Energy & Fuels 1995,9, 97-103
97
NO Reduction by Activated Carbons. 2. Catalytic Effect of Potassium M. Jose Illan-Gbmez, Angel Linares-Solano," Ljubisa R. Radovic,? and Concepcih Salinas-Martinez de Lecea Department of Inorganic Chemistry, University of Alicante, Alicante, Spain Received June 24, 1994@
The results of an investigation are reported concerning the NO reduction activity of different carbons loaded by ion exchange with varying amounts of a potassium catalyst. The samples were characterized by physical adsorption of C02 (at 0 "C) and N2 (at -196 "C) and by chemisorption of C02 a t 250 "C. The reactivity of the pure carbons was determined in both NO and COS. The catalytic effect of potassium in NO reduction was evaluated 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 t o be determined. Potassium was found to be an excellent catalyst for the NO reduction by carbon. At temperatures below 600 "C, the products of the reaction were primarily N2 and C02. The catalytic activity, and thus the concentration of catalytically active sites, is dependent on both catalyst dispersion and carbon reactivity.
Introduction In a previous study,l the somewhat surprising existence of a correlation between carbon reactivity in NO and the BET surface area of a wide range of activated carbons was reported. The catalytic effect of residual potassium in some of the carbons-those produced by chemical activation with KOH-was also found and briefly analyzed. The finding that potassium catalyzes NO reduction by carbon is not surprising. Alkali metals in general and potassium in particular are well-known as catalysts of carbon gasification, and the NO-carbon reaction is a gasification reaction. Indeed, comparisons of NO reduction by carbon with the more common carbon gasification reactions in C02,02 and H2O are available in the In carbon gasification reactions, the role of potassium has been studied in great detai1.8-12 Its catalytic effect + Permanent 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)IIlan-Gbmez, M. J.; Linares-Solano, A.; Salinas-Martinez de Lecea, C.; Calo, J. M. Energy Fuels 1993,7,146. (2)Shelef, M.; Otto K. J. Colloid Interface Sci. 1969,31,73. (3) Edwards, H.W. AIChE Symp. Ser. 1972,68,No. 126,91. (4)Song, Y.H.;Beer, J. M.; Sarofim, A. F. Combust. Sci. Technol. 1981,25,237. (5)Chan, L. K.; Sarofim, A. F.; Beer, J. M. Combust. Flame 1983, 52,37. (6) Suuberg, E. M.; Teng, H.; Calo, J. M. 23rd Symposium (Internatmnall Combustion [Proceedings]; The Combustion Institute: Pittsburgh, 1991;p 1199. (7)DeGroot, W.F.; Richards, G. N. Carbon 1991,29,179. (8) McKee, D.W. In Chemistry and Physics of Carbon; Walker, P. L.. Jr.. Thrower, P. A,, Eds.: Marcel Dekker: New York. 1988,Vol. 16,p i. (9)Pullen, J. R. Catalytic Coal Gasification, IEA Report ICTISmR 26;International Energy Agency Coal Research: London, 1984. (10)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. (11)Kapteijn, F.; Moulijn, J. A. Fuel 1983,62, 221. @
0887-0624/95/2509-0097$09.00/0
is thought to be due to its very effective participation in an oxidation-reduction (redox) cycle, in which the catalyst is oxidized by the gaseous reactant and reduced by the carbon. The resulting carbon-oxygen surface complexes decompose t o yield CO and CO2. In this mechanism, the presence of metallic potassium is possible;8also, nonstoichiometric species of the form KOy, in which the O K ratio is lower than that found in the oxide (KzO), are thought t o play the role of oxygen acceptors.gJlJ2 In contrast, the catalysis of the NO-carbon reaction by potassium has hardly been studied in any detail. Kapteijn et al.13studied the reduction of NO over alkali metal-carbon systems in the presence and absence of CO and found that NO reacted in approximately equal amounts with the CO and the carbon. They also observed a large increase in NO reduction (and carbon reactivity) when potassium was added to the carbon. Okuhara et al.14 found that, upon potassium addition to an activated carbon, both the NO adsorption capacity and carbon reactivity increased. In these studies, however, the role of potassium in the reaction has not been clarified. In an attempt to improve our understanding of the catalytic effect of potassium in NO reduction by carbon, initiated in a previous study,l we have undertaken a more detailed investigation of the following issues: (i) the importance of carbon nature, and (ii)the effects of catalyst amount and dispersion. In this paper, we discuss primarily the former issue. The results obtained (12)Moulijn, J. A.; Kapteijn, F. In Carbon and Coal Gasification; Figueiredo, J. L., Moulijn, J. A., Eds.; Martinus Nijhoff Publishers: Dordrecht, The Netherlands, 1986;p 181. (13)Kapteijn, F.;Alexander, J. C; Mierop, G. A.; Moulijn, J. A. J . Chem. Soc., Chem. Commun. 1984,1084. (14)Okuhara, T.; Tanaka, K. J . Chem. Soc., Faraday Trans. 1986, 82, 3657.
0 1995 American Chemical Society
Illan-Gbmez et al.
98 Energy &Fuels, Vol. 9,No. 1, 1995
on the latter are presented in detail in the subsequent p~b1ication.l~
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.' Briefly, A and B are chars obtained from a phenol-formaldehyde polymer and almond shells, respectively,16 while K-UA1 is a coal-derived carbon whose activation was accomplished with KOH." Potassium was introduced by ion exchange (60 "C, 4 h) onto these carbons after they had been oxidized with HN03 to enhance their ion-exchange capacity.ls 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). Potassium acetate (0.5 M) was used as the catalyst precursor. The ionexchanged samples were washed with distilled water until complete elimination of K+ ions from the filtrate. Potassium content in the thus-obtained samples was determined by atomic absorption spectroscopy. For this purpose, potassium was extracted from the samples by refluxing them in 1M HC1 for 8 h. The potassium-loaded samples are designated by the parent carbon used, followed by their potassium content (in weight percent K). Carbon dioxide chemisorption experiments were carried out in a Stanton-RedcroR 780 thermobalance. After a heat treatment in Nz (60 mumin) at 20 W m i n to 900 "C, the samples were cooled to 250 "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 Nz at 250 "C. Adsorption of COz on carbon was negligible at 250 "C. Pore structure characteristics of the samples were determined by physical adsorption of COz (at 0 "C) and Nz (at -196 "C) in an automated volumetric apparatus (Autosorb-6, Quantachrome). The reactivity of the carbons in COz was determined by isothermal thermogravimetric analysis, following standard Briefly, the sample procedures described in detail e1~ewhere.l~ is heated at 20 "C/min in Nz (60 mUmin) to 900 "C; it is subsequently cooled to 800 "C and Nz is replaced by COZ(60 mumin). The kinetics of the NO-carbon reaction were studied at atmospheric pressure in a ked-bed flow reactor (15 mm, i.d.; ca. 300 mg sample) connected to a gas chromatograph (Hewlett Packard, Model 5892A). The gaseous products were analyzed using a Porapak Q 80/100 column and a thermal conductivity detector. Two types of experiments were carried out: (i) temperature-programmed reaction (TPR), consisting of heating the sample at 5 "C/min to 900 "C in a NO/He mixture (0.4% NO, 60 mumin); and (ii)isothermal reaction at 300,400,500, and 600 "C for a period of 2 h. In both types of experiments, the sample was first subjected to an in situ heat treatment in He, at 50 "C/min t o 900 "C for 10 min. In case (i), the ~~~
(15) Ill6n-G6mez, M. J.; Linares-Solano, A.; Radovic, L. R.; Salinas-
Martinez de Lecea, C. NO Reduction by activated carbons. 3.Influence of catalyst loading on the catalytic effect of potassium. Energy Fuels 1995, 9 , 104. (16)Linares-Solano, A.;Almela Alarc6n, M.; Salinas-Martinez de Lecea, C.; Mufioz-Guillena, M. J.; IllBn-G6mez, M. J. In Characterization of Porous Solids II; Rodriguez-Reinoso, F., et al., Eds.; Elsevier: Amsterdam, 1991,p 367. (17)111Bn-G6mez, M. J.;Muioz-Guillena, M. J.; Salinas-Martinez de Lecea, C.; Linares-Solano, A.; Martin-Martinez, J. M. Proc. Znt. Carbon Conf. Carbone '90,Paris,France, 1990, p. 68. (18)Schafer, H.N.S. Fuel, 1984, 63, 723. (19)Salinas-Martinez de Lecea, C.; Almela-Alarch, M.; LinaresSolano, A. Fuel 1990, 69,21.
Table 1. Principal Characteristics of Carbons Used
sample A(ox) B(ox) K-UAl(0x)
SBET,N~ SDR,CO~ COZ,TPD COTPD (mVg) (mz/g) @mol@ @mol/p) 878 547 1078
712
619 853
1810 2041 1420
2330 2226 2370
%K 4.9 3.9 1.9
temperature is lowered to 20 "C, He is replaced by the reactant mixture, and the TPR experiment is performed. In case (ii), the temperature is lowered to the desired level and the isothermal experiment is initiated by substituting the NO/He mixture for He.
Results and Discussion Sample Characterization. The porous structure of the carbons was analyzed in detail e1sewhere.l Table 1 summarizes their NZ and CO2 surface areas, determined using the BET and Dubinin-Radushkevich equation, respectively. Differences are seen to exist in their specific surface area and pore size distribution. Carbons A(ox) and K-UAl(ox)have a broader pore size distribution, as evidenced by the difference between their S N ~ and Scoz. In contrast, carbon B(ox) has somewhat narrower pores; its larger Scozis a consequence of the well-known problem of slow diffusion of NZinto narrow micropores at 77 K. The TPD spectra exhibit features that are typical of carbons treated with HNOB, both with regard to the quantities of CO and C02 evolved and to their general shapes.20 The quantities of CO and C02 evolved (see Table 1) are primarily a measure of carbonyl and carboxyl groups, respectively, present on the carbon surface. As a result of the oxidative treatment of the carbons, these values are high in all cases. Nevertheless, there are appreciable differences in the COzyielding groups between carbon K-UAl(ox) and the other two carbons. As indicated in the Experimental Section, potassium was introduced by ion exchange from an acetate solution. It is reasonable to expect that, under the experimental conditions used (unadjusted pH), the degree of exchange is related to the quantity of COZevolved in TPD.18>20 However, the data in Table 1 show that the carbon with the largest quantity of CO2-yieldinggroups (B(ox)) does not have the largest potassium content. (The lowest potassium content of carbon K-UAUox) does agree with its lowest concentration of CO2-yielding groups.) This is due to the fact that, in addition to the surface chemistry,21,22 the porosity of the carbon affects catalyst loading. The narrow microporosity of carbon B(ox) apparently makes some of the carboxyl groups inaccessible to the catalyst precursor. The C02 gasification reactivities of carbons A and B a t 800 "C were found to be quite similar, 0.01 and 0.03 h-l, respectively. They were both considerably lower than that of carbon K-UAl(O.15 h-l). Also, the steadystate NO reduction activities (per gram of C) were in the following order: K-UA1 > A > B.l The activities of the same carbons after oxidation were in the following order: K-UAl(ox) > B(ox) > A(ox1.l (20)Otake, Y.;Jenkins, R. G. Carbon 1995,31, 109. (21)Solar, J. M.; Leon y Leon, C. A.; Osseo-Asare, K.; Radovic, L. R. Carbon 1990,28, 369. (22)RomBn-Martfnez, M. C.; Cazorla-Amor68, D.; Linares-Solano, A.;Salinas-Martinez de Lecea, C. Carbon, 1993,31, 895.
NO Reduction by Activated Carbons
Energy & Fuels, Vol. 9,No. 1, 1995 99
% NO reduction
n
Ova
"100
200
300
400
500
600
700
800'
T CC)
Figure 2. TPR molar flow ( n )profiles (Nz, N O , NzO, CO, COz) for potassium-loaded oxidized carbon A having 4.9% potassium, A(ox)-4.9.
/
T ("C) %NO reduction
100,
100
,
fl
m
300
400
500
i 7
600
700
800
T W)
Figure 1. Temperature-programmedreaction (TPR) profiles of NO reduction for oxidized and potassium-loaded activated carbons: (a) A (b) B and B-32%; (c) K-UA1.
Temperature-Programmed Reaction Studies. Figure 1 shows that the TPR curves of potassiumcontaining samples are quite different from those of their parent carbons. Generally speaking, the former exhibit maximum NO reduction a t low temperatures, an intermediate temperature region in which the reactivity is negligible, and a high-temperature region in which the degree of reduction increases with temperature until 100% NO conversion level is reached. The fundamental differences with respect to the parent carbons are the appearance of the reduction maximum at low temperature and a significant reduction in the temperature at which 100%reduction is achieved (T~oo). From a comparison of the TPR profiles of the potassium-containing samples, it is seen that carbon B(ox)3.9exhibits somewhat different behavior: at temperatures lower than 200 "C, NO reduction is not observed. If it is considered that this TPR profile is shiRed toward higher temperatures (by about 200 "C) with respect to the other two potassium-loaded carbons, it can be concluded that the catalytic effect of potassium is
manifested in the same way for all samples: a shift in the TPR profile toward lower temperatures resulting in an important change in the TPR profile. In an earlier publication,l the two regimes of reactivity behavior evident in the TPR profiles of pure carbons (e.g., sample A(ox) in Figure l a ) were attributed to a significant change in the reaction mechanism at 600680 "C. In the presence of potassium, these regimes are not observed. The mechanistic implications of this result are considered elsewhere.23 The delay observed in the TPR profile of sample B(ox)3.9 was thought to be attributable t o the same reason as its lower-than-expectedpotassium loading (see above), i.e., diffusional limitations caused by its narrow micr0porosity.l This hypothesis was substantiated by the following experiment. A sample of carbon B(ox) was activated in CO2 to 32% burnoff; the surface areas of the resulting carbon were 798 m2/g for C 0 2 and 1490 m2/gfor N2. It was oxidized in the same manner as the other carbons and potassium was introduced by impregnation (resulting in 4.3% K). Figure l b shows that this sample exhibits a TPR profile quite similar to those of potassium-loaded A(ox) and K-UAl(ox) samples, and very different from that of K-loaded B(ox). The analysis of reaction products, mainly NzO, N2, NO, CO2 and CO, provides essential complementary information. In comparison with the results obtained in the absence of a catalyst,' the appearance of N20 is a novel feature. Kapteijn et al.13 and Okuhara et al.14 also detected N2O among the reaction products of NO reduction by potassium-loaded carbons at low temperatures. Figure 2 shows the typical evolution of products, using potassium-loadedcarbon A(ox) as an example. For all potassium-loaded carbons, three regions of reactivity are observed. (i) In the first stage, N2 and N2O are the only products. No oxygen-containing products, other than N20, are observed; oxygen from NO reduction is retained by the potassiudcarbon system. considering the two possible types of NO chemisorption on carb o n ~ , this ~ ~ stage * ~ ~ appears to involve irreversible chemisorption of NO, oxygen retention and evolution of nitrogen-containing compounds. (23) Radovic, L. R.; Illhn-G6mez,M. J.; Linares-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. (24) Teng, H.; Suuberg, E. M. Znd. Eng. Chem. Res., 1993,32,416. (25) Teng, H.; Suuberg, E. M. J. Phys. Chem. 1993,97,478.
Illan-Gc5mez et al.
100 Energy & Fuels, Vol. 9, No. 1, 1995 (ii) In the second stage, N2 continues to evolve, but it is accompanied by C02 evolution. The temperature of onset of C02 evolution is lower than that found in the absence of potassium, further illustrating the catalytic effect of potassium. It is different for each sample, being 280, 300, and 400 "C for K-UAl(ox)-1.9,A(oxh4.9, and B(ox)-3.9, respectively. This is an indication of the importance of carbon nature in catalyzed NO reduction. For the most reactive carbon, K-UAl(ox), the temperature at which C02 evolution begins is the lowest. However, the trends observed do not follow those of the NO reduction reactivity of the pure carbons. (The implications of the fact that neither of these two reactivity parameters is related to carbon's ability to accept oxygen from the catalyst are intriguing, but are beyond the scope of the present study.) The quantity of C02 evolved does not correspond stoichiometrically t o that of N2; an excess of C 0 2 is observed, which must be related to oxygen retention in the first stage. (iii)In the third and last stage, at temperatures above 500 "C, N2 evolution becomes constant, C02 evolution decreases, and CO becomes the dominant oxygencontaining product. The balances of oxygen and nitrogen provide interesting information which allows us to explain this distribution of products. For example, the oxygen balance is negative at temperatures below 300 "C, which indicates its accumulation on the surface. Upon heat treatment of the samples prior to reaction, the catalyst precursor is most likely reduced by carbon to a substoichiometric oxide, KZOy,l1J2or to metallic potassium.8 It can be concluded, therefore, that oxygen initially accumulates on the potassium surface, forming a more oxidized species (&Oy+l), which is eventually reduced by the carbon, when oxygen is finally transferred to the carbon active sites, thus completing the catalytic cycle. This hypothesis is supported by the study of Moulijn and coworkersl1J2 in which a similar redox cycle was found to be operative in the potassium-catalyzed C02 gasification. At temperatures above 500 "C, the oxygen balance becomes positive as a consequence of the decomposition of carbon-oxygen surface complexes and C02 and CO evolution. Above 700 "C, and in agreement with the results obtained in the absence of a catalyst,l the quantity of oxygen evolved, mainly as CO, becomes much higher than that expected from NO reduction. Apparently, the high-temperature surface complexes, which remain on the carbon surface subsequent t o the initial heat treatment to 900 "C at 50 "C/min, are destabilized in the presence of NO and desorb at lower temperatures. The balance of nitrogen is also negative in the first reaction stage, though less so than that of oxygen. Thus either nitrogen-containing species are also retained on the surface (as a consequence of dissociative NO adsorption) or reversible chemisorption of N024p25is taking place. This phenomenon appears to be dependent on the nature of the carbon, since it is not observed for sample B ( o ~ ) - 3 . 9 After . ~ ~ this first stage, the nitrogen balance becomes positive as the retained species desorb from the surface. In order to verify that potassium is indeed reduced by the carbon during the initial heat treatment of the samples, the products of this initial TPD treatment were analyzed by mass spectrometry. Figure 3 illustrates
10
% vovg
co, --co
0
100
200
300
400
500
a)
600
700
800
900
600
700
800
900
T W)
"0
100
200
300
400
500
TW )
Figure 3. Temperature-programmeddesorption (TPD) gas composition profiles (COz, CO) for oxidized carbon A(ox) (a) and potassium-loadedoxidized carbon, A(ox)-4.9 (b) prior to their use in NO reduction.
typical results. The spectra of potassium-containing samples are quite different from those obtained in the absence of the catalyst. In a qualitative sense, the main difference is in the CO spectrum, which exhibits two peaks, at ca. 790 and 840 "C, in excess (Figure 3a) of CO found in the absence of the catalyst. These temperatures coincide indeed with the reduction range reported in the literature. (For example, from the diagram of Elligham,26potassium oxide is reduced at 820 "C.) The appearance of two distinct peaks suggests the existence of two potassium species, e.g., having different particle sizes, whose extent of potassiud carbon contact may be different. The lower-temperature CO peak is thus ascribed tentatively to the smaller (26) Gilchrist, J. D.International Series on Materials Science and Technology, Hopkins, P. W.Ed.; Pergamon Press: London, 1980.
NO Reduction by Activated Carbons
"0
20
40
80
60
Energy &Fuels, Vol. 9, No. 1, 1995 101
loo
120
t (min)
Figure 4. Isothermal NO reduction activity as a function of time for carbon A(ox)-4.9 at different temperatures. Table 2. Steady-State Catalytic Activity of Potassium in NO Reduction at 300-600 "Ca sample A(ox)-~.~ B-32%(0~)-4.3 B(oxb3.9 K-UAl(ox)-l.9
R300
R400
R500
R600
16 4 0 15
62 39 9 46
95 a4 58 62
100 87 100 87
"R = reactivity after 2 h, % NO reduced. catalyst particles, whose interfacial contact is greater, resulting in easier catalyst reduction. Isothermal Reaction Studies. Isothermal reactivity experiments were performed in order to obtain easily comparable catalyst activity data. Figure 4 shows typical data. The expected activity increase with temperature, analogous to that observed in TPR experiments, is observed. Furthermore, a t all temperatures, 100% NO is initially reduced. At 600 "C the reactivity of all samples is very high and constant throughout the 2 h interval. As the temperature decreases, the degree of catalyst deactivation increases, and a low level of constant activity is reached after ca. 60 min. This deactivation is particularly evident for sample B(ox), suggesting that the completion of the catalytic cycle is not possible at 300 "C on this carbon, and the catalyst remains in the oxidized state. This sample is also the only one that does not exhibit the characteristic high initial activity at all temperatures. Table 2 summarizes the catalytic activity data. At constant temperature the samples are seen to have different reactivities, which could be attributed to their different potassium contents and/or to the differences in carbon nature and potassiudcarbon contact. Sample A(ox)-4.9is seen to be the most reactive. It should be noted, however, that the reactivity order here is somewhat different from that deduced from TPR results. For example, at 300 "C sample A(ox)-4.9 exhibits a TPR minimum (0% NO reduction), while achieving 16% reduction in the isothermal experiment. This discrep-
ancy is typical of thermally activated p r o c e s s e ~ .In ~~ contrast, the potassium-loaded carbon B(ox) achieves 60% reduction at 300 "C in TPR, while its isothermal reactivity a t 300 "C is negligible. To explain this discrepancy, it must be considered that only NO chemisorption is occurring on this sample at 300 "C. This process stops when the catalyst becomes saturated. No further reaction is possible until COz evolution starts, as observed in the isothermal experiment (see Figure 2). Indeed, the NO uptakes before the onset of C02 evolution, in the isothermal experiment, and before 300 "C in the TPR experiment are in reasonable agreement (400 vs 330 pmoVg of C). These results suggest the importance of conducting both types of reactivity experiments, which are seen to provide complementary information. The analysis of reaction products during isothermal experiments is instrumental in interpreting the reactivity variations with time. Figure 5 shows the results obtained for the most reactive sample; the results for the others were very similar. A delay in the evolution of COZ with respect t o Nz, also observed in TPR experiments, is seen to be more pronounced as the reaction temperature decreases. At 300 "C, the only initial product is Nz, as a consequence of dissociative chemisorption of NO, accompanied by oxygen accumulation on the reduced catalyst and its eventual transfer to the carbon active sites. Subsequently, COz evolution is observed and new carbon sites, necessary for activity maintenance, are created. As the accumulation of surface oxygen becomes faster, with increasing reaction temperature, the delay in COZ evolution becomes less pronounced. This important finding can be understood in the light of the often-invoked similarity between NO reduction by carbon and other carbon gasification reactiorm6 In carbon gasification by oxygen in particular, it is well-known that oxygen surface complexes act as both reaction inhibitors and reaction intermediates.28 Oxygen accumulation on the surface in the form of stable C-0 complexes, while temporarily reducing the number of reactive sites, induces surface heterogeneit^,^^ which in turn makes the carbon more reactive. This dual role of oxygen surface complexes is also clearly seen in NO reduction, in both TPR and isothermal experiments (Figures 2 and 5). The formation of these complexes is a necessary consequence of NO dissociation. But NO chemisorption becomes less effective on the oxygen-rich surface, as evidenced by the rapidly increasing NO concentration in the gaseous product in the absence of COS evolution (Figures 2 and 5a). Once COZevolution begins, the reactivity at low temperatures decreases much more slowly (Figure 5a), and a t higher temperatures, when both CO and COn are seen to evolve, it is maintained over extended periods of time. The mechanistic implications of these findings are explored in more detail elsewhere.23 In order to ascertain whether the reactivity differences observed for different carbons are related only to their different potassium contents, the kinetic data were expressed as specific activities, i.e., per gram of catalyst. (27) Falconer, J. L.; Schwarz, J. A. Catul. Rev. Sci. Erg.,1983,25, 141. (28)Radovic, L. R.;Lizzio, A. A.; Jiang H. In Fundamental Issues in Control of Carbon Gasification Reactiuity; Lahaye J., Ehrburger, P., Eds.; Kluwer Academic Publishers: Dordrecht, The Netherlands, 1991; p 235. (29) Boudart, M. J.Am. Chem. SOC.1962,74, 3556.
Illan-Gbmez et al.
102 Energy &Fuels, Vol. 9, No. 1, 1995 rmoVg Cis)
095
0
20
40
60 t
@"Ug
80
100
120
0
I
0
20
40
(min)
60 t
(min)
t
(min)
80
100
120
80
100
120
Cis) -NO
20
40
-N2
60
b)
C02
80
100
120
0 0
20
t (min)
40
60
Figure 6. Isothermal molar flow (n)profiles (NO, N2, C 0 2 ) for NO reduction on sample A(ox)-4.9 at 300 "C (a), 400 "C (b), 500 "C (c), and 600 "C (d). Figure 6 shows the dependence of specific activity on catalyst loading a t 400 and 500 "C. No clear relationship exists. Thus, sample B(ox)-3.9 exhibits minimum activity, possibly due to diffusional limitations in its narrow micropores, while sample K-UAl(ox)-l.9exhibits unexpectedly high activity. The results obtained with samples B(ox)-3.9 and B-32%(ox)-4.3(see Table 2 and previous section) suggested the importance of the porous structure of the carbon, in agreement with our results on uncatalyzed NO reducti0n.l However, other factors must also be important, as evident from the pore structure parameters found for samples A(ox)-4.9 and K-UAl(ox)-l.g. Their COz and NZsurface areas are not very different: 593 and 503 mzlg and 844 and 593 m21 g, respectively. In contrast, the specific activity of the latter sample is much higher than that of the former. The activity sequence shown in Figure 6, which reflects the differences in the concentration of catalyti-
cally active sites,30could be related to different degrees of catalyst dispersion or the presence of different catalyst species in different carbons. An attempt t o determine catalyst dispersion was made by COz chemia method that was successfully sorption at 250 0C,31 used to characterize the dispersion of CaO on carbons.32 Assuming that one C 0 2 molecule adsorbs on one KzO site to form the surface carbonate, the data in Table 3 were obtained. It is seen that the C0&20 ratio decreases with decreasing catalyst loading. This trend is contrary to that found for catalytic activity (Figure 6). Its tentative interpretation is offered below. (30)Cazorla-Amor6s,D.; Linares-Solano, A.; Radovic, L. R.; SalinasMartinez de Lecea, C. Calcium as Catalyst for Carbon Gasification: Energy Fuels, submitted for publication. (31)Ratcliffe, C. T.; Vaughn, S. N. ACS Preprints Diu. Fuel Chem., 1986, 30, 304.
(32)Linares-Solano, A.; Almela-Alarc61-1,M.; Salinas-Martinez de Lecea, c. J. Cutal. 1990,125, 401.
NO Reduction by Activated Carbons 20
Energy &Fuels, Vol. 9,No. 1, 1995 103
Activity (Irmolig W s ) Carbon 8ubatrate
* m * c *SOOT
(a) high catalyst dlapwsion
(b) low catalyst dispersion
Figure 7. Schematic representation of the catalystkarbon interface: (a)high catalyst dispersion,(b) low catalyst disper\
I 0' 0
\ / 1
3
2
4
I I
5
sion. in agreement with the results in Figure 6. In carbons A(ox)-4.9and B(ox)-3.9,on the other hand, the high Cod K20 ratio of the former suggests a low concentration of KOy sites at the catalystharbon interface. A direct confirmation of this explanation will have to await more direct titrations of CAS, e.g., using transient kinetics technique^.^^-^^
%K
Figure 6. Plot of specific catalytic activity vs. potassium loading on the carbon at 400 and 500 "C. (The specific activity was determined at 2 h from the beginning of the experiment.) Table 3. Results of COa Chemisorption on Ion-Exchanged Potassium-Loaded Carbons sample amount adsorbed kmoYg) C02/K20 A(ox)-4.9 289 0.46 B(ox)-3.9 160 0.32 K-UAl(~ ~ 4 - 1 . 9 37 0.15
The concentration of catalytically active sites (CAS) is related t o the concentration of catalyst active sites (catalyst dispersion or surface area) as well as to the nature of the carbon itself. Catalyst dispersion controls the processes of oxidation and reduction of the catalyst because the transfer of oxygen from the oxidized catalyst (K20 or substoichiometric K,O,+l) t o the carbon occurs at the catalyst'carbon interface. This interfacial surface area increases as the catalyst particle size decreases or as its dispersion increases. Furthermore, more reactive carbons are expected to be more effective in reducing the catalyst. Taking into account these factors, it is conceivable that C 0 2 titrates only those sites that are not in contact with the carbon, e.g., K20 sites. This is in agreement with the intuitively obvious notion that the potassium species closer to the catalystharbon interface are likely t o be the more reduced ones. These concepts are summarized in the schematic representation shown in Figure 7. The low ratio COfi20 for sample K-UAl(ox)-1.9(Table 31, together with the high reactivity of this carbon (lowest temperature of C02 appearance in TPR, highest reactivity in both NO and COz), is thus interpreted as evidence for the largest proportion of catalytically active sites (e.g., KrOy sites),
Conclusions The investigation of the catalytic effect of potassium in NO reduction on carbons of different porous structure and reactivity has led to the following conclusions: (i) Potassium catalyzes the NO-carbon reaction by decreasing very significantly the temperature necessary to reduce NO. At temperatures below 600 "C, the products of the reaction are primarily N2 and C02. (ii) The first stage of the reaction is the dissociative chemisorption of NO, accompanied by the formation of oxygenated surface species. Subsequently, oxygen is transferred to the carbon and desorbs as CO2 and/or CO. (iii) The catalytic effect of potassium seems to depend on the reactivity of the carbon as well as on its dispersion. Both factors control the NO reduction effectiveness by determining the fraction of potassium that is transformed into the active reduced form under reaction conditions. The best index of carbon reactivity in NO reduction seems to be the temperature of first appearance of COn in a temperature-programmed NO reduction experiment. Acknowledgments. This study was made possible by the financial support from DGICYT (projectsAMB921032-C02-02) and (CE91-0011-C03-01). The thesis grant for M.J.I.G. and an invited research grant t o L.R.R. from Generalitat Valenciana are also gratefully acknowledged. EF940122A (33)Lizzio, A.A.;Radovic, L. R. Znd. Eng. Chem. Res. 1991,30,1735. (34)Cazorla-Amor6s, D.;Linares-Solano,A.; Marcilla-Gomis,A. F.; Salinas-Martinez de Lecea, C. Energy Fuels, 1991,5, 799. (35)Cazorla-Amor6s, D.;Linares-Solano, A.; Salinas-Martinez de Lecea, C.; Kapteijn, F. Carbon, 1994,32,423.
