Forced thermal cycling of catalytic reactions: Experiments and modelling

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Catalysis Communications 8 (2007) 1985–1990 www.elsevier.com/locate/catcom

Forced thermal cycling of catalytic reactions: Experiments and modelling Søren Jensen a, Jakob L. Olsen b, Sune Thorsteinsson a, Ole Hansen a, Ulrich J. Quaade a

b,*

MIC – Department of Micro and Nanotechnology, Technical University of Denmark, 2800 Kgs. Lyngby, Denmark b Department of Physics, NanoDTU, Center for Individual Nanoparticle Functionality (CINF), Technical University of Denmark, 2800 Kgs. Lyngby, Denmark Received 25 January 2007; received in revised form 15 March 2007; accepted 20 March 2007 Available online 30 March 2007

Abstract Recent studies of catalytic reactions subjected to fast forced temperature oscillations have revealed a rate enhancement increasing with temperature oscillation frequency. We present detailed studies of the rate enhancement up to frequencies of 2.5 Hz. A maximum in the rate enhancement is observed at about 1 Hz. A model for the rate enhancement that includes the surface kinetics and the dynamic partial pressure variations in the reactor is introduced. The model predicts a levelling off of the rate enhancement with frequency at about 1 Hz. The experimentally observed decrease above 1 Hz is explained by dynamic thermal limitations of the reactor.  2007 Elsevier B.V. All rights reserved. PACS: 82.65.+r; 82.65.s; 82.40.Bj Keywords: Microreactor; Catalysis; Temperature cycling

1. Introduction Unsteady-state processing has long been recognized as an important way to improve the performance of physical and chemical processes [1], organic [2] as well as inorganic, catalytic as well as non-catalytic [3]. The improved performance can be in the form of stabilization of spontaneously oscillatory systems [4], increase of reaction rates, and improvement of selectivities. The improvements are obtained by periodically varying the reactant concentrations/flow rates [5], pressure [6], or the reaction temperature [7]. For catalytic reactions, the improvements in many cases stem from a periodic removal of inhibiting species that build up on the catalyst surface over time. As an example, periodic variation of the feed composition during ammonia synthesis has resulted in a 5–50 fold production rate increase [8]. *

Corresponding author. E-mail address: [email protected] (U.J. Quaade).

1566-7367/$ - see front matter  2007 Elsevier B.V. All rights reserved. doi:10.1016/j.catcom.2007.03.026

Temperature oscillations affect the time-averaged reaction rate. At low frequencies, the reaction rates during oscillation are equal to the ones found during steady state operation at the same temperatures, and the time-averaged rate is the so-called quasi steady-state rate, rQSS. At very high (MHz) frequencies, where the characteristic time constants for the surface processes are larger than the oscillation period, the adsorbed species on the surface will only experience the average temperature of the oscillation, and the time-averaged rate is the so-called relaxed steady-state rate, rRSS. Depending on the exact nature of the reaction, there are possibilities for rate enhancement at intermediate frequencies. By numerical solution of a Langmuir–Hinschelwood reaction scheme for a catalytic system subjected to forced temperature oscillations, rate enhancement ascribed to dissimilar adsorption/desorption dynamics of the species involved has been observed for a certain range of kinetic parameters [9]. The effects of temperature oscillations on oxidation of CO over platinum have been studied experimentally in

S. Jensen et al. / Catalysis Communications 8 (2007) 1985–1990

well-controlled UHV experiments on single crystal surfaces [10,11]. The number of direct experimental verifications of rate enhancement at atmospheric pressure has been very limited since the large thermal inertia of typical experimental setups smoothens out the thermal oscillations at frequencies much lower than where rate enhancement sets in. In this situation, the reaction rate is the relaxed steady-state rate, rRSS, although the reason for the species experiencing only the average oscillation temperature is not related to surface chemistry time constants but to system heat transfer time constants. Access to microreactors [12] with lower thermal inertia has increased the practical upper limit of frequencies that can be investigated. Here we use a microreactor to investigate oxidation of CO over a supported Pt/Al2O3 catalyst subjected to a forced temperature oscillation as a model reaction. Rate enhancements have been observed for this system, but no detailed high-frequency dependence has been reported [13,14]. Here, the reaction rate’s dependence on the oscillation frequency is mapped into the regime above 1 Hz, and the frequency spectrum’s dependence on oscillation amplitude investigated. Rate enhancements up to 70% are observed, followed by a decrease at frequencies above 1 Hz. The rate enhancement is confirmed by a physical model, which is solved numerically. The observed highfrequency decrease is attributed to a decrease of the temperature oscillation amplitudes at higher frequencies, which is confirmed by IR measurements. When extended to include amplitude decreases at high frequencies, the model reproduce the data. 2. Experimental In order to carry out investigations of the effect of fast temperature oscillations with large amplitudes, we have fabricated microreactors with integrated mass spectrometer interfaces, heaters and temperature sensors. As example of the reactor performance, temperature oscillations with an amplitude of 32 C and frequencies exceeding 1 Hz – a temperature rate of change of more than 150 C/s – have been realized. The microreactor chip, shown in Fig. 1, measures 0.9 mm by 16 mm by 20 mm. The structural (reaction chamber, in- and out let, and mass spectrometer interface) and functional (thermometer and heater) parts are realized on a silicon substrate, which is sealed by a transparent Pyrex lid. The details of the fabrication protocol are given in [15]. The lateral distance from the heater element to the thermometer is 100 lm, corresponding to only millisecond heat transfer delays. The reactor chamber is separated from the heater by approximately 200 nm silicon dioxide and 30 lm of silicon, providing even shorter heat transfer times. 2.1. Thermal reactor characterization

Front

Rear MS Interface Inlets

Thermometer contacts

Outlet

Reaction chamber Heater 10 mm 1.5 mm Pyrex 320 μm

30 μm Si

200 nm SiO2

100 μm

200 nm NiSi

Fig. 1. The reactor seen from the front (left) and rear (right). The crosssection at the bottom is taken at the dashed line in the left picture.

V ðtÞ ¼ V 0 þ v sinð2pftÞ;

ð1Þ

across the heater element, where t is time, V0 the dc offset, v the ac amplitude, and f the oscillation frequency. The reactor temperature is approximately proportional to the dissipated power, but since v  V0, the contribution at twice the drive frequency is very small, and we expect the temperature to vary around an offset temperature according to T ðtÞ ¼ T 0 þ A sinð2pftÞ;

ð2Þ

where T0 is the offset temperature, and A the temperature oscillation amplitude. The sine shape of the temperature oscillation has been confirmed experimentally as shown in Fig. 2. 220 210 200 Temperature (°C)

1986

190 180 170 160 150 140 0

0.5

1

1.5

2

2.5

Time (s)

During the experiments, the reactor temperature is varied by applying a sinusoidally varying voltage,

Fig. 2. Example of a measured temperature oscillation () together with a sine function (solid line). There is a good correspondence between the two.

S. Jensen et al. / Catalysis Communications 8 (2007) 1985–1990

The temperature in the reactor is measured by measuring the four-point resistance of the NiSi thermometer. The resistance of NiSi varies linearly with temperature, and the relation has been calibrated by furnace experiments [15]. The temperature is recorded using LabView software capable of sampling up to 14 times/s. The applied voltages are adjusted manually, and it is possible to realize temperature oscillations within a precision of 0.5 C. 2.2. IR investigations Prior to the chemical experiment, the reactor’s response to applied temperature oscillations is investigated using an infrared camera. The camera measures thermal radiation in the wavelength interval 7.5 lm < k < 13 lm at a frame rate of 50 s1. Since the spectral transmissivity of the Pyrex lid is zero for k > 5 lm it is not possible to image the reactor from the top, and measurements are therefore performed on the rear of the reactor. A temperature oscillation with an amplitude of 10 C and an offset of 180 C, as measured using the middle third of the thermometer, is applied, and IR image sequences are recorded at frequencies of 0.01 Hz, 0.1 Hz and 1 Hz. The thermometer measurement is monitored at all frequencies. The recorded images show that the average temperature over the middle third of the thermometer oscillates as expected. At measurement points 1 mm away from the heater, we note up to 50% amplitude loss and phase lags on the order of 0.1 s at 1 Hz. The reactor bottom is much closer to the heater than 1 mm, and the effects inside the reactor are expected to be less severe. However, since the thermal contact from the reactor bottom to the catalyst support is less than perfect, we still expect to see some reduction of the oscillation amplitude. 2.3. Catalytic reaction For investigation of the effects of fast temperature cycling of catalytic reactions, we study oxidation of CO over a supported Pt/Al2O3 catalyst [14], 1 CO þ O2 ! CO2 ; 2

ð3Þ

as a model reaction. The feed gas used is a stoichiometric mix of 5% CO and 2.5% O2. In addition, the gas contains 2.5% Ar for calibration, since the flow rate through the mass spectrometer interface is temperature dependent [16], and the carrier gas is He. With flow rates around 1 mL/min, a diffusion coefficient of 6 · 105 m2/s, and reactor dimensions given in Fig. 1, the reactor approaches plug flow behaviour with a Bodenstein number between 5 and 15, depending on the amount of catalyst occupying the reactor volume. The output gas is analyzed using a mass spectrometer. Blind-activity measurements in an empty reactor and in one with the support matrix without catalyst show no conversion.

1987

Since the reaction rate depends exponentially on temperature, the mean reaction rate during an oscillation period will always be larger than the reaction rate at the mean temperature of the period. To measure reaction rate increases beyond this trivial increase, we calculate a reaction rate enhancement, U¼

hrðT 0 ; A; f Þi  rQSS ðT 0 ; AÞ ; rQSS ðT 0 ; AÞ

ð4Þ

between the measured, time-averaged reaction rate at the frequency in question, f, and the quasi steady-state rate, rQSS(T0, A). In practice, rQSS(T0, A) is obtained as the time-averaged rate measured at frequencies where the reactor follows steady-state behaviour, typically 10–20 mHz, and with the same amplitude and offset as the fast oscillation. The measured data is subject to uncertainty due to drift in the system and random fluctuations of e.g. flow controllers. These effects are minimized by letting the system warm up and by averaging the measured concentrations over time. By repeated measurements at different constant parameter settings, the uncertainty is estimated to be less than 5% in total. 3. Results The frequency dependence of the reaction rate enhancement has been investigated for a number of different parameter settings [15]. Variations of gas flow rate and temperature oscillation offset do not change the shape of the frequency spectrum curve unless they are accompanied by substantial changes in the CO conversion. Variations of the oscillation amplitude have a large effect at low conversion, but only a limited effect at high conversions (>30%). The current investigations are therefore limited to studies of the frequency spectrum as a function of oscillation amplitude at low conversions (<12%). The experiments are carried out using a setup similar to the one described in [15]. The reaction rate enhancement as a function of frequency for different oscillation amplitudes is shown in Fig. 3. The measured rate enhancement is zero at low frequencies, starts to increase around 30 mHz, peaks at 0.8 Hz, whereafter it decreases. This general behaviour is observed for all oscillation amplitudes. As the amplitude is increased from 5 C to 20 C, the rate enhancement peak level increases gradually from less than 0.2 to 0.7, where the time-averaged reaction rate is 70% higher than in quasi steady state. There are no signs of any saturation of the peak level within the investigated interval. Investigations at higher amplitudes are limited by the reactor integrity. A similar investigation was carried out at an offset of 180 C, with a conversion at 1 Hz around 51%, and here no increase of the maximum rate enhancement with amplitude was observed [15]. The increase of the reaction rate enhancement peak level with amplitude thus seems to be limited by the conversion.

1988

S. Jensen et al. / Catalysis Communications 8 (2007) 1985–1990 1 5°C 10°C 15°C 20°C

Rate Enhancement

0.8

0.6 Offset: 160°C Flow: 0.975 mL/min 0.4

0.2

0 —2 10

—1

0

10

10

1

10

Frequency (Hz)

Fig. 3. Reaction rate enhancement as a function of frequency for different amplitudes. The solid lines represent data obtained from numerical solution of a model.

The observed rate enhancement decrease at higher frequencies could easily be due to earlier mentioned difficulties in maintaining the temperature oscillation amplitude inside the reactor as the frequency is increased. Further investigation of the rate enhancement behaviour at higher frequencies is therefore performed using numerical solution of a physical model. 4. Modelling Supplementary to the measurements of rate enhancement, a simple model is developed. Only little theoretical work on forced temperature oscillations is reported in the literature. In [9] a general Langmuir–Hinschelwood reaction mechanism is simulated with periodic oscillation of the temperature. In some ranges of the kinetic parameters and frequencies, deviation from steady state behaviour, including rate enhancement, is found. For CO oxidation the Langmuir–Hinschelwood reaction mechanism is [17]: CO þ  $ CO O2 þ 2 $ 2O 

ð5Þ



CO þ O ! CO2 ; with corresponding equations for the coverages, hCO and hO : dhCO  þ ¼ kþ 1 p CO h  k 1 hCO  k 3 hCO hO dt dhO 2  2 þ ¼ kþ 2 p O2 h  k 2 hO  k 3 hCO hO : dt þ=

ð6Þ

Here k 1 are the forward/backward rate constants for þ= CO-adsorption and k 2 the rate constants for O2 adsorption. The reaction rate is given by r ¼ k þ 3 hCO hO . All the rate constants are temperature dependent, and the temperature is given by T(t) = T0 + A sin (2pft), where T0 is the offset temperature, A the amplitude, and f the frequency. By solving the kinetic equations numerically along the lines of [9],

deviation from steady-state behaviour is found only for frequencies in the kHz range and above, even for the large variation of kinetic parameters found in the literature. Since rate enhancement is experimentally found at frequencies below 1 Hz, another mechanism must be sought. The partial pressures in the reactor vary with the reaction rate and adsorption/desorbtion rates of CO and oxygen. In the following we propose a model that, in addition to the surface kinetics, include these partial pressure variations. The change in the number of CO molecules ;gas depends on the flux into the reactor in the reactor dN CO dt minus the flux out of the reactor: kBJT ðpCO;0  pCO Þ, where pCO is now the time dependent partial pressure in the reactor, pCO,0 the constant inlet partial pressure, J the volumetric flow rate, and kB the Boltzmann constant. The partial pressures are assumed uniform throughout the reactor as ;gas in a continuously stirred tank reactor. Further, dN CO dt þ changes with the reaction rate as Nk 3 hCO hO and with changes in the coverage as N dhdtCO , where N denotes the number of sites in the reactor. Similar arguments hold for oxygen, and the following two equations for determining the partial pressures in the reactor as functions of time are obtained:   V dpCO J dhCO ¼ ðpCO;0  pCO Þ  N þ kþ h h CO O 3 k B T dt kBT dt   V dpO2 J 1 dhO ¼ ðpO2 ;0  pO2 Þ  N þ kþ h h : CO O 3 k B T dt kBT 2 dt ð7Þ To obtain results from the model, kinetic parameters are taken from [17] and reactor parameters from [14]. At frequencies below 10 mHz no deviation from steady state behaviour is observed, and the rate enhancement at a specific temperature oscillation, T(T0, A, f) is calculated as U¼

rave ðT 0 ; A; f Þ  rave ðT 0 ; A; f0 Þ ; rave ðT 0 ; A; f0 Þ

ð8Þ

where f0 is some frequency below 10 mHz. Details about the model and the analysis will be given elsewhere [18]. The model can not be expected to reproduce the measured rate enhancements exactly since the reactor model is based on a continuously stirred tank reactor and the measurements are performed under conditions that approach plug flow. However, looking at Fig. 3, where model predictions for the rate enhancement at four different amplitudes (5 C, 10 C, 15 C, 20 C) are shown, the general trend of increasing rate enhancement in the frequency range 0.01– 1 Hz and the levelling off at about 1 Hz is reproduced. The rate enhancement at high frequencies is mainly an indirect consequence of the CO coverage. As seen in Fig. 4, the CO coverage is low when the temperature is high. At low frequencies this is much more pronounced. Since the barrier for CO2 formation decreases with increasing CO coverage [17] the rate constant actually decreases with temperature at low frequencies and increases with

S. Jensen et al. / Catalysis Communications 8 (2007) 1985–1990

6 —1 Rate Constant (10 s ) CO Coverage

1.0 0.9 0.8

10°C, 5 Hz 20°C, 5 Hz 20°C, 0.01 Hz

T(t)

10

5

0 0

0.5

1 Normalized Time (t/Tp)

1.5

2

Fig. 4. CO coverage, hCO, and rate constant, k þ 3 , as functions of time during three temperature oscillations with three different combinations of amplitude and frequency. The temperature variation is indicated with a dashed line. The time is normalized with the oscillation period time, Tp = 1/f.

0.3 Offset: 160°C Flow: 0.975 mL/min

Rate Enhancement

0.25

0.15 0.1 0.05 0 –2

—1

0

10

tion. Rate enhancement has been observed at frequencies up to 2.5 Hz. The rate enhancement’s dependence on the oscillation frequency has been mapped and the frequency spectrum’s dependence on oscillation amplitude investigated. Rate enhancements up to 70% are observed, followed by a decrease at frequencies larger than 1 Hz. The rate enhancement at sub-Hz frequencies is confirmed by a physical model that includes the surface kinetics and the temporal partial pressure variation inside the reactor. The experimentally observed high-frequency decrease is attributed to a decrease of the temperature oscillation amplitude observed at higher frequencies using an infrared camera. When extended to include such amplitude decreases, the model reproduce the data. Acknowledgements S. Jensen is funded by the Danish Research Council for Technology and Production Sciences (DRCTPS). Center for Individual Nanoparticle Functionality is sponsored by the Danish National Research Foundation. References

0.2

10

1989

10

1

10

Frequency (Hz)

Fig. 5. Rate enhancement as a function of frequency for an amplitude of 10 C and an offset of 160 C. The solid line shows data obtained using a model including a 1 C/Hz amplitude decrease.

temperature at high frequencies. Thus the rate constant changes from being out of phase with the temperature at low frequencies to being in phase at high frequencies. While the model predicts almost constant rate enhancement at frequencies above 1 Hz, the experimental data show a clear decreasing trend. As mentioned earlier, inspections of the reactor with an infrared camera during temperature oscillations show that the amplitude actually decreases with increasing frequency, despite efforts to keep it constant using the integrated temperature sensor. If a decrease of the oscillation amplitude of 1 C/Hz is introduced in the model, a high-frequency decrease of the rate enhancement as observed in the experimental data and shown in Fig. 5 is obtained. 5. Conclusion We have investigated oxidation of CO over a supported Pt/Al2O3 catalyst subjected to a forced temperature oscilla-

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