Ind. Eng. Chem. Rod. Res. D e V . 1982, 21, 56-59
56
of S, as suggested by the data of Figure 4. The suppression of hydrogenolysis by S is in a way similar to the effect produced on chloriding the Pt-A1203 catalyst with CCl., in H2 In the latter case, the coke deposited from CCld has been shown to be primarily responsible for the suppression of hydrogenoiysis activity (Menon et al., 1979). A high-temperature pretreatment (>500 OC) in H2 also attenuates the hydrogenolysis, presumably due to a self-inhibition by a stronger chemisorption of H2 (Menon and Froment, 1979) at the catalytic sites of Pt responsible for C-C bond fission. The similarity of the results obtained on pretreating the catalyst by the above three different methods is somewhat analogous to a generalizing hypothesis just forwarded by Biloen et al. (1980). These authors propose that the beneficial effects on catalyst selectivity and stability, brought about by widely different catalyst modifiers (Re, Au, Sn, S, C of coke, etc.) to Pt catalysts, are largely due to one common cause, namely, the division of the Pt surface into very small ensembles of 1-3 Pt atoms. Acknowledgment This work was undertaken thanks to a "Center of Excellence" Grant awarded by the Belgian Ministry of Scientific Affairs within the framework of the "Action Concertie Interuniversitaire Catalyse". One of us (G.B.M.) is grateful to the Belgian "Instituut tot Aanmoediging vmr
het Wetenschappelijk Onderzoek in Nijverheid en Landbouw" for a grant over the period 1977-79. We thank Dr. W. F. de Vleesschauwer of Texaco Research Center, Gent, for analysis of the sulfur content in our catalyst samples.
Literature Cited Benson, J. E.; Bodart, M. J . Catel. 1965, 4 , 704. Biloen, P.; Helle, J. N.; Verbeek, K.; Dautzenberg, F. M.; Sachtk, W. M. H. J . Catal. 1980. 63, 112. Blakely, D. W.; Somorjal, 0.A. J . Catal. 1978, 42, 181. Boudart, M. A&. Catel. 1969, 20, 153. De Pauw, R. P.; Froment. G. F. Chem. fng.Sc/. 1975, 30, 789. Flscher, T. E.; Keleman, S. R. J . Catal. 1978, 53, 24. Freei, J. J . Catel. 1972, 25, 139. Gonzales-Tejuca, L.; Aka, K.; Namba, S; Turkevich, J. J . phys. Chem. 1977, 87,1399. Gonzales-Tejuca, L.; Turkevich, J. J . Chem. Soc.,Faraday Trans. 7 1978, 7 4 , 1064. Menon. P. G.: Sieders. J.; Streefkerk, F. J.; Van Keulen, G. J. M. J . Catal. 1 9 7 3 ~ 2 9 ,isa. Menon, P. G.; Rased, J. Roc. Int. Congr. Catel. 6th 1977, 1601. Menon, P. G.: De Pauw, R. P.; Froment, G. F. Ind. Eng. Chem. Prod. Res. D e v . 1979, 78, 110. Menon. P. G.; Froment, 0. F. J . Catal. 1979, 59, 138. Smith, R. L.; Naro, P. A.; SHvestry, A. J. J . Catel. 1971, 20, 359. c Somorjai, G. A. J . Catel. 1972, 27, 453. Thomas, L. C.; Chamberlln, G. J. "Cobrlmetric Chemical Analytical Methods", published by The Tintometer LM.. Salisbury, England, and distributed by John Wlley 8 Sons, London, 1974, p 198. Truesdale, E. C. Ind. Eng. Chem.. Anal. Ed. 1930, 2, 299.
Received for review April 27, 1981 Accepted September 24, 1981
Complete Conversion of NO over the Composite Catalyst Supported on Active Carbon Tomoyukl Inul, Toshlro Otowa, and Yoshlnobu Takegaml +
Lbpafiment of Hydrocarbon Chemlstiy, Faculty of Englneerlng, Kyoto Unherslty, YoshMa, Sakyo-ku, Kyofo 606 Japan
Complete conversion of low concentrations of NO (such as 0.6%) was achieved by a composite catalyst supported on active carbon without additional reductive gases. Several weight percent of Ni or Co was supported as the catalyst substrate, and small amounts of a rare earth oxMe and a platinum metal were used as co-catalyst components. The reaction of NO and active carbon occurred above 300 OC,and the complete conversion of NO was achieved at about 400 O C at a high hourly space velocity. In particular, above 440 O C , NO was converted quantitattvely Into N, and COP NO conversion was retarded by the coexistence of O,, but the NO was still rapidly converted beiow 420 O C in succession to the complete O2 conversion. The morphological observations of the change of active carbon during the reaction support the pitting mechanism similar to the catalytic oxidation of graphite. The composite catalyst also exerted high activity In the surface reactions of NO with H, and CO.
Introduction Many studies have been conducted recently to remove detrimental NO in exhaust gases from various internal combustion engines and furnaces by means of catalytic conversion (Dwyer, 1972; Shelef, 1975). The most simple method to remove NO is believed to be the direct decomposition of NO into N2 and O2 (reaction I). Some reduced 2N0 N2 + 0 2 .(I) transition metal catalysts have high activities for NO decomposition but are deactivated rapidly owing to the combination of metals with the oxygen released by the decomposition of NO. In contrast, oxide catalysts such as perovskite are stable against oxidative reaction conditions but have low activities for NO decomposition (Moser, 1975; Voohoeve, 1977). Indirect decomposition of NO via +
0196-4321/82/1221-0056$01.25/0
reactions with reducing agents such as H2, NH3,or CO are also thermodynamically more favorable than the direct decomposition of NO into N2 and 02.Therefore, NO has usually been made to react with some gaseous reducing agent such as hydrogen, ammonia, or methane (Adlhart et al., 1971; Katzer, 1975). However, this method is troublesome to apply for small sources or moving vehicles. In this paper, a novel and simple method is presented to convert NO,, irrespective of the coexistence of 02,into innoxious N2 and C 0 2 (reactions I1 and 111)without any 2N0 C N2 + COZ (11)
+ 2N20 + C
--
2Nz + COZ
(111)
additional gaseous reducing agents. These reactions were achieved at considerably low temperatures by a catalytic 0 1982 American Chemical Society
Ind. Eng. Chem. Prod. Res. Dev., Vol. 21, No. 1, 1982 57
NO,-carbon reaction with a continuous flow method at atmospheric pressure. Certain composite-catalyst systems involving the irongroup metals, lanthanum oxide, and platinum-group metals were applied in this study. These catalyst systems had been previously found to exert high activities, both in methanation of carbon oxides (Inuiet al., 1979a; 198Oa) and in direct hydrogenation of carbon (Inui et al., 1979b; 1980b). Experimental Section Catalysts. Catalyst mass, that is, catalyst supported on active carbon, was prepared by the impregnation of active carbon with metallic salta. A-3 active carbon, 30-60 mesh in size, prepared by Shimadzu Seisakusho Co. Ltd., was used as the reaction material and, concurrently, as the catalyst support. It had a BET surface area of 1230 m2 g-l, a porosity of 0.46, a bulk density of 0.38 g ~ m - a~ , macropore of 3 pm diameter, and a micropore of 26 A diameter. The volumetric ratio of the macropore to the micropore was 0.53. A water-soluble component was not detected. Volatile matter was negligible below 500 "C in the stream of nitrogen containing 10% hydrogen. The active carbon was immersed in an impregnating aqueous solution of nitrate salt or chloride of catalyst metal. It was dried thoroughly by stirring well over a boiling water bath. Then it was exposed to a saturated vapor of 10% aqueous ammonia solution at 20 "C for 2 min (Inuiet al., 1979b) followed by heating in a nitrogen stream containing more than 10% hydrogen. Ammonium complexes formed by this treatment were decomposed more easily into metal oxides. The oxides were reduced into the metallic state with the exception of the lanthanum oxide. Several weight percent of an iron-group metal was supported as the main component of the catalyst. The atomic ratios of La and Pt were set at ca. 0.2 and 0.04 of the substrate, respectively. The mixed solution was used for the preparation of a two-component catalyst, e.g., Co-La203. However, in the case of a three-component catalyst such as Co-La203-Pt, the platinum-group metal was first dispersed on the support by the method described above, and another two components were then supported together. The changes of shape of the active carbon and the catalyst accompanying the progress of the reaction were observed using a scanning electron microscope, HitachiAkashi MSM 4C-102, which has a resolution limit of 70
A.
Apparatus and Technique. An ordinary flow-reaction apparatus was used at atmospheric pressure. A quartz tube with an inner diameter of 4 mm was chosen as the reactor. A 15-35 mg (0.05-0.11 cm3) portion of catalyst was packed in the reactor. The corresponding catalyst bed length was 4-9 mm. The catalyst bed temperature was measured by a thermocouple shielded with stainless steel (0.6 mm outer diameter) inserted into the exit side of the catalyst bed. NO, N2, and, occasionally, H2 were fed from each cylinder, through pressure reducers, precise needle valves, and flow meters. The flow rate of the gas reaction mixture was set at 50 cm3 (STP)min-' so that the initial space velocity ranged from 27 0oO to 64 0oO h-' according to the initial volume of catalyst mass. Results and Discussion Catalytic Decomposition of NO and N20 on Nonsupported Catalysts. An example of time dependence of NO decomposition at 500 "C, using a reduced Ni-La203 catalyst without active carbon, is shown in Figure 1. After the complete decomposition of NO continued for 17 min,
801
5
lb
i! $40
115
REACTION T I M E
(min)
Figure 1. Time dependence of NO decomposition on an Ni-Laz03 catalyst: 0.86% NO; SV 13400 h-l.
TEMPERATURE
("c)
Figure 2. Comparison of the activity of NO and N20 decomposition for oxygenated catalysts without support: SV 20000 h-l; 0.75% NzO or N O initial catalyst composition: ((3) 22% Ni-61% Laz03-0.18% Rh; (0)26% Ni-74% LazOs; (A)1.1% Rh/SiOz; solid line, NzO decomposition; broken line, NO decomposition. 100,
60 a z
m -
A
/P-
/
-300
400 TEMPERATURE
500 ("C)
I 600
Figure 3. NO conversion activities for various catalysts of CcLazOs-Pt system supported on the active carbon: 0.63% NO; SV 20000 h-l; ( 0 )4.6% Co-2.5% La203-0.7% Pt; (A)0.7% Pt; ((3) 4.6% Co-2.7% LazO3; (0) 4.7% Co; (0)active carbon.
the activity disappeared abruptly. The total amount of oxygen released from the decomposed NO nearly corresponds to the amount of oxygen required for the nickel sesquioxide Ni2O3 formation from the metallic Ni. This oxygenated catalyst still exerted sufficient activity for N 2 0 decomposition, as shown in Figure 2. However, very small activity was achieved for NO decomposition even at high temperatures such as 600-700 "C. When a small amount of rhodium metal supported on silica was added to the powdery Ni-La203 by mixing in an agate mortar, both N20 and NO decomposition were enhanced more than the s u m of each activity. Despite the considerable improvement in NO decomposition activity compared with the ordinary perovskite-type catalyst, it was still insufficient for practical use. Catalytic Reduction of NO and N20 with Active Carbon. The novel method described here concerns the use of active carbon as the reducing agent for NO (reaction 11) and N 2 0 (reaction 111). The case of cobalt as the catalyst substrate is shown in Figure 3 (Inui et al., 1980~).A remarkable enhancement in the NO conversion activity was observed when a small amount of La203 and a platinum-group metal such as Pt were combined. As is clearly indicated in this figure, the rate of NO conversion increased in the following order:
SO
Ind. Eng. Chem. Rod. Res. Dev.. Vol. 21.
NO. 1.
1982
TEMPERATURE
("C)
TEMPLRATURE
Figure 1, Comparison of the effects of precious metals combined with Ni-La,O,: 0.63% NO, SV 20000 h-'; ( 0 )Ni-Le,O,-Pt; (A) Ni-La*O,-Pd; ( 0 )Ni-I+O,-Rh; ( 0 )Ni-L+03-Ru; (e)Ni-La,O,; ( 0 )4.7% Nk (A) active carbon.
I'
,
I
I
I
I
I
I
lo
.. 5
1 2 3 4 TIME (h) Figure 5. Change of reaction rate with earbon consumption: 1.8% NO, initial SV 20000 h-': solid line, NO Conversion; broken line, carbon conversion; dot-dashed line, the rate of NO conversion; catalyst. 4.6% Cw2.55 La2O,4.1% Pt.
active carbon (AC) < Co/AC < Co-La203/AC < low concentration Pt/AC < Co-La203-Pt/AC. The activity of Co-Pt/AC catalyst system was located between Pt/AC and Co-La203-Pt/AC systems, however, in comparison with Co-La203-Pt/AC system, the activity was not stable. Furthermore, the activity of the three-component catalyst was greater than the sum of the activities of the three catalyst components. Such a synergistic effect is similar to that in methanation of carbon oxides and in direct hydrogenation of carbon over the same catalyst system as mentioned above (Inui et al., 1979a,b; 1980a,b). When nickel was used in place of cobalt aa the catalyst substrate, a synergisticeffect was also observed. However, the rate-increase effect for precious metals varied from metal to metal as shown in Figure 4. Various kinds of catalysts were tested under the same reaction conditions noted in Figure 3. The order of their activities is Ni 5 Fe 5 Co (combined with La,03-Pt) Ru < Rh < Pd < Pt (chloride combined with Ni-La203) Ce203 TbO, < La203 (combined with Co) The catalyst activity depended upon the anions of the starting material of the catalyst. For example, in the case of Rh, the use of nitrate caused a higher activity than the use of chloride. This difference is considered to be due to the different crystallite size of the salts formed by drying in the surface of the support (Inui et al., 197913). The change in the reaction rate accompanying the carbon consumption at 500 "C is shown in Figure 5. The rate of NO conversion based on the residual catalyst weight increased until 70% carbon consumption. In other words, 80% conversion of NO was still achieved even when only 30% carbon remained. In comparision with the CoLa203-Pt/AC system, the decrease in the reaction rate of
1°C)
Figure 6. Change in product compasition with increase in temperature: 0.58% N O SV 20500 h-'; (0) converted N O (A) N,O; ( 0 )CO,; catalyst, 4.6% Cw2.5% LazOJ4.7%Pt.
Figure 7. NO conversion in the presence of oxygen: SV 20000 h-l; (0, 0 ) 0.6% NO-0.2% 0 2 ; (A, A) 0.3% NO-2.07~0%; broken line, Os conversion; solid line, NO conversion; catalyst, 4.6% Cw2.5% La,O34.7% Pt.
Figure 8. SEM photographs of the catalyst before and after the reaction: catalyst. 4.6% Cw2.5% La20,4.7% Pt: (a) before the reaction; (b) after 40 min (25% conversion); (c) after 90 min (67% conversion).
the Co-Pt/AC system accompanying the carbon consumption was observed as more being rapid; 100% NO conversion was also observed at 40000 h-' space velocity. This figure also shows that the active carbon is consumed almost completely when the reaction gas is allowed to flow for about 5 h. Figure 6 shows the variation of products with increase in temperature. N20 was produced in the range of relatively low NO conversion, which is from 340 to 440 OC. However, above 440 "C, C02 and N2 were stoichiometrically formed, under the consumption of the active carbon. If oxygen is added to the reaction gas containing NO, reduced metallic catalysts may he rapidly deactivated because oxygen adsorption on these catalysts will occur in preference to the adsorption of NO. However, as can be seen from Figure 7, in spite of the presence of oxygen with an O2 to NO molar ratio of between 0.3 and 7, NO was still able to be completely converted helow 420 "C subsequent to the complete conversion of oxygen. Morphological Observation. Photographs of the scanning electron microscope in Figure 8 show morphology of the catalyst before the reaction and after 40 and 90 min of reaction time. Pitting by catalyst particles was observed in the early stage of carbon consumption (b), and in (c), many pore openings have been generated by pitting.
Id.Eng.
O
as shown in Figure 9, complete conversion was easily achieved at lower temperatures such as 180 and 350 "C, respectively, without any carbon consumption. Therefore, this catalyst system could be recommended for practical applications such as emission control of small-scale sources or vehicles by the catalytic surface reaction or the gasaolid reaction.
u 100
200
TEMPERATURE
Literature Cited
300
("C)
Figure 9. Catalytic reduction of NO with Hzor CO over the composite catalyst: SV 20000 h-l; (0) 1.8% NO + 4.3% H,; (A)1.8% NO 4.3% CO; catalyst, 4.6% ( 2 ~ 2 . 5 %La2034.7%Pt.
+
This fact indicates that the reaction proceeds from the surface in contact with the catalyst crystallite into the bulk of the active carbon as though a hole is being dug by the catalyst crystallites. Such a contact mechanism/pitting mechanism is commonly accepted in the catalytic oxidation of graphite, which was studied applying in situ electron microscopy technique (Baker et al., 1976). When this composite catalyst, Co-La,O,-Pt supported on active carbon, was used for the steady-state surface reaction of NO with H, (reaction IV) or CO (reaction V) NO + Hz -+ '/2Nz + HzO (IV)
NO + CO -+ '/2N2
59
Chem. Prod. Res. Dev. 1982,21, 59-63
+ COZ
Adihart, 0. J.; Hindin, S. G.; Kenson. R. E. Chem. Eng. frog. 1971, 67(2), 73. Baker, R. T. K.; France, J. A.; Rouse, L.; Wake, R. J. J. Catal. 1976, 41, 22. Dwyer, F. G. Catal. Rev. 1972, 6, 261. Inui. T.; Funabiki, M.; Suehiro, M.; Sezume, T. J. Chem. Soc. Faraday Trans. 11979a, 75, 787. Inui, T.; Ueno, K.; Funabiki, M.; Suehiro, M.; Sezume, T.; Takegami, Y. J. Chem. Soc. Faraday Trans. 11979b, 75, 1495. Inui, T.; Funabiki, M.; Takegami, Y. Ind. Eng. Chem. Prod. Res. Dev. I980a, 19, 385. Inui, T.; Funabiki, M.; Takegami, Y. J . Chem. SOC.Faraday Trans. 1 IQeOb, 76. . - , 2237. .. Inui, T.; Otowa, T.; Takegami, Y. J . Chem. SOC. Chem. Commun. 1980~. 94. Katzer, J. R. "The Catalytic Chemistry of Nitrogen Oxides"; Klimisch, R. L.; Larson, J. G., Ed.; Plenum Press: New York-London, 1975; p 133. Moser, W. R. "The Catalytic Chemistry of Nitrogen Oxides";Kiimisch, R. L.; Larson, J. G., Ed.; Plenum Press: New York-London, 1975; p 33. Shelef, M. Catal. Rev. 1975. 11, 1. Voohoeve, R. J. H. "Advanced Materials in Catalysis"; Burton, J. J.; Garten. R. L., Ed.; Academic Press; New York, 1977; p 129.
___
Received for review June 1, 1981 Accepted November 6, 1981
(V)
GENERAL ARTICLES Molecular Weight Distributions of Cationic Polymers by Aqueous Gel Permeation Chromatography Irvln J. Levy and Paul L. Dubln' Clairol Research Laboratoty, Sbmford, Connecticut 06922
Aqueous size exclusion chromatography on nonionic semirigid gels (PW columns, Toyo Soda Corp.) has been successfully applied to a variety of industrially significant cationic polymers. Ionic effects, including polyelectrolyte expansion and Donnan equilibrium "salt peaks", were controlled with mobile phases of moderate ionic strength (0.1 or 0.2 M NaCI). Commercial poly(ethy1enimine) samples exhiblt broad bimodal distributions. Other samples characterized display symmetrical chromatograms and normal elution behavior. Comparisons of molecular weights measured by gel permeation chromatography (GPC) with expected values based on other methods suggest that separations are controlled chiefly by molecular exclusion and not strongly influenced by adsorption or partition.
Introduction Cationic polyelectrolytes have become increasingly valuable industrial chemicals in such important areas as water clarification, sewage sludge dewatering, paper processing, and other applications which involve flocculation. Current usage trends from the water treatment industry lead to a projected market of $120 million for these poly-
* Department of Chemistry, Indiana University-Purdue University at Indianapolis, Indianapolis, IN 46223.
mers by 1990 (Chem. Eng. News, 1981). Over 26 years ago Michaels and Morelos (1955) noted that increased polyelectrolyte chain length corresponded to greater flocculation of clay particles. The results of more recent investigations tend to support this observation. Tilton et al. (1972) found that flocculation of algae occurred with greater efficiency as the molecular weight of the flocculant, poly(ethy1enimine) (PEI), increased from 800 to 2000; however, no appreciable effect was noted at higher molecular weights. Anthony et al. (1975) reported a strong inverse relationship between molecular weight and 0 1982 American Chemical Society
