Ind, Eng. Chem. Prod. Res. Dev. 1081, 20, 91-95
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Reduction of Nitric Oxide with Ammonia on Vanadium Oxide Catalysts Supported on Homogeneously Precipitated Silica-Titania Tsutomu Shlkada, Kaoru Fujlmoto, Talsekl Kunugl, and H l r o s Tomlnaga Department of Synthetic Chemistry, Faculty of Engineering, Universlty of Tokyo, liongo, Bunkyo&,
Tokyo 113, Japan
Shojl Kaneko and Yasushl Kubo Department of Industrial Chemistry, Faculty of Engineering, Shizuoka Universlty, Johoku, Hamamatsu-shi, 432, Japan
Reduction of nitric oxide with ammonia on vanadlum oxide catalysts supported on a silica-titania complex oxMe were studied using simulated flue gases. Catalysts have been developed which are highly active at 200 "C and resistant to sulfur dioxide. The catalysts were obtained by supporting vanadlum oxide, more than 15% by weight, on the silica-titania by a homogeneous precipltatlon method and then by calcining at 350 "C. Oxygen in small concentration In the gas phase greatly enhanced the reduction rate, whereas steam and sulfur dioxide showed few adverse effects on the rate. A test to determine the life of the catalyst, performed at 230 "C using a flue gas containing 700 ppm of sulfur dioxide, demonstrated that its activity was stable for more than 500 h. The amount of accumulated sulfateson the used catalyst was quite small compared to that on a y-alumlna-supported catalyst.
Introduction In recent years numerous processes have been developed for the removal of nitrogen oxides in the flue gases from stationary sources. One of the most promising approaches is the catalytic reduction of nitrogen oxides with ammonia The most important features which the catalyst should provide are, among others, a high activity at low temperature in order to economize fuel for heating up the flue gases and a long stream life. Vanadium(V) oxide supported on y-alumina has been known to show a high catalytic activity for the reduction of nitric oxide in flue gas with ammonia (Bauerle et al., 1975, 1978; Fujimoto et al., 1977a). However, in the presence of sulfur dioxide in the gas phase, ammonium hydrogen sulfate is formed and accumulates on the catalyst surface and reacts with y-alumina to give aluminum ammonium sulfate, which plugs the micropores of the catalyst and thus leads to an irreversible loss of the catalytic activity (Fujimoto et al., 1977b). On the other hand, silica gel, which is highly stable in an acidic environment, has been reported to be a poor catalyst when impregnatedwith vanadium oxide (Kasaoka et al., 1977). The authors have found that the activity of a vanadium oxide catalyst is increased by supporting vanadium oxide, more than 20% by weight, on a silica gel with micropores of a mean diameter larger than 100 A, followed by calcining it in the temperature range of 250 to 350 "C (Shikada et al., 1978a,b). However, the catalytic activity is not high enough for practical use. Besides, titania-supported vanadium oxide catalysts have been reported to be highly active above 350 "C (Kasaoka et al., 1977),but the use of titania in practical applications presents some obstacles, such as a lack of abrasion resistance and a high price. In the present work, catalysts have been developed which are highly active at 200 "C and highly resistant to sulfur dioxide. The catalysts consist of 15% by weight of vanadium oxide and silica-titania complex oxides which are prepared by a homogeneous precipitation method. The relation between the catalytic activity and the method of preparation is described together with the influence of coexisting gases on the activity and the catalyst life.
Table I. Physical Properties of Supports and Catalyst
support or cat. SiO,.TiO, V 2 0,-Si0 ,. TiOZC SiO, gel
sp surf area, m'/g 240 245
pore vol, cm3/g mean less pore than diam,a total 200 A A 1.45 0.54 242 0.81 0.39 132
Dmax,b A
140 100
270 1.10 0.83 162 140 1.00 154 120 260 a Pore diameter calculated by the equation 4Vp/S,. Pore diameter where distribution function is maximum. V,O,/SiO TiO, = 1 / 5 weight ratio; calcination temperature, 350 O b . r-A1203
Experimental Section Catalysts. The silica-titania complex oxide support was prepared by the homogeneous precipitation method (Gorden et al., 1959) from the mixed solution of acidified sodium metasilicate and titanium tetrachloride using urea as the neutralizer, followed by calcination at 600 "C. The method of preparation of the support is shown in Figure 1. Catalysts were prepared by impregnating the support (between 20 and 40 mesh) with vanadyl oxalate from its aqueous solution containing a calculated amount of vanadium, followed by drying in an air oven at 120 "C for 24 h and calcining at a prescribed temperature for 3 h. In this study, the silica-titania complex oxide was composed of 50 mol % silica and 50 mol % titania. The reference Catalysts were prepared by the same procedure but using commercially available y-alumina (Shokubai-kasei ACBM-1) and silica gel (Davison ID) as the supports. The physical properties of the supports are shown in Table I. Apparatus and Procedure. Figure 2 shows the reaction apparatus which is a conventional flow type system equipped with devices supplying reactant gases and operated at atmospheric pressure. The reactor is made of Pyrex glass with an inner diameter of 10 mm. The weight of the catalyst employed was 3 g and the length of the catalyst bed was about 6 cm. The reactor was heated with an electric heater controlled by an electronic thermosetter.
o 1 9 ~ 4 3 2 ~ 1 a i 1 1 2 2 o - o o 9.oom ~ ~ o ~ 0 1981 American Chemical Society
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Ind. Eng. Chem. Prod. Res. Dev., Vol. 20, No. 1, 1981
+WASHING
-DRYING (lPO°C,
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------, IMPREGNATION--.--*
DRYING -CALCINATION (120'C, 24h) (3h)
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C I T Y GAS
HOMOGENEOUS PRECIPITATION
Figure 1. Flow diagram of preparation of V205-Si02.Ti02catalyst.
.
NO
SO2
+
+
N2
N2
I_
Figure 3. Reaction apparatus for the long run: (a) burner; (b) cooler; (c) pump; (d) fdter; (e) flow meter; (f) reactor; (g) ammonia absorber; (h) temperature recorder; (i) temperaturecontroller; 6) NO/NO, meter.
a
Fwure 2. Reaction apparatus for activity measurement: (a) silica gel column; (b) flow meter; (c) injedor; (d) evaporator; (e) reactor; (f) ammonia absorber; (9) temperature recorder; (h) temperature controller; (i) NO/NO, meter.
Reaction conditions, unlees cited otherwise, were as follows: 300 ppm of NO, 450 ppm of NH3, 100 ppm of S02, 5% 0 2 , 10% H20, balance N2, and 20000 cm3/g of cat.h(STP) space velocity (flow rate of the total gas was 60 L/h). In the test of the life of the Catalysts, the reaction apparatus shown in Figure 3 was employed. The simulated flue gas was obtaiaed by adding the prescribed concentrations of NO and SO2 into the gas supplied by the combustion of city gas. Ammonia water was injected into the vaporizer with a small rotary pump and then fed to the reactor. The reaction gas finally contained 300 ppm of NO, 400 ppm of NH3, 700 ppm of SO2, 7.5% 02,5.6% COz, and 15% H20 in N2 Space velocities were varied from 20000 to 60000 cm3/g of cat-h(STP). Analytical. Nitric oxide and nitrogen dioxide were analyzed with a chemiluminescence NO/NO, meter (Beckman Model 951). Analysis of nitrous oxide was made with a gas chromatograph-mass spectrometer. With all catalysts tested, the amounts of nitrogen dioxide and nitrous oxide were less than 1%of the nitric oxide reacted in the temperature range from 130 to 350 OC. Ammonia was determined using an ion meter after being absorbed in diluted aqueous boric acid solution. Physical properties of catalysts such as specific surface area, pore structure, and thermal stability were determined with a vacuum gas adsorption apparatus using nitrogen gas, a mercury penetration porosimeter, and a thermal balance, respectively. Results and Discussion Physical Properties of the Supports and the Catalysts. Table I shows the specific surface area (S), pore volumes (V&, mean pore diameters (Dp = 4Vp/& ) and maximum pore diameters in the pore-size dktrigution function (D,) of the supports used, and the standard
(i) Figure 4. Pore-size distributions of support and catalyst. PORE DIAMETER
catalyst which is prepared by impregnating 15% by weight of V205and calcining at 350 O C for 3 h. Figure 4 shows the pore-size distributions of the silica-titania support and the standard catalyst. As can be seen in Figure 4, the pore-size distribution of silica-titania was changed by the impregnation with vanadium oxide from a broad distribution which ranges from 40 to 250 A, with a peak at 140 A to a sharp distribution which extends from 40 to 150 A with a peak at 100 k The volume of micropores (leas than 200 A) and macropores (more than 200 A) decreased by 28% and 44%, respectively, suggesting that vanadium oxide precipitates in both micropores and macropores, while the specific surface area increases from 240 to 245 m2/g by the impregnation. The increase in the specific surface area in spite of the decrease in the pore volume leads us to a conclusion that a fairly large fraction of the catalyst surface is occupied by micro particles of vanaium oxide. Effects of Supports. Figure 5 shows the activities of three vanadium oxide catalysts supported on the silicatitania complex oxide, the y-alumina, and the silica gel, respectively. Silica-supported vanadium oxide catalysts exhibit high activities only when the silica gel ssesses micropores of a mean diameter larger than 100 and the activity is similar to that of the y-alumina-supportedvanadium oxide catalyst (Shikada et al., 1978a). As can be seen in Figure 5, the silica-titania-supported catalyst is more active than the silica and the y-alumina-supported catalysts and gives the same conversion level at about 60 "C lower temperature when compared to the other two. Effect of Vanadium Oxide Content. Figure 6 shows the catalytic activities of the three catalysts as a function
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Ind. Eng. Chem. Prod. Res. Dev., Vol. 20, No. 1, 1981 a3 100 I
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CALCINATION TEMPERATURE ('C)
Figure 7. Effect of calcination temperature on catalytic activity of Vz06-Si0z~Ti0zcatalyst. 1
100,
150 02(5%)
on
REACTION TIME ( h )
0
10
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CONTENT OF V 2 0 5 (wtrl
Figure 6. Effect of Vz05 content on catalytic activities of V206 catalysts supported on SiOpTiOz, yAl208,and SiOz at 200 OC.
of vanadium oxide content. With the y-alumina or the silica-titania-supported catalyst, the activity increases h o s t linearly with an increase in vanadium oxide content up to 10% by weight, reaches a maximum at around 15% by weight, and then decreases. On the other hand, with the silica-supported catalyst the activity is quite low below 5 % by weight of vanadium oxide, increases quickly with increasing vanadium oxide content between 10 and 15%, and is independent of the vanadium oxide content above 20%. The marked decrease in the activities of y-alumina and silica-titania-supported catalysts with increasing vanadium oxide content above 20% by weight is inferred to be caused by the plugging of micropores of the supports with vanadium oxide. Change in the Catalytic Properties with Calcination Temperature. Figure 7 shows the activity of the silica-titania-supported catalyst as a function of calcination temperature. The activity increases markedly with a rise in the calcination temperature up to 300 "C,reaches a maximum at 350 "C, and then decreases with increasing calcination temperature. An attempt has been made to clarify the reason why the activity changes with the calcination temperature. Table I1 shows that the specific surface area of the catalyst is a maximum when it is calcined at 350 "C and decreases with a rise in the calcination temperature at or above 400 "C. The specific activities (rate of NO reduction per unit surface area) are also shown in Table 11. It should be noted that the specific activities are almost independent of the calcination temperature in the range of specific surface area from 51 to 245 m2/g. In this connection, the surface areas of the support materials,
Figure 8. Dynamic response to on and off of oxygen flow at 200 OC; 40000 cm3/g of cat-h space velocity; 300 ppm of NO, 450 ppm of NH,, 100 ppm of SOz, 10% HzO in Nz. Table 11. Specific Surface Area and Specific Activity as a Function of Calcination Temperature convpf sp activity,= NO, mol % g-mol/m2.h calcin s~area. temp,OC m'/g 150°C 1 8 0 ° C 1 5 0 ° C 180°C
300 360 400 450 560 660
163 245 178 167 148 51
57.3 70.0 58.5 55.7 45.0
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90.9 99.9 93.3 86.0 55.7 28.3
0.94 0.77 0.88 0.89 0.82
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1.09 1.09 1.40 1.36 1.30 1.49
Rate of NO reduction per unit surface area.
silica-titania, remained nearly constant upon calcination at temperatures ranging from 200 to 650 "C. Besides, the main part of the catalyst surface can be thought to be covered with vanadium oxide particles as has been pointed out in a preceding section (Physical Properties of the Supports and Catalysts). Accordingly, the change in the specific surface area of the catalyst may be mostly attributed to that in the surface area of the vanadium oxide. Effects of 02,H20, and SO2. Real combustion gases contain oxygen and steam in fairly high concentrationsand also sulfur dioxide in low concentration (several hundreds of ppm). Therefore, the effects of 02,HzO, and SO2 on the activity of the standard catalyst were tested at their various levels of concentrations. It is well known that a small amount of oxygen in the gaa phase greatly enhances the reaction rate of the y-alumina or the titania-supported vanadium oxide catalyst (Bauerle et al., 1975; Kasaoka et al., 1977). Figure 8 shows the dynamic response of NO concentration in the effluent gas with and without oxygen feed. While 5 vol % oxygen
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Ind. Eng. Chem. Prod. Res. Dev., Vol. 20, No. 1, 1981
e = : = . = -
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sv
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600
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REACTION TIME (h) CONCENTRATION OF O2 ( ~ 0 1 % )
Figure 9. Effect of O2 concentration on NO conversion at 180 "C; 60000 cm3/g of cat-h space velocity. CONCENTRATION OF H20 ( ~ 0 1 % )
0
5
10
15
Table 111. Change in physical Properties during the Use of Catalvsts
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z 40-
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1500
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2000
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CONCENTRATION OF SO2 (ppm)
Figure 10. Effects of H 2 0 and SO2 concentrations on NO conversion at 180 "C.
is present in the feed, NO concentration at the outlet of the reactor is 7 ppm (NO conversion 97.7%). If oxygen feed is stopped, NO concentration in the outlet gas increases gradually to reach a steady-state concentration of 106 ppm (NO conversion 64.7%) after 90 min. The increase in the outlet NO concentration is attributed to the consumption of adsorbed oxygen and the reduction of V20, which has been proven by ESR measurement. When oxygen is fed again, NO concentration at the outlet decreases quickly and reaches 7 ppm again. As can be seen in Figure 9, low concentration of oxygen in the gas phase greatly accelerates the NO-NH3 reaction. Kinetic analysis gave a rate equation that included a term of 0.21 order with respect to oxygen partial pressure. It has been previously reported that the consumption ratio of NO to NH, is 1 to 1 in the presence of oxygen whereas the ratio is 3 to 2 in the absence of oxygen, indicating that the reaction stoichiometry in the presence of oxygen is 4NO + 4NH3+ O2 4N2 + 6H20. Thus oxygen in the reaction is thought to play a role of reactant as well as of oxidizing agent of the catalyst to keep the vanadium oxide at the 5th valence state. As can be seen in Figure 10 both steam and sulfur dioxide exhibit only small suppressive effects on the activity under the set of reaction conditions. Catalyst Life in a Combustion Gas Containing SO2. It is well known that the activity of y-alumina-supported vanadium oxide catalyst for NO reduction remains unchanged in the SO2free combustion gas but drops quickly
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Figure 11. Changes in catalytic activities of V2OScatalysts supported on Si02-Ti02and alumina with reaction time at 230 "C;300 ppm of NO, 400 ppm of NH3,700 ppm of SOg, 7.5% 02,5.6% Cog, 15% H20 in N2.
cat. and reaction conditions
conv cat. of NO, condition mol %
V,05-Si0,~Ti0, 230"C,550 h VzO5-A1zO3 I 230°C,136 h
before use after use beforeuse after use
100
100 100 36.5
SP
re1 wt
1.00 1.18 1.00
1.21
surf pore area, vol, m'/g cm3/g 148 1.24 9 5 0.73 102 0.35 30 0.05
with reaction time if sulfur oxides exist in the gas (see dotted line in Figure ll),especially at reaction temperatures lower than 250 "C (Fujimoto et ai., 1977b). The decrease of the activity is caused mainly by the plugging of catalyst micropores by both sulfates of ammonium and by the chemical change in y-alumina (the formation of NH4Al(S04)2and A12(S04)& The sulfate ions are formed from SO2by its catalytic oxidation on V20, when the feed gas contains sulfur dioxide and oxygen. Figure 11 shows the catalytic activity of the standard silica-titania-supported catalyst as a function of reaction time. NO removal at a level of almost 100% was maintained for 500 h even in the presence of 700 ppm of SO2when the space velocity was 20000 cm3/g of cabh. The change in the activity with reaction time is clear when the space velocity is raised to 60000 cm3/g of cat-h. While the conversion level of reduction decreases rapidly from about 98% to about 62% within 200 h, after that the catalyst maintains a constant NO removal rate for more than 350 h. The stable activity after 200 h is worthy of remark. With all catalysts tested, hydrated ammonium sulfate was deposited at the outlet of the reactor where the telliperature was about 115 OC. The changes in physical properties of the catalysts, both before and after the long-run experiments, are shown in Table 111. With the y-alumina supported catalyst, after only 136 h of experimental use, the weight increased by 21% and the specific surface area and the pore volume decreased by 70% and 86%, respectively. However, the weight increase in the silica-titania-supported catalyst was 18% and the decreases in the specific surface area and the pore volume were only 36% and 41%, respectively, after a reaction time of 550 h. The striking differences indicate that on both catalyats some material has deposited to plug some part of the porea and to reduce their effective surface. The deposition, however, is more drastic and deteriorative on the y-alumina-supported catalyst. The differential thermogravimetric curves of the used catalysts are given in Figure 12. With the y-alumina
Ind. Eng. Chem. Prod, Res. Dev. 1981, 20, 95-101
23OoC
V205-Si02. TiOl
500 h
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0
I
I
I
I
I
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TEMPERATURE ('C)
Figure 12. Differential thermogravimetric curves of the catalysts; heating rate, 20 OC/min, in air.
supported catalyst the weight losses are observed at three temperature ranges, which can be attributed to the dehydration (100-300 "C), the evaporation of NH4HS04and decomposition of NH4A1(S04),(450-600"C), and the decomposition of A12(S04)3(650-800 "C). With the silicatitania-supported catalyst, on the other hand, the weight losses which can be attributed to only the dehydration (50-200 "C) and the evaporation of NH4HS04(450-600 "C) are observed. Since the only stable ammonium salt of sulfur oxides under the reaction temperature is ammonium hydrogen sulfate (NH4HS04),it may adhere on the catalyst surface and may accumulate on the catalyst in the long run as the liquid (mp 146.9 "C). The accumulation of a large amount of sulfate compounds on the y-alumina-supportedcatalyst within a short process time may well be caused by the formation of thermally stable "-&A(so4)2and A12(S04)3, which are the
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reaction products between NH4HS04and A1203. On the other hand, since silica-titania is resistant to NH4HS04, the ammonium hydrogen sulfate on the catalyst may come out of the catalyst surface (vapor pressure 380 ppm/230 "C) as it is formed on the catalyst and reach an equilibrium concentrationafter a certain length of process time. Thus, constant activitiesare kept on the silica-titania-supported catalysts. Conclusions (1) Silica-titania-supported vanadium oxide catalysts were more active than silica and y-alumina-supported catalysts and gave the same conversion level at about 60 "C lower temperature when compared to the other two. (2) Oxygen in the gas phase greatly enhanced the reduction rate. Either steam or sulfur dioxide showed little adverse effects on the reaction. (3) The catalysts of the present work kept constant activities for more than 500 h at 230 "C using a simulated flue gas containing 700 ppm of sulfur dioxide. Literature Cited
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Bauerle, G. L.; Wu, S. C.; Nobe, K. I d . €m. Chem. Rod. Res. h v . 1975. 14, 268. BaUerle, G. L.; WU, S. C.; Nobe, K. Id.Eng. Chem. Rod. Re$. Dw. 1978, 17. .. , i.i.7. .
Fujlmoto, K.; Shikada, T.; Kunugi, T.; Tominaga, H. Nentyo Kyokakhi 1977a, 56, 267. Fujlmoto, K.; Shlkada, T.; Kunugl, T.; Tomlnaga, H. " y o Kyokalshl1977b, 56, 666. Gorden, L.; Saiutsky, M. L.; Willard, H. H. "Preclpttation from Homogeneous Sdutbn", Wlley: New York, 1959; Chapter 2, pp 6-45. Kasaoka, S.; Yamanaka, T.; Sasaoka, E. Nentyo Kydalshil877, 56, 816. Shkada, T.; Fujimoto, K.; Kunugi, T.; Tominaga, H. Nentyo Kyokaishi 1978a, 57, 923. .Shlkada, T.; Fujimoto, K.; Kunugi, T.; Tomlnaga, H. Nentyo KyokaishlIS78b, 57. 991.
Receiued for review April 16, 1980 Accepted July 28, 1980
Exchange-Coupled Clusters as Catalyst Precursors: Comparison of Monomeric and Trimeric Acetylacetonates C. Dlnaker Rao and Howard F. Rase' Department of Chemical Engineering, The UnlversRy of Texas at Austin, Austin, Texas 78712
Exchange-coupled cluster compounds, which do not have direct metal-to-metal bonds as in the familiar metaktom cluster compounds, were considered as catalyst precursors. One such cluster, a trimeric nickel acetylacetonate, was compared with a monomeric nickel acetylacetonate by dispersing each on a sodium form of Y-zeolite. When benzene hydrogenation was used as a test reaction, the catalyst prepared from the monomer was found to be more active than that prepared from the Mmer. Extenshre tests revealed that the monomer-based catalyst exhlbtted a broad nickel particle size distribution while the trimer-based catalyst had uniform nickel crystallite sizes, over three times in diameter than the average for the monomer-based catalyst. The resulting lower surface area and differing binding energy are postulated as causes for lower activity. An aiternative explanation based on steric considerations is also presented.
Heterogeneous catalysis by metal clusters has been a subject of great interest in recent years. The small uniform aggregates of metal atoms less than 10 A in size are prepared by depositing organometallic cluster compounds on a carrier and then decomposing and reducing to yield the 0198-4321/81/1220-0095$01.00/0
desired tiny metal clusters uniformly dispersed over the carrier surface so as to produce a highly active catalyst. The cluster compounds used have three or more metal atoms bound to all or most of the others (Cotton and Wilkinson, 1966a). Both the physical and catalytic prop@ 1981 American Chemical Society
