Ind. Eng. Chem. Res. 1994,33, 2868-2874
2868
Treatment of Activated Carbon To Enhance Catalytic Activity for Reduction of Nitric Oxide with Ammonia Bon Jun Ku? Joong Kee Lee? Dalkeun Park,'**and Hyun-Ku Rheet Korea Institute of Science and Technology, P.O. Box 131, Cheongryang, Seoul 130-650, Korea, and Department of Chemical Engineering, Seoul National University, Kwanak-ku, Seoul 151-742, Korea
Catalytic activity of activated carbon treated with various techniques was examined in a fxed bed reactor for the reduction of nitric oxide with ammonia a t 150 "C. Activated carbon derived from coconut shell impregnated with a n aqueous solution of ammonium sulfate, further treated with sulfuric acid, dried at 120 "C, and then heated in a n inert gas stream at 400 "C, showed the highest catalytic activity within the range of experimental conditions. The enhancement of catalytic activity of modified activated carbon could be attributed to the increase in the amount of oxygen functional groups which increased the adsorption site for ammonia. Catalytic activity of activated carbons depended on the surface area and the oxygen content as well.
Introduction The use of activated carbon as an adsorbent for SO2 removal and as a catalyst for NO, reduction with ammonia has been recognized as one of the advanced technologies for the cleanup of flue gas (Knoblauch et al., 1981). The active carbon process has the advantages of producing useful byproducts instead of waste, lower energy requirements, and suitability to retrofit applications. Activated carbons, however, should be applied at low space velocity below 1000 h-l for NO, reduction because of their low catalytic activity. Hence, enhancement of catalytic activity of activated carbon for NO reduction is required through proper modification of the carbon characteristics. Mochida et al. have reported that the mechanism of reduction of NO with NH3 on carbon is reaction between ammonia on the acidic or dehydrogenative site and nitric oxide adsorbed on the oxidative site of the carbon surface (Mochida et al., 1985). Therefore, it was understood that surface functional groups containing oxygen are closely related to the catalytic activity of activated carbon. These groups are expected to change the interaction between the carbon surface and the reactants through variation of adsorption and reaction characteristics (Singoredjo et al., 1993; Ahmed et al., 1993; Fujitsu et al., 1993; Juntgen and Kuhl, 1982; Lee et al., 1993). In the present study, activated carbon derived from coconut shell was treated with O2-NH3, HzS04, and (NH4)2S04 t o introduce surface oxygen functional groups. Changes in chemical and physical properties of activated carbon with various treatments were measured with the aid of elemental analyses, temperature-programmed desorption, adsorption characteristics of NO, "3, S02, and C 0 2 , and acidhase titration, and the effects of various parameters on catalytic activity of treated activated carbon were also investigated.
Experimental Section Activated Carbon Sample and Procedures of Treatments. Activated carbon, produced from coconut shell, was used for this study. This carbon has an ash content of 1.4% and nitrogen content of 0.3%. The ash ~
~~
* To whom all correspondence
should be addressed.
Seoul National University. t Korea Institute of Science and Technology. +
0888-5885/94/2633-2868$04.50IQ
Table 1. Ash Analysis of Coconut Shell Activated Carbon component wt% component wt% Si02 0.27 K20 1.13 A1203 0.021 CaO 0.033 0.062 Fez03 0.021 MgO 0.044 Ti02 0.0004 SO3 Na203 0.012
analysis of activated carbon sample is listed in Table 1. The metal compounds in ash may act as a catalyst for the reduction of nitric oxide with ammonia, but the reaction temperature of 150 "C is too low for them to be an active catalyst. Therefore, the investigation on the change in the metal component of treated carbon samples was not performed. The activated carbon was crushed and sieved, and then only the fractions in size range of 0.105-1.41 mm were subjected t o surface modification through three different procedures of oxidation and ammonia treatments. The first method was oxygen-ammonia treatment: the activated carbon was filled in a fured bed reactor, heated to temperatures ranging from 200 to 800 "C under nitrogen flow, and then activated with gas mixture containing 3.6% 0 2 , 7.5% "3, and nitrogen balance at space velocity of 636 h-l. The second method was H2SO4 treatment: the sample was treated by immersion in 8 wt % sulfuric acid solution, drying at 120 "C, and heat treatment at 400 "C under nitrogen stream until SO2 gas in the offgas was not detected through a SO2 gas analyzer. The third method was ("&S04-H2S04 treatment: carbon sample was soaked with the aqueous solution of (NH4)2S04,further treated with 8 wt % H2SO4 solution, dried at 120 "C, and then treated at 400 "C until no further SO2 gas was evolved. Table 2 shows the surface area and the ultimate analyses of the original and the modified activated carbons obtained by various treatment methods. Reduction of Nitric Oxide with Ammonia. Figure 1 shows the schematic diagram of the fixed bed reactor system for nitric oxide reduction with ammonia. The reactor was made of a quartz tube with 1.8 cm i.d. and 40 cm length. Reactant gases such as nitric oxide, ammonia, oxygen, and nitrogen gas were fed to the reactor through mass flow controllers. Unreacted ammonia gas from the reactor was completely removed by 4% boric acid solution so as not to interfere in measurement of NO concentration with a nondispersive infrared
0 1994 American Chemical Society
Ind. Eng. Chem. Res., Vol. 33, No. 11, 1994 2869 Table 2. Treatment Conditions and Properties of Activated Carbon Samples treatment conditions temp method
sample
original 02-NH3
("C)
time (h)
0 2
concn(%) concn(%)
burnoff
surf.areas
(%)
(m2/g) 888 905 901 970 928 855 969 997 1018 1183
Sob
treatment
S1 S2 S3 S4 S5 S6 S7
HzS04 treatment (NH4)2SO&I2SO4 treatment
S8 S9
200 300 400 600 800 400 400 400 400
15 15 15 4 4 15
4 20 30
3.6 3.6 3.6 3.6 3.6 0.0 3.6
7.5 7.5 7.5 7.5 7.5 7.5 0.0
0.3 1.6 5.9 8.3 13.1 2.1 10.3
ultimate anal. (wt %)
0 10.7
N 0.30
nd nd 16.95 13.24 nd 11.97 13.60 15.10 24.58
ndc nd 6.15 0.30 nd 0.43 0.30 0.50 0.30
NO conv(%) 18.1 24.3 31.1 39.3 31.8 30.6 26.8 30.1 47.2 67.0
COz adsorption a t 25 "C. No treatment. Not determined.
c
I
I I
I
I 11
14
Figure 1. Schematic diagram of the experimental apparatus for NO reduction with ammonia. 1,moisture trap; 2,oxygedmoisture trap; 3,4,6,8;mass flow controller; 5, 7,rotameter; 9,preheater; 10,furnace; 11, temperature controller; 12,N H 3 trap; 13,gas sampler; 14, NO, analyzer.
gas analyzer. The experimental conditions of NO reduction were reaction temperature 150 "C; carbon loading 25 g; NO concentration 500 ppm; NH3 concentration 500 ppm; oxygen concentration 3 vol % and total gas flow rate (at standard temperature and pressure) of 16.67 cm3*s-l. Adsorption Measurements. Surface areas of activated carbons were obtained by COZadsorption a t 25 "C using a conventional volumetric adsorption apparatus and applying the Dubinin-Radushkevich plot (Gregg and Sing, 1982). Both NO and NH3 adsorption isotherms were also determined with the same adsorption apparatus under the pressure ranging from 20 to 300 Torr at 25 and 150 "C, respectively, and by calculating the amount of adsorption gases with ideal gas law. Carbon samples were outgassed a t 150 "C to a pressure of Torr for 15 h before each adsorption experiment.
To achieve equilibrium, time span of 30 min was allowed for each adsorption point. The amount of SOZ gas adsorbed over activated carbon immersed in a stream of gas was measured with an electric torsion balance (Cahn 2000). About 0.5 g of sample was loaded in a basket suspended from the balance. Carbon was dried and outgassed a t 150 "C under nitrogen flow until a steady weight of sample was achieved. The weight increase of carbon sample due t o SO2 gas adsorption was recorded from the point of introduction of adsorbate gases a t the flow rate of 16.67 cm3.s-l containing 3000 ppm of SOz and 3 vol % 0 2 in nitrogen balance. Characterization of Activated Carbon. Changes in characteristics of the carbon samples through various treatments were measured using several Merent methods. Visual examination of the activated carbon samples
2870 Ind. Eng. Chem. Res., Vol. 33, No. 11, 1994 1 .o
1 .o
0.8
0.8
0.6
0.6
0"
0"
\
\
0
0
0.4
0.4
0.2
0.2
0.0
0
4
8
0.0 12
16
20
0
Figure 2. NO breakthrough c w e s over activated carbons treated with "3-02 as in Table 2. 0, SO; 0, S1; A, S3; 0, S4; x, S5.
was made with a Hitachi S-25OOC scanning electron microscope using a 30 kV beam. Ultimate analysis was employed to investigate change in oxygen and nitrogen content of the samples caused by treatments. The amount of surface oxygen groups was analyzed by temperature-programmed desorption (TPD) and acidbase titration. TPD experiment was carried out under 30 cm3.min-l of helium flow over 0.1 g of activated carbon sample while raising temperature at the rate of 10 "C-min-l up to 1100 "C. Gases evolved from TPD, CO and C02, were measured with gas chromatography using a thermal conductivity detector and a Porapak Q column at 40 "C. Identification of surface acid groups was also performed, according to the method of Boehm (Boehm, 19661, by neutralization of a 0.5 g sample with excess amount of 0.05 N NaOH solution (50 mL), followed by back-titration with 0.1 N hydrochloric acid.
Results and Discussion Effects of Treatment on Catalytic Activity. Figure 2 shows NO breakthrough curves from the fixed bed of the original carbon and the carbons treated with 0 2 NH3 at various temperatures. The carbons treated with O2-NH3 exhibited higher catalytic activity than that of the original. During the initial period of the NO reduction, complete removal of NO was achieved, but NO concentration increased eventually with time and reached a steady state after about 8 h. During the initial period of complete removal, NO seems to be removed by adsorption as well as selective catalytic reduction (SCR). When the adsorption sites of NO were completely occupied, the major portion of NO removal would be controlled by the catalytic reduction over the activated carbon surface. As one can see in Table 3, the amount of NO removal by adsorption increased with heat treatment temperature but the maximum NO removal ratio of SCR to adsorption appeared at 400 "C. The amount of NO adsorbed was determined by the graphical integration from the starting time of reaction to steady state assuming that the SCR reaction rate is the same throughout the reaction time. The amount of NO removal by SCR reaction was also obtained with the
4
8
12
16
20
Time (hr)
Time (hr)
Figure 3. Comparison of NO breakthrough curves over activated and (NH4)$304 as in Table carbons treated with "3-02, 2. 0, SO; a, S3; 0 , S8; B, S9. Table 3. Amount of NO Adsorbed and NO Removal Ratio of SCR to Adsorption of Original Carbon and Carbons Treated with Oz-NHs during Induction Period of NO Reduction with Ammonia treatment temp ("C) original 200 400 600 800
amt of NO ads (mmoVg of AC)
NO removal ratio of SCR to adsn
0.158 0.082 0.181 0.267 0.264
0.77 1.32 2.84 1.01 0.95
same method as mentioned above. The induction period of complete removal, as shown in Figure 2, was longer with higher treatment temperature. These results are well matched with the amount of NO adsorbed of the carbon samples in Table 3. However, the maximum NO removal ratio of SCR to adsorption for the carbon sample treated at 400 "C suggests that the adsorption sites of NO are different from those of SCR reaction. Otherwise, the NO adsorption on the carbon should be insignificant in the reaction mechanism for NO reduction. Catalytic activity seemed t o be dependent on the surface area of the treated carbon samples only when the temperature is a variable in the 02-NH3 treatment. However, the relationship between the catalytic activity and the surface area of the carbon samples treated at 400 "C in gases of different compositions, S3, S6, and S7 in Table 2, could not be established in the same way. As shown in Figure 3, NO breakthrough curves over the samples treated with H2S04 (S8 in Table 2) and (NH4)2S04/H2S04(S9 in Table 2) were plotted together with those over the sample treated with 02-NH3 and the original (S3 and SO in Table 2). Heat treatment temperature was the same at 400 "C. Patterns of breakthrough curves over the samples treated with H2SO4and (NH.&SO4/H2S04, 58 and S9 in Table 2, were quite different from those over the samples treated with O2-NH3 and the original. As can be seen in Figure 3, NO concentration for samples S8 and S9 reached the steady state value immediately while NO conversion was substantially enhanced when compared with that of S3. The difference in the catalytic activity and breakthrough curve patterns were closely related to the
Ind. Eng. Chem. Res., Vol. 33,No. 11, 1994 2871 2.5 1
1
4.0
n
n
2
0
a 2.0
11.4
7
I
/
1 '0.
a n,
1 '.
2'3.0
EE
E E
1.5
W
0
U
v
0
U
0)
n
e0
v)
72
P)
& 1.0
n
0
cn
0,
E
I
al
2.0
e
5:
v)
TI 0
0
0
n 1 .o
0
2 z
I
z
0
z 0.5
0.0
0.0
0
50
100
150
200
250
Pressure (torr)
300
Pressure (torr) Figure 4. NO adsorption isotherms of carbons treated with various techniques as in Table 2 . 0 , SO; A, 53;x, 55; W, S9.Solid line, 25 "C; dashed line, 150 "C.
surface properties and adsorption characteristics, which will be discussed in detail in the following sections. Adsorption Characteristics. Figure 4 shows NO isotherms of the original carbon and the carbons treated with O2-NH3 and (NH~)ZSO&IZSO~ a t 25 and 150 "C, respectively. A larger amount of NO adsorption was obtained for carbon samples treated with 02-NH3 compared to the original carbon while the NO adsorption amount for the sample treated with (NH4)zSOdHzSO4 was smaller. These results indicate that acidic/ basic surface characteristics of the activated carbons play an important role in the NO adsorption; carbon samples changed to acidic surface by (NH4)2SOdHzS04 treatment adsorbed less NO while samples treated with 02-NH3, having a basic surface, exhibited the opposite trend. Therefore, the difference in the amount of NO adsorption between the sample treated with 02-NH3 (S3 in Table 2) and those with acid treatments (S9 in Table 2) could explain the difference in patterns of NO breakthrough curves during the initial period in Figure 3. Figure 5 shows NH3 isotherms of the original carbon and the carbons treated with 02-NH3, HzS04, and (NH4)2SOazS04 at 25 and 150 "C, respectively. The heat treatment temperature for all the samples in the figure was the same at 400 "C except for the original carbon. Both acidic solution treatment and 0z-NH3 treatment gave rise to increase in the amount of NH3 adsorption. Thus it can be seen that oxygen during the O2-NH3 treatment caused both the acidic sites and basic sites to increase. The order of the amount of NH, adsorption corresponds to the catalytic activity of the modified carbon samples. Therefore it can be construed that NH3 adsorption be a determining step in the NONH3 reaction over activated carbon. The SO2 adsorption experiments for the activated carbons were carried out t o investigate the effects of treatments on oxidation activity and adsorption capacity. The amount of adsorbed gases on the carbon samples in a stream of S0z-02-N~ mixture increases with time due to accumulation of SO3 formed by catalytic oxidation of S02. Figure 6 shows the amount of SO2 adsorbed on various carbon samples versus time at 150 "C. The SO2 adsorption trends are similar to NO
Figure 5. NH3 adsorption isotherms of carbons treated with various techniques as in Table 2 . 0 , SO; A, S3;0 , S8; . , S9. Solid line, 25 "C;dashed line, 150 "C. 401
I
n
230
.
0.0
0.5
1.o
1.5
2.0
Time (hr) Figure 6. SO2 adsorption isotherms of carbons treated with various techniques as in Table 2 . 0 , SO; A, S3; O,S4; x, S5; 0 , 58;
,. s9. adsorption except for the carbons treated with O2-NH3. The carbons treated with O2-NH3 at 600 and 800 "C exhibited a larger amount of SO2 adsorbed than the one treated with O2-NH3 at 400 "C. Those results indicate that the modification of the carbon surface to basic is more effective at temperatures above 600 "C in the 0,NH3 treatment. Morphology of Carbons. Scanning electron micrography was employed to illustrate the difference in morphologies of the activated carbon samples treated with various methods. Figure 7a shows the surface morphology of the original activated carbon which has rather rough surface and narrow slit pores. As shown in Figure 7b, heat treatment at 400 "C in the 02-NH3 atmosphere led the surface morphology to develop new pores with more rounded shape. Figure 7c shows the surface morphology of the carbon sample treated with H2SO4. The surface was corroded and some pores were collapsed by the action of sulfuric acid. In contrast with Figure 7c, the surface of the carbon treated with ("412S04/H2S04was not corroded; instead pores were devel-
2872 Ind. Eng. Chem. Res., Vol. 33, No. 11, 1994
I:
\
. .
'
.. ,
Figure 7. Scanning electron rnicrographys of activated carbons. (a) SO (b) 53;(e) Sa,(d) S9.
oped further and larger pores could be observed in Figure 7d. The difference in specific surface areas of the carbon samples in Table 2 correspond to change in surface morphology. Consequently, treatments of activated carbon led to an increase in the specific surface area and change in surface morphology, thereby affecting catalytic activity of the activated carbons for NO reduction. Oxygen Functional Groups. The nitrogen content of the carbons after treatments did not correspond to catalytic activities of them, hut dependence of their catalytic activities on oxygen content of the carbon samples was evident. However, it was insufficient to explain catalytic activities of carbon samples only with them because oxygen contained as inorganic was also included in the ultimate analysis of oxygen. Therefore, an attempt was made to explain the difference in catalytic activities with the aid of TPD and acid-base titration. Gases evolved by TPD from the activated carbons were CO and COz. The oxygen functional groups such as carboxyl anhydride are evolved as COz whereas hydroxyl and carbonyl groups are evolved as CO (Otake and Jenkins, 1993). Therefore, the relationships of CO and COz from TPD to the catalytic activity of the carbons were examined, respectively. Figure 8 shows the COn profiles from the carbon samples. As can be
c C 3
h
Figure 8. COz profiles by TPD far carbons treated with various techniques as in Table 2. 0, SO; a,S3;0. S8; . , S9.
seen in Figure 8, the COz peak of the carbon samples treated with 02-NH3 is shifted to higher temperature compared to that of the original carbon and shows a
Ind. Eng. Chem. Res., Vol. 33, No. 11, 1994 2873
.-
CI
C
3
70
.-e CI
f0
-60
334
c .g 50
P
0)
s> 40 0
g 50 20
$
I"
300
500
700
Temperature
900
1100
("C)
Figure 9. CO profiles by TPD for carbons treated with various techniques as in Table 2. 0, SO; A, S3;0. S8; H, S9.
different pattern with almost the same magnitude. However, the COZ peaks from the carbon samples treated with HzS04 and (NH~)ZSO~/HZSO~ are significantly smaller than that of the original carbon. Thus the amount of carboxyl groups, evolving as COZby TPD, decreased after treatments with chemicals such as HzSO4 and (NH~)ZSO~/HZSO~. Considering higher catalytic activities of the carbon samples treated with H2SO4 and (NH4)zSOd/HzS04than those of the original carbon and the carbons treated with OZ-NH~,the effects of carboxyl groups on the NH3 adsorption and the reactivity for NO reduction are insignificant within the range of our experimental conditions. As can be seen in Figure 9, the CO profiles from the carbon samples, representing hydroxyl and carbonyl groups of surface oxygen functional groups, are different from that of the original carbon. The CO peaks from the carbons treated with various methods are significantly higher than that from the original carbon, and the amounts of CO evolved are closely related to the catalytic activity of carbon samples. Thus it can be seen that CO gas evolved by TPD has strong effects on both catalytic activities and adsorption characteristics of the carbon samples. Therefore the acidhase neutralization method was employed to identify the relationship between the oxygen functional groups such as carbonyl groups and the catalytic activity of carbon samples. As shown in Figure 10, the NO conversion over the carbons correlated well with the amount of sodium hydroxide neutralization.
Conclusions Catalytic activities of the activated carbons prepared HzSO4, and (NH&SOdH2SO4 treatments with 02-"3, were studied in a fxed bed reactor for the reduction of nitric oxide with ammonia a t 150 "C. Among the treated carbons tested, the one treated with (NH4)zSOdJ HzS04 solution and sequentially heated a t 400 "C showed the highest catalytic activity and the largest amount of NH3 adsorbed while the amounts of NO and SO2 adsorbed were very low. It was shown that various kinds of factors such as surface area, oxygen content,
10 20 30 40 50 the amount of NaOH neutralization (meq/100g AC)
0
Figure 10. Relationship between NO conversion and amount of NaOH neutralization of carbon samples. 1, original carbon (SO); (S8); 4,( " ~ ) z ~ O ~ / H (S9). Z ~ O ~ 2, OZ-"~ (s3); 3,
the amounts of NO, ,302 and NH3 adsorbed, and the oxygen functional groups evolved as CO by TPD affect the reactivity for NO reduction. Oxygen functional groups evolved as CO gas by TPD represent hydroxyl and carbonyl groups. They are introduced to the activated carbon surface by the action of oxidative function of oxygen and sulfuric acid during treatment. These oxygen functional groups increased the surface acidity and the amount of NH3 adsorbed while decreasing the amounts of SO2 and NO adsorbed. In conclusion, the improvement in catalytic activity of activated carbon could be attributed to the formation of acidic oxygen functional groups on the carbon surface which enhanced the adsorption of "3. Other factors such as surface area and oxygen content of activated carbon also have effects on the catalytic activity for NO reduction.
Literature Cited Ahmed, S. N.; Baldwin, R.; Derbyshire, F.; McEnaney, B.; Stencel, J. Catalytic Reduction of Nitric Oxide over Activated Carbons. Fuel 1992,72, 287-292 . Boehm, H. P. Chemical Identification of Surface Groups. In Advances in Catalysis; Eley, E. D., Pines, H., Weisz, P. B., Eds.; Academic Press: New York, 1966; Vol. 16, p 179. Fujitsu, H.; Mochida, I.; Verheyen, T.V.; Perry, G. J.; Allardice, D. J. The Influence of Modifications to the Surface Groups of Brown Coal Chars on Their Flue Gas Cleaning Ability. Fuel 1993,72, 109-113. Gregg, S. J.; Sing, K. S. W. Adsorption, Surface Area and Porosity; Academic Press: New York, 1982; pp 218-227. Juntgen, H.; Kiihl, H. Mechanisms and Physical Properties of Carbon Catalyst for Flue Gas Cleaning. In Chemistry and Physics of Carbon; Thrower, P. A., Ed.; Academic Press: New York, 1982; Chapter 2. Knoblauch, K.; Richter, E.; Jiintgen, H. Application of Active Coke in Processes of S02- and NOx Removal from Flue Gases. Fuel 1981,60,832-838. Lee, J. K.;Park, T.4.; Park, D.; Park, S. Catalytic Activity of Chars Prepared by Fluidized Bed Pyrolysis for the Reduction of Nitric Oxide with Ammonia. Ind. Eng. Chem. Res. 1993,32, 18821887. Mochida, I.; Ogaki, M.; Fujitsu, H.; Komatsubara, Y.; Ida, S. Mechanism in the Reduction of Nitrogen Monoxide with Am-
2874 Ind. Eng. Chem. Res., Vol. 33,No. 11, 1994 monia on the Coke Activated with Sulfuric Acid. Nippon Kagakukai-shi 1985,4, 680-684.
Catalytic Reduction of NO with 222.
"3.
Carbon 1993, 31, 213-
Otake, Y.; Jenkins, R. G . Characterization of Oxygen-containing Surface Complex Created on a Microporous Carbon by Air and Nitric Acid Treatment. Carbon 1993,31, 109-121.
Received for review March 31, 1994 Revised manuscript received J u n e 28, 1994 Accepted July 18,1994@
Singoredjo, L.; Kapteijn, F.; Moulijn, J. A.; Martin-Martinez, J.M.; Boehm, H.-P. Modified Activated Carbons for the Selective
Abstract published in Advance ACS Abstracts, September 15, 1994. @
