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Ind. Eng. Chem. Res. 1996, 35, 4394-4397
Effect of Tungsten Oxide Addition on the Catalytic Activity of γ-Al2O3 for NOx Reduction from Fuel-Lean Gas Mixtures R. J. Kudla, S. Subramanian, M. S. Chattha, and T. E. Hoost* Chemical Engineering Department, Ford Motor Company, MD 3179 SRL, P.O. Box 2053, Dearborn, Michigan 48121
Non-zeolitic catalysts are increasingly being considered as durable alternatives in automotive lean-NOx catalysis. This study focused on increasing the acidity of γ-Al2O3 through the addition of tungsta (WO3) and increasing the resulting catalytic activity using simulated lean-burn engine exhaust. The effects of WO3 loading, space velocity, catalyst temperature, hydrocarbon and sulfur dioxide concentrations, and hydrothermal aging on the NOx reduction activity of WO3/ Al2O3 composite oxides were studied. It was found that tungsta-modified alumina compositions had higher NOx removal activity under lean-burn conditions than the unmodified γ-Al2O3. Maximum NOx conversion occurred at a 1 wt % loading of WO3 on γ-Al2O3 at a temperature of 580 °C. Introduction
Experimental Section
To meet upcoming stringent federal and California emission requirements, there has been a considerable research effort in automotive catalysis. Operating under fuel-lean conditions (air/fuel ratios often greater than 20:1) affords reduction of carbon dioxide emissions and improved fuel economy [1]. Thus, attempts are being made to develop catalyst systems for diesel, twostroke, and lean-burn gasoline engines, all of which operate in this lean region. However, creating a thermally stable catalyst that can reduce nitric oxide (NO) in a net-oxidizing environment (oxygen in excess of the stoichiometric amount necessary for complete combustion) is a technological challenge. The ability of hydrocarbons (HC) to participate in the selective catalytic reduction (SCR) of nitric oxide under oxidizing conditions has been found over both zeoliteand non-zeolite-based catalysts [2-5]. This method of SCR has even been found to be effective when using refractory oxides such as alumina (Al2O3), titania, and zirconia, with alumina exhibiting the greatest reduction of NO [6]. Because of its low cost and thermal durability, alumina may be a practical lean-NOx catalyst carrier. The acidity of alumina contributes to its ability to selectively reduce NO [7]. It is well known that tungsta (WO3) exhibits acidic properties and that the addition of WO3 to Al2O3 results in increased acidity [8]. Zhang et al. have shown that WO3 supported on Al2O3 has high HC cracking activity and stability under high-temperature steam treatments [9]. Also, a study by Hilbrig et al. reported that titania supported tungsta catalysts are very efficient for various acid-catalyzed reactions [10]. The aim of this study is to increase the acidity of Al2O3 through the addition of tungsta and investigate its catalytic activity using simulated lean-burn vehicle exhaust. The effects of WO3 loading, space velocity, catalyst temperature, HC (propylene, C3H6, and propane, C3H8) concentration, sulfur dioxide (SO2) concentration, and hydrothermal aging on the NO reduction activity of WO3/Al2O3 composite oxides have been studied. A comparison of the performance of WO3/Al2O3 catalysts with that of unmodified γ-Al2O3 was investigated.
Catalyst Preparation. The precursor for WO3 used in this study, ammonium metatungstate (AMT), with molecular formula (NH4)6H2W12O40‚18H2O, was obtained from EM Science. Desired wt % loadings of WO3 on defumed γ-Al2O3 (Degussa Corp., Al2O3-C) were obtained by impregnating the Al2O3 powder, particle size 20-40 mesh and nominal surface area of 100 m2/ g, with the appropriate concentration of AMT solution using the incipient wetness technique. These samples were then dried at 100 °C and calcined for 5 h at 500 °C. A series of WO3/Al2O3 composite oxides with WO3 loadings ranging from 0.1 to 3 wt % were prepared. Flow Reactor Studies. Catalytic activity tests were conducted using an integral flow reactor [11]. The powder catalyst was packed in a horizontal quartz glass reactor tube (22 mm i.d.) and held in place by plugs of quartz wool. The temperature was reproducibly obtained from a thermocouple placed 32 mm behind the catalyst bed. Nitrogen was used as the carrier gas, and a total gas flow rate of 3000 cm3 STP/min was used for all the experiments. An analyzer system located downstream also used on-line, continuous monitoring of the reactor effluent. Carbon monoxide and hydrocarbon concentrations were determined by infrared (Beckman Model 868) and flame ionization (Beckman Model 400) detectors, respectively. The amount of oxygen was measured using a polarographic oxygen sensor (Sensormedics OM-11EA). The chemiluminescent method (Beckman Model 951A) was used for detecting nitric oxide. When in the NOx ([NOx] ) [NO] + [NO2]) mode, the sample is routed through a converter consisting of a molybdenum/carbon catalyst, where NO2 is converted to NO. By monitoring both NO and NOx conversions with the NO analyzer, the amount of NO2 present in the gas stream can be determined. 1. Effect of WO3 Loading and Temperature. Five WO3/Al2O3 catalysts with 0.1, 0.5, 1, 2, and 3% by weight loadings of WO3 on γ-Al2O3 were examined for NO and NOx reduction activity. The “standard” composition of the synthetic lean-burn vehicle exhaust used in this study was as follows: 500 ppm C3H8, 1000 ppm C3H6, 4000 ppm CO, 1333 ppm H2, 500 ppm NO, 4% O2, 12% CO2, 10% H2O, 20 ppm SO2, and N2 to balance. The standard space velocity (SV) was 9800 h-1. The standard gas mixture had a redox ratio (R value) of 0.24, which represents an overall net oxidizing environment.
* Corresponding author: Dr. Ed Hoost. Tel.: (313) 248-2332. Fax: (313) 594-2963. E-mail:
[email protected].
S0888-5885(96)00163-7 CCC: $12.00
© 1996 American Chemical Society
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Figure 3. Effect of SO2 concentration on NO conversion over K-Al2O3 and 1% WO3/Al2O3 catalysts, using SO2 test conditions (see Experimental Section). Temperature, 580 °C.
Figure 1. Effect of WO3 loading on NO (A,C) and NOx (B,D) conversion as a function of temperature at standard feed conditions (see Experimental Section).
Figure 2. Effect of C3H6 and C3H8 concentration on NOx conversion over K-Al2O3 and 1% WO3/Al2O3 catalysts. Standard feed conditions, except SV ) 19,600 h-1 and HC concentration is varied. Temperature, 580 °C.
The redox ratio is the ratio of reducing components to oxidizing components found in the gas mixture, and it is determined from the partial pressures of the feed gases [11]. The catalyst temperature was raised in incremental steps from 450 to 700 °C. At each temperature, the observed HC, NO, and NOx conversions were obtained after steady state was achieved. 2. Effect of HC and SO2 Concentrations. The effects of C3H6 and C3H8 were obtained by varying the concentration of either propylene or propane from the “standard” feed conditions mentioned above. The catalyst temperature was maintained at 580 °C in these studies. For Figure 3, the SO2 concentration was gradually increased from 0 to 20 ppm while the other
Figure 4. Effect of space velocity on NO and NOx conversion over K-Al2O3 and 1% WO3/Al2O3 catalysts. Standard feed conditions, except SV is varied. Temperature, 580 °C.
gases were held at the following “SO2 test” concentrations: 900 ppm C3H8, 1000 ppm C3H6, 15 000 ppm CO, 5000 ppm H2, 450 ppm NO, 4% O2, 20% H2O, and N2 to balance. The SO2 test space velocity was 9800 h-1. 3. Effect of Space Velocity. Space velocity was investigated by changing the volume of catalyst powder used while keeping the total gas flow rate constant. The monolith equivalent SVs reported in Figure 4 were determined on the basis of the coating of a 400 cells/ in.2 cordierite monolith (bulk density, 0.585 g/cm3) with 35% by weight γ-Al2O3. Space velocity calculations were based on the total volume of the monolith. The catalyst temperature was held at 580 °C in these experiments. Calculating the SV of the powder sample onthe basis of a γ-Al2O3 washcoated cordierite monolith represents a more realistic value for automotive applications. For example, if the SV was based on the packed density (0.585 g/cm3) of γ-Al2O3 powder and the amount of catalyst used, the reported monolith equivalent SV would be 9800 h-1, whereas the actual powder-based SV in the reactor would be 38 600 h-1. 4. Effect of Catalyst Aging. γ-Al2O3 and 1% WO3/ Al2O3 catalyst samples were hydrothermally aged in an air and 10% H2O environment at 615 °C for 98 and 250 h, respectively. The activity of these samples was then evaluated on the flow reactor at SV 9800 h-1 and a temperature of 580 °C and is shown in Figure 5. 5. Pulse Flame Combustor Study. A pulse flame combustor was used to evaluate the activity of a K-Al2O3 washcoated cordierite monolith (400 cells/in.2) impregnated with 1% WO3 by weight. A description of this apparatus has been reported elsewhere [12]. The performance of the catalyst was carried out in a gas stream with a redox ratio of 0.50, produced by the
4396 Ind. Eng. Chem. Res., Vol. 35, No. 12, 1996
Figure 5. Effect of hydrothermal aging on K-Al2O3 and 1% WO3/ Al2O3 catalysts. Standard feed conditions. Temperature, 580 °C.
Figure 6. Pulse flame reactor conversion of NO and HC over K-Al2O3 and 1% WO3/Al2O3 catalysts. See Experimental Section for pulsator feed concentrations. SV ) 6700 h-1.
combustion of pure isooctane. This resulted in the following inlet concentrations: 450 ppm HC, 450 ppm NO, 2.5% CO, and 4% O2. The catalyst was tested for NO and HC activities as a function of temperature at SV 6700 h-1 (Figure 6). Results and Discussion Effect of WO3 Loading and Temperature. Using the full feed gas mixture, it was found that the maximum NOx conversion (27%) and NO conversion (8%) occurred at 580 °C in an empty reactor tube. The NOx and NO conversions reported in the following figures are observed values and are not adjusted for the above homogeneous reaction. Figure 1 depicts NO and NOx conversions as a function of temperature at various weight loadings of WO3 on γ-Al2O3. Maximum NOx conversion (Figure 1B) gradually increased to 1% WO3 loading, where it reached a high of 74% (at SV ) 9800 h-1), and then began to decrease at higher levels of WO3 loading. In the temperature range of 550-700 °C, NOx conversion was at least 20% for all catalysts tested. At 580 °C, a maximum was observed in the NOx conversion. Beyond 580 °C, NOx conversion decreased due to rapid total oxidation of hydrocarbons at higher temperatures [2]. As can be seen in Figure 1C,D, the activity of the 1% WO3/Al2O3 catalyst was significantly greater than that of the unmodified γ-Al2O3. Effect of HC and SO2 Concentrations. The reaction temperature for these experiments was maintained at 580 °C, which is the temperature at which maximum catalytic activity occurred. Figure 2 shows that an increase in C3H6 concentration resulted in an increase
in NO conversion. An increase in the concentration of the slower burning C3H8 resulted in a less pronounced increase in NOx conversion (Figure 2). The higher reactivity of C3H6 when compared to that of C3H8 for the SCR of NO over an alumina catalyst has been reported in the literature [7]. Also, an earlier study found that a WO3/Al2O3 catalyst was not active for propane oxidation around the stoichiometric region [13]. Once again, Figure 2 shows that the 1% WO3/Al2O3 catalyst was more active than the unmodified γ-Al2O3 at the various hydrocarbon concentrations. When NO conversion was evaluated for unmodified and modified γ-Al2O3 at various WO3 loadings in the presence of 0-20 ppm SO2, there was very little change in the catalyst performance (Figure 3), whereas for copper-exchanged ZSM-5 zeolite, Iwamoto et al. reported that the addition of SO2 resulted in partial deactivation of the catalyst [14]. Effect of Space Velocity. NO and NOx conversions were examined at three SVs for both the 1% WO3/Al2O3 and γ-Al2O3 catalysts, and the results are shown in Figure 4. As the SV is increased from 10 000 to 20 000 h-1, there was a continuous decline in conversion for both catalysts. At the highest SV, the 1% WO3/Al2O3 and γ-Al2O3 catalysts had 34% and 24% NOx conversions, respectively. Therefore, even with the increase in activity due to the addition of WO3, large volumes of this catalyst would be necessary under actual lean-burn vehicle conditions. Effect of Catalyst Aging. Although the 1% WO3/ Al2O3 catalyst showed a slight decrease in NOx conversion with hydrothermal aging, it still remained more active than the unmodified γ-Al2O3 catalyst (Figure 5). On the other hand, whereas Cu-ZSM-5 initially has a higher NO conversion than the Al2O3-based catalysts, the zeolite has very poor thermal durability and undergoes irreversible deactivation in the presence of water [15]. Pulse Flame Combustor Study. The results from the evaluation of the 1% WO3/Al2O3 monolith catalyst at SV ) 6700 h-1 are shown in Figure 6. NO and HC conversions are plotted as a function of temperature. The HC conversion steadily increased with increasing temperature. NO converted in the 450-750 °C temperature range, with maximum activity occurring at 600 °C. These results correspond to the flow reactor data, except that the window for NO conversion was broader in the pulsator study. This could be the result of the mixture of HCs with varying degrees of SCR for NO that results from the combustion of isooctane [5]. Figure 6 also depicts a similar experiment that was performed at the identical SV using an Al2O3-only washcoated monolith catalyst. The same trends as in the 1% WO3/ Al2O3 pulsator experiment were observed, except that the window of NO conversion was not as broad for the Al2O3 catalyst. Also, the maximum NO conversion for the Al2O3 catalyst was 56%, whereas for the WO3/Al2O3 catalyst it was similar at 58%. Thus, the pulsator study did not reveal a significant activity difference between the two catalysts as was observed in the flow reactor experiments. The wider conversion window of WO3/ Al2O3 observed from the pulsator study would be a desirable feature for automotive applications. Summary Under simulated lean-burn conditions, tungsta-modified alumina compositions were shown to have higher NOx removal activity under simulated lean-burn condi-
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tions than unmodified γ-Al2O3. In the flow reactor evaluations, the highest NOx conversion was measured for at a 1% weight loading of WO3 on Al2O3 at a temperature of 580 °C. For all of the catalysts tested, NO and NOx conversions were observed in the 550700 °C temperature range. Increasing the amount of C3H6 in the feed gas for the selective catalytic reduction of NOx led to increased NOx reduction activity. The addition of C3H8 caused a less pronounced increase in NOx conversion. The activity of the 1% WO3/Al2O3 catalyst was decreased only slightly in the presence of SO2 and upon hydrothermal aging but was highly dependent on space velocity. Pulsator results indicate that, although the operating window is broader for the 1% WO3/Al2O3 than for the unmodified γ-Al2O3 catalyst, even higher NOx conversion would be needed at higher space velocities to have a feasible lean-burn catalyst. Acknowledgment We thank W. Chun for providing the pulse flame combustor data. Literature Cited (1) Cooper, B. Automot. Eng. 1992, 100, 9. (2) Truex, T. J.; Searles, R. A.; Sun, D. C. Platinum Met. Rev. 1992, 36, 2. (3) Hamada, H.; Kintaichi, Y.; Sasaki, M.; Ito, T.; Tabata, M. Appl. Catal. 1990, 64, L1. (4) Held, W.; Konig, A.; Richter, T.; Puppe, L. SAE Tech. Pap. Ser. 1990, 900496.
(5) Subramanian, S.; Kudla, R. J.; Chun, W.; Chattha, M. S. Ind. Eng. Chem. Res. 1993, 32, 1805. (6) Iwamoto, M.; Hamada, H. Catal. Today 1991, 10, 57. (7) Hamada, H.; Kintaichi, Y.; Sasaki, M.; Ito, T.; Tabata, M. Appl. Catal. 1991, 75, L1. (8) Brady, R. L.; Southmayd, D.; Contescu, C.; Zhang, R.; Schwarz, J. A. Appl. Catal. 1991, 129, 195. (9) Zhang, R.; Jagiello, J.; Hu, J. F.; Huang, Z. Q.; Schwarz, J. A. Appl. Catal. 1992, 84, 123. (10) Hibrig, F.; Gobel, H. E.; Knozinger, H.; Schmelz, H.; Lengeler, B. J. Phys. Chem. 1991, 95, 6973. (11) Gandhi, H. S.; Piken, A. G.; Shelef, M.; Delosh, R. G. SAE Tech. Pap. Ser. 1976, 760201. (12) Siegl, W. O.; McCabe, R. W.; Chun, W.; Kaiser, E. W.; Perry, J.; Henig, Y. I.; Trinker, F. H.; Anderson, R. W. J. Air Waste Manag. Assoc. 1992, 42, 912. (13) Adams, K. A.; Gandhi, H. S. Ind. Eng. Chem. Prod. Res. Dev. 1983, 22, 207. (14) Iwamoto, M.; Yahiro, H.; Shundo, S.; Yu-u, Y.; Mizuno, N. Appl. Catal. 1991, 69, L15. (15) Petunchi, J. O.; Sill, G.; Hall, W. K. Appl. Catal. B 1993, 2, 303.
Received for review March 22, 1996 Revised manuscript received August 16, 1996 Accepted August 26, 1996X IE960163P
X Abstract published in Advance ACS Abstracts, November 15, 1996.
