Ind. Eng. Chem. Prod. Res. Dev. 1985, 2 4 , 226-233
226
oxidation of the adsorbed intermediate species to maleic anhydride. This redox cycle comprising three kinds of ions is made easier by the ability of the B phase to maintain relatively high contents of vanadium ions with valence states different from VI" without apparent modification of the structure. The phosphorus content strongly affects the activity and selectivity of the catalyst, determining the temperature range in which maleic anhydride can be obtained in high yields. With P:V ratios higher than 1.0, it is necessary to operate at higher temperatures, because the excess of phosphorus decreases the activity by hindering the participation of VIv in the redox cycles. However, a slight excess of phosphorus is necessary to control the rate of oxidation of VIv to V", since an excessive oxidation of the surface is responsible for the oxidation of maleic anhydride to carbon oxides. Thus, maleic anhydride is not oxidized even a t high temperatures. The excess of phosphorus therefore principally affects the first part of the scheme of Figure 12. A defect of phosphorus, on the contrary, does not influence this first part of the scheme (the activity and the rate of VI" reduction are similar in the catalyst with P:V atomic ratio 1.0 and 0.95) but modifies the stabilization of VN in respect to its oxidation to V" and the ratio between the rate of maleic anhydride formation and decomposition at high temperature. Registry No. P, 7723-14-0;V, 7440-62-2;1-butene,106-98-9; maleic anhydride, 108-31-6; butadiene, 106-99-0.
Bremer, M. J.; Dria, D. E. U S . Patent 4398535, 1983. Brkic, D.; Trifirb, F. Ind. End. Chem. Prod. Res. Dev. 1979, 18, 333. Riva, A,; Trifir6, F. Ind. Eng. Chem. Prod. Cavani, F.; Centi, G.; Manenti, I.; Res. Dev. l983a, 2 2 , 565. Cavani, F.; Centi, G.; Manenti, I.; Riva, A,; Trifird, F. "Proceedings, 9th Iberoamerican Symposium on Catalysis"; Lisboa July 1984a; p 973. Cavani. F.; Centi, G.; Trifir6, F. Ind. Eng. Chem. Prod. Res. Dev. 1983b, 22, 570. Cavani, F.; Centi, G.; Trifirb, F. Appl. Cafal. 1984b, 9 , 191. Centi, G.: Fornasari, G.; Trifird, F. J . Catal. 1984a, 89, 44. Centi, G.; Galassi, C.; Manenti, I.; Riva, A,; Trifir6, F. I n "Studies in Surface Science and Catalysis"; Poncelet, G.; Grange, P.; Jacobs, P. A,, Ed.; Vol. 16; Elsevier Science Pub.; Amsterdam 1983; p 543. Centi, G.; Manenti, I . ; Riva, A,; Trifird, F. Appl. Cafal. 1984b, 9 , 177. Centi, G.; Trifird, F.; Vaccari, A,; Pajonk, G. M.; Teichner, S. J. Bull. SOC. Chim. Fr. 1981, 7 - 8 , 1. Hanke, W.; Bienert, R.; Jerschkewitz, H. G. 2.Anorg. Allg. Chem. 1975, 414, 109. Hodnett, B. K.; Delmon, B. Appl. Cafal. 1984, 9 , 203. Hodnett, B. K.; Permanne, Ph.; Delmon, B. Appl. Cafal. 1983, 6 , 231. Hofmann, H.; Emig, G.; Rijder, W. "Proceedings, 8th International Symposium on Chemical Reaction Engineering"; Edimburg, Sept 1984; p 419. Hucknall, D. J.; "Selective Oxidation of Hydrocarbons"; Academic Press: London, 1974. Katsumoto. K.; Marquis, D. M. U S . Patent 4 132670, 1979. King, E. F.; Good,M. L. Specfrochim. Acta 1973, 29, 707. Martini, G.; Morselli, L.: Riva, A,; Trifirb, F. Reacf. Kinet. Cafal. Left. 1978. 8 , 431. Martini, G.; Trifird, F.; Vaccari, A. J . Phys. Chem. 1982, 8 6 , 1573. Morselli, L.; Riva, A,; Trifird, F.; Zucchi, M.; Emig, G. Chim. Ind. (Milan) 1979, 10, 791. Morselli, L.; Trifirb, F.; Urban, L. J . Catal. 1982, 75, 112. Nakamura, M.; Kawai, K.;Fujwara, Y. J . Catal. 1974, 34. 345. Niwa, M.; Murakami, Y. J . Cafal. 1982, 7 6 , 9. Ruggeri, 0.; Trifid, F. Appl. Catel. 1981, 7 . 395. Poli, G.; Resta. I.; Poli, G.; Ruggeri, 0.; Trifird, F. "Proceedings, 9th International Symposium on Reactlvity of Sollds"; Cracow, 1980; p 512. Schneider, R . A. U S . Patent 4043943, 1977. Varma, R . L,; Saraf, D. N. Ind. Eng. Chem. Prod. Res. Dev. 1979, 18, 7.
Received for review August 1, 1984 Revised manuscript receiued November 20, 1984 Accepted December 8, 1984
Literature Cited Ai, M. Bull. SOC. Chim. Jpn. 1970a, 43, 3490. Ai, M.; Boutry, P.; Montarnal. R. Bull. SOC. Chim. Fr. 197Ob, 8 - 9 , 2775. . - 9 , 2783. Ai, M.: Boutry, P.; Montarnal, R. Bull. SOC. Chim. Fr. 1 9 7 0 ~ 8 Ai, M.; Suzuki, S. Bull. SOC. Chim. Jpn. 1974, 4 7 , 3074. Bailhausen. C. J.; Gray, H. B. Inorg. Chem. 1962, 7 , 111. Bhargava, R . N.; Condrate, R. A. Appl. Specfrosc. 1977. 31. 230. Bordes, E.; Courtine, P. J . Cafal. 1979, 57, 236.
The present work was realized out with the contribution of the research program "Progetto Finalizzato per la Chimica Fine" of the National Research Council, Rome, Italy.
Mechanistic Model of the Selective Catalytic Reduction of Nitric Oxide with Ammonia Ronald J. Wllley,' John W. Eldrldge, and J. R. Klttrellt Chemical Engineering Department, University of Massachusetts, Amherst, Massachusetts 0 1003
The kinetics of iron-chromium-catalyzed selective catalytic reduction of NO by NH3 in the presence of oxygen and water were studied. A CSTR was used with activated 4304s catalytic screens attached to the rotating shaft. Reaction rates were measured as a function of concentrations of NO, NH, O,, and HO , to examine plausible mechanisms. Adsorption of NO conformed to a Freundlich isotherm while ammonia adsorption followed by surface dissociation was modeled with a Langmuir isotherm. Oxygen was needed to regenerate active sites, which it is speculated are reduced by the hydrogen atoms from the ammonia dissociation in a redox mechanism. Water adsorbed competitively, thereby retarding the rate. Neither H, nor NO , had a significant effect on the rate, while NO, appeared to produce some enhancement. A dual-site model with adsorption and dissociation of NH, NO adsorption, lattice O2replacement, and competitive water adsorption was used to describe the reaction rate data.
Introduction Control of NO, emissions from stationary combustion sources has become an important national and interna-
* Department of Chemical Engineering, Northeastern University, Boston, MA 02115. KSE, Inc., Amherst, MA 01004. 0196-4321/85/1224-0226$01.50/0
tional problem. Not only do nitrogen oxides create a direct health hazard 8s air Pollutants, but they also Play a key role in smog formation. Through their Promotion of SOB formation, their acidic nature, and their strong oxidizing effect on surface minerals, they can play a major role in acid rain formation and effects. For many applications, the postcombustion process of selective catalytic reduction (SCR) of NO, with NH, is 0 1985 American Chemical Society
Ind. Eng. Chem. Prod. Res. Dev., Vol. 24, No. 2, 1985 227
deemed to be highly promising for major reductions of such emissions, beyond those attainable via combustion modifications. Many mechanistic studies of this SCR have been made, several of which are highlighted below. Otto et al. (1970, 1972,1973)passed isotopically labeled 15NH3with NO, or NH3 and 16N0over copper, platinum, and rhodium catalysts. The copper was unsupported while the platinum and rhodium were on an alumina support. Based on their results, a mechanism was proposed in which NH, dissociates to NH2, which then reacts with an absorbed NO to form Nz and HzO. The remaining adsorbed H surface species reacts with a second adsorbed NO to form an HNO complex which dissociates to N 2 0and H20. That work excluded the presence of oxygen. Markvart and Pour (1967), Katzer (1975), and Bauerle et al. (1978) have pointed out that oxygen has a positive influence on the rate of reduction. Since postcombustion gases contain oxygen, a relevant reaction model and mechanism should reflect oxygen participation. The reaction stoichiometry when oxygen is present has been proposed (Rosenberg, 1978) to be 4N0
+ 4NH3 + O2
-
4N2 + 6H20
(1)
The experimental work of this study also supports this stoichiometry. Katzer (1975) used the mechanism of Otto et al. to develop a model which is first order in NO and half order in NH,. He also proposed that the reaction rate in the presence of oxygen would be half order in oxygen. Inomata (1980) and Miyamoto (1982) proposed for V205 catalyst in the presence of oxygen that ammonia adsorbs via a Langmuir isotherm, and that NO reacts from the gas phase or adsorbs only lightly. Their mechanistic explanation suggests how oxygen enters into the reaction; however, their final rate model does not include an oxygen term. Takagi et al. (1977) concluded that oxygen facilitates the adsorption of NO as NO2 and that NH3 adsorbs as NHI. For an iron oxide-titanium oxide catalyst, Kat0 et al. (1981) proposed that adsorbed NH, and NO react to form N2 and HzO and that the remaining surface hydride reacts with oxygen to form water. The oxygen comes from either the surface or from the catalyst lattice. However, they did not present a kinetic model. Meier and Gut (1978) derived a mechanism and model using oxygen. For an aluminasupported platinum catalyst, based on the reaction 4N02 + 4NH3 + 302
-
4N20 + 6Hz0
(2)
their model was
They used reaction 2 because of the amount of N 2 0 observed in the product. As indicated above, views on the mechanism of the reduction of NO with NH,, and the role of oxygen have varied with researchers, catalysts, and conditions studied. It is our purpose to present a kinetic study of the mechanism by using an iron-chromium catalyst which is a highly active catalyst for this reaction (Willey, 1984).
Experimental Section The catalyst used in this study was 18 mesh, 0.017-in. wire, 430 stainless steel screen treated by a method described by Baldi and Damiano (1981). Basically, the method involves diffusion coating the screen with aluminum and then etching most of the aluminum away with a caustic solution. This catalyst had 0.5 w t % of aluminum
MAGNE-DRIVE PULLEY ' 4 ~ 9 9 16 cm'
Figure 1. Aluminum CSTR with catalytic screens attached to magne-drive shaft.
remaining after etching and a BET surface area of 2 m2/g. Further characterization of the surface of this catalyst by SEM and microprobe is presented elsewhere (Willey et al., 1985). The mechanistic study was conducted using a 992-cm3 aluminum, Carberry-type, continuous stirred tank reactor, portrayed in Figure 1. Four catalyst screens, 3.5 X 11.4 cm each, with a total weight of 24.2 g, were attached to the stirrer shaft and rotated at 1000 rpm. Tracer studies supported the perfect mixing assumption in the data analysis. Aluminum was used for the reactor body because it is catalytically inert. The stainless steel stirrer shaft and support frame for the catalytic screens were painted with Thurmalox silicone ceramic coating to render them catalytically inactive. The feed gas was supplied from cylinders, with the exception of water vapor which was generated by a water vaporizer. The pilot plant schematic is shown in Figure 2. Nitric oxide concentrations were measured by Thermo Electron chemiluminescent NO-NO, gas analyzers. Since analysis was by gas chromatography for oxygen, nitrogen, nitrous oxide, and some ammonia measurements, helium was used as the carrier gas. GC packings used were MS 5A for O2 and N2, and Poropak QS for NzO and for high concentrations of NH, (above 3%). Lower NH, concentrations were measured with both the chemiluminescent analyzers and Matheson-Kitagawa ammonia tubes, with good agreement. Water vapor concentrations were measured by adsorption on molecular sieves, by an EG&G dew point hygrometer, and by flow rates, the latter method proving to be the most reliable. Typical feed composition consisted of helium containing 10% oxygen, 3600 ppm water vapor, 1500 ppm NH,, and either 500 ppm or 5000 ppm NO. Reactant functionality testing required adjusting gas concentrations in the reactor to constant values for all but one component. This allowed relating rate data directly to one variable concentration. The total gas flow rate, which in the range used did not significantly affect the reaction rate, was essentially the same in all functionality experiments, at approximately 62 cm3/s measured at 298 K and 1 atm. The reactor temperature was maintained constant within f3 K and averaged 564 K, well below that at which ammonia oxidation becomes significant over this catalyst. It was found that the screens required initial conditioning under reaction conditions to stabilize their catalytic activity. These screens were conditioned for a total of 36
228
Ind. Eng. Chem. Prod. Res. Dev., Vol. 24, No. 2, 1985 GAS CHROMATWRAW
t
t t l t t t t
@ 4
He
CIH
0, N O
wr
TEST METER
I
ROTAMLTEI)
NH,
CHEMILUMINESCENT ANALYZER
I
-41
n 11
CATALYTIC /SCREENS
ROTAMETER
N2 r i 2 0
CONTROL VALVE PRESSURE GAUGE
II AUTOCLAVE-ENQINEERS REACTOR
Figure 2. Flowsheet of experimental reactor system.
h over three conditioning runs. The last conditioning run showed no further change in activity over a period of 10 h before the functionality experiments discussed below were begun. The experiments were designed to investigate the kinetic effects of four main variables: concentrations of NO, NH3, Oz, and H20. Also, experiments were conducted to determine if the presence of Hz, NOz, or NzO affected the rate of reduction of NO. Results and Discussion The NO, NH,, 02, and HzO were blended into the helium a t flow rates adjusted to hold the reactor concentrations of any three as constant as possible while varying that of the fourth. As the reactor concentration of each was varied separately, the effect on the rate of NO reduction was measured. Objectives were to determine if a single- or dual-site model explained the data, to determine the roles of oxygen and water in affecting the rate, and to examine reaction stoichiometry. Nitric Oxide Functionality. The functionality of NO was determined by keeping NH3, 02,and HzO concentrations in the reactor at 1400 to 1600 ppm for NH3, 10% for oxygen, and 3000 to 4500 ppm for HzO. Figure 3 shows the rate of NO conversion increasing with NO partial pressure at slightly less than first order. In this figure, the measured rates have been corrected to a common base of concentrations for NH3 and HzO (1500 and 3600 ppm, respectively) by their functionality models, to be discussed later. Table I lists functionalities of NO considered. For the indicated F test results, parameter estimates were derived from nonlinear least-squares analyses. Model 3-NO, which represents NO adsorption as a Freundlich isotherm, best describes the data, and so was used as the basis for the curve in Figure 3. It was also used for the NO functionality in the general kinetic model derivation presented below. Otto and Shelef (1970) also have reported that NO adsorption on iron oxides can be described by a Freundlich isotherm. Ammonia Functionality. Ammonia functionality was determined by keeping reactor concentrations at 490 to 560 ppm for NO, 10% for Oz, and 3000 to 4000 ppm for H20. Since rate is very dependent on NO concentration, all rates were also corrected to a common NO concentra-
PPm
2000
6000
4000
000 c Model 3-NO 01 Table 1
Adlusted lo constant reactor cordfiions of:
NH3
152 Pa (1500DDm)
o2
10132Pa
30
365 Pa
(
10%
)
(3600Dpm)
Carrw: Helium T
200
- 564 K
400 PNo ,Pa
600
Figure 3. Effect of NO partial pressure on NO reduction at conand H,O partial pressures. stant NH3, 02,
tion of 500 ppm by Model 3-NO. Rates were also adjusted to a common water concentration of 3600 ppm by procedures discussed below. The results shown in Figures 4a and b indicate a very strong influence of NH3 partial pressure on the rate of NO reduction at low NH3 concentrations. The rate rises to a maximum at NH3 concentrations of about 400 ppm and then declines as NH3partial pressure is further increased. Table I lists functionalities of NH3 considered, based on models from the literature, applied to constant NO partial pressure. Model 1-NH3represents a Rideal-Eley single site mechanism as shown by Miyamoto (19821, or a bifunctional catalyst mechanism as shown by Takagi (1977). Model 2-NH3can be derived from the Otto-Shelef mechanism by assuming that the rate-limiting step is the reaction between NH2(ads) and NO(ads), coupled with a steady-state balance on the H(ads) species as shown by Katzer (1975). Model 3-NH3 can be derived by assuming an associative adsorption of ammonia and a surface species directly proportional to NH3 partial pressure as shown by Gupta (1970).
Ind. Eng. Chem. Prod. Res. Dev., Vol. 24, No. 2, 1985 2
% NH,
E
4
8
7h 0 2
5
10
229
20
15
Adjusted to constant reactor conditions of:
NO 0,
10132.0 Pa
H,O
365.0 Pa
50.7 Pa
( 5 0 0 ppm) (
10% )
( 3 6 0 0 ppm)
C a r r i e r : Helium T - 564 K
a
-
0 100.
0 L.
O-. 0
I
I
2
4
P NH, 3"
200
,
I
I 8
I 6
600 ppm NO
2
kPa
400
400
600
o 0
-
o
5
10
P,
e
Model 2-NH,
e
t
l
15
20
,kPa
Figure 5. Effect of O2partial pressure on rate of NO reduction at constant NO, NH3, and H20 partial pressures.
of Table 1
I
I
b 100
-
NO
300
I
I.
c \
m
El
0
20
60
40
RH^^
I
Adjusted to constant r e a c t o r c o n d i t i o n s of:
60
z
200
50.7 P a
( 500 p p m )
NH,
152.0 Pa
0,
tOi32.OPa
M o d e l 3-H,O
C a r r i e r : Helium
of T a b l e 1
T
-
(1500 p p m ) (
10%
1
564 K
Pa
Figure 4. Effect of NH3 partial pressure on rate of NO reduction at constant NO, 02,and H 2 0partial pressures: (a) full range of data; (b) low partial pressure range.
Model %NH3 gave the superior fit and is shown as a solid line in Figures 4a and b together with the raw data points. An explanation for the decline in reaction rate when NH, partial pressure is high is that adsorbed NH3 or its fragments can block sites for NO adsorption, thus retarding the rate of reaction. These results support the dissociation of NH3 on the surface of the catalyst as a mechanistic step. Therefore, they also support work presented by Katzer (1975), Gland (1978), and Takoudis (1983). Transient reaction rate experiments were performed to assist in understanding the adsorption process. One experiment in which the rate of reaction was followed after NH, was removed from the reactant stream showed initially an increase in the rate of NO reduction as the catalyst surface cleared itself of adsorbed NH,, thereby unblocking sites for NO absorption. The rate passed through a maximum then declined as expected since no more NH, was being supplied. The results of this experiment indicate that ammonia is adsorbed on the same type of site as those on which NO is adsorbed. Oxygen Functionality. Oxygen functionality was evaluated at two levels of reactor concentration of NO, viz., approximately 500 and 5000 ppm NO. All rates were corrected to common concentrations of 500 ppm NO, 1500 ppm NH,, and 3600 ppm H20 for the first level of NO, and 5000 ppm NO, 2000 ppm NH,, and 3600 ppm H20 for the second level of NO. The results, shown in Figure 5, indicate that the rate of NO reduction increases at less than first order as oxygen partial pressure is increased. Of the several models evaluated, the three which fit best are given in Table I. Model 1-O2is a simple empirical model similar to a Freundlich isotherm. Model 2-O2 is a
..
P
H,O
,kPa
Figure 6. Effect of H 2 0 partial pressure on rate of NO reduction at constant NO, NH3, and O2 partial pressures.
half-order expression suggested by Katzer (1975), which includes an adsorption term for oxygen. Model 3-02is a derivable model based on lattice oxygen replacement. All three models fit the data equally well with no significant difference indicated by the F statistic. Since model 3-02 is consistent with the mechanism presented later as the basis for the general kinetic model, it was used to compute the solid lines shown in Figure 5. Water Functionality. The constant concentrations of NO, NH,, and O2used for studying the effect of water were 500 ppm NO, 1500 ppm NH,, and 10% 02. The result, shown in Figure 6, was that the presence of water retarded the rate of reaction. The three functionalities given in Table I showed no significant difference in their quality of data representation. Since model 3-H20is consistent with the overall mechanism presented below, in which water enters by competitive adsorption in a dual site mechanism, it was selected for display as the solid line in Figure 6. Kato et al. (1981) noted that water retarded the rate of reaction, which they also explained as competitive adsorption. Effects of Hydrogen, Nitrogen Dioxide, and Nitrous Oxide. Hydrogen, nitrogen dioxide, and nitrous oxide were added separately to the reactant gas stream with NO and NH, present. The objective was to determine if any of these gases influenced the rate of NO reduction. The results are summarized in Table 11.
230
Ind. Eng. Chem. Prod. Res. Dev., Vol. 24, No. 2, 1985
Table I. Individual Functionalities Considered for NO, NH,, 0,, and H,O model no.
model rNO
deg of freedom
resid mean square
F ratio
F stat 95% conf
Functionalities of NO Considered (at Constant NH,, U,, and H,O) 1-NO 2-NO
kPN 0
2
2293.5
23.31
9.55
2
2635.0
26.78
9.55
3
98.4
Functionalities of NH, Considered (at Constant NO, 0,, and H,O) kl“ H , 7 7055.6 14.30
3.79
Kl
+
KNOPNO kPN n
3-NO
1-NH, 2-NH,
3-NH3 15869.7
7
1-0, 2-0,
3-0,
1-H,O 2-HZO 3-H,O
a
32.16
Functionalities of 0, Considered ( a t Constant NO, NH,, and H,O) ( 2 Data Sets) 6‘ 687.5 1.37 kP03’” 6b 833.0 1.01 i P O, 5” 499.7 5b 2.04 1691.9 K , t K,Po,~” kP, 6‘ 545.6 1.09 6b 827.8 ( 1 + K3P021‘4)2 Functionalities of H,O Considered ( a t Constant NO, NH,, and 0 , ) kP, 3 90.4 k 2 146.9 1.62
k
2 ( K , + KwPw)Z Refers to runs with 5 0 0 ppm NO reactor concentration.
228.8
2.53
3.79
4.95 4.28 4.39 4.95
9.55 9.55
Refers t o runs with 5000 ppm NO reactor concentration.
Table 11. Effects of Hz, NOz, and NzO on Rate of NO Reduction He carrier N2 carrier without H2 with 2.4% H2 without NO2 with 100 ppm NOz 555 555 566 566 T,K flow rateo 0.1746 0.1746 0.1033 0.1033 224 212 57 61 rate N O conv.*
He carrier without N20 with 8000 ppm N20 560 560 0.524 0.524 325 325
“Total flow rate at 294 K (including carrier), m3/h. ’In pmol/(h 9).
Addition of 2.4% of hydrogen was accompanied by a slight retardation in the rate of NO reduction. This may be partially explained by a slight increase in space velocity which could not be measured exactly. Because only definietly significant responses were being sought, it was concluded that molecular hydrogen played no significant role in the reduction of NO with NH3 in this investigation. Without ammonia present but with hydrogen present, the reduction rate was nearly zero. Hence, under these conditions with this catalyst, H2 did not significantly reduce NO. The HNO surface species shown in the mechanism of Otto et al. (1970, 1972, 1973) was postulated by them on the basis that the addition of H2 did reduce NO to NzO over platinum in the oxygen deficient environment. When approximately 100 ppm NOz was added to the feed containing 285 ppm NO (resulting in reactor concentrations of 10 ppm NOz and 20 ppm NO) and with an oxygen concentration of 14.8%, a slight increase in NO and NO, (as NO + NOz) reduction rates was observed. The purpose was to see if the oxidative power of NOz could be used to increase the effectiveness of the catalyst, since
oxygen itself had a positive influence. The results indicated that this may indeed be the case. The addition of up to 8000 ppm N 2 0 showed no effect on the rate of reduction of NO with NHB. It was also determined, by direct effluent gas composition measurements, that NzO formation by reaction was negligible at these conditions with this catalyst. The rates were significantly different among the three conditions examined (H2 vs. NO2 vs. N20) because NO reactor concentrations were significantly different for each set. Within a set (the without vs. with conditions), the rates were adjusted to a common NO reactor concentration for proper comparison. Examination of Reaction Stoichiometry. Reaction stoichiometry was examined by means of a mass balance across the CSTR. These experiments utilized feed concentrations around 5000 ppm NO with somewhat lower concentrations of O2and NH3,so that oxygen consumption could be followed accurately by gas chromatographic measurements. The atomic nitrogen balance was within 10% for most points. The resulting data on the molar ratio
Ind. Eng. Chem. Prod. Res. Dev., Vol. 24, No. 2, 1985 231 Table 111. Reaction Rate Data for Kinetic Model Evaluation data point 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36
OPa = 0.101325 l 1200 c
> 1000 al
-
X
NO concn, ppm' 500
NH3 concn, ppm' 50
470 550 490 510 500 490 560 510 500 580 470 582 510 518 490 6100 1475 47 515 490 900 500 500 505 520 500 500 4610 4460 4950 6200 4850 4910 5200 5370
210 900 20 000 7 857 700 30 000 82 000 50 500 1600 1330 1440 1520 1412 1330 1600 1625 1463 1400 1710 1900 2 080 2 220 1595 1587 1580 1500 2 000 1720 2 240 2 200 1866 2 160 2 240 2 480
O2 concn, ppma
H 2 0 concn, ppm'
obsd rate, w " / ( h g)
pred rate, pmol/(h g)
100000 100 000 100 000 100 000 100 000 100000 100 000 100000 100 000 100 000 102 000 102 000 102 000 102 000 117 000 100 000 90 000 92 000 100 000 100000 105 000 8 750 8 750 682 197 000 100000 34 000 100 000 228 360 1006 970 2 178 2 361 90 551 91 300
3 708 4 023 3 933 3 393 3 663 3 858 3 243 3 198 3 618 1290 151000 2 733 19 000 64 000 108 000 3 588 6 933 4 345 2 925 3 573 3 813 3 393 3 198 2 845 3 790 3 678 3 318 3 858 885 1140 1455 1425 1275 2 055 5 205 7 458
215.4 270.0 269.6 148.3 209.1 252.7 125.3 108.2 206.3 300.5 117.7 260.2 221.3 154.7 131.0 256.4 1260.7 483.5 57.2 251.6 250.6 155.2 109.4 26.7 255.2 228.2 162.0 271.2 191.2 246.2 318.2 315.0 416.2 438.1 1161.4 1132.2
207.6 257.5 279.0 117.7 168.0 266.0 100.7 74.9 209.9 278.6 104.7 242.0 243.9 160.2 126.6 246.5 1246.1 512.4 39.0 253.8 239.1 195.3 125.6 53.0 281.9 250.5 189.5 245.2 174.8 204.0 315.5 361.0 409.2 418.5 1154.9 1149.8
ppm. 4
O
0
9
-
U
support the stoichiometry of reaction 1 as being that prevailing in an excess of oxygen over this catalyst. Proposed Mechanism. The results of this work support the following: (a) NO adsorption can be described by a Freundlich isotherm. (b) NH, adsorbs dissociatively on the surface. (c) O2 enhances the rate of reaction. (d) H 2 0 retards the rate of reaction. From these results the following mechanism is proposed
6000
NO
k
+ s -f NO.S k-4
(4)
k
NH3 + S & NH3-S
(5)
N H 2 8 + N O 3 -!L N2 + H20-S+ S
(7)
k-5
0
200
400
600
800
1000 1200 1400
O B S E R V E D R A T E , )Ifnolea/ hr 9
Figure 7. Agreement of kinetic model, eq 13, with all observed rates of NO reduction.
of O2 consumed to NO reduced averaged 0.24. The data on the molar ratio of NH, consumed to NO reduced averaged 0.80. Given the difficulty of accurate ammonia measurements with several analytical methods attempted, it is not surprising that the 95% confidence region encompasses ammonia consumption ratios (moles of NH3 consumed per mole of NO reduced) of both 1:1 and 2:3, which are the ratios in the stoichiometry of reaction 1and the oxygen-free equation, 6 N 0 + 4NH3 5Nz + 6H20, respectively. The more accurately measurable oxygen consumption ratio (moles of O2consumed per mole of NO reduced) averaging 0.24, however, is very close to the stoichiometric ratio of reaction 1. These results therefore
-
OH43 + S'
H-S + S
4s'
+0 2
4s
7
k-IO
k
H20& aH 2 0 + S
XT
(11)
Combination of the appropriate multiples of these equations results in the overall net stoichiometric equation of reaction 1. Mechanistic Model. From the above mechanism, the following kinetic model is derived with these postulations:
232 Ind. Eng. Chem. Prod. Res. Dev., Vol. 24, No. 2, 1985
(a) Reaction 7 is rate determining. (b) Active sites involve lattice oxygen shared with adjacent sites. (c) The Freundlich isotherm describes NO adsorption. (d) The NO adsorption is also proportional to the concentration of active sites on the catalyst. (e) Water adsorbs competitively on the active sites. NO = ~ k , ( p ~ # p ~ ~ 3 p o , ) " 2 1+/ (K##p## [l
+
K N H , ~ N+ H ,KwPw +
at equal rates, NH2.S by reaction 7 and H-S by reactions 8 and 9. Furthermore, the steady-state balance on 0H.S requires that the rates of reactions 8 and 9 be equal. Therefore k,[NHz*S][NO*S]= 2ka[H*S][S] (15) where the brackets indicate concentration of the species on the catalyst. Solving for [HmS]
CY(PNH,/P##)'/' @ ( P N H , P ~ #-k) 'Kx4Pg4)2) / ~ ] ~ ( ~ (12) where K, = adsorption equilibrium constant, k , = lumped rate constant = ( ~ ~ , ~ & , & # K N H , K O ,X) ~C~t ~, ki = forward rate constant for step i above, C, = total concentration of sites (oxidized and reduced), a = lumped constant = (2k$(G/k7)'/2X (KNH /K#$)'J2, and P = lumped constant = (k7K6/2k8)lI2X (dN~3K#)'/2.Details of this derivation are presented in the Appendix. The preliminary fit of this model by nonlinear least squares indicated that KNO, K N H 3 , and @ were near zero. Thus the model simplified to ?NO
From reaction 6 and eqn 16
With NO adsorption described by a Freundlich isotherm and NH, and H 2 0 adsorption described by Langmuir isotherms [NO41 = K##P##[S] [NH,*SI = KNH3PNH3[Sl
+ KWPW + a ( P N H 3 / P g # ) '"1 2(1+ Kg4Pg4)'}( 13)
= ~~s(P##PNH,PO,)"~)/~[~
Equation 13 was then fitted to the raw data from all 36 sets of conditions used for determining the individual functionalities, and the results are given in Table 111. The parameter values for 95% confidence from this fit are
k,, pmol/h g Pa(l
+
1/2M =
[HzO*S]= KwPW[S] Substituting eq 18 and 19 into eq 17 gives
or
2.73 f 0.65
N = 1.57 f 0.051 K,, Pa-' = 1.23 X cy,
$-
1/2n9 =
f 4.1 X
0.56 f 0.069
Kg4, Pa-l/* = 0.13 f 0.014
(14)
The predicted rates given in Table I11 were calculated with these parameter values and are plotted versus the observed rates in Figure 7. This shows that the final model describes the experimental observations very well. Summary The effects of four variables, NO, NH,, 02,and H 2 0 concentrations, on the rate of NO reduction by NH, over etched metal catalysts were examined. The results indicate that NO adsorption is required and follows a Freundlich isotherm. They also support NH3 adsorption by dissociation into NH2 and H. Data reflecting oxygen participation are consistent with either lattice oxygen transfer or adsorbed oxygen on a neighboring site of different type. Water appears to be adsorbed competitively on active sites. Functionality experiments support a dual-site rate-determining step between adsorbed NO and adsorbed NH2. A mechanism is presented which is consistent with all observations, and a kinetic model derived from this mechanism predicts the observed rates very well. It was also observed that neither H2 nor NzO had any significant effect on the rate of NO reduction, while NO2 appeared to produce some enhancement of the rate. Appendix Derivation of Kinetic Model. Reaction 7 is taken as rate controlling. The H-S species is converted irreversibly to H20& by the sequence of reactions 8 and 9, with the O H 6 species a short-lived intermediate. All other reactions are taken to be at pseudo-equilibrium under steady-state reaction conditions. At steady state, since the N H Z 4and H-S species are created at equal rates by the net forward effect of reaction 6, they must be converted
Since step 7 is rate controlling rNo = k,[NHyS] [NO-SI Substituting eq 18 and 21 into eq 27 gives
Ind. Eng. Chem. Prod. Res. Dev., Vol. 24, No. 2, 1985 233
Since O H 4 is a short-lived intermediate, its concentration can be neglected compared with other species. [coxdl represents total concentration of oxidized sites per gram of catalyst. Solving for [SI in eq 30 and substituting into eq 28 leads to rNO
=~k’(~~#~NH3)1~2[~oxd~2~/
-
(31) From reaction 10, where it is assumed that the total concentration of oxidized sites is proportional to the gas-phase concentration of oxygen, it is found that
k1,[S’I4Po, = k-10[Coxd14
(32)
or
K1 = parameter used in reduced reaction models shown in Table I, greater than 1.0 K2 = parameter used in reduced reaction models shown in Table I, K3 = parameter used in reduced reaction models shown in Table I,
Ks = equilibrium constant for reaction 6, dimensionless Ki = adsorption equilibrium constant for component i, Pa-l m = exponent parameter used in functionality fits for O2 and H 2 0 in Table I N = Freundlich isotherm constant, dimensionless Pi = partial pressure of component i, Pa PNo = partial pressure of NO, Pa rNo = rate of NO reduction, pmol/(h g) S = empty oxidized catalyst site, molecule S’ = reduced catalyst site, molecule [SI = concentration of empty oxidized catalyst sites, pmol/g [S’] = concentration of reduced catalyst sites, pmol/g T = temperature, K yi = mole fraction of component i, dimensionless W = subscript used to represent water (H20) Greek Letters = lumped constant defined in eq 12, Pac-1/2+1/2hr, 0 = lumped constant defined in eq 12, Pa(-1/2-1/w Registry No. NO, 10102-43-9; NH3,7664-41-7; 02,7782-44-7; HzO, 7732-18-5; iron, 7439-89-6; chromium, 7440-47-3. CY
[S’l =
(K0,p0,)-1/4[coxd1
(33)
An overall site balance requires reduced plus oxidized sites to equal a constant level, Ct.
[ctl = [S’l + [ c o x d l
(34)
Substituting eq 33 into eq 34 gives
[ctl = (KO~Oo,)-1/4[c~~d1 + [coxdl
(35)
so 1/4
[coxdl
=
1
ICt]
+ (Ko$’o,)’/~
(36)
Equation 36 can be substituted into eq 31 to give the final rate equation shown above (eq 12). Nomenclature [A-SI = concentration of adsorbed molecules on the catalyst, Pmol/g [cox,] = concentration of total oxidized catalyst sites, pmol/g [C,]= total concentration of catalyst sites (oxidized and reduced), wmol/g k = lumped forward rate constant used in various reduced reaction models of Table I Y ,
.
.
kl ‘= rate constant for eq 3, (pmol Pa3)/(h g) ki = forward reaction rate constant for reactions 4 to 11 k-; = reverse reaction rate constant for reactions 4 to 11 k,’= lumped rate constant defined in equation 12, pmol/(h gPa(l+ll~)
Literature Cited Baldi, A. L.; Damiano, V. V. U S . Patent 4292208, 1981. Bauerie, G. L.; Wu, S. C.; Nobe, K. Ind. Eng. Chem. Prod. Res. Dev. 1978, 17, 117. Bodenstein, M. 2 . Elektrochem. 1918, 2 4 , 183. Gland, J. L.; Korchak, V. N. J. Catal. 1978, 5 5 , 324. Gupta, B. P. Ph.D. Dissertation, University of Cincinnati, Cincinnati, OH, 1970. Inomata, M.; Miyamoto, A,; Murakami, Y. J. Catal. 1980, 6 2 , 140. Kato, A,; Matsuda, S.; Nakajima, F.; Imanarl, M.: Watanabe, Y. J. Phys. Chem. 1981, 8 5 , 1710. Katzer, J. R. I n “The Catalytic Chemistry of Nitrogen Oxides”, Kiimish, R. L.; Larson, J. G., Ed.; Plenum Press: New York, 1975; p 133. Markvart, M.; Pour, V. J. Catal. 1967, 7 , 279. Meier, J.; Gut, G. Chem. Eng. Sci. 1978, 33, 123. Miyamoto, A.; Yamazaki, Y.; Hattori, T.; Inomata, M.; Murakami, Y. J. Catal. 1982, 74, 144. Otto, K.; Sheief, M.; Kummer, J. T. J. Phys. Chem. 1970, 74, 2690. Otto, K.; Sheief, M. J. Phys. Chem. 1972, 7 6 , 37. Otto, K.; Sheief, M. Z . Phys. Chem. (Frankfurt am Main) 1973, 8 5 , 308. Rosenberg, H. S.;Curran, L. M.; Slack, A. V.; Ando, J.: Oxiey, J. H. Paio Alto, California, Oct 1978, EPRI Report FP-925. Seiyama, T.; Arakama, T.; Matsuda, T.; Takita, Y.; Yamazoe, N. J. Catal. 1977, 48, 1. Takagi, M.; Kawai, T.; Soma, M.; Onishl, T.; Tamaru, K. J. Catal. 1977, 50, 441. Takoudis, C. G.; Schmidt, L. D. J. Phys. Chem. 1983, 8 7 , 958. Wiiiey, R. J. Ph.D. Dissertation, University of Massachusetts, Amherst, MA, 1984. Wiiiey, R. J.; Conner, W. C.; Eidridge, J. W. J. Catal. 1985, 92, 136.
Received for review October 2 , 1984 Accepted February 6, 1985
The authors wish to acknowledge partial support from Northeast Utilities Services Co. of Hartford, CT, and Alloy Surfaces Co., Inc., of Wilmington, DE.
