Ind. Eng. Chem. Res. 1995,34, 2923-2930
2923
KINETICS, CATALYSIS, AND REACTION ENGINEERING Infrared Thermography and Fourier Transform Infrared Spectroscopic Study of CO Oxidation over a Catalyst Washcoat Supported on a Metal Substrate Feng &in and Eduardo E. Wolf* Department of Chemical Engineering, University of Notre Dame, Notre Dame, Indiana 46556
Spatiotemporal oscillations on a RWSiOz washcoat catalyst supported on a n Al disk are studied using infrared thermography to visualize the temperature patterns. The dynamics of the temperature pattern oscillations were analyzed with the empirical eigenfunction method. During self-sustained and forced oscillations, these patterns are characterized by a pulsating oscillation with one hot spot near the reactor inlet. Proper vibration of the 0 2 and CO flow rates when the reaction is near transitions between steady states or in a n oscillatory regime decreases the ignitiodextinction temperature, increases the time average temperature and CO conversion, and dramatically suppresses the self-sustained oscillations. In situ Fourier transform infrared spectroscopic results indicate that feed vibrations prevent the formation and propagation of high CO coverage on the catalyst surface.
Introduction The oxidation of CO on metal catalysts is one of the important reactions in automobile emissions control. Complex dynamic behavior, such as chaotic selfsustained oscillations, occurs during the reaction. Ertl and co-workers found self-sustained oscillations of CO oxidation on different Pt single crystal surfaces under high vacuum conditions and reported a transition from regular oscillations t o a state of deterministic chaos (Ertl, 1990). At atmospheric pressure and on supported catalysts, the oscillations are affected by many factors, such as the catalyst structure, size, and distribution of crystallites, and thermal and gas phase communication. Wolf and co-workers (Kaul and Wolf, 1985; Sant and Wolf, 1988) using selected area FTIR and localized surface temperature measurements, have shown that self-sustained oscillations on Pt and Rh supported catalyst wafers involve spatially propagating regions of nonuniform coverage and surface temperature. Schmitz and co-workers (D'Netto et al., 1984)introduced infrared thermography (IRT) t o observe temperature patterns during the H2-02 reaction on a Pt catalyst. Since then, IRT has become a powerful tool in studies of exothermic catalytic reactions with spatial temperature distributions (Kellow and Wolf, 1990, 1991; Philippou et al., 1991; Lane and Luss, 1993). Oscillations have been successfully suppressed by video-feedback control using IRT t o monitor the surface temperature (&in et al., 1994). A review by Schuth, Henry, and Schmidt (1993) provides an overview of the field of oscillatory reactions. A recent book by Slinko and Jaeger (1994) summarizes the literature of oscillatory catalytic reactions both on single crystals and on supported catalysts. In spite of the extensive studies of oscillations during CO oxidation, the detailed mechanism of this phenomenon on supported catalysts is not fully understood. Recently, electrically heated catalytic converters (EHCs) have been shown effective in reducing automo-
* To whom correspondence
should be addressed.
0888-588519512634-2923$09.00/0
bile cold start emissions, which are responsible for a significant fraction of CO and HC emissions during FTP tests. Metal foils are used in EHCs as a support of the catalytically-active .washcoat as well as an in situ heating element. A previous study (Sant and Wolf, 1988) showed that the communication between catalyst particles affected the reaction dynamics in a catalyst wafer of RWSiOz. Using a metal substrate to support the washcoat may affect the reaction dynamics and oscillations. As a first step to study the dynamics and the characteristics of the CO oxidation on the metal supported catalyst washcoat converter, this paper presents results using the IRT technique to monitor the CO oxidation on a thin RWSiOZ catalyst washcoat layer supported on an aluminum disk. Moreover, the effects of feed vibration on the ignition and extinction behavior of the reaction are also studied in this paper using IRT and in situ FTIR techniques.
Experimental Section Apparatus. The experimental setup for IRT studies is similar to that previously described (Kellow and Wolf, 1991). The main component is a continuous flow reactor made of two flanges with CaF2 windows that allows an IR camera (AGA 782) to image the thermal patterns on the catalyst wafer (2.5 cm). Two heaters, connected to a temperature controller, are used to maintain the temperature of the reactor at a set value. The feed gases enter the reactor from either the bottom left corner or the top right corner. Flow rates are regulated by an on line IBM computer through electronic flow controllers (PF 914). The IR camera measures the infrared radiation emitted from the catalyst surface, point by point, via a scanning optical system. The temperature image thus collected is digitized and stored in an on line IBM personal computer. Each image has 105 x 68 pixels, and the spatial resolution is about 0.24 mm horizontally and 0.37 mm vertically. Real time temperature images are displayed on an on line television monitor and recorded in a videocassette recorder.
0 1995 American Chemical Society
2924 Ind. Eng. Chem. Res., Vol. 34, No. 9, 1995
Transmission Fourier transform infrared spectroscopy (FTIR) studies were conducted during reaction over a thin 5% RWSiO2 wafer without metal support. The same reactor as in the IRT studies was used in the FTIR measurements, but it was placed in the sample compartment of a FTIR spectrometer (Mattson Galaxy 6020) to measure the IR absorbance of CO adsorbed on Rh. The C02 concentration at the reactor exit is measured by an infrared analyzer (Beckman 856). Catalyst. To simulate the washcoat catalysts supported on a metal substrate, a catalyst wafer was made by pressing a thin layer of catalyst powder (RWSiOz) on an aluminum disk. Eighty milligrams of Al powder was pressed at 12000 psia t o form an Al disk of 2.5 cm in diameter. The catalyst washcoat was prepared by impregnating Si02 in powder form with an aqueous solution of Rh(N03)3 until incipient wetness. A proper amount of Rh(N03)3solution and Si02 powder was used such that the Rh content was 5% by weight. After drying, the powder was calcined in 0 2 at 300 “C for 3 h, and reduced in H2 at 350 “C for 6 h. Then, 25 mg of RWSiOz powder was uniformly spread on the Al disk and pressed at 7000 psia t o form the wafer. The catalyst wafer was pretreated in situ by passing 9 mumin 0 2 and 30 mumin N2 at 220 “C for 0.5 h. After flushing in N2, it was reduced again in 17.5 mumin H2 and 22.5 mumin N2 at 230 “C for 6 h. Before the experiments, the reactor was cooled t o 70 “C in N2 flow. In case of the temperature programmed reaction (TPR) experiment, the reactant’s feed flow rate was fixed at 12 mL/min 02,2 mL/min CO, and 90 mumin N2, while the reactor temperature was increased until ignition. Upon reaching an oscillatory regime, the reactor temperature was maintained constant.
Data Reduction by the Empirical Eigenfunction Method To analyze the time series of the two dimensional (105 x 68 pixels per image) temperature images, we applied
the empirical eigenfunction method. This method was used to analyze the coherent structures of jet flow pictures (Sirovich et al., 1990) and of the spatial patterns in catalytic reactions (Chen et al., 1993; Graham et al., 1993; Krischer et al., 1993). The following description briefly outlines the procedure for the purpose of subsequent discussion. A detailed description has been presented elsewhere (Chen and Chang, 1992; Chen et al., 1993). After taking the time average of the IR image time series i
N
and obtaining the deviation variable & ( x y ) = @&y)@(x,y),a two-point spatial correlation function is defined as K(XJ&’J’) = ( J n ( X J )
Jn(X’J’))
=
Using it as a kernel, the following integral eigenvalue problem is obtained:
where {Yi] are termed empirical eigenfunctions and pi are the corresponding eigenvalues. Each image can be approximated with M eigenfunctions and their time dependent coefficients a&)
c M
cPn(XJ)
= &X$)
+
a#,)
Yi(X$)
(4)
i=l
It can also be shown that pi represents the projection of the energy on the Yi base if the energy is defined as
(di2>= ( ( ~ ~ ( x J ) , y i ( x J ) ) ( ~ n ( x $ ) , y i = ( xiuiY ) )( )5 ) where “+)” means taking average over all N . The energy ratio N
e, = pi/>i i=l
gives the probability that the { i n } fall in the direction of empirical eigenfunction Yi. The dynamics of the images thus can be well estimated through the coefficients of the first M eigenfunctions when C r l e i > 0.9. In this work, the empirical eigenfunctions are calculated from about 500 images acquired during oscillations with a 2 s interval between every image. In most of our experiments, M = 1-4 is enough to capture more than 90% of the energy.
Results Self-sustained Oscillations. Large amplitude selfsustained oscillations were observed when the gas flow rates of N2, 0 2 and CO were 90, 12, and 2 cm3/min, respectively. Figure 1shows the temperatyre patterns and the time average temperature image & y ) during self-sustained oscillations at a reactor temperature of 118 “C. In this figure, the time interval between acquiring images is 8 s. The images are characterized by a hot spot located near the lower right corner corresponding to the position of the inlet where the reactants enter a t about 45” into the reactor. During oscillations, the temperature of the whole wafer and that of the hot spot changed, but the shapes of the temperature patterns were similar although there was a small shift of the position of the hot spot. Figure 2a shows the first two empirical eigenfunctions (Yl,Y2)with Y1 containing over 93% and Y2 containing 4% of the total energy (see eq 5). Y1 has only one peak near the reactor inlet while Y2 has a peak on the upper left side and a valley on the lower right. Empirical eigenfunctions of the self-sustained oscillations were calculated from 470 temperature images during 940 s. Projection coefficients (al(t),a&)) of the first two eigenfunctions and the exit C02 concentration are plotted in parts b, c, and d of Figure 2, respectively. The phase space trajectory between al(t)and a&) in Figure 3 indicates that the dynamics of the oscillatory process follow a homoclinic orbit. The projection coefficient of the first eigenfunctional(t)(Figure 2b) is very similar to the CO2 concentration time series (Figure 2d) except for a 10 s time delay between each C02 peak and the corresponding a1 peak. This delay corresponds to the time gap between the acquisition of the IR image, which is an in situ measurement, and the C02 measurement, which is taken downstream of the reactor. Vibrating the Feed in an OscillatoryRegime. It has been observed that a forced oscillation on the inlet
Ind. Eng. Chem. Res., Vol. 34,No. 9,1995 2925
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Figure 1. Twelve temperature images acquired during the self-sustained oscillations and an average temperature image. The arrow indicates the flow direction. The maximum temperature in the average image is 139 "C. The reactor temperature is 118 "C. Feed rates of N2, 0 2 and CO are 90, 12, and 2 cm3/min, respectively. The time interval between images is 8 s.
flow rates of CO and 0 2 suppressed the self-sustained oscillationsby 90%on a Rh/Si02 catalyst wafer (&in and Wolf, 1995). In this work with a catalyst washcoat supported on a metal disk, we obtained a larger suppression when a proper sine wave was superimposed on the 0 2 and CO feed rates as following:
Qco = QEo(l + A sin((2dr)t))
(7)
where Q is the flow rate of the feed gases and Q8, = 12 cm3/min, QN,= 2 cm3/min, and QN, = 90 cm3/min. z is the period and A is the amplitude of the vibration. We set the phase difference of CO and 0 2 flow rates a t n (9 = n). Figure 4a,b shows a dramatic suppression of the CO, oscillations by a feed vibration with z=10 s and A = 10%. The reactor temperature is 118 "C and flow rates are Q& = 12 cm3/min,Q& = 2 cm3/min, and QN,= 90
cm3/min. Figure 4c shows the first two empirical eigenfunctions of residual temperature oscillations after the self-sustained oscillations being suppressed by feed vibration. Four eigenfunctions are required to capture 90% (Y1 73.7%, Y2 8.1%, Y 3 5.1%, Y4 3.3%) of the energy for the suppressed oscillations of the temperature patterns shown in Figure 5. The time averaged temperature image obtained with the vibrating feed (Figure 5) has the same shape as that of the selfsustained oscillations (Figure l), but the temperature of the hot spot is higher. The time average concentration of CO2 rises from the 0.59% corresponding to the stationary feed with self-sustained oscillations, to the 0.86% of the vibrating feed case, increasing by 45%. The increase is not due to an increase in catalytic activity but to the elimination of the low activity states during the oscillations. Vibrating the Feed in a Steady State Regime. When the reactor temperature is higher than 160 "C, a t the same flow rate of 0 2 , CO, and N2 as in the
2926 Ind. Eng. Chem. Res., Vol. 34, No. 9, 1995 Yl
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Figure 4. (a) Exit CO2 concentration at a steady state feed. (b) Exit COz concentration with a vibrating feed (A = 108, t = 10 s). (c) Contour plots of the first two eigenfunctions of the image time series with the vibrating feed. The reactor temperature is 118 "C. Base feed rates are Q8, = 12 cm3/min, Qeo = 2 cm3/min, and Q N ~ = 90 cm3/min.
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a1 Figure 3. Phase space trajectories of the image time series projected on the first two empirical eigenfunctions.
previous section, the reaction is stabilized at a high state and no self-sustained oscillations are observed. Under this high steady state condition, when the flow rates are periodically vibrated, the C02 production and the temperature patterns oscillate periodically with the same period as the external feed vibrations. The dynamics of the temperature patterns is dominated by a pulsating oscillation with the first eigenfunction containing over 90% of the total energy. The shape of the first eigenfunction is very similar to those shown in Figure 2a and Figure 4b. Time average COn production was about the same with or without feed vibrations under this high reactor temperature condition. When the reactor temperature is lower than 105 "C, i.e., in the low steady state before ignition, the reaction stayed in the low state with no response to the feed vibrations. Vibrating the Feed Near Ignition and Extinction
Transitions. To study the effect of the feed vibrations on the dynamic behavior during ignition and extinction of the reaction, the same temperature-ramp experiment was conducted under stationary and vibrating feed conditions. The reactor temperature was increased at a rate of 4 "C/min starting at 104 "C. It was found that feed vibration lowered both the ignition and extinction temperature. In Figure 6a, when a vibration was applied with z = 10 s andA = 0.10 (curve 31, the ignition occurs at 112 "C,i.e., 7 "C lower than without feed vibration (curve 1). Figure 6b shows the C02 concentration with and without vibration during the cooling process of the temperature ramp. When z = 10 and A = 0.1 (curve 31, vibrating the feed results in a 9 "C decrease in the extinction temperature versus the case without vibration (curve 1). To test whether the decrease of ignition and extinction temperature is due t o the effect of the change of the OdCO ratio during the feed vibration, we run another stationary feeding experiment with a 10% increase in 0 2 feed and a 10% decrease in CO feed in reference to the conditions corresponding to curve 1. With the higher OdCO ratio (curve 21, both the ignition and extinction temperature decreased when compared to the lower OdCO ratio case (curve 11, but they were still higher than the feed vibration case (curve 3). In Situ FTIR Study with and without Feed Vibration. In situ FTIR studies were conducted t o relate the C02 oscillations with changes in the CO surface coverage. The reactor temperature was 118 "C and the gas flow rates of N2, 0 2 , and CO were 90, 12, and 2 cm3/min,respectively. The catalyst used is a 5% RWSiOz thin wafer without the metal disk so that transmission FTIR experiments can be performed. The reaction over the RWSiOz wafer with and without the Al support showed similar responses to the feed vibration. Figure 7 shows four spectra taken at a 15 s intervals during the cooling of one large self-sustained oscillation, i.e. in the region where C02 concentration
Ind. Eng. Chem. Res., Vol. 34,No. 9, 1995 2927
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time average image Figure 5. Temperature images and time average image at vibrating feed (A = lo%, t = 10 s). The maximum temperature in the average image is 142 "C.The reaction conditions are the same as Figure 4b. The time interval between two images is 16 s.
decreases. The spectra shows four bands corresponding to three forms of CO adsorbed: dicarbonyl CO (2092, 2029 cm-l), linear CO (2064 cm-l), and bridged CO (-1900 cm-l). The dicarbonyl CO (2095, 2029 cm-l) bands are seen a t both high (curve 1) and lower state (curves 3, 4); however, the linear (2064 cm-l) and bridged CO (-1900 cm-l) bands only appeared a t low steady state. Whenever the linear and bridged CO bands appeared, there was a large decrease in the CO2 band (2363 cm-l) and the cooling process started. When the reignition occurred, the linear and bridge CO (2064 and -1900 cm-l) disappeared immediately with a jump in the CO2 band. Figure 8 shows the CO and CO2 bands under the same reactor conditions as in Figure 7 but with a vibrating feed with z = 10 s and A = 0.1. The intensity of the linear and bridge CO bands is seen a t the highest and lowest values during small range oscillations due to the feed vibration. The spectra at Figure 9 are a closer look a t the CO coverage during a feed vibration experiment in the low steady state before ignition (110 "C reactor temperature). It-shows that there are small amplitude oscillations of the intensity of the linear adsorbed CO band (2064 cm-l). However, when the reactor temperature was higher than 160 "C, i.e., in the high steady state, only the nonreactive CO dicarbonyl bands (2095, 2029 cm-l) are seen and they did not change at all although the CO2 band oscillated with the same frequency as the feed vibration.
Discussion Our previous study has shown the effect of feed vibrations on the CO conversion and temperature pattern oscillations on a Rh/SiO;! wafer when the system was in oscillatory regime (&in and Wolf, 1995). The present paper extends the results to the effects of feed vibrations on ignitiodextinction transitions over a washcoat catalyst supported on a metal substrate. Moreover, in situ FTIR experiments were carried out in this study to provide additional information about the effect of vibrating state feed on this catalytic reaction. The empirical eigenfunctions method provide an effective way to analyze temperature patterns acquired during the self-sustained oscillations. The dominant eigenfunction (Figure 2a, Yl), which contains over 93% energy, shows that the oscillations of the temperature images are characterized by in phase temperature changes over the whole wafer with the largest oscillation a t the hot spot near the reactor inlet. These patterns differ from those observed in a wafer without the metal support (&in and Wolf, 1995) in that the movement of the hot spot during oscillations is less pronounced. The possibility that the hot spot location results from a local nonuniform catalyst distribution was tested by switching the reactor inlet to the outlet. The hot spot moved to the new inlet after the switching which indicates that
2928 Ind. Eng. Chem. Res., Vol. 34,No. 9, 1995
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Figure 8. IR spectra under feed vibration condition. T = 118 "C, A = ~. 10%. t = 10 s. Curves 1 and 2 show the range of oscillations on the CO coverage. ~~
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Figure 6. Exit C02 concentration a t steady feed and vibrating feed during the same reactor temperature ramp. 1.Stationary feed. Q8, = 12 mumin, Qeo = 2 mumin, QN, = 90 mumin. 2. Stationary feed. 88, = 13.2 mumin, Qeo = 1.8 mL/min, QN,= 90 mumin. 3. Vibrating feed. Q8, = 12 mumin, 860= 2 mumin, Q N ~= 90 mumin, A = lo%, 5 = 10s.
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Wave number (cm-1) Figure 7. Four spectra taken a t a 15 s interval during the cooling process of self-sustained oscillations starting from the high steady state (curve 1).The reactor temperature was 118 "C and gas flow rates of Nz, 0 2 , and CO were 90, 12, and 2 cm3/min, respectively.
the spatial patterns observed are the results of the coupling of catalytic activity with heat transfer, mass transfer, and reactor geometry. Since 93%of the energy is captured using only the first eigenfunction (Y1) to estimate each image as
4,(xy) = &xy) + a,(t,) Yu,(xy) analyzing the al(t)coefficient time series will get most
Figure 9. IR spectra under feed vibration condition before ignition. T = 110 "C, A = 1070,t= 10 s. Curves 1 and 2 show the range of oscillations on the CO coverage.
of the relevant information about temperature image oscillations with a data compression ratio of 8192:l. Both the al(t) and CO2 time series show that the oscillations consist of two main processes: (i) a partial extinction from a high state t o a lower intermediate state along with a cooling of the surface and decrease in C02 production, and (ii)a reignition process from the lower intermediate state to a high state with an increase in the temperature and COz concentration. The key to understanding the oscillatory process is to determine what triggers the reignition process. The second eigenfunction (Y2)shown in Figure 2 with a small peak on the upper left and a valley on the lower right corresponds to an out of phase temperature change of these two regions. Two thermocouples located near the peak and valley respectively might record an out of
Ind. Eng. Chem. Res., Vol. 34, No. 9, 1995 2929 phase change when a&) changes with time much larger than al(t). Out of phase changes of temperature at different points on Pt/SiOz and Rh/SiOz wafers have been previously observed during the oscillations (Kaul and Wolf, 1985; Sant and Wolf, 1988). For this washcoat catalyst, it is expected that out of phase changes would be overwhelmed by the in phase change represented by Y1 since Y2 only contains about 4% of the total energy. However, Y2 is important during the reignition because there are very sharp spikes in the plot of az(t) when such transitions occur (Figure 2c). Temperature images (Figure 1,panels 9, 10, 11)show that reignition consists of two steps: a local expansion of the hot spot (panels 9 and 10); and a movement of the hot spot toward the inlet (panels 10 and 11). Thus it is clear that the shape of Y2 and sharp spikes in a2(t) are caused by the movement of hot spot which cannot be captured by the main eigenfunction (Y1) only. The reconstructed phase space trajectory (Figure 3) is more informative about the dynamics with 97% of the energy being captured. Low trajectories in Figure 3, corresponding to changes in al(t) with a small change in &), represent the cooling of the surface from high steady state to a lower intermediate state. It shows that the cooling processes for all the oscillations follow a common temperature pattern changes. The reignitions from the lower intermediate state t o high steady state correspond t o changes in al(t) and a2(t) and follow trajectories of variable sizes which start in different sections of the cooling trajectory. The longer the cooling, the larger the reignition trajectory, and the larger amplitude in the C02 oscillation. It is thus clear that the differences in the amplitude of the self-sustained oscillations are determined by the extent of the cooling process or conversely by the point and conditions where reignitions occur, stopping the cooling process. While there are some similarities in the long-lasting self-sustained oscillations of the C 0 2 concentration between the catalyst on the metal support and the catalyst wafer without an Al disk, the movement of the temperature patterns is less pronounced in the latter case. The temperature images for the self-supported Rh/SiOz catalyst wafer changed more actively including a pronounced movement of the hot spot during the reignition process of the oscillations (&in and Wolf, 1995). For this metal-supported washcoat layer, the position of the hot spot changes very little during the oscillations as shown in Figure 1. This indicates that the Al disk provides a more uniform temperature environment to the catalyst washcoat which synchronizes the temperature patterns better than in a selfsupported Rh/SiOz catalyst wafer. The effect of vibrating the feed in suppressing oscillations is rather remarkable with an increase in time average CO conversion as shown in Figure 4. Comparison of the temperature images in Figure 5 with those in Figure 1 shows that the increase in CO2 production is also accompanied by a higher catalyst temperature. The first eigenfunction (Figure 4c, Y1, 73.7% energy) of the suppressed oscillations corresponds t o the in phase temperature changes on the wafer with the hot spot being the most active region. The similarity between the shape of the dominant eigenfunction with (Figure 4c, Y1) or without feed vibration (Figure 2a, "1) indicates that feed vibration does not alter the main character of temperature pattern oscillations although the oscillation amplitude has been dramatically suppressed. However, the second eigenfunction changes its
symmetry after starting feed vibration. Thus the feed vibration alters the small movement of the hot spot which occurs during the reignition under stationary feed (Figure 1,panels 9-11). FTIR studies (Figure 7) showed much higher coverage of linearly adsorbed CO in the low state than in the high state during the oscillations. It is known that high CO coverage inhibits the adsorption of oxygen and thus it decrease the reaction rate. During the cooling period of the oscillations, the FTIR results clearly show that CO2 production decreases as linear and bridged CO bands appeared. At the reignition, the linear and bridged CO peak disappeared immediately. After application of a fast (t < 20 s) feed vibration, the 2064 and -1900 cm-' bands do not increase. This implies that a fast periodical vibration in the gas phase concentration prevents the accumulation and propagation of high CO coverage on the surface, particularly the linear and bridged adsorbed CO, that leads t o the low steady state. In the oscillatory regime, a local increase in the CO surface coverage decreases the local reaction rate, inducing an increase in the local gas phase CO concentration and leading to further increase and propagation of CO coverage. The process results in extinction of the reaction if the temperature decreases sufficiently low. The reignition process, the nature of which is not known, stops this trend and returns the reaction to the high steady state. It is clear that vibrating the feed prevents the propagation of regions of high CO coverage and keeps the reaction vibrating at around the high state. The frequency of the feed vibration is related t o the rate of the propagation of CO concentration and may vary with the type of catalysts used. When operating a t the high steady state a t high reactor temperature (> 160 "C), feed vibrations induced C 0 2 oscillations in phase with vibrations of CO flow rate. The linear and bridge CO bands (2064 and 1900 cm-l) were not observed whereas the unreactive carbonyl bands (2095,2025 cm-l) were unchanged as the C 0 2 band oscillated following the feed vibration under high reactor temperature. This indicated that under these conditions the CO coverage was sufficiently low not to detect the linear and bridge bands. The decrease of ignition and extinction temperature (Figure 6) caused by the feed vibration indicates that the reactor performance can be improved by using external feed vibration. An earlier ignition may result in lower cold start emissions in an automobile catalytic converter. IRT studies shows that ignition starts from a point and quickly propagates to the whole wafer. Since the surface is covered by CO at the low steady state, one necessary condition for a local ignition is the desorption of CO and adsorption of oxygen. A decrease in the CO/O2 ratio in the feed will help the CO desorption and decrease the ignition temperature. The vibrating feed induced a periodical change of the CO/Oz ratio in a range of [COmin/O2max,CO,ax/O2minl SO the ignition can occur at a lower temperature than at a fixed C0/02 ratio. Thus it is easy to understand the observation that ignition temperature decreased with lower CO/O;?ratio under stationary feed cases (Figure 6, curve 2). However, this cannot explain the further decrease of the ignition temperature by feed vibration when comparing with the stationary feed at a COmin/Ozmax ratio (Figure 6, curves 2 and 3). This further decrease in ignition temperature has to be some effect of unsteady state at the surface level which does not correlate with the changes in the gas phase. FTIR study of feed vibration
2930 Ind. Eng. Chem. Res., Vol. 34, No. 9, 1995
before the ignition (Figure 9) shows that there are small oscillations in the intensity of the linear CO band which correlate with changes in the gas phase. Thus, near the ignition point when the surface coverage is changing, the oscillations in the CO surface coverage caused by feed vibration facilitates more CO desorption and adsorptiodpropagation of oxygen than under stationary conditions. The decrease of extinction temperature by feed vibration is understandable in light of the previous discussion regarding feed vibration preventing the cooling process during self-sustained oscillations and keeping the reaction at high state. The extinction process is caused, again, by the formation of high CO coverage on the catalyst surface. When a feed vibration is superimposed, forced oscillations in the gas phase prevent the gradual formation of high CO coverage on the catalyst surface, thus requiring a lower temperature for extinction. The FTIR results do not have the spatial resolution of the IRT results; thus, unfortunately, we cannot study in the same detail the propagation of the surface coverage with the temperature propagation and with the frequency of CO vibrations in the feed. To solve the riddle of oscillatory reactions in supported catalysts, there is a need to develop a spectroscopic techniques with spatial resolution. It is also interesting that vibrations in the feed can alter the dynamic of the reaction near transition between steady states. This presents opportunities for increasing catalyst performance using unsteady state operations.
Conclusions As in the case of a self-supported Rh/SiOz wafer, selfsustained oscillations are observed on a washcoat Rh/ Si02 catalyst supported on an Al disk. The C02 time series during oscillations are similar for both catalysts; however, the temperature patterns in the latter case do not move as much as in the former. Proper feed vibrations near transitions between steady states lead to a decrease of the ignition and extinction temperatures, suppression of the oscillations, and an increase of CO conversion. In situ FTIR study suggests that vibrating the feed prevents the formation and propagation of high CO coverage keeping the reaction near the high steady state at a lower temperature.
Acknowledgment The financial support of NSF Grant CTS 92:00210 is gratefully acknowledged.
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Received for review April 7, 1995 Accepted May 5, 1995 @
I39402372
Abstract published in Advance A C S Abstracts, August 1, 1995. @
