the Competence Centre for Catalysis, which is financially supported by the
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Swedish National Energy Administration and the member companies: AB Volvo, Johnson Matthey-CSD, Saab Automobile AB, Eka Chemicals AB, Perstorp AB and MTC AB.
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