where PH, = hydrogen partial pressure, psia; K = equilibrium constant, psia; and ( ) = mole fraction in the product. The data have been presented in this fashion in Figure 6 and it appears that eq 2 adequately describes the results a t each temperature. The change in slope of the line represents the effect of temperature on the equilibrium constant which is given by the relationship
Acknowledgments
h cP
Conclusions The data presented in this study offer support for the reaction network shown below in which the vertical or diagonal reaction paths occur on acidic catalytic sites and the horizontal paths occur on different types of metal sites. CH CH' @ Bz
C+-
mation of the MCP/MCP- equilibrium and suggest that the CH/CH= equilibrium is also established. Furthermore, it appears that catalytic reforming of MCP with the platinum-rhenium catalyst used in this study occurs by reaction mechanisms that are not inconsistent with those obtained using the standard platinum-alumina catalyst (Selman, 1972).
iC 6 + nC6
L MCP
\
nC6
11
e MCP'
\\/ nC8'
Evidence has been presented that decyclization of M C P occurs on hydrogenolysis sites to produce a distribution of Cg paraffins which can undergo further reaction on the same type of sites to produce Cg- products. There is substantial evidence that most of the n-hexane product is formed by a different mechanism, one that possibly occurs on acidic sites from M C P = or MCP and with hexene-2 as an intermediate product. Experimental data offer confir-
The authors are indebted to Exxon Research and Engineering Company for sponsoring this project and to the Department of Chemical Engineering at Louisiana State University for the use of their facilities. Literature Cited Bryant, P. A , , Voorhies, A , , J r . , AlChEJ., 1 4 , 852 (1968). Ciapetta, F. G., Hunter, J. E.. lnd. Eng. Chem.. 45 ( 1 ) . 147 (1953). Haensel, V., Donaldson. G. R., lnd. Eng. Chem.. 43 ( 9 ) . 2102 (1951). Hindin. S. G., Weller, S. W., Mills, G. A , , J. Phys Chem . 62, 244 (1958). Hougen, 0. A . , Watson, K. M.. "Chemical Process Principles, Part I l l , Kinetics and Catalysis," Wiley, New York, N . Y . , 1947. Jacobson, R. L., Kluksdahl, H. E.. McCoy, C. S., Davis, R. W., 34th Midyear Meeting of American Pet. Inst.. Div. of Ref., Chicago, Ill., May 13, 1969. Mills, G. A . . Heinemann. H., Milliken, T. H..Oblad, A . G., lnd. Eng. Chern.. 45 ( l ) , 134 (1953). Selman, D. M.. Ph.D. Dissertation, Louisiana State University, Baton Rouge, La., 1972. Smith, R . L., Naro, P. A,, Silvestri, A . J., J. Catal.. 20, 359 (1971).
Received f o r revieu M a y 16. 1974 Accepted D e c e m b e r 13, 1974 Presented a t t h e D i v i s i o n of P e t r o l e u m C h e m i s t r y , 165th N a t i o n a l M e e t i n g of t h e A m e r i c a n C h e m i c a l Society, Apr 8-13, 1973, D a l l a s , Texas.
Reduction of Nitric Oxide with Ammonia on Noble Metal Catalysts George L. Bauerle, S. C. Wu,and Ken Nobe* School of Engineering and Applied Science, Universify of California, Los Angeles, California 90024
The reduction of NO with N H 3 on alumina-supported Pt, Pd, Ru, and Rh catalysts has been studied in flow reactors between 120 and 400°C. The presence of 0 2 in the gas mixture enhances the reduction of NO on Pt and Pd below about 230"C, while 0 2 inhibits NO reduction on Ru and Rh. In the presence of 0 2 , considerable N2O is produced on both Pt and Pd. Using simulated power plant.stack gas containing 14% C02, 5% H 2 0 , 3% 0 2 and with NO varying from 250 to 1000 ppm, conversion of NO is a function of the NH3,"O inlet ratio from 0.5 to 2.3. The presence of 0 2 was also observed to accelerate the decomposition of NO on Pt in simulated flue gas. At 300"C, maximum decomposition occurs with 2% 0 2 present in the flue gas.
Introduction The catalytic reduction of nitric oxide in exhaust streams of combustion processes has been studied extensively in recent years. A primary goal has been to develop catalysts that promote the selective reduction of NO to Nz in the presence of oxygen with various reducing agents. Jones et al. (1971) found that under strict control of carburetion of an automobile engine some selective reduction of NO on Pt can be achieved. Shelef et al. (1969) reported partial selective reduction of NO with CO on a chromia catalyst. Their conclusions were based on an observed dip in the outlet NO concentration vs. temperature curve at low temperatures. This observation has also been
made in our laboratory with several catalysts (Bauerle et al., 1972) and has been attributed to the overlap in the temperature ranges in which the respective CO-NO and CO-02 reactions predominate. While there has been little success in the selective removal of NO using the more commonly studied reducing agents, ammonia has been purported to be selective for NO reduction in the presence of 0 2 (Andersen et al., 1961; Markvart and Pour, 1967, 1969; Nonnenmacher and Kartte, 1966; Schmidt and Schulze, 1968; Yamanaka, 1971; Kudo et al., 1974). Consideration of NH3 as a reducing agent for NO has been given to processes for nitric acid tail gas cleanup (Gillespie et al., 1972); such considInd. Eng. Chem., Prod. Res. Dev., Vol. 14, No. 2, 1975
123
TEMPERATURE
(OC)
.,
Figure 1. Effect of 0 2 on the reduction of NO with NH3'on Ru: 20,000 hr- (STP) space velocity; 1000 ppm NO, 1200 ppm NH3 in N2 carrier gas: 0 , 0 , NO concentration; 0 , NH3 concentration; A , N2O concentration, o , NO2 concentration; open points, 3000 ppm 02,closed points, zero 0 2 ; dashed line represents maximum NzO production based on reduction of NO; dotted line represents maximum E20 production by oxidation of NH3 with 0 2 based on total NH3 oxidized. eration has also been given to power plant flue gas control (Bartok et al., 1969) and to automobile exhaust control (Griffing et al., 1969). Markvart and Pour's results (1967) contradicted those of Michaelova (1939) who reported NO reduction with NH3 is inhibited by oxygen. The former found that NO reduction on Pt is, in fact, enhanced by the presence of oxygen. The production of N20 during the catalytic reduction of NO with NH3 on noble metals in the absence of 0 2 has been reported (Otto et al., 1970, 1973); similarly, the formation of N20 a t low temperatures during oxidation of ammonia with 0 2 has been reported (DeLaney and Manogue, 1972, and references therein). It is of interest to investigate in more detail the selecThe relationship between tive reduction of NO with "3. the selectivity and/or enhancement of the NH3-NO reaction in the presence of 0 2 and the appearance of N2O in the exhaust products is of particular interest. Platinum, palladium, ruthenium, and rhodium catalysts were investigated for activity in a dry NO-Oz-NH3 system. In addition, platinum was studied more extensively in simulated power plant stack gas. Experimental Section The noble metal catalysts were obtained from Engelhard Industries. Each was in the form of 3.2-mm cylindrical pellets containing 0.5 wt 90metal on alumina. The test apparatus, used in studies with dry feed gas and incorporating a gas mixing manifold and a Pyrex upflow reactor, has been described previously (Bauerle et al., and 1972). Nitrogen carrier gas was used and NO, "3, 0 2 were introduced into the manifold through individual metering valves and rotameters. Flow rates of NO were verified with a soap-bubble flow meter. The reactor was operated isothermally with a series of meter-relays and heating tapes. Analyses of NO, N02, and N20 were made by nondispersive infrared spectroscopy, continuous visible colorimetry, and vapor chromatography (using a 3-m column of 124
Ind. Eng. Chem., Prod. Res. Dev., Vol. 14, No. 2, 1975
TEMPERATURE
('C)
Figure 2. Effect of 0 2 on the reduction of NO with NH3 on Rh. Same symbols and conditions (except 1100 ppm "3) as in Figure 1.
Porapack Q), respectively. Ammonia was determined by bubbling a portion of the inlet or outlet gas stream through 2% boric acid and titrating the resultant solution with 0.03 N HC1 using bromcresol green as an indicator. A wet test-meter a t the end of the train indicated the volume of gas passed. Flow rate of the carrier gas was 275 1. (STP)/hr. Fourteen grams of catalyst was used in each case. Nominal concentrations of NO and NH3 were 1000 and 1150 ppm, respectively, with variable O2 concentration ranging from 0 to 3000 ppm. In the simulated flue gas studies, a stainless steel reactor of 12-mm diameter and 14-ml volume was employed. The reactor, along with a 6-m, stainless steel preheater coil, was immersed in an electrically heated, fluidized bath of powdered alundum. The NO was added to the mixing manifold as before. Addition of a measured flow of air provided 0 2 . Water was injected with a pump, through a flow meter into an electrically heated vaporized tube located immediately upstream of the preheater. Ammonia (in the form of a prepared, gaseous N H S - N ~mixture) was admitted at the vaporizor. The vaporizor was maintained a t about 380°C. In the studies with simulated flue gas, ammonia and nitric oxide concentrations were varied from 500 to 1600 ppm and 250 to 1000 ppm, respectively. Carbon dioxide (14%), water (5%), and oxygen (3%) in nitrogen carrier gas constituted the remainder of the simulated flue-gas stream. Standard flow rate was 283 1. (STP)/hr (space velocity = 20,000 hr l ) . In some tests, SO2 was added to the gas mixture at the entrance to the vaporizer. Analyses were performed as described above with the addition of nondispersive infrared methods for C02 and S02. When $302 was included, NH3 analyses were performed using an Orion specific ion electrode. The sample stream leading to the bubbler was heated to prevent condensation and/or salt formation. With both NO and 0 2 present in the feed stream, a small amount of NO2 was produced in the process lines and analytical equipment. The point of production of NO2 was traced to the NO2 analyzer itself which incorporated a 1-m optical cell about 2.5 cm in diameter operating a t room temperature. Residence time in the cell at the sample flow rate used was about 15 min (constant NO2 con-
TEMPERATURE
(OC)
Figure 3. Effect of 0 2 on the reduction of NO with NH3 on Pd. Same symbols and conditions as in Figure 1. centration was a prime criterion for steady-state in this study). In the calibration of the instruments it was determined that the measured concentration of NO2 (about 150 ppm maximum with an NO inlet concentration of 1000 ppm) was reproducible and second order in NO concentration and first order in 0 2 concentration, reflecting the well known kinetics of homogeneous NO oxidation. A Calibration curve was developed to determine NO concentration a t the reactor inlet and outlet based on the combined readings of the NO and NO2 analyzers. NO2 concentration values reported in this paper represent the amount measured less the amount known to be produced homogeneously in the analytical system. Results Preliminary tests with both reactor systems showed that the homogeneous reduction of NO with NH3 was negligible between 200 and 500°C. Inlet and outlet NH3 analyses agreed closely indicating that there was neither significant NH3 decomposition nor condensation of H2O in the sample lines. Effect of 0 2 on t h e Reduction of NO in Dry Gas Mixtures. Figures 1 through 4 show the results of NO reduction with NH3 on the four noble metal catalysts in the simple test gas mixtures. Outlet concentrations of NO, NzO, "3, and NO:! in the absence and presence of 0 2 at various reaction temperatures are given. In these tests, H20 and COz were not present in the inlet stream. The four noble metal catalysts exhibited differing 0 2 effects on performance characteristics. With ruthenium catalyst, Figure 1 shows that the addition of 0 2 retarded the reduction of NO. In the absence of 0 2 , the continued decrease in NH3 concentration with increase in temperature beyond the point of total conversion of NO (265°C) indicates decomposition of "3. Outlet NO concentration in the presence of 0 2 reached a minimum a t 330°C; ammonia conversion was somewhat lower when 0 2 was admitted. Although no X Z O was detected in the absence of 0 2 , about 175 ppm of i V 2 0 was produced in the presence of 02.
Rhodium catalyst (Figure 2) performed similar to Ru. As with ruthenium, a minimum in N O outlet concentration and a maximum in NzO production occurred in the presence of 0 2 . Without 0 2 , conversion of NO was complete a t 230°C. Ammonia conversion was not changed
TEMPERATURE
?C 1
Figure 4. Effect of 0 2 on the reduction of NO with NH3 on Pt. Same symbols and conditions (except 1100 ppm "3) as in Figure 1.Half-filled points, 784 ppm 0 2 . much by 0 2 addition up to 230°C. However, in the absence of 0 2 , NH3 outlet concentration leveled off a t about 500 ppm a t the point of total conversion, indicating little NH3 decomposition up to 270°C; however, substantial ammonia decomposition occurred above 275°C. Some NO2 was detected in the presence of 02, while N2O and NO2 were not found in the reaction products in the 02-free tests for both Rh and Ru. With Pd (Figure 3), NH3 decomposition did not occur in the absence of 0 2 below 325°C. When 0 2 was added to the reaction mixture, the NO-NH3 reaction was enhanced. Considerable N2O was produced, reaching a maximum of 750 ppm a t 200°C. Nitric oxide outlet concentration increased slowly from zero between 150 and 280°C and subsequently increased sharply. At 400"C, NO concentration was 1200 ppm, indicating the oxidation of NH3 to NO. Almost 200 ppm NO2 was produced a t 400°C. Up to about 200°C the behavior of platinum catalyst was very similar to palladium. The introduction of 0 2 enhanced the NO-NH3 reaction. The influence of temperature on N O and NH3 concentrations in the absence of 0 2 was quite different with Pt than with the other catalysts; with Pt both N O and NH3 outlet concentrations reached minima a t 280°C. The data points on the ascending legs of the concentration curves for NO and NH3 in the absence of 0 2 were not steady-state values. Both concentrations were increasing very slowly with time; for instance, the points a t 360°C (400 ppm NO and 800 ppm "3) were taken after about 20 hr elapsed time. Shortly after stabilizing the temperature for these tests the NO and NH3 concentrations had been initially about 200 and 400 ppm, respectively. In the presence of 0 2 the slow drift in outlet concentrations did not occur; in this regard, Pt and Pd were very similar in their behavior. Nitrogen dioxide production on Pt in the presence of 0 2 was below 40 ppm over the temperature range studied. An additional set of points for NO concentration in the presence of 780 ppm of 0 2 also is shown in Figure 4; it is seen that the behavior is similar to that with 3000 ppm present. Maximum NO conversion occurs a t a slightly higher temperature with the lower 0 2 level. Figures 1 through 4 indicate that enhancement by 0 2 for noble metal catalysts may be closely related to the production of N2O. Otto et al. (1973) reported considerInd. Eng. Chem.,Prod. Res. Dev., Vol. 14, No. 2, 1975
125
loo[
1OOr
80
-ae
b
60
Y
i3
401
A 0
800 500
2ol
0
250 1000
"0
04
'
08
20000 20000 20000 10000
0 900
10000
8 "
'
12 NHJ/NO
"
16
'
2.0
l
'
24
20
Figure 5 . Reduction of NO with NHs on Pt in simulated flue gas: 250°C; 14% C O z , 5% HzO and 3% 0 2 in Nz carrier gas.
able N20 production on Ru in a dry, 02-free and COz-free recycle reactor; in the present study, little N 2 0 was produced in the absence of 0 2 , HzO, and COZ with a single-pass flow reactor. Otto, e t al. (1973) further showed that, in the absence of 0 2 , N 2 0 production decreases sharply when reduction of NO is complete. Addition of 0 2 did not have a large effect on NH3 conversion on either Ru or Rh up to 230°C, a s shown in Figures 1 and 2, suggesting that oxidation of NH3 with both NO and O2 is a t approximately the same rate as the NH3-NO reaction in the absence of 0 2 . This result is consistent with surface dissociation of NH3 as the rate-determining step, and negligible difference in the rate of removal of the dissociation fragments with either NO or 0 2 . With Pt and Pd catalysts, minima in NO concentration in the presence of 0 2 were also obtained as shown in Figures 3 and 4 . However, unlike Ru and Rh, a definite decrease in NO output occurs compared to the 02-free condition during most of the descending portion of the NO output versus temperature curves. Furthermore, more 3 " is consumed with addition of 0 2 . Reduction of NO on Pt in Simulated Power Plant Flue Gas. The platinum catalyst was tested in simulated flue gas containing 14% COz, 5% HzO, and 3% 0 2 in N2 carrier gas. Figure 5 shows typical results of tests a t 250°C in which NO concentration was varied from 250 to 1000 ppm and the NH3/NO ratio was varied from about 0.5 to 2.3. Space velocity was 10,000 or 20,000 hr-1 (STP). It is noted that conversion of NO is not a strong function of NO concentration, but appears to be a function of the NO/"3 ratio. Space velocity has only a moderate effect over the range studied. In order to explore more fully the effect of 0 2 on NO reduction, tests were run with the Pt catalyst using simulated flue gas with varying 0 2 content. Specifically, the tests were conducted a t 200 and 250"C, a t which points the preliminary studies showed enhancement and inhibition by 0 2 , respectively (Figure 4). Nitric oxide concentration was set a t 1000 ppm and NH3 concentration was maintained with either the approximate stoichiometric level for comFigure 6 shows plete conversion to Nz or with excess "3. the results of these tests. At 200"C, addition of 0 2 caused a sharp increase in NO conversion with both levels of "3, while a t 250"C, the conversion of NO decreased when 0 2 was added. At both temperatures and NH3 concentrations, introduction of 0 2 caused a sharp increase in NzO production. In all cases, NO conversion and N20 production were not significantly affected above an 0 2 inlet concentration of about 5000 ppm. In view of the results in Figure 4 with 784 ppm 0 2 , the slopes of the curves near zero 0 2 concentration in Figure 6 may be much steeper than shown. 126
Ind. Eng. Chem., Prod. Res. Dev., Vol. 14, No. 2, 1975
200 200
1100
0
250
700
0
250
1100
I
O2
Figure 6. Effect of
700
0
0
RATKI
200
2 a
2 CONCENTPATON
(%I
concentration on reduction of N O on Pt in simulated flue gas: 14% Con, 5% HzO, 1000 ppm NO in Nz carrier gas; open points, NO reduction; closed points, NzO produced. 0 2
Effect of SO2 on Reduction of NO on Pt in Simulated Flue Gas. The effect of SO2 on the reduction of NO with NH3 on Pt in simulated flue gas was investigated. Table I summarizes the results. At 209"C, the introduction of SO2 into the gas mixture resulted in a sharp, immediate drop in NO conversion, followed by a more gradual decrease in NO removal. When SO2 was admitted, N20 production decreased and excess NH3 appeared a t the outlet. The SO2 outlet concentrations in Table I are not quantitative since the SO2 analyzer was located downstream of the cold trap; a t the latter point some SO2 was removed by reaction with NH3 and absorption in the condensed water. These data are presented to show qualitative trends with the measured outlet concentrations of N20 and "3. The outlet values of "3, which were determined with a specific ion electrode, include free NH3 dissolved and ionized in boric acid as well as dissolved ammonium salts presumably formed by homogeneous reaction with sulfur oxides before or in the bubbler. In progressing from 209°C to higher temperatures, (runs 3 through 5), total ammonia output decreased while indicated SO2 increased. Such a result could be interpreted as being indicative of increased reaction of NH3 with NO as well as with 0 2 with a lesser amount of NH3 left to react homogeneously with SOz. However, a t 400"C, a t which point NH3 was almost totally consumed in the reactor, SO2 outlet concentration decreased abruptly to zero. At the same time, the colorimeter for NO2 analysis indicated a high concentration of an opaque component in the gas stream. The interfering component was a white gas which was presumably SO3. Prior to run 7 the reactant gas mixture was replaced by air and the reactor was maintained a t 400°C for 21 hr. Evolution of the opaque, white gas persisted for several hours. Subsequent tests a t 400, 250, and 205°C in the absence of SO2 were in accord with data taken earlier indicating that, at least for short-term exposure to SOz, the catalyst regained its original activity. The abrupt decrease in SOz, which cannot be attributed to reaction with NH3 is ascribed to the formation of SO3 a t 400°C. The persistence of SO3 in the gas stream long after the test gas was replaced by air indicates the decompostion of a salt that had been previously formed either on the catalyst or in the preheater. During the tests, the vaporizer and process lines were maintained a t about 380°C (i.e., hotter than the reactor during most of the SO2 runs). Inlet NH3 analyses by the Orion probe showed that deposition of a salt did not occur upstream of the preheater.
Table I. Effect of SO2 on Reduction of NO with NH3 on Pt Catalyst in Simulated Flue Gas Outlet gas composition
Inlet g a s composition Run
no.
Space Temp, velocity,‘ “C hr-’, STP
NO, ppm
NH,, PPm
PPm
NO, PPm
N20, PPm
NH,, PP m
SO,,“ ppm
Conv. of NO, %
0 ... 76 238 689 799 0 993 20,000 209 80 36 345 663 278 783 1000 209 20,000 1043 320 23 389 804 318 783 1000 20,000 1043 3 209 450 49 108 537 415 783 1000 20,000 1043 4 260 520 51 1000 516 458 N. A. 783 5 300 20,000 1043 13 0 18 860 347 783 1000 20,000 1043 6 400 0 891 290 0 10 988 858 7e 400 20,000 0 292 682 0 ... 70 988 858 8 250 20,000 0 184 0 ... 81 988 858 808 9 205 20,000 0 ... 88 1232 0 119 787 1003 10 250 10,000 76 67 N.A. 329 N . A. 1232 1200 10,000 1003 llf 250 600 N. A. 41 633 N. A. 1261 1200 12f 250 10,000 1064 0 1261 198 N. A. N. A . ... 81 13h 250 10,000 1064 86 ... 90 1189 0 787 124 989 14’ 250 10,000 a Feed gas also contained 14% C02, 3% 0 2 , 5% HzO in Nz. Rotameter-determined values. Indicated values only. Some removal of S O z in the cold-bath. Runs 2 through 6 at 1-hr intervals. e Reactor purged with air at 400°C for 21 hr prior to this run. r Readings taken 1h hr after “SO2 on.” g Readings taken 18 hr after “SO2 on.” Readings taken 1 hr after “SO2 off.” Catalyst heated to 400°C in S02-free gas mixture prior to testing.
1 2‘
...
A second series of tests (run 10 et seq) was performed at 250°C to determine the effect of longer-term exposure of Pt to SOz. In this case, a larger excess of NH3 was employed. The decrease in N O conversion was not as drastic as a t 209°C and NO conversion leveled off a t about 40%. Again, excess NH3 appeared a t the outlet when SO2 was present, and a s before, evolution of so3 was observed when the catalyst was heated to 400°C. Data for run 14 indicate recovery of catalytic activity after heating the catalyst. Decomposition of NO o n P t i n Simulated F l u e Gas. T o supplement the study of NO reduction with ”3, several experiments were conducted to assess the extent of NO decomposition on Pt in simulated flue gas. Figure 7 shows these results for space velocities of 10,000 and 20,000 hr ( S T P ) . The data points were obtained in two series of tests performed several months apart on separate samples of the Engelhard Pt catalyst. The product stream contained negligible N20 and NO2 even though the entire test series was performed over a temperature range favoring oxidation of NO. Figure 8 shows that, under the conditions employed in the present study, 0 2 promotes the decomposition of 1000 ppm of N O on P t in simulated flue gas. An apparent maximum in conversion occurs a t about 2% 0 2 a t 300°C. The d a t a indicate that at relatively low space velocities (at least compared with automotive applications) the decomposition of dilute concentrations of NO on Pt should not yet be discounted a s a potential means of pollutant control.
01
200
“
”
250
”
300
TEMPERATURE
350 (OC
‘
1
400
t
Figure 7. Decomposition of NO on Pt in simulated flue gas: 1470 COZ,5% HzO, 3% 0 2 and lOOOppm NO in NZcarrier gas.
0‘ 0
I
I
O2
I
2
I
3
CONCENTRATlON (Yo)
Figure 8. Effect of
0 2 on decomposition of NO on Pt in simulated flue gas: 300°C; 14% Con, 5% HzO, 1000 ppm NO in Nz carrier gas; 20,000 h r - l (STP)space velocity.
Discussion Although the N2 produced was not measured directly in this study, it is imformative to calculate the outlet concentration of the N2 produced (exclusive of the carrier gas) based on a material balance of the other reaction products. Figure 9 shows that with all four catalysts, the calculated production of N2 was less in the presence than in the absence of 0 2 . The production of Nz on P d leveled off a t about 850 ppm in the absence of 0 2 , which is in accord with the amount predicted by the reaction 6N0 + 4NH3 = 5N2 + 6HzO (1) Figure 3 shows t h a t the unreacted NH3 did not decom-
pose on P d . For Rh the onset of decomposition of unreacted NH3 above 275”C, a s shown in Figure 2, leads to additional N2 production. The calculated concentration of Nz produced on Ru in the absence of 0 2 reaches 1100 ppm which is the value predicted for complete reduction of NO to N2 plus complete decomposition of residual NH3 (Figure 1). Calculated NZ production on Pt in the absence of 0 2 goes through a maximum, as expected from the data given in Figure 4. The above results indicate t h a t the decrease in Nz production on Pt and Pd with addition of 0 2 can be attributed to the increased production of NzO. On the other Ind. Eng. Chem., Prod. Res. Dev., Vol. 14, No. 2, 1975
127
tion of Nz and NzO from the NO-NH3 reaction in the absence of 0 2 based on isotope labeling experiments. The mechanism included primary and secondary reaction steps for formation of each product as follows (adsorption and desorption steps have been omitted)
1t,* 1000
-f
9P
t
",(ads)
+
NO(ads)
",(ads)
(NO*NH,)(ads) = N, 6oo-
+ H(ads)
(4)
= (NOeNHz)(ads)
(5)
= ",(ads)
+
(NO*NH,)(ads) = NzO
H,O(ads),primary N, step (6)
+ 2H(ads), secondary NzO step
zN 400-
NO(ads)
+ H(ads)
2HNO(ads) = N,O(ads) 200
(9 1
2H(ads)
+
2HNO(ads) =
100
200
300
+
N,
1
2HzO(ads),secondary Nz step (10)
400
TEMPERATURE ('C)
Figure 9. Production of Nz during reduction of NO with NH3 on noble metal catalysts: 20,000 h r - 1 (STP) space velocity; 1000 ppm NO, 1100-1200 ppm NHs in Nz carrier gas; 0 , Pd; A, Ru; 0 ,Rh; v , Pt; open points, 3000 ppm 0 2 ; solid points, zero 0 2 .
hand, for Ru and Rh catalysts, the decrease in N2 production is attributed to a decrease in the reduction of NO. The latter conclusion is indicated by the calculated Nz level attained for Ru and Rh in the presence of 0 2 a t higher temperatures. A value of 600 ppm of N2 would be predicted for either decomposition or oxidation with 0 2 of 1200 ppm of NH3 to N2. As shown in Figure 9, the Nz produced on both Ru and Rh in the presence of oxygen a t higher temperature is approximately 600 ppm while the corresponding concentration of NO approach inlet conditions as shown in Figures 1 and 2 . The dashed and dotted lines in Figures 3 and 4, representing maximum N20 produced by the reactions 8N0 + 2NH3 5N,O + 3H,O (21 and 2NH3 + 202 N,O + 3H,O (3) respectively, are both too close to the measured N2O concentration curves to determine which of the two reactions is dominant for production of N2O on Pt and Pd. It is evident that both reactions contribute, since N2O production exceeds the amount predicted by either eq 2 or 3 separately. Other data, however, suggest that the NH3-02 reaction is the primary source of N2O. The SO2 studies (Table I) showed that, in the presence of S02, N2O production on Pt decreases and NH3 appears in the reactor effluent (usually, NH3 consumption on Pt is complete in S02-free simulated flue gas). These results suggest that SO2 is a poison for the "3-02 reaction on Pt. Table I shows that NO conversion in the presence of SO2 is very close to that obtained in the decomposition studies, as shown in Figure 7, implying that the NH3-NO reaction is also poisoned. Similar observations have been made by Kudo et al. (1974), who reported that the addition of 0 2 to a reacting NH3-NO ixture on Zr-doped lanthanum copper oxide shifted the maximum NO conversion point to lower temperatures. Introduction of SO2 caused the NO conversion curve to return to the 02-free position. Although N20 was not present in Kudo's study, it is apparent that introduction of SO2 precluded the enhancement effect by poisoning the "3-02 reaction. Otto et al. (1970) proposed a mechanism for the produc-
-
+
128
H,O(ads), primary NzO step
-
t 0
+
(7) (8)
= HNO(ads)
Ind. Eng. Chem., Prod. Res. Dev., Vol. 14, No. 2, 1975
For the' overall reaction, the surface dissociation of "3 (eq 4) is considered the rate-controlling step. NH3 oxidation with 0 2 has been studied extensively and mechanisms have been proposed (e.g., Bodenstein, 1941; Zawadzki, 1950). More recent studies have shown that the mechanism of NH3 oxidation may simply involve the initial direct formation of NO on the catalyst surface (f\rutt and Kapur, 1968, 1969; Fogel et al., 1964a,b). The direct formation of NO has been considered to include the following steps, by Nutt and Kapur (1969) O,(ads)
+
NO(ads)
",(ads)
+
",(ads)
+ H,O(ads) = N,(g) + H,O(ads)
= NO(ads)
and by Fogel et al. (196413) ",(ads) + O,(ads) = NO(ads)
+
(12)
+
H(ads) (13) ",(ads) + O(ads) = NO(ads) + H,(ads) + H(ads) (14) Equation 12 is equivalent to the combined eq 5 and 6 in the NO reduction mechanism of Otto et al. (1970). Nutt and Kapur (1968) found no N 2 0 in the reaction products. Fogel et al. (1964a,b) operating a t about the same pressure as Nutt and Kapur ( = l o Torr) also did not obtain any N2O in the products and proposed the initial, direct formation of KO. The formation of h'20 during oxidation of NH3 a t higher pressure has been well established (e.g., DeLaney and Manogue, 1974; Schriber and Paravanno, 1967). The latter proposed a mechanism for production of N2O involving the reaction between NH3 and the nitroxyl radical NH,(g)
+
H,O(ads)
(11)
+ H20(g) N,O(g) + 4H(ads)
O2 = HNO(ads)
(15)
NH,(g) + HNO(ads) = (16) However, a step such as (16) is not necessary to explain N 2 0 production. As shown in eq 9 the production of N2O as in eq from HNO need not require reaction with 3" 16. It is of interest to consider mechanisms consistent with the observed enhancement and inhibition phenomena based on previously proposed mechanisms for the separate NO-NH3 and O2-NH3 reactions. Even though surface dissociation of NH3 may be the rate-controlling step in the reduction of NO, it is apparent that catalytic activity for NH3 decomposition in the absence of 0 2 is not a criterion for either selectivity or enhancement. This can be seen by comparing Pt and Pd, which showed enhancement, with Rh, which showed inhibition, for NO reduction in the presence of 0 2 . All three catalysts showed little activity for NH3 decomposition below 270°C. On the other hand,
Ru was quite active for NH3 decomposition; however, its performance in the presence of 0 2 was similar to Rh. If dissociation of NH3 is rate controlling, the presence of 0 2 must accelerate this step on both Pt and Pd. Markvart and Pour (1967) surmised that the enhancement effect on Pt was due to acceleration of the NH3 dissociation step by 0 2 preferentially reacting with one of the dissociation fragments and increasing the surface coverage of the other fragments. If NO then preferentially reacts with the remaining fragments, the reduction of NO is accelerated. Based on the mechanism of Otto et al. (eq 4 through 10) either H or NH2 could serve as NH3 dissociation fragments for preferential reaction with 0 2 . Since Fogel et al. (1964a,b) found considerable amounts of atomic oxygen on the catalyst surface, reaction of oxygen and ammonia fragments such as O(ads) + NH2(ads) = HNO(ads) + H(ads) (17) is possible. Thus, the reaction of oxygen with NH2 could cause the observed high N2O production (eq 9) with the HNO derived from both NH3 (eq 17) and NO (eq 8). Similarly, if Pt and Pd have high selectivity for promoting the reaction between atomic hydrogen and molecular or atomic oxygen, the production of NH2 could be increased (eq 4), leading to high NO removal (eq 5), and N20 production by the secondary step (eq 7 ) . There are not, a t present, sufficient data to determine which preliminary oxidation step occurs (i.e., reaction of 0 2 or 0 with NH2 or H ) . It is known that the rate of decomposition of NH3 on P t is controlled by the rate of desorption of H2 (Hinshelwood and Burk, 1925). Thus, steps leading to reaction of H(ads) with 0 2 to produce a gas phase product could favor an increase in the rate of NH3 surface dissociation. In this case the initial reaction between oxygen and the H fragment is a precursor to enhanced removal of NO. However, considering only primary reaction steps for production of N20 and N2, as suggested by Pusateri et al. (1974), the higher N2O and lower N2 production, in the presence of 0 2 , tend to favor the preliminary selective reaction of NH2 and 0 2 . That is, if increased decomposition of the surface complex (NO.NH2) is involved (reactions 6 and 7), the N2 produced relative to N2O should increase. The reverse was observed for both Pt and P d . The direct formation of NO as proposed by Nutt and Kapur, and Fogel (eq 11, 13, and 14) may begin to occur in the temperature range where the NO outlet concentration increased slowly with temperature (150 to 270°C for Pd as shown in Figure 3 ) . In this region, NO removal by other reaction steps continues competitively. Above 270°C for Pd, NO concentration increased sharply with temperature indicating that reaction steps for NO removal had subsequently effectively ceased. Production of N2 on Ru and Rh at the stoichiometric amounts for either NH3 decomposition or oxidation with 0 2 above 300"C, as shown in Figure 9, suggests that reactions of NO have ceased. Unlike Pt and Pd, less N2O was produced on Ru and Rh than the maximum possible from the reduction of NO. Thus, over part of the temperature range, reaction between NO and NH3 yielded a considerable fraction of the produced biz. If it is assumed that the N 2 0 produced on Ru and Rh is not derived from inlet NO, the maximum amount of N2 produced from reaction between inlet NO and NH3 can be calculated using the measured NO conversion and eq 1. Figure 10 shows this calculated N2 produced on Ru and Rh as well as that determined by the overall material balance (also given in Figure 9 ) . At low temperatures, the calculated N2 production on both Ru and Rh by eq 1 accounts for a large portion of the NZ determined by material balance. Good
-
800
r
-B
'I
400-
200
303 TWERATURE
400
(9:I
Figure 10. Comparison of Nz produced on Ru and Rh based on material balance with maximum Nz produced if all reacted NO is reduced t o Nz (eq 1): A , Ru; 0 ,Rh; 20,000 hr (STP) space velocity; 1000 ppm NO, 1100-1200 ppm NH3 in Nz carrier gas; open points, Nz calculated by eq 1; closed points, Nz calculated from
material balance. agreement is obtained for Ru up to about 230°C. On the other hand, for Rh, the N2 curves are parallel up to that temperature but are offset about 50 ppm. For Rh and Ru, maxima in N2 calculated by eq 1 are obtained a t 300 and 325"C, respectively, indicating that NH3 is the primary source of N2 a t the higher temperatures. The net effect of 0 2 on Ru and Rh catalysts is to decrease the rate of NO-NH3 reaction with little change in the product distribution. The conversion of NH3 is approximately the same or slightly less than in the 02-free case. The accelerating effect of 0 2 on the NO-NH3 reaction by preferential reaction with fragments of NH3 dissociation does not occur on Ru and Rh. For both of these catalysts the direct formation of NO from NH3 and 0 2 may occur throughout the temperature range studied. It is also possible that either ( a ) surface dissociation of NH3 is not rate controlling or ( b ) selective reaction between the different decomposition fragments and K O and 0 2 , as with Pt and Pd, does not occur. Finally, one has some basis for suggesting that the 0 2 enhancement on Pt is primarily related to the 0 2 effect on the decomposition of NO rather than on the NH3-N0 reaction. It seems plausible that the superposition of a small but inhibited NH3-NO reaction rate in the presence of 0 2 with the decomposition of NO would appear as a net, enhanced reduction of NO. However, other data obtained in this laboratory with non-noble metal catalysts tend to refute this possibility. The enhancement effect has been observed with catalysts which show absolutely no activity for either NO decomposition or for NzO production in synthetic flue gas. The maximum in NO conversion on Pt in the absence of 0 2 as shown in Figure 4 was totally unexpected. Several replicate experiments as well as additional tests with other concentrations of NH3 verified the existance of this phenomenon. The experimental data exhibited an almost exact stoichiometric balance between NO and NH3 according to eq 1 for both ascending and descending branches of the curves. The observed effect is apparently related to the relative adsorption characteristics of NO and NH3 on Pt. It is likely that the relative equilibrium coverage of one of these species increases with temperature to the point that the rate of reaction between adjacently adsorbed NO and NH3 dissociation fragments increases. Based on the mechanisms given above and the fact that NH3 decomposition was not observed on Pt in the absence of 02, the data tend to indicate selfpoisoning by NO. T h a t is, since NH3 adsorption and disInd. Eng. Chem., Prod. Res. Dev., Vol. 14, No. 2 , 1975
129
sociation appear to be rate-controlling, the outlet NH3 concentration vs. temperature curve should not have passed through a minimum. Published data on the catalytic decomposition of NO appear to fall into two categories. Data obtained with pure NO or a t high concentrations indicate that catalytic decomposition of NO is too slow for practical use (e.g., Shelef et al., 1969). Studies a t low NO concentrations (i.e.,
