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6. The resin capacity was unaffected even at high burner fuel oil concentration (17 500 ppm). Acknowledgment The authors are indebted to the sponsoring companies of the University of Tulsa Environmental Protection Projects program for the funds for equipment and support required to complete this study. Thanks are due to the following companies for materials used in this study: Rohm and Haas Company for ion-exchange resins; Calgon Corporation for CL-68 corrosion inhibitor; and Dow Chemical Company for phenate sample. Thanks are also due to the Chemistry Discipline of The University of Tulsa for use of some of the experimental apparatus and Mr. David Siebert a t Cities Service Oil Company for the
scanning electron microscope services. Special thanks are due to Mr. John Byeseda for many helpful comments. Literature Cited Donohue, J. M., Ind. Water Eng., 15 (4), 8-13 (July/Aug 1978). Sabadell, J. E., Ed., "Traces of Heavy Metals in Water Removal Processes and Monitoring", €PA Report No. 902/9-74-001 (Nov 1973). Siebert, D. A.. "Removal of Chromates from Cooling Tower Waste Streams by Ion-Exchange", Technical Report No. 76-6, University of Tulsa Environ. Protection Projects, 1976. Yadeta, E., Sylvester, N. D.,"Chromate and Cyanide Removal from Industrial Wastewater, A State-of-+Art Review", Technical Report No. 753, University of Tulsa Environ. Protection Projects, 1975. Yamamoto, D., Koichi, Y., Osamu, A,, "Recovery of Chromate from Coollng Tower Blowdown by Ion-Exchange Resins", presented at the Cooling Tower Institute Annual Meeting, Houston, Texas, 1975.
Receiued for review April 14, 1978 Accepted January 11, 1979
CATALYST SECTION Automotive Exhaust Emissions Control Using the Three-way Catalyst System. 1. Computer Simulation of the NO-H2-02 Reaction on Pt/AI2O3 Catalyst Akira Miyamoto," Bunkei Inoue, and Yuichi Murakami Department of Synthetic Chemistty, Faculty of Engineering, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464, Japan
Using a method of computer simulation, the kinetics of the NO-H2-0, reaction on Pt/AI,O, has been studied. Inlet concentrations of NO and H, were 1000 ppm and 1.08 % , respectively. Inlet concentration of O2 (Coo) was varied in the range 0-1.1 %. When Coo, < 0.4%, exit concentration of NH, was kept high irrespective of Cbo,,, although NO was almost completely removed. When Coo* > 0.55 % , a considerable amount of NO remained unreacted while the formation of NH, was small. These data were analyzed by the computer simulation method, and simulated results agreed well with those of the experiments. The calculations have also shown the existence of a "window" where all of the NO, NH3, H, and 0,can be removed simultaneously. Furthermore, NH, has been found to play an important role as an intermediate in the reaction, and the H2-0, reaction has been shown to proceed more readily than the NO-H, reaction. On the basis of the simulated results, directions for the improvement of R/A1203 as a catalyst have been suggested. It has been concluded that the computer simulation method is promising for the analysis of a complex reaction system, such as the three-way catalyst system.
Introduction A single bed three-way catalyst system is one of the most promising methods under consideration for the control of automobile exhaust emissions by catalysts. As indicated in previous literature (for example, Shelef, 1975; Weigert and Koberstein, 1976; Gandhi et al., 1976), this technique brings about the simultaneous removal of nitric oxide, carbon monoxide, and hydrocarbons. This is done in a single bed by the use of an oxygen sensor in the exhaust system with feedback to the carburetor in order to hold the air/fuel ratio of the exhaust near the stoichiometric point. Much research has been done on this system, mainly from the practical point of view. Fundamental studies, however, have scarcely been reported in the literature. Jones et al. (1971) showed that the formation of NH3 due to the NO-H2 reaction on Pt/A1203is suppressed by 0019-7890/79/1218-0104$01.00/0
the presence of 02.Fedor et al. (1975) also found that, for the synthetic exhaust gas, the formation of NH3 is decreased by the addition of 02.Similarly, Mannion et al. (1975) showed that in the reaction of synthetic exhaust gas on supported Ru catalysts, NO can be removed without the formation of NH3 near the stoichiometric point. Furthermore, Klimisch and Taylor (1973) demonstrated that NH3 is an important intermediate in the reduction of NO in the synthetic exhaust gas. Klimisch and KOmarmy (1975) inferred from the durability data that, in the reaction of synthetic exhaust gas, the oxidations of H2, CO, and hydrocarbons proceed more readily than the reduction of NO. Taylor (1975) obtained more systematic data on the NO-H2-02 reaction, the NO-H2-CO-02 reaction, and the reaction of synthetic exhaust gas on alumina supported Rh, Pt, and Pd catalysts, and the data indicate that the behavior of the NO-H2-02 reaction on
0 1979 American Chemical Society
Ind. Eng. Chem. Prod. Res. Dev., Vol. 18, No. 2, 1979
Rh/A1203is similar to that of synthetic exhaust gas which contains not only NO, H2, and O2but also CO, H 2 0 , and hydrocarbons. In connection with the data by Taylor (1975), Klimisch and Barnes (1972) showed that the water gas shift reaction (CO + H 2 0 C 0 2 + H,) can readily be equilibrated and H2 thus formed can easily reduce NO to "3.
It must be kept in mind in developing fundamental studies of the three-way catalyst system that the reactions involved in this system are far more complex than previous ones usually considered in chemical industries, and that the number of elementary steps in the former reactions is consequently numerous. Since the three-way catalyst system is itself complex in nature, the reduction of the reactions in this system to a few elementary reactions may lead to a meaningless oversimplification. Therefore, a method available for the treatment and the analysis of complex reactions is highly desirable in addition to the traditional methods to analyze an elementary reaction or a simple reaction composed of a few elementary steps. Taking into account an increasing ability of a computer, a computer simulation method may give a solution to the problem. Thus, we have studied, as a first step, the NO-H2-02 reaction on Pt/A1203in order to judge whether such an approach proposed is promising for the analysis of the mechanism of a complex reaction system, such as the three-way catalyst system. This is because, as mentioned above, the NO- H2-02 reaction has been shown to be similar in several points to the reaction of synthetic exhaust gas. Of course, the NO-H2-02 reaction on P t / A1203is still a complex reaction with many elementary steps and the complete analysis of the mechanism of this reaction seems not to be easy. Therefore, we present the following questions as basic problems in the investigation of the mechanism of the NO-H2-02 reaction on Pt/A1203: (1) Is the behavior in the NO-H2-02 reaction similar to that in the reaction of the synthetic exhaust gas or not? (2) How does O2suppress the formation of NH3 due to the NO-H2 reaction? (3) Does the Hz-O2 reaction proceed more readily than the reduction of NO or not? (4) How are both NO and NH3 removed near the stoichiometric point? ( 5 ) Does NH3 play an important role in the removal of NO in the NO-H2-02 reaction on Pt/Al,O, or not? In order to give solutions to the above problems, we have investigated the NO-H2--02reaction on Pt/A1203catalyst experimentally, and at the same time have proposed a computer simulation method. Furthermore, on the basis of the simulated results, we have given directions for the improvement of the Pt/A1203 catalyst. E x p e r i m e n t a l Section Catalyst a n d Reagents. The Pt/A120, catalyst (Pt loading of 0.5 wt. %) was obtained from Nippon Engelhard and was granulated in the range 0.3--0.6 mm. Before use, the catalyst was reduced in an H2 stream a t 500 "C for 4 h. Commercial NO (4.85% in He balance), H 2 (99.99% purity) and O2 (99.9% purity) were used as reactants without further purification. N2 as a balance gas was supplied commercially and was passed through a reduced copper column to remove 02,an impurity in the N2. Apparatus a n d Procedure. Experiments were carried out with an ordinary flow reactor. Inlet concentrations of NO and H2 were 1000 ppm and 1.08%,respectively. Inlet concentration of O2 (Coo,) was varied in the range 0-1.1%. Concentrations of NO and NH3 were determined with the phenol disulfonic acid method and Nessler's method, respectively. H2 and O2 were analyzed using gas chromatographs with thermal conductivity detectors a t room temperature. Columns used were molecular sieve type 5A
105
Scheme I . Mechanism of the NO-H,-0, Reaction on Pt/A120,. (Species in parentheses represent adsorbed species.)
NH,
--kl_
( N H , ) -t ( H )
122
NO
k3
(NO)
k,
(NH,)
k5
.r
(NO) -----+
N,'t II,O
k6
( N H I ) -i. (NO) ----+ N,O .i 2 ( H ) k7 2(NO) t 2 ( H ) ---+ N,O .t II, 0 k 2(NO) i 4(H)---+ N, + 2 H i 0 k, 0, 2(0) k 10 lz,, H2O 2 ( H ) 4- (O)-+
121 2
H, F==* 2(H) k 13 k 14
(NO) t 4 ( H ) ---+ ( N H , )
iH,O
k!,
(NH,) .I- 2 ( 0 ) ( N O ) t H,O k I* H, + (0)---+ H,O k 17 0 , t 4(I-I) ---+ 2H,O --+
for H2 with N, as the carrier gas and molecular sieve type 13X for O2 with He as the carrier gas. Method of Computer Simulation Mechanism of t h e NO-H2--02Reaction. In order to analyze the kinetics of the NO-H2-02 reaction by the computer simulation method, the mechanism of the NO--H2--02reaction on Pt/A120, must be established. As yet no mechanism of the NO-H2-02 reaction has been proposed. Consequently, we adopted the mechanism shown in Scheme I referring to the mechanism of the NO--NH3reaction on supported precious metal cat,alysts proposed by Ott,o et al. (1970) and Otto and Shelef (1972, 1973), and to that of the oxidation of NH, on Pt/A1203 proposed by Ostermaier et al. (1974). In Scheme I, species represented in parentheses refer to adsorbed species, and ki (i = 1, 2, ..., 17) is the rate constant for the ith step. Although the validity of this mechanism has not been proved kinetically, the NO--H2,NO-NH,, NH3-02, and H2-02 reactions are all taken into consideration in the mechanism. Therefore, considering the present state of the investigations where no quantitative analyses of the kinetics of the NO---H2--02 reaction on Pt/A1203have heen made, the mechanism seems, to a first approximation, t,o be adequate. As to t.he reduction of (NO) with (H), i.e., as a steps 7 and 8, Otto et al. (1970) assumed ("0) surface intermediate in the reactions. Therefore, it rnay be more rigorous to consider ("0) as an adsorbed species in the calculation. These same rate expressions were was taken into consideration, obtained when ("0) provided we assumed that the coverage of ("0) was small enough. Similarly, the recent studies on the adsorption and the reaction of NO on Pt (eg., Pirug and Bonzel, 1977; Bonzel et al., 1978) suggest that the adsorbed nitrogen species, i.e. (N), should be taken into consideration in the reaction mechanism. The processes through (N) species, however, may be approximately represented by the direct processes from (NO) to N,O, N2, and (NH,), steps 7, 8, and 14 in Scheme I, respectively, since the coverage of (NO) or (N) in the reduction of NO on Pt catalyst is considered to be small (e.g., Shelef and Gandhi,
106
Ind. Eng. Chem. Prod. Res. Dev., Vol. 18, No. 2, 1979
1972; Taylor and Klimisch, 1973; Pirug and Bonzel, 1977). Since we did not measure the concentrations of N 2 0 and Nz,we neglected the reduction of NzO to N2. Otto et al. (1970) had also neglected this process in the mechanism of the NO-NH3 reaction on Pt/AlZO3. In conclusion, we adopted for this study the mechanism shown in Scheme I owing to the simplicity in the reaction model. Method of Computation. If we assume a stationary state in the plug flow reactor, concentrations of reactants and products (C,'s) a t the various WIF are given by the following simultaneous differential equations
!OX
,
,
,
(1)
where j represents NO, NH3, Hz, or 02. In this study, reaction rates of individual steps were formulated by applying the Langmuir approach, assuming a homogeneous surface, to the mechanism shown in Scheme I. Namely, R.'s were calculated in the following manner. The appiication of steady-state conditions to the reaction mechanism shown in Scheme I gives the following simultaneous equations a t a given WIF
i
!C
'NLFT
RJ
I
&4-&!g---d"
dCj d( W / F )
,
NC
'
Lq
Figure 1. NO reduction by H2 with various 0, concentration at 400 O C over Pt/AlZ0,: W / F = 236.5 g.s/mol; CON0 = 1000 ppm; C O H ~= 1.08%. Experimental values: 0,NH,; 0, NO; A, Hz; and A, 02. Calculated values: -.-, NH3; -, NO; - - -, H,; and --, 0,.
0 1'4
ET
O:
:obb
r:
1
:
Figure 2. NO reduction by H2with various O2concentration at 400 "C over Pt/Al2O3: W / F = 47.3 g.s/mol; CoNO= 1000 ppm; CoH = 1.08%. Experimental values: 0,NH,; 0,NO; A, Hz; and A, Calculated values: NH,; -, NO; - - -, H,; and --, 0,.
b,.
-e-,
+ 2k68"26'~~o~2 - 2 k 7 8 ~ 0 ~-6 ' ~ ~ 4k86'~0% - 2k110~~00 ~~ + 2k12P~~6'~' -
~&.JH~~H
k130~ - ~4 k 1 4 6 ~ 0 6-' ~4k17P0~6'~~ ~ =0
(4)
d6'0 N,= 2k9P020v2- k,06'02 - k110H26'o- 2 k 1 5 6 ' ~ ~ ~oo~ dt k16pH26'0 = 0 (5) 0NH2
+ ON0 +8H + 60' + 6'v = 1
(6)
In order to obtain eq 2-5 from the reaction mechanism shown in Scheme I, we must know the rate equations for each individual step. Unfortunately, however, the rate equations have not been reported. Thus, to a first approximation, we assume a rate equation for an individual step as shown in the individual term in eq 2-5. For instance, the rate equation of step 1 is proportional to P" Bv2 which corresponds to the dissociative adsorption of dH3 to (NH,) and (H). Solving eq 2-6 with respect to 0" $NO, OH, 6'0, and 6'" for a given set of k:s, we can calcufate R,'s as follows. RNO RNH3
=
= -k3PNOOV + -k1PNH30V2
k46'NO
(7)
+ k26'NH26'H
(8)
Thus, if inlet concentrations of NO, H2, and O2 are given, Cj's can be calculated as functions of W / F by eq 1-10. Equation 1 was solved numerically with the method of
finite differences, and the solutions of eq 2-6 were obtained by Newton's method. Numerical calculations were carried out with a FACOM 230-75 digital computer (Computation Center, Nagoya University). In the present work, the effects of external and internal diffusions were not separated from the data of the NO-H2-02 reaction on Pt/A1203,so the rate constants, k,, can be considered to contain the effects of diffusions. Furthermore, the adiabatic temperature rise was ignored in the calculation, since Pt/Al2O3was diluted with a great amount of fused alumina in the catalyst bed. Results a n d Discussion Experimental Results of t h e NO-H2-02 Reaction on Pt/A1203. Before proceeding with the catalytic experiments, we confirmed that no homogeneous reactions took place under the experimental conditions of the present work. Thus, the results indicated below are due to the catalytic reactions. Results of the NO-H2-02 reaction on Pt/AlZO3catalyst a t 400 "C are shown in Figures 1-3, where the values of WIF were 236.5, 47.3, and 7.0 g-s/mol, respectively. As shown in Figure 1, when Coo2= 0, NO was almost completely removed, while a significant amount of NH3 was produced. Also, more than 70% of the inlet H2 remained unreacted. Even if Coo2was increased to about 0.4% in the "rich" region, the outlet concentration of NH3 remained high irrespective of Cooz,although the concentration of residual Hz was decreased linearly with respect to Coo2. On the other hand, when Coo was increased to more than 0.55% (the "lean" region?, a considerable amount of NO was observed while the producticn of NH3 did not substantially occur. Comparing the data of the NO-Hz-02 reaction shown in Figure 1 with the data of the NO-H2-CO-02 reaction and of the synthetic exhaust gas
1 I
J
L
/ - - - - - -
r---------I
'
-05 0
-N
-
-
\ u N
u
I
0 .',-E- 72
COhC, 1
[Q
0
'
Figure 3. NO reduction by H2with various 02 concentration at 400 "C over Pt/A120,: W / F = 7.0 g.s/mol; C'NO = 1000 ppm; C " H ~= 1.08%. Experimental values: 0,NH,; 0,NO; A, Hz; and A, 02. Calculated values: NH,; -, NO; - - -, H2; and ---, 02. -e-,
Table I. Rate Constants for t h e NO-H,-0, Reaction on Pt/Al,O, a t 4 0 0 ' C (mol/s.g)a N
o c
klP0
0.571
k,
14.275
k,,
0.2855
P,, = 1 x
k,
0.571
k*
142.75
k 14 11.42
k3Po
0.571
k9Po 5.71
k,,
2.855
k,
0.571
k,,
0.2855
k1R"
0.571
k,
2.855
k,,
0.571
k6
,"
2
2.855
0.5
u.N m= \ u
k1,P"
N
5.71
u -
kl2,
0.571 I
atm.
obtained by Taylor (1975) and Schlatter and Taylor (1977), we can find similarities between these data. The same correlation can also be seen in the data of the reactions on Rh/AlZO3obtained by Taylor (1975). These correlations suggest that the NO-Hz-O2 reaction is adequate to investigate the possibility of the extension of the computer simulation method to the analysis of the three-way catalyst system. Surveying the data a t WIF = 47.3 g.s/mol shown in Figure 2 , we note the following points. First, unreacted NO decreased with the increase of Cooz in the "rich" region. Second, as shown in Figure 2 , both the outlet concentration of O2 in the "rich" region and that of Hz in the "lean" region were almost zero. When WIF was further decreased, as shown in Figure 3, the outlet concentration of NH3 lowered and the concentration of unreacted NO increased. Even in the "rich" region, residual O2was observed, while, too, a small amount of unreacted H z was detected in the "lean" region. Schemes of the NO-HZ-O2 Reaction on Pt/AlZO3. The set of rate constants (hi's)for the NO-Hz-O2 reaction on Pt/Al2O3 a t 400 "C was determined by the computer simulation method coupled with the trial and error method in order to give the calculated results which agreed best with experimental results shown in Figures 1-3. Calculated results are also shown in Figures 1-3 and the rate constants obtained are indicated in Table I. As shown in Figures 1-3, calculated results are in fairly good agreement with the experimental results. In determining the rate parameters in Table I, we made trial and error calculations first, by a factor varying the relative rate constants to h,Po, of 10" ( n = integer) and, secondly, by a factor of from 2 to 9. At this stage of trials, the calculated values in Figures 1-3 were obtained. Therefore, the rate constants in Table I show some repetitions. Of course, it may be best to determine the rate constants by the least-squares method, and the application of this method to the present system seems not to be very difficult. However, we did not try the latter method in this study, since, as mentioned above, calculated results by the trial and error method are in fairly good agreement with experimental ones, and the exper-
100 W/F
200 ( G.SEC/MOL
I0
)
Figure 5. Calculated effects of W/F on the NO-H2-02 reaction over Pt/A1203at 400 "C: C", = 0.324% ("rich region); -, NO; -, NH3; - - -,Hz; 02. -a*-,
-
M
500
z
p Y
3
100 W/F
200 ( G.SEC/WL
Figure 6. Calculated effects of W/F on the NO-H2a2reaction over Pt/A1,03 at 400 "C: Cooz= 0.4968% (stoichiometric point); -, NO; "3; - - -,H2; 02.
-a-,
-e*-,
I
II
