partial pressure but unselective when air was used, has been investigated. The following points concerning their catalytic behavior have been stressed: (1) Lattice oxygen is responsible for selective oxidation to butadiene. (2) Adsorbed forms of oxygen are responsible for substantial production of CO, COz, and maleic anhydride. (3) The production of maleic anhydride seems to require low coverage of the catalyst surface and, we think, the presence of molecular adsorbed oxygen species. (4) The partial pressure of oxygen is a key to modify the selectivity to the desired products (carbon oxides, maleic anhydride, or butadiene). Acknowledgments We thank Professor Pasquon for encouragement and useful discussions and Dr. P. L. Villa for help in characterization of a-CoMoOl and Italian Centro Nazionale delle Ricerche (Rome) for financial support.
Callahan, J. L., Grasselli, R. K., A.l.Ch.E. J., 9, 755 (1963). Courtine, P., Cord, P. P., Pannetier, G., Daumas, J. C., Montarnal, R., Bull. SOC. Chim. Fr., 4816 (1968). Dewing, J., "Catalytic Oxidation Principles and Processes," London, July, 1970. Keizer, K . , Batlst. Ph. A., Schuit, G. C., J. Catal., 15, 256 (1969). Matsuura, I., Schuit, G. C. A , , J. Catal., 25, 314 (1972). Pasquon, I., Trifiro, F., Centola. P., Chim. lnd., 49, 1151 (1967). Pasquon, I., Trifiro, F., Caputo, G., Chim. lnd., 55, 168 (1973). Peacock, J. M., Parker, A. J , Ashmore, P. G., Hockey, J. A,, J. Catal., 15,398 (1969). Schwets, V. A., Kazansky, V. E., J. Catal., 25, 123 (1972). Tajbl, D. G..Simons, J. B., Carberry, J. J., Ind. Eng. Chem., Fundam., 5, 171 (1966). Trifiro, F., Pasquon, I., J. Cafal., 12 (4), 412 (1968). Trifiro, F., DeVecchi, V., Pasquon, i., J. Catal., 15, 8 (1969). Trifiro, F., Banfi. C., Caputo, G., Forzatti, P., Pasquon, I., J . Catal., 30 (3), 393 (1973). Trifiro, F., Caputo, G., Villa, P. L., J. Less-Common Metals, 36, 305 (1974). Weiss, F., Marion, J., Cognion, J. M . , "Annual Meeting of the Chemical Society," Manchester, Oct 14, 1972.
Literature Cited Receiuedfor review M a r c h 28, 1974 Accepted October 2, 1974
Boutry, P , Courty, Ph , Daumas, J. C , Montarnal, T., Bull SOC. Chim Fr., IO, 4050 (1968).
Catalytic Reduction of Nitric Oxide on Ruthenium Richard
L. Klimisch and Kathleen C. Taylor*
Physical Chemistry Department General Motors Research Laborafor/es General Motors Technical Center. Warren. Michigan 48090
The details of nitric oxide catalytic reduction by carbon monoxide and hydrogen over a supported ruthenium catalyst have been investigated. The feed stream used in these studies resembled automotive exhaust. The effect of space velocity and various reactant concentrations on the ammonia/nitrogen product distribution were studied along with the effect of these various parameters on the rate of nitric oxide removal. The pertinent chemical reactions in this system and the limitations imposed by an exhaust gas feed stream are discussed.
Introduction Recent studies have shown that ruthenium catalysts selectively convert NO to N2 in feed streams resembling automotive exhaust (Klimisch and Taylor, 1973; Taylor and Klimisch, 1973; Shelef and Gandhi, 1972). The problem of ammonia formation, which has plagued other catalysts, is minimized with ruthenium. It was noted by Klimisch and Taylor (1973) that ruthenium catalysts exhibit two states which differ in activity characteristics. One state can be obtained by treating the catalyst with oxygen at low temperature and the other is obtained when the catalyst is treated with a reducing feed stream at high temperature ( >650"C). These transformations are reversible since they have been repeated a number of times for an individual catalyst sample. The phenomenon was only eliminated when the catalyst was exposed to sulfur containing materials (Taylor and Klimisch, 1973). The application of physical methods of surface examination to characterize the two states of the catalyst gave no evidence that the so-called oxidized form of the catalyst is an oxide under the reaction conditions for NO reduction (Taylor, et al., 1974). A recent publication from this laboratory described how various reactant parameters influence the product distribu26
Ind. Eng. Chern., Prod. Res. Dev., Vol. 14, No. 1, 1975
tion for NO reduction over a reduced ruthenium catalyst supported on silica-alumina (Taylor and Klimisch, 1973). A complementary study of the oxidized form of a ruthenium on alumina catalyst is the subject of the present work. Specifically, this study will examine product distribution as a function of reactant concentration and will also examine the question of ammonia intermediacy in the reduction of NO to N2. Experimental Section The ruthenium catalyst was prepared by impregnation of 1k-in. alumina spheres (Kaiser KC/SAF) with an aqueous solution of ruthenium trichloride (RuC13.1-3H20, Alfa Inorganics). The catalyst was dried in air at 25°C and then calcined in air a t 500°C for 4 hr (gas hourly space velocity = 500). The catalyst was made up to contain 0.1 wt 70ruthenium. The total ruthenium content determined by X-ray fluorescence was 0.081 w t %. The catalytic reactor and gas analysis techniques have been described before (Klimisch and Barnes, 1972). The gas hourly space velocity (GHSV) was 38,000 for most experiments. The feed stream referred to as the standard feed stream contained 0.10% NO, 1.0% CO, 0.3% HP, 10%
I \
A-A
co
Catalyst Temperature (OCi
Catalyst Temperature l°C)
Figure 1. Reduction of NO by Hz over 0.3% Pt-Alz03: 0.1% NO and 0.3% Hz in an Nz atmosphere: GHSV = 38,000; 0, NO; A , 2xN20; 0,"3.
0
Figure 3. NO reduction over reduced Ru-Al~03. Feed stream: 0.1% NO, 1.0% CO, 10% COz, and 10% HzO in an Nz atmosphere: GHSV = 38,000; 0 , NO; A , CO; 0 ,"3.
500
400
300
200
-
Catalyst Temperature ( C l
Figure 2. Reduction of NO by H2 over 0.1% Ru-Al203. Feed stream: 0.1% NO and 0.3% Hz in an N2 atmosphere: GHSV = 38,000; 0 , NO; A , "3; ---, oxygen treated; - - - -, reduced.
COz, and 10% HzO in a nitrogen atmosphere. All data points represent steady-state conversion.
Results and Discussion The apparently unique selectivity of ruthenium in N O reduction is seen most dramatically in the Hz-NO reaction. Thus, hydrogen removes nitric oxide at very low temperature over platinum catalysts (Figure 1) converting most of the NO to ammonia. This behavior is almost identical for palladium catalysts and is quite similar for other transition metal catalysts as well. In contrast, when this same reaction is carried out over ruthenium (Figure 2), only minor amounts of ammonia are formed. This behavior does not change appreciably for the two states of the ruthenium catalyst. A characteristic difference between the two forms of the ruthenium is observed in the water-gas shift reaction. The reduced ruthenium catalyst has no activity for this reaction whereas the oxygen treated catalyst does (Taylor and Klimisch, 1973). It was suggested previously (Klimisch and Barnes, 1972) that a scheme involving the water-gas shift reaction (1) and reaction 2 is responsible for ammonia formation when Hz is not present in the feed stream. CO t H,
+
H20 NO
--t
--t
CO:,
NH,
+
+ H,
(1)
HZO
(2 )
However, a sequence involving reactions 1 and 2 is not responsible for NH3 formation with the reduced catalyst. If NO is added to the feed stream over the reduced catalyst, a small amount of CO is removed which corresponds to the amount of NO reacted (Figure 3). An alternate mechanistic pathway involving an isocyanate intermediate is
Catalyst Temperature l°Cl
rTg.\
I
y,\ O\
Reduced Catalyst
?
200
300
400
Catalyst Temperature ( O C i
Figure 4. Nitric oxide variation over Ru catalysts. Feed stream: variable NO, 1.0% CO, 0.3% H2, 10% C02, and 10% HzO in an Nz atmosphere; GHSV = 38,000; inlet NO: 0 , 500 ppm; A , lo00 ppm; 0,2000 ppm.
probably more consistent with these observations (Unland, 1973; London and Bell, 1973). Moreover, when both CO and Hz are present in the feed stream over the reduced catalyst, NH3 formation is greater than the sum of the NH3 shown in Figure 2 and Figure 3. It has been suggested that adsorbed CO increases the selectivity for NHs formation oia the NO-Hz reaction by virtue of lowering the probability of nitrogen pairing (Shelef and Gandhi, 1972). The major reaction of interest in these studies was nitric oxide reduction. The temperature required to remove NO increases with increasing NO concentration as shown in Figure 4. As indicated in Figure 4, the two forms of the ruthenium catalyst show only slight differences in their activity for NO removal. Nitric oxide concentration does, however, have a significant effect on product distribution. Ind. Eng. Chem., Prod. Res. D e v . , Vol. 14, No. 1, 1975
27
Oxygen Treated Catalyst
%
I
I
200
500
300 400 Catalyst Temperature ( O C )
Catalyst Temperature (OCI 1000 1
100
I
H
Reduced Catalvst
Reduced Catalyst
-
0
E
5
50
I
m
c
"
s I W
z Ob
I
I
I
O I ! 200
A
i
I
i
300
400
500
Catalyst Temperature I°C)
Figure 7. NO reduction and NH3 formation with variable NH3 over Ru catalysts. Feed stream: A , 0% NH3 or 0 , 0.05% NH3 in 0.1% NO, 0.3% Hz, 10% COz, and 10% H2O in an N2 atmosphere; GHSV = 38,000.
Oxygen Treated Catalyst
315OC 0
O-0 /
A
S
A
A 0
o , * O ,
:
75
50
IO0
125
Gas Hourly Space Velocity l h r - 1 ) /lo3
Reduced Catalyst
; 20
650°C 30
40
50
;,I 60
(hr-1)/ l o 3 Figure 6. NO reduction to NH3 with variable space velocity over 0.1% Ru-AlZOs Ru catalysts. Feed stream: 0.1% NO, 1.0% CO, 0.3% Hz, 10% CO2, and 1HzO in an N2 atmosphere. Gas Hourly Space Velocity
28
Ind. Eng. Chem., Prod. Res. Dev., Vol. 14, No. 1, 1975
As the nitric oxide concentration is increased, the fraction converted to ammonia decreases for both forms of the catalyst. This effect is not large but is consistent with the simple nitrogen pairing explanation proposed by Shelef and Ghandi (1972). The most interesting characteristic of the oxygen catalyst, however, is the absence of ammonia in the exit gases at higher temperature (above 500°C). In contrast, ammonia formation over the reduced form of the catalyst does not decrease as the temperature is increased from 300 to 6 0 ° C (Figure 5 ) . There is also a small amount of nitrous oxide (NzO) in the product gases which reaches a maximum of 100 ppm a t 300°C for both forms of catalyst. This product disappears a t higher temperature and probably arises from the reaction between ammonia and unreacted nitric oxide. It was shown earlier that the oxygen treated form of the catalyst is more active for ammonia decomposition than the corresponding reduced Ru-Al203. This suggests the possibility of two paths to nitrogen (reaction 3) over oxy-
NO
-
N,
gen-treated Ru-Al203 which would explain the concentration effects noted above. Indeed, a study of space velocity lends credence to this suggestion. Thus, increased ammonia formation with increasing space velocity (Figure 6) is observed for oxygen-treated Ru-Al203. Admittedly, the space velocity effect in Figure 6 is not large, so additional evidence was sought to confirm ammonia intermediacy. Thus, experiments were carried out in which a fairly large amount of ammonia (500 ppm) was
added to the feed stream. This experiment simulates the conditions as they would exist halfway through the reactor and bears directly on the question of ammonia intermediacy since it determines the efficiency of the catalyst to remove ammonia under reaction conditions (Bemstein, et al., 1973). The results for the ammonia addition experiments are dramatically different for the two states of the catalyst (Figure 7). Thus, most of the ammonia passes through the reactor unchanged when the catalyst is in the reduced state. In contrast for the oxidized catalyst, the ammonia did not pass through the reactor but was efficiently removed at temperatures above 350°C. The presence of only 350 ppm of NH3 in the product a t -250°C is due to our inability to analyze for NH3 in the presence of large amounts of NO. An NH3-NO reaction over the Pt catalyst used t u oxidize NH3 back to NO for the NH3 analysis results in a decrease in the amount of NH3 determined. This problem does not exist at higher temperatures where all the N O is reacted. These additional ammonia experiments strongly support the intermediacy of ammonia for oxygen-treated Ru-Al203. The fact that the added ammonia does not affect the temperature of NO removal (Figure 7) argues against a change in the mechanism for NO reIt has been suggested though moval with the added "3. that an NO-NH3 mechanism may operate at low temperature to some extent all the time (Otto and Shelef, 1973). In both states ruthenium is very selective in the conversion of nitric oxide to molecular nitrogen. The unique selectivity of ruthenium catalysts in NO reduction is undoubtedly related to the unique chemistry of ruthenium
with nitrogen ligands (Cotton and Wilkinson, 1972). Thus, the strong NO-ruthenium bonding as well as the remarkable interactions of Nz with ruthenium are consistent with the Nz selectivity of ruthenium catalysts in NO reduction. Ammonia intermediacy does not appear to be involved in NO reduction over reduced Ru-Alz03 (Taylor and Klimisch, 1973) and is only a minor pathway to nitrogen over the oxygen treated Ru-Al203 catalyst. Acknowledgment The authors acknowledge the indispensable assistance of Mr. R. M. Sinkevitch in all aspects of this work. Literature Cited Bernstein. L. S., Lang, R. J.. Lunt. R. S., Musser, G. S., Society of Automotive Engineers, Detroit, Mich., May 17, 1973. Paper No. 730576. Cotton, F. A,, Wilkinson, G., "Advanced Inorganic Chemistry," 3rd ed, p 1012, Wiley, NewYork, N. Y., 1972. Klimisch. R. L., Barnes, G. J., Environ. Sci. Techno/.,6, 543 (1972) Klimisch, R. L., Taylor, K. C., Environ. Sci Techno/.. 7, 127 (1973). London, J. W., Bell, A. T., J . Catal.. 31, 96 (1973). Otto, K . , Shelef, M., Z. Phys. Chem. (Frankfurt am M a i n ) , 85, 308, (1973), Shelef, M . , Gandhi, H. S., Ind. Eng. Chem.. Prod. Res Develop.. 11, 393 (1972). Taylor, K. C., Klimisch, R. L., J. C a t a i , 30, 478 (1973). Taylor, K. C . , Sinkevitch, R. M., Klimisch. R. L., to be published in J . Catal.. 1974. Unland M.. Science, 179, 567 (1973).
Received for reuiew April 17, 1974 Accepted October 4,1974
Presented a t the California Catalysis Society Fall Meeting, California Institute of Technology, Pasadena, Calif., Nov. 2, 1973.
Reduction of Nitric Oxide by Monolithic-Supported Palladium-Nickel and Palladium-Ruthenium Alloys Calvin H. Bartholomew' Research and Development Laboratory Corning Glass W o r k s Corning. New Yorb 14830
The effect of reaction parameters such as temperature, space velocity, and pollutant concentration on the performance of monolithic-supported Pd-Ni and Pd-Ru alloys in reduction of NO has been studied in a laboratory reactor. Freshly prepared Pd-Ni and Pd-Ru catalysts convert 100% NO (1000 ppm) with less than 5% ammonia formation in 0.4% 02 (1% CO and 250 ppm of C3H6) at 600 and 480°C, respectively. Conversions of NO, CO, and C3H6 decrease slightly with increasing space velocity. The effect of dynamic exposure to 100 ppm of SO2 is to lower slightly but reversibly the NO and C3H6 conversions while significantly and irreversibly lowering conversion of CO. After 100 hr of exposure in a reducing en ine exhaust the conversions of NO, CO, and C3H6 over monolithic-supported Pd-Ru/AlzOp and PdNiaNiAlz04 are lowered 5-20% compared to freshly prepared samples. The leading 1.9-cm section appears less active than the adjacent 1.9-crn section suggesting deactivation by poisoning and/or loss of ruthenium. Pd-Ni is significantly more stable catalytically on a NiA1204 wash-coated monolith compared to a surface-treated uncoated monolith.
Introduction
control. Monolithic supports are thin-walled, multichanne1 The application of monolithic supports ( ~ ~et al,, ~ l ~ ceramic ~ , structures which can be coated with any of a number of high surface area materials such as alumina or 1973) as a of supporting catalysts is relatively new silica and which offer the advantages of low pressure drop and has been limited thus far to automotive emissions and high geometrical surface as compared to pellet supports. These advantages are particularly important in the ' Address correspondence to the author at the Department of Chemical Engineering Science, Brigham Young University, Provo, Utah 84602. catalytic reduction of NO, normally carried out at high I n d . Eng. Chem., Prod. Res. Dev., Vol. 14, No. 1. 1 9 7 5
29
