Al2O3 Catalyst: Reaction Kinetics and

Bifurcation Analysis on Pt and Ir for the Reduction of NO by CO. Dinesh Mantri , Viral Mehta , Preeti Aghalayam. The Canadian Journal of Chemical Engi...
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Ind. Eng. Chem. Res. 1997, 36, 4609-4619

4609

NO Reduction by CO over a Pt/Al2O3 Catalyst: Reaction Kinetics and Experimental Bifurcation Behavior Ramakant R. Sadhankar† and David T. Lynch* Department of Chemical and Materials Engineering, University of Alberta, Edmonton, Alberta, Canada T6G 2G6

The kinetic behavior of NO reduction by CO over a Pt/Al2O3 catalyst has been investigated in the temperature range of 465-520 K. The NO + CO reaction exhibits isothermal steady-state multiplicity. A high-conversion steady state was obtained starting with a net oxidizing feed composition ([NO]0 > [CO]0), and a low-conversion steady state was obtained for a net reducing initial feed composition ([NO]0 < [CO]0). A significant amount of N2O was observed as a reaction product, particularly for the high-conversion steady states. The selectivity toward N2O decreased with increasing NO conversion. For the high-conversion steady states, the N2O selectivity decreased rapidly with a decrease in the feed [NO]0/[CO]0 ratio below a critical value of 1.5. The N2O + CO reaction was found to occur to a significant extent. The steady-state bifurcation behavior has been used to discriminate among three kinetic models. Increasingly stringent regulations on NOx emissions from automobile exhaust have led to several studies on the reduction of nitric oxide (NO) over noble metal catalysts in the last 2 decades. In automobile catalytic converters, NO is reduced by CO and/or H2 in the presence of water vapor over supported catalysts containing noble metals such as platinum, palladium, and rhodium (Taylor, 1984). The reaction between NO and CO has been shown to exhibit very complex behavior. Several studies have reported self-sustained rate oscillations for the NO + CO reaction, both on crystalline platinum (Adlhoch and Lintz, 1976, 1981; Fink et al., 1991a; Schwartz and Schmidt, 1987, 1988) as well as on supported platinum catalysts (Schu¨th and Wicke, 1989). In addition, the reaction has also been shown to exhibit reaction rate multiplicity on crystalline platinum surfaces (Bolten et al., 1985; Fink et al., 1991a; Schwartz and Schmidt, 1987, 1988). Temperatureprogrammed reaction studies (Fink et al., 1990, 1991b; Lesley and Schmidt, 1985) have revealed the “explosive” nature of the reaction from the rapid evolution of the CO2 and N2 products in an extremely narrow temperature range. Forced composition cycling studies (Sadhankar and Lynch, 1996a,b) have shown long-term transients and resonant behavior. The mechanism underlying these complex phenomena cannot be explained by simple Langmuir-Hinshelwood kinetics. An additional difficulty in understanding the reaction mechanism arises from the fact that nitrous oxide (N2O) has been observed as a reaction byproduct in the overall NO + CO reaction on platinum, particularly at temperatures below 580 K (Shelef and Otto, 1968; Klein et al., 1985; Kudo et al., 1990; Muraki and Fujitani, 1986; Sadhankar and Lynch, 1996b). At a typical operating temperature of approximately 800 K in an automobile catalytic converter, the overall * Author to whom correspondence should be addressed. Phone: (403) 492-3596. Fax: (403) 492-0500. E-mail: [email protected]. † Current address: Chemical Engineering Branch, Chalk River Laboratories, Atomic Energy of Canada Ltd., Chalk River, Ontario, Canada K0J 1J0. S0888-5885(97)00138-3 CCC: $14.00

NO + CO reaction is represented by

2CO + 2NO f 2CO2 + N2

(1)

The formation of N2O at lower temperatures can be represented by

CO + 2NO f CO2 + N2O

(2)

It has been suggested by Cho et al. (1989) that the N2O formed by the above reaction is subsequently reduced by CO according to

CO + N2O f CO2 + N2

(3)

The N2O + CO reaction (eq 3) is thus considered to be an important subset of the overall NO + CO reaction. Because of its importance in the NO + CO reaction, the N2O + CO reaction over supported platinum catalyst has been studied previously (Sadhankar et al., 1994), and it has been shown that the steady-state multiplicity behavior of the N2O + CO reaction could be used to discriminate among several rival mechanisms. Therefore, steady-state bifurcation information could also be useful in the study of the NO + CO reaction. In this study, it was found that the NO + CO reaction exhibits steady-state multiplicity in the temperature range of 465-520 K. A significant amount of N2O was also observed, particularly for the high-conversion steady states obtained for the feed containing excess NO. The steady-state bifurcation behavior has been used to discriminate among three kinetic models. Experimental Methods The experiments were carried out using a recycle reactor containing 20 g of 0.5 wt % Pt/γ-Al2O3 catalyst supplied by Engelhard. The equipment was the same as previously described by Sadhankar et al. (1994) and Sadhankar and Lynch (1996a,b). The recycle pump delivered a recycle flow of 600 cm3‚s-1. The catalyst used in this study was taken from the same batch of catalyst that was used previously for the study of the N2O + CO reaction (Sadhankar et al., 1994). The gas mixtures were purchased from Linde and © 1997 American Chemical Society

4610 Ind. Eng. Chem. Res., Vol. 36, No. 11, 1997

included mixtures of 1.99%, 2.02%, or 4.97% CO in N2 and 2.04%, 5.09%, or 5.13% NO in N2. Prepurified N2 was used to makeup the total flow to the reactor, which was always maintained at 185 cm3(STP)‚min.-1 The catalyst was pretreated for 2 h using a gas mixture of 2% N2O in N2 before the start of each experiment. The reactor pressure was always maintained at 103 kPa. The experiments were carried out under isothermal conditions at four different temperatures of 465, 485, 505, and 520 K, respectively. The temperature was measured by four type J thermocouples inserted into the reactor, and the resistance heater was controlled by a Barber Coleman Model 520 solid-state controller which maintained the reactor temperature within (1 K. The maximum recorded temperature difference across the reactor was 2 K. At the operating conditions, the recycle ratio was calculated to be in the range of 103-116. The methods used were the same as those described previously for the N2O + CO reaction system (Sadhankar et al., 1994). The experiments were carried out in the same temperature range (465-520 K) as that used for the earlier study of the N2O + CO reaction to study the bifurcation behavior. It was also necessary to do the experiments in the selected temperature range in order to study the role of N2O formation in the NO + CO reaction at low temperatures. At each of the three temperatures, namely, 465, 485, and 505 K, the steadystate measurements were made following stepwise increases or decreases in the feed CO concentration while holding the NO concentration in the feed constant. At each of these three temperatures, the experiments were performed using three different values of the feed NO concentration, namely, 0.45%, 0.7%, and 1.2%. This results in nine sets of experimental data for the steadystate bifurcation measurements. The steady-state measurements were made after 4 h following a step change in the feed CO concentration. In the last set of experiments carried out at 520 K, the feed CO concentration was held constant while the feed NO concentration was varied in a stepwise fashion. Four different sets of experiments were carried out at 520 K using four different feed CO concentrations of 0.235%, 0.31%, 0.435%, and 0.685%, respectively. Thus, in total, 13 sets of experiments, each comprised of 2 subsets of experiments corresponding to the stepwise increase and the stepwise decrease, respectively, of the concentration of the variable component (NO or CO) in the feed, were carried out in the temperature range of 465-520 K. Results and Discussion NO Conversion. NO conversion was calculated from the measured values of the NO concentration in the reactor and the calculated values of NO concentration in the feed. The feed NO concentration can be calculated from the flow rate of NO. It can also be calculated from the measured concentrations of CO2, N2O, and NO in the reactor as explained below. For the reactions given by eqs 1-3, the material balances for elemental carbon and oxygen are given by eqs 4 and 5,

[CO]0 ) [CO] + [CO2]

(4)

[CO]0 + [NO]0 ) [NO] + [CO] + [N2O] + 2[CO2] (5) respectively, where it is assumed that the inlet and exit volumetric flow rates are identical due to the very dilute mixtures of reactants used in this study (otherwise the inlet concentrations are multiplied by Q0 and the exit

Figure 1. NO conversion multiplicity for the NO + CO reaction at 465-505 K. The lines indicate the predictions of the proposed model.

concentrations by Q). By subtracting eq 4 from eq 5, eq 6

[NO]0 ) [NO] + [N2O] + [CO2]

(6)

is obtained to calculate the feed NO concentration from the measured concentrations in the reactor. It was found that the NO conversion calculated by these two methods matched closely, especially for the high-conversion data. The NO conversions reported in this study are the average values of the NO conversions calculated by the above two methods. The NO conversions for the experiments carried out in the temperature range of 465-505 K are summarized in Figure 1. The high-conversion branches in Figure 1 correspond to the experiments which were started with approximately 0.1% CO in the feed. It can be seen from

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Figure 1 that the NO conversion increased with the stepwise increase in the feed CO concentration on the high-conversion branch. The increase in feed % CO beyond a certain limit caused the NO conversion to drop from a high value to a low value. The feed % CO at which the abrupt drop in NO conversion occurred is referred to as the high-to-low conversion bifurcation point. For example, in Figure 1a, the high-to-low conversion bifurcation occurred when the feed CO was increased from 0.97% to 1.02%, for the feed containing 1.2% NO as shown by the downward arrow. The highto-low conversion bifurcation points shifted in the direction of higher feed % CO with both increasing feed % NO and increasing temperature. It can also be seen from Figure 1 that the NO conversion on the highconversion branch decreased with an increase in the feed % NO. For example, at 505 K, with approximately 0.3% CO in the feed, the NO conversions were 90% for 0.45% NO, 65% for 0.7% NO, and 40% for 1.2% NO, respectively. From the product of the NO conversion and the feed NO concentration, it is readily seen that the absolute amount of NO reacted increased with increasing % NO in the feed, even though the NO conversion decreased as the % NO increased. It is also apparent from Figure 1 that the NO conversions on the high-conversion branch are not affected significantly by temperature except for a noticeable shift in the highto-low bifurcation points toward higher feed % CO. The low-conversion branches of Figure 1 correspond to the experiments which were started with approximately 1.2% CO in the feed. The stepwise decrease in feed % CO did not significantly affect the NO conversion. It can be seen from Figure 1 that the NO conversion increased with a decrease in the feed NO%, and this behavior is similar to that observed on the highconversion branch. It can also be seen that the lowconversion branches were relatively insensitive to the changes in temperature. The low-to-high conversion bifurcation occurred when the feed CO% was decreased to a value at which there was an abrupt increase in CO conversion to near 100%. This was accompanied by a relatively small increase in NO conversion. The upward pointing arrow in Figure 1a indicates the low-to-high conversion bifurcation for the feed containing 1.2% NO. The low-to-high conversion bifurcation points shifted slightly in the direction of higher feed % CO with an increase in feed % NO and an increase in temperature, which is consistent with that for the high-to-low conversion bifurcation points. All of the nine low-to-high conversion bifurcation points shown in Figure 1 occurred in the range of feed CO between 0.09% and 0.2%. An additional four sets of experiments were carried out at 520 K by reversing the roles of NO and CO in the feed. The NO conversion data for these experiments are summarized in Figure 2a. For the experiments at 520 K, the CO concentration in the feed was held constant while varying the NO concentration in steps. Therefore, Figure 2a has a totally different appearance when compared to Figure 1. The high-conversion branches in Figure 2a correspond to the experiments which were started with approximately 2.5% NO in the feed and in which the % NO in the feed was decreased in a stepwise fashion. It can be seen that the NO conversion on the high-rate branch increases with an increase in feed % CO or a decrease in the feed % NO. The high-to-low conversion bifurcation points shift in the direction of increasing feed % NO with an increase

Figure 2. Steady-state multiplicity for the NO + CO reaction at 520 K. The lines indicate the predictions of the proposed model.

in % CO in the feed. The low-conversion branches of Figure 2a correspond to the experiments which were started with approximately 0.2% NO in the feed. The NO conversion on the low-conversion branch was near 10% over most of the range of concentrations examined, with only a slight decrease in NO conversion with increasing feed % CO seen in Figure 2a. The low-tohigh conversion bifurcation for the feed containing 0.235% CO occurred when the feed NO was increased from 1.34% to 1.44%. The low-to-high bifurcation points for the other three experiments carried out with feed CO of 0.31%, 0.435%, and 0.685%, respectively, were not obtained even when the feed NO was increased to 2.5%. Because of the limitations on the flowmeter, NO gas analyzer (maximum range 0-2.5%), and the maximum NO concentrations in the gas mixtures, the lowto-high bifurcation points were not determined for feeds containing more than 0.235% CO. The dependence of

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Figure 3. CO conversion multiplicity for the NO + CO reaction at 465-505 K. The lines indicate the predictions of the proposed model.

the bifurcation points on feed composition and temperature was similar to that observed earlier for the N2O + CO reaction in the same temperature range (refer to Figure 3 of Sadhankar et al. (1994)). CO Conversion. The CO conversions are summarized in Figures 2b and 3 for the experiments carried out at 520 and 465-505 K, respectively. The CO conversions on the high-conversion branch were always near 100%, as can be seen from Figures 2b and 3. On the low-conversion branch of Figure 3, the CO conversion increases gradually with a decrease in feed % CO and then abruptly increases to 100% at the bifurcation point. Therefore, the CO conversion, instead of NO conversion, was used as a criteria to bracket the lowto-high conversion bifurcation points. It can be seen from Figure 3c that the three low-to-high bifurcation points at 465 K occurred within a narrower range of

feed % CO, as compared with the three bifurcation points at 505 K (Figure 3a). The low-conversion branches of Figure 2b show that the CO conversion decreases with an increase in feed % CO, which confirms the inhibiting effect of CO on the reaction. These results are very similar to those for the N2O + CO reaction from an earlier study (Sadhankar et al., 1994). A comparison of CO conversion at 520 K with the prior study (Sadhankar et al., 1994) indicates that the NO + CO reaction is more strongly inhibited by CO than the N2O + CO reaction in the low-conversion steady-state region. This could be due to the N2O and NO decomposition reactions requiring different numbers of catalytic sites. Therefore, the rate of CO2 formation for the NO + CO reaction is lower than that for the N2O + CO reaction in the lowconversion steady-state region. However, for the highconversion steady states, the rate of CO2 formation is limited by near complete conversion of CO and is therefore approximately equal for both of the reactions. N2O Formation. The reactor N2O formation is summarized in Figures 2c and 4 for the experiments carried out at 520 and 465-505 K, respectively. As can be seen from Figure 4, on the high-conversion branch, N2O in the reactor increases with an increase in feed % CO and reaches a maximum, after which it decreases with further increases in feed % CO. The maxima in the N2O curves shift in the direction of increasing feed % CO with an increase in feed NO%. Higher feed NO% also led to higher N2O% in the reactor. On the low-conversion branch of Figure 4, the N2O% in the reactor was very low and N2O measurement was limited by the range of the gas analyzer (lowest range 0-0.5%). Although a slight trend of increasing % N2O with increasing % NO in the feed is apparent from Figure 4, there is not a marked dependence of % N2O on the feed % CO or the temperature for the lowconversion branch. From Figure 2c, it can be seen that, on the highconversion branch at 520 K, the % N2O in the reactor decreases gradually with a decrease in % NO in the feed and then decreases rapidly near the bifurcation point, where % NO approaches % CO in the feed. On the lowconversion branch of Figure 2c, where [NO]0/[CO]0 < 1, the N2O formation is consistently low and the % N2O decreases slightly with an increase in feed % CO. These observations are consistent with those seen in Figure 4. An analysis of the data in Figure 4 shows that the maxima in the high-conversion curves occur when the feed concentration ratio [NO]0/[CO]0 is approximately equal to 1.5. For [NO]0/[CO]0 > 1.5, the N2O formation increases with increasing % CO in the feed. This observation is consistent with the data in Figure 2c, where N2O formation increases with % CO for a fixed % NO in the feed. For feed compositions with a [NO]0/ [CO]0 ratio of 1.5 or less, both the NO and CO conversions are nearly 100%, as can be seen from Figures 1 and 3, respectively. Therefore, from the carbon and oxygen material balance equations (4) and (5), it can be easily shown that the N2O formation for a part of the high-conversion branch can be described by

[N2O] ) [NO]0 - [CO]0

for

[NO]0 [CO]0

< 1.5 (7)

Equation 7 predicts that, for a feed concentration ratio of [NO]0/[CO]0 ) 1.5, the N2O concentration will be onehalf of the feed CO concentration. Further analysis of

Ind. Eng. Chem. Res., Vol. 36, No. 11, 1997 4613

Figure 5. Prediction of eqs 7 and 8 for the reactor N2O concentration for the high-conversion steady-state of the NO + CO reaction at 505 K.

Figure 4. Multiplicity of the rate of N2O formation for the NO + CO reaction at 465-505 K. The lines indicate the predictions of the proposed model.

the high-conversion steady-state data in Figure 4 also showed that these data points could be approximately described by a straight line passing through the origin with a slope of 0.5 for [NO]0/[CO]0 ratios of greater than 1.5. Therefore, the remaining part of the high-conversion curves (not described by eq 7) can be described by

[N2O] )

[CO]0 2

for

[NO]0 [CO]0

g 1.5

(8)

The reactor N2O concentration calculated by eqs 7 and 8 is compared with the data at 505 K in Figure 5, which shows a good agreement with the experimental data. The earlier study of the NO + CO reaction (Sadhankar and Lynch, 1996b) using forced concentration cycling showed evidence of the formation of N2O and

its subsequent reduction by CO, thereby supporting the theory of two consecutive reactions described earlier by eqs 2 and 3, respectively. Cho (1992, 1994) also reached a similar conclusion about the existence of two consecutive reactions after a mechanistic analysis of the NO + CO reaction system. Shelef and Otto (1968) observed that the N2O concentration passed through a peak as the reactor temperature was increased when a feed containing a stoichiometric excess of NO was reduced by CO over a supported Pt catalyst. They suggested that the observed N2O peak can be explained by considering that the reaction proceeds according to two consecutive reactions (eqs 2 and 3) over the entire temperature range. For the two consecutive reactions given by eqs 2 and 3, it can be seen from the reaction stoichiometry that approximately 75% of the feed CO will react by reaction 2 for a feed ratio of [NO]0/[CO]0 ) 1.5 (with near-complete conversions of both NO and CO). The high-to-low conversion bifurcation occurs when the feed composition approaches stoichiometric proportions; i.e., [NO]0/[CO]0 = 1. For a feed with a [NO]0/[CO]0 ratio of unity, 50% of the feed CO would react via reaction 2. Therefore, approximately 50-75% of the feed CO reacts via the NO + CO reaction (eq 2) and the remaining 25-50% reacts via the N2O + CO reaction (eq 3) for the high-conversion steady states. This simplified analysis shows that the N2O + CO reaction (eq 3) occurs to a significant extent and is therefore an important subreaction of the overall NO + CO reaction system. N2O Selectivity. The N2O selectivity is defined as the fraction of the total NO converted to N2O, according to

YN2O )

2[N2O] ([NO]0 - [NO])

(9)

The N2O selectivity on the high-rate branch as a function of the ratio of NO to CO in the feed is shown

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[CO]0 feed ratios, which could cause a large error in the denominator of eq 9 because of the subtraction of two similar quantities. Therefore, large errors could occur in calculation of the N2O selectivity for high [NO]0/[CO]0 ratios in the feed. Similar large errors are likely to occur for the N2O selectivity on the low-conversion branch, where the NO conversions are consistently low. Therefore, the N2O selectivity for the low-conversion branches is not discussed here. However, the data show that the N2O selectivity for the low-conversion branch is higher than that observed for the high-conversion branches. Near the bifurcation point, the N2O selectivity increases strongly with an increase in [NO]0/[CO]0 as can be seen from Figure 6, following which it reaches a plateau. The lowest N2O selectivity occurs at the high-to-low conversion bifurcation point where the NO conversion is the highest. Muraki et al. (1986) also observed that the N2O selectivity increased with an increase in the [NO]0/[CO]0 ratio in their study of the NO + CO reaction on a Pd/Al2O3 catalyst. The inverse dependence of the N2O selectivity on the NO conversion is consistent with the observation from the transient response experiments reported earlier (Sadhankar and Lynch, 1996b). From Figure 6, it appears that the N2O selectivity on the high-conversion branch is not significantly affected by the temperature in the range of 465520 K. A similar observation regarding an insignificant influence of temperature on N2O selectivity for the NO + CO reaction on Rh(111) catalyst has been made by Belton and Schmieg (1993). The prediction of N2O selectivity from eqs 7 and 8 is also shown in Figure 6 by dotted lines. Equations 7 and 8 predict that the N2O selectivity increases rapidly with an increase in the [NO]0/[CO]0 ratio from 1 to 1.5, after which it does not change with further increases in the ratio. There appears to be better agreement between the predictions of eqs 7 and 8 and the data at higher temperatures (i.e., 505 and 520 K) as compared with the data at lower temperatures. Model Formulation Langmuir-Hinshelwood-Hougen-Watson (LHHW) Mechanism. Various mechanisms have been proposed to describe the NO + CO reaction on noble metal catalysts depending upon the experimental conditions used in the studies. For the purpose of discussing the experimental results in this study, a mechanism consisting of seven reaction steps has been used:

Figure 6. N2O selectivity of the NO + CO reaction (high-rate branch only). The curves indicate the predictions of the proposed model. The dotted straight lines show the predictions of eqs 7 and 8.

in Figure 6. All of the data points in Figure 6 appear to fall on a single curve, except for some scatter for high [NO]0/[CO]0 ratios in parts b-d in Figure 6. The data scatter for high [NO]0/[CO]0 ratios is partly attributed to the uncertainty in the measurement of low concentrations of N2O in the reactor at these feed compositions. In addition, the NO conversion is low for high [NO]0/

CO(g) + M a CO-M

(10)

NO(g) + M a NO-M

(11)

NO-M + M f N-M + O-M

(12)

N-M + N-M f N2(g) + 2M

(13)

NO-M + N-M f N2O(g) + 2M

(14)

N2O(g) + M f N2(g) + O-M

(15)

CO-M + O-M f CO2(g) + 2M

(16)

Similar mechanisms have been used previously (Lorimer and Bell, 1979; Cho et al., 1989) with some differences in the reaction step for the N2O dissociation. The

Ind. Eng. Chem. Res., Vol. 36, No. 11, 1997 4615 Table 1. Kinetic Parameters rate const k-1 k-2 k3 k4 k5 k6 k7 a

465 K

485 K

505 K

520 K

Ei,a kJ‚mol-1

ki0,b s-1

0.48 340 61 000 20 2 600 0.5 5 500

0.72 615 212 046 46.5 7 000 1.08 12 000

1.042 1 050 667 800 101 17 500 2.2 24 612

1.35 1 500 1.49 × 106 174 33 000 3.6 40 682

37.8 54.3 116.8 79.1 92.9 72.2 73.1

8 440 4.3 × 108 1.6 × 1013 3.0 × 105 1.4 × 109 2.5 × 106 1.8 × 107

Activation energy. b Preexponential factor.

proposed mechanism includes three steps, namely, (10), (15), and (16), respectively, which were used previously to describe the steady-state multiplicity of the N2O + CO reaction under identical conditions (Sadhankar et al., 1994). The other steps are necessary to describe the NO + CO reactions according to eqs 1-3. It has been previously shown (Sadhankar et al., 1994) that the external and internal mass-transfer resistances are negligible under the experimental conditions used in this study. Therefore, it has been assumed that the experimental results obtained in this study represent the intrinsic kinetics of the reaction. The recycle reactor has been assumed to be an ideal CSTR. The mass balance equations based on the preceding mechanism are given in the Appendix. Equation 6, used earlier to describe the calculation of NO conversion, can be easily derived from eq 27 if it is assumed that the feed flow rate is equal to the flow rate at the reactor exit; i.e., Q0 ) Q. The assumption of a constant flow rate through the reactor is justified because of the low concentrations of the reactants ( 1.5. Conclusions Reaction rate multiplicity for the NO + CO reaction with respect to variation in temperature has been reported in the literature (Schwartz and Schmidt, 1987, 1988; Fink et al., 1991a). Apart from a previous note (Bolten et al., 1985) on NO + CO isothermal rate multiplicity on polycrystalline Pt, this is the first detailed report on the NO + CO reaction isothermal steady-state multiplicity with respect to variation in

feed composition, over a supported Pt catalyst, in the temperature range 465-520 K where significant amounts of N2O formation have been observed. As with the N2O + CO reaction described in an earlier study (Sadhankar et al., 1994), a high-conversion steady state is obtained when the initial feed contains NO in excess of CO (net oxidizing feed). For the initial feed containing CO in excess NO (net reducing feed), low steady-state conversions were obtained. In all of the experiments, N2O was formed as a reaction product, although significantly larger amounts of N2O were observed for the highconversion steady states. An interesting observation for the high conversion steady state was that the reactor N2O concentration increased with an increase in the feed CO (with constant % NO in the feed) up to a certain maximum and then gradually decreased with further increases in the feed CO. A simplified analysis of the data suggests that the rate of N2O formation decreases rapidly as the ratio of NO-to-CO concentrations in the feed drops below a critical value of approximately 1.5. The N2O selectivity was found to be a strong function of the ratio of NO to CO in the feed. The N2O selectivity was the lowest for a feed composition near the high-tolow conversion bifurcation point where the NO conversion was a maximum. The effect of temperature on N2O selectivity was negligible in the temperature range of 465-520 K. A comparison with a previous study (Sadhankar et al., 1994) showed that, in the region of low-conversion steady states, the NO + CO reaction appeared to be more strongly inhibited by CO than the N2O + CO reaction. Multiplicity occurred for a wider range of feed compositions for the NO + CO reaction as compared to that for the N2O + CO reaction. The N2O + CO reaction occurs to a significant extent and is therefore an important subreaction of the overall NO + CO reaction at the temperatures used in this study. Three kinetic models were examined to describe the experimental data. A LHHW kinetic model based on a reaction mechanism consisting of seven reaction steps could not fully describe the temperature and feedcomposition effects on the steady-state bifurcation. A model incorporating two main features, namely, the CO self-exclusion effect and the adsorbate-induced Pt surfacephase transition (1 × 1 T hex), gave an improved description of the experimental observations. Acknowledgment This work has been supported by the Natural Sciences and Engineering Research Council of Canada. Nomenclature a ) total surface area of supported catalyst, 9.4 m2 in this study Ei ) activation energy for the forward reaction described by eq 9 + i E-i ) activation energy for the reverse reaction described by eq 9 + i ki0 ) preexponential factor for the rate constant ki, s-1 k-i0 ) preexponential factor for the rate constant k-i, s-1 k1 ) CO adsorption rate constant, 6.87SCOT0.5/L, m3‚mol-1‚s-1 k-1 ) CO desorption rate constant, k-10 exp(-E-1/RT), s-1 k2 ) NO adsorption rate constant, 6.64SNOT0.5/L, m3‚mol-1‚s-1 k-2 ) NO desorption rate constant, k-20 exp(-E-2/RT), s-1 k3 ) NO-M dissociation rate constant, k30L-1 exp(-E3/RT), s-1

4618 Ind. Eng. Chem. Res., Vol. 36, No. 11, 1997 k4 ) nitrogen desorption rate constant, k40L-1 exp(-E4/RT), s-1 k5 ) N2O formation rate constant, k50L-1 exp(-E5/RT), s-1 k6 ) N2O dissociation rate constant, k60(RT/P) exp(-E6/ RT), s-1 k7 ) CO2 formation rate constant, k70L-1 exp(-E7/RT), s-1 L ) adsorption capacity of platinum surface, 2 × 10-5 mol‚m-2 M ) vacant site on the platinum surface NCO ) CO self-exclusion factor, 1.05 in this study P ) reactor pressure, 103 kPa in this study Q0 ) total feed flow rate to the reactor at reactor conditions, 1.11 × 10-8T m3‚s-1 Q ) exit flow rate from the reactor, m3‚s-1 rCO, ads ) rate of CO adsorption, mol‚m-2‚s-1 R ) gas constant, 8.314 J‚mol-1‚K-1 SCO ) CO sticking probability on platinum, 0.01 in this study SNO ) NO sticking probability on platinum, 0.01 in this study T ) reactor temperature, K YN2O ) N2O selectivity [X] ) concentration of the gas-phase species X (X ) CO, NO, N2, N2O, or CO2) in the reactor, mol‚m-3 [X]0 ) concentration of the gas-phase species X (X ) CO, NO, N2, N2O, or CO2) in the feed, mol‚m-3 X(g) ) gas-phase species X (X ) CO, NO, N2, N2O, or CO2) X-M ) adsorbed species X (X ) CO, NO, nitrogen, or oxygen) on the platinum catalyst surface Greek Letters θCO ) fractional CO surface coverage θN ) fractional nitrogen surface coverage θNO ) fractional NO surface coverage θO ) fractional oxygen surface coverage θV ) fraction of vacant sites on the surface

Appendix: Equations The material balance equations for the adsorbed species CO, atomic oxygen, NO, and atomic nitrogen are given respectively by

k1L[CO]θV - k-1LθCO - k7L2θOθCO ) 0

(19)

k3L2θNOθV + k6L[N2O]θV - k7L2θOθCO ) 0 (20) k2L[NO]θV - k-2LθNO - k3L2θNOθV - k5L2θNOθN ) 0 (21) k3L2θNOθV - k4L2θN2 - k5L2θNOθN ) 0

(22)

The material balance equations for the gas-phase NO, N2O, and CO2 species are given respectively by

Q0[NO]0 - Q[NO] ) ak2L[NO]θV - ak-2LθNO

(23)

Q[N2O] ) ak5L2θNOθN - ak6L[N2O]θV

(24)

Q[CO2] ) Q0[CO]0 - Q[CO] ) ak7L2θCOθO (25) The fractional concentration of vacant surface sites, θV, is given by

θV ) 1 - θCO - θO - θNO - θN

(26)

The steady-state gas-phase material balance is given by

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Received for review February 18, 1997 Revised manuscript received August 8, 1997 Accepted August 9, 1997X IE970138Z

X Abstract published in Advance ACS Abstracts, October 1, 1997.