Catalytic Reduction of NO with Propene over Coprecipitated CuZrO

Chapter 8

Catalytic Reduction of NO with Propene over Coprecipitated CuZrO and CuGa O Downloaded by UNIV OF CALIFORNIA SAN FRANCISCO on December 6, 2014 | http://pubs.acs.org Publication Date: May 5, 1995 | doi: 10.1021/bk-1995-0587.ch008

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Mayfair C. Kung, Kathleen Bethke, David Alt, Barry Yang, and Harold H. Kung Ipatieff Laboratory, Center for Catalysis and Surface Science, Northwestern University, Evanston, IL 60208-3000

Copper-zirconium oxide and copper-gallium oxide prepared by coprecipitation and gelation were found to be active catalysts for the selective reduction of NO to N with propene in the presence of a large excess of O . The most active catalysts were comparable in activity to the Cu-ZSM-5 catalyst. The dispersion of the Cu ions and the efficiency for propene to react with NO instead of O increased with decreasing Cu content in the catalyst, although the activity decreased. The apparent activation energy for propene oxidation was 1 7 ± 3 kcal/mol for the 2.1 wt.% and 2 0 ± 3 kcal/mol for the 33 wt.% Cu-ZrO . The apparent activation energy for NO conversion to N was 1 0 ± 3 kcal/mol for the 2.1 wt.% and 1 4 ± 3 kcal/mol for the 33 wt.% Cu-ZrO catalysts. The activity of the matrix oxides (ZrO and Ga O ) in reducing NO with propene was much higher than in reducing NO, and the difference between the two matrix oxides correlated with the different response of NO reduction activity on the Cu-Ga O and Cu-ZrO to increasing O partial pressure. 2

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Nitric oxide (NO) is an atmospheric pollutant, and its efficient removal from engine exhaust or from the flue gas of boilers and power generation plants is important for the environment. In recent years, much attention in catalytic N O removal has centered on its reduction with environmentally acceptable hydrocar bon molecules in the presence of a large excess of oxygen (lean condition). The lean condition applies to flue gas from power plants, which typically contain a few percent oxygen, and the exhaust of lean burn automobile engines which are under development. Recently, Cu-ZSM-5 catalysts were found to have a high activity for N O reduction under the lean condition (1-3). To date, they are still among the most active catalysts known (4). Unfortunately, Cu-ZSM-5 catalysts suffer from the lack of hydrothermal stability (5), such that they are not suitable for the extended usage required for automobile exhaust converters. 0097-6156/95A)587-0096$12.00A) © 1995 American Chemical Society

In Reduction of Nitrogen Oxide Emissions; Ozkan, U., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1995.

Downloaded by UNIV OF CALIFORNIA SAN FRANCISCO on December 6, 2014 | http://pubs.acs.org Publication Date: May 5, 1995 | doi: 10.1021/bk-1995-0587.ch008

8. KUNG ET AL.

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Catalytic Reduction of NO with Propene

One strategy for the search of catalysts to replace Cu-ZSM-5 would be to assume that the unusually high activity of Cu-ZSM-5 is due to the high dispersion of the C u ions which are stabilized by the framework charge of the zeolite, and to mimic this situation by dispersing C u ions in other matrices. Using this approach, we have investigated the N O reduction reaction on mixed oxides of C u and Z r prepared by the sol gel method (6,7). It was found that such catalysts are quite active in the reduction of N O with propene. In fact, N i and Co ions well dispersed in Z r 0 were found also to be effective N O reduction catalysts. Walker et al. (8) have independently reported the high activities of C u - Z r 0 catalysts, and Montreuil et al. (9) have reported improved performance of Cu-ZSM-5 catalyst in the presence of C u - Z r 0 . These interesting results prompted further research to characterize C u - Z r 0 and other similar catalysts. This paper reports the results of the investigation of C u - Z r 0 and C u - G a 0 catalysts for the reduction of N O by propene. 2

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Experimental The catalysts were prepared by coprecipitation and gelation of the nitrate salts of C u (Fisher Scientific, A C S reagent grade) and Z r (Aldrich, 99.99% Z r O ( N 0 ) ) with urea at about 373 Κ using the methods of Amenomiya et al. (10). For catalysts containing G a , the corresponding nitrate salts were used. After precipitation, the solid was boiled vigorously for 3-4 h to remove excess urea. Then the precipitate was suction filtered, washed with water, filtered again, and dried in air at 373 K . The dried powder was ground and heated in flowing air at a rate of 1 K / m i n to 623 Κ and then at that temperature for 3 h. Afterwards, the solid was quenched cooled to room temperature. Before reaction, the catalyst was treated in a flow of 1000 ppm Q H * and 1% O in He at 573 K . This procedure was used to eliminate the residue nitrate left from the preparation procedure and the treatment was terminated when N production from nitrate reaction with C He ceased. Cu-ZSM-5, with a C u loading of 3.2% and a S i / A l ratio of 70, was supplied by G M Corporation, and was the same as the one used by Cho (11). Reaction tests were conducted using a fused silica U-tube reactor. In most experiments, 1 g of catalyst was tested with a feed of 1000 ppm N O , 1000 ppm propene, and 1% 0 in He. N , 0 , and C O in the gaseous product were analyzed with a 1.5 ft. Carbosphere column linked in series with a 1.5 ft. molecular sieve 5A column at room temperature. C 0 and hydrocarbons were analyzed with a 10 ft. Porapak Q column at 403 K . N 0 was analyzed by the same Porapak column at 303 K . For activation energy studies, 25 mg of C u - Z r 0 with particles of 170 + mesh size was used and the 0 concentration in the feed was 2%. N O conversion to N was kept below 10% and the propene conversion was kept below 30% to ensure that the reactor was being operated in the differential regime. For the data reported, % N O conversion to N and % N O competitiveness are defined as: 3

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% N O conversion % N O competitiveness

= 2*^*100/^0* = 2*N *100/(n*reductant consumed) = 2*N *100/(9*propene consumed)

(1)

2

2


(2)

98

N O REDUCTION x

In these equations, N is the molar rate of N produced, N O * is the molar feed rate of N O , reductant consumed is the molar rate of consumption of propene, and η is the number of Ο atoms required to completely convert the reductant (propene) into C O and H O , which is 9 for the case of propene. The % N O competitiveness measures the effectiveness (selectivity) of the hydrocarbon reductant to convert N O to N versus reacting with 0 . With this definition, reactions leading to the formation of N 0 would not contribute to the % N O competitiveness. The term "% selectivity" is not used to avoid confusion with the term "selective reduction" of N O to N . X-ray photoelectron spectroscopy (XPS) analyses were performed in a V G Scientific spectrometer and sensitivity factors of 4.0 for C u 2 p and 1.1 for Z r 3 d were used. Electron paramagnetic resonance (EPR) analyses were performed with a modified Varian E-4 X-band spectrometer. Temperature programmed reduction (TPR) was performed in the tubular flow reactor using 5% H in argon as the reductant. 2

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3/2

5/2

2

Results and Discussion C u - Z r 0 . As reported earlier (6,7), coprecipitated C u - Z r O catalysts were active for the selective reduction of N O to N by propene. The activity for N production on all the C u - Z r 0 samples showed a volcano-shaped dependence on temperature, with a broad maximum located at temperatures where the propene conversions were close to completion. This behavior was similar to those observed on many of the selective N O reduction catalysts such as Cu-ZSM-5 (1) and C u / S i 0 - A l 0 (12). The activity, and thus the temperature of maximum N O conversion, and the % N O competitiveness depended on the C u loading (Table I). As the C u loading was increased, both the temperature of maximum N O conversion and the % N O competitiveness decreased. This indicates that increased activity for both propene oxidation and N O reduction occurred with increased C u loading, and that the propene oxidation activity increased faster than the N O reduction activity, resulting in lower efficiency in converting N O to N . Under comparable conditions (space velocity 10,400 h ' \ 642 K ) , a 3.2 wt.% Cu-ZSM-5 catalyst gave 72% N O conversion, 100% propene conversion, and 8.3% N O competitiveness (6). Thus the performance of the most effective C u - Z r 0 sample (2.1 wt.% Cu-ZrO ) was comparable to that of Cu-ZSM-5. The higher competitiveness in utilizing N O to oxidize propene on lower C u loading samples was also observed at low conver sions. This is shown in Table II where the data were obtained by extrapolation of the results in Fig. 1. These data also show that the % N O competitiveness decreased with increasing temperature on both samples. It is interesting to note that, on the 2.1 wt.% C u - Z r 0 at 601 K , the % N O competitiveness at low conversion and for high conversion are comparable. This reaction trend for C u - Z r 0 is consistent with the model proposed earlier (6), that highly dispersed C u ions are desirable for selective reduction of N O with propene. This model assumes that C u O is not efficient for N O reduction because the C u ions are too easily reduced and the lattice oxygen are very reactive 2

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Table I: N O Conversion and % Competitiveness as a Function of C u Loading for Cu-Zr0 2


Wt. % Cu

Cu-Zr0 0.0 2.1 6.0 7.6 8.9 11 15 24 33

W/F g-min/cc

SV h

BET

0.010 0.010 0.014 0.0098 0.010 0.011 0.0094 0.0098 0.010

13300 13100 9680 13600 13300 12600 14400 13800 13000

167.6 163.5 92.2 138.3 157.8 151.5 160.5 117.5 135.9

1

mVg

8

Τ

*• max

κ

%NO* Conv.

b

%NO % CH» Compet. Conv. 3

6

c 2 d

784" 601 561 538 536 518 511 511 504

25" 68 58 64 58 54 52 46 43

34 91 92 90 81 82 91 90 91

642

72

100

8.4" 8.6 7.7 8.3 7.9 7.9 6.8 5.9 5.5

e

Cu-ZSM-5 3.2 0.0027 a. b. c. d. e.

10400

8.3

A t T ^ , temperature of maximum N O conversion. % N O competitiveness at T , ^ . Feed: 0.1% N O , 0.1% propene, 1% 0 , balance He. Not at T . , , T ^ > 784 K . Feed: 0.1% N O , 0.1% propene, 2.5% O , balance He. Sample provided by General Motors Corporation. 2

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such that the reaction of adsorbed propene and hydrocarbon intermediates with lattice oxygen competes effectively with adsorbed oxygen and adsorbed N O , thereby lowering the efficiency of the reductants to remove N O . Therefore, reducing the reactivity of lattice oxygen for hydrocarbon oxidation would increase the efficiency of N O removal. One method to achieve this is to disperse C u ions in a nonreducible matrix, such as Ζ ι Ό . According to the model, the competitive ness for N O utilization would decrease with increasing C u loading because of increasing extent of clustering of C u - O species which results in an increase in the amount of reactive lattice oxygen ions. The effect of C u loading on the apparent activation energy of N O conversion to N and propene conversion was also examined. Fig. 1 shows the Arrhenius plots for propene and N O reaction for the 2.1 wt.% C u - Z r 0 and 33 wt.% C u - Z r 0 . For the 2.1 wt.% sample, the apparent activation energy for N O conversion to N was 10 ± 3 kcal/mol, and that for C Ho conversion was 17 ± 3 kcal/mol. For the 33 wt.% sample, they were slightly higher, being 14 ± 3 kcal/mol 2

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-ι

550

100

600

700 750 800

550

b

600

+

700 Temperature (Κ)

650

* 6% 0 2

+ 1%02

750

800

2

Arrhenius plots ofpropene and NO reaction rates for a) 2.1 wt.% and b) 33 wt. % Cu-Zr0 .

Temperature (Κ)

650

10

20

30

40

> 60 c ο ϋ 50

70

80 4-

90

100


8. KUNG ET A U

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Table II. Comparison of % N O Competitiveness as a Function of Temperature for 2.1 wt.% and 33 wt.% C u - Z r 0 . (1000 ppm N O , 1000 ppm Q H * 2% 0 ) 2

% N O Comnet."

Temp. Κ

Catalyst wt.% C u


2

2.1 33

504 504

14 3.9

2.1 33

553 553

7.5 2.3

2.1

601 601 601

4.5 5.6 1.5

33 a.

Determined from the Arrhenius plots of data obtained at low N O and C H conversions. Determined from high conversion data at the temperature of maximum N O conversion. 3

b.

b

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and 20 ± 3 kcal/mol, respectively. It is interesting to note that the values for propene conversion were higher than for N O conversion on the catalysts. In contrast, Bennett et al. (13) reported that for a 5 wt.% Cu-ZSM-5 catalyst, the apparent activation energies for N O and propene reactions were 25 ± 1 kcal/mol and 11.2 ±0.7 kcal/mol, respectively. That is, the value for N O conversion was much higher than that for propene conversion. The reason that the activation energy for N O conversion appeared to be lower than that for propene on the C u - Z r O systems is because the efficiency of using the activated hydrocarbon decreased with elevation in temperature (Table II). Since reaction of lattice oxygen with surface hydrocarbon intermediates involves breaking strong bonds in the lattice, it would have higher activation energies (i.e. stronger temperature dependence) than reactions involving adsorbed N O or adsorbed oxygen. The fact that the activation energy for N O conversion is higher than that for conversion on Cu-ZSM-5 implies a higher N O competitiveness at higher temperatures. E P R spectroscopic data of samples in Table I provided evidence that the dispersion of C u ions decreased with increasing C u content. Figure 2 shows the E P R signals of some of the samples. The spectrum of the 2.1 wt.% sample was quite symmetric and relatively narrow. As the C u loading increased, the spectrum became broader and it was increasingly evident that the spectrum did not return to the baseline, suggesting the presence of a very broad feature. The broadness was attributed to an increase in the extent of Cu-Cu nuclear spin-spin interactions with increasing C u loading. For CuO, the nuclear spin-spin interaction was so strong that the E P R signal was too broad to be observable. The E P R data of z



102

N O REDUCTION x

Fig. 2.

EPR spectra at 77K. a: 3.2 wt.% Cu-ZSM-5; b: 2.1 wt. % Cu-Zr0 ; c: 8.9wt.% Cu-Zr0 ; d: 14.7wt.% Cu-Zr0 . 2

2


2

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these C u - Z r O samples, when coupled with the X P S data (6) that showed no evidence of preferential segregation of C u on the surface of these samples, suggested that the fraction of surface C u ions in a highly dispersed state decreased with increasing C u content. However, even for the 2.1 wt.% C u - Z r 0 sample, the well defined hyperfine structure observed for C u ions in Cu-ZSM-5 (14) was not observed. Thus, although well dispersed, the C u ions in this sample were sufficiently close to have some nuclear spin-spin interaction. The T P R data also supported this picture. Fig. 3 shows that the T P R profile for the 7.4 wt.% Cu-ZrÔ sample contained only one reduction peak, which was assigned to the reduction of well dispersed C u ions. For the higher C u loading samples, a higher temperature reduction peak appeared which increased in size with increasing C u loading. The spectrum of the 33 w t % C u - Z r O sample is shown in Fig. 3. The higher temperature peak could be assigned to reduction of clusters of C u - O units or small crystallites of C u O . These data were similar to those reported by Amenomiya et al. (10). The amount of hydrogen consumed in the T P R for both samples corresponded to complete reduction of C u to its zero valent state. z

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2 +

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C u - G a ^ . The C u ions in a 1.9 wt.% C u - G a ^ sample, with a surface area of 48.3 m /g, were also well dispersed as indicated by the C u E P R signal of the sample. Similar to the 2.1 wt.% C u - Z r O sample, the g, hyperfine splitting was not well resolved, suggesting that most of the C u ions were not isolated. The catalyst was effective in the catalytic reduction of N O to N with propene under the lean condition. The data in Table III compare the catalytic behavior of CuG a ^ and C u - Z r 0 . Under comparable conditions, C u - G a ^ was more effective than C u - Z r 0 . In particular, the decline in N O conversion for C u - G a ^ was less rapid with increasing 0 partial pressure than for Cu-ZrO . 2

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Z r 0 and G a 0 . One possible explanation for the observed differences in the catalytic behavior of C u - Z r 0 and C u - G a 0 was that the matrices participated in the overall steady state reaction in varying degrees and that the % N O competi tiveness was different for reactions that took place over the matrices and the C u active sites. Fig. 4a and 4b are temperature profiles of C H and N O conversions, at 1% and 6% O in the feed, over Z r 0 . Propene conversion was a strong function, whereas N O conversion was apparently independent, of 0 concentration in the feed. The latter is probably a coincidental consequence of the decrease in the % N O competitiveness being offset by the increase in hydrocarbon conversion with higher 0 partial pressure. The N O conversion was only noticeable above 625 Κ and was practically zero at temperatures where C u - Z r 0 catalysts usually operate (Table I). Table I V shows the catalytic data for G a ^ at different 0 concentrations in the feed at 709 K . After reaction in a feed containing 1% 0 , G a ^ turned from white to a light tan with black specks interspersed in between. The 1.9 wt.% C u - G a 0 catalyst was light green after reaction, suggesting that C u helped in preventing carbon deposition. Unlike Ζ ι Ό , the N O conversion for G a 0 improved with higher 0 concentration and was 100% (with >90% selectivity to 2

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TPR profiles of Cu-Zr0

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samples, a: 7.4wt.% Cu-Zr0 ;

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b: 33 wt. % Cu-Zr0 .

Heating rate = 2.5 Klmin.


Ο

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S

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Table ΠΙ. Reduction of N O with Propene over 2.1 wt.% C u - Z r 0 and 1.9 wt.% C u - G a 0 . (1000 ppm NO, 1000 ppm Q H J . 2

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% N O Compet.

Catalyst*

%o

Cu-Zr0 Cu-Zr0 Cu-Zr0

1.0 3.1 6.2

0.01(13,100) 0.01(13,100) 0.01(13,100)

601 601 601

68 38 20

8 5 2

1.0 4.1 6.7

0.01(6,100) 0.01(6,100) 0.01(6,100)

621 621 621

77 43 29

10 6 3

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Cu-Ga 0 Cu-Ga 0 Cu-Ga 0 a. b. c. d.

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W/P(SV)

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% N O Conv.

b

g-min/cc. Space velocity, h . Temperature of maximum N O conversion determined when 0 was 1%. B E T area was 163.5 m / g for C u - Z r 0 and 48.3 m / g for C u - G a ^ . l

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Table IV. Selective Reduction of N O over G a 0 . (1000 ppm N O , 1000 ppm C 3 H , 709 K , W/F=0.01g-min/cc) 2

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%o

% N O Conversion

2

1.0 4.0 6.7

55 90 91

% N O Compet.

% Q H j Conv. 63 84 92

10 12 11 2

a. b.

Surface area = 38 m /g. This was the temperature of maximum N O conversion determined for a 6.7% 0 feed. 2

N , rest to N 0 ) at 0 >4%. Comparing the data in Table I V with those for CuG a 0 in Table ΙΠ, it can be seen that the G a 0 was less active than C u - G a 0 . Its temperature of maximum N O conversion was 88 Κ higher. However, the data in Table V , which compare the N O reduction activity, at 1% O in the feed, of G a ^ and 2.4 wt.% C u - G a ^ at 648 Κ (near the temperature of maximum N O conversion for C u - G a ^ ) show that even at this low temperature, the rate of N production over the matrix was significant compared with the C u supported catalyst. In addition, the data in Tables ΙΠ and I V show that G a 0 , relative to C u - G a 0 , had much higher % N O competitiveness at higher 0 partial pressure. Thus^ reaction on the G a 0 matrix plays an important role in the overall N production on the C u - G a Ô sample, particularly at high 0 partial pressures. 2

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Fig. 4. 6

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Dependence of a) % CjH conversion, b) % NO conversion to N on oxygen concentration over Zr0 . Reaction conditions: 1000ppm propene, 1000ppm NO, W/F = 0.01 g-minlcc.


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ON

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N O vs. N 0 . The above discussion over the degree of participation of the matrix in the steady state reaction did not take into account N O formation over the catalysts and the different reactivity of N 0 , as compared with N O , towards the reductant hydrocarbon. It has been observed that the oxidation of N O to N 0 is rapid over some catalysts in conditions similar to those used in N O reduction in excess oxygen (15,16). The effectiveness of C u in catalyzing N 0 formation cannot be properly assessed from the data of Hamada and coworkers (15) as the reaction conditions they used were such that close to equilibrium distribution of N 0 and N O were achieved over both Cu-ZSM-5 and H-ZSM-5. However, Arai et al. (17) and Karisson et al. (18) demonstrated that C u in 13X zeolite was responsible for catalyzing N 0 formation from N O and 0 . Shimokawabe et al. (19) have shown that at 773 K , the areal rate of N 0 decomposition was CuO> > Z r 0 > A l 0 > > >SiO . Thus, if one of the consequences of adding C u to an inert oxide matrix was to increase the N 0 / N O ratio in the gas phase, then the contribution of the matrix to the overall catalytic reaction may be altered by the increased level of N 0 . It has been found that on Cu-ZSM-5 using isobutane as the reductant, the reduction of N O was slightly faster than the reduction of N O (20). Iwamoto et al. (21) found that for the selective reduction of N O by ethene over Cu-ZSM-5 catalysts, N 0 was more effective than N O between 350-500 K . Thus, it is possible that N O reduction on C u - Z r 0 and C u - G a ^ proceeded via first the formation of N0 . In order to evaluate this possibility, the rates of reduction of N O and N 0 with propene were compared over G a 0 and Z r 0 . For the purpose of ensuring identical reaction conditions, experiments were conducted on the same apparatus 2

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Table V . Comparison of Rates of Reduction of N O and N 0 . (Feed = 1000 ppm N O , 1000 ppm propene, 1% 0 ) . 2

x

Catalyst

Zr0

Reactant

2

2.1 wt.%Cu-Zr0 2

Ga 0 2

3

2

a.

3

%NO Compet.

Propene Conv.%

Temp. Κ

NO, Conv.%

NO N0 * 2

598 598

0 24

28

0 10

NO

601

66

8

97

NO N0 '

648 648

21 58

14 22

17 30

NO

632

77

9

92

2

1.9 wt.%Cu-Ga 0

2

-

Feed contained an equilibrium mixture of 680 ppm N 0 , 320 ppm N O . 2


108

NO

REDUCTION

X

using the same feed of N O , O , and propene, except that in the experiments with N 0 , the N O and O stream was first passed through a reactor containing 1 g C o / A l 0 catalyst at 573 Κ to achieve the equilibrium composition of N O / N O at that temperature. The conversion of N O to N 0 over the C o / A l 0 catalyst was independently confirmed by monitoring the gas mixture with Fourier transform infrared spectroscopy. The resulting mixture was then mixed with propene and passed over the Z r O or G a ^ catalyst. Thus, the feed to these matrix oxides consisted of 1000 ppm propene, 1% O , and about 680 ppm N 0 and 320 ppm N O in He. On both G a 0 and Z r 0 , the rate of formation of N from N O was much higher than from N O (Table V ) . In fact, these rates suggested that the contribu tion of the matrix to the overall N production would be substantial if the C u in the mixed oxide catalysts had catalyzed the oxidation of N O to the equilibrium ratio of N 0 / N O . However, our IR data suggested that C u - G a 0 was not a very effective catalyst for the oxidation of N O to N 0 . Even in the presence of 4% O , the overall N O conversion to N 0 was only about 40% at 573 K . Thus, the experiments in which efficient N 0 formation was ensured by pre-oxidation of N O on C o / A l 0 before entering the reactor placed an estimate of the upper limit of the degree of participation of the matrix in the production of N over a Cucontaining catalyst. z

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Summary and Conclusion It was demonstrated in this study that C u ions highly dispersed in matrices that are not active in propene combustion, such as Z r O and G a ^ , are active catalysts for propene reduction of N O to N in the lean condition. For the C u - Z r 0 catalysts, it was found that increasing the C u content resulted in catalysts that were more active in both propene combustion and N O reduction. But the increase in propene combustion activity was faster than N O reduction, thus the resulting efficiency in N O removal decreased. E P R spectra and T P R profiles showed that catalysts with higher C u contents contained larger fraction of C u ion in clusters. Under similar conditions, C u - G a ^ was more efficient in N O reduction than C u - Z r 0 . It was also less sensitive to suppression by high oxygen partial pressure. The differences in the two catalysts may be due to the degree of participation of the matrices in N production relative to Cu. In the absence of efficient oxidation of N O to N 0 , Z r 0 was considered too inactive to contribute to the overall N production at the temperatures where C u - Z r 0 was active. However, the G a 0 matrix contributed to the overall N production at tempera tures where C u - G a ^ operated. N 0 was a much more effective oxidizer of the hydrocarbon than N O over both G a ^ and Z r 0 . If N O in the feed could be efficiently oxidized to N 0 , then even Z r 0 would contribute to the overall N O conversion on C u - Z r 0 . Results of this study suggest that the possible role of transition metal in these mixed oxide catalysts are to: 1) prevent coking, 2) generate N 0 , and 3) form N . The role of a desirable matrix oxide is to disperse the metal function as well z

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8. KUNG ET AL.

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as participate in N production. Synergism results when N O produced on the transition metal enhances the contribution of the matrix oxide in N production. 2

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Acknowledgments This work was supported by Engelhard Industries, General Motors Corporation and NSF grant CTS-9308465. We thank Professor Can Li for testing the N O formation ability of some catalysts with IR.


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Literature Cited 1. 2. 3. 4. 5. 6. 7. 8.

9. 10.

11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21.

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