molar ratio of WCl6 and EtaAl and that of WCle and 1-octene
Acknowledgment
at 1.0 and 120, respectively. In Figure 4 the yield of oligomer and octylbenzenes is plotted against the molar ratio of water
T h e authors gratefully acknowledge the useful discussions with Akira Matsumoto of Osaka University on the interaction of tungsten halides with oxygen. Bruce DeBona of PPG Industries, Inc., Springdale, Pa., assisted in language translation and description of the experiments.
and WCl6. The system of Et3A1-WC16 is by no means a Friedel-Crafts catalyst; however, the system of Et3L-i-O~-WC16 or EtsA1Hz0-WC16 behaves in some cases as a Friedel-Crafts catalyst. For the formation of a Ziegler-Natta-type tungsten complex, the reduction of WV1 cation to a low oxidation state is necessary. However, if Et& is destroyed by reaction with oxygen or water, reduction to form such a Ziegler-Natta catalyst is not possible, and WCl6 added afterward acts as a Friedel-Crafts catalyst in the presence of some cocatalyst, such as water or hydrogen chloride. Thus, the formation of alkylbenzenes in the previous report (Uchida et al., 1971a) should be correlated with the fact that Et3A1 used in the previous report was not distilled before use.
literature Cited
Levandos, G. S., Pettit, R., J.Amer. Chem. SOC.,93,7088 (1971a). Levandos, G. S., Pettit, R., Tetrahedron Lett., 1971b, p 789. Olive, G. H., Olive, S., Angew. Chem., 83, 129 (1971). Uchida, Y., Hidai, M., Tatsumi, T., Bull. Chem. SOC.Jap., 45, 11,58 (1972). Uchida, A., Mukai, Y., Hamano, Y., Matsuda, S., Ind. Eng. Chem. Prod. Res. Develop., 10, 369 (1971a). Uchida, A,, Hamano, Y. Mukai, Y., Matsuda, S., ibid., 372 (1971b). RECEIVED for review May 1, 1972 ACCEPTEDSeptember 29, 1972
Ammonia Formation in Catalytic Reduction of Nitric Oxide by Molecular Hydrogen 11. Noble Metal Catalysts Mordecai Shelefl and Haren S. Gandhi Scientific Research &a$,
Ford Motor Co., Dearborn,
Mich. 48121
The study of ammonia formation in the reduction of NO by hydrogen is extended to include the noble metal catalysts, Pt, Pd, Ru, and Os. As in the case of the base metal oxide catalysts, NH3 formation first increases and subsequently decreases with increasing temperature. O n Pt and Pd catalysts, NH3 formation decreases in the presence of CO, whereas it is enhanced on a Ru catalyst. O f the catalysts studied, Ru has the best selectivity toward the “defixation” of NO to molecular nitrogen. Small amounts of oxygen have little effect on the catalytic behavior of noble metals, provided an overall stoichiometric excess of hydrogen is present.
T h i s study of NH3 formation in the NO-HZ reaction on supported noble metal catalysts is a sequel to the study on base metal catalysts presented in Part I (Shelef and Gandhi, 1972). These two classes of catalysts differ in several aspects, such as changes of oxidation state with changes of environment, interaction with the support, resistance to poisoning, catalyst loading, and compatibility with monolithic supports; these differences are the reasons for the separate studies. The thermodynamic considerations and literature survey of Part I apply as well to the present paper. Experimental
Table I describes the catalysts employed in this study. The Pt catalyst was obtained from Universal Oil Products (Des Plaines, Ill.) and has been described in more detail earlier (Shelef e t al., 1969), but its Pt content is not known. Both the P d and R u catalysts were purchased from Engelhard InTo whom correspondence should be addressed.
dustries (Newark, N.J.). The R u surface area as measured by hydrogen chemisorption was 30.6 pmol/g cttalyst, which corresponds to a n average particle size of 13 A and a 62% dispersion. The Os catalyst was prepared by dissolving osmium tetroxide @so4) in water, impregnating the Engelhard support with the solution, drying a t room temperature overnight, and reducing in a stream of hydrogen. I n the reduction process the temperature was slowly increased to prevent sublimation of the volatile oso4. Experimental apparatus and procedures, conditions of the experiments, and analytical methods were the same as in Part I (Shelef and Gandhi, 1972). The space velocity of all experiments was 20,000 hr-l unless otherwise stated. Results and Discussion
As in Part I, ammonia was asssumed to be the sole product with “fixed” nitrogen. The “unfixed” product was mainly Nz, Ind. Eng. Chem. Prod. Res. Develop., Vol. 1 1 , No.
4, 1972 393
Table 1. Description of Catalysts Catalyst designation
Active ingredient
Pt
Pt Pd
Pd, 1%
Ru
R u , 0.5%
Os
Os, 0 . 2 %
Table II. Temperature of 90% NO Conversion in NO-Hz, NO-H2-C0, and NO-CO Systems, “ C
Surface area,
Inlet levels: KO, 1000 ppm; H P , 1.4y0; CO, 1.4%
m 2/s
Support
illumina spheres d = in. Alumina spheres d = ’/I6 in. Alumina pellets d = a / 3 ~ in., 1 = ‘/0 in. Engelhard alumina spheres d = ‘/I6 in.
Catalysts
Unknown
NO-Hz
Pt Pd Ru os
202 98
NO-Ha-CO
280 40 70
L
L
0
Vl
20 30
IO
1
I
I
100
200
I
I
I
I
I
300 400 500 600 700 800 Temperature, ‘C
1
I
100
I
I
I
I
I
I
200 300 400 500 6 0 0 700 800 Temperature,
co, 1.5%
O C
Figure 1 . Selectivity for “unfixed” nitrogen-containing products in NO-H2 reaction as function o f temperature. Inlet concentration: NO, 1000 ppm; Hz, 1.4%
and some N20 may have been formed a t the lower temperatures. The changes in selectivity tolvard the desired “unfixed” products with process variables such as temperature, KO concentration a t the inlet, CO presence, and space velocity were followed. The selectivity, S, for unfixed products is defined as:
;Is analyses for K2 and by the difference,
?;20
were not made, S was computed
]
- NO (outlet) - NH3 (outlet) s = [NO (inlet) x 100 NO (inlet) - NO (outlet) Similarly to the base metal oxide catalysts, NH3 formation passes through a peak with increasing temperature on the noble metal catalysts. This i s reflected in Figure 1 by the 394
Figure 2. Selectivity for “unfixed” nitrogen-containing products as function of temperature in presence of carbon monoxide. Inlet concentration: NO, 1000 ppm; HB, 1.4%;
Ind. Eng. Chem. Prod. Res. Develop., Vol. 1 1 , No. 4, 1972
selectivity dip. The most important result is the low NHI formation with the R u catalyst. Platinum and palladium catalysts behave alike and form large amounts of NH3 over a wide temperature range under the reducing conditions of Figure 1. The Os catalyst does not begin to be active until much higher temperatures than the other catalysts, and its SH3 forming tendency falls between that of R u and that of Pt or Pd. The relative activity of the catalysts in the NO-H2, NOHz-CO, and SO-CO systems, as expressed by the temperature of 90% conversion, is given in Table 11. I n the system SO-H2 the catalytic activity decreases in the order Pd > R u > P t > os. The introduction of carbon monoxide causes considerable poisoning of the Pt and Pd catalysts but not of the R u or Os catalyst. This reversible poisoning of Pt and Pd surfaces by CO under reducing conditions is well-known both in CO oxidation by osygen (Bond, 1962b) and by nitric oxide (Jones et al., 1971) and hampers the use of pure P t catalyst for the removal of nitric oxide in the first bed of a dual-bed catalytic eshaust-treatment system. The low temperature of
loor 90
1000 I
1
c
I100
Pd
80 70
1.4% H2,1.4%C0 a t inlet
60
-60 0
ae .-h 50
L
-50
-
.-w 0
$
- 4 0 %
40
z
rn
-30
30
- 20
20 IO
ic z
t
1
100
1
200
I
A I
I
I
I
I
1
1000
300 400 500 600 700 800 Temperature,
2000
;
2
1'" 1
I
3000
4000
INLET NO CONCENTRATION, ppm
O C
Figure 3. Effect of oxygen presence on selectivity as function of temperature. Lines redrawn from Figure 1. Points obtained a t inlet concentration: NO, 1000 ppm; Hz, 1.4%;
I
t
r
Figure 4. Ammonia formation as function of inlet NO concentration over Ru catalyst a t 371 "C
02,0.25-0.30% 2500-
90% KO reaction over the R u catalyst in the NO-CO system is a n additional indication of the resistance of this catalyst to the reversible inhibition by CO. The effect of the presence of CO in the system on the extent of NH? formation is shown in Figure 2. Here, the contrast between Ru, on the one hand, and Pt or Pd, on the other, is sharp. With the R u catalyst the behavior is similar to t h a t noted over the base metal catalysts; carbon monoxide enhances S H 3 formation, accentuating the selectivity dip. As in the case of the base metal oxide catalysts, this enhancement of N H 3 formation must be attributed to competitive adsorption of carbon monoxide which lowers the surface concentration of adsorbed nitric oxide molecules and thereby the probability of pairing-up of E-atoms required for the defixation to Kzor N20. Oxygen addition a t the 0.25-0.30% level to the stream carrying 1000 ppm NO and 1.4% HZhas little effect, if any, on t h e selectivity. This is clear from Figure 3, where the points obtained in the presence of oxygen fall close t o the lines redrawn from Figure 1. I n this case, R u behaves as a noble metal. On base metal catalysts, as noted previously (Shelef and Gandhi, 1972), the presence of oxygen causes a shift of the activity and of the "3-formation curves to higher temperatures. The effect of the nitric oxide concentration on iYH3 formation was followed a t temperatures which correspond to the ascending branch of the selectivity curves in Figure 1. At relatively elevated temperatures the increase in the NO surface coverage associated with the higher KO content in the gas phase is being offset in part by desorption. Therefore, i t can be expected that the selectivity to ?Jz nil1 not rise so sharply with NO concentration as a t the lower end of the temperature range. Severtheless, as shown in Figure 4,for the R u catalyst the proportion of S O converted to NH3 decreases monotonically with NO concentration, and the absolute amount of NH3 formed goes through a maximum, as was ob-
2250
-
2000
-
leo 170 0 z 0
60 2
2 1250 L
1.4%CO a t inlet
0 0
40 0
LL
1000
2000
3000
4000
INLET NO CONCENTRATION, ppm
Figure 5. Ammonia formation as function of inlet NO concentration over Pt catalyst a t 463°C
served over base metal oxide catalysts. When CO is added, the proportion of NO converted to NH3 is increased, but the shape of the curves is unchanged. Over the Pt catalyst the effect of NO concentration is entirely different than with R u (Figure 5 ) . The absolute amount of 3" formed rises continuously with increase in NO concentration, and the proportion of NO converted to NHj remains nearly constant. I n the absence of carbon monoxide, 70-80% of the NO is converted to NH3 a t all concentrations. The temperature of this experiment, 463"C, is apparently Ind. Eng. Chem. Prod. Res. Develop., Vol. 1 1 , No.
4, 1972 395
Table 111. Comparison of Effect of Space Velocity on Selectivity in NO-H2 Reaction and on NH3 Decomposition NHx, pprn
...
38.5
...
A.
...
68.6
54.5
...
B Ab
1090 ...
18.8 56.5 53.5 8.3 89.2 87.2 84.4
9.2 46.0 50.7 5.5 84.2 77.3 78.5
6.25 ... ... ... ...
T, ‘C
Index
Pt
463
Ab
Pd
513
A. Ru
371
Space velocity, hr-Ia
s x 108 60.5
Catalyst
B Ab
a t inlet
I
.
.
1090 ...
A. ... B 1070 A. Selectivity in NO-HZ reaction, yo. B. Extent of NH8 decomposition, yo. a Figures in parentheses indicate NO conversion (70) if less than 100%. b Inlet composition, 1000 ppm NO, 1.4% Hz. c Inlet composition, 1000 ppm NO, 1.4% Hz, and 1.4% CO.
such that there is no substantial increase of the NO surface coverage on Pt with increased NO pressure. The introduction of CO lowers the proportion of NO converted to NHa to 60-70% a t all NO concentrations. This decrease is probably the result of direct reduction of NO by CO. The formation of NH3 with increasing NO concentration over the Pd catalyst is similar to that of Pt. With respect to NH3 formation, the Os catalyst resembles the R u catalyst. As was done in the work on the base metal oxides in Part I (Shelef and Gandhi, 1972), the extent of selectivity over the noble metal catalyst with varying space velocity was compared with the extent of KHa decomposition under similar conditions. These experiments are summarized in Table 111. The extent of decomposition of NH3 in a stream of NZ 1.4% Hz is compared with the selectivity for nitrogen formation in KO reduction in a stream of N Z 1.4% HPor of NP 1.4% HP 1.4% CO. Only over the catalyst which shows a n extent of ammonia decomposition close to that of the selectivity for NZ can the “defixation” route involve the NHS decomposition as an intermediate stage. The inspection of the data in Table I11 shows that in the case of Ru, the N H Bdecomposition can largely account for the mechanism leading to the hightemperature defixation of the nitrogen in NO, similarly to the case of the MOD catalyst in the base metal series. The outstanding ability of Ru, as compared to other noble metals, to decompose NH3is well-known (Amano and Taylor, 1954). Similarly to the MOD catalyst, the supported R u catalyst in pelleted form produces in the NO-CO-HZ system large amounts of hydrocarbons, presumably methane. A t 371O the amount of hydrocarbons formed, measured by the flame ionization detector as ppm C, varied from -5000 in a n inlet composition of 3550 ppm NO, 1.4% CO, and 1.4% HZdown to -3600, when the inlet level of NO was lowered to 250 ppm. The high activity of R u in the hydrogenation of carbon monoxide is a well-documented phenomenon (Bond, 1962a). Formation of hydrocarbons is a detrimental property for a n auto exhaust catalyst, and with other preparations of Ru-contain-
+
396
+
+ +
Ind. Eng. Chern. Prod. Res. Develop., Vol. 11, No. 4, 1972
104
1,s
x
104
2
x
104
25.0 (92.8). 46.0 (92.8). 5.32 24.8 46.8 3.2 80.4 47.7 54.4
. . .
64.5
4
x
104
20.8 (74,2). 39.1 (68.3)” ...
15.3 30.4 ...
57.0 36.8 ...
ing catalysts, it can be completely eliminated without adversely affecting the high artivity for NO reduction or the selectivity. Further investigation of CO hydrogenation over R u catalysts is being carried out. The initial results indicate that the reaction is structure sensitive, Le., strongly dependent on the particle size of Ru. Presence of a high concentration of water vapor in the exhaust also tends to suppress the formation of methane. Some Practical Considerations
The use of the common noble metal catalysts, Pt and Pd, under reducing conditions for YO removal is severely hampered by CO poisoning and high NH3 formation. I n contrast, Ru, which in certain respects is more akin to such base metals as F e or Ki (Amano and Taylor and references therein, 1954), is not poisoned by CO, has a remarkable selectivity for NO defixation, and is active in NO reduction. On the other hand, in comparison with base metal oxide catalysts, R u retains the advantages of noble metals, such as compatibility with monolithic supports and resistance to deactivation by small concentra tions of oxygen. Nevertheless, important practical problems of which we are aware, such as stability and resistance to poisoning and to various operating conditions, still remain to be solved. An iiitense effort is underway in this direction. literature Cited
Amano, A,, Tt:ylor, H., J. Amer. Chem. SOC.,7 6 , 4201 (1954). Bond, G. C., Catalysis by Metals,” pp 353 ff, Academic Press, London, England, 1962a. Bond, G. C., ibid., p 461, 196210. Jones, J. H., Kummer, J. T., Otto, K., Shelef, &I,,Weaver, E. E., Environ. Sci. Technol., 5 , 790 (1971). Shelef, M., Gandhi, H. S., Ind. Eng. Chem. Prod. Res. Develop., 11 (l),2 (1972). Shelef, M., Otto, K., Gandhi, H. S., Atmos. Environ., 3, 107 (1969). RECEIVED for review May 12, 1972 ACCEPTEDAugust 7, 1972
