Energy &Fuels 1995,9, 112-118
112
NO Reduction by Activated Carbons. 4. Catalysis by Calcium M. Jose IllBn-G6mez, Angel Linares-Solano,* Ljubisa R. Radovic,? and Concepcih Salinas-Martinez de Lecea Department of Inorganic Chemistry, University of Alicante, Alicante, Spain Received July 6, 1994@
The effect of calcium as catalyst of the NO-carbon reaction has been investigated. Three carbons of different origin and surface properties were loaded by ion exchange with calcium acetate. The effect of catalyst loading was also investigated in one of the carbons. The samples were characterized by physical adsorption of COz (at 0 "C) and N2 (at -196 "C) and by chemisorption of COz at 300 "C. The NO-carbon reaction was studied in a fxed-bed flow reactor a t atmospheric pressure using two types of experiments: (i) temperature-programmed reaction in a NO/He mixture; and (ii) isothermal reaction at 300-600 "C. The reaction products were monitored in both cases, thus allowing detailed oxygen and nitrogen balances to be determined. Calcium was found to catalyze NO reduction by carbon through a mechanism that is consistent with the formation of intermediate CaO(0) surface species. Nevertheless, the calcium species present on the carbon surface before NO reduction (CaO) are much less effective than the potassium species (elemental potassium or potassium suboxide) in chemisorbing NO, as a result of which they transfer much less oxygen to the carbon active sites. The results show also that the porous structure and the surface chemistry of the carbon determine, in a complex way, the catalyst loading and dispersion, as well as the catalystkubstrate contact, and hence control the catalytic activity of calcium in NO reduction by carbon.
1. Introduction The challenges in finding efficient and economic ways t o remove nitrogen oxides (NO,) from exhausts of both stationary and mobile sources have led to several different approaches. The flue gas NO, clean-up technique currently receiving the most attention is selective catalytic reduction (SCR).ls2 The SCR technology was developed primarily in Japan, where the first installation of a coal-fired unit started operation a t the end of 1980. In the SCR method, ammonia is injected into the exhaust gases in the presence of a catalyst, resulting in the reduction of NO and NO2 to Nz and HzO. The catalysts can have various chemical compositions and geometry. The main types are noble metals, supported metal oxides (e.g., titania, vanadia, iron oxide), zeolites, and activated carbons. The use of carbon as both reducing agent and SCR catalyst has been shown to be fea~ible.~-'OThe latter is a commercially available technology.2J1 NeverthePermanent address: Department of Materials Science and Engineering, The Pennsylvania State University, University Park, PA 16802. Abstract published in Advance ACS Abstracts, December 1,1994. (1)Bosch, H.; Janssen, F. Catal. Today 1988,2,369. (2) Hjalmarsson, A. K.; Sound, H. N. "NOx control installations on coal-fired plants", IEA Report IEACW34, International Energy Agency, London. (3)Chan, L. K.; Sarofim, A. F.; Beer, J. M. Combust. Flame 1983, 52,37. (4) Mochida, I.; Ogaki, M.; Fujitsu, H.; Komatsubara, Y.; Ida, S.Fuel 1985,64,1054. ( 5 )Furusawa, T.; Tsunoda, M.; Tsujimura, M.; Adschiri, T. Fuel 1985,64,1306. (6) Teng, H.; Suuberg, E. M.; Calo, J. M. Energy Fuels 1992,6,398. (7) Singoredjo, L.; Kapteijn, F.; Moulijn, J. A.; Martin-Martinez, J . M.; Boehm, H. P. Carbon 1993,31,213. (8)Yamashita, H.; Yamada, H.; Kyotani, T.; Tomita, A.; Radovic, L. R. Energy Fuels 1993,7,85.
less, the well-known variability of carbon as a material12 (e.g., pore size distribution, structure, inorganic impurities) presents both challenges regarding its reproducible behavior and opportunities for its further optimization. In the first paper in this series,1° we discussed the role of carbon porosity and surface area in NO reduction by activated carbons that were found to have very different reactivities. It is also desirable to gain a better understanding of the potential catalytic effects of various inorganic constituents of carbon, which are either inherently present or can be deliberately added, if warranted. In this context, we studied in some detail the catalytic effects of potassium.13J4 In the present paper, we analyze the catalytic effect of calcium, a ubiquitous impurity in coal-derived carbons and a very good carbon gasification ~ata1yst.l~ In the remaining two papers of the series, we discuss the catalytic effects of transition metals16 and explore the mechanisms of both uncatalyzed and catalyzed NO reduction by carb0n.l'
+
@
(9)Ahmed, S. N.; Baldwin, R.; Derbyshire, F.; McEnaney, B.; Stencel, J. Fuel 1993,72,287. (10)Illh-G4mez, M. J.; Salinas-Martinez de Lecea, C.; LinaresSolano, A.; Calo, J. M. Energy Fuels 1993,7,146. (11)Jiintgen, H. In Chemistry and Physics ofcarbon: Thrower, P. A., Ed.; Marcel Dekker: New York; 1989; Vol. 22, p 145. (12) Marsh, H., Ed. Introduction to Carbon Science; Buttenvorths: London; 1989. (13) Ill&n-G6mez,M. J.; Linares-Solano, A.; Radovic, L. R.; SalinasMartinez de Lecea, C. NO Reduction by Activated Carbons. 2. Catalytic Effect of Potassium. Energy Fuels 1996,9, 97. (14) Illh-G6mez, M. J.; Linares-Solano, A.; Radovic, L. R.; SalinasMartfnez de Lecea, C. NO Reduction by Activated Carbons. 3. Influence of Catalyst Loading on the Catalytic Effect of Potassium. Energy Fuels 1995,9, 104. (15)Cazorla-Amor4s, D.; Linares-Solano, A.; Radovic, L. R.; SalinasMartinez de k e a , C. Calcium as Carbon Gasification Catalyst. Energy Fuels, submitted for publication.
0887-0624/95/2509-0112$09.00/00 1995 American Chemical Society
NO Reduction by Activated Carbons While potassium is known to be a very good catalyst for NO reduction by ~ a r b o n , ~ ~the J ~catalytic J ~ J ~ effects of other carbon gasification catalystsz0have not been studied in detail. In fact, the effectiveness of potassium in both NO reduction by carbon and carbon gasification with the more common oxidizing gases ((202, 0 2 , Hz0) has prompted our interest in the catalytic effect of calcium, whose role in carbon gasification is very well underst00d.l~ Well-dispersed calcium oxide formed upon pyrolysis of lignitesz1was found to be efficient in both in situ sulfur capturez2 and NO, r e d ~ c t i o n . Yamashita ~ et al.8923also showed the effectiveness of calcium-loaded carbon in NO, reduction in the presence of 0 2 . The catalytic role of calcium was found to be analogous to the role it has in carbon gasification, that of increasing the concentration of carbon-oxygen complexes on the carbon surface. It was of interest, therefore, to explore the possibility of a redox mechanism in calcium~ ~well - ~ ~as the importance catalyzed NO, r e d u c t i ~ n , as of calcium-carbon contactz7in determining the kinetics of this reaction.
2. Experimental Section Three carbons of different origin and surface properties were studied: A, B and K-UA1. The methods of their preparation have been described in detail elsewhere.1° Briefly, A and B are chars obtained from a phenol-formaldehyde polymer and almond shells, respectively,2*while K-UA1 is a coal-derived carbon whose activation was carried out with KOH.= Calcium was introduced by ion exchange (at 60 "C for 4 h) onto these to increase carbons after they had been oxidized with their ion-exchange capacity.30 The oxidized samples are referred to as A(ox), B(ox), and K-UAl(ox). The degree of their oxidation was quantified using temperature-programmed desorption (TPD) in a flow of He (60 mumin) at 20 "C/min, the desorption products were analyzed by a quadrupole mass spectrometer (Micromass PC, VG Quadrupoles). Calcium acetate (1.5 M) was used as the catalyst precursor. The ionexchanged samples were finally washed with distilled water. (16)Illln-G6mez, M. J.; Linares-Solano, A,; Radovic, L. R.; SalinasMartinez de k e a , C. NO Reduction by Activated Carbons. 6.Catalytic Effect of Transition Metals. Energy Fuels, submitted for publication. (17) Radovic, L. R.; Illln-G6mez, M. J.; hares-Solano, A.; SalinasMartinez de Lecea, C. NO Reduction by Activated Carbons. 7. Some Mechanistic Aspects of Uncatalyzed and Catalyzed Reaction. Energy Fuels, submitted for publication. (18)Kapteijn, F.; Alexander, J. C.; Mierop, G. A.; Moulijn, J. A. J. Chem. SOC.,Chem. Commun. 1964,1084. (19)Okuhara, T.; Tanaka, K. J.Chem. SOC.,Faraday Trans. 1986, 82,3657. (20)Mims, C. A. In Fundamental Issues in Control of Carbon Gasification Reactivity; Lahaye, J., Ehrburger, P., Eds.; Kluwer Academic Publishers: Dordrecht, The Netherlands, 1991;p 383. (21)Radovic, L. R.; Walker, P. L., Jr.; Jenkins, R. G. Fuel 1963,62, 209. (22) Freund, H.; Lyon, R. K. Combust. Flame 1982,45,191. (23)Yamashita, H.; Yamada, H.; Tomita, A. Appl. Catal. 1991,78, L1. (24)Radovic, L. R.; Walker, P. L., Jr.; Jenkins, R. G. J. Catal. 1985, 82,382. (25) Kyotani, T.; Hayashi, S.; Tomita, A. Energy Fuels 1991,5,683. (26)Kyotani, T.; Hayashi, S.; Tomita, A,; MacPhee, J. A.; Martin, R. P. Fuel 1992,71,655. (27) Cazorla-Amor6s, D.; Linares-Solano, A.; Salinas-Martinez de Lecea, C.; Joly, J. P. Carbon 1991,29,361. (28) Linares-Solano, A.; Almela Alarch, M.; Salinas-Martinez de Lecea, C.; Muiioz-Guillena,M. J.; Illh-G6mez, M. J. In Characterization of Porous Solids II; Rodriguez-Reinoso F., et al.,Eds.; Elsevier: Amsterdam, 1991;p. 367. (29)Illb-G6mez, M. J.; Muiioz-Guillena, M. J.; Salinas-Martinez de Lecea, C.; Linares-Solano, A.; Martin-Martinez, J. M. Proc. Int. Carbon Conf. Carbone '90,Paris, Fr 1990,68. (30)Schafer, H. N. S. Fuel 1984,63,723.
Energy &Fuels, Vol. 9, No. 1, 1995 113 Table 1. principal Characteristics of Carbons Used
sample A(ox)
B(ox) K-UAl(0x)
sBET,Ns
SDR,COa
(mZ/g) 878 547 1078
(m2/g) 712 619 853
CO2,TPD (umoVg) 1810 2041 1420
COTPD (umoVg) 2330 2226 2370
% Ca
3.6 0.6 1.9
Carbon A was selected for a study of the effect of catalyst loading. Calcium was therefore added to this carbon using impregnation from solution, whose concentration was adjusted to yield the desired quantity of catalyst. Instead of washing the samples to remove excess catalyst, as done in ion exchange, the samples were dried by bubbling nitrogen through them until all the water had been removed; they were subsequently oven-dried at 110 "C for 12 h. The samples are designated by the parent carbon used, followed by their calcium content (in wt % Ca). The calcium content was determined in all samples by atomic absorption spectroscopy. For this purpose, each sample was converted to ash in a muffle furnace at 850 "C over a period of 12 h, and the ash was subsequently dissolved and analyzed. Pore structure characteristics of the carbons were determined by physical adsorption of C02 (at 0 "C) and NZ(at -196 "C) in an automated volumetric apparatus (Autosorb-6,Quantachrome). Carbon dioxide chemisorption experiments were carried out in a Stanton-Redcroft 780 thermobalance, as described in detail elsewherea31After a heat treatment in Nz (60 mumin) at 20 " C h i n to 900 "C, the samples were cooled to 300 "C and contacted with COz (60 mumin) for a period of ca. 30 min. The gaseous atmosphere was then switched back to Nz for ca. 10 min; after weight stabilization, the quantity chemisorbed on the sample was determined from the weight difference between the two steps in N2 at 300 "C. The kinetics of the NO-carbon reaction were studied at atmospheric pressure in a flow reactor (15 mm id.; ca. 300 mg of sample) connected to a gas chromatograph. The gaseous products (Nz, CO, NO, C02, and N2O) were analyzed using a Porapak Q 80/100 column and a thermal conductivity detector. Two types of experiments were carried out: (i)temperatureprogrammed reaction (TPR), consisting of heating the sample at 5 " C h i n to 900 "C in a NO/He mixture (0.4% NO, 60 mL1 min); (ii)isothermal reaction at 300, 400, 500, and 600 "C for a period of 2 h. In both types of experiments, the samples were subjected to an in situ heat treatment in He, at 50 "C1 min to 900 "C for 10 min. In case (i), the temperature was lowered to 20 "C, the reactant mixture was substituted for He, and the TPR experiment was performed. In case (ii), the temperature was lowered to the desired level and the isothermal experiment was initiated by substituting the NO/He mixture for He. In either case, the maximum amount of carbon consumed at the end of an experiment was less than 5%.
3. Results and Discussion 3.1. Effect of Carbon Nature. Sample Characterization. Table 1 summarizes the principal characteristics of the parent carbons. In agreement with the results on the exchange of K+ ions onto these same carbons,13the quantity of ion-exchangedcalcium is seen to depend on the nature of the carbon. It is reasonable to expect that, under the experimental conditions used (unadjusted pH), the degree of exchange is related to the concentration of carboxyl groups present on the (31)Linares-Solano, A.; Almela-Alarch, M.; Salinas-Martinez de Lecea, C. J. Catal. 1990,125,401.
Illan-Gbmez et al.
114 Energy & Fuels, Vol. 9, No. 1, 1995 Table 2. Summary of Data on Calcium Dispersion on Ion-Exchanged Carbons C02 chemisorbed dispersion sample
OlmoYd
(mol CaOJmol CaO)
A( OX)-^. 6 B(ox)-O.6 K-UAl(OX)- 1.9
392 44 71
0.61 0.41 0.21
100
'k NO reduction =A(OX)-3.6
B(ox)-O.6
-K-UAI(ox)-1.9 -A(ox) 60
B(ox) -K-UAl(ox)
40
surface of the oxidized carbon,32which is in turn roughly proportional to the quantity of C02 evolved in TPD.33p34 However, the data in Table 1show that the carbon with the largest quantity of Con-yielding groups (B(ox))has the lowest calcium content. This is attributed to the narrow microporosity of this carbon (as evidenced by the fact that its CO2 surface area is higher than its N2 surface area35 1, which makes some of the carboxyl groups inaccessible to the catalyst precursor. The potassium loading on the same carbon was higher (3.9 wt %), as expected from the different COO-/metal ion stoichiometry and the relative sizes of the two cations ) 0.96 nm). (rK+(aq)= 0.38 nm VS r ~ a + 2 ( a q = Selective chemisorption of C02 has been shown to provide meaningful data on the dispersion of calcium species on carbon surfaces.31 On the basis of 1:l stoichiometry, the quantity of CO2 chemisorbed (Table 2) corresponds to the amount of surface CaO present. Tables 1 and 2 show a complex relationship between the nature of the carbon and catalyst loading, on one hand, and catalyst dispersion on the other. From earlier, more detailed studies of this issue, one could have expected that dispersion would be independent of catalyst loading on these high-surface-area substrate^^^ and a t these relatively low catalyst loadings.32 Obviously, however, the details of the porous structure of the substrate and its surface chemi~try3',~~ play an important role in determining the dispersion. The highest dispersion is seen to be obtained on the substrate A(ox),on which essentially all the carboxyl groups (1.8 mmol COO-/g of C; 2 mol COO-/mol Caf2) are exchanged. Presumably, the uniform distribution of carboxyl groups over the entire surface of this highsurface-area carbon (Table 1) is responsible for the preservation of high dispersion upon heat treatment to 900 0C.39In contrast, the lowest dispersion is obtained on the substrate K-UAl(ox), where only ca. 68% of the carboxyl groups have been exchanged with Ca2+. The reactivity of this carbon is much higher than that of carbon A (e.g., 0.15 vs. 0.01 h-l at 800 "C in 1atm C02). Therefore, the less uniform distribution of carboxyl groups, presumably concentrated closer to the external surface of the particles of the more reactive carbon, may be responsible for the greater degree of sintering of CaO particles upon heat treatment to 900 "C. (32) Linares-Solano, A.; Salinas-Martinez de Lecea, C.; CazorlaAmor6s, D.; Joly, J. P.; Charcosset, H. Energy Fuels 1990,4, 467. (33) Puri,B. R. In Chemistry and Physics of Carbon; Walker, P. L., Jr., Ed.; Marcel Dekker: New York; 1970; Vol. 6, p. 191. (34)Otake,Y.; Jenkins, R. G. Carbon 1993, 31, 109. (35) Rodriguez-Reinoso,F.;Linares-Solano, A. In Chemistry and Physics of Carbon; Thrower, P. A., Ed.; Marcel Dekker: New York, 1988; p 1. (36) Hippo, E. J.; Jenkins, R. G.; Walker, P. L., Jr. Fuel 1979, 65, 776. (37) Solar, J. M.; Leon y Leon, C. A.; 08seo-Asare, K.; Radovic, L. R. Carbon 1990,28, 369. (38) Romln-Martinez, M. C.; Cazorla-Amor6s, D.; Linares-Solano, A.; Salinas-Martinez de Lecea, C. Carbon 1993,31,895. (39) Huffman, G . P.; Huggins, F. E.; Jenkins, R. G.;Piotrowski, A.; Lytle, F.W.; Greegor, R. B. Fuel 1986, 65, 1339.
20
0 100
206
300
400
500
600
700
800
T('C)
Figure 1. Temperature-programmed reaction (TPR) profiles of NO reduction for carbons in the presence and absence of ion-exchanged calcium catalyst.
Temperature-Programmed Reaction Studies. Figure 1 shows the TPR profiles for NO reduction of Ca-exchanged samples and their parent carbons. In all cases, the catalytic effect of calcium is seen to manifest itself in a substantial shift of the curves toward lower temperatures. Both the temperature at which the reaction starts and the temperature required for 100% reduction (2'100) are lower than those of the parent carbons. In contrast to the results obtained with potassium-loaded carbons,13J4only a typical Arrheniustype behavior is observed, in which the reactivity increases with increasing temperature. Calcium species on the carbon surface (e.g., CaO) are much less effective in chemisorbing NO. In the case of potassium, a maximum in the TPR profile was observed a t temperatures as low as 100 "C;13 when this chemisorption capacity was reached, the % NO reduction first decreased with increasing temperature and subsequently increased. Only sample A(ox)-3.6is seen to exhibit, to some extent, the more complex behavior analogous to that of the potassium-catalyzed reaction. Indeed, this sample did show measurable quantities of NO chemisorbed a t 60 "C. As expected, the greatest catalytic effect in Figure 1 is seen to be exhibited by the sample having the highest calcium content and highest dispersion. A more detailed comparison of the effectiveness of calcium and potassium as NO reduction catalysts is provided e1~ewhere.l~ The gas composition profiles in Figure 2 provide important complementary information. In many respects, they are more similar to those of pure carbonslO than to those of potassium-loaded carbons.13J4 In the presence of potassium, three stages were observed, the first one, a t low temperatures, being due to NO chemisorption in which N2 and some N20 were the only products observed. In the presence of calcium, only two stages are observed, corresponding to the second and third stage in potassium-catalyzed NO reduction: (i) In the first stage, N2 and C02 are the dominant products. Initially, in contrast to pure carbons,1° there is a significant delay in C02 evolution with respect to N2. At higher temperatures, COz in excess is observed, its evolution exceeding that of N2 and displaying a well defined peak at ca. 660 "C. (ii) In the second stage, at temperatures above ca. 700 "C, Nz evolution becomes constant, C02 evolution decreases and CO becomes the dominant oxygen-containing product.
NO Reduction by Activated Carbons
O8
0.4 Oa6
Energy &Fuels, Vol. 9,No. I, 1995 116
1
i---\ OJ
03
0 100
OS
Od 0.4
Md
300
400
500
600
700
800
0 900
-
I
h 0.5
03
0 100
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200
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700
8I
0 900
500
600
700
800
900
1
oa 0.6
OA
03
0 100
t o the accumulation of oxygen on the surface. This oxygen is eventually transferred to the carbon (re)active site to complete the catalytic redox cycle.17 In contrast to the case of potassium-catalyzed NO reduction, where both facile NO chemisorption and early reduction of oxidized potassium species take place (with decomposition of carbon-oxygen surface complexes occurring a t temperatures as low as 300 "C), Figure 2 suggests that the completion of the catalytic cycle in the case of calcium-catalyzed NO reduction is more difficult. It is limited by the amount of oxygen formed on the catalyst surface (rather than by its transfer to the carbon surface). This interpretation is examined in more detail below. Under the conditions of sample preparation and pretreatment used in this study, calcium acetate is converted to Ca0.31s40This oxide is thought to participate in the catalytic redox cycle analogous to that proposed for the carbon-oxygen r e a c t i ~ n ~ (CaO ~-~~ CaO(0)). However, because CaO(0) is less favored thermodynamically than the catalytically active potassium species (a substoichiometric oxide &Oy+l),oxygen transfer to the carbon, with subsequent formation of the C(0)surface complexes, occurs instantaneously. (Much less C(0) is formed, however, compared to potassiumcatalyzed reaction.) As the oxygen eventually accumulates on the carbon surface, COZevolution in TPR should thus begin a t temperatures comparable to those observed in potassium-catalyzed NO reduction, i.e., ca. 300 "C. The late appearance of COz in the TPR profiles (ca. 500 "C in Figure 2a,c) is thought t o be an experimental artifact caused by the great affinity of CaO toward COz.31 (In calcium-catalyzed NO reduction, the first appearance of COZin TPR cannot be used, therefore, as an index of carbon reactivity.)l0J3J4J7 As the COz is evolved, it is probably trapped on the surface by chemisorption on CaO, forming CaC03.31 Only at higher temperatures does this C02 appear among the reaction products, when this chemisorption capacity is exceeded andor when surface carbonate decomposition becomes important. Indeed, the COz evolution peaks at ca. 660 "C coincide with the temperature range of surface carbonate decompositi~n.~~ A quantitative comparison of the amount of oxygen retained in the region of C02 delay (349 pmollg of C) and the amount of excess COZevolved subsequently (353 pmollg of C), for sample A(oxb3.6 (Figure 2a), supports the above interpretation. Furthermore, when these values are compared to the total quantity of calcium in the sample (900 pmollg of C), it is confirmed that only a fraction of the catalyst is involved in COz retention. It is important to note the gradual decay in NO concentration, i.e., a gradual increase in catalytic activity (Figure 2a,c in particular), which begins at temperatures that are much lower than that of first appearance of COz. This delay in COZevolution with respect to the NO concentration decay further supports the above interpretation. It is attributed to a competition between the formation of the active surface CaO(0) species and the formation of inactive calcium carbonate, which temporarily inhibits the reaction at intermediate temperatures. It is contrary to the behavior of both pure and potassium-loaded carbons,1°J3 where no such deactivation occurs and rapid NO decay (i.e., rapid in-
T rc)
Figure 2. TPR molar flow (n)profiles for the three calciumexchanged carbons: (a) A(ox)-3.6;(b) B(ox)-0.6; (c) K-UAl(ox)1.9.
It should be noted that the above-described characteristics are less clearly noted for sample B(ox)-0.6,both because of its low calcium content and because of probable diffusional limitations imposed on its reactivity by its narrow microporosity (see Table 11, also observed in the presence of ~ 0 t a s s i u m . l ~ The absence of N20 among the reaction products is in agreement with our results with potassium-loaded carbons.13J4 When dissociative NO chemisorption occurs at temperatures under 200 "C, Nz and NzO evolve during the process; in contrast, when chemisorption takes place at higher temperatures, only N2 is produced. This is also in agreement with the findings of Okuhara and Tanakalg that adsorbed Nz0 decomposes to NZand surface oxygen a t temperatures above 280 "C. In the presence of potassium, when significant NO reduction was observed at temperatures below 280 "C, NzO was found t o be a significant product. The delay in COZevolution with respect to Nz, also observed in the presence of potassium,13J4is attributed
(40) Gregg, S.J.; Ramsey, J. D.J. Chem. SOC.1970, A17,2784.
Illan-Cf6mez et al.
116 Energy &Fuels, Vol. 9, No. 1, 1995 0.6
ivity @moUg C/s) -A(ox)-3.6
Table 3. Steady-State Catalytic Activity of Calcium ion NO Reduction at 400-600 "Ca
-
B(oxbO.6
~9
K-UAl(ox)-13
~
~
sample A(0x)-3.6
B(ox)-O.6
0.5
K-UAl(ox)-l.9
~
R4oo 0.3 0.0 0.2
R5oo
R'500
Rsoo
22.0 0.0 5.2
6.11 0.0 2.74
42.9 35.0 26.7
" R = reactivity after 2 h, in f.pmoYg of C/s) reactivity after 2 h, in pmoYg of C d s .
0.4
03
0.2
0.1
0
~
20
40
60
80
loo
120
t (min)
Figure 3. Isothermal NO reduction activity as a function of time for the three calcium-exchanged carbons at 500 "C.
crease in catalytic activity) is observed as soon as COz begins to evolve from the carbon surface. The TPR results of Figure 2 also provide an indication of the catalytic activity of the different samples. Thus, at temperatures less than 500 "C, the reactivity follows the sequence A(ox)-3.6> B(oxk0.6 > K-UAl(ox)-1.9.The fact that this sequence is in qualitative agreement with the catalyst dispersion data and not with the C02 chemisorption data (Table 2) requires an explanation, which is offered below. According to the two steps in the postulated reaction mechanism, two parameters, in addition to the nature of the carbon substrate, are important in determining catalytic activity: the extent of the catalyst surface area, where CaO(0) formation occurs, and the CaO/carbon contact (interfacial surface area), through which the oxygen transfer occurs. Their relative importance and relationship to the catalytic surface area41,42seem t o be dependent on the reaction temperature. It is also complicated by the formation of inactive calcium carbonate on a portion of the catalyst surface. At low temperatures, because the amount of CaO(0) formed is small, the contact seems to be more important. In calcium-catalyzed carbon gasification with C02, the area of catalyst'carbon contact was also found to best represent the catalytic surface area, and it was quantified both by a deconvolution of TPD peaks43and using other transient kinetics method^.^^,^ No such attempt was made in the present study to confirm the above interpretation. Isothermal Reaction Studies. Isothermal reactivity experiments were performed t o obtain easily comparable catalytic activity data. Figure 3 shows typical results. A significant loss of activity with time on (41)Carberry, J. J. J . Catal. 1988, 114, 277. (42) Lizzio, A. A.; Radovic, L. R. Ind. Eng. Chem.Res. 1991,30,1735. (43) Cazorla-Amor&, D.; Linares-Solano,A.; Marcilla-Gomis,A. F.; Salinas-Martinez de Lecea, C. Energy Fuels 1991, 5 , 796. (44) Cazorla-Amor6s, D.; Linares-Solano, A.; Salinas-Martinez de Lecea, C.; Kapteijn, F. Carbon 1994, 32, 423.
x 100.
R6oo 11.9 58.3 14.1
R' =
stream is observed for all samples, in agreement with the behavior of both pure carbondo and (to a lesser extent) potassium-loaded ~ a r b o n s . ' ~ JSteady-state ~ activities of the samples, achieved after ca. 40-50 min, are compared in Table 3. At 400 "C the activity is very low. This result is consistent with the TPR data and the argument that catalyst deactivation occurs by formation of surface carbonate. At 500 "C the catalytic activity sequence is seen to correlate with the CO2 chemisorption capacity and not with the catalyst dispersion data, contrary to the TPR findings at lower temperatures. This points to a greater importance of the catalyst surface area than of catalystdcarbon contact and suggests that at higher temperatures NO chemisorption becomes the ratedetermining step (RDS) in the reaction sequence. Having in mind the definition of RDS (the step that consumes the largest portion of the overall concentration gradient), the above finding is also consistent with the argument presented earlier that oxygen transfer to the carbon is instantaneous, due to the low stability of CaO(0). These and other mechanistic implications are discussed in detail e1~ewhere.l~ Suffice it to say here that as the reaction temperature further increases, the RDS may be affected by the nature and reactivity of the carbon, as evident from the data in Table 3 for 600 "C. Because the contribution of the uncatalyzed, direct NO reduction on carbon becomes significantloa t ca. 600 "C, comparisons of catalytic effectiveness of calcium (i.e., per gram of Ca) are thus more meaningful at the lower temperatures. In agreement with the above discussion, no clear correlation between R'500 and CaO dispersion exists, even though the most highly dispersed catalyst was found to be the most active (A(ox)-3.6). To analyze the difference in catalytic effectiveness of calcium and potassium, their specific activity at 400 "C has been calculated for the same substrate, ionvs exchanged carbon K-UAl(ox). The result (4 x 5x mol NO reducedlmol metal/s, respectively) is in agreement with the TPR results discussed above. We conclude, therefore, that the principal reason for the lower activity of calcium resides not in its inability to transfer oxygen to the carbon surface, but rather in its much lower ability to chemisorb NO. The latter is in turn due to the different intermediates formed in the two redox cycles: the formation of CaO(0) is much less favorable than the formation of K,O,+1. 3.2. Effect of Catalyst Loading. Catalyst Loading and Dispersion. In order to eliminate the effects of the carbon substrate, three catalysts prepared from a single carbon, A(ox), were used in this portion of the study. Two were prepared by impregnation, at loadings below and above the calcium exchange capacity of the carbon, A(ox)-1.5and A(ox)-6.0,respectively. The third one was the previously used ion-exchanged sample A(0x)-3.6.
Energy &Fuels, Vol. 9, No. 1, 1995 117
NO Reduction by Activated Carbons
I
__---_._-
03
0 100
200
300
500
)oo
600
100
800
900
Figure 4. TPR profiles of NO reduction for calcium-loaded carbons having different catalyst loading. (Catalysts A(ox)-1.5 and A(ox)-6.0were prepared by impregnation; catalyst A(ox)3.6 was prepared by ion exchange.)
The calcium oxide dispersion results, obtained from COz Chemisorption,were 0.60 and 0.56 for A(ox)-1.5and A(ox)8.O, respectively. High dispersion is thus maintained in all cases, in agreement with other studies.31 Temperature-Programmed Reaction Studies. Figure 4 shows the TPR profiles for NO reduction of the three samples with increasing calcium content. As expected, the sample with the lowest catalyst loading exhibits behavior that is quite similar to that of pure carbons,1° with no activity at low temperatures. As the catalyst loading increases, the NO chemisorption capacity of the samples becomes measurable and the profiles are more similar to those of potassium-loaded carbons, characterized by high NO reduction activities a t low temperatures which decrease with temperature. In the presence of potassium, however, it was found that catalyst loading affects the TPR profile only in the lowtemperature (NO chemisorption) region.14 The fact that changes in calcium loading affect the NO reduction process to a greater extent than changes in potassium content is also illustrated in Figure 5 . At a low loading (Figure 5b), the evolution of products is similar to that observed for the parent carbon (Figure 5a), the only difference being that the events occur at lower temperatures in the former case. In contrast, at high loading (Figure 5c) the evolution of products is similar to that of the calcium-exchanged sample (Figure 2a). At low temperatures both show a delay in CO2 evolution with respect t o N2. At high temperatures there is excess COZwith a peak at ca. 650 "C, which is not as well defined as that for calcium-exchanged carbon. A comparison of the quantities of retained oxygen in the first stage (298 vs 349 pmoVg of C for carbons A(ox)-6.0and A(oxb3.6, respectively) suggests that the fraction of CaO that is capable of retaining COz is not determined by the calcium loading in the carbon. In the case of potassium,14 a relationship was found between the oxygen retained and the NO reduction activity. This possibility is considered below for calcium as well. Isothermal Reaction Studies. In a previous study of the behavior of this series of calcium-impregnated carbons in gasification with COz,45it was found that the catalytic activity exhibited a saturation point, beyond which further increases in loading resulted in negligible (45) Salinas-Martinez de Lecea, C.; Almela-Alarc6n, M.; LinaresSolano, A. Fuel 1990, 69,21.
15
O 0.6
a
*
L
1 0.4 05 01
n
"I00
u)o
300
400
500
600
100
800
n 900"
T PC)
Oa 0.6
0.4
I
/ \
1
05
03
0 100
200
300
400
100
600
700
800
0 900
T PC)
Figure 6. TPR molar flow (n) profiles for calcium-loaded carbons having different catalyst loading: (a) A(ox) (b) A(ox)1.5; (c) A(ox)-6.0. (Catalysts A(ox)-1.5 and A(ox)-6.0 were prepared by impregnation.)
reactivity increase. This saturation point coincided with the Ca2+ion-exchangecapacity of the carbon. The TPR results presented above (Figure 5 ) suggested a similar behavior of these carbons in NO reduction. Figure 6 illustrates the results obtained in isothermal reactivity experiments at 400, 500, and 600 "C. The specific activity parameter plotted on the ordinate was obtained by integrating the amount of NO reduced in the period from the beginning of the steady-state activity (ca. 40 min) until the end of the experiment (120 mid. Beyond ca. 3.6% Ca, the calcium added t o the carbon does not enhance its NO reduction capability. By analogy with the findings in a detailed study of this issue in catalyzed CO2 gasification,15f2 it is concluded that, above this saturation limit, calcium deposits on top of the ionexchanged calcium, contributes to an increase in crystallite size, but does not increase the area of catalyst/
118 Energy &Fuels, Vol. 9, No. 1, 1995
Illan-Gbmez et al.
4. Conclusions
1
w 1000
"I 0
4
2
6
C Ca
Figure 6. Relationship between the NO reduction activity and calcium loading.
substrate contact, which was found to be the key reactivity-determining parameter. The activity decrease at the highest loading, apparently most pronounced at intermediate temperatures, obviously warrants further studies, as does the relationship between catalyst dispersion and catalystkarbon contact. It is interesting t o note though that the similarity in the high-temperature activity of samples containing 3.6 and 6.0% Ca is consistent with their comparable oxygen retention during TPR experiments. Oxygen retention has been shown to be a measure of dissociative NO chemisorption.14 This correlation confirms the conclusion, discussed in detail elsewhere,14 that dissociative NO chemisorption is the appropriate technique for titrating the active sites in NO reduction. It also reinforces the argument that an oxidized species, e.g., CaO(O), is the catalytically active species that transfers oxygen t o the carbon surface.
The findings in this study, along with those of complementary studies of NO reduction by carbon in the absencelo and presence13J4J6 of catalysts, have allowed us to reach several specific and general conclusions about calcium-catalyzed NO reduction.15J7 Welldispersed calcium oxide on the carbon surface is not effective in dissociating NO at temperatures below 300 "C. This is the principal reason for its lower activity when compared to potassium catalysts. The mechanism of this modest catalytic effect seems to involve a redox cycle analogous to that responsible for calcium-catalyzed gasification of carbon with 0 2 . Once the relatively unstable intermediate CaO(0) species is formed, it readily transfers oxygen to the carbon surface. The relative importance of the catalyst surface area and the catalysthubstrate contact (interfacial surface area) seems to depend on the reaction temperature. Currently available results suggest that the latter factor is relevant a t low temperatures and the importance of the former increases with increasing temperature. Due to the high affinity of CaO toward COz, and contrary to the cases of both uncatalyzed and potassiumcatalyzed NO reduction, the temperature of first evolution of COz in a temperature-programmed reaction experiment is not a good index of carbon reactivity toward NO. The result of increasing catalyst loading is analogous to that observed in calcium-catalyzed gasification of carbon with COz: a very sharp saturation effect beyond the ion-exchange capacity of the carbon substrate.
Acknowledgments. This study was made possible by the financial support from DGICYT (projects -921032-(202-02 and CE91-0011-C03-01). The thesis grant for M.J.I.G. and an invited research grant to LRR from Generalitat Valenciana are also gratefully acknowledged. EF940134W
