2930
Ind. Eng. Chem. Res. 1994,33, 2930-2934
Steady State Isotopic Transient Kinetic Analysis (SSITKA) Investigation of NO Reduction with CO over Perovskite Catalysts? Meltem tical, Rachid Oukaci,' and George Marcelin Chemical & Petroleum Engineering Department, University of Pittsburgh, Pittsburgh, Pennsylvania 15261
Sanjay K. AgarwaP Research Triangle Institute, Research Triangle Park, North Carolina 27709
The reduction of NO with CO on a perovskite catalyst, LaCo03, was investigated by means of steady state isotopic transient kinetic analysis (SSITKA). Steady state reactions were carried out at temperatures of 300-700 "C,while the isotopic transient experiments were carried out at temperatures of 500-700 "C. Isotopic switches of lzCO to 13C0or 14N0to 15N0were performed after the reaction reached steady state. I n order to determine the effect of readsorption, experiments at various temperatures were carried out as a function of space velocity. The SSITKA results for NO reduction with CO indicated that all the species (reactants and products) had measurable surface lifetimes and concentrations on the catalyst surface at various temperatures. Both the steady state and the SSITKA results were consistent with Nz0 being a precursor in the formation of NZ a t temperatures below 600 "C. SSITKA results indicated that the entire catalyst surface was involved in the reaction, and that, at high temperature, COZdesorption became limiting for the overall reaction. This was most likely due to the rapid formation and slow decomposition of carbonates on the surface.
Introduction The removal of NO, from exhaust gases in conventional combustion systems has received a great deal of attention in recent years, mainly due t o increasingly stricter environmental regulations and a need to reduce pollutant emission levels. It is known that NO is thermodynamically unstable relative t o its molecular elements but kinetically difficult t o decompose both thermally and in the presence of catalysts. Because of its kinetic stability, a major focus of work has been the reduction of NO, with other reducing agents, preferably those present in exhaust gas streams, such as CO or hydrocarbons. There have been a number of studies pertaining to the reduction of NO with CO on noble metal catalysts, particularly rhodium and platinum. These studies have addressed topics such as kinetics (e.g., Cho (1992), Scharpf and Benziger (1992), and Bense et al. (1989)), influence of metal particle size (Oh et al. (1991)), and support effect (Kudo et al. (1990)). Supported noble metal catalysts rapidly lose their activities at elevated temperatures due to sintering of the active component which renders their use under such conditions impossible. Additionally, these noble metals are in limited supply and are rather expensive. Oxide catalysts have also gained importance for this type of reaction. Oxides have the advantages of temperature stability and lower cost over supported metals. A variety of reports exists in the literature describing the use of different oxide catalysts for NO reduction, in particular perovskite-type oxides. The majority of these investigations are screening studies (e.g., Mizuno et al. (19921, Tabata and Misono (19901, and Viswanathan (199211, and only a few address fundamentals (Ladavos + Paper presented at the US-Russia Workshop on Environmental Catalysis in Wilmington, DE, January 14-16,1994. * To whom all correspondence should be addressed. Present address: Exxon Chemical Company, P. 0.Box 536, Lindon, NJ 07036.
*
and Pomonis (1992)) and surface chemistry (Pena et al. (1987) and Tascon et al. (1985)) for this reaction. This paper presents a study on the reduction of NO with CO on a perovskite catalyst, LaCoO3, investigated by means of steady state isotopic transient analysis (SSITKA). The isotopic transient technique has been successfully used for this type of reaction on a threeway auto exhaust catalyst (Oukaci et al. (1992))and for other types of reactions such as CO hydrogenation (e.g., Hoost and Goodwin (19921, De Pontes et al. (1987), and Zhang and Biloen (1986)),oxidative coupling of methane (Peil et al. (1990)),and ammonia synthesis (Nwalor and Goodwin (1989)). It involves a change in the isotopic composition of the reactants during the reaction without disturbing the steady state condition. This technique gives the opportunity to directly measure the steady state surface concentrations, coverages, and lifetime of reaction intermediates, thus providing useful information about the surface chemistry.
Experimental Procedure Catalyst. The preparation of Lac003 catalyst has been described in detail by Agarwal et al. (1994). Briefly, the catalyst was prepared from an aqueous solution containing appropriate amounts of the nitrate salts of La and Co. The corresponding hydroxides were precipitated by adding tetramethylammonium hydroxide to the solution. The solution was then filtered, and the precipitate was washed with distilled water. The resulting catalyst was dried at 120 "C for 12 h and calcined 900 "C for 10 h. The surface area of the catalyst was measured by NZ physisorption a t - 196 "C, and powder X-ray diffraction was used to determine the phase composition of the catalyst. Reaction Studies. Steady state isotopic transient technique was used to determine the surface lifetimes (z), average site activities (k = reciprocal of surface life time), surface concentrations ( N ) , and surface coverages (0) of the species during the reaction of NO reduction with CO.
0888-5885/94/2633-2930$04.50/00 1994 American Chemical Society
Ind. Eng. Chem. Res., Vol. 33, No. 12,1994 2931 1.0
Vent
0.8
0.6 0.4
0.2 0.0
0
2
5
8
10
Time (sec)
Figure 2. Normalized isotopic transient curves for CO, COz,and Ar at 500 "C. 1.0
INI
n.
v c
LL
Figure 1. Schematic of the reaction system.
Steady state and transient experiments were performed in a microreador consisting of a 4 mm i.d. straight quartz tube. The simplified schematic of the transient experiments is shown in Figure 1. Ultra high purity gases were used for the reactions. The labeled gases, W O (containing 12% 13C180)and 5% ISNO in He, were obtained from Isotec Inc. The nonlabeled gases, W O with 5% Ar and 5% I4NO in He, were obtained from Linde. In order to determine the gas phase hold-up, nonreactive inert tracers were used. For the isotopic switches of CO, Ar was used as an inert tracer, whereas Ne was used for the NO switches. Helium was used as diluent in all cases. The transient experiments entailed a switch of the isotopic composition of the reactants ('TO to I3CO, or 14N0to ISNO)a t steady state followed by continuous monitoring of the reactants and products of interest using a quadrupole mass spectrometer interfaced with a microcomputer for data acquisition. Steady state studies were carried out a t temperatures between 300 and 700 "C. The SSITKA studies were carried out a t temperatures between 500 and 700 "C with 5-21 mg of catalyst loading and a WHSV between 30 and 200 h-I. Reactant concentrations were between 1and 1.5%with a NO/CO ratio of 1. The total pressure was 1.8 bar. In order to determine the possible effect of readsorption of reactants and products, experiments were carried out a t various space velocities. Achievement of steady state was monitored by online gas chromatographic (GC) analysis of the reaction products using a Varian 3700 GC equipped with a thermal conductivity detector (TCD). The reactor effluent was separated using a 100/120 mesh Carbosieve 53-11(V8 in. x 12 ft) column which was kept a t 30 "C for 15 min and subsequently ramped to 140 "C with a heating rate of 5 Wmin. Under these conditions, the sequence of elution was Nz, CO, NO, COz, and Nz0. Subsequently, isotopic switches from W O to I3CO and sequentially from "NO to l5NO were performed at each temperature. In a separate set of experiments, the steady state decomposition and reduction of NzO with CO were also studied. The experiments were carried out with and without catalyst at 500 "C with a total flow of 68 cm3/ min and a catalyst loading of 15 mg. The NzO feed
Time (sec)
Figure 3. Normalized isotopic transient curves for NO, Nz. NzO, and Ne at 500 "C.
concentration was 1% with a N20/CO ratio of 1 for the reduction experiments.
Results and Discussion The surface area of the Lac003 catalyst was determined to be 1.5mz/g by Nz physisorption. XRD patterns of the fresh samde calcined a t 900 "C confirmed the perovskite structiure. Reaction data ~ ~ ~ ~ Studies: . . Data ~ Treatment. . ~ The ~ raw ~ obtained from the mass spectrometer was first smoothed using a Fourier transform algorithm. Subsequently, normalization of the transients was performed by first subtracting the baseline (level of constant minimum intensity) and then dividing every data point of the transient by the maximum intensity of that same transient. Although some species show fragments with identical m/e ratio in the mass spectrometer analysis (such as WOz and I4NzO),the fact that the C-switches and N-switches were performed independently still allowed evaluation of such transients because the intensity of the mle ratio of interest would reach a certain constant level atter the transient. This approach does not work for species where such a differentiation was not possible V4NO and ISNz)and only the complementary transients P N O and "Nz) were evaluated. Typical transient curves obtained at 500 "C for a IzCO to '3CO switch and a 14N0to l5NO switch are shown in Figures 2 and 3, respectively. The data is plotted as the normalized gas phase concentration of a species (F(t))as a function of time after the isotope switch. Since the traces of nonlabeled and labeled species were perfectly symmetrical, only decay curves are plotted. Average surface lifetimes (t)for all species of interest (12C0,'3C0,12C0~,13COz,15N0,14Nz,14Nz0,and 15NzO) were calculated by integrating the area between the transient of each particular species and that of the inert gas tracer (Ar or Ne) which corresponds to the gas phase hold-up. Average site activities were calculated by ~~~~~
~~
2932 Ind. Eng. Chem. Res., Vol. 33, No. 12, 1994
50
c
1 Ing P lo
401
N.0
'
t
J
40
Table 1. Results of N20 Decomposition and Reduction with CO at 600 "Cin the Presence and Absence of the CatalysP decomposition reduction without with without with cat. cat. cat. cat. conversion (%I
co
NzO product composition (mol %) Nz 0 2
coz
a
200
300
400
500
600
700
800
("C) Figure 4. Steady state results for NO CO reaction as a function of temperature. NO conversion (a), CO conversion (O), N2 yield (v),N2O yield (e).WHSV = 57 h-l, 1%N0/1% CO. Temperature
+
Ni = tiFCS/W
+ CO - C 0 2 + N 2 0 (300 "C) (I) 2 N 0 + 2CO - 2C0, + N, (700 "C) (11) 2N0
The results of the decomposition and reduction of N20 with CO at 500 "C in the presence and absence of the catalyst are compiled in Table 1. The conversions for the decomposition of N2O and the reduction of N2O with CO in the absence of the catalyst were approximately the same. This suggests that thermal decomposition of N20 was not significantly influenced by the presence of CO and that the reaction path was not altered. It is
27
70
69
30
31
14 16
88 86
50 0
50
0 50
50
WHSV = 68 h-l, N2O concentration = 1%, and NzO/CO = 1.
Table 2. Results of Steady State Transient Kinetic Analysis Studies for CO Switches' temp ("C) 500 600 700
taking the reciprocals of the surface lifetimes, k = 117. The surface concentration of labeled intermediates was calculated according to where Ni = surface concentration of labeled products @mol/g),ti = surface lifetime of labeled products (s), F = flow rate (NO or CO) @mol/s), C = conversion (NO or CO), S = selectivity, W = catalyst weight (g). The surface coverage of intermediates was estimated based on the surface area of the catalyst and the crosssectional areas of the corresponding species (CO = 20.9 A2, C02 = 20.6 A2, NO = 14.5 A2, N2 = 16.3 A2, and N2O = 23.1 A2). Steady State Reactions. The catalytic activity of the LaCoOa catalyst for the reduction of NO with CO at various temperatures expressed as the conversion together with N2 and NzO yields, calculated as the product of NO conversion (CO conversion for C02) and the selectivity to the corresponding product, are shown in Figure 4. Conversions of both CO and NO increased with increasing temperature. The same was observed for the N2 yield which increased from 0% a t 300 "C to 52% at 700 "C. The N2O yield, however, decreased with increasing temperature. Below 500 "C, the N20 yield was higher than the Nz yield, and N2O was produced exclusively a t 300 "C. Above 500 "C, the N20 yield was lower than the N2 yield and became 0 a t 700 "C. This suggests that at lower temperatures N2 is not a primary product but is formed by the decomposition of N2O. Similar conclusions were drawn by Tascon et al. (1985), by Cho (1992), and by Ladavos and Pomonis (1992). It should be noted that the measured conversions for CO and NO in the two extreme cases (300 and 700 "C)also supports the conclusion that two different reactions occur dependent on the temperature:
13
0
ZCO ( 8 ) ZCOZ ( 8 )
0.4 1.0
kco WS) kco2(1M NCO@ m o W NCO,@ m o W
2.5 1.0 2.3
5.6
0.2 0.9 5.0 1.1
2.4 10.3
0.1 0.7 10.0 1.4 4.5 28.4
Total pressure = 1.8 bar, WHSV = 145 h-l.
interesting that in the presence of CO no oxygen was detected in the reaction products, i.e., C02 was exclusively formed. This suggests that oxidation of CO by 0 2 occurs rapidly under these conditions. The presence of the catalyst had a positive effect on NzO decomposition as can be seen from the approximately doubled activity in Table 1. The presence of CO, however, had a dramatic effect as it increased the N2O conversion from 16% without catalyst to 86% with catalyst. Also in this case only N2 and C02 were detected as reaction products. The increased conversion of N2O for the decomposition in the presence of the catalyst is tentatively attributed t o the ability of the catalyst surface t o abstract oxygen from the N2O. Oxygen vacancies which are believed to be the active sites for this reaction have been claimed to play an important role for NO reduction (Viswanathan (1992)) and may also be the active sites for N2O decomposition. However, an easier abstraction of oxygen due to the presence of oxygen vacancies on the catalyst surface cannot explain the additional activity boost by CO. The data suggest that CO eases the removal of oxygen from the surface, thus keeping the number of available sites (oxygen vacancies) high. Other experiments, in which pure CO was contacted with the catalyst for several minutes, revealed the high oxygen abstraction capability of CO through the formation of CO2. SSITKA Results. The results of the isotopic transients in terms of surface lifetime (t), average site activity (k = lh), and surface concentrations of the reactants (CO and NO) and products (C02, Nz, and N2O) are compiled in Table 2 for the C-containing species and in Table 3 for the N-containing species. All species had measurable surface lifetimes and concentrations under the conditions applied. For all species the surface lifetime decreased as the temperature increased from 500 to 700 "C. The rather small values for the surface lifetimes of NO and CO indicate that either adsorption/ desorption of these species occurred fast or that NO and CO were irreversibly adsorbed. The decrease o f t for NO and CO with increasing temperature may be
Ind. Eng. Chem. Res., Vol. 33, No. 12, 1994 2933 Table 3. Results of Steady State Transient Kinetic Analysis Studies for NO Switchee temp ("C)
ZNzO (9)
kNo (l/S) kNz (l/S) kNzo (l/S) NNO Ocmollg) N NOcmoYg) ~ NN~O Ocmolld
500
600
700
0.5 1.2 0.4
0.4 1.1 0.2
0.3 0.3 im
2.0 0.8 2.5 4.5 4.1 2.6
2.5 0.9 5.0 6.3 10.9 1.3
.-c
5
3.3 3.3
u 0 0
.c
>
14.9 11.8
ln
1.6
-
1.4
*
1.2
-
0.6 0.4 1.0
0.8
a Total pressure = 1.8 bar, WHSV = 145 h-l, im = immeasurably small.
0.0
.
..p
L
0.2 I
0
5
10
15
20
25
30
35
1 OOO/WHSV (h)
considered an indication for faster adsorptioddesorption at high temperatures. The C02 transient lagged behind the CO transient (see also Figure 21, suggesting that the surface reaction and the desorption of C02 cannot be both fast steps. Either the surface reaction or the desorption of CO2 or both steps were slow. Comparing the variation of site activities as a function of temperature in Table 2, k for C02 remained almost constant and was paralleled by a signiqcant increase in the surface concentration of intermediates leading to C02 with increasing temperature. This may indicate that at higher temperatures desorption of the product becomes limiting for the overall reaction. It was concluded by Tejuca et al. (1984) that at high temperatures (500 "C) CO:! adsorbs on Lac003 catalyst in the form of bridged carbonates that interact with the metal cations (Co2+ and La3+) in the catalyst. It was also shown by Pena et. al. (1987) that carbonates are strongly bound to the surface of perovskite-type oxides up t o 600 "C. Hence, it is conceivable that the rate of formation of carbonates from adsorbed CO:! was high and that they were very stable. The reason for the high surface concentration of C02 may be due to the slow decomposition of carbonates as compared to the oxidation of CO at high temperatures. From Table 3, it can be seen that the average site activity, k, for N2O formation increased much faster than that for N2 formation with increasing temperature. Simultaneously, the surface concentration of N20 decreased and became immeasurable at 700 "C. The increasing rate constants and decreasing surface concentrations indicate that the decomposition of N20 was accelerated even more with increasing temperature. This is in agreement with the steady state results which suggested that the Nz0 was an intermediate for the formation of N2. It is interesting to note that the average site activity for the formation of N2 increased strongly between 600 and 700 "C, the temperature where N2O was no longer detected in the gas phase. Although this could be explained by a very rapid N20 decomposition, it has been proposed by Ladavos and Pomonis (1992) that at higher temperatures N2 is formed directly from NO. In order to determine the effect of readsorption on the SSITKA results, experiments were performed at 500 "C a t various WHSV. For species that do not pass through a repetitive adsorptioddesorption cycle on their way through the catalyst bed, one would expect to see no influence of the residence time on the average surface lifetime. It can be seen from Figure 5 that the surface lifetimes for CO and NO (reactants) essentially did not change with varying WHSV, whereas there was a
Figure 5. Readsorption phenomena of the reaction species at 500
"C. Table 4. Corrected Values for Reaction Intermediates at 500 "C surface species
co c02 NO Nz NzO
surface lifetime,
surface concentration, N Olmok)
surface coverage,
t(s)
average site activity, k (l/s)
0.5 0.5 0.5 0.7 0.2
2.0 2.0 2.0 1.4 5.0
2.8 2.9 4.5 1.2 2.3
0.24 0.24 0.26 0.20 0.09
e
considerable decrease in surface lifetimes with increasing WHSV for the reaction products ( ( 3 0 2 , N2, and NzO). This indicates that the products readsorbed readily on the catalyst surface. The fact that CO and NO did not readsorb suggest that once they adsorbed on proper sites they reacted. Table 4 shows the actual surface lifetimes (corrected for readsorption effects by extrapolation to infinite WHSV), concentrations, and coverages for all the species detected at 500 "C. The surface concentration of NO was about 50% higher than that of CO, suggesting a higher adsorption constant of NO. This is in agreement with the work of Pena et. a1 (1987) who suggested that NO is bound more strongly to the surface of these oxides than CO. A n estimation of the surface coverage reveals that the entire surface was covered by the various species. Thus, it may also be concluded that the entire catalyst surface participates in the reaction. In order to determine the extent of readsorption as a function of temperature, experiments for CO transients were also performed at high temperatures. Figure 6 shows the surface lifetimes of CO and C02 as a function of 1/WHSV at 500,600, and 700 "C. It can be seen that the slope of the curves for C 0 2 flattened as the temperature increased, indicating that the effect of readsorption of COSbecame less important at high temperatures. This is consistent with the model of a surface increasingly covered with COz in the form of carbonates. Conclusions
The reaction of NO with CO on Lac003 was investigated by means of SSITKA. All reaction intermediates had measurable surface lifetimes and concentrations on the catalyst surface at various temperatures. Both the steady state data and the SSITKA results suggest that N2O was an intermediate in the formation of N:! at temperatures below 600 "C. Above 600 "C N2 could be formed directly from NO. The SSITKA results indicated
2934 Ind. Eng. Chem. Res., Vol. 33,No. 12, 1994 Kudo, A.; Steinberg, M.; Bard, A. J.; Campion, A.; Fox, M. A.; Mallouk, T. E.; Webber, S. E.; White, J. M. Reduction at 300 K of NO by CO over Supported Platinum Catalysts. J. Catal. 1990, 125,565-567. Ladavos, A. K.; Pomonis, P. J.; Effects of substitution in Perovskites Laz.xSnNi04- on their Catalytic Action for the Nitric Oxide Carbon Monoxide Reaction. Appl. Catal. B: Environ. 1992,1,101-116. Mizuno, N.; T a n k a , M.; Misono, M. Reaction between Carbon Monoxide and Nitrogen Monoxide over Perovskite-type Mixed Oxides. J . Chem. SOC., Faraday Trans. 1992,88(11,91-95. Nwalor, J. U.;Goodwin, J. G., Jr.; Biloen, P. Steady-State Isotopic Transient-Kinetic Analysis of Iron-Catalyzed Ammonia Synthesis. J. Catal. 1989,117,121-134. Oh, S. H.; Eickel, C. C. Influence of Metal Particle Size and Support on the Catalytic Properties of Supported Rhodium: CO0 2 and CO-NO Reactions. J. Catal. 1991,128,526-536. Oukaci, R.; Blackmond, D. G.; Goodwin, J. G., Jr.; Gallaher, G. R. Steady-State Isotopic Transient Kinetic Analysis Investigation of CO-02 and CO-NO Reactions over a Commercial Automotive Catalyst. ACS Symp. Ser. 1992,No. 495,61-72. Peil, K.P.; Goodwin, J. G., Jr.; Marcelin, G. Surface Concentrations and Residence times of intermediates on Smz03 During the Oxidative Coupling of Methane. J. Am. Chem. SOC.1990,112, 6129. Pena, M. A,; Tascon, J. M. D.; Fierro, J. L. G.; Tejuca, L. G. A Study of NO an CO Interactions with L&nO3. J . Colloid Interface Sci. 1987,Vol. 119,No. 1, 100-107. Scharpf, E. W.; Benziger, J. B. wnetics of the C O N 0 Surface Reactions by the Direct Observation of the Adsorbed Species. J. Catal. 1992,136,342-360. Tabata, K.; Misono, M. Elimination of Pollutant Gases-Oxidation of CO, Reduction and Decomposition of NO. Catal. Today 1990, 8,249-261. Tascon, J. M. D.; Tejuka, L. G.; Rochester, C. H. Surface Interactions of NO and CO with L&o3 Oxides. J . Catal. 1985,95, 558-566. Tejuca, L.G.;Rochester, C. H.; Fierro, J. L. G.; Tascon, J . M. D. Infrared Spectroscopic Study of the Adsorption of Pyridine, Carbon Monoxide and Carbon Dioxide on the Perovskite-type Oxides L&o3. J. Chem. SOC.,Faraday Trans. 1 1984,80, 1089-1099. Viswanathan, B. CO Oxidation and NO Reduction on Perovskite Oxides. Catal. Rev.-Sci.Eng. 1992,34(4),337-354. Zhang, X.;Biloen, P. A Transient Kinetic Observation of Chain Growth in the Fischer-Tropsch Synthesis. J. Catal. 1986,98, 468-476.
+
'J 1.4
0.4 0.2
0.0
0
5
10
15
20
1 OOO/WHSV
25
30
35
(h)
Figure 6. Effect of temperature on readsorption for CO and COz. Open symbols and filled symbols represent CO and COz, respectively.
that at high temperature C02 desorption became limiting for the overall reaction, which was most likely due to the rapid formation and slow decomposition of carbonates on the surface.
Acknowledgment The financial support provided by Gas Research Institute under Contract No. 5092-260-2333is gratefully acknowledged.
Literature Cited Agarwal, S. K.; Jang, B. W. L.; Oukaci, R.; Riley, A.; Marcelin, G. NO, Control by Catalytic Combustion of Natural Gas. Environmental Catalysis;Armor, J. N., Ed.; ACS Symposium Series 552;American Chemical Society: Washington, DC, 1994; pp 224-232. Banse, B. A.; Wickham, D. T.; Koel, B. E. Transient Kinetic Studies of the Catalytic Reduction of NO by CO on Platinum. J. Catal. 1989,119,238-248. Cho, B. K. Mechanistic Importance of Intermediate NzO CO in Overall NO CO Reaction System. I. Kinetic Analysis. J. Catal. 1992,138,255-266. De Pontes, M.; Yokomizo, G. H.; Bell, A. T. A Novel Method for Analyzing Transient Response Data Obtained in Isotopic Tracer Studies of CO Hydrogenation. J. Catal. 1987,104,147-155. Hoost, T. E.;Goodwin, J. G., Jr. Reaction Analysis of Potassium Promotion of Ru-Catalyzed CO Hydrogenation Using SteadyState Isotopic Transients. J . Catal. 1992,137,22-35.
+
+
Received for review March 31, 1994 Revised manuscript received August 19, 1994 Accepted September 8,1994*
* Abstract published in Advance ACS Abstracts, November 1, 1994.
