The selective catalytic reduction of NO by propylene Pt supported on ...

6:ENVIRONMENTAL ELSEVIER

Applied Catalysis B: Environmental 14 (1997) 203-209

The selective catalytic reduction of NO by propylene over Pt supported on dealuminated Y zeolite Michael D. Amiridisa>*, Kenneth L. Robertsa, Carmo J. Pereirab aDepartment of Chemical Engineering, Universiry ofSouth Carolina, Columbia, SC 29208, USA b Research Division, WR. Grace and Co.-Corm., Columbia, MD 21044, USA

Received 23 October 1996; received in revised form 27 February 1997; accepted 1 March 1997

Abstract The reactivity of Pt supported on a stable dealuminated Y zeolite (Pt/DeY) for the Selective Catalytic Reduction of NO by hydrocarbons (HC-SCR) has been investigated with a monolith sample. The results show that the PtDeY catalyst has substantial activity for this reaction at temperatures between 200 and 300°C. Furthermore, the presence of water and sulfur dioxide, at levels similar to the ones expected in vehicle exhaust gas, does not significantly affect the performance of the catalyst, which makes it a promising candidate for further commercial development. In the same temperature range, oxygen promotes the rate of the NO reduction by assisting in the activation of the hydrocarbon. NOa is also formed under the conditions studied as a result of the oxidation of NO. In the presence of the hydrocarbon however, it is preferentially reacting with the hydrocarbon, and reduces primarily back to NO. High selectivities were observed toward the formation of NZO, which is a primary product of the hydrocarbon-SCR reaction. 0 1997 Elsevier Science B.V. Keywords:

Selective catalytic reduction;

Nitric oxide; Propylene;

1. Introduction NO, emissions are today regulated through a complex network of federal and local laws [l]. Emission control is primarily achieved by the use of heterogeneous catalytic processes which reduce NO, to Nz. The selective catalytic reduction of NO by hydrocarbons (HC-SCR) under excess oxygen (i.e. ‘lean’) conditions has potential applications in both mobile and stationary emission control, and certain advan-

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0926-860X/97/$17.00 0 1997 Elsevier Science B.V. All rights reserved. PII SO926-3373(97)00023-4

Platinum; Zeolite

tages over the currently utilized commercial technologies, such as the ‘three way’ noble metal catalysts and the selective catalytic reduction of NO by ammonia (NHs-SCR) [ 1,2]. The first reports regarding HC-SCR were communicated by Held and coworkers [3] and Iwamoto and coworkers [4] in 1990. Since these early reports, significant attention has been given to this area as outlined in our recent review of the pertinent literature [2]. Most of the focus of the research conducted so far on HC-SCR has been on Cu-ZSM-5, and as a result most of the detailed studies published examine this system (e.g. [5-lo]). The reason for this preference is the high initial activity of Cu-ZSM-5-based catalysts

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Catalysis B: Environmental

for HC-SCR. Nevertheless, there are significant limitations in the utilization of such catalysts for commercial applications due to their low hydrothermal stability in realistic exhaust compositions [ 111, and their undesirable selectivities for byproducts such as CO and potentially HCN [ 121. Although these issues currently continue to be under investigation in both industrial and academic laboratories, an increasing number of catalyst manufacturers appears to believe that “Cu/ZSM-5 is not suitable for widespread application on vehicles” u31. Relatively limited attention has been given in the literature to the utilization of noble metal-based catalysts for HC-SCR, despite the fact that some of the earliest patents in this area cover the use of such catalysts [ 14,151. The maximum activity of noble metal catalysts has been reported to be similar to the activity of Cu-ZSM-5. Moreover, the temperature at which this maximum occurs is significantly lower (200-350°C) than the corresponding temperature of maximum activity for Cu-containing [ 16181, which makes catalysts (350-500°C) noble-metal-based catalysts prime candidates for commercial application in diesel and lean-bum gasoline vehicles. Little is also known about the mechanism of NO reduction by hydrocarbons over noble metal-based catalysts. Workers at Johnson Matthey and the University of Reading [13,19,20] have proposed a redox mechanism similar to that proposed previously for the non-selective reduction of NO by CO and H2 over Pt-based systems. The key step in this mechanism is the reduction of oxidized Pt sites by the hydrocarbon, to generate active metallic sites. Subsequently, NO adsorbs and dissociates on these reduced Pt sites, and the resulting species recombine to release N2 and N20 leaving the Pt site reoxidized. In this work, we investigated the HC-SCR reactivity of a Pt catalyst supported on a stable dealuminated Y zeolite. Activity measurements were conducted with a monolith sample, and the results were compared to the corresponding results obtained with Cu/ZSM-5. Further activity measurements were also conducted in simplified gas streams, in an effort to gain a better understanding of the different elements of the HC-SCR reaction mechanism.

I4 (1997) 203-209

2. Experimental 2.1. Catalyst preparation The Pt catalyst examined in this work was prepared by impregnation of a tetraammineplatinum (II) hydroxide ([Pt(NH&](OH),) solution onto a stable dealuminated Y (DeY) zeolite support, and contained 1.2% Pt by weight. The Pt dispersion was estimated to be approximately 20% by hydrogen chemisorption. The zeolite used as the support was prepared from a USY sample (Davison Chemical Co.), which was steam-dealuminated, and further washed with an acidic solution to remove non-framework Al. A combination of X-ray diffraction and elemental analysis results showed that the dealuminated zeolite had a framework Si/Al ratio of about 14, and approximately, only 3 non-framework Al atoms per unit cell. Following impregnation, the catalyst was calcined for 1 h in air at 55O”C, ball-milled for 2 h in a 3.5% HNOs solution (based on the weight of the solids) to accommodate the adhesion to the metal foil, and finally, applied on a proprietary corrugated metal foil (21 in. length by 1 in. width) made by Grace Emission Control Products. Three successive layers were applied to the metal foil having a total weight of approximately 1.2 g. Following the application of each layer, the foil-washcoat combination was calcined for 20 min in air at 500°C. At the end of this process the foil was rolled and encased into a stainless steel cylindrical monolith of 1 in diameter by 1 in height. Two additional Pt catalysts were prepared for comparison purposes according to the exact same procedure. A commercially available alumina (MI-386; Davison Chemical Co.), and a ZSM-5 zeolite sample (Si/Al ~20, Mobil) were used as the supports. The Cu/ZSM-5 catalyst used for the comparison with Pt/DeY was prepared by impregnation of a copper acetate ((CHsC02)&u.xH20) solution onto a H-ZSM-5 zeolite (SilAlx20, Mobil) and contained approximately 5.4% Cu by weight. The preparation closely followed the steps described by d’Itri and Sachler [6], including drying the impregnated zeolite overnight at 30°C in a rotavap, calcining it in flowing O2 from 25 to 500°C at 0.7‘C/min and at 500°C for 2 h, and finally, cooling it to room temperature in Oz. A metal monolith containing the Cu/ZSM-5 catalyst


was then prepared following a procedure identical the one described above for the Pt catalysts. 2.2. Activity


14 (1997) 203-209

205

to

tests

Testing was conducted in an one-pass flow reactor at a space velocity of 40 000 hh’ (based on monolith volume). The reacting gases were mixed and preheated prior to the reactor entrance. Analyzed certified mixtures (Matheson) were used as the sources of NO, CsHdCsHs, 02, and SOZ, while N2 was used as the carrier gas. Water was introduced to the system through a high performance pump (Shimadzu). Three thermocouples were used to monitor the temperature in the preheating zone, the entrance and the exit of the monolith. The concentrations of the various components of the reacting gas mixture were analyzed at both the inlet and outlet of the reactor by the use of a series of on-line analyzers, including a Therm0 Electron chemiluminescent analyzer for NO,, a Shimadzu flame ionization detector for the hydrocarbon species, Thermoenvironmental Corporation infrared analyzers for CO and N20, and a Horiba paramagnetic analyzer for OZ. ‘Blank’ activity tests were conducted with a monolith coated with the zeolite support, prior to its impregnation with Pt, and yielded NO reduction levels below 3% in the 200-45O”C temperature range.

-I 150

200

250

300

350

400

450

Temperature (C) Fig. 1. CsH6 oxidation (O), NO reduction (m). and overall NO conversion (0) as functions of temperature (1600 ppm NO, 400 ppm W-k,

1% OZ),

25

3. Results and discussion NO (reduction) and propylene (oxidation) conversions were measured over the Pt/DeY catalyst under ‘lean conditions’ at different temperatures (Fig. 1). A maximum in NO reduction is observed with temperature at approximately 275°C. Similar maxima have been reported with other SCR catalysts which utilize either hydrocarbons or ammonia as the reducing agents [2]. NO reduction to the left of the maximum is limited by the activation of the NO-propylene reaction. To the right of the maximum, however, the rate is limited by the availability of propylene, which is consumed in a competing reaction by oxygen. The light-off of propylene closely follows the increase of NO reduction to the left of the maximum. Propylene conversions at temperatures higher than the temperature of maximum NO reduction are near 100%. The catalyst oxidizes the propylene completely to CO2 and

0

200

250

300

350

400

450

500

Temperature (C) Fig. 2. NO reduction over Pt/DeY (m. 0) and Cu/ZSM-5 (0, 0) as a function of temperature (Filled symbols: 1850 ppm NO, 300 ppm CsH6, 100 ppm C3Hs, 1% 0,; Open symbols: 1850 ppm NO, 300ppm C&,, 100ppm CsHs, 1% Olr 10% H20, 20ppm SW.

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14 (1997) 203-209

30

25

20 g g 15 P $ 3

10

B 5

n ”

200

250

300

350

400

4;o

Temperature (C) Fig. 3. NO reduction over Pt/DeY (m), WZSM-5 l& ppm C3Hs. 1% 02).

(v),

and Pt/MI-386

water, and no traces of CO or other partial oxidation products were observed within our detection limits. A comparison between the performance of the platinum zeolite system studied and Cu/ZSM-5 is attempted in Fig. 2. The results are in agreement with previously published reports [ 16,171 and show that although maximum NO reduction for the Cu/ZSM-5 system occurs at a higher temperature, it is comparable in magnitude to the NO reduction observed with the platinum zeolite system. In contrast to Cu/ZSM-5 however, the Pt/DeY catalyst does not seem to be affected by the presence of water and SOa. Experiments on long term deactivation (i.e. 200 h on stream under reaction conditions in the presence of water and SO*) also indicate that the Pt/DeY catalyst is significantly more stable than Cu/ZSM-5. These results become more important in view of the fact that the biggest obstacle in the commercial development of a

(A) as a function

of temperature

(1850 ppm NO, 300 ppm C3Hs,

viable ‘lean NO,’ catalyst has been, so far, the poor hydrothermal stability of transition metal ionexchanged zeolites. The performance of the Pt/DeY catalyst also compares favorably with the performance of the other two Pt catalysts tested, as shown in Fig. 3, indicating that the nature of the support affects the NO reduction activity. The observed NO conversions for the Pt/DeY catalyst in the presence and absence of oxygen are shown in Fig. 4. The presence of oxygen in the gas phase was found to enhance significantly the NO reduction activity at low temperatures. Since NO reduction in the presence of oxygen is limited at high temperatures by the availability of propylene, when oxygen is not present the NO reduction continues without competition at the higher temperatures. As a result, higher NO conversions were observed at temperatures greater than 325°C in the absence of oxygen. A maximum



207

14 (1997) 203-209

.

.

.I . . . .

200

250

300

350

400

Temperature

150

200

250

300

350

400

450

500

100

500

.

Temperature (C) Fig. 4. NO reduction as a function of temperature (Filled symbols: 1600 ppm NO, 400 ppm C&, 1% 02; Open symbols: 1600 ppm NO, 400 ppm C3Hs).

450

(C)

. 90 .

.

g

.

0

2?

s

80

h

.T

in NO reduction with temperature was also observed in the absence of oxygen, but at a much higher temperature (i.e. 425450°C). It is possible that this maximum is due to coking of the catalyst at these high reactor temperatures. Significant amounts of N20 are formed over the Pt/DeY catalyst. Although N20 emissions are not currently regulated in the United States, N20 has received attention for troposcopic ozone destruction and as a ‘greenhouse effect’ gas. The selectivity of the Pt/DeY catalyst toward N20 formation in the presence and absence of oxygen is shown as a function of temperature in Fig. 5. In the absence of oxygen and at low temperatures, N20 is the main reaction product (Fig. 5a). The selectivity toward N20 however, decreases with increasing temperature as a likely result of the increased activation of the hydrocarbon, which leads to a further reduction to N2. In the presence of oxygen (Fig. 5b), both N20 and N2 are primary products. The selectivity toward N20 in this case, follows the opposite trend and increases with increasing temperature, reaching nearly 100% at temperatures in excess of 350°C. Under these conditions the hydrocarbon conversion is almost complete, with

.z 2 x

70

.

60 1

1

50 I 200

250

300

Temperature

(C)

350

Fig. 5. N20 selectivity as a function of temperature. (a) 1600 ppm NO, 400 ppm C3Hs. (b) 1600 ppm NO, 400 ppm CxH6, 1% 02.

propylene oxidized primarily through a competing reaction with 02, which may be the reason for the lack of any further reduction of N20 to N2. The effect of NO on the hydrocarbon light-off is shown in Fig. 6. The presence of NO is found to retard the hydrocarbon light-off by approximately 50°C. This effect was observed both with ethylene (CzHd) and propylene (&He). These results suggest that the NO molecules are adsorbed on, and at low temperatures block, the sites responsible for the hydrocarbon oxidation. Similar inhibiting effects of NO on hydrocarbon oxidation have been recently reported over Ptand Cu-ZSM-5 catalysts [21,22].

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.g E 2 g u !z = ?!a

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70 60 50 40

4

30

3

20

150

200

250

300

350

Temperature (C) Fig. 6. C3H6 ( ?? , 0) and CZH4 (0, 0) oxidation as a function of temperature (Filled symbols: 1200 ppm C, 1% 0,; Open symbols: 1600 ppm NO, 1200 ppm C, 1% 02).

The overall NO conversion (i.e. NO reduction to Nz and NZO, as well as NO oxidation to NOz) with temperature is shown in Fig. 1. In this graph then, the net NOz formation is represented by the difference between the NO conversion and the NO reduction curves. As is seen in Fig. 1, no NOz is found in the reactor products at temperatures below the temperature of maximum NO reduction. At temperatures above the temperature of maximum NO reduction however, there is a considerable amount of NOz present in the reactor products as a result of the oxidation of NO over the noble metal catalyst. Experiments in the absence of the hydrocarbon indicate that NO is catalytically converted to NO2 even at temperatures below the temperature of maximum NO reduction, suggesting that NO2 may be more Table 1 NOa reduction

by propylene

14 (1997) 203-209

reactive, and hence been preferentially reduced by the hydrocarbon. To further explore the role of NO2 as a potential reaction intermediate for SCR, we conducted experiments in which NO in the reactor inlet was replaced by NO*. The results of theses experiments are summarized in Table 1. These results show that significant reduction of NOz by propylene (or similarly, significant oxidation of propylene by NO*) takes place even in the absence of a catalyst, as conversions of 35 and 51% of NO2 to NO were observed at 200 and 250°C respectively. The presence of 1% oxygen appears to promote slightly the NOz-propylene reaction only at the high temperature, probably due to a faster activation of the hydrocarbon. The only reduction product observed in the absence of a catalyst was NO. On the contrary, in the presence of the catalyst a small amount of NO2 is also further reduced to N2 and NZO. In the absence of oxygen for example, at 200°C 53% of the NO2 proceeds to form NO, while an additional 11% is converted to N2 and N20. At 250°C the conversion of NO2 is complete (96% toward NO and the remaining 4% toward N2 and N20). The presence of 1% oxygen does not appear to affect the overall reduction of NOz, but it affects the product selectivity, as it promotes the formation of N2 and NzO. This is probably a result of the oxygen promotion on the NO-propylene reaction (i.e. in the presence of oxygen the NO produced from NO2 is further reduced to N2 and N20 as suggested by the results in Fig. 4). A comparison between the data obtained with NO2 versus the data obtained with NO in the feed shows that NOz reacts faster and preferentially with the hydrocarbon. The main product of the NOz-propylene reaction however, is NO, which is in agreement with previous reports over a Cu-ZSM-5 catalyst [23]. Most reaction mechanisms proposed for transition metal containing zeolites (see Ref. [2] and references therein) suggest that the role of oxygen in the hydro-

(400 ppm NO*, 100 ppm C3Hs)

T(T)

CO*

Catalytic NO? to NO

NO* to NZ, N20

C1H6 oxidation

Gas phase NO2 to NO

NO2 to Nz, N20

C3H6 oxidation

200

WI0 02 1% 02 WI0 02 1% o*

53% 51% 96% 84%

11% 14% 4% 15%

29% 34% 41% 100%

35% 33% 51% 62%

0% 0% 0% 0%

11% 11% 16% 27%

250

M.D. Amiridis et al. /Applied Catalysis B: Environmental 14 (1997) 203-209

carbon-SCR reaction is to oxidize NO to NOz, with NO2 in turn, being the main reaction intermediate (i.e. NO+Oxygen+N02+hydrocarbon=+N~/N~O). On the contrary, the data in Table 1 and the close proximity of the hydrocarbon light-off and NO reduction curves (Fig. 1) suggest that over noble metal catalysts - such as the Pt/DeY zeolite examined in this work - the promoting effect of oxygen is probably related to the activation of the hydrocarbon.

4. Conclusions Activity measurements show that a Pt/DeY catalyst has substantial activity for the selective catalytic reduction of NO by propylene at temperatures between 200 and 300°C. Furthermore, the presence of water and sulfur dioxide, at levels similar to the ones expected in vehicle exhaust gas, does not significantly affect the performance of this catalyst, which makes it a promising candidate for further commercial development. In the same temperature range, oxygen promotes the rate of the NO reduction by assisting in the activation of the hydrocarbon. NzO is the primary product of the hydrocarbon-SCR reaction. Its formation is favored at low temperatures and at high hydrocarbon conversions (hydrocarbon-deficient conditions). NO2 is also formed under the conditions studied as a result of the oxidation of NO. In the presence of the hydrocarbon however, it is preferentially reacting with the hydrocarbon, and reduces primarily back to NO.

Acknowledgements The authors gratefully acknowledge the experimental support of Keith Halle; useful discussions with Dr. J.E. Kubsh; and W.R. Grace and Co., CN, for permission to publish this work. Part of the studies at the University of South Carolina was financially supported by a grant from the University of South Carolina Research and Productive Scholarship Fund.

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