CATALYZED NITRIC OXIDE REDUCTION WITH CARBON MONOXIDE R O B E R T A.
B A K E R ' A N D
R O B E R T C. D O E R R Z
The Franklin Institutp, Philadriphia, Pa.
The reduction of nitric oxide by carbon monoxide over a copper chromite catalyst from 1 15' to 2 7 0 ' C. and up to 36,000hr.-l space velocity has been defined. Reduction of 9070 at space velocities of 16,000 hr,-I is obtained at temperatures exceeding 200" C. At a given space velocity stepwise NO reduction is observed as a function of temperature. Partial reduction of NO to N20 takes place at low temperatures, followed by complete reduction to NSas temperature increases. At 12,000 hr.' inlet NO concentrations between 500 and 9000 p.p.m. do not affect reduction efficiency at temperatures exceeding 150" C. Excess CO has no effect. Water does not affect NO reduction but leads to ammonia formation. Oxygen if present preferentially oxidizes CO to Con. There must be'sufficient CO to react with the 0 2 as well as NO for effective.reduction of the NO. This system has practical significance for NO removal from automobile exhaust.
and homogeneous catalytic reduction of oxides of nitrogen have been the subject of a number of laboratory investigations. Wise and Frech ( 9 ) showed NO reduction to be heterogeneous except at high temperatures, when it is autocatalytic. Yuan et 01. (70) also found NO reduction to be surface-catalyzed in a zero-order reaction a t temperatures of less than 1100' C . ; above 1400' C. a homogeneous second-order reaction occurred. These results were similar to those of Fraser and Daniels ( 4 ) ,who studied a variety of catalysts. Sakaida et al. ( 6 ) using platinum-nickel-alumina catalysts determined that NO decomposition was second order a t 425' t o 540' C. and 1 to 15 atm. Ayen and Peters ( 7 ) investigated NO reduction with hydrogen. Batta, Solymosi, and Szabo ( 3 ) ,studying nitrous oxide decomposition on doped cupric oxide, found that oxygen caused deviations from firstorder behavior but altered activation energy only slightly. Sourirajan and Blumenthal (7) studied NO decomposition between 250' and 1000° C. at concentrations of 300 to 2100 p,p.m. and reported CuO-silica to be the best catalyst. Previous studies a t these laboratories have been devoted to determination of the feasibility of eliminating oxides of nitrogen from automobile exhaust. Taylor (8) studied a variety of catalysts and found that several promoted N O reduction in the presence of carbon monoxide. Roth and Doerr (5) extended this work by determining that CO, NO, Nz mixtures react over chromite catalysts with initial yields of CO2 greater than may be accounted for by the NO consumed. They showed that the catalyst may exist in the oxidized or reduced states. This paper describes a detailed investigation of the decomposition of nitric oxide in the presence of carbon monoxide, using a commercial copper chromite catalyst. This system is characteristic of automobile exhaust and therefore is of practical significance. ETEROGENEOUS
Experimental
The experimental system utilized reagent tank gases, including nitric oxide, carbon monoxide, and nitrogen. Flow was regulated manually and monitored by calibrated rotamPresent address, Mellon Institute, Pittsburgh, Pa. Present address. Dobbins Vocational High School, Philadelphia. Pa. 1
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I h E C PROCESS D E S I G N A N D DEVELOPMENT
eters and oil manometers. Gases first entered a preheater, then the reactor. Both were housed in a chamber made by enclosing a 2400-watt Glas-Col heater in an aluminum shell approximately 28 cm. in diameter and 66 cm. high. A stirrer, mounted through the insulated base, assured uniform heat distribution. An indicating-controller provided for temperature regulation. The preheater brought the inlet gases to the desired temperature prior to entry to the reactor. which consisted of a thin-walled stainless steel tube. 2.54 cm. in diameter and 61 cm. long, packed with 5-mm. borosilicate glass beads. T h e reactor was made of a section of 304 stainless steel) 3.08 cm. in diameter by 38.1 cm. Threaded end caps facilitated catalyst change and service. One end piece was modified to take a Megopak, iron-constantan thermocouple, encased in a 1.6-mm. stainless steel sheath. The thermocouple was fastened through the end cap with a Conax fitting and asbestos gland to keep it gas tight. .42.4-mm. stainless steel tube along the longitudinal axis served as a well for movement of the thermocouple. This permitted monitoring of the bed temperatures as a function of depth. The thermocouple was connected to a continuous recorder with 5-second readout. The 50-ml. catalyst bed was supported on approximately 60 ml. of 5-mm. borosilicate glass beads. The catalyst \cas a nominal 3.2-mm. cylinder. Exit and inlet gases were sampled in Scotchpak plastic bags (2) of approximately 13-liter capacity. Analyses
Analyses were made by infrared spectroscopy using a PerkinElmer Model 21 spectrometer equipped with twin long-path, 40-meter cells and sodium chloride optics. Infrared absorption was measured as follows: CO: 4.6 microns; C o n , 2.6 microns; NO, 5.2 microns; NOn, 6.1 microns; and S 2 0 , 7.6 microns. Gas samples taken in Scotchpak bags were immediately transferred to the spectrometer cells to minimize conversion of S O to NO:!. An appropriate gas sample was expanded into fhe previously evacuated cell and the cell was then pressurized to 1 atm. with nitrogen. Spectra were compared with previously determined standards prepared in N01), nitrogen. Decomposition is based on NO,(NO although NO:! formed in handling the sample was usually very low (less than 30 p.p.m, of NO:! in ,1500 p.p.m. of N O ) .
+
I n reporting analytical results reductions exceeding 90% are reported as >90%. The small sample sizes used, low initial concentrations, and the large volume of the long-path cell precluded quantitative measurement of trace amounts. Since differentiation of removals >90% was of little interest, no further resolution was deemed necessary.
Procedure
'The preheater and reactor were first adjusted to the desired temperature level. CO, NO, and N2 were then added a t the desired composition and space velocity. T h e temperature profile a t the top, middle, bottom, and ahead of the catalyst bed was measured. Samples were taken after equilibrium existed for a t least 10 minutes. Blank runs showed that the stainless steel reactor or lines did not promote reaction without the catalyst.
00 -
x\
60 40 -
Catalyst
20 -
'The catalyst used was a commercial copper chromite, C u O 203, manufactured by the Harshaw Chemical Co. The manufacturer gives the composition as 82% C u O and 17% Cr-203. Fresh catalyst properties were measured as follows: hardness, 11 kg.; surface area, 4 sq. meters per gram; pore volume, 0.16 cc. per gram; and mean pore radius, 780 A.
0'
,b 16,
\ 2kO
4!70
3hO
4bO
5kO
620 640
TIME (MINJ
Figure 1. Carbon monoxide uptake by fresh copper chromite catalyst
The oxidation-reduction state of the catalyst was found to influence reaction efficiency. Reducing tests with unconditioned fresh catalysts show erratic results. Since the fresh catalyst was received in the oxidized state, as partial reduction in the CO-rich atmosphere occurred, catalytic behavior might modify. The time rate of catalyst reduction will be a function of temperature and CO concentration. This aspect of catalyst conditioning was not studied in detail, but a n indication of the process is seen in Figure 1. With 1% CO in nitrogen at 240' C. and 8000-hr.? space velocity, the catalyst uptake of CO continued for 640 minutes of conditioning. Addition of 1500 p , p , m , of N O to the conditioning gas stream did not modify the time-CO assimilation relationship over the first 6 hours. Finally the CO consumed reached a steady rate corresponding to that required for reducing the nitric oxide. Reduction is postulated to be important in providing an active surface. The state of the catalyst may be expected to be related to the length of time in use and the equilibrium between oxidation-reduction mechanisms operating.
Space velocity. 8000 hr. -1 Temperature. 24OoC. Gas composition 0. I % C O i n N z X. 1 % C O , 1500 p.p.m. NO in N?
Results
Catalytic reduction of oxides of nitrogen in the presence of carbon monoxide is summarized in Figure 2, which illustrates the interrelationship among reduction efficiency, inlet temperature, and space velocity. Initial gas composition was 1500 p , p , m , of N O and 1% C O in nitrogen. At 8000-hr.? space velocity and 122' C. as much as 85y0 NO reduction will occur. The effect of varying inlet N O concentration may be seen in Figure 3. At temperatures below 146' C., a slight decrease in removal is suggested with increased inlet N O concentration. Above 146' C. varying N O concentration from 500 to 9000 p.p.m. did not significantly modify reduction a t 12,000 hr.-I. During this investigation the over-all reduction of N O by CO was monitored:
2 NO
+ 2 CO --%N2 + 2 C o n + 178.5 kcal. cat.
+ co -$v20
+ COZ
Figure 2.
x ~ 0 3HR:~
Catalytic reduction of oxides of nitrogen
Temperatures a r e o f inlet gases to reactor measured 0.7 cm. b e f o r e catalyst Inlet gases. 1 C O , 1500 p.p.m. NO in N P
%
(1)
However, it was of interest to determine if partial reduction involving other oxides of nitrogen was involved in the over-all reaction. For example : 2 NO
SPACE VELOCITY
tt 70 2 60 0
W
202°C. 146OC. 1 5oc.
127°C.
A-
t xO0-
I
(2)
ka
N10
+ CO of:N2 + C O S
NO PBM.
(3)
The partial reduction was confirmed. Figure 4 indicates the reduction of 1500 p.p.m. in 1% CO a t 12,000-hr.-' space velocity. There is a sharp rise of N20 as Reaction 2 initiates
Figure 3.
Effect of NO concentration on catalytic reduction
Temperatures a r e o f inlet gases to reactor measured 0.7 cm. b e f o r e catalyst NO in N2 nlet gases. 1 % C O Space velocity. 12,000 hr.-'
+
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I
1500
i
O
I
400
TEMPERATURE ('Cj
Figure 4.
Catalytic reduction of oxides of nitrogen
%
Inlet gas. 1 CO, 1500 p.p.m. NO in N2 Space velocity. 12,000 hr. -I Temperature, OC. X. Measured 0.7 cm. before catalyst b e d 0. Maximum
at approximately 115' C. An apparent peak of 450 p.p.m. of S20 was found at 130' C . This point corresponds to an over-all reduction of approximately 70y0 of the initial 1500 p.p.m. of S O concentration. .4bout 86y0 of the N O removed is accounted for by the N20 measured. The NzO concentration drops sharply as temperature increases, becoming less than 50 p.p.m. a t about 165' C. As over-all N O reduction approaches completion, N20 in the effluent gases disappears. Figure 4 also indicates the temperature elevation associated with exothermic heat release. Maximum bed temperature is plotted with the corresponding inlet temperature. Differential temperature increments vary from approximately 10' C. a t 115' C. inlet temperature to over 25" C. at inlet temperatures exceeding 145' C. The N 2 0 levels as a function of the temperature for various space velocities are shown in Figure 5. The important effect of temperature, particularly a t the levels immediately above reaction initiation, is immediately evident. Peak N 2 0 concentrations occur a t decreasing temperatures with decreasing space velocities. At lower temperatures Reaction 2 dominates. At temperatures greater than 160' C . Reaction 3 occurs with such velocity that the intermediate product, NzO, is not detectable. The rate approaches that for the over-all reduction given by Reaction 1.
Figure 5. Nitrous oxide formation during catalytic reduction of nitric oxide Inlet gases. Temperature,
,'%
CO, 1500 p . p m NO in N2 C., measured 0.7 cm. before catulyst b e d
effect except for higher bed temperatures resulting from the exothermic heat release of the CO oxidation. Discussion
Examination of these data for kinetic factors indicated firstorder behavior at temperatures greater than 160' C. At lower temperatures marked departure from LangmuirHinshelwood behavior was observed. An activation energy of only 3.0 kcal. per gram mole was calculated, while values of approximately 19 kcal. per gram mole have been cited (4, 9 ) for N O decomposition in the absence of a reducing gas. The intrinsic rate constants merit further evaluation. External mass transfer or intraparticle diffusion factors may have been influenced by the experimental conditions. The mechanism for limiting reaction rate at lower temperatures with formation of N20 may be postulated, since catalyst conc'itioning (partial reduction) is necessary. The catalyst contains copper, Cu', a p-type semiconductor with no unpaired d electrons, and Crf3, p-intrinsic semiconductor with 3 unpaired d electrons. The p-type catalysts conrain excess oxygen which reacts with adsorbed CO to form COS. The oxygen deficiency thus created is satisfied when oxygen is stripped from the chemisorbed NO to create active oxygen.
+ ( e ) e NzO + 0 N2 + 0 N20 + (e) C o + 2 0 - e co3-2 cos-' t 0 - + CO2 + ( e ) 2 NO
Excess CO, Woter, and Oxygen
CO in quantities exceeding stoichiometric requirements, 6% CO with 1500 p.p.m. of NO, did not affect nitric oxide reduction with conditioned catalysts. The presence of water vapor in the inlet gases did not affect reduction of 1500 p.o.m. of N O at 268' C. with either 1% or 6y0CO in the inlet gases but did lead to ammonia formation. At temperature levels where N O reduction is essentially complete, the ammonia concentration did not vary between 8000- and 20,000-hr.? space velocity. The quantity appeared to be a function only of the water added. The water-ammonia relationship merits further investigation. Oxygen in the inlet gases will effect NO reduction, since it preferentially reacts with the CO present to form COz. This copper chromate catalyst is also an excellent oxidation catalyst. Consequently sufEcient C O must be present to satisfy the stoichiometric oxygen demand and to accomplish NO reduction. Tests showed that 1% oxygen with 1% C O and 1500 p.p.m. of NO in Nz,at temperatures and space velocities where N O reduction is generally complete, will virtually eliminate N O reduction. However, if 6y0 C O is present there is no 190
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PROCESS DESIGN A N D DEVELOPMENT
The last step assumes release of CO2 formed. If the CO2 desorption is slow, active sites hill be unavailable and the process will be retarded or CO2 poisoned, particularly at temperatures below 160' C. At higher temperatures C O P is readily desorbed and reduction is not inhibited. This study made with tank gases has been extended to automobile exhaust. The reduction of oxides of nitrogen is comparable. This has practical significance. I t removes a component involved in formation of photochemical smog. These results are to be published separately. Acknowledgment
Some of these results were obtained during an investigation of copper-containing catalysts for reduction of nitric oxide in leaded automobile exhausts sponsored by the International Copper Research Association, Inc.
literature Cited
(1) Ayen, R. J., Peters, M. S., IND.ENG.CHEM.,PROCESS DESIGN DEVLLOP. 1, 204 (1962). (2) Baker, R . A., Doerr, R. C., Intern. J . Air Pollution 2, 142 (1 959). (3) Batta, I., Solymosi, F., Szabo, Z . G., J . Catalysis 1, 103 (1962). (4) Fraser, .J. M., Daniels, F., J . Phys. Chem. 62, 215 (1958). (5) Roth, J., Doerr, R.. Ind. E n g . Chem. 53, 293 (1961). (6) Sakaida, R. R.. Rinker, R. G., LVang, Y . L., Corcoran, \V. H., '4.I.Ch.E. J . 7, 658 (1961). (7) Sourirajan, S., Blumenthal, J . L., Intern. J . A i r . Water Po'ollution 5 , 24 (1961).
(8) Taylor, F. R., "Elimination of Oxides of Nitrogen from Automobile Exhaust," Air Pollution Foundation, Rept. 28
(1959). (9) \Vise, H.. Frech, M. F.. J . Chem. Phys. 20, 22 (1952). (10) Yuan, E. L.: Slaughter, J. I., Koerner, 14'. E., Daniels, F., J . Phys. Chem. 6 3 , 952 (1959).
RECEIVED for review March 26, 1964 ACCEPTED July 23, 1964 Division of Water and M'aste Chemistry, 147th Meeting, ACS, Philadelphia, Pa., April 1964.
CATALYTIC DISSOCIATION OF NITROGEN DIOXIDE LEONARD L . W I K S T R O M AND
K E N NOBE
Department of Engineering. C;1iiersztj of California. Los Angrles 2J, Calzf
The partial and complete dissociation of NO1 with CuO-alumina and CeO1-alumina catalysts was studied with an isothermal flow reactor. The initial concentration and temperature were varied between 720 and
2200 p.p.m. and 304" and 520' C., respectively, at carrier gas flow rates ranging from 10 to 77.8 mi. (STP) per second. At low temperatures and high flow rates, the rates were zero order. Considerable decomposition of NOZwith CuO catalysts, even in excess air, was observed. The results of this investigation along with those reported previously for hydrocarbon oxidation indicate that both hydrocarbons and oxides
of nitrogen may be simultaneously eliminated to a considerable degree with CuO catalysts.
YVESTIGATIONS
of heterogeneous dissociation of nitric oxide,
1- S O . and nitrogen dioxide, Son,have not been too numerous
until recently. I n the past few years? however, with the increase in air pollution and evidence that oxides of nitrogen are significant air pollutants, interest in the chemistry of nitrogen oxides has increased considerably. The thermal dissociation of nitrogen dioxide has been investigated by Bodenstein (3),Rosser and Wise (7), and .4shmore and Burnett (2). Heterogeneous dissociation of nitric oxide has been studied by Green and Hinshelwood ( 6 ) :Daniels and his coworkers (5, 75), Wise and Frech (73),and Sakaida et ai. (8). Fraser and Daniels (5) studied the dissociation of NO in the temperature range 740' to 1040' C . with various metal oxide catalysts. They reported that the reaction was zero-order for all the catalysts studied, with activation energies ranging from 16 to 60 kcal. per mole. They. concluded that the rate of reaction was adsorption-contrdled. More recently, Yuan et al. (75) studied N O dissociation in a packed Alundum vessel in the temperature range 700' to 1800' C. Below 1100' C . the reaction was surface-catalyzed and zero-order with a n activation energy of 31.6 kcal. per mole; above 1400' C., homogeneous and second-order with an activation energy of 63.1 kcal. per mole. Sakaida et al. (8) investigated the heterogeneous decomposition of N O with a platinum nickel-alumina catalyst. They observed the reaction to be second-order in the temperature range 425' to 540' C . and the pressure range 1 to 15 atm. In recent years a t L C L A , there have been several investigations on heterogeneous decomposition of N O with various supported metal oxide catalysts. Sourirajan and Blumenthal ( 7 7 . 72) compared the effectiveness of a number of metal oxides with various carriers in the temperature range 250' to
1000" C. and initial N O concentrations ranging from 300 to 2100 p.p.m. Their most active catalyst was the CuO-silica gel ( 3 to 7). After a review of the literature, it is evident that, although there have been a number of studies on the catalytic dissociation of NO, SO2 dissociation by catalysis has not been studied to any great extent. I t was the purpose of this investigation, therefore, to study the kinetics of the catalytic dissociation of Sop. These studies were carried out with both CuOalumina (1 to 1) and CeOn-alumina (1 to 1) catalysts. Experimentation
The system, shown in Figure 1, was a typical chemical flow reactor, consisting of a 70-cm. Vycor reactor tube. 22 mm. in diameter with a centrally located thermocouple well traversing the top 40 cm. of the tube. T h e reactor tube was surrounded by an electrical furnace and the temperature was controlled by a 240-volt Variac. \Vith this arrangement, the temperature throughout the catalyst bed was maintained Lvithin = 2 O C . The bottom 50 cm. of the reactor tube was filled \vith ceramic beads which served as a preheater for the incoming gas. T h e catalyst bed of 25 ml. of catalyst was placed on top of the preheater. The temperature in the preheater and the catalyst bed was measured by six Chromel-Alumel thermocouples which were placed in the thermocouple \\.ell. The first two thermocouples were in the preheater section, \vhile the four remaining Lvere equally spaced throughout the catalyst bed. These temperatures were recorded on a 16-channel L 8r N Speedomax recorder. T h e incoming gas mixture was metered by a calibrated capillary flo\vmeter. The reaction products leaving the reactor tube were passed through a 50-ml. glass wool filter and then split into two streams. One passed through a Beckman Model 15a infrared analyzer equipped with sapphire \vindo\vs and a N O detector; the other, through a Beckman Model 77 colorimeter for the detection of NO?. T h e N O concentration was knoivn within + 10 p,p.m. At concentrations belo\v 300 VOL. 4
NO. 2
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