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Pulsed activation in heterogeneous catalysis
Applied Thermal Engineering 57 (2013) 180e187
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Applied Thermal Engineering journal homepage: www.elsevier.com/locate/apthermeng
Pulsed activation in heterogeneous catalysis J. Stolte a, L. Özkan a, *, P.C. Thüne b, J.W. Niemantsverdriet b, A.C.P.M. Backx a a b
Department of Electrical Engineering, Eindhoven Univ. of Technology, The Netherlands Department of Chemistry and Chemical Engineering, Eindhoven Univ. of Technology, The Netherlands
a r t i c l e i n f o
a b s t r a c t
Article history: Received 29 September 2011 Accepted 19 June 2012 Available online 6 July 2012
This paper describes a novel form of dynamic operation named pulsed activation method. It can be viewed as a form of periodic operation in which very fast temperature pulsing is used to induce chemical reactions directly and locally as needed. The main goal in this method is to activate catalytic reactions at will and within a time scale such that physical transport related dynamics cannot follow. A proof of principle experimental setup has been built to realize pulsed activation on heterogenous catalytic reactions. The temperature of the catalytic surface is pulsed at higher frequencies and amplitudes than have been reported before. As an example, oxidation of CO over a Pt catalyst is investigated. Ó 2012 Elsevier Ltd. All rights reserved.
Keywords: Periodic operation Temperature pulsing Local heating Heterogenous catalysis Microreactors
1. Introduction Chemical systems are characterized by their nonlinear dynamics and multi-scale nature. They naturally exhibit various types of periodic phenomena and involve mechanisms that span several orders of magnitude in length and time. In catalytic reactions as an example, one or more reactant molecules diffuse to the catalyst surface and adsorb onto an active site (reaction center). At this site, the reaction occurs and subsequently the products move away from the surface and desorb into the surrounding phase. The length scale for these events ranges from 0.1 nm up to the reactor size (10 m). Simultaneously, during a reaction the time scales involved range from femto and pico seconds for the motion of valence electrons and atoms in a molecule up to seconds/minutes or even infinity if the reactants end up in unwanted byproducts (Chorkendorff and Niemantsverdriet [5]). In catalytic systems, the “slow” transport of reactants toward the catalyst surface limits the “fast” conversion rate of reactants. Additionally, transport of heat such as reaction heat occurs at a rate substantially lower than speed of the actual reaction. The time scales of elementary reactions on the catalyst’s active sites can themselves also vary due to the diffusion of adsorbed species which is needed for reaction. On the other hand, according to the transition state theory (Atkins [2]), the typical
* Corresponding author. Tel.: þ31 40 2473284; fax: þ31 40 2434582. E-mail address: [email protected] (L. Özkan). 1359-4311/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.applthermaleng.2012.06.035
requirement for chemical reactions is bringing molecules to an energy level which exceeds the activation energy threshold. The conventional way of providing energy to molecules is conductive heating, i.e. increasing the temperature will increase the number of molecules that are able to react. Conductive heating is not selective and results in heating of both the reaction site with bulk of the reactor. Recently, several mechanisms are being studied in providing energy to molecules for overcoming the energy barrier namely, electromagnetic fields, electric fields, acoustic methods and lasers (Zare [18], Zhang et al. [19], Durka et al. [7]) but these are not in the scope of this paper. There also exist examples of dynamic temperature operation, such as feed flow temperature variation (Dorawala and Douglas [6]) or the coolant temperature (Chang and Schmitz [4]). However, the realization of temperature forcing was not considered practical since “Any large thermal inertia tends to defeat the effect of sudden changes in this variable” (Silveston et al. [17]). With the improved heat transfer capabilities of microstructure devices, some of the recent research efforts are directed toward forced temperature cycling in microreactors (Brandner et al. [3], Luther et al. [14]), where cycling periods are several seconds and amplitudes are tens of degrees Celcius. The pulsed activation method is a type of dynamic operation in which very fast temperature pulsing is used to induce chemical reactions directly and locally as needed. In this method, we distinguish two different temperature regimes which alternate very fast in the reactor. The first regime is the base regime and the second regime is the pulsed regime. Fig. 1 conceptually shows the catalytic surface temperature during base and pulsed regimes in
J. Stolte et al. / Applied Thermal Engineering 57 (2013) 180e187
Fig. 1. A typical temperature profile of catalytic surface. Temperature switching is instantaneous.
the ideal case of an instantaneous temperature switching. The base regime is the regime the reactor is subjected for the majority of the time during the dynamic operation. It is characterized by mild conditions with negligible overall reaction rate. In the base regime, the transport of molecules to and from the surface can occur, so that reactants are available on the surface. When the desired surface concentrations are achieved, the system can be switched to the pulsed regime. In contrast, the pulsed regime is characterized by locally very high temperatures. The switch from the base regime to the pulsed regime is realized by introducing a burst of energy at specific locations within the reactor. The release of energy is so fast that the system does not have time to reach new equilibrium conditions during the switching time. Therefore, at the start of the pulse the system has different surface concentrations than those which are associated with high energy steady state conditions. In locations where there is a high energy density there is plenty of energy available for chemical reactions to be activated so the reaction rates are locally high. Note that the bulk of the reactor material is not subjected to this high energy density at all and continuously remains at or close to the mild conditions of the base regime even during the pulsed regime. In pulsed activation, the additional operation parameters in the form of pulse frequency and the amplitude (energy content of pulses) are introduced. Also, the reactor conditions in the base regime can be varied just like in any other traditional steady state operation. Setting pulse frequency or amplitude of the pulses to zero means that the reactor operates in conditions at which the reaction rate is negligibly small. Assigning these additional parameters to non-zero values leads to new modes of operation not possible in steady state operation. Since the reactor conditions alternate between these two regimes, the process operation is no longer at steady state and exhibits transient dynamics. These transients result in conditions which cannot be achieved in either of the two regimes separately. Due to this exploitation of nonlinear dynamics in the system, we expect the reaction rates and selectivity to be fundamentally different from those obtained in stationary operation. The operating window of reactors is in general limited by the design specifications. For example, the maximum temperature and pressure at which a reactor can be safely operated depends on the materials used for the reactor walls or seals. Under steady state operation the reaction conditions can never exceed this safe region of operation. On the other hand, the melting temperature for many catalysts is well above that of many typical reactor/sealing materials. Therefore, with pulsed activation the high energy reaction conditions can be created locally at the reaction zone only. This will give the possibility to drive the reaction locally and temporarily to reaction conditions which are outside the safe operating region for the whole reactor. The main advantage of this approach is the elimination of the need of the relatively slow physical transport mechanisms to control the course of chemical reactions. Due to high inertias of
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traditional reactors, the time scale of these mechanisms are much higher than reactions kinetics. When pulsed activation is used, the reaction rate is negligible in the base state, but with each pulse a certain amount of reactant conversion is achieved and after each pulse the base conditions are restored again quickly. Through variation of the amount of pulses per unit of time the reaction rates can be controlled precisely and very nearly instantaneously without the need for physical transport mechanisms. In this way, each mechanism can be affected and optimized separately. In this work, we consider the oxidation of CO as a test reaction for the implementation of the pulsed activation method. CO oxidation over platinum is one of the most studied reactions (Engel and Ertl [8], Herz and Marin [11], Ertl et al. [9], Rinnemo et al. [16]). It is of direct relevance in the removal of CO from waste gases, and the removal of CO from the H2 streams for fuel cells. Platinum is an active catalyst in this reaction, allowing studies with platinum wires and foils. The reaction will also run over a sputtered platinum strip as it is presented in this study. The oxidation of CO has previously been investigated in the context of dynamic operation (forced periodic operation) (Abdul-Kareem et al. [1]). In the work of Abdul-Kareem et al. [1], V2O5 is used as a catalyst and the temperature is switched between two values with a difference of 10e20 K and with cycle periods between 1 h and 8 h. The CO2 production under the temperature switching is comparable to that of steady state operation. Recently, a number of temperature forcing studies have been performed in microreactor setups (Brandner et al. [3], Hansen et al. [10], Jensen et al. [12], Luther et al. [13,14]). In all these studies an increase in reactivity under periodic temperature operation is observed however different explanations are provided. What mechanism causes the increase is not clear, and simulation results depend greatly on the model used. The paper is organized as follows: Section 2 presents the experimental setup built to realize this concept and to establish a first proof of principle. In Section 3 we present and discuss experimental results. Finally, in Section 4 conclusions are presented.
2. Experimental setup The experimental setup built to realize the pulsed activation method is shown in Fig. 2 with key functional parts indicated. As we use this setup to explore temperature pulsing which can have varying effects on heterogeneous catalytic reactions, it is desired to design a reactor that can handle a wide range of operating conditions. The setup consists of mainly three components: The
Fig. 2. Overview picture of the complete proof of principle setup.
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microreactor embedded with the pulsing device, the pulsing electronics and the measurement and data collection. As catalyst platinum is chosen due to its robustness and relatively low complexity. 2.1. Microreactor embedded with pulsing device The microreactor shown in Figs. 3 and 4 is a proof of principle reactor. Therefore, the reactor volume and throughput are not important yet. The reactor volume therefore is a variable which can be chosen freely. A wafer with the Pt catalyst attached to the wafer surface as a thin layer is embedded in a stainless steel reactor. The bulk temperature in this reactor is controllable. On top of it, a stainless steel lid is placed at a short distance. This lid has been gold plated to prevent chemical reactions at the surface. Between the wafer and the lid a closing ring is used to make the reactor gas proof. The lid and the wafer are pressed against the closing ring, forming a reaction chamber. The reactants enter this reaction chamber through a gas inlet at one side of the strip, and the products together with non reacted reactants leave the reactor at the gas outlet at the other end of the platinum strip. The reactor chamber has the dimensions of 0.25 mm height, 6 mm of width and 29 mm of length. The height of the reactor chamber is chosen as small as possible just preventing electrical breakdown of the gas in the chamber at the extreme of conditions applied. The value of 0.25 mm is experimentally found to work. The total volume of the reactor is approximately 43.5 ml. The Pt catalyst is in the form of a strip with dimensions of 0.20 nm height, 4 mm of width and 20 mm of length. The pulsed activation method requires a sharp temperature pulse. Therefore, the catalyst has to be heated and cooled down very fast. Fast heating can always be achieved by creating a high current density in the material since there is no fundamental limit on current density up to the limit that can be handled by the conductor. On the other hand, cooling down the way it is applied here is a passive process in which thermal energy needs to be transported from the metal into the surroundings. The main bottleneck in creating the sharp temperature pulse is the cooling down quickly using natural heat transport mechanisms. In order to overcome this bottleneck, the geometry is chosen such that it favors fast cooling of the platinum. Therefore, a strip of platinum is mounted directly on top of the wafer. The wafer itself functions both as a structural support and as a heat sink simultaneously. Between the platinum strip and the silicon wafer a thin (silica) isolation layer is applied for thermal and electrical isolation of the platinum. In addition to its usual catalytic task of increasing rate of reactions, the Pt strip is also the electrical heating element and acts as an electrical resistance to which an electrical potential is applied at gold plated electrical contacts at both ends. The temperature pulses
Fig. 4. Key parts of the experimental reactor with a coin for size reference.
are invoked by applying a controlled voltage to the platinum strip. The advantage of this design is that it allows a large flexibility in pulse repetition frequency, as well as pulse amplitude. It is therefore possible to explore pulsed operation under a wide range of pulsing conditions. There is also some flexibility in the pulse length through the various isolation layer thicknesses, but other than that the pulse length is fixed throughout the design. By using the catalyst as the pulsing device it also now becomes a dynamic component in the reaction instead of remaining a passive element. 2.2. Pulsed electronics Creating the temperature pulse in the setup only requires the automated application of a high voltage to the platinum strip which generates short energy bursts with a high current. A common solution to create bursts of energy is to store energy in a capacitance and then discharge this using an RLC circuit when needed. The simplified electrical schematic for this method is shown in Fig. 5, in which the resistance R represents the platinum strip. To analyze the circuit of Fig. 5, the initial conditions are such that the initial current in the inductance is zero, and the initial voltage across the capacitance is Vco. Using elementary circuit theory (Nilsson and Riedel [15]) equation (1) is derived, which describes the voltage over the platinum strip VR as a function of the initial capacitance voltage Vco in the laplace domain.
VR ¼
sR=L VC0 s2 þ sR=L þ 1=LC
(1)
Equation (1) represents a standard second order system. The transfer function’s denominator defines the time domain behavior. In particular, this type of second order systems are often interpreted to have a quality factor Q (dimensionless), and a natural frequency
L C
Fig. 3. Schematic of the prototype reactor built to test pulsed temperature operation.
R
Fig. 5. Electrical schematic of basic pulsing circuit. The platinum strip is represented as a resistance.
J. Stolte et al. / Applied Thermal Engineering 57 (2013) 180e187
u0[1/s]. The natural frequency determines the frequency at which the second order system oscillates, while the quality factor is roughly equal to the amount of periods before the system comes to a rest. Equation (2) equates the standard second order denominator for these quantities to the denominator of 1.
s2 þ s
u0 Q
þ u20 ¼ s2 þ sR=L þ 1=LC
(2)
Equation (2) can be solved for u0 and Q, and their solutions are given by 3 and 4 respectively.
u0 ¼ Q ¼
rffiffiffiffiffiffi 1 LC 1 R
(3)
rffiffiffi L C
(4)
As this application is set up for single pulse operation, we require a simple damped pulse which completely discharges the capacitor. This behavior corresponds to a quality factor of Q ¼ 0.5 (critically damped system). A higher quality factor (underdamped) gives rise to oscillations with more than one pulse, while a lower quality factor (overdamped) will result in a more slowly varying pulse hence a very slow release of energy. The averaged measured resistance of the platinum strip applied in the setup was 12.7 0.2U (some variation within the batch of chips) at room temperature. With a capacitance of 1.5 mF and an inductance of 47 mH the quality factor is just below 0.5, with a natural frequency of 120 kHz. Fig. 6 shows a comparison between the solution to equation (1) and the experimentally measured voltage at the platinum strip terminals for an initial capacitance voltage of 350 V. Considering the degree of the approximation of equation (1), the match between simulation and practice is reasonable. In particular, the peak location and the general shape of the curve correspond well to that of the simulation. Several important effects have been neglected in equation (1). For instance, the resistivity of the platinum strip is taken to be constant while in practice it depends on temperature. Furthermore, the impedance of the wiring from the electronics to the platinum strip and the connectors are neglected. The inductance used in this setup is specified to work up to 14 A, at higher currents, the iron saturates however, which introduces a nonlinearity. In practice, the current can go up to 80 A for an initial capacitance voltage of 1 kV. If saturation of the inductance becomes a problem, it can be solved by using multiple inductances in parallel. This adaptation was not required for the measurements in this thesis.
300 Simulated Measured
250
Vcat [V]
200 150 100 50 0 0
2
4 time [s]
6
8
10 × 10− 5
Fig. 6. Voltage over the catalytic strip for 350 V initial capacitance voltage.
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The RLC circuit is built into a 19 inch rack, together with a 1 kV, 100 mA source by Applied Kilovolts (HW001PCP). Insulated gate bipolar transistors (IGBTs) are used to switch the capacitance between the source and the platinum strip, since they have a desired small maximum voltage drop of approximately 2 V when conducting. Therefore IGBTs cause relatively low energy losses when the current is high (i.e. during the pulse). The maximum repetition frequency is set to 100 Hz such that there is always enough time for the platinum strip to cool down to nominal conditions and for the capacitance to recharge. High voltage cabling connected the electronics to the wafer. Voltage and current limits were controllable through an analog input to the hardware board. The measured voltage and current are available through analog outputs. The switching of the IGBTs is controlled by digital inputs using special designed electronics. These analog and digital lines are connected to the PCI 6030 board in the LabVIEW PC. 2.3. Measurement and data collection A flexible but sensitive measurement device is needed for analysis of the effects of temperature pulses on the chemical reactions in the reactor. For this purpose the Pfeiffer Omnistar GSD 320, with a quadrupole mass spectrometer is used. The Omnistar consists of the mass spectrometer itself, integrated with the required pumps, valves and a heated capillary. The mass spectrometer has a range of 200 atomic mass units (amu). The Omnistar continuously takes in a small flow of gas through its capillary, from which the mass spectrometer analyzes the mass spectrum. By repeated sampling, the presence of molecules of a certain mass can be tracked over time. The capillary of the mass spectrometer is placed right at the flow exit of the reactor, such that the response time of the measurement is minimized. Because it is placed as close to the reactor as possible, the response time of the mass spectrometer is in the second range. However, the pulsed phenomena occur at microsecond time scale. Therefore it is fundamentally impossible to use the mass spectrometer to track intra pulse behavior. Effectively, the measurement reflects the cumulative (low pass filtered) effect of multiple pulses on the total conversion of species throughout the reactor. If conditions are similar throughout the reactor, the contribution of individual pulses can be computed from the cumulative conversion. In addition to the components described above, the setup includes a temperature controller and a DC heat supply. The temperature controller is installed to keep the temperature of the reactor at the base regime. A thermocouple positioned just below the wafer is used to measure the temperature as close as possible to the surface. The tip of the thermocouple is placed at 0.5 mm below the bottom of the wafer, which is approximately 1 mm below the platinum strip. The temperature controller provides the power needed to the heating rods which are positioned in the middle of the reactor base, approximately 1 cm below the platinum surface. When applying temperature pulses to the platinum strip, a small amount of heat is added very locally in time and space. This heat quickly spreads into the supporting wafer and from there into the bulk of the reactor. This means that over the course of many pulses there is a slight elevation of temperature in the platinum and the wafer as compared to the case when no heat is added directly to the platinum strip. Specifically, when significant power is added to the platinum strip, the strip and the wafer can be expected to have a slightly higher temperature than the reactor base. Differences in the steady state thermal energy distribution can have an effect on the chemical reactions on the surface. The focus of this work is on the direct effect of pulsed heating only, and therefore this change in steady state heat distribution is unwanted.
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J. Stolte et al. / Applied Thermal Engineering 57 (2013) 180e187 Table 1 Operational parameters for CO oxidation reaction. Value 3 kPa 20 kPa 180 kPa 0.4 1/s 150e210 C
40
Hence, an additional DC voltage source is included in the setup to compensate for undesired differences in the steady state heat distribution. The DC source guarantees that the time averaged amount of energy added to the platinum strip is constant throughout an experiment. When a high amount of energy is added through pulses the energy added with the DC source is small and vice versa. This way the average heat distribution is kept constant and any observed effects can be uniquely attributed to the high temperature of the pulses. For this DC compensation, a Delta Elektronika SM 70-AR-24 power source, which can deliver up to 70 V at a current of up to 12 A, is used. The output of the source is connected to the platinum strip through a high voltage diode. The unit is automated using the PSC-232 extension from Delta Elektronika. All the components of the setup are automated and controlled via Labview. 3. Experimental results and discussion 3.1. Reaction conditions and performance measures The CO oxidation experiments for waste removal and in fuel cell experiments typically use a small fraction of CO and an excess of oxygen. These conditions were also used here and the specific operational parameters are shown in Table 1. The total pressure is approximately 2 atm in order to maintain an overpressure in the reactor so that small leaks do not immediately lead to a large inflow of gas. Argon is used as a carrier gas, and care is taken to stay well outside the region of explosive concentrations. In the work of Hansen et al. [10], Jensen et al. [12], the expression given in equation (5) is used to determine the relative rate enhancement F:
F ¼
hrðT0 ; A; f Þi rQSS ðT0 ; AÞ rQSS ðT0 ; AÞ
(5)
normalized CO 2 production
Parameter Pco PO2 PAr f/V T0
30
20
10
0 300
250
210 200
150
Pulse amplitude [mJ]
195 100
180 50
165 0
150
Temperature [ οC]
Fig. 8. Normalized CO conversion as function of pulse amplitude.
where is the time averaged rate at a given frequency f, and rQSS(T0,A) is the time averaged rate as the frequency approaches zero (quasi steady state (QSS)). Both rates are dependent on the pulse amplitude A and the base reactor temperature T0. The quasi steady state rate is measured by applying a frequency of 10 mHz. However, the experiments performed for pulsed activation method have a different goal which is to show that it is indeed possible to create conversion through pulsing the catalyst temperature, and to switch the reaction rate near instantaneously. In this setup it is not possible to create high catalyst temperatures at low frequencies. Therefore, a different measure is introduced which is called the relative pulse effectiveness h as in equation (6).
h¼
hrðT0 ; A; pÞi rSS ðT0 Þ rSS ðT0 Þ
where is the average pulsed reaction rate, depending on the base temperature T0 in [ C], the pulse amplitude A [J] and the period p [s]. A positive value for h indicates a positive effect, and a negative value means that pulses actually decrease the reaction rate. In a perfect pulsed operation experiment, the rate is zero when no pulses are applied. Therefore, the relative pulse effectiveness will be infinity in the ideal case. However, the setup used here is the first of its kind and is far from perfect. Therefore, it may be expected
× 10− 11 6
CO 2 production (mass spec. data)
5
4
3
2
1
0 0
5
10
15
Time [h] Fig. 7. CO2 production at stepwise increase of pulse amplitude versus time.
(6)
Fig. 9. Normalized pulse effectiveness versus temperature and amplitude.
J. Stolte et al. / Applied Thermal Engineering 57 (2013) 180e187
185
Fig. 10. Normalized CO conversion per pulse q versus temperature and amplitude.
there is some base conversion in the reactor even when no pulses are applied. Another measure that will be investigated is the relative conversion per pulse q, for which the expression is given in equation (7):
q ¼ hp
(7)
The relative conversion per pulse q relates the relative pulse effectiveness to the pulse period p. It measures the added effect of one pulse, as compared to the steady state conversion at the same base reactor temperature. A value of unity for q means that one pulse creates as much product as is created in steady state operation in 1 s. 3.2. Carbon monoxide oxidation with pulsed activation 3.2.1. Pulse amplitude The CO2 formation is monitored for a stepwise increase of the pulse amplitude. Specifically, the pulse energy is increased from zero to the maximum in six steps. The maximum is defined as the value at which the reactor still operates reliably and was found to be 300 mJ. The pulse frequency is kept constant at 20 Hz. Each new setting is maintained for 5 min. The total energy applied to the platinum strip is kept constant over time using the DC source. The DC source always complements the energy added with pulsing to
Fig. 11. Normalized CO conversion versus temperature and frequency.
Fig. 12. Normalized pulse effectiveness versus temperature and frequency.
6 W. In Fig. 7, the raw data from the mass spectrometer for the mass 44, which represents the CO2 concentration in the reactor, versus time is presented. The recipe explained above is repeated 3 times at a base temperature of 210 C. The repeatability of the experiments can be clearly distinguished in this plot. Fig. 8 shows the mass spectrometer CO2 signal for this experiment, normalized to the base conversion at no pulses and a temperature of 150 C. It can be seen that the rate is effected significantly by the temperature pulses, once the pulse energy reaches 200 mJ per pulse. For pulse energies of 150 mJ and lower, the impact is only small at all base temperatures. The rate of reaction without pulses also increases with reactor temperature. Evaluation of h and q as discussed in the previous section gives more insight in the relative effect of the pulses at each temperature. Fig. 9 shows the relative pulse effectiveness h for this experiment. The relative pulse effectiveness shows roughly the same behavior at each temperature. For pulse energies up to 150 mJ the effectiveness of the pulses is near zero as compared to the steady state conversion. For pulses of 200 mJ the conversion added by pulsing is around 0.3 times that of steady state operation at the same temperature, for 250 mJ and 300 mJ the value quickly rises to between 4 and 5 times as effective as steady state operation. Note that the odd value at 250 mJ amplitude and 180 C base temperature can be attributed to the mass spectrometer artifact. Clearly,
Fig. 13. Normalized CO conversion per pulse q versus temperature and frequency.
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The pulse parameters are changed every 5 min, and the CO2 production changes with it, producing the staircase like response. The transition from one level to the next is near instantaneous. Moreover, the levels of the reaction rates are flat and constant over time as long as the pulsing parameters are not changed. By constructing a mapping of reaction rate to pulse frequency, it is possible to create any desired reaction near instantaneously by selecting the corresponding pulse frequency. 4. Conclusions and outlook
Fig. 14. Pulse effectiveness h measured over time.
temperature pulses can be used to increase the production of CO2 by at least a factor of four at all temperatures within this low conversion operating range. Fig. 10 shows the relative conversion per pulse. Since all data was taken at the same flow rate and at the same pulse frequency, q is identical in shape to h, only the scaling is different. For 300 mJ pulses at this frequency, each pulse produces over 20% of the steady state production per second. We have established that high energy pulses have the greatest impact on the observed conversion. The next question is how the value for q varies with pulse repetition rate. Is the added conversion a pure per pulse phenomenon, indicating that all the surface processes are again near their equilibrium? If this is the case, a purely linear dependence of the conversion on pulse frequency is expected. 3.2.2. Pulse frequency To inspect the dependence of the CO2 production on the frequency of the pulses, the pulse amplitude is fixed at 300 mJ per pulse, and the pulse frequency is varied in six equidistant steps. The maximum frequency applied is 20 Hz, corresponding to a repetition period of 50 ms. This is the same frequency as the one that was used in the previous experiment. Fig. 11 shows the CO2 production, normalized to the steady state value at 150 C in this experiment. Fig. 11 shows a consistent result. Within this range of parameters, the reaction rate always increases with temperature, and the rate always increases with pulse frequency. Moreover, the increase with pulse frequency seems approximately linear. Fig. 12 shows the relative CO production for this experiment. In this figure, it can be seen that by varying the pulse repetition rate, the CO2 production by pulses can be varied up to approximately four times the steady state production for all the temperatures. In Fig. 13 we can observe that the per pulse production varies slightly in this range, and that it actually increases somewhat with frequency, especially at the higher temperatures. 3.2.3. Dynamic response of the reaction rate An important goal of pulsed temperature operation is to obtain tight control over the rate of reaction in time. Therefore, the dynamics of the surface reactivity when the pulse parameters are changed are of interest. Fig. 14 shows the value of h as measured in the conversion versus pulse frequency experiment at 180 C.
We have introduced the pulse activation method and presented a proof of principle reactor in which temperature pulses are realized at higher frequencies and amplitudes than have been reported before. The oxidation of CO over a Pt catalyst has been investigated as a test reaction. It was observed that the higher the pulse energy the higher the conversion of CO is. Similar observations can also be made in the case of frequency, that is the reaction rate increases with increasing pulse frequency. Most importantly, it was found that the reaction rate can be influenced almost instantaneously. This means that the reactions can be activated/ deactivated at will which is one of the main goals of the pulsed activation method. This study works as a proof of principle for the pulse activation method in heterogenous catalysis. Further experiments on different test reactions are necessary to derive more detailed conclusions and a better understanding of mechanisms involved. An important improvement toward this will be measurement of surface temperature at the time scale of the temperature pulses. Nomenclature
h u F f/V q A C f L P Q R r T V p
relative pulse effectiveness frequency, Hz relative rate enhancement 1/residence time, 1/s relative conversion per pulse pulse amplitude, J capacitance, F frequency, Hz inductance, H pressure, Pa quality factor resistance, ohm time averaged rate temperature, C voltage, volt period, s
References [1] H.K. Abdul-Kareem, R.R. Hudgins, P.L. Silveston, Forced cycling of the catalytic oxidation of CO over a V2O5 catalyst-II temperature cycling, Chem. Eng. Sci. 35 (10) (1980) 2085e2088. [2] P. Atkins, Physical Chemistry, Freeman, New York, 1998. [3] J.J. Brandner, G. Emig, M.A. Liauw, K. Shubert, Fast temperature cycling in microstructure devices, Chem. Eng. J. 101 (2004) 217e224. [4] M. Chang, R.A. Schmitz, An experimental study of oscillatory states in a stirred reactor, Chem. Eng. Sci. 30 (1) (1975) 21e34. [5] I. Chorkendorff, H. Niemantsverdriet, Concepts of Modern Catalysis and Kinetics, Wiley, 2003. [6] T.G. Dorawala, J.M. Douglas, Complex reactions in oscillating reactors, AIChE Journal 17 (4) (1971) 974e981. [7] T. Durka, T. Van Gerven, A. Stankiewicz, Microwaves in heterogeneous gasphase catalysis: experimental and numerical approaches, Chem. Eng. Technol. 32 (2009) 1301e1312. [8] T. Engel, G. Ertl, Elementary steps in the catalytic oxidation of carbon monoxide on platinum metals, In: Advances in Catalysis, vol. 28, Academic Press, 1979, Ch. 1, pp. 1e78.
J. Stolte et al. / Applied Thermal Engineering 57 (2013) 180e187 [9] G. Ertl, P.R. Norton, J. Rüstig, Kinetic oscillations in the platinum-catalyzed oxidation of co, Phys. Rev. Lett. 49 (2) (July 1982) 177e180. [10] H.A. Hansen, J.L. Olsen, S. Jensen, O. Hansen, U.J. Quaade, Rate enhancement in microfabricated chemical reactors under fast forced temperature oscillations, Catal. Comm. 7 (2006) 272e275. [11] R.K. Herz, S.P. Marin, Surface chemistry models of carbon monoxide oxidation on supported platinum catalysts, J. Catal. 65 (1980) 281e296. [12] S. Jensen, J.L. Olsen, S. Thorsteinsson, O. Hansen, U.J. Quaade, Forced thermal cycling of catalytic reactions: experiments and modelling, Catal. Comm. 8 (2007) 1985e1990. [13] M. Luther, J.J. Brandner, L. Kiwi-Minsker, A. Renken, K. Schubert, Forced periodic temperature cycling of chemical reactions in microstructure devices, Chem. Eng. Sci. 63 (23) (2008) 4955e4961.
187
[14] M. Luther, J.J. Brandner, K. Shubert, A. Renken, L. Kiwi-Minsker, Novel design of a microstructured reactor allowing fast temperature oscillations, Chem. Eng. J. 135S (2008) S254eS258. [15] J.W. Nilsson, S.A. Riedel, Electric Circuits, Prentice Hall, 2011. [16] M. Rinnemo, D. Kulginov, S. Johansson, K.L. Wong, V.P. Zhdanov, B. Kasemo, Catalytic ignition in the COeO2 reaction on platinum: experiment and simulations, Surf. Sci. 376 (1e3) (1997) 297e309. [17] P.L. Silveston, R.R. Hudgins, A. Renken, Periodic operation of catalytic reactors e introduction and overview, Catal. Today 25 (1995) 91e112. [18] R.N. Zare, Laser control of chemical reactions, Science 279 (1998) 1875e1879. [19] X.L. Zhang, S.-M. C., M.P. Mingos, D. Hayward, Oxidative coupling of methane using microwave dielectric heating, Applied Catalysis A: General 249 (2003) 151e164.
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Applied Thermal Engineering journal homepage: www.elsevier.com/locate/apthermeng
Pulsed activation in heterogeneous catalysis J. Stolte a, L. Özkan a, *, P.C. Thüne b, J.W. Niemantsverdriet b, A.C.P.M. Backx a a b
Department of Electrical Engineering, Eindhoven Univ. of Technology, The Netherlands Department of Chemistry and Chemical Engineering, Eindhoven Univ. of Technology, The Netherlands
a r t i c l e i n f o
a b s t r a c t
Article history: Received 29 September 2011 Accepted 19 June 2012 Available online 6 July 2012
This paper describes a novel form of dynamic operation named pulsed activation method. It can be viewed as a form of periodic operation in which very fast temperature pulsing is used to induce chemical reactions directly and locally as needed. The main goal in this method is to activate catalytic reactions at will and within a time scale such that physical transport related dynamics cannot follow. A proof of principle experimental setup has been built to realize pulsed activation on heterogenous catalytic reactions. The temperature of the catalytic surface is pulsed at higher frequencies and amplitudes than have been reported before. As an example, oxidation of CO over a Pt catalyst is investigated. Ó 2012 Elsevier Ltd. All rights reserved.
Keywords: Periodic operation Temperature pulsing Local heating Heterogenous catalysis Microreactors
1. Introduction Chemical systems are characterized by their nonlinear dynamics and multi-scale nature. They naturally exhibit various types of periodic phenomena and involve mechanisms that span several orders of magnitude in length and time. In catalytic reactions as an example, one or more reactant molecules diffuse to the catalyst surface and adsorb onto an active site (reaction center). At this site, the reaction occurs and subsequently the products move away from the surface and desorb into the surrounding phase. The length scale for these events ranges from 0.1 nm up to the reactor size (10 m). Simultaneously, during a reaction the time scales involved range from femto and pico seconds for the motion of valence electrons and atoms in a molecule up to seconds/minutes or even infinity if the reactants end up in unwanted byproducts (Chorkendorff and Niemantsverdriet [5]). In catalytic systems, the “slow” transport of reactants toward the catalyst surface limits the “fast” conversion rate of reactants. Additionally, transport of heat such as reaction heat occurs at a rate substantially lower than speed of the actual reaction. The time scales of elementary reactions on the catalyst’s active sites can themselves also vary due to the diffusion of adsorbed species which is needed for reaction. On the other hand, according to the transition state theory (Atkins [2]), the typical
* Corresponding author. Tel.: þ31 40 2473284; fax: þ31 40 2434582. E-mail address: [email protected] (L. Özkan). 1359-4311/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.applthermaleng.2012.06.035
requirement for chemical reactions is bringing molecules to an energy level which exceeds the activation energy threshold. The conventional way of providing energy to molecules is conductive heating, i.e. increasing the temperature will increase the number of molecules that are able to react. Conductive heating is not selective and results in heating of both the reaction site with bulk of the reactor. Recently, several mechanisms are being studied in providing energy to molecules for overcoming the energy barrier namely, electromagnetic fields, electric fields, acoustic methods and lasers (Zare [18], Zhang et al. [19], Durka et al. [7]) but these are not in the scope of this paper. There also exist examples of dynamic temperature operation, such as feed flow temperature variation (Dorawala and Douglas [6]) or the coolant temperature (Chang and Schmitz [4]). However, the realization of temperature forcing was not considered practical since “Any large thermal inertia tends to defeat the effect of sudden changes in this variable” (Silveston et al. [17]). With the improved heat transfer capabilities of microstructure devices, some of the recent research efforts are directed toward forced temperature cycling in microreactors (Brandner et al. [3], Luther et al. [14]), where cycling periods are several seconds and amplitudes are tens of degrees Celcius. The pulsed activation method is a type of dynamic operation in which very fast temperature pulsing is used to induce chemical reactions directly and locally as needed. In this method, we distinguish two different temperature regimes which alternate very fast in the reactor. The first regime is the base regime and the second regime is the pulsed regime. Fig. 1 conceptually shows the catalytic surface temperature during base and pulsed regimes in
J. Stolte et al. / Applied Thermal Engineering 57 (2013) 180e187
Fig. 1. A typical temperature profile of catalytic surface. Temperature switching is instantaneous.
the ideal case of an instantaneous temperature switching. The base regime is the regime the reactor is subjected for the majority of the time during the dynamic operation. It is characterized by mild conditions with negligible overall reaction rate. In the base regime, the transport of molecules to and from the surface can occur, so that reactants are available on the surface. When the desired surface concentrations are achieved, the system can be switched to the pulsed regime. In contrast, the pulsed regime is characterized by locally very high temperatures. The switch from the base regime to the pulsed regime is realized by introducing a burst of energy at specific locations within the reactor. The release of energy is so fast that the system does not have time to reach new equilibrium conditions during the switching time. Therefore, at the start of the pulse the system has different surface concentrations than those which are associated with high energy steady state conditions. In locations where there is a high energy density there is plenty of energy available for chemical reactions to be activated so the reaction rates are locally high. Note that the bulk of the reactor material is not subjected to this high energy density at all and continuously remains at or close to the mild conditions of the base regime even during the pulsed regime. In pulsed activation, the additional operation parameters in the form of pulse frequency and the amplitude (energy content of pulses) are introduced. Also, the reactor conditions in the base regime can be varied just like in any other traditional steady state operation. Setting pulse frequency or amplitude of the pulses to zero means that the reactor operates in conditions at which the reaction rate is negligibly small. Assigning these additional parameters to non-zero values leads to new modes of operation not possible in steady state operation. Since the reactor conditions alternate between these two regimes, the process operation is no longer at steady state and exhibits transient dynamics. These transients result in conditions which cannot be achieved in either of the two regimes separately. Due to this exploitation of nonlinear dynamics in the system, we expect the reaction rates and selectivity to be fundamentally different from those obtained in stationary operation. The operating window of reactors is in general limited by the design specifications. For example, the maximum temperature and pressure at which a reactor can be safely operated depends on the materials used for the reactor walls or seals. Under steady state operation the reaction conditions can never exceed this safe region of operation. On the other hand, the melting temperature for many catalysts is well above that of many typical reactor/sealing materials. Therefore, with pulsed activation the high energy reaction conditions can be created locally at the reaction zone only. This will give the possibility to drive the reaction locally and temporarily to reaction conditions which are outside the safe operating region for the whole reactor. The main advantage of this approach is the elimination of the need of the relatively slow physical transport mechanisms to control the course of chemical reactions. Due to high inertias of
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traditional reactors, the time scale of these mechanisms are much higher than reactions kinetics. When pulsed activation is used, the reaction rate is negligible in the base state, but with each pulse a certain amount of reactant conversion is achieved and after each pulse the base conditions are restored again quickly. Through variation of the amount of pulses per unit of time the reaction rates can be controlled precisely and very nearly instantaneously without the need for physical transport mechanisms. In this way, each mechanism can be affected and optimized separately. In this work, we consider the oxidation of CO as a test reaction for the implementation of the pulsed activation method. CO oxidation over platinum is one of the most studied reactions (Engel and Ertl [8], Herz and Marin [11], Ertl et al. [9], Rinnemo et al. [16]). It is of direct relevance in the removal of CO from waste gases, and the removal of CO from the H2 streams for fuel cells. Platinum is an active catalyst in this reaction, allowing studies with platinum wires and foils. The reaction will also run over a sputtered platinum strip as it is presented in this study. The oxidation of CO has previously been investigated in the context of dynamic operation (forced periodic operation) (Abdul-Kareem et al. [1]). In the work of Abdul-Kareem et al. [1], V2O5 is used as a catalyst and the temperature is switched between two values with a difference of 10e20 K and with cycle periods between 1 h and 8 h. The CO2 production under the temperature switching is comparable to that of steady state operation. Recently, a number of temperature forcing studies have been performed in microreactor setups (Brandner et al. [3], Hansen et al. [10], Jensen et al. [12], Luther et al. [13,14]). In all these studies an increase in reactivity under periodic temperature operation is observed however different explanations are provided. What mechanism causes the increase is not clear, and simulation results depend greatly on the model used. The paper is organized as follows: Section 2 presents the experimental setup built to realize this concept and to establish a first proof of principle. In Section 3 we present and discuss experimental results. Finally, in Section 4 conclusions are presented.
2. Experimental setup The experimental setup built to realize the pulsed activation method is shown in Fig. 2 with key functional parts indicated. As we use this setup to explore temperature pulsing which can have varying effects on heterogeneous catalytic reactions, it is desired to design a reactor that can handle a wide range of operating conditions. The setup consists of mainly three components: The
Fig. 2. Overview picture of the complete proof of principle setup.
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microreactor embedded with the pulsing device, the pulsing electronics and the measurement and data collection. As catalyst platinum is chosen due to its robustness and relatively low complexity. 2.1. Microreactor embedded with pulsing device The microreactor shown in Figs. 3 and 4 is a proof of principle reactor. Therefore, the reactor volume and throughput are not important yet. The reactor volume therefore is a variable which can be chosen freely. A wafer with the Pt catalyst attached to the wafer surface as a thin layer is embedded in a stainless steel reactor. The bulk temperature in this reactor is controllable. On top of it, a stainless steel lid is placed at a short distance. This lid has been gold plated to prevent chemical reactions at the surface. Between the wafer and the lid a closing ring is used to make the reactor gas proof. The lid and the wafer are pressed against the closing ring, forming a reaction chamber. The reactants enter this reaction chamber through a gas inlet at one side of the strip, and the products together with non reacted reactants leave the reactor at the gas outlet at the other end of the platinum strip. The reactor chamber has the dimensions of 0.25 mm height, 6 mm of width and 29 mm of length. The height of the reactor chamber is chosen as small as possible just preventing electrical breakdown of the gas in the chamber at the extreme of conditions applied. The value of 0.25 mm is experimentally found to work. The total volume of the reactor is approximately 43.5 ml. The Pt catalyst is in the form of a strip with dimensions of 0.20 nm height, 4 mm of width and 20 mm of length. The pulsed activation method requires a sharp temperature pulse. Therefore, the catalyst has to be heated and cooled down very fast. Fast heating can always be achieved by creating a high current density in the material since there is no fundamental limit on current density up to the limit that can be handled by the conductor. On the other hand, cooling down the way it is applied here is a passive process in which thermal energy needs to be transported from the metal into the surroundings. The main bottleneck in creating the sharp temperature pulse is the cooling down quickly using natural heat transport mechanisms. In order to overcome this bottleneck, the geometry is chosen such that it favors fast cooling of the platinum. Therefore, a strip of platinum is mounted directly on top of the wafer. The wafer itself functions both as a structural support and as a heat sink simultaneously. Between the platinum strip and the silicon wafer a thin (silica) isolation layer is applied for thermal and electrical isolation of the platinum. In addition to its usual catalytic task of increasing rate of reactions, the Pt strip is also the electrical heating element and acts as an electrical resistance to which an electrical potential is applied at gold plated electrical contacts at both ends. The temperature pulses
Fig. 4. Key parts of the experimental reactor with a coin for size reference.
are invoked by applying a controlled voltage to the platinum strip. The advantage of this design is that it allows a large flexibility in pulse repetition frequency, as well as pulse amplitude. It is therefore possible to explore pulsed operation under a wide range of pulsing conditions. There is also some flexibility in the pulse length through the various isolation layer thicknesses, but other than that the pulse length is fixed throughout the design. By using the catalyst as the pulsing device it also now becomes a dynamic component in the reaction instead of remaining a passive element. 2.2. Pulsed electronics Creating the temperature pulse in the setup only requires the automated application of a high voltage to the platinum strip which generates short energy bursts with a high current. A common solution to create bursts of energy is to store energy in a capacitance and then discharge this using an RLC circuit when needed. The simplified electrical schematic for this method is shown in Fig. 5, in which the resistance R represents the platinum strip. To analyze the circuit of Fig. 5, the initial conditions are such that the initial current in the inductance is zero, and the initial voltage across the capacitance is Vco. Using elementary circuit theory (Nilsson and Riedel [15]) equation (1) is derived, which describes the voltage over the platinum strip VR as a function of the initial capacitance voltage Vco in the laplace domain.
VR ¼
sR=L VC0 s2 þ sR=L þ 1=LC
(1)
Equation (1) represents a standard second order system. The transfer function’s denominator defines the time domain behavior. In particular, this type of second order systems are often interpreted to have a quality factor Q (dimensionless), and a natural frequency
L C
Fig. 3. Schematic of the prototype reactor built to test pulsed temperature operation.
R
Fig. 5. Electrical schematic of basic pulsing circuit. The platinum strip is represented as a resistance.
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u0[1/s]. The natural frequency determines the frequency at which the second order system oscillates, while the quality factor is roughly equal to the amount of periods before the system comes to a rest. Equation (2) equates the standard second order denominator for these quantities to the denominator of 1.
s2 þ s
u0 Q
þ u20 ¼ s2 þ sR=L þ 1=LC
(2)
Equation (2) can be solved for u0 and Q, and their solutions are given by 3 and 4 respectively.
u0 ¼ Q ¼
rffiffiffiffiffiffi 1 LC 1 R
(3)
rffiffiffi L C
(4)
As this application is set up for single pulse operation, we require a simple damped pulse which completely discharges the capacitor. This behavior corresponds to a quality factor of Q ¼ 0.5 (critically damped system). A higher quality factor (underdamped) gives rise to oscillations with more than one pulse, while a lower quality factor (overdamped) will result in a more slowly varying pulse hence a very slow release of energy. The averaged measured resistance of the platinum strip applied in the setup was 12.7 0.2U (some variation within the batch of chips) at room temperature. With a capacitance of 1.5 mF and an inductance of 47 mH the quality factor is just below 0.5, with a natural frequency of 120 kHz. Fig. 6 shows a comparison between the solution to equation (1) and the experimentally measured voltage at the platinum strip terminals for an initial capacitance voltage of 350 V. Considering the degree of the approximation of equation (1), the match between simulation and practice is reasonable. In particular, the peak location and the general shape of the curve correspond well to that of the simulation. Several important effects have been neglected in equation (1). For instance, the resistivity of the platinum strip is taken to be constant while in practice it depends on temperature. Furthermore, the impedance of the wiring from the electronics to the platinum strip and the connectors are neglected. The inductance used in this setup is specified to work up to 14 A, at higher currents, the iron saturates however, which introduces a nonlinearity. In practice, the current can go up to 80 A for an initial capacitance voltage of 1 kV. If saturation of the inductance becomes a problem, it can be solved by using multiple inductances in parallel. This adaptation was not required for the measurements in this thesis.
300 Simulated Measured
250
Vcat [V]
200 150 100 50 0 0
2
4 time [s]
6
8
10 × 10− 5
Fig. 6. Voltage over the catalytic strip for 350 V initial capacitance voltage.
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The RLC circuit is built into a 19 inch rack, together with a 1 kV, 100 mA source by Applied Kilovolts (HW001PCP). Insulated gate bipolar transistors (IGBTs) are used to switch the capacitance between the source and the platinum strip, since they have a desired small maximum voltage drop of approximately 2 V when conducting. Therefore IGBTs cause relatively low energy losses when the current is high (i.e. during the pulse). The maximum repetition frequency is set to 100 Hz such that there is always enough time for the platinum strip to cool down to nominal conditions and for the capacitance to recharge. High voltage cabling connected the electronics to the wafer. Voltage and current limits were controllable through an analog input to the hardware board. The measured voltage and current are available through analog outputs. The switching of the IGBTs is controlled by digital inputs using special designed electronics. These analog and digital lines are connected to the PCI 6030 board in the LabVIEW PC. 2.3. Measurement and data collection A flexible but sensitive measurement device is needed for analysis of the effects of temperature pulses on the chemical reactions in the reactor. For this purpose the Pfeiffer Omnistar GSD 320, with a quadrupole mass spectrometer is used. The Omnistar consists of the mass spectrometer itself, integrated with the required pumps, valves and a heated capillary. The mass spectrometer has a range of 200 atomic mass units (amu). The Omnistar continuously takes in a small flow of gas through its capillary, from which the mass spectrometer analyzes the mass spectrum. By repeated sampling, the presence of molecules of a certain mass can be tracked over time. The capillary of the mass spectrometer is placed right at the flow exit of the reactor, such that the response time of the measurement is minimized. Because it is placed as close to the reactor as possible, the response time of the mass spectrometer is in the second range. However, the pulsed phenomena occur at microsecond time scale. Therefore it is fundamentally impossible to use the mass spectrometer to track intra pulse behavior. Effectively, the measurement reflects the cumulative (low pass filtered) effect of multiple pulses on the total conversion of species throughout the reactor. If conditions are similar throughout the reactor, the contribution of individual pulses can be computed from the cumulative conversion. In addition to the components described above, the setup includes a temperature controller and a DC heat supply. The temperature controller is installed to keep the temperature of the reactor at the base regime. A thermocouple positioned just below the wafer is used to measure the temperature as close as possible to the surface. The tip of the thermocouple is placed at 0.5 mm below the bottom of the wafer, which is approximately 1 mm below the platinum strip. The temperature controller provides the power needed to the heating rods which are positioned in the middle of the reactor base, approximately 1 cm below the platinum surface. When applying temperature pulses to the platinum strip, a small amount of heat is added very locally in time and space. This heat quickly spreads into the supporting wafer and from there into the bulk of the reactor. This means that over the course of many pulses there is a slight elevation of temperature in the platinum and the wafer as compared to the case when no heat is added directly to the platinum strip. Specifically, when significant power is added to the platinum strip, the strip and the wafer can be expected to have a slightly higher temperature than the reactor base. Differences in the steady state thermal energy distribution can have an effect on the chemical reactions on the surface. The focus of this work is on the direct effect of pulsed heating only, and therefore this change in steady state heat distribution is unwanted.
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J. Stolte et al. / Applied Thermal Engineering 57 (2013) 180e187 Table 1 Operational parameters for CO oxidation reaction. Value 3 kPa 20 kPa 180 kPa 0.4 1/s 150e210 C
40
Hence, an additional DC voltage source is included in the setup to compensate for undesired differences in the steady state heat distribution. The DC source guarantees that the time averaged amount of energy added to the platinum strip is constant throughout an experiment. When a high amount of energy is added through pulses the energy added with the DC source is small and vice versa. This way the average heat distribution is kept constant and any observed effects can be uniquely attributed to the high temperature of the pulses. For this DC compensation, a Delta Elektronika SM 70-AR-24 power source, which can deliver up to 70 V at a current of up to 12 A, is used. The output of the source is connected to the platinum strip through a high voltage diode. The unit is automated using the PSC-232 extension from Delta Elektronika. All the components of the setup are automated and controlled via Labview. 3. Experimental results and discussion 3.1. Reaction conditions and performance measures The CO oxidation experiments for waste removal and in fuel cell experiments typically use a small fraction of CO and an excess of oxygen. These conditions were also used here and the specific operational parameters are shown in Table 1. The total pressure is approximately 2 atm in order to maintain an overpressure in the reactor so that small leaks do not immediately lead to a large inflow of gas. Argon is used as a carrier gas, and care is taken to stay well outside the region of explosive concentrations. In the work of Hansen et al. [10], Jensen et al. [12], the expression given in equation (5) is used to determine the relative rate enhancement F:
F ¼
hrðT0 ; A; f Þi rQSS ðT0 ; AÞ rQSS ðT0 ; AÞ
(5)
normalized CO 2 production
Parameter Pco PO2 PAr f/V T0
30
20
10
0 300
250
210 200
150
Pulse amplitude [mJ]
195 100
180 50
165 0
150
Temperature [ οC]
Fig. 8. Normalized CO conversion as function of pulse amplitude.
where
h¼
hrðT0 ; A; pÞi rSS ðT0 Þ rSS ðT0 Þ
where
× 10− 11 6
CO 2 production (mass spec. data)
5
4
3
2
1
0 0
5
10
15
Time [h] Fig. 7. CO2 production at stepwise increase of pulse amplitude versus time.
(6)
Fig. 9. Normalized pulse effectiveness versus temperature and amplitude.
J. Stolte et al. / Applied Thermal Engineering 57 (2013) 180e187
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Fig. 10. Normalized CO conversion per pulse q versus temperature and amplitude.
there is some base conversion in the reactor even when no pulses are applied. Another measure that will be investigated is the relative conversion per pulse q, for which the expression is given in equation (7):
q ¼ hp
(7)
The relative conversion per pulse q relates the relative pulse effectiveness to the pulse period p. It measures the added effect of one pulse, as compared to the steady state conversion at the same base reactor temperature. A value of unity for q means that one pulse creates as much product as is created in steady state operation in 1 s. 3.2. Carbon monoxide oxidation with pulsed activation 3.2.1. Pulse amplitude The CO2 formation is monitored for a stepwise increase of the pulse amplitude. Specifically, the pulse energy is increased from zero to the maximum in six steps. The maximum is defined as the value at which the reactor still operates reliably and was found to be 300 mJ. The pulse frequency is kept constant at 20 Hz. Each new setting is maintained for 5 min. The total energy applied to the platinum strip is kept constant over time using the DC source. The DC source always complements the energy added with pulsing to
Fig. 11. Normalized CO conversion versus temperature and frequency.
Fig. 12. Normalized pulse effectiveness versus temperature and frequency.
6 W. In Fig. 7, the raw data from the mass spectrometer for the mass 44, which represents the CO2 concentration in the reactor, versus time is presented. The recipe explained above is repeated 3 times at a base temperature of 210 C. The repeatability of the experiments can be clearly distinguished in this plot. Fig. 8 shows the mass spectrometer CO2 signal for this experiment, normalized to the base conversion at no pulses and a temperature of 150 C. It can be seen that the rate is effected significantly by the temperature pulses, once the pulse energy reaches 200 mJ per pulse. For pulse energies of 150 mJ and lower, the impact is only small at all base temperatures. The rate of reaction without pulses also increases with reactor temperature. Evaluation of h and q as discussed in the previous section gives more insight in the relative effect of the pulses at each temperature. Fig. 9 shows the relative pulse effectiveness h for this experiment. The relative pulse effectiveness shows roughly the same behavior at each temperature. For pulse energies up to 150 mJ the effectiveness of the pulses is near zero as compared to the steady state conversion. For pulses of 200 mJ the conversion added by pulsing is around 0.3 times that of steady state operation at the same temperature, for 250 mJ and 300 mJ the value quickly rises to between 4 and 5 times as effective as steady state operation. Note that the odd value at 250 mJ amplitude and 180 C base temperature can be attributed to the mass spectrometer artifact. Clearly,
Fig. 13. Normalized CO conversion per pulse q versus temperature and frequency.
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The pulse parameters are changed every 5 min, and the CO2 production changes with it, producing the staircase like response. The transition from one level to the next is near instantaneous. Moreover, the levels of the reaction rates are flat and constant over time as long as the pulsing parameters are not changed. By constructing a mapping of reaction rate to pulse frequency, it is possible to create any desired reaction near instantaneously by selecting the corresponding pulse frequency. 4. Conclusions and outlook
Fig. 14. Pulse effectiveness h measured over time.
temperature pulses can be used to increase the production of CO2 by at least a factor of four at all temperatures within this low conversion operating range. Fig. 10 shows the relative conversion per pulse. Since all data was taken at the same flow rate and at the same pulse frequency, q is identical in shape to h, only the scaling is different. For 300 mJ pulses at this frequency, each pulse produces over 20% of the steady state production per second. We have established that high energy pulses have the greatest impact on the observed conversion. The next question is how the value for q varies with pulse repetition rate. Is the added conversion a pure per pulse phenomenon, indicating that all the surface processes are again near their equilibrium? If this is the case, a purely linear dependence of the conversion on pulse frequency is expected. 3.2.2. Pulse frequency To inspect the dependence of the CO2 production on the frequency of the pulses, the pulse amplitude is fixed at 300 mJ per pulse, and the pulse frequency is varied in six equidistant steps. The maximum frequency applied is 20 Hz, corresponding to a repetition period of 50 ms. This is the same frequency as the one that was used in the previous experiment. Fig. 11 shows the CO2 production, normalized to the steady state value at 150 C in this experiment. Fig. 11 shows a consistent result. Within this range of parameters, the reaction rate always increases with temperature, and the rate always increases with pulse frequency. Moreover, the increase with pulse frequency seems approximately linear. Fig. 12 shows the relative CO production for this experiment. In this figure, it can be seen that by varying the pulse repetition rate, the CO2 production by pulses can be varied up to approximately four times the steady state production for all the temperatures. In Fig. 13 we can observe that the per pulse production varies slightly in this range, and that it actually increases somewhat with frequency, especially at the higher temperatures. 3.2.3. Dynamic response of the reaction rate An important goal of pulsed temperature operation is to obtain tight control over the rate of reaction in time. Therefore, the dynamics of the surface reactivity when the pulse parameters are changed are of interest. Fig. 14 shows the value of h as measured in the conversion versus pulse frequency experiment at 180 C.
We have introduced the pulse activation method and presented a proof of principle reactor in which temperature pulses are realized at higher frequencies and amplitudes than have been reported before. The oxidation of CO over a Pt catalyst has been investigated as a test reaction. It was observed that the higher the pulse energy the higher the conversion of CO is. Similar observations can also be made in the case of frequency, that is the reaction rate increases with increasing pulse frequency. Most importantly, it was found that the reaction rate can be influenced almost instantaneously. This means that the reactions can be activated/ deactivated at will which is one of the main goals of the pulsed activation method. This study works as a proof of principle for the pulse activation method in heterogenous catalysis. Further experiments on different test reactions are necessary to derive more detailed conclusions and a better understanding of mechanisms involved. An important improvement toward this will be measurement of surface temperature at the time scale of the temperature pulses. Nomenclature
h u F f/V q A C f L P Q R r T V p
relative pulse effectiveness frequency, Hz relative rate enhancement 1/residence time, 1/s relative conversion per pulse pulse amplitude, J capacitance, F frequency, Hz inductance, H pressure, Pa quality factor resistance, ohm time averaged rate temperature, C voltage, volt period, s
References [1] H.K. Abdul-Kareem, R.R. Hudgins, P.L. Silveston, Forced cycling of the catalytic oxidation of CO over a V2O5 catalyst-II temperature cycling, Chem. Eng. Sci. 35 (10) (1980) 2085e2088. [2] P. Atkins, Physical Chemistry, Freeman, New York, 1998. [3] J.J. Brandner, G. Emig, M.A. Liauw, K. Shubert, Fast temperature cycling in microstructure devices, Chem. Eng. J. 101 (2004) 217e224. [4] M. Chang, R.A. Schmitz, An experimental study of oscillatory states in a stirred reactor, Chem. Eng. Sci. 30 (1) (1975) 21e34. [5] I. Chorkendorff, H. Niemantsverdriet, Concepts of Modern Catalysis and Kinetics, Wiley, 2003. [6] T.G. Dorawala, J.M. Douglas, Complex reactions in oscillating reactors, AIChE Journal 17 (4) (1971) 974e981. [7] T. Durka, T. Van Gerven, A. Stankiewicz, Microwaves in heterogeneous gasphase catalysis: experimental and numerical approaches, Chem. Eng. Technol. 32 (2009) 1301e1312. [8] T. Engel, G. Ertl, Elementary steps in the catalytic oxidation of carbon monoxide on platinum metals, In: Advances in Catalysis, vol. 28, Academic Press, 1979, Ch. 1, pp. 1e78.
J. Stolte et al. / Applied Thermal Engineering 57 (2013) 180e187 [9] G. Ertl, P.R. Norton, J. Rüstig, Kinetic oscillations in the platinum-catalyzed oxidation of co, Phys. Rev. Lett. 49 (2) (July 1982) 177e180. [10] H.A. Hansen, J.L. Olsen, S. Jensen, O. Hansen, U.J. Quaade, Rate enhancement in microfabricated chemical reactors under fast forced temperature oscillations, Catal. Comm. 7 (2006) 272e275. [11] R.K. Herz, S.P. Marin, Surface chemistry models of carbon monoxide oxidation on supported platinum catalysts, J. Catal. 65 (1980) 281e296. [12] S. Jensen, J.L. Olsen, S. Thorsteinsson, O. Hansen, U.J. Quaade, Forced thermal cycling of catalytic reactions: experiments and modelling, Catal. Comm. 8 (2007) 1985e1990. [13] M. Luther, J.J. Brandner, L. Kiwi-Minsker, A. Renken, K. Schubert, Forced periodic temperature cycling of chemical reactions in microstructure devices, Chem. Eng. Sci. 63 (23) (2008) 4955e4961.
187
[14] M. Luther, J.J. Brandner, K. Shubert, A. Renken, L. Kiwi-Minsker, Novel design of a microstructured reactor allowing fast temperature oscillations, Chem. Eng. J. 135S (2008) S254eS258. [15] J.W. Nilsson, S.A. Riedel, Electric Circuits, Prentice Hall, 2011. [16] M. Rinnemo, D. Kulginov, S. Johansson, K.L. Wong, V.P. Zhdanov, B. Kasemo, Catalytic ignition in the COeO2 reaction on platinum: experiment and simulations, Surf. Sci. 376 (1e3) (1997) 297e309. [17] P.L. Silveston, R.R. Hudgins, A. Renken, Periodic operation of catalytic reactors e introduction and overview, Catal. Today 25 (1995) 91e112. [18] R.N. Zare, Laser control of chemical reactions, Science 279 (1998) 1875e1879. [19] X.L. Zhang, S.-M. C., M.P. Mingos, D. Hayward, Oxidative coupling of methane using microwave dielectric heating, Applied Catalysis A: General 249 (2003) 151e164.