Selective Catalytic Reaction of Hydrogen with Nitric Oxide in the Presence of Oxygen James H. Jones, Joseph T. Kummer, Klaus Otto, Mordecai Shelef,’ and E. Eugene Weaver Research and Engineering Center, Ford Motor Co., Dearborn, Mich. 48121
Under certain conditions, many supported noble metal catalysts are capable of promoting the removal of nitric oxide from automobile exhaust, even in the presence of large amounts of oxygen. Laboratory experiments showed that gaseous hydrogen is capable of such a selective catalytic reduction under certain conditions. Extension of these experiments to engine dynamometer tests has confirmed this observation. The catalysts which exhibit the selective nitric oxide reduction are very sensitive to poisoning, particularly by sulfur. The selective removal of nitric oxide by molecular hydrogen occurs only at relatively low temperatures, thus limiting its use in treating auto exhaust. The hydrogen-nitric oxide interaction in the presence of oxygen results in only minimal formation of ammonia, which is a major product under reducing conditions. At higher temperatures, close to those prevailing in the exhaust catalyst during normal operation, the nitric oxide removal on certain catalysts in the presence of oxygen is due partly to the participation of nitric oxide, along with the oxygen, in the oxidation of carbon monoxide and partly to the reduction by hydrogen formed “in situ” by the water-gas shift reaction.
T
he work described in this paper is concerned primarily with assessing the effect of hydrogen and water on the catalytic removal of nitric oxide. The scheme of the catalytic treatment of the exhaust must take into account the chemical system as a whole and consider the ways in which the behavior of the chemical species under study is coupled with that of other exhaust constituents. Therefore, the subject matter is somewhat broader than the catalytic reaction between hydrogen and nitric oxide. This investigation is the result of collaboration between two research groups, one at the laboratory employing synthetic gas mixtures and the other using engine dynamometers. To emphasize the mutual feedback of information, the presentation follows the gradual examination of the questions as they arose in the course of the study.
fairly close to those obtained with stoichiometric carburetion. This effect was noted both in multi- and single-cylinder engine dynamometer tests. In multicylinder engine dynamometer screening tests, two catalysts exhibited this behavior. In both, the active ingredient was platinum. One sample was supplied by the Universal Oil Products Co. (UOP) under the code number PZ-1-168 and the other by the Harshaw Chemical Co. A more detailed description of the UOP catalyst is given by Shelef et al. (1969). There are only limited data available on the Harshaw catalyst and their inclusion here serves only to demonstrate that the observed behavior is not restricted to a particular Pt catalyst. Figure 1 shows that,at conditions close to stoichiometric,there is a definite peak in the removal of NO for both Pt catalysts. This notable conversion of NO in the region close to stoichiometric carburetion was largely unexpected, as the exhaust at these mixture ratios contains appreciable amounts of oxygen. Figure 2 gives the measured relationship between carbon monoxide and oxygen in the exhaust. The best-fit curve drawn through the data points, although showing a considerable amount of experimental scatter, indicates an appreciable amount of oxygen at CO content of 0.5 to 1.O %. The removal of a substantial percentage of the NO, using the platinum catalysts at relatively high oxygen levels, led to the search for an explanation. This phenomenon of NO removal in the case of the UOP catalyst was noted to persist even after the catalyst had suffered an accidental overheating (>1600°F). It also occurred initially when leaded fuel was used as shown on Figure 3. In this case and also with the overheated catalyst, the curve is more sharply peaked, and the removal of NO does not extend into the region of rich carburetion observed under the conditions of Figure 1. The bottom curves on Figure 3 clearly demonstrate the vulnerability of the NO reduction to lead poisoning. This, however, does not diminish the importance of the observations and further study was deemed worthwhile.
80
Reduction of Nitric Oxide in Engine Exhaust over a Noble Metal Catalyst
Under conditions of rich carburetion, when there is a large excess of reducing species present in the exhaust and the oxygen levels are low, many transition metal oxide catalysts are capable of removing nitric oxide from the exhaust stream (Shelef and Kummer, 1969). This was the expected and observed behavior when a number of these catalysts were screened in actual engine tests. The behavior was, however, entirely different when the active ingredient of the catalyst was platinum. In this case, a characteristic peak of the catalyst efficiency for NO removal was noted at exhaust compositions Author to whom correspondence should be addressed 790 Environmental Science & Technology
,
100 I
I P a t Stoichiometric Carburetion
0
z
20 Harshaw Pt
, I 1.0
I I I .5 20 I N L E T CARBON MONOXIDE, mole % I
0
0.5
:
5
Figure 1. Conversion of NO as a function of CO concentration at catalyst inlet, for Pt containing catalyst Multicylinder engine; nonleaded fuel (iso-octane); simulated cruise, 30 mph; catalyst temperature, 900°F
3.5I
I
I""
80 -
0 INITIAL A 18 5 HOURS CYCLE
V 43 HOURS C Y C L E
4.0
0
0
0.5
1.0
I .5
c
'
Stoichiometric
0.4
2.0
I N L E T CARBON M O N O X I D E , m o l e %
ae -
Figure 2. Cross-correlation for inlet oxygen vs. inlet CO using isooctane fuel in multicylinder tests
3.0
3.3
0)
E
$
0)
0
N
In the multicylinder engine dynamometer screening tests for catalysts, the oxygen content shown in Figure 2 was obtained by a n indirect computation from material balances and the verification of the observed selectivity was considered necessary. Therefore, single-cylinder engine dynamometer tests were conducted with a direct determination of oxygen concentrations. The nitric oxide removal was again noted in these experiments. The exhaust composition of the single-cylinder engine, shown as a function of air-fuel ratio, is given in Figure 4. Here CO, 02, and NO are measured values while those of Hz are calculated. The carbon mass balances, performed by computer, were within a 3 % error. The data in the figure confirm, by direct measurement, the presence of appreciable oxygen near the stoichiometric point. By presenting the disappearance of NO as a function of the measured O2 concentration in the exhaust (Figure 5 ) , we again observe the sharp peak around the stoichiometric ratio, confirming the authenticity of the previous observations. Figure 6 shows the removal of NO over the UOP catalyst as a function of temperature. At an inlet concentration of 0.5 % O2and temperatures >800"F, the removal of NO was approximately 90%. The ability of the platinum catalysts to remove NO, near the stoichiometric point along with CO and hydrocarbons served as a basis for a patent application (Lassen and Weaver, 1969). The observed NO reduction in the presence of oxygen presented intriguing questions as to the nature of this phenomenon. The remainder of the report is devoted to our search for the answer as well as presenting some of the associated investigations involved in this search. The prevailing opinion in the literature (Shelef and Kummer, 1969), our own results on a different Pt catalyst (Shelef et al., 1968), and finally some of our own preliminary experiments using CO as a reducing agent for NO in competition with oxygen had led LIS t o assume initially that carbon monoxide would always react preferentially with oxygen to the
0 . 2.0
0.2 E.
N 1
0 2
6 0 I .o
3.I
14
13
16
15
17
AIR TO F U E L WEIGHT RATIO
Figure 4. hleasured exhaust composition vs. A/F ratio in a singlecylinder engine (H?calculated) 100
80
ae J
2 60 0
2 W
a 0
z
4o
-
20
0
0
I
I
I
05
IO
15
20
I N L E T E X H A U S T O X Y G E N , mole %
Figure 5. Conversion of NO as a function of 0,concentration at catalyst inlet. Catalyst PZ-1-168 Single-cklinder engine; nonleaded fuel; catallst temperature, 850'F; spaceielocitj, 14,000 to 16,000 hr-' Volume 5, Number 9, September 1971 791
Table I. Description of Catalysts Used in Studying the NO-H2 Reaction Chemical analysis, Catalyst wt % o f Space velocity, designation active metal hr-1 PZ-1-168 AmCyO.l%Pt AmCy0.1zPd
1800-1 8000 9000 9000
contains Pt 0 . 1 % Pt 0 . 1 % Pd
Table 11. Poisoning of Selective H2-NO Reaction by CO Space Inlet Outlet velocity, Temp., concna concn, "F NO, Z CO, Z, NO, Z Catalyst hr-1 AmCyO.l%Pd 9000 386 0.17 0 0.04 0.24 0.09 1.8 0.12 PZ-1-168 9000 365 0.17 0 0.07 0.10 0.11 PZ-1-168 1800 314 0.17 0 0,006 0.27 0.16 a At constant levels of 0.5 % H Pand 0.8 02.
40
g
-
20
V
0
z
20
Total O p e r a t i n g Time - 3 3 5 h r r . I
0
200 400 600 800 CATALYST INLET TEMPERATURE , O F
0
Figure 14. Effect of catalyst inlet temperature on the removal of nitric oxide by hydrogen in engine tests. Catalyst PZ-1-168 H Jdrogen inlet level, 8000 ppin
Lead S t e r i l e Fuel 0 0 2 % Sulfur
01 0
1
I 5
I I 10 15 TEST DAYS
I 20
I 25
Figure 15. The effect of the sulfur content of the fuel on the catalyst life in engine tests. Catalyst PZ-1-168 Space velocity, 14000 hr -1; inlet temperature, 350'F; hydrogen inlet level, 8000 ppm Volume 5, Number 9, September 1971 795
c
loo
20
n "
0
200
CATALYST
400
600
800
IN L E T TEMPERATURE , O F
Figure 16. Effect of inlet oxygen concentration on the removal of nitric oxide by hydrogen in engine tests. Catalyst PZ-1-168, isooctane fuel Hydrogen inlet level, 8000 ppm
loo
temperature of maximum NO removal was observed (Figure 17). At higher CO levels there is a marked resemblance to the original observation of NO removal near to the stoichiometric engine operation. The shift of the NO removal peak away from the low-temperature region is consistent with the laboratory observations on the poisoning of the reaction between nitric oxide and hydrogen by the addition of CO (Table 11). It appears that the process of NO removal at higher CO levels in Figure 17 is the same as that initially observed for exhaust gas at high temperatures as indicated in Figure 1 . The engine dynamometer tests have confirmed the main results of the laboratory experiments: the selectivity of hydrogen for the reaction with NO in the presence of oxygen on noble metal catalysts, the limitation of this selectivity to low catalyst temperatures, and the extreme susceptibility of this selective activity to poisons. Nevertheless, the cause of the original observations still remained unresolved because the selectivity of the molecular hydrogen for NO was noted at temperatures much below those noted for the NO removal in Figures 1 and 5 . Further, in a series of experiments carried out independently using the engine dynamometer installation, some removal of NO was observed under conditions at which the measured level of molecular hydrogen at the catalyst inlet was virtually zero, again at air-fuel ratios close to stoichiometric carburetion. Barring the possibility of NO decomposition which we have found to be very slow (Shelef et al., 1969), we either had to reconsider our previous assumption concerning the ineffectiveness of carbon monoxide for selective NO removal (at least insofar as certain noble metal catalysts are concerned) or to examine more closely the role of the vast amounts of water vapor present in the exhaust.
I I n l e t CO Concentration
$ 80 J
a > 60 w
a: 40
Role of Water Vapor in the Reniocal of NO on a Pt Catalyst 20
+
/
n v
0
200
400
600
800
CATALYST I N L E T TEMPERATURE ,'
Figure 17. Effect of inlet carbon monoxide concentration on the removal of nitric oxide by hydrogen in engine tests. Catalyst PZ-1168, iso-octanefuel Hydrogen inlet level, 8000 ppm
-
2.5%-nlat CO
r u
-m
n--
N
0 "
0
. E . B
--c coz
2.0-
0
-
''3
.
4 00
E
--0--co
d
i +
The composition of the exhaust, particularly under conditions of rich carburetion, is far to the left in the water gas shift reaction equilibrium H 2 0 C O S H? C02.This composition is essentially frozen-in when the exhaust is rapidly quenched after combustion. Recently, it has been disclosed in a patent (Gross et a]., 1968), that a wide class of catalysts is capable of shifting this equilibrium almost completely to the right at temperatures easily attainable in the exhaust system. The cited patent treats the thermodynamics of the water-gas shift, under the exhaust conditions, in a detailed manner. The hydrogen formed in the water-gas shift reaction accomplishes efficient removal of N O by its reduction by hydrogen on the same catalyst. This participation of the water vapor in the catalytic treatment of the exhaust shows that it cannot be regarded as inert. It seemed plausible t o us that if the catalyst PZ-1-168 was active for the water-gas shift reaction, the hydrogen formed could accomplish the partially selective removal of NO observed in the original tests. The hydrogen derived from the water-gas shift reaction could react with the NO before being desorbed as molecules into the gas phase and therefore its activity could, conceivably, extend into higher temperatures than that noted when the hydrogen is initially present in molecular form where it must be adsorbed dissociatively in order to react. Figure 18 shows the activity of the PZ-1-168 catalyst in the water-gas shift reaction. The inlet composition was 2.53 % co and 8.67 H?O in He, About 1600 ppm of NO was added to assess the behavior of this constituent. The exit gas composition was monitored as a function of temperature, passing the gas over the catalyst at a space velocity of 10.000 hr.-]
.
_ - 0,
-.20
-Hp 0 . NO
0-
z
LL 0
.
600 800 TEMPERATURE , O F
io00
Figure 18. Water-gas shift reaction on catalyst PZ-1-168 with simultaneous removal of NO Space velocity, 10000 hr-l; water level at inlet 8.7% 796 Environmental Science & Technology
+
Table V. Comparison of the Removal of NO and O2in Dry and Wet Synthetic Streams over Catalyst PZ-1-168 Space velocity, hr-1
Inlet compo~ition,~ Z
~~
CO
NO
0 2
1.05 1.09 1.1 1.04 1.01
0.19 0.20 0.21 0.18 0.18
0.45 0.47 0.51 0.47 0.49
Temp.
50z removal, “F HzO NO Oz
100
I-
. I 2 ae
80
-I
I W
2000 6000 6000 6000 6000
8.7 ... 3.7 ... 8.7
540 610 583 622 568
520 583 574 568 550
Balance He.
a
K
0
.5
I.o
1.5
2.0
2.5
3.0
:3
I N L E T C A R B O N M O N O X I D E , mole %
The results show that the catalyst was active for the water-gas shift. The NO was reduced almost completely at approximately 750”F, which was expected under the prevailing reducing conditions. There is a good carbon material balance since the C 0 2 formed corresponds approximately to the CO converted. On the other hand, the required carbon dioxidehydrogen equivalence is not fulfilled strictly, even assuming that the NO is wholly reduced by H2. One should not conclude, however, that the shift reaction on a platinum catalyst is faster than CO oxidation by oxygen in a dry medium. That this is not the case was pointed out (Gross et al., 1968) and verified experimentally in our laboratory. To prove the effect of water vapor in the phenomenon noted in the first section of the paper, conditions closer to those found in the exhaust needed to be simulated in the laboratory. First, we have followed, as a function of temperature, the behavior of synthetic mixtures of 1.0% CO, 0.5% 0 2 , 0.15 to 0.20% NO in He with 9 % H20 and without H?O at space velocities of 2000,6000, and 10,000 hr.-I As an example, typical data, obtained at 6000 hr,-l are given in Figure 19. I t is immediately apparent that, even in dry streams at COjO, ratio close to stoichiometry, there is a definite participation of the NO in the oxidation of CO. The decrease of NO began at somewhat higher temperatures than that of oxygen and it participated in the CO oxidation in smaller absolute amounts. When the NO concentration in the gas stream was increased to a level comparable to that of oxygen, the preference for the CO-O2 reaction became more pronounced. Thus, when a dry mixture containing 0.82 % 0 2 , 1.72 % NO, and 1.75 % CO was passed over the PZ-1-168 catalyst at 1100 hr,-l 89 to 90% of
I””
-
5 80-
a
I
-
2
60E w
a 0
40 -
z
20 -
-
0
Q
I C
L
Figure 20. Comparison of “wet” and “dry” synthetic mixtures in the removal of NO over catalyst PZ-1-168 at a space velocity of 10000 hr -l Temperature, 660” Z+ 15°F
the oxygen reacted at 620°F while only approximately 15% of the NO was removed. It can be inferred from the experimental results that the order of the NO reduction by CO on the PZ-1-168 catalyst has a low fractional value and, hence, the relative amount reacted in the presence of oxygen will increase as the NO concentration in the gas stream decreases. The order of NO decomposition on the same catalyst was determined as 0.3 to 0.45 (Shelef et al., 1969). Superimposed onto the participation of NO in the dry reaction is the effect of water vapor. This is already seen on Figure 19, where the wet curve of NO removal lies to the leftLe., at lower temperatures-of the dry one. Table V gives the comparison between nitric oxide and oxygen removal at a space velocity of 6000 hr-l in wet and dry conditions for two different levels of water vapor concentration. With use of the point of 50% removal as a reactivity index, it is clear that the presence of water enhances the removal of NO and also slightly that of oxygen. The shifting of the 50% removal point in the presence of approximately 9 % water vapor by 60°F is quite significant if one notes that the lowering of the space velocity by a factor of 3 (to 2000 hr-l) causes a shift in this reactivity index by 30 O F . Finally, to evaluate the effect of the water vapor as a function of the exhaust composition change, when changing the A/F ratio, the following isothermal runs were carried out. With the aid of Figure 4 blends of CO and O2were made corresponding to the A/F ratio between 13.5 and 17.0, at a constant nitric oxide content of approximately 2000 ppni. It is realized that the NO content also changes as a function of the A/F ratio but a constant value was adopted to facilitate the blending of the mixtures. The mixtures reacted over PZ1-168 catalyst at a constant temperature of 660°F and a space velocity of 10,000 hr.-I Figure 20 gives the results of these experiments plotted in the same coordinates as Figure l-i.e., percent NO removal as a function of inlet CO. The 0% inlet concentration associated with a given experimental point is marked on the figure. The points obtained in presence of 8.7 H 2 0 are the open triangles and in the dry conditions, the closed circles. There is a large scatter of the experimental points due to the difficulty in maintaining isothermal conditions in an exothermic reaction and to the low precision of the mass spectrometric analysis at the low NO content. I n spite of the scatter of t h e data, the following facts emerge. At low CO levels found under Volume 5, Number 9, September 1971 797
Table VI. Formation of NH,in the H2-NO Reaction on Catalyst PZ-1-168 NO inlet
concn,
NO
reacted,
PPm
7 3
3600 2000 905 460 245
92.5 92.5 93 .O 91.5 91.5
NH3 formed, ppm
3160 1735 730 41 3 206
Reacted Inlet NO NO converted converted to “3, X to “3, Z 87.8 86.7 80.6 89.8 84.1
94.9 93.7 86.7 98.1 91.9
Carrier gas: N2; H, inlet concentration: 1 . 4 3 z ; Space velocity: 20,000 hr-l; Reaction temperature: 550’F Table VII. Formation of NHI in the H2-NO Reaction on Catalyst PZ-1-168 in the Presence of Oxygen NO inlet concn,
NO reacted,
ppm
Z
3450 1900 885 500 245
82.5 83.5 77.5 64.0 32.6
Reacted Inlet NO NO formed, converted converted ppm to “3, Z to NH3, Z NH8
95 69 28 19 13
2.75 3.63 3.16 3,80 5.30
3.33 4.35 4.08 5.94 16.3
Carrier gas: N2; HZinlet concentration: 1.43 %; 0 2 inlet concentration: 0.9-1 .O %; Space velocity: 20,000 hr-l; Reaction temperature: 392’F
very lean conditions, the removal of NO is small, but not absent, under both wet and dry conditions. At high CO levels, the difference between wet and dry point tends to diminish. The largest effect of the water occurs somewhat to the rich side of the stoichiometric carburetion, between 1 and 2 % CO in the gas. The best curve drawn through the wet points reaches a maximum at about 1.2% CO, as opposed to 0 . 7 z CO on Figure 1. While this disagreement can be due to a number of causes, such as differences in analytical methods, divergences in the actual composition of the exhaust and the synthetic streams, or finally the differences in the temperatures, we believe that the water-gas shift reaction makes a contribution to the reduction of NO over a noble metal catalyst in a certain range of exhaust compositions. This contribution is superimposed onto the participation of NO along with oxygen in the reaction with carbon monoxide under exhaust conditions produced by carburetion close to stoichiometry. Conclusions
In engine tests under certain conditions of carburetion, close to the stoichiometric air-fuel ratio, marked removal of nitric oxide is noted in the presence of oxygen when passing the exhaust over noble-metal catalysts. At low temperatures (600”F) the removal of NO from the exhaust is ascribed to its simultaneous participation, with oxygen, in the oxidation of carbon monoxide and to 798
Environmental Science & Technology
its reaction with hydrogen generated “in situ” by the watergas shift reaction. The work brings into focus the complex changes in the actual effects of the various exhaust constituents and of the catalyst as the conditions of the treatment (temperature, composition) change. Addendum
The question of the formation of ammonia in the reaction between nitric oxide and hydrogen arose during the presentation at the Denver meeting of ArchE-IMIQ in August 1970. The N H 3 formation, under reducing conditions on a Pt catalyst, is documented in the literature (Kokes, 1966). In the catalytic treatment of exhaust under reducing conditions, the ammonia formation has been observed on a wide variety of catalysts, both in this laboratory and elsewhere, although published data are scarce as yet. The hydrogen for the ammonia formation is derived, in the exhaust treatment, either from molecular Hspresent in exhaust from rich carburetion or from the water-gas shift reaction. A series of experiments was performed with use of the PZ-l168 catalyst in which the exit gases were analyzed specifically for N H 3 by a titrimetric method using bromo-cresol green indicator. As seen from Table VI, in a reducing atmosphere without the presence of oxygen at 550”F, almost all the NO is converted to ammonia. The data in Table VII, however, indicate that in slightly oxidizing atmosphere, in the composition and temperature ranges of the observed selectivity, the conversion to ammonia is negligible. This is a striking result, the mechanism of which is currently under study. Acknowledgment
We thank Ann Piken for her able help in the laboratory tests; J. S. Ninomiya and Amos Golovoy for the data they obtained on the single-cylinder engine; and Haren Gandhi for the ammonia formation work. Literature Cited
Berkman, S., Morrell, J. C., Egloff, G., “Catalysis,” Reinhold, New York, 1950, p 391ff. Gross, G. P., Biller, W. F., Greene, D. F., Kearby, K. K., U S . Patent 3,370,914, February 27, 1968. Kokes, R. J., J. Phys. Chem. 70,296 (1966). Kummer. J. T., Shelef, M.. Patent Application 819,330, __ . . May 25, 1969. Lassen. H. G.. Weaver. E. E.. Patent Amlication 826.482 * May16, 1969. Otto, K., Shelef, M., Kummer, J. T., J. Phys. Chem. 74,2790 (1970). Sergeant, G . A., Bartlett, A. F. F., J. Appl. Chem. 5, 208 (1955). Shelef, M., Kummer, J. T., “The Behavior of Nitric Oxide in Heterogeneous Catalytic Reactions,” Preprint 13f, 62nd Annual Meeting AIChE, Washington, D.C., November 16-20, 1969. Shelef, M., Otto, K., Gandhi, H., Atmos. Enciron. 3, 107 (1969). Shelef, M., Otto, K., Gandhi, H., J . Catalysis 12, 351 (1968). Yarrington, R. M., Bambrick, W. E., American Cyanamid Co., Stamford, Conn., private communication, 1969. ’
Receiced.for reciew June 4, 1970. Accepted December 7, 1970. Prepared for the session “Control of Nitrogen Oxides Pollution” at the joint meeting o f American Institute o f Chemical Engineers and Instituto Mexican0 de Ingenieros Quimicos, Denver, Colo., August 30-September 2, 1970.
