Energy & Fuels 1995,9, 540-548
540
NO Reduction by Activated Carbons. 5. Catalytic Effect of Iron M. Jose Illan-G6mez, Angel Linares-Solano," Ljubisa R. Radovic,? and Concepci6n Salinas-Martinez de Lecea Department of Inorganic Chemistry, University of Alicante, Alicante, Spain Received November 30, 1994@
The effect of iron as catalyst of the NO-carbon reaction has been investigated. A coal-derived carbon was loaded with iron using different methods and different precursors. A brief exploratory study was also conducted with pitch-derived carbon fibers. The iron-loaded andlor parent carbons were characterized by physical adsorption of CO2 (at 0 "C) and N2 (at -196 "0, X-ray absorption fine structure spectroscopy (XAFS),and chemisorption of CO at 25 "C. The NO-carbon reaction was studied in a fixed-bed flow reactor at atmospheric pressure using two types of experiments: (i) temperature-programmed reaction (TPR) in a NO/He mixture, and (ii)isothermal reaction a t 300-600 "C. The reaction products were monitored in both cases, thus allowing detailed oxygen and nitrogen balances to be determined. Iron was found to catalyze NO reduction by carbon through an oxidationheduction (redox) mechanism similar to that reported previously for potassium- and calcium-catalyzed reaction. Nevertheless, the iron species present on the carbon surface before NO reduction (Fe,O, or FeO) are less effective than the potassium species (elemental potassium or potassium suboxide) in chemisorbing NO, as a result of which they transfer less oxygen to the carbon active sites. The results show also that the nature of the catalyst precursor, the catalyst preparation conditions and the reducibility of the catalyst by the carbon determine the chemical state of the catalyst, its dispersion and catalysthubstrate contact, and hence control the catalytic activity of iron in NO reduction by carbon.
1. Introduction
The first stage in the NO-carbon reaction is the chemisorption of NO molecules on the carbon ~ u r f a c e . l - ~ In a previous paper,4 we reported that NO chemisorption on activated carbons was enhanced in the presence of metallic impurities, in agreement with the results of other studies.5-10 Indeed, in a n extensive study of the enhancement of NO adsorption capacity of activated carbons and activated carbon fibers, Kaneko and Inouye7 have reported the effectiveness of iron added in the form of different precursors (a-FeOOH, P-FeOOH, y-FeOOH, and Fe304). Iron's potential as a NO reduction catalyst also stems from its effectiveness as a catalyst for carbon gasification. In numerous studies of catalyzed carbon gasification-in C02,02, and H2O-it has been found that this effectiveness depends on iron's oxidation state. The presence of highly dispersed zerovalent, metallic iron is essential for high catalytic a c t i ~ i t y . l l - ~ ~
* Author to whom correspondence should be addressed. ' Permanent address: Department of Materials Science and Engineering, The Pennsylvania State University, University Park, PA 16802. @Abstractpublished in Advance ACS Abstracts, April 15, 1995. (1)Teng, H.; Suuberg, E. M.; Calo, J. M. Energy Fuels 1992,6, 398. (2) Teng, H.; Suuberg, H. Ind. Eng. Chem. Res. 1993, 32, 416. (3) Teng, H.; Suuberg, E. M. J . Phys. Chem. 1993, 97, 478. (4) Illan-Gomez, M. J.; Linares Solano, A,; Radovic. L. R.; Salinas Martinez de Lecea, C. Energy Fuels 1995, 9, 104. (5)Kaneko, K.; Inouye, K. Carbon 1984,24, 772. (6) Kaneko, K.; Inouye K. J . Chem. Technol. Biotechnol, 1987, 37, 11. ( 7 )Kaneko, K.; Inouye, K. Adsorpt. Sci. Technol. 1988, 5, 11. ( 8 )Kaneko, K.; Fukuzaki, N.; Ozeki, S. J . Chem. Phys. 1987, 87, 776. (9) Kaneko, K. In Characterization of Porous Solids; Unger, K. K., et al., Eds.; Elsevier Science Publishers: Amsterdam, 1988; p 183.
0887-0624/95/2509-0540$09.00/0
In carbon gasification, iron (as well as many other catalysts) participates in a n oxygen-transfer process,12-15,20which includes a redox cycle between two iron species in different oxidation states, Fe,O, and Fe,O,(O). Therefore, the activity of the metal is conditioned by its capacity to accept oxygen. In recent studies, Tomita and co-workers21,22found that, in ironcatalyzed carbon gasification with HzO, the fraction of metal reduced after heat treatment in inert atmosphere at 650 "C depends on the catalyst precursor used and the nature of the carbon. Similarly, Kaneko and Inouye7 reported that the degree of enhancement of NO adsorption capacity of activated carbon fibers depends on the iron species selected (a-FeOOH or P-FeOOH). (10) Kaneko, K., Ozeki, S.; Inouye K. Atmos. Environ. 1987, 21, 2053. (11)Everett, D. H.; Powl, J. C. J . Chem. SOC.Faraday Trans. 1976, 72, 619. (12) Walker, P. L. Jr.; Shelef, M.; Anderson, R. A. In Chemistry and Physics ofCarbon; Walker Jr., P. L., Ed.; Marcel Dekker: New York, 1968; Vol. 4,p 287. (13) Mckee, D. W. In Chemistry and Physics of Carbon, Walker, Jr., P. L., and Thrower, P. A,, Eds.; Marcel Dekker: New York, 1981, Val. 16, p 1. (14) Huttinger, K. J.;Alder, J.; Hermann. G. In Carbon and Coal Gasification; NATO AS1 Series, Figuereido, J . L., Moulijn, J . A,, Eds.; Martinus Nijheff Publishers: Dordrecht, The Netherlands; 1986; p 213. 115) Kasaoka, S.; Sakata, Y.; Yamashita, H.; Nishino T. Int. Chem. Engl. 1981, 21, 419. (16) Ohtsuka, Y.; Kuroda, Y.; Tamai, Y.; Tomita, A. Fuel 1986, 65, 1476. 117) Ohtsuka, Y.; Tamai, Y.; Tomita, A. Energy Fuels 1987, I , 32. (181Furimsky, E.; Sears, P.; Suzuki, T.; Watanabe, Y. Energy Fuels 1988, 2 , 634. 119) Suzuki, T.; Inoue, K.; Watanabe, Y. Energy Fuels 1988,2,673. 120) Yamashita, H.; Yoshida, S.; Tomita, A. Energy Fuels 1991, 5, 52. ( 2 1 )Yamashita, H.; Tomita, A. Ind. Eng. Chem. Res. 1993,32,409. (22) Yamashita, H.; Ohtsuka, Y.; Yoshida, S.; Tomita, A. Energy Fuels 1989, 3, 686.
0 1995 American Chemical Society
Energy & Fuels, Vol. 9, No. 3, 1995 641
NO Reduction by Activated Carbons
Considering the facts that iron enhances NO adsorption on activated carbons and that it is a good carbon gasification catalyst, it is of interest to analyze its catalytic effect in NO reduction by carbon. In the present study, we first compare the effectiveness of iron added to carbon in two very different manners. We then analyze the effects of iron precursor and iron loading on the kinetics of NO reduction. 2. Experimental Section Two carbons of different origin and surface properties were studied: F and K-UA1. The former is a pitch-based carbon fiberz3while the latter is an anthracite-derived carbon whose activation was carried out with KOH.24 Iron was loaded onto the carbon by the following methods. For carbon F, it was added in the process of fiber preparation, at the stage of fiber fusion and spinning.z3 This sample is designated as F-Fe. For sample K-UA1, iron was introduced using three different precursors: a-FeOOH, P-FeOOH, and Fe(N03)3. The synthesis of a-FeOOH and P-FeOOH was accomplished directly in contact with the carbon, as f01lows:~(i) starting with iron (+3) sulfate, at pH = 13 and 30 "C, a-FeOOH was precipitated over a period of 6 h; (ii) starting with FeC13, in the presence of urea, at pH = 2 and 100 "C, P-FeOOH was precipitated over a period of 4 h. The samples were subsequently washed with distilled water (2 L/g of carbon) and dried overnight at 110 "C. Carbon loading with iron nitrate was done by impregnation from aqueous solution; the solvent was removed by bubbling nitrogen through the sample and subsequently drying it overnight at 110 "C. These samples are designated as K-UA1-aFe, K-UA1-PFe, and K-UA1-Fe. Iron content in the samples was determined by atomic absorption spectroscopy. For this purpose, a sample of ash, obtained by combustion of each iron-loaded carbon at 800 "C in a muffle furnace, was dissolved in concentrated solution of HC1 and analyzed. The catalyst particles on the carbon surface were characterized by X-ray absorption fine structure (XAFS)spectroscopy and CO chemisorption was used for the chemisorption experiments. The EXAFS spectra were obtained in the National Laboratory for High-Energy Physics (KEK-PF), Tsukuba, Japan. A mass spectrometer (Micromass PC, VG Quadrupole) connected to a flow reactor was used for chemisorption experiments; a 10% CO/90% He mixture (60 cm3(STP)/min) was passed over the sample (ca. 200 mg) maintained at 25 "C, until complete CO breakthrough. Because the amounts chemisorbed were relatively small, it was deemed to be more precise to determine them by integrating the CO peaks obtained during subsequent temperature-programmed desorption (in He at 20 "C/min, up to 500 "C). The kinetics of the NO-carbon reaction were studied at atmospheric pressure in a flow reactor (15 mm i.d.; ca. 300 mg sample) connected to a gas chromatograph (Hewlett Packard 5890A). The gaseous products were analyzed (every 3 min) using a 2 m Porapak Q 80/100 column at 30 "C 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); (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 t o an in situ heat treatment in He, at 50 "C/min to 900 "C for 10 min. In case (i),the temperature was lowered t o 20 "C, the reactant mixture was substituted for He, and the TPR experiment was per(23) Otani, S.; Oya, A. Proc. 3rd Japan-US Conf Composite Mater., Tokyo, 1986,1. (24) Illan-Gomez, 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, 1990,68.
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Figure 1. Temperature-programmed reaction (TPR) profiles of NO reduction for pitch-derived carbon F (a), and coal-derived carbon K-UA1 (b), in the presence and absence of iron. formed. In case (ii), the temperature was lowered to the desired level and the isothermal experiment was initiated by substituting the NOMe 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. Catalytic Effect of Iron. Samples K-UA1 and F, having widely differing physical surface proper tie^,^^ were used to study the catalytic effect of iron. The former sample is a microporous activated carbon possessing high adsorption capacity (SN~ = 1790 m2/g;Scoz = 1785 m2/g). The latter consists of carbon fibers having negligible N2 adsorption capacity (at -196 "C), but whose C02 adsorption capacity at 0 "C is considerable (SCO,= 507 m2/g);these numbers suggest that sample F possesses very narrow micropores, in which the diffusion of N2 at -196 "C is very slow. Figure 1 shows the temperature-programmed reaction (TPR) profiles for the iron-loaded carbons, together with those for the parent iron-free carbons. No significant change is observed in these NO reduction vs temperature profiles when iron is added to sample F (5,2%Fe); the observed sigmoidal profiles (Figure la) are typical for carbons in the absence of NO reduction catalysts.25 The catalytic effect of iron is manifested in a displacement of the profile toward lower temperatures. Appreciable NO reduction is seen to start at -300 "C; complete reduction occurs at -720 "C Ke., -160 "C below that observed for sample F). Figure l b shows that, in the case of sample K-UA1(4,7%Fe), the catalyst modifies both the characteristic temperatures and the entire TPR profile. This is similar to the behavior of (25) IllAn-G6mez, M. J.; Salinas Martinez de Lecea, C.; Linares Solano, A,; Calo, J. M. Energy Fuels 1993,7, 146.
Illan-Gomez et al.
"C) is characterized by the evolution of NZand NzO as the only reaction products. The N20/N2 ratio is higher in the presence of iron than in the presence of potas0.8 - 15 sium? This stage thus appears to consist of dissociative chemisorption of NO accompanied by N2O and N2 evolution and oxygen accumulation on the surface. In the subsequent chemisorption stage (280 < T < 400 "C), only N2 evolution is observed, accompanied by significant catalyst deactivation as a consequence of further surface oxygen accumulation. In the third stage (400 < T < 630 "C), C02 evolution is observed, accompanied by the recovery of catalyst effectiveness. In agreement "100 200 300 400 500 600 700 800 900, ~ratio ~,~~,~~ with K- and Ca-catalyzed NO r e d u ~ t i o n , ~the T ('C) (C02 evolved)/(Nz evolved) is greater than 1. In the last Figure 2. TPR molar flow (n) profiles of the products of NO stage (T> 630 "C), N2 evolution becomes constant, C02 reduction by the iron-loaded carbon F-Fe. evolution continues, and CO also appears among the n Olmollg Cis) n (IrmoUg C/s) reaction products, becoming dominant above -750 "C. 25 IY-NO - CO ."CO,-&O Iron-catalyzed NO reduction by K-UA1-Fe is unique in the appearance of a clearly defined CO peak, at -800 0.8 "C, superimposed on the peak characteristic of the uncatalyzed NO reduction. The unique CO evolution 0.6 1 should be related with the reduction of the oxidized iron species by carbon, as supported by the following two ob0.4 servations: (a) the maximun temperature evolution of the CO agrees with the reduction temperature of iron 0.2 oxide by carbon29and (b) quantification of the CO peak (subtracting the CO corresponding t o the iron-free , _.-, 0 0 parent carbon) gives a pmol of CO/pmol of iron ratio 100 200 300 400 500 600 700 800 900 close t o 1. T ('C) Oxygen and nitrogen balances can be obtained readily Figure 3. TPR molar flow ( n ) profiles of the products of NO reduction by the nitrate-derived iron-loaded carbon K-UA1from the data presented above, and they provide adFe . ditional interesting information. For example, for sample K-UA1-Fe, the oxygen balance is negative between 200 potassium as a catalyst of NO reduction by ~ a r b o n . ~ , ~and ~ , 400 ~ ~ "C, as a consequence of its accumulation on the The characteristic low-temperature maximum occurs a t surface. The number of moles of accumulated oxygen -325 " C ,while 100% reduction takes place at -600 " C , (obtained by integration of the TPR profile in the much lower temperature than for the parent carbon (Kappropriate temperature interval) corresponds to the UA1, -750 "C). Important complementary information number of moles of iron contained in the sample. This is contained in Figures 2 and 3 which show the evolution suggests that oxygen is retained by the iron; being in of gaseous products during TPR. reduced form (subsequent to sample pretreatment), its For iron-loaded sample F-Fe (Figure 21, the evolution affinity toward oxygen is greater than that of the carof gaseous products during TPR is similar to that bon active sites. At temperatures above 600 "C, the observed for the parent carbon.25 In the presence of oxygen balance becomes positive; as the temperature iron, all the events Le., N2 and C02 evolution) are increase, previously retained oxygen is transferred to displaced toward lower temperatures. Between 200 and the carbon active sites and desorberd as COS and CO. 400 "C, a small quantity of N2 0 is also observed among In agreement with the results obtained both in the the products. absence of a catalyst25and in the presence of potasFor iron-loaded sample K-UA1 (Figure 3), the evolusium and c a l c i ~ m , much ~ , ~ ~more , ~ ~oxygen appears in tion of gaseous products is very different from that of this stage than can be accounted for from its accumuthe parent carbon. In agreement with the results for lattion based on monitoring of the reaction products potassium-catalyzed NO reduction by carbon,4*25'26 the with the gas chromatograph. In contrast for the relamain feature of these low temperature profiles is the tively inactive sample F-Fe, this excess oxygen is not appearance of N20 as a major product. Similar findings observed. were reported by Imai et al.;27these authors observed The nitrogen balance is more straightforward in the N2O evolution between 100 and 300 "C during NO entire range of temperatures. In contrast to the Kreduction by a-FeOOH-doped activated carbon fibers. , ~ , ~in~agreement with the catalyzed NO r e d ~ c t i o n and On closer inspection of Figure 3, an interesting differresults for Ca-catalyzed reaction,28 there is no retenence is observed with respect to K-catalyzed NO reduction of nitrogen-containing products. Possible mechat i ~ n : (a) ~ , a~ less ~ pronounced displacement of the nistic implications of these results are analyzed elseprofiles toward lower temperatures and (b) the appearwhere.30 ance of an additional TPR feature or reaction stage. Indeed, the first, low-temperature stage (100 < T < 280 (28) Illan-Gomez, M. J.; Linares Solano, A,; Radovic. L.: Salinas -Nz
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(26) Illan-Gbmez, M. J.;Linares Solano, A.; Radovic, L. R.; Salinas Martinez de Lecea, C. Energy Fuels 1995,9 , 97. 127) Imai, J.; Takaomi, S.; Kaneko, K. Int. Symp. Carbon Tsukuba, 1990,288.
Martinez de Lecea, C. Energy Fuels 1995,9, 112. (29) Gilchrist, J. D. Extraction Metallurgy; Int. Ser. Mater. Sci. Technol., Vol. 30; Pergamon Press: London, 1980. (30)Illan-Gbmez, M. J.; Linares Solano, A,; Radovic, L. R.; Salinas Martinez de Lecea, C. Energy Fuels, submitted for publication.
Energy & Fuels, Vol. 9, NO.3, 1995 643
NO Reduction by Activated Carbons 100
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Figure 4. Dependence of the TPR profiles of NO reduction by iron-loaded coal-derivedcarbon on the nature of the catalyst precursor.
Following the pattern of the very informative studies of the potassium-catalyzed NO chemi~orption,~ the behavior of sample K-UA1-Fe was analyzed during NO chemisorption at 60 "C. Interestingly, even though iron clearly enhances the dissociative NO chemisorption, the quantity of NO retained, -104 pmollg, is not much larger than that retained in the parent iron-free carbon (K-UA1, -85 pmol/g). Given the one-to-one relationship between the number of moles of accumulated oxygen and iron in the sample, the low-temperature NO reduction maximum observed in the TPR profile (Figure 3) is attributed to the saturation of the reduced iron species with oxygen produced by dissociative NO chemisorption which occurs at temperatures higher than 60 "C. The above results reflect the importance of the method of catalyst incorporation into the carbon. The sample preparation step obviously determines the location of the catalyst within the porous structure of the carbon. Iron must be distributed in quite different ways in the two samples analyzed here. In the carbon fibers, sample F,23a large portion of the iron, which is added to the carbon precursor prior to carbonization, does not seem t o be present on the surface and is thus inaccessible to the gaseous reactants. In the anthracite-derived activated carbon, when iron is added by impregnation, the catalyst remains accessible to the gaseous reactants and is thus more effective. 3.2. Influence of Iron Catalyst Precursor. It is well documented in the literature on iron-catalyzed carbon gasification that the chemical state of iron is - ~ ~that the important for its catalytic a ~ t i v i t y l ~and fraction of the active species (reduced metal) depends on the catalyst precursor used as well as on the preparation method and carbon nature.21,22 On the basis of this knowledge, it was deemed necessary to determine how the chemical nature of the catalyst precursor affects the activity of iron in the NO-carbon reaction. Considering the results discussed in section 3.1, sample K-UA1 was chosen for this purpose. The two iron precursors are the same ones used previously by Kaneko and Inouye' a-FeOOH and /3-FeOOH. Iron loadings were 4.7 and 3.2 wt % for samples K-UA1-aFe and K-UA1-PFe,respectively. For the sake of completeness, the results obtained when iron nitrate was used as the catalyst precursor (see section 3.1) are also included. TPR Results. Figure 4 shows a comparison of the TPR profiles of the three iron-loaded samples with that
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Figure 5. TPR molar flow (n)profiles of the products of NO reduction by the FeOOH-derived iron-loaded carbon K-UA1: (a) K-UA1-aF'e; (b) K-UA1-PFe.
of the parent carbon, while Figure 5 presents the product evolution profiles for the two additional ironloaded samples. Sample K-UA1-aFe exhibits similar behavior t o that of K-TJA1-Fe (Figures l b and 31, although the characteristic points are displaced toward higher temperatures. Significant reduction starts at slightly above 200 "C, the low-temperature reduction maximum is observed at -430 "C, while 100% reduction is aEhieved at -735 "C, which is close to the temperature needed for 100% reduction of the parent carbon K-UA1(-750 "C). The most important difference with respect to sample K-UA1-Fe is the absence of the CO evolution peak at high temperatures. Sample K-UA1-PFe exhibits behavior that is very different from that of other samples in this series, and quite similar to that of the parent carbon (K-UA1).25The small catalytic effect of iron is clearly detected, however, in that the onset of reduction is observed at somewhat lower temperature; similarly to the behavior of sample K-UA1-aFe, 100% reduction is achieved at a temperature which is quite close to that of the parent iron-free carbon (-730 vs -750 "C). The most important phenomena observed in the presence of iron can thus be summarized as follows: 1. Nitrate- and a-FeOOH-derived iron is effective in dissociative NO chemisorption and the corresponding TPR profiles exhibit an NO reduction maximum at low temperatures. Dissociative NO chemisorption does not occur on P-FeOOH-derived iron. 2. Only nitrate-derived iron has a major catalytic effect in NO reduction by carbon. This is manifested in C02 evolution at temperatures that are much lower than those observed for the parent carbon25(Figure 3). 3. a-FeOOH- and /3-FeOOH-derived iron is not a good catalyst for this reaction; at 600 "C and above the TPR
544 Energy & Fuels, Vol. 9, No. 3, 1995
Illan-G6mez et al.
Arbritary Uniu
Fe (NO,),.9H,O
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Figure 6. Reference FT-EXAFS spectra for iron-containing species.
profiles are coincident with that observed for the parent carbon (Figure 4). In a n attempt to rationalize these observations, a study of the nature and distribution of the iron species, both as-prepared and subsequent to heat treatment, was carried out. The analysis of the latter samples is important because the catalyst can undergo significant changes in its oxidation state (as a consequence of its reduction by carbon) as well as in its particle size (as a consequence of heat-induced sintering). XAFS spectroscopy was used to analyze the freshly prepared samples. Figure 6 shows the Fourier transform EXAFS spectra of the reference iron species (Fe(N03)3*9H20, a-FeOOH, a-FezO3, FeO, y-FezO3, and a-Fe), while Figure 7 shows FT-EXAFS spectra of the three iron-loaded samples. The FT-EXAFS spectrum of sample K-UA1-Fe (Figure 7a) exhibits two peaks, one a t -1.7 if (with a shoulder at 1.3 if) and the other at -2.7 if. The position of the first one coincides with that obtained by Yamashita and Tomita21for a coal impregnated with Fe3+nitrate, which in turn appears in the spectra of (Fe(N03)3*9HzOand a-FeOOH (Figure 6 ) and corresponds to the Fe-0 distance. The peak at -2.7 if is attributed to that of the Fe-Fe distance in the FeO reference sample. This peak is a n evidence of a partial reduction of Fe(N03)3 to FeO during sample preparation. The sample was dried overnight a t 110 " C . The reduction of Fez03 t o FesO4 is thermodynamically feasible at such a low temperature.29 Figure 7b shows the corresponding spectra of sample K-UA1-aFe. The FT-EXAFS spectrum contains the same peaks as that of sample K-UA1-Fe, suggesting that the same iron species are present in both samples. The higher intensity of the peak a t -2.7 if indicates that the FeO particle size is larger in the former sample. The FT-EXAFS spectrum of sample K-UA1-PFe (Figure 7c) exhibits two peaks between 1and 2 A which are
'1
0
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8,
C)
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Figure 7. FT-EXAFS spectra for iron-loaded K-UA1 carbons: (a) K-UA1-Fe; (b) K-UA1-aFe; (c) K-UA1-BFe.
thought to be equivalent to the peak a t -1.7 A and the shoulder seen in the spectra of the other samples, and has been assigned to the Fe-0 distance. The absence of the peak at -2.7 A, corresponding to the Fe-Fe distance in FeO, indicates the absence of the reduced iron species in this sample.
Energy & Fuels, Vol. 9,No. 3, 1995 545
NO Reduction by Activated Carbons
The following two conclusions can be drawn from the XAFS results presented above: (a) the particle size of iron species of sample K-UA1-aFe seems to be larger I than that of the sample K-UA1-Fe and (b) the iron species of samples K-UA1-Fe and K-UA1-aFe show a much higher propensity for reduction than iron species of sample K-UA1-PFe. Thus samples K-UA1-Fe and K-UA1-aFe contain two iron species, one being Fe3+(aFeOOH or Fe(N0313) and the other being Fez+ (FeO); therefore, partial reduction of the iron precursor by the carbon has taken place during the preparation of these samples. On the other hand, sample K-UA1-PFe contains only Fe3+ species. These are intriguing findings. They suggest the need to explore in more detail the capability of different carbonaceous solids (e.g., activated carbons, activated carbon fibers, etc.) to reduce the various catalyst precursors. The available literature is scant and provides little guidance in this regard. In a recent study, Fu et al.31reported that activated carbon fibers are capable of reducing, at low temperature (3075 "C), different metallic ions to a lower oxidation state, and even to zerovalent state, in agreement with the EXAFS results presented above for the Fe3+ catalyst precursors. Given the difficulties in performing in situ heat treatments (in He at 900 "C) prior to EXAFS analysis, the heat-treated samples were characterized by CO chemisorption. Selective chemisorption is a well established tool for determining the dispersion of supported metal catalysts. Chemisorption of CO on supported iron catalysts is also well e s t a b l i ~ h e d ,even ~ ~ though one 0.1 must bear in mind the uncertainties with regard to the adsorption stoichiometry (Fe/CO being 1 for linear adsorption of CO, and 2 for bridge-bond adsorption), as well as the effects of chemisorption temperature and 0.05 particle size on the adsorption s t ~ i c h i o m e t r y . ~ ~ Figure 8a shows the CO desorption spectra for the three iron-loaded samples and the parent carbon. Sample K-UA1 exhibits a small CO peak with a maximum at -180 "C; sample K-UA1-Fe exhibits a CO 100 200 300 400 500 desorption peak with a maximum at -220 "C. The CO T ('C) peak of samples K-UAl-aFe and K-UA1-PFe (-180 "C) Figure 8. (a) Dependence of CO temperature-programmed is similar t o but more intense than that of the parent desorption (TPD)profiles on the nature of the catalyst precurcarbon. sor for iron-loaded carbon K-UA1. (b) CO desorption profiles for iron-loaded carbon K-UA1-Fe subsequent to oxidation in In an attempt to correlate the CO peak with the air (at 25 "C) and reduction in Hz (at 450 "C). oxidation state of the iron species on the carbon surface, CO chemisorption experiments were carried out subseTable 1. Quantification of CO Desorption Peak quent to the following pretreatments of the sample Subsequent to CO Chemisorption on Iron-Loaded Carbons K-UA1-Fe: (a) air exposure at 25 "C to ensure the formation of the oxidized iron species (Fez03 x H ~ O ) ; ~ ~ CO chemisorbed (b) reduction in H2 at 450 "C for 18 h, to ensure iron sample hmoLlpmo1 Fe) x lo2 reduction. Figure 8b shows the CO desorption profiles K-UA1-Fe 1.67 subsequent to CO chemisorption on thus pretreated K-UA1-aFe 0.36 K-UA1-BFe 0.35 samples. Interestingly, the desorption profile of the reduced sample does exhibit the CO peak, but its that the CO desorption peak is the signature of reduced intensity is much lower compared to that of the sample iron species; its lower intensity for the Hz-pretreated which was not subjected t o the reduction pretreatment sample may be attributed to the catalyst particle (Figure 8a). On the contrary, the air-exposed sample does not exhibit the CO peak. These results suggest sintering during the reduction in Hz. The integration of the CO desorption peak can thus, (31) Fu, R.; Zeng, H.; Lu,Y. Carbon 1993,31, 1089. in principle, be related to the fraction of reduced iron (32) Delannay, F., Ed. Characterization of heterogeneous catalysts; on the particle surface. Table 1 summarizes the relMarcel Dekker: New York, 1984; p 310. (33) Wade, K. In Transition Metal Cluster; Jhonson, B. F. G., Ed.; evant results. It is seen that iron particles on sample Wiley: New York, 1980; p 193. K-UA1-Fe contain the largest fraction of reduced iron (34) Cotton, F. A.; Wilkinson, G. Auanced Inorganic Chemistry, 4th ed.; Wiley: New York, 1980; p 796. on the surface, while samples K-UA1-aFe and K-UA1-
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Illan-Gomez et al.
546 Energy & Fuels, Vol. 9, No. 3, 1995 Actlvity @moVg CIS)
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Figure 9. Isothermal NO reduction activities as a function of time for nitrate-derived iron-loaded carbon K-UA1-Fe.
PFe either contain no reduced iron or the particle size of the iron species is so large that the amount of CO chemisorbed is very small. These data are consistent with the reactivity data, as well as with the EXAFS results. With the reactivity data, as determined by TPR, at low temperatures (e300 "C), where it is reasonable to assume that NO chemisorption is the ratedetermining ~ t e p , ~sample , ~ ~ K-UA1-Fe , ~ ~ , ~exhibits ~ the highest NO reduction capacity; a t high temperatures (>500 "C), only sample K-UA1-Fe exhibits a reactivity that is higher than that of the iron-free carbon, and the characteristic TPR temperatures are much lower than those exhibited by the iron-free carbon. CO chemisorption results are consistent with the EXAFS results which showed that (i) the iron species of samples K-UA1-Fe and K-UA1-aFe are reduced easier than those of sample K-UA1-PFe (in the former, Fez+species were detected) and (ii) that the particle size of the iron species of sample K-UA1-Fe is smaller than that of the K-UA1-aFe. On the other hand, the fact that NO chemisorption occurs at higher temperature on sample K-UA1-aFe than on sample K-UA1-Fe (see Figure 4) seems to confirm the lower dispersion (larger particle size) of the Fe2+ species on sample K-UA1-aFe which in turn makes more difficult its reduction by the carbon subsequent to the NO chemisorption, by carbon. Consequently, sample K-UA1-Fe will be the one that presents higher disociative NO chemisorption and higher activity for NO reduction a t any given reaction temperature, as is discussed below. The analysis of the reaction products evolved during the heat treatment prior t o the reactivity tests (see Experimental Section) confirms the conclusions about the effects of iron on the NO-carbon reaction based on EXAFS and CO chemisorption data. These data, showing that the sample K-UA1-Fe is the most reactive one because catalyst reduction on it is most effective, are presented in detail elsewhere.35 It is interesting to point out that these conclusions are similar to the ones obtained for other carbon (35)Illan-Gomez, M. J.; Linares-Solano, A,; Salinas-Martinez de Lecea, C. Energy Fuels, submitted for publication.
0'
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Figure 10. Dependence of the isothermal NO reduction activity (at 600 "C) vs time plot on the nature of the catalyst precursor for carbon K-UA1. 0.6
Activity @moVg Cis)
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Figure 11. Effect of catalyst loading on the isothermal NO reduction activity (at 600 "C) vs time plot for carbon K-UA1aFe.
gasification reactions:12-19the presence of reduced iron is necessary for the metal to be effective in the NOcarbon reaction. Isothermal Reactivity Results. By monitoring the NO reduction activity at 300-600 "C, complementary reactivity data were obtained. Figure 9 illustrates typical results for the most reactive sample. The activity vs time curves are qualitatively similar to those reported for the potassium-catalyzed r e a c t i ~ neven ~ , ~though ~ the deactivation observed here is more pronounced. The high-activity interval (100% reduction) is seen to become longer as the reaction temperature increases. In this interval, the only reaction product is N2, and oxygen accumulation on the surface takes p l a ~ e . ~ , ~ ~ - ~ ~ , ~ Figure 10 compares the isothermal NO reduction activity maintenance curves (at 600 "C) for the three iron-loaded samples and the parent, iron-free carbon.
NO Reduction by Activated Carbons
Energy & Fuels, Vol. 9, No. 3, 1995 547
t
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Figure 12. Evolution of products during isothermal NO reduction (at 600 "C)of iron-free and iron-loaded carbon K-UA1: (a) K-UA1; (b) K-UA1-Fe; (c) K-UA1-aFe; (d) K-UA1-PFe.
Samples K-UA1-aFe and K-UA1-PFe are initially more reactive than the parent carbon, but the steady-state activities of these three samples are very similar. This observation suggests that upon retention of oxygen during NO reduction, the iron species becomes oxidized and therefore inactive. The observed loss of catalytic activity is due to the inability of the oxidized iron species to transfer oxygen to the carbon and thus close the catalytic redox ~ y ~ l e Figure . ~ 11 ~provides ~ ~ ad, ~ ditional evidence for the above interpretation. A sample with a higher iron content (8.2%Fe) was prepared using a-FeOOH. Its activity is seen to be initially higher than that containing 4.7% Fe, confirming the role of iron as oxygen acceptor in NO reduction; nevertheless, the steady-state activity of the two samples is practically the same, highlighting the inability of the iron species in both samples to transfer oxygen to the carbon and recover their reduced state. Figure 12 shows the isothermal product evolution profiles for the four principal samples studied. At steady state, sample K-UA1-Fe is seen to produce the largest quantity of CO2 (Figure 12b). The evolution of
COa, being the result of the reduction of the oxidized iron species by carbon, is evidence for the closure of the catalytic redox cycle and a necessary condition for the maintenance of high steady-state activity. In contrast to this behavior, samples K-UA1-aFe (Figure 12c) and K-UA1-PFe (Figure 12d) exhibit relatively low steadystate levels of C02 evolution, similar to that of the ironfree carbon (Figure 12a), and thus a low catalytic ~activity. ~ ~ It~is interesting ~ ~ ~ t o note that C 0 2 evolution for sample K-UA1-aFe is initially delayed with respect to the evolution of N2 (Figure 12c), in agreement with the TPR results shown in Figure 5a for sample K-UA1-aFe. This confirms the interpretation offered earlier that here the catalyst retains the oxygen produced by dissociative NO chemisorption and, at steady-state, the ability of the catalyst to close the redox cycle explains its effectiveness in NO reduction. Finally, the steady-state activities of the iron-loaded samples-0.34 pmol NO/g of C/s for K-UA1-Fe, 0.10 pmol NO/g of C/s for K-UA1-aFe, and 0.13 pmol NO/g of C/s for K-UA1-PFe; (see Figure lO)-are seen to be in good qualitative agreement with the CO chemisorption data
548 Energy & Fuels, Vol. 9, No. 3, 1995
(Table 1); i.e., sample K-UA1-Fe is the only one whose reactivity is much higher than that of the iron-free carbon. 4. Summary and Conclusions
Iron acts as a catalyst of the NO-carbon reaction by undergoing an oxidationheduction cycle which is analogous to that proposed for potassium- and calciumcatalyzed NO r e d ~ c t i o n . ~Both , ~ ~ the NO reduction temperature and the product distribution are modified in its presence. The first step in the catalytic cycle is the dissociative chemisorption of NO, accompanied by N2 and NzO evolution; the reduced iron species (FexOy or Fe) accepts oxygen deposited on its surface and is transformed into a n oxidized species (FexOy+lor FeO). The catalytic cycle is closed when the oxygen is transferred from the catalyst to the carbon and the iron species recovers its reduced state. By analogy with our findings in potassium- and calcium-catalyzed NO reduct i ~ n the , ~ catalystkarbon , ~ ~ interface and interfacial area are the determining factors whether the latter step will occur. The inference from the results presented in this study is that, for iron-based catalysts prepared from FeOOH, catalyst reduction under reaction conditions occurs with greater difficulty than when potassium is used. Furthermore, all iron-based catalysts prepared in this study had a lower NO chemisorption capacity and are thus less effective than their potassium-based counterparts. Not surprisingly, catalyst particle size has been shown t o be a n important factor determining the effectiveness of iron in NO reduction: nitrate-derived- and
Illan-Gbmez et al.
a-FeOOH-derived catalyst species were the same, yet their catalytic effects were very different. As the particle size increases, the catalystkarbon interfacial or contact area decreases, resulting in a less effective transfer of oxygen from the catalyst to the carbon. The chemical nature of the catalyst precursor was shown t o be important as well: both the oxidation state of the catalyst prior to NO reduction and its particle size are dependent on it. The fraction of the catalyst that is in its active (reduced) state depends in turn on its reducibility by the carbon and its particle size. The latter two factors also control the ease with which the catalytic redox cycle is closed under reaction conditions, when the oxidized iron species transfers oxygen to the carbon. The above-described mechanistic features of NO reduction catalyzed by iron and other transition metals, and alkali and alkaline earth metals, as well as those of uncatalyzed NO reduction by carbon, are discussed in more detail elsewhere.30
Acknowledgment. This study was made possible by the financial support from DGICYT (projects AMB921032-(302-02 and CE91-0011-C03-01). The thesis grant for M.J.1.-G. and a n invited research grant to L.R.R. from Generalitat Valenciana are also gratefully acknowledged. The authors are grateful to A. Oya (Gunma University, Japan) for performing F-Fe sample preparation, and H. Yamashita (Osaka Prefectural University, Japan) for the XAFS experiments. EF940219X
