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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

aDepartment of Electrical Engineering, Eindhoven Univ. of Technology, The NetherlandsbDepartment of Chemistry and Chemical Engineering, Eindhoven Univ. of Technology, The Netherlands

a r t i c l e i n f o

Article history:Received 29 September 2011Accepted 19 June 2012Available online 6 July 2012

Keywords:Periodic operationTemperature pulsingLocal heatingHeterogenous catalysisMicroreactors

* Corresponding author. Tel.: þ31 40 2473284; fax:E-mail address: l.ozkan@tue.nl (L. Özkan).

1359-4311/$ e see front matter � 2012 Elsevier Ltd.http://dx.doi.org/10.1016/j.applthermaleng.2012.06.03

a b s t r a c t

This paper describes a novel form of dynamic operation named pulsed activation method. It can beviewed as a form of periodic operation in which very fast temperature pulsing is used to induce chemicalreactions directly and locally as needed. The main goal in this method is to activate catalytic reactions atwill and within a time scale such that physical transport related dynamics cannot follow. A proof ofprinciple experimental setup has been built to realize pulsed activation on heterogenous catalyticreactions. The temperature of the catalytic surface is pulsed at higher frequencies and amplitudes thanhave been reported before. As an example, oxidation of CO over a Pt catalyst is investigated.

� 2012 Elsevier Ltd. All rights reserved.

1. Introduction

Chemical systems are characterized by their nonlinear dynamicsand multi-scale nature. They naturally exhibit various types ofperiodic phenomena and involve mechanisms that span severalorders of magnitude in length and time. In catalytic reactions as anexample, one or more reactant molecules diffuse to the catalystsurface and adsorb onto an active site (reaction center). At this site,the reaction occurs and subsequently the products move away fromthe surface and desorb into the surrounding phase. The length scalefor these events ranges from 0.1 nm up to the reactor size (10 m).Simultaneously, during a reaction the time scales involved rangefrom femto and pico seconds for the motion of valence electronsand atoms in a molecule up to seconds/minutes or even infinity ifthe reactants end up in unwanted byproducts (Chorkendorff andNiemantsverdriet [5]). In catalytic systems, the “slow” transport ofreactants toward the catalyst surface limits the “fast” conversionrate of reactants. Additionally, transport of heat such as reactionheat occurs at a rate substantially lower than speed of the actualreaction. The time scales of elementary reactions on the catalyst’sactive sites can themselves also vary due to the diffusion ofadsorbed species which is needed for reaction. On the other hand,according to the transition state theory (Atkins [2]), the typical

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All rights reserved.5

requirement for chemical reactions is bringing molecules to anenergy level which exceeds the activation energy threshold.

The conventional way of providing energy to molecules isconductive heating, i.e. increasing the temperature will increasethe number of molecules that are able to react. Conductive heatingis not selective and results in heating of both the reaction site withbulk of the reactor. Recently, several mechanisms are being studiedin providing energy tomolecules for overcoming the energy barriernamely, electromagnetic fields, electric fields, acoustic methodsand lasers (Zare [18], Zhang et al. [19], Durka et al. [7]) but these arenot in the scope of this paper. There also exist examples of dynamictemperature operation, such as feed flow temperature variation(Dorawala and Douglas [6]) or the coolant temperature (Chang andSchmitz [4]). However, the realization of temperature forcing wasnot considered practical since “Any large thermal inertia tends todefeat the effect of sudden changes in this variable” (Silveston et al.[17]). With the improved heat transfer capabilities of microstruc-ture devices, some of the recent research efforts are directedtoward forced temperature cycling inmicroreactors (Brandner et al.[3], Luther et al. [14]), where cycling periods are several secondsand amplitudes are tens of degrees Celcius.

The pulsed activation method is a type of dynamic operation inwhich very fast temperature pulsing is used to induce chemicalreactions directly and locally as needed. In this method, wedistinguish two different temperature regimes which alternatevery fast in the reactor. The first regime is the base regime and thesecond regime is the pulsed regime. Fig. 1 conceptually shows thecatalytic surface temperature during base and pulsed regimes in

Fig. 1. A typical temperature profile of catalytic surface. Temperature switching isinstantaneous.

Fig. 2. Overview picture of the complete proof of principle setup.

J. Stolte et al. / Applied Thermal Engineering 57 (2013) 180e187 181

the ideal case of an instantaneous temperature switching. The baseregime is the regime the reactor is subjected for the majority of thetime during the dynamic operation. It is characterized by mildconditions with negligible overall reaction rate. In the base regime,the transport of molecules to and from the surface can occur, so thatreactants are available on the surface. When the desired surfaceconcentrations are achieved, the system can be switched to thepulsed regime. In contrast, the pulsed regime is characterized bylocally very high temperatures. The switch from the base regime tothe pulsed regime is realized by introducing a burst of energy atspecific locations within the reactor. The release of energy is so fastthat the system does not have time to reach new equilibriumconditions during the switching time. Therefore, at the start of thepulse the system has different surface concentrations than thosewhich are associated with high energy steady state conditions. Inlocations where there is a high energy density there is plenty ofenergy available for chemical reactions to be activated so thereaction rates are locally high. Note that the bulk of the reactormaterial is not subjected to this high energy density at all andcontinuously remains at or close to the mild conditions of the baseregime even during the pulsed regime.

In pulsed activation, the additional operation parameters in theform of pulse frequency and the amplitude (energy content of pul-ses) are introduced. Also, the reactor conditions in the base regimecan be varied just like in anyother traditional steady state operation.Setting pulse frequency or amplitude of the pulses to zero meansthat the reactor operates in conditions at which the reaction rate isnegligibly small. Assigning these additional parameters to non-zerovalues leads to newmodes of operation not possible in steady stateoperation. Since the reactor conditions alternate between these tworegimes, the process operation is no longer at steady state andexhibits transient dynamics. These transients result in conditionswhich cannot be achieved in either of the two regimes separately.Due to this exploitation of nonlinear dynamics in the system, weexpect the reaction rates and selectivity to be fundamentallydifferent from those obtained in stationary operation.

The operating window of reactors is in general limited by thedesign specifications. For example, the maximum temperature andpressure at which a reactor can be safely operated depends on thematerials used for the reactor walls or seals. Under steady stateoperation the reaction conditions can never exceed this safe regionof operation. On the other hand, the melting temperature for manycatalysts is well above that of many typical reactor/sealing mate-rials. Therefore, with pulsed activation the high energy reactionconditions can be created locally at the reaction zone only. This willgive the possibility to drive the reaction locally and temporarily toreaction conditions which are outside the safe operating region forthe whole reactor.

The main advantage of this approach is the elimination of theneed of the relatively slow physical transport mechanisms tocontrol the course of chemical reactions. Due to high inertias of

traditional reactors, the time scale of these mechanisms are muchhigher than reactions kinetics. When pulsed activation is used, thereaction rate is negligible in the base state, but with each pulsea certain amount of reactant conversion is achieved and after eachpulse the base conditions are restored again quickly. Throughvariation of the amount of pulses per unit of time the reaction ratescan be controlled precisely and very nearly instantaneouslywithout 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 reactionfor the implementation of the pulsed activation method. COoxidation over platinum is one of the most studied reactions (Engeland 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, andthe removal of CO from the H2 streams for fuel cells. Platinum is anactive catalyst in this reaction, allowing studies with platinumwires and foils. The reactionwill also run over a sputtered platinumstrip as it is presented in this study. The oxidation of CO haspreviously been investigated in the context of dynamic operation(forced periodic operation) (Abdul-Kareem et al. [1]). In the work ofAbdul-Kareem et al. [1], V2O5 is used as a catalyst and thetemperature is switched between two values with a difference of10e20 K and with cycle periods between 1 h and 8 h. The CO2

production under the temperature switching is comparable to thatof steady state operation. Recently, a number of temperatureforcing 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 periodictemperature operation is observed however different explanationsare provided. What mechanism causes the increase is not clear, andsimulation results depend greatly on the model used.

The paper is organized as follows: Section 2 presents theexperimental setup built to realize this concept and to establisha first proof of principle. In Section 3 we present and discussexperimental results. Finally, in Section 4 conclusions are presented.

2. Experimental setup

The experimental setup built to realize the pulsed activationmethod is shown in Fig. 2 with key functional parts indicated. Aswe use this setup to explore temperature pulsing which can havevarying effects on heterogeneous catalytic reactions, it is desired todesign a reactor that can handle a wide range of operating condi-tions. The setup consists of mainly three components: The

Fig. 4. Key parts of the experimental reactor with a coin for size reference.

J. Stolte et al. / Applied Thermal Engineering 57 (2013) 180e187182

microreactor embedded with the pulsing device, the pulsing elec-tronics and the measurement and data collection. As catalystplatinum is chosen due to its robustness and relatively lowcomplexity.

2.1. Microreactor embedded with pulsing device

The microreactor shown in Figs. 3 and 4 is a proof of principlereactor. Therefore, the reactor volume and throughput are notimportant yet. The reactor volume therefore is a variable which canbe chosen freely.

A wafer with the Pt catalyst attached to the wafer surface asa thin layer is embedded in a stainless steel reactor. The bulktemperature in this reactor is controllable. On top of it, a stainlesssteel lid is placed at a short distance. This lid has been gold plated toprevent chemical reactions at the surface. Between the wafer andthe lid a closing ring is used to make the reactor gas proof. The lidand the wafer are pressed against the closing ring, forming a reac-tion chamber. The reactants enter this reaction chamber througha gas inlet at one side of the strip, and the products together withnon reacted reactants leave the reactor at the gas outlet at the otherend of the platinum strip. The reactor chamber has the dimensionsof 0.25 mm height, 6 mm of width and 29 mm of length. The heightof the reactor chamber is chosen as small as possible just pre-venting electrical breakdown of the gas in the chamber at theextreme of conditions applied. The value of 0.25 mm is experi-mentally found to work. The total volume of the reactor isapproximately 43.5 ml.

The Pt catalyst is in the form of a strip with dimensions of0.20 nm height, 4 mm of width and 20 mm of length. The pulsedactivation method requires a sharp temperature pulse. Therefore,the catalyst has to be heated and cooled down very fast. Fastheating can always be achieved by creating a high current densityin the material since there is no fundamental limit on currentdensity up to the limit that can be handled by the conductor. On theother hand, cooling down the way it is applied here is a passiveprocess in which thermal energy needs to be transported from themetal into the surroundings. The main bottleneck in creating thesharp temperature pulse is the cooling down quickly using naturalheat transport mechanisms. In order to overcome this bottleneck,the geometry is chosen such that it favors fast cooling of the plat-inum. Therefore, a strip of platinum is mounted directly on top ofthe wafer. The wafer itself functions both as a structural supportand as a heat sink simultaneously. Between the platinum strip andthe silicon wafer a thin (silica) isolation layer is applied for thermaland electrical isolation of the platinum.

In addition to its usual catalytic task of increasing rate of reac-tions, the Pt strip is also the electrical heating element and acts asan electrical resistance to which an electrical potential is applied atgold plated electrical contacts at both ends. The temperature pulses

Fig. 3. Schematic of the prototype reactor built to test pulsed temperature operation.

are invoked by applying a controlled voltage to the platinum strip.The advantage of this design is that it allows a large flexibility inpulse repetition frequency, as well as pulse amplitude. It is there-fore possible to explore pulsed operation under a wide range ofpulsing conditions. There is also some flexibility in the pulse lengththrough the various isolation layer thicknesses, but other than thatthe pulse length is fixed throughout the design. By using thecatalyst as the pulsing device it also now becomes a dynamiccomponent in the reaction instead of remaining a passive element.

2.2. Pulsed electronics

Creating the temperature pulse in the setup only requires theautomated application of a high voltage to the platinum strip whichgenerates short energy bursts with a high current. A commonsolution to create bursts of energy is to store energy in a capaci-tance and then discharge this using an RLC circuit when needed.The simplified electrical schematic for this method is shown inFig. 5, in which the resistance R represents the platinum strip.

To analyze the circuit of Fig. 5, the initial conditions are such thatthe initial current in the inductance is zero, and the initial voltageacross the capacitance is Vco. Using elementary circuit theory(Nilsson and Riedel [15]) equation (1) is derived, which describesthe voltage over the platinum strip VR as a function of the initialcapacitance voltage Vco in the laplace domain.

VR ¼ sR=Ls2 þ sR=Lþ 1=LC

VC0 (1)

Equation (1) represents a standard second order system. Thetransfer function’s denominator defines the time domain behavior.In particular, this type of second order systems are often interpretedto have a quality factor Q (dimensionless), and a natural frequency

L

RC

Fig. 5. Electrical schematic of basic pulsing circuit. The platinum strip is represented asa resistance.

J. Stolte et al. / Applied Thermal Engineering 57 (2013) 180e187 183

u0[1/s]. The natural frequency determines the frequency at whichthe second order system oscillates, while the quality factor isroughly equal to the amount of periods before the system comes toa rest. Equation (2) equates the standard second order denominatorfor these quantities to the denominator of 1.

s2 þ su0

Qþ u2

0 ¼ s2 þ sR=Lþ 1=LC (2)

Equation (2) can be solved for u0 and Q, and their solutions aregiven by 3 and 4 respectively.

u0 ¼ffiffiffiffiffiffi1LC

r(3)

Q ¼ 1R

ffiffiffiLC

r(4)

As this application is set up for single pulse operation, werequire a simple damped pulse which completely discharges thecapacitor. 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 lowerquality factor (overdamped) will result in a more slowly varyingpulse hence a very slow release of energy.

The averaged measured resistance of the platinum strip appliedin the setup was 12.7 � 0.2U (some variation within the batch ofchips) at room temperature. With a capacitance of 1.5 mF and aninductance of 47 mH the quality factor is just below 0.5, witha natural frequency of 120 kHz. Fig. 6 shows a comparison betweenthe solution to equation (1) and the experimentally measuredvoltage at the platinum strip terminals for an initial capacitancevoltage of 350 V.

Considering the degree of the approximation of equation (1), thematch between simulation and practice is reasonable. In particular,the peak location and the general shape of the curve correspondwell to that of the simulation. Several important effects have beenneglected in equation (1). For instance, the resistivity of the plat-inum strip is taken to be constant while in practice it depends ontemperature. Furthermore, the impedance of the wiring from theelectronics to the platinum strip and the connectors are neglected.The inductance used in this setup is specified to work up to 14 A, athigher currents, the iron saturates however, which introducesa nonlinearity. In practice, the current can go up to 80 A for an initialcapacitance voltage of 1 kV. If saturation of the inductance becomesa problem, it can be solved by using multiple inductances inparallel. This adaptation was not required for the measurements inthis thesis.

MeasuredSimulated

Vca

t[V

]

time [s] × 10− 50 2 4 6 8 10

0

50

100

150

200

250

300

Fig. 6. Voltage over the catalytic strip for 350 V initial capacitance voltage.

The RLC circuit is built into a 19 inch rack, together with a 1 kV,100 mA source by Applied Kilovolts (HW001PCP). Insulated gatebipolar transistors (IGBTs) are used to switch the capacitancebetween the source and the platinum strip, since they havea desired small maximum voltage drop of approximately 2 V whenconducting. Therefore IGBTs cause relatively low energy losseswhen the current is high (i.e. during the pulse). The maximumrepetition frequency is set to 100 Hz such that there is alwaysenough time for the platinum strip to cool down to nominalconditions and for the capacitance to recharge.

High voltage cabling connected the electronics to the wafer.Voltage and current limits were controllable through an analoginput to the hardware board. The measured voltage and current areavailable through analog outputs. The switching of the IGBTs iscontrolled by digital inputs using special designed electronics.These analog and digital lines are connected to the PCI 6030 boardin the LabVIEW PC.

2.3. Measurement and data collection

A flexible but sensitive measurement device is needed foranalysis of the effects of temperature pulses on the chemicalreactions in the reactor. For this purpose the Pfeiffer Omnistar GSD320, with a quadrupole mass spectrometer is used. The Omnistarconsists of the mass spectrometer itself, integrated with therequired pumps, valves and a heated capillary. The mass spec-trometer has a range of 200 atomic mass units (amu). The Omnistarcontinuously takes in a small flow of gas through its capillary, fromwhich the mass spectrometer analyzes the mass spectrum. Byrepeated sampling, the presence of molecules of a certain mass canbe tracked over time. The capillary of the mass spectrometer isplaced right at the flow exit of the reactor, such that the responsetime of the measurement is minimized. Because it is placed as closeto the reactor as possible, the response time of the mass spec-trometer is in the second range. However, the pulsed phenomenaoccur at microsecond time scale. Therefore it is fundamentallyimpossible to use the mass spectrometer to track intra pulsebehavior. Effectively, the measurement reflects the cumulative (lowpass filtered) effect of multiple pulses on the total conversion ofspecies throughout the reactor. If conditions are similar throughoutthe reactor, the contribution of individual pulses can be computedfrom the cumulative conversion.

In addition to the components described above, the setupincludes a temperature controller and a DC heat supply. Thetemperature controller is installed to keep the temperature of thereactor at the base regime. A thermocouple positioned just belowthewafer is used tomeasure the temperature as close as possible tothe surface. The tip of the thermocouple is placed at 0.5 mm belowthe bottom of the wafer, which is approximately 1 mm below theplatinum strip. The temperature controller provides the powerneeded to the heating rods which are positioned in the middle ofthe 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. Thisheat quickly spreads into the supporting wafer and from there intothe bulk of the reactor. This means that over the course of manypulses there is a slight elevation of temperature in the platinum andthewafer as compared to the casewhen no heat is added directly tothe platinum strip. Specifically, when significant power is added tothe platinum strip, the strip and the wafer can be expected to havea slightly higher temperature than the reactor base.

Differences in the steady state thermal energy distribution canhave an effect on the chemical reactions on the surface. The focus ofthis work is on the direct effect of pulsed heating only, and there-fore this change in steady state heat distribution is unwanted.

Table 1Operational parameters for CO oxidation reaction.

Parameter Value

Pco 3 kPaPO2

20 kPaPAr 180 kPaf/V 0.4 1/sT0 150e210 �C

norm

aliz

edC

O2

prod

uctio

n

Pulse amplitude [mJ] Temperature [οC]150

165180

195210

300250

200150

10050

0

0

10

20

30

40

Fig. 8. Normalized CO conversion as function of pulse amplitude.

J. Stolte et al. / Applied Thermal Engineering 57 (2013) 180e187184

Hence, an additional DC voltage source is included in the setup tocompensate for undesired differences in the steady state heatdistribution. The DC source guarantees that the time averagedamount of energy added to the platinum strip is constantthroughout an experiment. When a high amount of energy is addedthrough pulses the energy added with the DC source is small andvice versa. This way the average heat distribution is kept constantand any observed effects can be uniquely attributed to the hightemperature of the pulses. For this DC compensation, a DeltaElektronika SM 70-AR-24 power source, which can deliver up to70 V at a current of up to 12 A, is used. The output of the source isconnected to the platinum strip through a high voltage diode. Theunit is automated using the PSC-232 extension from Delta Elek-tronika. All the components of the setup are automated andcontrolled via Labview.

3. Experimental results and discussion

3.1. Reaction conditions and performance measures

The CO oxidation experiments for waste removal and in fuel cellexperiments typically use a small fraction of CO and an excess ofoxygen. These conditions were also used here and the specificoperational parameters are shown in Table 1. The total pressure isapproximately 2 atm in order to maintain an overpressure in thereactor so that small leaks do not immediately lead to a large inflowof gas. Argon is used as a carrier gas, and care is taken to stay welloutside the region of explosive concentrations.

In the work of Hansen et al. [10], Jensen et al. [12], the expres-sion given in equation (5) is used to determine the relative rateenhancement F:

F ¼ hrðT0;A; f Þi � rQSSðT0;AÞrQSSðT0;AÞ

(5)

CO

2pr

oduc

tion

(mas

ssp

ec.

data

)

Time [h]

0 5 10 15

× 10− 11

0

1

2

3

4

5

6

Fig. 7. CO2 production at stepwise increase of pulse amplitude versus time.

where <r(T0,A,f)> is the time averaged rate at a given frequency f,and rQSS(T0,A) is the time averaged rate as the frequency approacheszero (quasi steady state (QSS)). Both rates are dependent on thepulse amplitude A and the base reactor temperature T0. The quasisteady state rate is measured by applying a frequency of 10 mHz.However, the experiments performed for pulsed activation methodhave a different goal which is to show that it is indeed possible tocreate conversion through pulsing the catalyst temperature, and toswitch the reaction rate near instantaneously. In this setup it is notpossible to create high catalyst temperatures at low frequencies.Therefore, a different measure is introduced which is called therelative pulse effectiveness h as in equation (6).

h ¼ hrðT0;A;pÞi � rSSðT0ÞrSSðT0Þ

(6)

where <r(T0,A,p)> is the average pulsed reaction rate, dependingon the base temperature T0 in [�C], the pulse amplitude A [J] and theperiod p [s]. A positive value for h indicates a positive effect, anda negative value means that pulses actually decrease the reactionrate. In a perfect pulsed operation experiment, the rate is zerowhenno pulses are applied. Therefore, the relative pulse effectivenesswill be infinity in the ideal case. However, the setup used here is thefirst of its kind and is far from perfect. Therefore, it may be expected

Fig. 9. Normalized pulse effectiveness versus temperature and amplitude.

Fig. 10. Normalized CO conversion per pulse q versus temperature and amplitude.

Fig. 12. Normalized pulse effectiveness versus temperature and frequency.

J. Stolte et al. / Applied Thermal Engineering 57 (2013) 180e187 185

there is some base conversion in the reactor even when no pulsesare applied. Another measure that will be investigated is the rela-tive conversion per pulse q, for which the expression is given inequation (7):

q ¼ hp (7)

The relative conversion per pulse q relates the relative pulseeffectiveness to the pulse period p. It measures the added effect ofone pulse, as compared to the steady state conversion at the samebase reactor temperature. A value of unity for q means that onepulse creates as much product as is created in steady state opera-tion in 1 s.

3.2. Carbon monoxide oxidation with pulsed activation

3.2.1. Pulse amplitudeThe CO2 formation is monitored for a stepwise increase of the

pulse amplitude. Specifically, the pulse energy is increased fromzero to the maximum in six steps. The maximum is defined as thevalue at which the reactor still operates reliably and was found tobe 300 mJ. The pulse frequency is kept constant at 20 Hz. Each newsetting is maintained for 5 min. The total energy applied to theplatinum strip is kept constant over time using the DC source. TheDC source always complements the energy added with pulsing to

Fig. 11. Normalized CO conversion versus temperature and frequency.

6W. In Fig. 7, the raw data from themass spectrometer for the mass44, which represents the CO2 concentration in the reactor, versustime is presented. The recipe explained above is repeated 3 times ata base temperature of 210 C. The repeatability of the experimentscan be clearly distinguished in this plot.

Fig. 8 shows the mass spectrometer CO2 signal for this experi-ment, normalized to the base conversion at no pulses anda temperature of 150 �C. It can be seen that the rate is effectedsignificantly by the temperature pulses, once the pulse energyreaches 200 mJ per pulse. For pulse energies of 150 mJ and lower,the impact is only small at all base temperatures. The rate ofreaction without pulses also increases with reactor temperature.

Evaluation of h and q as discussed in the previous section givesmore 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 ateach temperature. For pulse energies up to 150 mJ the effectivenessof the pulses is near zero as compared to the steady state conver-sion. For pulses of 200 mJ the conversion added by pulsing isaround 0.3 times that of steady state operation at the sametemperature, for 250 mJ and 300 mJ the value quickly rises tobetween 4 and 5 times as effective as steady state operation. Notethat the odd value at 250 mJ amplitude and 180 �C base tempera-ture can be attributed to the mass spectrometer artifact. Clearly,

Fig. 13. Normalized CO conversion per pulse q versus temperature and frequency.

Fig. 14. Pulse effectiveness h measured over time.

J. Stolte et al. / Applied Thermal Engineering 57 (2013) 180e187186

temperature pulses can be used to increase the production of CO2by at least a factor of four at all temperatures within this lowconversion operating range.

Fig. 10 shows the relative conversion per pulse. Since all datawas taken at the same flow rate and at the same pulse frequency, qis identical in shape to h, only the scaling is different. For 300 mJpulses at this frequency, each pulse produces over 20% of the steadystate production per second.

We have established that high energy pulses have the greatestimpact on the observed conversion. The next question is how thevalue for q varies with pulse repetition rate. Is the added conversiona pure per pulse phenomenon, indicating that all the surfaceprocesses are again near their equilibrium? If this is the case,a purely linear dependence of the conversion on pulse frequency isexpected.

3.2.2. Pulse frequencyTo inspect the dependence of the CO2 production on the

frequency of the pulses, the pulse amplitude is fixed at 300 mJ perpulse, and the pulse frequency is varied in six equidistant steps.The maximum frequency applied is 20 Hz, corresponding toa repetition period of 50 ms. This is the same frequency as the onethat was used in the previous experiment. Fig. 11 shows the CO2production, normalized to the steady state value at 150 �C in thisexperiment.

Fig. 11 shows a consistent result. Within this range of parame-ters, the reaction rate always increases with temperature, and therate always increases with pulse frequency. Moreover, the increasewith pulse frequency seems approximately linear.

Fig. 12 shows the relative CO production for this experiment. Inthis figure, it can be seen that by varying the pulse repetition rate,the CO2 production by pulses can be varied up to approximatelyfour times the steady state production for all the temperatures.

In Fig. 13 we can observe that the per pulse production variesslightly in this range, and that it actually increases somewhat withfrequency, especially at the higher temperatures.

3.2.3. Dynamic response of the reaction rateAn 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 parametersare changed are of interest. Fig. 14 shows the value of h asmeasured in the conversion versus pulse frequency experimentat 180 �C.

The pulse parameters are changed every 5 min, and the CO2production 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 overtime as long as the pulsing parameters are not changed. By con-structing a mapping of reaction rate to pulse frequency, it ispossible to create any desired reaction near instantaneously byselecting the corresponding pulse frequency.

4. Conclusions and outlook

We have introduced the pulse activation method and presenteda proof of principle reactor in which temperature pulses arerealized at higher frequencies and amplitudes than have beenreported before. The oxidation of CO over a Pt catalyst has beeninvestigated as a test reaction. It was observed that the higher thepulse energy the higher the conversion of CO is. Similar observa-tions can also be made in the case of frequency, that is the reactionrate increases with increasing pulse frequency. Most importantly,it was found that the reaction rate can be influenced almostinstantaneously. This means that the reactions can be activated/deactivated at will which is one of the main goals of the pulsedactivation method.

This study works as a proof of principle for the pulse activationmethod in heterogenous catalysis. Further experiments on differenttest reactions are necessary to derive more detailed conclusionsand a better understanding of mechanisms involved. An importantimprovement toward this will be measurement of surfacetemperature at the time scale of the temperature pulses.

Nomenclature

h relative pulse effectivenessu frequency, HzF relative rate enhancementf/V 1/residence time, 1/sq relative conversion per pulseA pulse amplitude, JC capacitance, Ff frequency, HzL inductance, HP pressure, PaQ quality factorR resistance, ohmr time averaged rateT temperature, �CV voltage, voltp period, s

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