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initial composition of the kaolin and the Si/A1 ratio of the zeolite can be misleading. Acknowledgment The authors express their appreciation to the Instituto Mexicano del Petrdleo for permission to publish the results of this study. Literature Cited
depends on the amount of Na20, Table VI. For short aging times, the formation of cubic P zeolite is favorable, so more time is necessary to destroy the kaolin structure providing the amounts of silica and alumina units necessary to build in this case the Y zeolite. The results of kaolin AH confirm the previous remarks; taking into account that this kaolin is not pure kaolinite, their initial crystallinity determines the solubility of the Si and Al and, then, the composition of the resulting zeolite, Tables VI1 and VIII. Table VI11 shows that the Si/Al dissolved fraction of kaolin varies with the kaolin/NaOH ratio; indeed the X zeolite is obtained first, then the low Si/A1 ratio Y, and finally no zeolite can be crystallized. Hydroxysodalite zeolite, whose generation is favored by high silica content of the gel (Breck, 1974), is always detected.
Aieiio, R.; Colella, C. Chim. Ind. 1970, 6 0 , 587. Bajpafi, P. K.; Rao, M. S.;Gokhale, K. V. G. K. Ind. Eng. Chem. prod. Res. Dev. 1978, 17, 223. Barrer, R. M. "Molecular Sieves"; Society of Chemical Industry: London, 1968. Breck, D. W. J . Am. Chem. SOC. 1983, 8 5 , 5963. Breck, D. W. "Zeolite Molecular Sieves";: Wiley: New York, 1974. Brindiey, G. W.; Nakahira, M. Nafure (London) 1958, 181, 1333. Brindiey, G. W. I n "The X-Ray Identification and Crystal Structures of Clay Mineral, 2nd. ed.; Brown, G., Ed.; Jarroll and Sons: Norwich, 1961; Chapter 11. Ciric, J. J . Co//oid. Interface Sci. 1988, 2 8 , 315. Dempsey, E.; Kuehl, G. H.; Olson, D. M. J . Phys. Chem. WSg, 7 3 , 387. Griesmer, G . P.; Rhodes, M. B.; Kiyonagak, M. B. Pet. Refiner 1980, 3 9 , 125. Grim,R. "Clay Minerallogy"; McGraw Hill: New York, 1953. Haden, W. L.; Dzierzanowski, F. J. U S . Patent 3 391 994, 1968; 3 657 165, 1972: 3657 154. 1972. "Powder Diffraction File Search Manual";: Joint Committee of Powder Diffraction Standards, Pennsylvania, 1975. Kerr. G. T. J . Phys. Chem. 1988. 7 2 , 1385. Kostinko, J. Am. Chem. Soc. Prepr. Dlv. Pet. Chem. 1982, 2 7 , 487. Leveder, V. I. Sew. Geol. i . Geograf. 1958, 4 , 24. Schwochow, F.; Puppe, L. Angew. Chem. Int. Ed. Engl. 1975, 14(g),620. ZBnith, J.; Rlvera, B.; Bosch, P.; Schifter, I. Revista Instituto Mexicano del Petrdleo 1981, 13, 43. Zhdanov, S. P. Adv. Chem. Ser. 1971, No. 101, 20.
Conclusion In this work it has been shown that kaolin is used as a supply of the silica and the alumina units necessary to synthesize the zeolites. The thermal treatment of the kaolin is needed to destroy the initial crystallinity and to favor the dissolution of highly reactive species, but it has to be such that the sample does not recrystallize in a more stable phase. However, it is impossible to use all the A1 and Si atoms that compose the kaolin; only a fraction of the original material is useful. This remark can be related to the suggestion of Brindley and Nakahira (1958) that around 925 "C new phases are formed. Finally, it has then to be kept in mind that the correlations established between the
Received for review October 7 , 1982 Revised manuscript received March 4, 1983 Accepted March 16, 1983
Activity and Durability of Iron Oxide-Titanium Oxide Catalysts for NO Reduction with NH, Aklra Kato,' Shlmpel Matsuda, and Tomolchl Kamo Hitachi Research Laboratory, Hltachi Ltd., Kujl-cho, Hitachi-shi, Ibaraki-ken, 3 19- 12 Japan
The reduction of NO with NH:, on iron oxide-titanium oxide catalysts was studied with a flow reactor between 300 and 450 O C . Preparation of the catalysts has much influence on the activity: The activity was found to be enhanced with increase in the content of S O : in the catalyst. The content of SO, affected the amount of adsorbed NH3. The iron oxide-titaniumoxide catalyst whose major component was TiO, showed high durability to the SO, poisoning. On the other hand, the iron oxide catalyst lost its activity in the presence of SO3 due to the physical property change caused by the formation of iron sulfate from reaction of Fe203with SO3.
It has been known that NO, are selectively reduced by NH3 in the presence of a large excess of oxygen (Bartok et al., 1969). Several catalysts, for example, V, Mo, W oxides (Nonnenmacher and Kartte, 1966), platinum metals (Anderson et al., 1962), CuO (Griffing et al., 1969), V205-A1203 and Fe-Cr (Bauerle et al., 1975; Wu and Nobe, 1977), Cr203-A1203(Niiyama et al., 1977), and zeolite (Seiyama et al., 1977) have been known to enhance the NO,-NH3 reaction. A catalyst used in a commerical plant must possess high activity and selectivity, since the volume of flue gas to be treated is extraordinarily large. In addition, the catalyst must be resistant to the SO, poisoning, since sulfur dioxide and sulfur trioxide are usually contained in an oil or coal-fired boiler flue gas.
Introduction Nitrogen oxides (NO,) from stationary combustion facilities such as power plant boilers comprise a considerable part of total NO, emitted to the atmosphere. The emission standards have been set forth for boilers and other stationary emission sources in Japan, and they will be stricter in the future. Several methods for the control of NO, have been proposed and tested using pilot plants (Bartok et al., 1969). It has been found that the selective catalytic reduction (SCR) process is most feasible for industrial application. By the end of 1981 several tens of commercial plants based on the SCR process were constructed in Japan, the largest one treating 2 million normal cubic meter per hour (Nm3/h) of flue gas at a power station (700 MW) (Kuroda and Nakajima, 1978). 0196-4321/83/1222-0406$01.50/0
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1983 American Chemical Society
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Table I. Catalysts Used in the Present Work no.
Ti/Fe at. ratio
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
9:l 9:l 9:l 9:l 8:2 8:2 8:2 9:l 9:l 9:l 9:l 9:l 9:l 99:l 98:2 95:5 8:2 6:4 1:9 0:l
prep
procedure a W.M. W.M. W.M. W.M. P.
P. P.
C.P. C.P. C.P.
P.
starting material Fe a-FeOOH FeSO, Fe2(S04)3
FeSO;(NH,),SO, FeSO, FeCl, Fe(NO313 FeSO, Fe(N03)3 FeSO, FeSO,
Ti TiO( OH),.nH,O TiO(OH);nH,O TiO( OH),.nH,O TiO(OH);nH,O TiO(OH);nH,O
TiO(OHj;.nH;O TiO( OH);nH,O TiCl" Tic1;
Tioso,
TiO, TiO, FeSO, TiO( OH);nH,O FeSO, TiO(OH),~nH,O FeSO, FeSO, TiO(OH);nH,O S O ( OH),.nH,O FeSO, FeSO, TiO(OH),.nH,O FeSO, TiO( OH),.nH,O FeSO, W.M.: wet mixing; P.: precipitation; C.P.: coprecipitation. Treated with concentrated H,SO, at 200 "C. P. P. W.M. W.M. W.M. W.M. W.M. W.M. W.M.
In the early stage of the process development several catalysts, for example, V, Mo, and W oxides supported on A1203 carrier, and Fe203-based catalysts were tested. The life of these catalysts was found to be short, because they were susceptible to the SO, poisoning (Matsuda et al., 1978). A series of catalysts consisting mainly of titania have been developed (Nakajima et al., 1978). The Ti02-based catalysts show a high activity, selectivity, and resistance to the SO, poisoning. Among them, iron oxide-titanium oxide catalyst has been found to be one of the most promising catalyst for commercial application. In this paper fundamental aspects of the preparation of iron oxide-titanium oxide catalyst and durability of the catalyst are presented. Experimental Section Catalysts. The catalysts used in this study are summarized in Table I. Iron(I1) sulfate, iron(II1) sulfate, iron(I1) chloride, iron(II1) nitrate, and iron(I1) ammonium sulfate (Mohr's salt) were used as starting materials of iron oxide. These reagents were extra pure grade and were obtained commercially. Iron oxide hydrate was also used, which was precipitated from iron(I1) sulfate with 1 N ammonia solution. Titanium(1V) chloride, titanium(1V) sulfate, titanium oxysulfate, and titanium oxide were used as starting materials of titanium oxide. These reagents were extra pure grade. Metatitanic acid (TiO(OH)?nH20) which contained 6-7 w t % SO4on the basis of TiOz was also used. Commercial titanium oxide was also used after treating with concentrated H2S04at 200 "C for 4 h to convert a portion of Ti02 into TiOS04and Ti(S04)z.The procedures for the preparation of the iron oxide-titanium oxide catalysts were as follows. (1) Wet Mixing Method. Iron compound was mixed with titanium compound by adding an appropriate amount of distilled water. The mixture was kneaded thoroughly, dried at 120 "C for 5 h, and calcined 300 "C for 4 h in air. Then the calcined powder was molded into a tablet of 6 mm diameter and 6 mm height by a pelletizing machine. Subsequently, the pellets were calcined at various temperatures, usually at 500 "C. (2) Precipitation Method. When metatitanic acid or titanium oxide was used as a starting material, this method was also used. Metatitanic acid or titanium oxide and the solution of iron salt were mixed together. Ammonia solution (1N) was added to the mixture until the pH of the
FeSO,
Figure I. Schematic diagram of experimental apparatus: (1) flow meter; (2)electric furnace; (3)water pump; (4) reactor; (5) catalyst; (6) ammonia trap; (7) NO, analyzer.
solution reached 7. The precipitate was washed with distilled water by the decantation method and then filtered. The filtrate was dried, calcined, and molded in the same manner as described in method 1. (3) CoprecipitationMethod. The iron salt and titanium salt were dissolved in distilled water or HCl solution which was used to dissolve TiC1,. Ammonia solution (1 N) was added to the solution until the pH of the solution reached 7. The precipitate was washed, filtered, dried, calcined, and molded in the same manner as described above. The pellets were crushed, a portion of 10-20 mesh being used in the measurement of catalytic activity. Reaction Apparatus. (a) Laboratory Scale Apparatus. The catalytic activity on the reaction of NO with NH, was studied in a conventional flow type apparatus, illustrated in Figure 1. A vertical 16 mm i.d. quartz tube reactor was heated to 300-450 "C with an electrical furnace. The reactor contained a catalyst bed of 4 mL. A gas mixture was passed through the catalyst bed at a rate of 240 NL/h, producing a gas space velocity of 60 000 h-' (NTP). A typical test gas contained 300 ppm of NO, 330 ppm of NH3, 500 ppm of SOz, 3% 02,10% H 2 0 and the remainder N2. When sulfur trioxide was contained in the gas mixture, a dilute sulfuric acid solution was pumped into the upper region of the reactor and vaporized at about 550 "C. The test gas was prepared by mixing gases from cylinders (pure grade) without further purification. The concentration of NO, was measured by a chemiluminescence analyzer. The concentration of N 2 0 was measured by gas chromatography after condensation of
408
Ind. Eng. Chem. Prod. Res. Dev., Vol. 22, No. 3, 1983 100,
,-L--LJo. 5QC
753
400
"e
A53
3Oc d
75d
4-0
LW>
330
,
'
'
350
40C
430
1
Figure 2. Reduction of NO with NH3on iron oxide-titanium oxide catalysts prepared from various iron oxide sources: space velocity, 60000 h-l; inlet gas composition, 300 ppm NO; 330 ppm NH,; 500 ppm SO,; 3% O, 12% CO,; 12% H20;the remainder N,.
NzO in a liquid Nz trap at -195 OC. The concentration of NH, was determined by the Nessler method after absorption with 0.5 wt % boric acid solution. Conversion of NO was estimated from the difference between inlet and outlet concentrations of NO. Nitrous oxide was not counted in the calculation of NO conversion, since the formation of N20 was very small (less than 5 ppm) in all experiments. Steady states of the activities were usually attained after the reaction of 1 h duration at various reaction temperatures. (b) Pilot Plant. A pilot plant which consisted mainly of a 150 mm i.d. reactor, an NH3 tank, a flow controlling unit, and analytical instruments was used for a life test of catalysts. The reactor which contained 10 L of catalyst (6 mm diameter X 6 mm long, pellet) was heated extern d y . The life test was carried out at a temperature of 350 "C and at a space velocity of 10000 h-l, by use of a flue gas from a residuum oil fired boiler. The composition of the flue gas was 110-150 ppm NO,, 660-750 ppm of SOz, 40-90 ppm of SO3, 4-790 Oz, 12-13% C02, 10-12% HzO, and the remainder Nz, to which NH3 was added in the NH3/N0 mole ratio of 1.0 f 0.2. It should be emphasized that the gas contained rather high concentrations of SO3. The concentration of dust particles in the flue gas was 20-50 mg/Nm3. No equipment was employed to remove particulates, though soot blowing through the catalyst bed was carried out occasionally. Analytical Methods. (a) X-ray Diffraction. Powder X-ray diffraction patterns of the catalysts were obtained with an X-ray diffractometer. Cu Ka radiation (Ni filter, voltage: 30 kV, current: 10 mA) was used. (b) Physical Properties of Catalysts. Specific surface area was determined by the standard BET technique (N2 adsorption method). Pore volume was determined with a mercury penetration porosimeter and N2 adsorption method. (c) Analysis of Sulfur Content in the Catalysts. It is considered that sulfur in the catalysts existed in the form Sulfur content in the catalyst was determined of SO-.: by the combustion-coulometry method. Results and Discussion Reduction of NO with NH, on Various Iron Oxide-Titanium Oxide Catalysts. Comparisons were made on the activity of catalysts prepared by various procedures. In Figure 2 the effects of starting materials of iron oxide on the activity are shown. Metatitanic acid was used as starting material of titanium oxide for no. 1-7 and titanium(IV) chloride was used for no. 8 and 9. As can be seen in Figure 2(a), the catalysts no. 2 and 3, prepared from FeSO, and Fe2(S04)3,respectively, showed the highest conversions. Catalysts no. 1 and 4, prepared from aFeOOH and FeS04-(NH4)2S04, showed slightly lower activities. Figure 2(b) shows that catalysts no. 5 and 7 have almost the same activity and catalyst no. 6, prepared from
300
350
400
450
React i o n T e m p e r a t u r e
C
Figure 3. Reduction of NO with NH3on iron oxide-titanium oxide catalysts prepared from various titanium sources; reaction conditions: the same as Figure 2.
I/
o
I
02
a4
a6
08
IO
a t o m c f r a c t i o n o f Fe
Figure 4. Effect of composition of catalyst: reaction temperature, 350 O C ; space velocity, 60000 h-l; inlet gas composition, the same as Figure 2 .
FeC12, showed lower activity. From the above results it is clear that the catalysts prepared from metatitanic acid as starting material of titanium oxide show similar activity regardless of various starting materials of iron oxide. In Figure 2(c) comparisons were made on the activity of the catalsyta prepared from FeS0, (no. 8) and Fe(N03)3(no. 9), respectively, as starting materials of iron oxide and TiCl,, as that of titanium oxide. There was remarkable difference between catalysts no. 8 and 9. Catalyst no. 9, prepared from Fe(N0J3 and TiCl, by the coprecipitation method, showed the lowest activity of all the catalysts pepared in the present study. In Figure 3 the effects of starting materials of titanium oxide are shown. Catalysts no. 10, 11, and 13 have almost the same activity above 400 OC. Catalyst no. 12 prepared from FeS0, and commercial TiOz (calcined above 700 "C) showed significantly lower activity than the other three. Catalyst no. 13, prepared from FeSO, and TiOz which was treated with concentrated H 8 0 4 at 200 OC, showed much higher activity than no. 12. Since TiOz obtained commercially has low specific surface area (
