I
JAMES F. ROT"
and ROBERT C. DOERR
Research Laboratories, The Franklin Institute, Phildelphia, Pa.
Air Pollufion Studies
Oxidation- Reduction Catalysis Chromite catalysts can be used to reduce nitrogen oxides and/or hydrocarbon content of combustion exhausts, may be useful in control of photochemical smog
THE
COMBUSTION of hydrocarbons by air in automobile engines, power plants, and other processes produces exhaust streams which contain as air pollutants nitrogen oxides, carbon monoxide, and hydrocarbons in varying but sometimes important concentrations. The elimination of some or all of these pollutants is of increasing interest in the control of photochemical smog and other air pollution problems. The present studies are an extension of earlier work in these laboratories by Taylor ( 7 ) . The results indicate that chromites can catalytically promote NO removal in the presence of reducing agents like CO when the over-all composition is reducing in nature. Oxidation of C O and hydrocarbons (saturated and unsaturated) by oxygen is promoted when the over-all composition is oxidizing in nature. Chromite catalysts prepared on an alumina support exhibit activity similar to that obtained with unsupported chromites. The practical application of chromite catalysts will req ire life tests under conditions of potential use. The results suggest three different ways in which chromite catalysts can
Both nitrogen oxides and hydrocarbon vapors are necessary for photochemical smog formation. Chromites can reduce air pollution from exhaust fumes by: Removing
Treating
nitrogen oxides hydrocarbons and CO nitrogen oxides, hydrocarbons, and CO
without added air with air addition in two stages, first without, then with air addition
stantial amounts of NO, CO, and hydrocarbons simultaneously, which should provide for exceptional reduction in smogforming potential. This should be achieved by a two-stage treatment in which exhaust is first treated without added air to remove NO, and then with added air to remove CO and hydrocarbons. No published information is available for describing any catalysts other than platinum capable of promoting the reduction of NO so readily. The-cata-
be useful in the treatment of automobile exhaust. Automobile exhaust is normally reducing in nature and treatment without added air results in a substantial reduction in nitrogen oxides content. By adding sufficient air to the exhaust to furnish an oxygen content of about 6Yo or more, treatment results in substantial oxidation of CO and hydrocarbons. Either of these two procedures leads to a reduction in the smog-forming potential of automobile exhaust. These results suggest a means of removing sub-
Present address, Monsanto Chemical Co., Research and Engineering Division, Dayton, Ohio.
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TIME
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Figure 1 . An appreciable fall off in the rate removal of nitric oxide from nitric oxide-nitrogen mixture is evident
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Figure 2. Pure CO removal a t about 270° C. using copper chromite G-13 confirms a reduction of the catalyst VOL. 53, NO. 4
APRIL 1961
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lyzed reduction of N O may also be applied to those propellent combustion systems in which NO is known to be an important combustion intermediate. NO reduction catalysts might be useful as additives to modify rates of combustion in such systems. Experimental
Equipment and Procedure. Studies with synthetic gas mixtures were conducted in a vertical reactor which consisted of a borosilicate glass tube having an inside diameter of 11/8 inches and a length of 26 inches. The reactor was partly filled with small glass beads as a medium for preheating the incoming gases, and the catalyst bed was placed directly over the beads. The entire reactor was surrounded by a tubular Glas-col heating mantle. Thermocouples were placed in the reactor to measure temperatures at the bottom and top of the catalyst bed. A bulk volume of 50 cc. of catalyst was used in all studies. Experiments were conducted by metering the gas composition of interest into the reactor through a calibrated flowmeter, and removing samples of the gas before and after its passage through the reactor. The samples were passed through a 1meter absorption cell and their infrared spectra obtained using a Perkin-Elmer Model 21 double-beam spectrometer. T h e spectrometer was equipped with a scale-expansion system that was sometimes used to confirm the analysis for low NO concentrations. All analyses were made employing the same absorption bands used previously (7) and employing standard infrared spectrometric techniques based on per cent transmittance us. partial pressure curves of the desired components. The sensitivity of the I-meter absorption cell was adequate for the concentration ranges encountered in this study except when there was over 9070 removal of NO. Differences between 90 and 100% removal of KO in the results reported are not necessarily significant. A few studies \vere conducted on carbon monoxide and hydrocarbons using actual automobile engine exhaust. T h e exhaust was taken from a 1947 eight-cylinder Ford engine running on a leaded gasoline. Engine temperature was 150° F., speed 1300 r.p.m., and the engine was run under no load. A portion of the exhaust was metered into a vertical catalytic reactor made of galvanized pipe but otherwise similar to the glass reactor used for the synthetic gas mixtures. Materials. Synthetic gas mixtures were supplied by The Matheson Co. These contained nominal concentrations of 6Y0 C O and 0.4% NO in prepurified nitrogen. Actual concentrations were determined by infrared analysis. A few runs were made with about 0.670 Phillips Hydrocarbon Mixture No. 40
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in nitrogen. I n hydrocarbon mixture, unsaturated hydrocarbons comprise about 52% of the total hydrocarbon content, the remainder consisting of several aliphatic saturated hydrocarbons. All commercial catalysts studied were in the form of l / S inch by I / s inch cylindrical tablets. Copper chromite G-13 and barium-promoted copper chromite G-22 were obtained from Girdler Catalysts, Chcmctron Corp., and copper catalyst Cu-0803 was obtained from the Harshaw Co. According to the manufacturer's description, G-13 contains a minimum of 50 wt. CuO, G-22 contains not less than 40 I v t . yo C u O and Cu-0803 consists of 10% C u O mounted on a high activity alumina. I t appears likely that the chromite catalysts G-13 and G-22 are largely if not entirely unsupported catalysts. For the present study several supported chromite catalysts were prepared. These were made using Houdry IOOS hard alumina pellets for the support. These pellets appear to be extrusions of about I / g inch in diameter and about 1/4 inch in length. The catalysts were prepared by impregnating the alumina pellets with an appropriate amount of a solution of copper and chromium nitrates. The composition of the impregnating solution was adjusted to yield a CuO/Crz03 ratio of 1.O. After impregnation, the pellets were dried overnight a t 100" C. and then calcined for 3 hours a t 450" C. These supported chromite catalysts bear the designation HM. Results a n d Discussion
Mechanism of NO Removal. I n addition to observing S O removal when a CO-SO-S2 mixture was reacted catalytically, Taylor ( 7 ) also observed extensive S O removal from A-0-Nz mixtures in the absence of any added reducing agent. For example, when 0.6y0N O in Nz was passed over bariumpromoted copper chromite at 215' C. and a space velocity (volume of gas per bulk volume of catalyst per hour)
Table I.
100
90
80
70
60 J
9
5
50
e
as 40
30
20
10
J 0
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1
2
3
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4
5
6
0
% ADDED O X Y G E N
Figure 3. The amountsof CO removed corresponded fairly well to the amounts attributed to oxidation by oxygen present
of about 10,000 hr.-l, there was a 77% removal of NO. This result suggested that chromites are able to catalyze the simple decomposition of NO into r\;2 and 0 2 , and furthermore that the mechanism of N-0 removal even in the presence of reducing agents might consist of an initial step in which NO is decomposed to yield oxygen and a subsequent step in which CO is catalytically oxidized to COZ. Decomposition of NO in the absence of added reducing agents was therrfore investigated. The results of this study are summarized in Table I . The first two runs show no removal of NO on chromite catalysts that were not pretreated in any way. Runs 3 to 5 show the results obtained on a catalyst that was first treated with carbon monoxide and presumably reduced. After thorough flushing with nitrogen, the same NO-NZ mixture yields appreciable amounts of N O removal. T o confirm that KO removal resulted from a reaction with the cata-
Nitric Oxide Removal b y Oxidation of Catalyst Made under various conditions
Run
No.
ReactantsQ
1
NO
+ N2
2
NO
+ N2
3
NO
4
+ Nz NO + Nz
5
NO
f NB
Space Vel.,
Hi-.-l
Catalyst
5000
Girdlercopper chromite (as received) Barium-promoted copper chromite (as received) Same, but prereduced Same, but prereduced Same
4740 4740 4740 4740
Temp., In
O
C.b
Out
%5
NO
Removal
234
252
0
340
345
0
302
305
44
270 270
270 265
62-15' 40
"In" temperature is a Initial reactant concentration was 0.6% NO and the balance N2. measured a t the bottom of the catalyst bed; "Out" temperature, a t the top of the catalyst bed. See Figure 1.
INDUSTRIAL AND ENGINEERING CHEMISTRY
OXIDATION-REDUCTION CATALYSIS
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NO
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Figure 4. Under similar conditions with the catalyst used in as received condition, the amount of CO removed is substantially greater
lyst itself, a study was made of NO removal as a function of time. These results are shown in Figure 1. An appreciable fall off in the rate of NO removal is evident as would be expected in the case of a direct reaction of N O with the catalyst. There is thus no evidence that the chromites can promote a catalyzed decomposition of NO under the range of conditions studied. Oxidation States of the Catalyst. Catalytic NO removal from CO-NO-Nz, mixtures is presumed to be due to the reaction 2CO
+ 2 N 0 + 2C02 +
N2
In all cases in which CO-NO-NZ mixtures are reacted over chromites, the initial contact with the catalyst yields a COZconcentration in the effluent that is considerably in excess of that which can be accounted for by the amount of NO consumed. This result together with those described for NONz mixtures indicate that the catalysts studied are capable of existing in an oxidized state and in a reduced state. T o further verify this conclusion a study was made in which pure C O was passed over a chromite catalyst and effluent CO concentration measured as a function of time. The results are shown in Figure 2 and tend to confirm a reduction of the catalyst. Pure oxygen, equal to the amount of C O used in reduction, was then reacted with the catalyst. This resulted in partial reoxidation of the catalyst which regained its ability to oxidize CO, although a t a rate less than that of the original unreduced catalyst. Apparently the oxidized catalyst is more readily reduced with CO than the reduced catalyst is oxidized with oxygen. Reactions in Presence of Oxygen. Many combustion exhausts contain im-
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Figure 5. An appreciable decrease in NO removal does not occur until the oxygen concentrationis 3% or greater, using pre-reduced catalysts
portant concentrations of oxygen which can influence the possibilities of catalytic removal of NO. I n the earlier work (7), t h e presence of 1.1% oxygen in a NOCO-Na mixture resulted in a marked decrease in NO removal with a chromepromoted iron oxide catalyst, whereas the presence of 1.470 oxygen in a similar NO-CO-Ng mixture did not decrease N O removal when using barium-promoted copper chromite catalyst. This suggested the possibility that different catalysts might have different sensitivities to N O removal in the presence of oxygen. I t was therefore decided to investigate the effect of oxygen on N O removal in more detail. Figures 3 through 6 show the results of studies with several catalysts in which varying amounts of oxygen were added to NO-CO-NZ mixtures. Space velocity was 10,000 hr.-L and the initial reactor temperature was about 250' C. in each case. The oxidation of the CO was .quite exothermic and the reactor temperature a t the time of sampling was up to several hundred degrees above the initial temperature, depending on the initial oxygen concentration. Every effort was made to standardize the procedure in these runs so as to minimize the effects of temperature variations. The second determination a t 0% oxygen shown in Figures 3, 5, and G was made after the sequence of runs with added oxygen. The results show that the ability of the catalyst to promote N O removal in -a reducing composition was not impaired by its exposure to the higher oxygen concentrations. The results in Figure 3 were obtained with barium-promoted copper chromite that was prereduced with CO. The
amounts of CO removed corresponded fairly well to the amounts that might be attributed to oxidation by the oxygen present. I n Figure 4 are given the results under similar conditions with the same catalyst used in an as received condition. I n this case, the amount of C O removed is substantially greater than that which can be attributed to reaction with the added oxygen. The difference is undoubtedly due to reaction of CO with the catalyst. The apparent oxygen concentration a t which a marked decrease in NO removal occurs is quite different in Figures 3 and 4 because of the difference in initial oxidation states of the catalyst. The results in Figures 3, 5, and 6 on! different catalysts, all prereduced with. CO, show that an appreciable decrease in N O removal does not occur until the oxygen concentration is 39?0 or greater, 3% oxygen being the stoichiometric amount needed for conversion of the C O present to COz. Thus, it appears that the effect of oxygen in decreasing NO removal by chromites is due to removal of CO from the system (7). The presence of a reducing agent appears to be necessary for N O removal. The results in Figures 3, 5, and 6 show some differences in the exact concentration of oxygen at which there is an appreciable decrease in NO removal. This may be due to differences in the rates at which the various catalysts pro-
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Figure 6. There are some differences in the exact concentration of oxygen at which there is an appreciable decrease in N O removal VOL. 53, NO. 4
APRIL 1961
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90
t-
Table IV. Oxidation of C O and Hydrocarbons in Automobile Exhaust
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Figure 7. HM-4 is as active as G-13 in promoting NO removal from NO-CONz mixtures
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$
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Space velocity was 3100 hr.-I; catalyst was copper chromite G-13.
7% Added Oxygen 0
7.3
I501
Temp., O C. 305 120 218 130 285
% Removal CO
Hydrocarbon
94
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0 6 88
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lower temperature of 110' C . was inferior to that of the supported chromite HM-4. HM-4 was the most active of the supported catalysts and it was 0 compared with the unsupported chroX mite G-13 in a series of runs at 110" C. 20 The results (Figure 7) indicate that 0 HM-4 is as active as G-13 in promoting , NO removal from hTO-CO-N2 mixtures. X X H M - 4 COPPER CHROMITE ON ALUMINA 0 0 GIRDLER COPPER CHROMITE G - 1 3 Figure 6 indicated a high order of activity for HM-4 in promoting C O n ; oxidation by oxygen. -4ctivity OS HM-4 0 3000 6000 9000 12,000 15,000 was evaluated with respect to hydroSPACE V E L O C I T Y ( HR:') carbon oxidation by oxygen. The results given in Table I11 show their catalytic activities determined. mote the oxidation of CO by oxygen, that Hhf-4 promotes the oxidation of Results with several supported copper the NO-CO reaction, or both. both olefins and saturates at temperaoxide and chromite catalysts are given Results with Supported Catalysts. tures of about 300' C. or higher. The in Table 11. In these studies the commercial unsupdata are too limited to decide whether The supported copper oxide catalysts, ported chromites were observed to underthe difference between HM-4 and G-13 is Cu-0803 and HM-3, show differences in go attrition after being subjected to significant. activity which may be due to differences several cycles of alternate oxidation and Results with Automobile Exhaust. in the properties of the support. Alreduction. Such instability would be Several runs were made to investigate though the supported copper oxide unfavorable for good catalyst life in a oxidation of C O and hydrocarbons in HM-3 exhibited good activity for NO fixed bed reactor. Several supported actual automobile exhaust using G-13 removal at 245' C., its activity at the catalysts were therefore prepared and catalyst. The results in Table IV confirm that CO is more readily oxidized than hydrocarbons, that CO oxidation is Table II. Supported Catalysts appreciable at temperatures above Effectively promote no removal 200' C., and that hydrocarbon oxidation Catalyst is appreciable at temperatures above Designation and Temp., C. % WO % Added Space VelocitZ250' C. Oxidation in the absence of Hr.-1 Oxygen (In) Removal Composition added oxygen must be due to the oxygen Copper oxide catalyst (Harshaw present in the exhaust itself. The de242 69 0 10,000 C ~ - 0 8 0 3 - ~ / sX " l/g'') 10,000 26 249 3 tailed results indicate that among the HM-2 (3.4% CuO 3.4TGCrnOa hydrocarbons the relative ease of oxida245 0 10,000 83 on Houdry IOOS hard alumina) tion is acetylene > olefins > saturates. 2 46 3 10,000 35 Acetylene appears to be readily oxidized HM-3 (3.4% CuO on Houdry IOOS 0 10,000 245 100 alumina) by G-13 at temperatures as low as 218" HM-4 (6.8% CuO 6.8% CrzOs C. a t a space velocity of 3000 hr.-I. 200 91 0 10,000 on Houdry 100s alumina) The requirement of added air for the 0 10,000 164 78 oxidation of CO and hydrocarbons in 105 0 10,000 32 2.9 10,000 104 78 chromite catalysts is common to cata110 0 0 10,000 HM-3 lytic and noncatalytic processes reported 8 0 1,500 110 by other investigators. Under conditions 0 92 1,500 107 HM-4 of deceleration in an automobile, for instance, there is simply not enough Table I l l . Hydrocarbon Oxidation oxygen present in the exhaust to comSpace velocity was 10,000 hr.-I pletely oxidize hydrocarbons no matter % Removal how effective the catalyst might be. Temp., c. % Added Hydrocarbona "CH"C Catalyst In Out Oxygen Olefinsb Literature Cited HM-4 342 312 4.2 100 95 (1) Taylor, Francis R., Rept. 28, Air
f
40
301
+
+
Girdler chromite (G-13)
256 357 251
224 319 214
4.1 3.8 4.3
100 100 100
76 100 94
The a The hydrocarbon mixture consisted of 0.6% Phillips mixture No. 40 in nitrogen. The "CH" band was measured a t 3.4 olefins were measured in the region 9.9-12 microns. microns.
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INDUSTRIAL A N D ENGINEERING CHEMISTRY
Pollution Foundation, Calif., September 1953.
San
Marino,
RECEIVED for review September 29, 1960 ACCEPTED January 30, 1961 T h e authors thank the Franklin Institute for support of these studies.
