Energy & Fuels 1995,9, 104-111
104
NO Reduction by Activated Carbons. 3. Influence of Catalyst Loading on the Catalytic Effect of Potassium M. Jose Illan-Gbmez, Angel Linares-Solano," Ljubisa R. Radovic,? and Concepci6n Salinas-Martinez de Lecea Department of Inorganic Chemistry, University of Alicante, Alicante, Spain Received June 27, 1994@
The results of an investigation are reported concerning the NO reduction activity of a carbon loaded by impregnation from solution with varying amounts of a potassium catalyst. The samples were characterized by COz chemisorption at 250 "C and NO chemisorption at 60 "C. The catalytic effect of potassium in NO reduction was evaluated at atmospheric pressure in a fEed-bed flow reactor. Two types of experiments were performed: (i)temperature-programmed reaction (TPR) in a NO/He mixture; and (ii) isothermal reaction at 300-600 "C. Both TPR and temperatureprogrammed desorption experiments were also conducted subsequent to NO chemisorption. The reaction products were monitored in all cases, thus allowing detailed oxygen and nitrogen balances to be determined. Three characteristic reactivity regions were found in the TPR profiles: (a) high catalytic activity at low temperatures due to dissociative NO chemisorption, accompanied by NZ and N2O evolution and oxygen accumulation on the catalyst surface; (b) severe catalyst deactivation a t intermediate temperatures as the catalyst becomes fully oxidized; (c) recovery of high catalytic activity at high temperatures as the catalyst is reduced by the carbon, accompanied by the evolution of COZand CO. An increase in potassium loading was found to affect only the first region, extending the period of high activity. A good correlation was found between steadystate isothermal catalytic activity and the amount of oxygen accumulated on the surface upon NO chemisorption a t 60 "C, especially for NO reduction at low temperatures (e.g., 300 "C). Therefore, dissociative NO chemisorption seems to be a promising technique for titrating the active sites in potassium-catalyzed NO reduction by carbon.
1. Introduction
In previous publications in this series on NO reduction by both pure and metal-doped activated carbons,lP2 potassium was found to decrease considerably the temperature a t which significant reduction takes place. The presence of the catalyst thus modifies the temperature-programmed reaction (TPR) profile as well as the evolution of reaction products. The first stage of the reaction was found to involve the irreversible chemisorption of accompanied by evolution of nitrogencontaining products (Nz and NzO) and oxygen accumulation on the catalyst, in a fashion similar to that observed in other catalyzed carbon gasification reactions.6-8 A delay in the evolution of COZ, with simultaneous catalyst deactivation, was observed in all cases. At higher temperatures (in TPR experiments) or at higher oxygen surface coverages (in isothermal experiments), COZ evolution eventually reaches levels + 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)Illan-Mmez, M. J.; Linares-Solano, A.; Salinas-Martinez de Lecea; Calo, J. M. Energy Fuels 1993,7,146. (2) Illln-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,-. (3)Teng, H.; Suuberg, E. M.; Calo, J. M. Energy Fuels 1992,6,398. (4) Teng, H.; Suuberg, E. M. J. Phys. Chem. 1993,97,478. ( 5 ) Teng, H.; Suuberg, E. M. Znd. Eng. Chem. Res. 1993,32, 416. (6) Kapteijn, F.;Moulijn, J. A. Fuel 1983,62,221. (7)Cerfontain, M. B.; Kapteijn, F.; Moulijn, J. A. Carbon 1988,26, 41. (8)Meijer, R.; Weeda, M.; Kapteijn, F; Moulijn, J. A. Carbon 1991, 29,929. @
that are higher than that corresponding - to Nz evolution and the stoichiometry of the reaction. The mechanistic implications of the above findings are explored el~ewhere.~ In the present study, it was of interest to clarify the influence of catalyst loading on the catalytic effect of potassium. In our previous work,2 this effect was coupled with that of the nature of the carbon and the two could not be deconvoluted. Furthermore, a more detailed analysis of the influence of potassium on NO chemisorption, the important first stage of the catalytic reaction, was also performed. It has been shown by Kaneko and co-workers1°-16 that transition metals (Cu, Ni, Co, Fe, Ti) enhance the NO adsorption capacity of activated carbon fibers. The extents of this enhancement were found to be different, with Cu and Fe being the most effective catalysts. In contrast, very little information is available in the literature on the catalytic effects of alkali and alkaline(9)Radovic, L.R.; Illln-Gmez, 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. (10)Kaneko, K.; Inouye, K. Carbon 1986,24,772. (11)Jbneko, K.; Inouye, K. Ads. Sci. Technol. 1988,5,11. (12)Kaneko, K.;Ohta, T.; Ozeki, S.; Kosugi,N.; Kuroda, H. Appl. Su$. Sci. 1988,33, 355. (13)Kaneko, K.;Yamamoto, H.; Suzuki, T.; Ozeki, S. 3rd Znternational Conference on Fundamentals of Adsorption; Bad Soden, Germany; 1989;p 355. (14)Kaneko, K. In Characterization ofPorous Solids; Unger, K . K., et al., Eds; Elsevier Science Publishers: Amsterdam, 1980;p 183. (15)Kaneko, K.; Ozeki, S.; Inouye, K. Atmos. Environ. 1987,21, 1877. (16)Wang, Z. M.; Shindo, N.; Otake, Y.; Kaneko, K. Carbon 1994, 32, 515.
0887-0624/95/2509-0104$09.00/0 0 1995 American Chemical Society
NO Reduction by Activated Carbons
Energy & Fuels, Vol. 9, No. 1, 1995 105
Table 1. Wncipal Characteristics of Carbons Used S N ~ Qcoz C02 chemisorbed sample (m2/g) (m2/g) OlmoVg) COfi20 K-UA1 K-UA1-2.8 K-UA1-4.6 K-UA1-7.4
2087 2102 1765 1749
1080 998 691 644
154 253 436
0.43 0.43 0.46
100
%NO reduction
80
60
40
earth metals. A notable exception is the elegant study of Okuhara and Tanaka,17 in which it was found that when a carbon is doped with 3.5% K (by weight), a significant NO adsorption enhancement occurs in the temperature range 25-200 "C.
- K-UAl -K-UAl-ZS
20
0 100
K-UA1-4.6 -K-UA1-7.4 VY
200
300
400
500
600
700
I
800
T CC)
2. Experimental Section A single carbon precursor, activated carbon K-UA1, was chosen for this study. It exhibited the highest potassiumcatalyzed NO reduction activity in a previous study.2 It is a coal-derived carbon whose activation was accomplished with KOH.18 The catalyst was introduced by excess-solution impregnation using potassium acetate (10 mL of solutiodg of carbon). Catalyst content was varied by using solutions of different concentrations. The impregnated samples were dried by first bubbling N2 through the solution; subsequent to solvent elimination in this fashion, they were left in an oven at 110 "C for a period of 12 h. Potassium content was determined by atomic absorption spectroscopy. For this purpose, potassium was extracted from the samples by refluxing them in 1 M HC1 for 8 h. The potassium-loaded samples are designated by the parent carbon used, followed by their potassium content (in wt % K). The three samples analyzed in detail in this study, with potassium contents of 2.8, 4.6, and 7.4%, will be referred to as K-UA1-2.8, K-UA1-4.6 and K-UA1-7.4. Their properties, along with those of the parent carbon, are summarized in Table 1. The kinetics of the NO-carbon reaction were studied at atmospheric pressure in a fixed-bed flow reactor (15 mm, id.; 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 subjected first t o a n in situ heat treatment in He, at 50 "C/min t o 900 "C for 10 min. In case (i), the temperature is lowered to 20 "C, the reactant mixture is substituted for He, 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. The NO chemisorption stage of the reaction was studied in three different experiments: (i) NO chemisorption at 60 "C until the point at which the outlet NO concentration becomes equal to the inlet concentration (NO breakthrough), with continuous analysis of all the gaseous products; (ii) temperature-programmed desorption (TPD) in He at 5 "C/min to 900 "C, subsequent to experiment (i); (iii) temperature-programmed reaction (TPR) at 5 "C/min, subsequent to experiment (i). Prior to experiment (i), the samples were heat-treated in He at 50 "C/min and held at 900 "C for 10 min. (17)Okuhara, T.; Tanaka, K. J. Chem. Soc., Faraday Trans. 1986, 82, 3657. (18)Illan-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 1990,p 68.
Figure 1. Temperature-programmed reaction (TPR) profiles of NO reduction for activated carbon and carbons derived from it by impregnation with potassium.
3. Results and Discussion 3.1. Temperature-ProgrammedReaction Studies. Figure 1 shows the changes in the TPR profiles of NO reduction with increasing catalyst loading. In agreement with the results obtained for potassiumloaded carbons prepared by ion exchange,2 three reactivity regions are observed for catalyzed NO reduction: (a) a maximum in NO reduction a t low temperatures, (b) an intermediate temperature region in which the reactivity becomes negligible, and (c) 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 carbon are the appearance of the reduction maximum at low temperature and a significant reduction in the temperature at which 100% reduction is achieved (Tloo). In comparison with the potassiumexchanged sample,2 somewhat lower activity is observed; the third region starts at ca. 280 "C for the ionexchanged sample while ca. 325 "C is needed for the sample prepared by impregnation. Potassium content is seen to have a dramatic effect in the first stage, and thus highlights the role of the catalyst in NO reduction. In contrast, the last stage of the reaction (T > 325 "C) is seen to be practically independent of potassium content. Consequently, as catalyst effectiveness in the first stage increases, the intermediate low-reactivity stage becomes increasingly narrow. The analysis of reaction products, mainly N20, N2, NO, CO2, and CO, provides essential complementary information, summarized in Figure 2. The same three regions of reactivity are observed: (i) At low temperatures, NO reduction is high, resulting in the production of N2 and N2O. Carbon oxides are not observed among the reaction products. This stage -~ of NO appears to involve i r r e ~ e r s i b l e ~chemisorption with oxygen retention by the catalyst. (ii) In the second, intermediate-temperature stage, N2 and N20 evolution continues, but severe catalyst deactivation is observed. (iii)In the third stage, N2 evolution becomes constant. It is accompanied by the evolution of CO2 and catalyst effectiveness is recovered. The quantity of C02 evolved does not correspond stoichiometrically to that of N2; an excess of COz is observed, which must be related to oxygen retention in the first stage. Eventually, C 0 2
Illan-Gmez et al.
106 Energy & Fuels, Vol. 9, No. I , 1995 n bmol/gUs)
n @mol/g C/s)
my
a)
CO -NO
0.6
C02-y0
0.5
\
0.4
0.4 OS
0.2
0.3
n
n
"I00
200
500
400
300
600
700
800-
T PC)
0.2
0.1
n
0.6 1
0
20
40
60 t
0.4
so
100
120
(min)
Figure 3. Isothermal NO reduction activity as a function of 0.5 0.2
0 100
0
200
400
300
500
600
700
800
T ("2) 1
n @mol& C/s)
n bmol/g Cis) C)
9%
CO -NO
2
CO,-\O
A
I\
0.6
I
0.4 OS
100
200
300
- .
400
500
*j*
600
700
0 800
T ("2)
Figure 2. TPR molar flow (n)profiles during NO reduction for potassium-impregnated carbons having increasing potassium loadings: (a) K-UA1-2.8; (b) K-UA1-4.6; (c) K-UA1-7.4.
evolution decreases and CO becomes the dominant oxygen-containing product. Even though the three samples share these common features, a more detailed analysis of Figure 2 shows clear effects of increasing potassium content: (a) In the first stage, NzO production decreases and Nz production increases with increasing % K. (b) Because the same carbon substrate is used,2 the evolution of COz and CO starts a t the same temperature for all the samples in this series. However, the COZexcess, characteristic of the third stage, increases with increasing % K (i.e., 159, 348, and 557 pmol/g C/s for K-UA1-2.8, K-UA1-4.6, and K-UA1-7.4, respectively). The oxygen and nitrogen balances are similar to those observed for potassium-exchanged carbons.2 They provide interesting information which allows us to complement the interpretation of the distribution of products presented above. For example, the oxygen balance is negative in the first reaction stage, which confirms its accumulation on the reduced potassium surface (e.g.,
time for potassium-impregnated carbon K-UA1-4.6.
KO,, where y 2 01, forming a more oxidized species (&O,+I).~ This oxygen is eventually transferred t o the carbon active sites and appears in the gas phase, at higher temperatures, as COZ and CO. The oxygen balance becomes positive in the latter reaction stages as a consequence of this decomposition of accumulated carbon-oxygen surface complexes. The balance of nitrogen is also negative in the first reaction stage, suggesting that, in addition to the dissociative NO chemisorption resulting in the formation of Nz and NzO, longer-lived adsorbed nitrogen species (e.g., NO(ad), N(ad), andor NzO(ad))are also retained on the surface. After this first stage, the nitrogen balance becomes positive as the retained species desorb from the surface. Possible mechanistic implications of these findings are discussed in more detail el~ewhere.~ 3.2. Isothermal Reaction Studies. Figure 3 shows typical isothermal reactivity data, which are similar to those obtained for both pure carbons1 and carbons loaded with calcium,lgiron,20and ion-exchanged potassium.2 Very high activity is seen to be maintained by this potassium-doped carbon at 500 "C and above. Figure 4 summarizes the catalytic activity data at low temperatures, at which the role of potassium content is more critical. 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 min). When the results for an ionexchanged sample, with 1.9 % K, are also included, a clear trend is observed at 400 "C. The decrease in specific activity (per gram of K)with increasing catalyst loading suggests a concomitant decrease in catalyst dispersion and highlights the importance of catalysU substrate interfacial area in determining the concentration of catalytically active sites (CAS). In contrast: at (19)Illhn-G6mez,M. J.; Linares-Solano,A.;Radovic, L. R.; SalinasMartinez de Lecea, C. NO Reduction by Activated Carbons. 4. Catalytic Effect of Calcium. Energy Fuels, 1995, 9, 112. (20) Illh-G6mez, M. J.; Salinas-Martinez de Lecea, C.; Oya, A.; Linares-Solano, A. Proc. X Z I Simp. Zberoamericano Catril., Segovia, Spain 1992,p 351.
Energy &Fuels, Vol. 9,No. 1, 1995 107
NO Reduction by Activated Carbons 100 H300°C A40D'C
0.5
80 0.4
60 03
0.2
40
20 H
H
0
0
2
4
6
8
10
%K
Figure 4. Correlations between NO reduction activity and catalyst loading. 300 "C the specific activity is practically invariant with
increasing catalyst loading for impregnated samples, and higher for the ion-exchanged sample, suggesting a good correlation with catalyst dispersion. To understand these intriguing results, which are at variance with the catalytic effect of ion-exchanged potassium on different carbon substrates,2 a more detailed study of NO chemisorption by these samples was undertaken, as discussed below. 3.3. NO Chemisorption Studies. NO Chemisorption. Figure 5 shows the evolution of gaseous products during the NO chemisorption process at 60 "C on potassium-loaded samples. The breakthrough of NO is seen to be delayed as catalyst loading increases, further supporting the contention that the catalyst is responsible for NO retention. The appearance of N2 and N2O indicates that NO dissociation occurs upon adsorption. The imbalance between NO adsorbed and N20 and N2 produced confirms the retention of oxygen and nitrogen on the catalyst surface. Table 2 summarizes the results of the quantification of nitrogen retention and N2 and N2O evolution during chemisorption of NO on both the potassium-loaded samples and the parent carbon. The large increase in the adsorption capacity (NOR)with increasing % K is evident. It is also interesting to note that, in contrast to the COfi20 ratio (Table 11, the NO/K ratio is seen to decrease monotonically as % K increases. In a complementary study,2 it was concluded that C02 chemisorption does not titrate the catalytically active sites (CAS). This is confirmed by the data in Figure 4. The large decrease in specific catalytic activity at 400 "C with increasing catalyst loading cannot be explained if C 0 2 is assumed to titrate the CAS. It can be explained, at least in a qualitative sense, by assuming that NO chemisorption is related to the CAS,i.e., a decrease in the NOK ratio is interpreted as a decrease in the concentration of reduced KOy species. The establishment of a better quantitative correlation requires that distinction be made between reversible (nondissociative) and irreversible (dissociative) NO chemisorption, as discussed below.
t (min)
IO
t (min)
Figure 6. Molar flows (n)of gases during chemisorption of NO at 60 "C on potassium-impregnated carbons having increasing potassium loadings: (a) K-UA1-2.8;(b) K-UA1-4.6; (c) K-UA1-7.4.
Illan-Gbmez et al.
108 Energy &Fuels, Vol. 9, No. 1, 1995
Table 2. Retained (R)and Evolved Species during NO Chemisorptionat 60 "Ca NOiK NOR (mol/mol) NzO
sample
K-UA1 K-UA1-2.8 K-UA1-4.6 K-UA1-7.4
1.4 1.2 0.9
85 970 1380 1740
In pmol/g of C. 2Nz.
excess
NR'
OR
85 85 52 649 224 146 980 288 230 1259 318
425 692 941
Nz
OR = NOR - N2O.
a
2000
-
321 400 481
OR^
helpful in clarifying these results:
0
NR = NOR - 2Nz0
-
NO @mol/g C) 400'C
V300'C
I500
1000
i
I ,
rate -gK
-
catalyst surface area catalytic surface area gK catalyst surface area
4
1
In conventional heterogeneous catalysis, the catalyst surface area (percentage exposed or dispersion) is determined most frequently by chemisorption of a suitable gas at around room temperature. The second term on the right is unity for structure-insensitive reactions and is dependent on catalyst dispersion for structure-sensitive reactions. In carbon gasification reactions, both uncatalyzed and ~ a t a l y z e dthere , ~ ~ is ~~~ ,~~ increasing evidence of "structure ~ e n s i t i v i t y "which complicates the establishment of structure/(re)activity correlations. Good quantitative explanations of observed trends in carbon reactivity or catalytic activity are rarely obtained by considering only the variations in low-temperature chemisorption capacity (e.g., of 0 2 for uncatalyzed gasification, or C02 for calcium-catalyzed gasification). For catalyzed gasification in particular, knowledge of the catalystkarbon contact, i.e., the interfacial surface area, is also necessary (though not always sufficient). Indeed, in the case of potassium, it was recently concluded2 that CO2 chemisorption titrates not the sites in this interfacial region (K,O,) but primarily the inactive (e.g., K20) sites, which are not in contact with the carbon substrate. Therefore, the relevant catalyst surface area here is that of the catalystlcarbon contact (e.g., substoichiometric oxide K,O,); it is determined by dissociative NO chemisorption. Whether dissociative NO chemisorption is a good measure of the catalytic surface area as well is still an open question, as can be concluded from a more detailed analysis of Figures 4 and 6. For example, the existence of a better correlation in Figure 6 at 300 "C than at 400 "C, correlation coefficients being 0.92 and 0.83, respectively, suggests that dissociative NO chemisorption is the rate-determining step (RDS) at lower temperatures (in which case the second term on the right may be irrelevant). At higher temperatures, the RDS may shift toward a greater importance of the oxygen transfer from the catalyst to the carbon. A more conclusive resolution of this issue will require the use of transient kinetics te~hniques.~~,~~,~~ TPD Subsequent to NO Chemisorption. Upon completion of the chemisorption process, i.e., after complete NO breakthrough is achieved, the samples were subjected to TPD. Figure 7 illustrates the spectra obtained for sample K-UA1-4.6 and the parent carbon, while Table 3 presents a quantitative summary of all the data. The appearance of NO confirms the importance of nondissociative chemisorption. As the potas(22) Carberry, J. J . J . Catal. 1988,114, 277. (23)Lizzio, A. A.; Radovic, L. R. I d . Eng. Chem. Res. 1991,30,1735. (24) Radovic, L. R.; Lizzio, A. A.; Jiang, H. In Fundamental Issues
Figure 6 shows that this excess oxygen on the catalyst in Control of Carbon Gasification Reactivity; Lahaye, J., Ehrburger, surface is a good measure of the concentration of P., Eds.; Kluwer Academic Publishers: Dordrecht, The Netherlands, catalytically active sites, particularly at the lower 1991; p 235. (25) Cazorla- Amords, D., Linares-Solano, A.; Radovic, L. R; Salinasreaction temperature. The following fundamental rate Martinez de Lecea, C. Calcium as Catalyst for Carbon Gasification. expression for catalyzed carbon g a s i f i ~ a t i o n ~ is , ~ ~ - ~Energy ~ Fuels, submitted for publication. (21) Kapteijn, F.; Moulijn, J. A. In Carbon and Coal Gasification, NATO AS1 Series E 105; Figueiredo, J. L., Moulijn, J. A., Eds.; Martinus Nijhoff Publishers: Amsterdam, 1986; p 291.
(26) Cazorla-Amor6s, D.; Linares-Solano, A.; Marcilla-Gomis,A. F.; Salinas-Martinez de Lecea, C. Energy Fuels 1991,5,799. (27) Cazorla-Amor6s, D.; Linares-Solano, A.; Salinas-Martinez de Lecea, C.; Kapteijn, F. Carbon 1994,32,423.
Energy & Fuels, Vol.9,No.1, 1995 109
NO Reduction by Activated Carbons
4
01 0
-co2---co--NO
100
m
300
100
200
300
I
400
500
600
700
800
400
500
600
700
800
03
03
0.1
n 0
T PC)
Figure 7. Temperature-programmed desorption molar flow (n) profiles for activated carbon K-UA1 (a) and potassiumimpregnated carbon K-UA1-4.6 (b). Table 3. Products Evolved upon TPD Subsequent to NO Chemisorption at 60 "C NO coz co sample &moVg) %NO, &moVg) &moVg) K-UA1 K-UA1-2.8 K-UA1-4.6 K-UA1-7.4
85 213 291 305
100 22 21 18
8 204 203 264
+
597 398 405 455
sium loading increases, however, the fraction of NO that adsorbs in this manner (% NO,) remains practically constant. The absence of NOin TPD spectra suggests that the predominant adsorption processes (on a surface site S) are
rather than
NO
+ 2s - S ( 0 ) + S(N)
sorption on potassium-loaded carbons indeed occurs primarily according to reactions 1. A comparison of NO desorption peaks for the two samples (Figure 7) shows both a marked intensity increase and a shift toward higher temperatures in the presence of potassium. In similar experiments, Okuhara and Tanaka17observed also an NO desorption peak at 360 "C, which was accompanied by another peak a t lower temperatures. The absence of the low-temperature peak (at ca. 100 "C) in Figure 7b suggests that potassium now occupies (or blocks) the carbon active sites that are capable of chemisorbing NO (Figure 7a), e.g., those sites associated with surface oxygen groups, and therefore inhibits the uncatalyzed NO adsorption. The complexity of the CO and C02 evolution profiles, in both TPD (Figure 7) and TPR (Figure 21, is a consequence of the well-known energetic heterogeneity of the carbon surface.30 In the present case, this heterogeneity is manifested, upon transfer of oxygen from the oxidized potassium catalytic species (e.g., &O,+l) t o the carbon active sites, in the formation of carbon-oxygen surface complexes that have different desorption activation energies. In this sense, the results in Figure 7 and Table 3 could, in principle, be discussed by invoking analogies2 with other carbon gasification reactions where oxygen transfer has been investigated in greater detai1.6-8~24~30,3i They are complicated, however, by the unknown distribution of oxygen, at any given time, between the catalyst and the carbon surface. The effect of potassium on C02 evolution and the CO/ C02 ratio is clear. As the oxygen coverage of the carbon surface increases in the presence of potassium, there is an increase both in the probability of forming the C(0) surface intermediate at two adjacent carbon active sites and in the occurrence of the reaction CO C(0) = C02; both processes favor increased CO2 evolution and a decrease in the CO/C02 ratio (Table 3). There is no clear correlation, however, between the amount of CO2 evolved in TPD, or the temperatures of its evolution either in TPD or TPR, and the catalytic activity. As expected, based on our previous study,2where different carbons were used and the catalytic activity was determined in part by the reactivity of the substrate, the influence of the carbon is much less important here than that of the catalystlcarbon interface. It is interesting to note in Table 3 the high CO yield (and the resulting very high CO/C02 ratio) in TPD of the pure carbon K-UA1. In a previous study,' excess CO was also reported during TPR. It was attributed to the destabilizing effect of NO on the high-temperature CO-forming complexes. The fact that a similar result is obtained during TPD, i.e., in the absence of NO, indicates that an additional effect is a t least as important, if not more so. This factor is thought to be the high affinity of a clean carbon surface toward NO. In other words, very reactive sites on the carbon surface, formed during surface cleaning prior to the NO chemisorption experiment, are responsible for the formation of relatively stable C-0 complexes upon first exposure
(2)
This is in agreement with the finding that on pure carbons there is a second-order dependence of chemi~ is contrary, sorption rate on NO partial p r e ~ s u r e . It however, to the recent findings of Chu and Schmidt28 and Suzuki et who report evidence of C(N) surface specie^.^ A very good correlation is seen to exist between the quantity of nitrogen retained on the surface (Table 2) and the quantity of NO desorbed in TPD (Table 3). This is convincing evidence that NO chemi(28) Chu, X.;Schmidt, L. D. Ind. Eng. Chem. Res. 1993,32,1359. (29) Suzuki,T.; Kyotani, T.; Tomita, A. Ind. Eng.Chem. Res. 1994, 33,2.
~~
~
(30) Calo, J. M.; Hall, P. J. In Fundamental Issues in Control of Carbon Gasifiatwn Reactivity; Lahaye, J.,Ehrburger, P., Eds.; Kluwer Academic Publishers: Dordrecht, The Netherlands, 1991; p 329. (31) Hiittinger, K. J. In Fundamental Issues in Control of Carbon Gusifiation Reactiuity; Lahaye, J., Ehrburger, P., Eds.; Kluwer Academic Publishers: Dordrecht, The Netherlands, 1991; p 163.
110 Energy &Fuels, Vol. 9, No. 1, 1995 100
Illan-Gbmez et al.
5%NO reductlon
80
60
O8
40
I
I
15
20 0
.M -40
05
-60
03
-80 -3""
100
200
300
400
500
600
n "100
200
300
400
500
T ('C)
n OLmol/g C/s)
chemisorption for potassium-impregnated carbons having increasing potassium content.
(32) Illan-Gbmez, M. J.; Linares-Solano, A.; Salinas-Martinez de Lecea, C. NO Reduction by Activated Carbons. 6. Catalytic Effect of Transition Metals. Energy Fuels, submitted for publication.
700
800
n 900"
T ('C)
Figure 8. TPR profiles of NO reduction subsequent to NO
to NO. (This effect cannot be detected by gas chromatography and does not affect, therefore, the nitrogen balances.) TPR Subsequent to NO Chemisorption. Figures 8 and 9 show the results of the temperature-programmed reaction experiments performed with the potassium-loaded carbons after NO chemisorption. The effect of increasing potassium loading on NO uptakes (and subsequent desorption) is evident. These experiments were designed in an attempt to differentiate and quantify the processes illustrated in Figures 1 and 2, which apparently occurred simultaneously. In Figure 8 three stages are again distinguishable but are different from those observed in Figure 1. At low temperatures (first stage), there is no NO reduction. At intermediate temperatures (second stage), "negative"reduction is observed, due to the desorption of NO that was retained on the surface during chemisorption. At high temperatures (third stage), the degree of NO reduction continuously increases until complete reduction is achieved at ca. 500 "C. These three stages are readily interpreted using the results of Figure 9 and comparing them with those shown in Figure 2. The main difference is seen to be the disappearance of the first reaction stage, in which NO reduction was accompanied by N20 and N2 evolution. Therefore, when the catalyst becomes inactive, as a consequence of its conversion to the oxidized state (upon retention of oxygen during NO chemisorption), negligible NO reduction is found (Figure 8) and negligible Nz and NzO evolution occurs (Figure 9). In contrast, once COZevolution becomes important (at ca. 300 "C in these experiments), the TPR profiles in Figures 2 and 9 are essentially identical. This set of experiments fully confirms the importance of dissociative NO chemisorption step and the necessity of having potassium in reduced form (potassium andl or substoichiometric potassium oxide) as the catalytically active species. This observation is in turn of paramount importance in understanding the different catalytic effects exhibited by different metals, which are controlled by their redox characteristic^.^^ A comparison of Figuress 9b and 7b shows that the NO maximum in the second stage appears at the same temperature, which confirms that this excess NO cor-
600
n OLmollg Us)
b)
2
-N, --CO --NO "'COz
0.8
0.6
0.4
0.2
0-
05 0.2
0 100
200
300
500
400
600
700
0 800
T
Figure 9. TPR molar flow (n)profiles during NO reduction subsequent to NO chemisorption for potassium-impregnated
carbons having increasingpotassium loadings: (a)K-UA1-2.8; (b) K-UA1-4.6;(c) K-UA1-7.4.
responds to the desorption of NO retained during chemisorption prior to TPR. In the third stage, the decrease in NO concentration coincides with the onset of Nz and CO2 evolution. At temperatures above 500 "C, CO evolution begins and above 700 "C CO becomes the dominant product. The evolution of CO2 exceeds that of Nz in proportion with potassium loading. Finally, the comparison of this excess with that observed in TPD subsequent to NO chemisorption indicates that the origin of CO2 is the same, i.e., decomposition of carbon-oxygen surface complexes formed upon oxygen transfer from the oxidized catalyst to the free carbon sites. 4. Conclusions
The complementary studies of both isothermal and temperature-programmed reduction of NO with potassium-exchanged2and potassium-impregnated carbons, combined with NO chemisorption followed by tempera-
Energy &Fuels, Vol. 9,No. I, 1995 111
NO Reduction by Activated Carbons ture-programmed desorption and temperature-programmed reaction, have allowed us to gain a very good understanding of the role of potassium in catalyzing NO reduction by carbon. In particular, a decoupling of the effects of catalyst loading (and dispersion) and carbon reactivity has allowed us to establish a correlation between catalytic activity and dissociative NO chemisorption. Furthermore, it is now clear why potassium is an excellent NO reduction catalyst. Potassium is very active in dissociating NO, even a t temperatures as low as 60 "C, because it is readily reduced by the carbon substrate. Its chemical characteristics are suitable for completing the catalytic oxidationheduction cycle. The first stage of the, reaction involves dissociative chemisorption of NO with oxygen retention by the catalytically active species. The oxygen accumulates on the reduced potassium surface (e.g., K,O,), forming a more oxidized species (&Oy+d.This
oxygen is eventually transferred to the carbon active sites, and appears in the gas phase, at higher temperatures, as C02 and CO. Also, nondissociative chemisorption of NO occurs a t temperatures lower than 300 "C. The absence of N2 in the TPD spectra, within the detection limits of the gas chromatograph used in this study, excludes the possibility of formation of C(N) groups under our experimental conditions.
Acknowledgment. This study was made possible by the financial support from DGICYT (projectsAMB921032-CO2-02)and (E91-0011-C03-01).The thesis grant for M.J.I.G. and an invited research grant to L.R.R. from Generalitat Valenciana are also gratefully acknowledged. EF940125N
