412
Energy & Fuels 1987,1, 412-416
NO Reduction by A1203-SupportedRhodium, Palladium, and Platinum. 1. Intrinsic Activities and Selectivities Harvey G. Stenger, Jr.,* and Jeffrey S. Hepburn Department of Chemical Engineering, Lehigh University, Bethlehem, Pennsylvania 18015 Received March 12, 1987. Revised Manuscript Received July 15, 1987 The reduction of NO with Hz by using alumina-supported Pt, Pd, and Rh catalysts at temperatures between 80 and 200 "C and near-atmospheric pressure has been investigated. The dependence of the rate of NO reduction and the rates of N20, N2, and NH3 formation on the temperature and the partial pressures of NO and H, was measured. The formation of primary products and the disappearance of NO were found to fit a simplified Langmuir-Hinshelwood model with the rate-limiting step between adsorbed NO and adsorbed atomic H. The intrinsic activity ranking of the catalysts tested was Pt > Pd > Rh. For all three catalysts, N 2 0was formed a t a rate two or three times faster than Nz and NH3 a t low conversions of NO, while at conversions greater than 50% N2 and NH3 were formed a t rates comparable to that of N20. Introduction Environmental concerns and pending legislation have resulted in a growing interest in the problem of nitric oxide emissions from stationary sources. One of the most complete methods of removing nitric oxide is by catalytic reduction with either Hz or NH3.1g2 It is well-known that alumina- and silica-supported noble metals are active NO reduction catalysts. A number of investigators have studied the catalytic reduction of nitric oxide using noble metal catalysts as applied to automobile exhaust.3+ Kobylinski and Taylor1 have investigated the reduction of NO with Hz, CO, and an equimolar mixture of H,/CO using supported Pt, Pd, Rh, and Ru catalysts. They found that CO inhibits NO reduction strongly when Pt and Pd are used but inhibits NO reduction to a lesser extent for Rh. Taylor et al.' investigated the pretreatment effects on supported platinum, palladium, and ruthenium catalysts. It was found that a dual-state behavior was shown to occur for Pt and Pd as well as for Ru catalysts. It was suggested that a transformation to the more active oxidized catalyst state involves a reconstruction of the surface and/or destruction of inhibiting metal support interactions. Hecker and Bel1899have quantitatively studied the kinetics of NO reduction with Rh/SiO, catalysts using H, and CO as reduction agents. In their work, rate data were fit to power law expressions. Bauerle and Nobelo and Bauerle et al.ll compared a large number of catalysts, including the noble metals, for NO reduction with H2and NH3 and found a ranking of Pd > Pt > Rh based on conversion-temperature data. However their work did not include metal dispersion data to allow calculation of turnover numbers nor did they propose a kinetic model to explain their activity and selectivity measurements. (1) Kobylinski, T. P.; Taylor, B. W. J. Catal. 1974, 33, 376. (2) Harrison, B.; Diwell, A. F.; Wyatt, M. Platinum Met. Reu. 1985, 29, 50. (3) Herz, R. K.; Sell, J. A. J. Catal. 1985, 94, 166. (4) Herz, R. K.; Klela, J. B. Ind. Eng. Chem. Prod. Res. Deu. 1983,22, 387.
(5) Herz, R. K. Ind. Eng. Chem. Prod. Res. Deu. 1981, 20, 451. (6) Schlatter, J. C.; Taylor, K. C. J. Catal. 1977, 49, 42. (7) Taylor, K. C.; Sinkevitch, R. M.; Klimisch, R. L. J. Catal. 1974,35, 34. (8) Hecker, W. C.; Bell, A. T. J. Catal. 1985, 92, 247. (9) Hecker, W. C.; Bell, A. T. J. Catal. 1983,84, 200. (10)Bauerle, G. L.; Nobe, K. Znd. Eng. Chem. Prod. Res. Deu. 1974, 13, 185. (11) Bauerle, G. L.; Wu, S. C.; Nobe, K. Ind. Eng. Chem. Prod. Res. Deu. 1975, 14, 123.
0887-0624/87/2501-0412$01.50/0
Muraki et a1.12and Muraki and Fujitani13 compared the noble metals ability to reduce NO with CO. Their ranking was Rh > Pd > Pt, nearly opposite that when reduction was accomplished by using Hz or NH3. This reversal in activity ranking was shown to be attributed to strong CO adsorption blocking sites for adsorption of NO. Shelef and Gandhi14reported results showing Pd to be more active than Pt for reducing NO with Hz on a volume of catalyst basis; however, they did not report platinum loadings or metal dispersions. This paper describes a series of complementary experiments in which the kinetics of NO reduction by H2with Pt/A1,03, Pd/Al2O3,and Rh/A1203catalysts are investigated on an equal and comparative basis. The goals of our work were to compare steady-state activity and selectivity under nearly identical conditions and to use this data to determine kinetic parameters for each catalyst. The motivation for this work is to build a kinetic model of intrinsic reactivity that can be used to study the effects of SO2 ~0isoning.l~Our long-term interest is the design of sulfur-tolerant catalysts for NO reduction in stationary sources such as power stations and industrial boilers. Since it is expected that retrofitting of catalytic reduction units will be done at or near the flue gas stack, our temperature range of interest is restricted to between 80 and 200 OC. Experimental Section Apparatus. The apparatus used for this study is shown in Figure 1. Nitric oxide in helium and hydrogen are fed to a catalyst bed through separate gas flow meters. A third gas stream containing only helium was also used to further dilute the inlet gas. The central component of the system is a microreactor, shown in Figure 2. In this microreactor the catalyst powder is maintained between two porous stainless-steel frits. The microreactor is positioned within a Lindburg furnace. The catalyst temperature is measured by using a thermocouple imbedded into the catalyst powder. This temperature is controlled by using the outside reactor wall temperature as the input to a digital temperature (12) Mwaki, H.; Shinjoh, H.; Yokota, K.; Fujitani, Y. Ind. Eng. Chem. Prod. Res. Deu. 1985, 24, 43. (13) Muraki, H.; Fujitani, Y. Ind. Eng. Chem. Prod. Res. Dev. 1986, 25, 414. (14) Shelef, M.; Ghandi, H. S. Ind. Eng. Chem. Prod. Res. Deu. 1972, 11, 393. (15) Hepburn, J. S.; Stenger, H. G., submitted for publication in Energy Fuels. (16) Farrauto, R. J. AIChE Symp. Ser. 1974, 70, 9. (17) Foley, J. M.; Katzer, J. R.; Manogue, W. H. Ind. Eng. Chem. Prod. Res. Deu. 1979, 18, 170.
0 1987 American Chemical Society
Energy & Fuels, Vol. 1, No. 5, 1987 413
NO Reduction by A120,-Supported Catalysts
Table 11. GC Calibration Factors (C,= mol of i mL-' (area of i)-I) CNO
3.54
Reeulator P....
I 11 1 NO
H2
C.",.
---c-+
X
lo-"
CNzO
2.23 X lo-"
2.74
CNZ X lo-"
CNH, 5.08 X lo-"
Y u r n a c e
Gauge
Ro tome'iep
Figure 1. Experimental system. Thermoc oup
Porous F r i t
w
I
e
o'"'""'""'""'""'"""''''" 90
Figure 2. Experimental reactor. Table I. Results of Catalyst Characterization catalyst wt% % dispersion Pt 0.10 29 Pd 0.057 28 Rh 0.083 24 controller (Omega CN-5000). A pressure gauge positioned at the reactor inlet enables the reactor feed pressure to be monitored. The total exiting flow rate is measured with a soap film bubble meter. Gases. All gases for this study were supplied by Linde Division, Union Carbide (NO, 5.0% in He; Hz, 99.995% pure; and He, 99.99% pure). Both hydrogen and nitric oxide were passed through molecular sieve (5A) traps. This procedure served to remove impurities, including moisture, from each gas. The 5.0% NO in He mixture was found to contain Nz as an impurity that could not be removed in the molecular sieve traps but was accounted for in determining the rate of Nz formation during NO reduction. Catalyst Preparation. A finely divided alumina powder (98% pure, particle size < 10 pm, Aldrich Chemical Co.) was impregnated with aqueous solutions of PtC14, Pd(N0J2.3Hz0, and RhC13-3Hz0to prepare three separate catalysts. The concentration of the respective solutions were adjusted to give approximately 0.1 wt % metal on each finished catalyst. The impregnated powder was dried at 80 "C for 1h and calcined in air at 500 "C for an additional hour. Each catalyst was analyzed for total metal loading by atomic adsorption. To determine metal dispersion, selective H2 chemisorption was performed on the Pt and Rh catalysts. The dispersion of the Pd catalyst was estimated through sulfur-poisoningdatal0 since chemisorption measurements on supported Pd are difficult to interpret." The results of the catalyst characterizations are given in Table I. Procedure and Analysis. Inlet and outlet gases were analyzed by gas chromatography (a Hewlett-Packard 5890 gas chromatograph with a thermal conductivity detector and a Hewlett-Packard 3393 integrator). It has been well established that the major products formed in noble-metal reduction of nitric oxide by hydrogen are Nz, NzO, NH3, and Hz0.1t899J8 NO and N2 were separated by using a molecular sieve (5A) column while N20 and NH3 were separated by using a Chromosorb 103 column. The gas chromatograph was calibrated with gas standards prior to each catalyst run. The results of these calibrations are given in Table
(18) Hepburn, J. S. Masters Thesis, Lehigh University, 1987.
120 130 140 Temperature eC)
150
160
Figure 3. Conversion of NO vs. temperature for platinum. Conditions are listed in Table 111. i ng
11.
110
100
Columns in the gas chromatograph were switched manually with valves; therefore, two samples were needed for a complete analysis of an outlet sample. Reactor inlet and outlet samples were taken by syringe at inlet and outlet sample ports on the reactor. For each run, 0.300 g of catalyst were loaded into the reactor and reduced in flowing hydrogen (30 standard cm3/min) a t 400 "C for 2 h and at 300 OC for an additional 24 h. Following catalyst reduction, the reactant gases, NO and Hz, were introduced into the reactor. The reactor was monitored for a period of 12-20 h to ensure that a steady state was achieved. For each catalyst, 30-40 data points at five or six different temperatures were obtained for each of five different combinations of inlet partial pressures of hydrogen and nitric oxide, thus giving a total of 150-200 data points for each catalyst. Hydrogen and nitric oxide partial pressures were in the range 13-55 kPa and 1-10 kPa, respectively. Because of differences in activity, the temperature ranges studied were different for each catalyst. Temperatures between 80 and 150 O C , 120 and 170 "C, and 150 and 200 OC were used for the Pt, Pd, and Rh studies, respectively.
Results Primary reactions occurring in t h e catalytic reduction of NO by Hz include
2N0 2N0 NO
+ 2H2 + H2
-
-+
+
Nz 2H20 N2O + HzO
+
+ 5/2H2
NH3 + HzO
(1) (2)
(3)
Secondary reactions that occur significantly involving NzO include
+
2NH3 + H2O (4) Nz + HzO (5) For each catalyst t h e conversion of NO as well a s the rate of formation of N20, N2, and NH, was measured as functions of temperature, feed composition, and space velocity. The conversion of NO is shown in Figures 3-5 for Pt, Pd, and Rh, respectively. T h e conditions for t h e data shown in Figures 3-5 are given in Table 111. Material balances based on total nitrogen (mol of N leaving reactor/mol of N entering t h e reactor X 100%) were consistently above 95%. I n general t h e Pt catalyst shows t h e highest level of activity with conversions of 100% reached for temperatures near 140 "C. On t h e basis of t h e temperature needed for complete conversion, Pt ranks above Pd, a n d R h is t h e least active of t h e three. To compare t h e selectivity of t h e three catalysts, t h e NzO 4H2 N2O Hz
+
+
+
turnover frequencies, Ni= mol of species i s-' (mol of noble metal exposed)-', for NO, N20, N2, a n d NH, are plotted
414 Energy & Fuels, Vol. 1, No. 5, 1987
.-
f
Stenger, Jr., and Hepburn
;;i
2-
4
60:
500 0
40 1 30 20 10 .
90
100
110
120 130 140 Temperature (%)
150
160
170
Figure 4. Conversion of NO vs. temperature for palladium. Conditions are listed in Table 111. .. . ..- . . . . . 100 . . , . 8 . 1 . .
s
'E s
0
0
8 . .
. . I
. I . .
. I . .
. I
' 8 . .
100
.
120 Temperature
130
140
(OC)
Figure 6. Turnover frequencies for NO, NzO, NH,, and N2 for platinum. Conditions are given in Table 111. 6
. I , ,
c n
90 1 RHODIUM 80 7060 1 50:
110
. . . . , . . . . , . . . . , . . . .
PALLADIUM
'I
No \
3
A /
40: 3020 10 140 150 160 170 180 190 200 210 220 230 240 Temperature (%)
Figure 5. Conversion of NO vs. temperature for rhodium. Conditions are listed in Table 111. Table 111. Reaction Conditions for Figures 3-8
Figure
curve
space velocity, L of gas mi& (total g of noble metal)-'
inlet PH2, inlet P N O , kPa kPa
Platinum 3 3 3 and6 3 3
A B C
4 4 4 4
A B C D E
D E
147 217 317 417 567
63.4 38.8 36.6 42.6 51.7
3.2 4.1 4.4 3.0 1.9
66.1 48.4 34.8 54.4 35.1
2.3 3.5 4.1 2.7 4.5
74.6 60.5 48.8 45.2 45.7
1.8 2.7 3.3 3.4 4.1
Palladium
4and 7
257 380 327 673 520
5and8
A B C D E
161 216 241 273 361
vs. temperature in Figures 6-8. The operating conditions for these runs are listed in Table 111. While not all the selectivity data obtained during our study are shown here, these three plots are representative of the complete set of runs listed in Table 111. It should be noted that the data shown in Figures 6-8 were obtained at a constant feed flow rate and constant inlet concentrations of H2 and NO. Therefore the extent of reaction is also increasing as temperature increases in Figures 6-8. For the case of platinum (Figure 6), Nz formation was minimal and in some cases was slightly negative from the consumption of nitrogen that was present as an impurity in the feed gas. The rate of NH3 continually rises even as the conversion of NO approaches 100% at 137 "C, while the rate of NzO reaches a maximum at 129 "C and then
eC)
1 RHODIUM
200
Rhodium 5 5 5 5
Temperature
Figure 7. Turnover frequencies for NO, N20,NH,, and Nz for palladium. Conditions are given in Table 111.
21 0
220 Temperature eC)
230
240
Figure 8. Turnover frequencies for NO, N20, NH,, and Nz for rhodium. Conditions are given in Table 111. diminishes. These trends were observed in the other four platinum runs listed in Table 111. For the Pt catalyst it was consistently observed that NH3was only formed when the conversion of NO was greater than 60% , independent of temperature, and that the rate of NzO formation always reached a maximum and then decreased at higher conversions. For Pt, Nz was never formed to an appreciable extent. It is postulated from these observations that the platinum catalyst forms N 2 0 as the primary reduction product (reaction 2), and that NH3 is formed as a secondary product, possibly from the sequential reaction of NzO (reaction 4). The rates of product formation for the palladium catalyst (Figure 7), behaved differently than those for platinum. For Pd, NH3 was formed to a negligible extent and was seldom detected, even a t high conversions and high temperatures. The rate of Nz formation was observed to increase continually even a t high conversions of NO, and the rate of N 2 0 asymptotically reached a maximum value. From these observations it is postulated that N20 is again the primary product (reaction 2) and N, is a secondary product (reaction 5).
Energy &Fuels, Vol. 1, No. 5, 1987 415
NO Reduction by A1203-Supported Catalysts
100
I
10
1000
I
\
I?
?
X 0 Y
1.90
2.10
2.30 2.50 I/T (K-1) x103
2.70
2.90
Figure 9. ka,NO vs. inverse temperature for Pt, Pd, and Rh when negative frrst order in NO and positive 0.5 order in H2is assumed.
The rhodium catalyst's selectivity properties were unique. Figure 8 shows for rhodium that the rates of all three reaction products increase in concert. This suggests that all three products, N2, NzO, and NH3, are formed in primary reactions.
Discussion If it is assumed, for each catalyst, that the rate-limiting step for the disappearance of NO is the reaction of an adsorbed NO molecule with an adsorbed H atom8 and that product adsorption is minimal, the turnover frequency of NO would be represented by
I
1E-1 2.00 2.10
2.20 2.30
I/T
2.40 2.50 2.60
2.70
I 2.80
(K-1) x103
Figure 10. ,k w. inverse temperature for Pt, Pd, and Rh when positive first order in NO and positive first order in H,is assumed. 100 1
1 1
2.50
2.60 I/T
2.70 (K-1) x103
2.80
Figure 11. k,'s for NO, NzO,and N2vs. inverse temperature for platinum.
kJ/mol for Pt, 93.5 kJ/mol for Pd, and 76.9 kJ/mol for Rh. Other reaction rate orders were used to calculate a reaction rate constant for the disappearance of NO, but none KNOpNo >> KH:~PH;~ KN$NO >> (7) were found to give as high a correlation as -1 for NO and +0.5 for H2. A naive choice of orders based on homogeThis reduces eq 6 to neous elementary kinetics would be +1order for both NO and HP. Figure 10 shows an Arrhenius plot of the ka,NO ~S~,N&H:'~~H?~ - ka,N8H:'5 calculated from this first-order assumption. Clearly the (8) O " = KNOpNO pNO correlation is poor and not linear. The linear relationship shown in Figure 9 does not confirm the proposed mechaFor all the runs listed in Table 111, the conversion of H2 nism (eq 8); however, it does support it. was less than 2%. Therefore the average partial pressure Similar to the analysis for NO disappearance, the rate of H2 between the inlet and outlet of the reactor can be of formation of the observed products can be analyzed. If used in calculating ka,NOin eq 8. The partial pressure of only the data a t conversions less than 40% are used in NO, however, varied significantly between reactor inlet and modeling the kinetics for product formation, then only NzO outlet depending on the conversion of nitric oxide. and N2 can be considered, since for all three catalysts it Therefore, the reactor could not be characterized as being was observed that NH3 was only formed at conversions of differential for NO. To calculate kaNO only that data where NO higher than 60%. If the rate-limiting steps for forming the conversion of NO was below 40% was used, and P N O was set equal to the linear average of P N Oand , ~P ~ N O , ~ ~N2 ~ .and N20 as shown by reactions 1 and 2 are similar to the rate-limiting step for consumption of NO, then the rate The calculated ka,NO's for each catalyst are plotted vs. of formation of each product can be written as inverse temperature in Figure 9. The plots are linear and can be fit with straight lines having small residual errors. ksr,iK~:'5P~:'5 k , , j P ~ ~ ' ~ Ni = =While this supports the model represented by eq 8, it does (10) KN$NO PNO not of course prove it. From Figure 9, the ranking of the intrinsic activity of the three metals are clear, with Pt Calculated ka,i)sfor N20 and N2 for Pt, Pd, and Rh are having the highest intrinsic activity, followed by Pd and plotted in Figures 11-13, respectively, together with ka,NO. then Rh. Since the rate of formation of N2 was negligible with Pt, The slopes of the lines in Figure 9 are equal to the no points are plotted for that case in Figure 11. Again, apparent activation energy divided by the gas law constant. as for NO, the calculated rate constants closely fit straight From the proposed model (eq 8) this apparent activation lines, which supports but does not validate that the rateenergy can be represented by determining step is between adsorbed NO and adsorbed H atoms. E, = E,, - O.5XH2 + A N 0 (9) The platinum results, shown in Figure 11,indicate that N20decreases in reaction rate relative to the NO reaction where X's are heats of adsorption and E,, is the activation rate a t higher temperatures. Since N2 and NH3 were not energy of the surface reaction. From the data in Figure detected for these runs, an error in the material balance 9 the values of E , for the three noble metals are 88.9 If the surface coverage of NO is much greater than that of Hzand if NO is assumed to nearly saturate the surface, then
Stenger, Jr., and Hepburn
416 Energy & Fuels, Vol. 1, No. 5, 1987 1000
1
PALLADIUM
4
NO
i
1 E-2 lE-l 1 E - 3 1 . 2.30
2.40 I/T
2.50 (K-1)
2.60 x 103
2.70
2.00
Figure 12. k i s for NO, NzO,and Nz vs. inverse temperature for
palladium. 1000
RHODIUM
.
1.90
2.00
2.10 2.20 I/T (K-1) 103
2.30
1
2.40
Figure 13. k,'s for NO, NzO, and N2vs. inverse temperature for
rhodium.
(although small) is apparent. It was found in our work as in the work of Hecker and BelIl9that small concentrations of NH, were difficult to detect by GC. This is most likely the cause for the discrepancy in Figure 11and suggests that NH, is being formed to a small extent a t the higher temperatures in the Pt runs even at conversions less than 40%. The palladium results in Figure 12 show a large shift in the selectivity from N 2 0 to N2 a t higher temperatures. This shift indicates that N2 would surpass N 2 0 in selec(19) Hecker, W. C.; Bell, A.
T.Anal. Chem. 1981,53, 817.
tivity at higher temperatures. Rhodium, as mentioned previously, appears to form each product with a constant selectivity, independent of temperature and conversion. Figure 13 supports this statement, since the apparent rate constanb for NO, NzO, and N2 are parallel over the range of temperatures studied.
Conclusion The kinetics of NO reduction and N20, N2, and NH, formation for alumina-supported Pt, Pd, and Rh catalysts were investigated for conditions where NO conversion was less than 40%. The rate of NO consumption and the rates of N 2 0 and N2 formation were accurately modeled by a simplified Langmuir-Hinshelwood mechanism. The resulting dependencies were found to be 0.5 order in H2 concentration and first order in NO concentration. In general Pt was found to exhibit the greatest NO reduction activity followed by Pd and then Rh. Under the conditions investigated, Pd was more selective toward the formation of N2 than Rh a t high conversions, while Pt formed only a very small amount of N2. N 2 0 was discovered to be the major reaction product formed over all three catalysts and appeared to be involved in secondary reactions to form N2 and NH,. Acknowledgment. Recognition is given to Air Products and Chemicals Co. for the graduate fellowship that enabled this work. Also, thanks is given to Mobil Research and Development Corp. for the metal analysis of the catalysts. Glossary Ci GC calibration factor E* apparent activation energy activation energy for surface reaction step E,, apparent reaction rate constant for species i k,i surface reaction rate constant for species i adsorption equilibrium constant for species i, Pa-' turnover frequency for species i, mol of i (mol of Ni metal)-l partial pressure of species i, Pa Pi heat of adsorption for species i Xi Registry No. NO, 10102-43-9; Pt, 7440-06-4; Pd, 7440-05-3;
2
Rh, 7440-16-6.
