NO Reduction by Activated Carbons. 7. Some Mechanistic Aspects of

158

Energy & Fuels 1996, 10, 158-168

NO Reduction by Activated Carbons. 7. Some Mechanistic Aspects of Uncatalyzed and Catalyzed Reaction M. J. Illań-Go´mez, A. Linares-Solano,* L. R. Radovic,† and C. Salinas-Martıńez de Lecea Department of Inorganic Chemistry, University of Alicante, Spain Received April 7, 1995X

In this last paper of a series devoted to uncatalyzed and catalyzed reduction of NO by carbon, we discuss some mechanistic aspects of this reaction, which is thought to be a viable alternative to selective catalytic reduction by ammonia or hydrocarbons. We present a summary of the results of isothermal and temperature-programmed reduction studies on a metal-free activated carbon and carbon loaded with K, Ca, Fe, Cu, Cr, Co, and Ni catalysts. All the results obtained on catalyzed NO reduction are consistent with a simple oxidation-reduction (redox) mechanism involving the following kinetically significant steps: (1) chemisorption of NO on the catalyst surface; (2) transfer of oxygen from the catalytically active sites to the carbon reactive sites; and (3) desorption of oxygen from the carbon surface. A comparison with the proposed mechanisms for selective catalytic reduction is offered, as well as a mechanistic interpretation of the NO reduction enhancement in the presence of O2.

Introduction Because of society’s increasing environmental awareness, removal of NOx from both stationary and mobile sources has been the subject of very intense research and development efforts in the recent past. Novel burner and engine design, adsorption, and thermal destruction are some of the more straightforward approaches that can be used to resolve this increasingly serious environmental problem.1-5 Catalysis is another approach, and it turned out to be quite a challenge.6-17 It can be used either instead of or in tandem with the other approaches. Decomposition of NO to N2 and O2,8,13,18-27while thermodynamically feasible over a wide * Author to whom correspondence should be addressed. † Permanent address: Department of Materials Science and Engineering, Fuel Science Program, Penn State University, University Park, PA 16802. X Abstract published in Advance ACS Abstracts, December 1, 1995. (1) Clarke, A. G.; Williams, A. Chem. Ind. 1991, 917. (2) Farrauto, R. J.; Heck, R. M.; Speronello, B. K. Chem. Eng. News 1992, Sept. 7, 34. (3) Anonymous. Modern Power Systems 1992, May, 87. (4) Price, H. J. Trans Inst. Chem. Eng. 1992, 70A, 192. (5) Cho, S. M. Chem. Eng. Prog. 1994, January, 39. (6) Bosch, H.; Janssen, F. Catal. Today 1987, 2, 369. (7) Ju¨ntgen, H.; Ku¨hl, H. In Chemistry and Physics of Carbon; Thrower, P. A., Ed.; Marcel Dekker: New York, 1989; Vol. 22, p 145 (8) Iwamoto, M.; Hamada, H. Catal. Today 1991, 10, 57. (9) Armor, J. N. Appl. Catal. B 1992, 1, 221. (10) Gutberlet, H.; Schallert, B. Catal. Today 1993, 16, 207. (11) Armor, J. N. In Environmental Catalysis; Armor, J. N., Ed.; American Chemical Society: Washington, DC, 1994; p 2. (12) Heck, R. M.; Chen, J. M.; Speronello, B. K.; Morris, L. In Environmental Catalysis; Armor, J. N., Ed.; American Chemical Society: Washington, DC, 1994; p. 215. (13) Iwamoto, M.; Yahiro, H. Catal. Today 1994, 22, 5. (14) Lowe, P. A. In Environmental Catalysis; Armor, J. N., Ed.; American Chemical Society: Washington, DC, 1994; p 205. (15) Lowe, P. A.; Ellison, W. In Environmental Catalysis; Armor, J. N., Ed.; American Chemical Society: Washington, DC, 1994; p 190. (16) Maxwell, I. E.; Naber, J. E.; Jong, K. P. d. Appl. Catal. A 1994, 113, 153. (17) Spitznagel, G. W.; Hu¨ttenhofer, K.; Beer, J. K. In Environmental Catalysis; Armor, J. N., Ed.; American Chemical Society: Washington, DC, 1994; p 172.

0887-0624/96/2510-0158$12.00/0

temperature range of practical interest and despite much recent progress,13 is still lacking promising catalyst formulations because of severe catalyst deactivation in oxygen-rich environments. On the other hand, the conceptually less attractive selective catalytic reduction (SCR) is a commercially available technology.3,5,7,12,14,15,17,28-30 In automobile exhaust cleanup, for example, current technology is a three-way noblemetal catalyst system that removes hydrocarbons, CO, and NOx with reasonable efficiency. However, when excess oxygen is present in the exhaust gases (e.g., in diesel or lean-burn engines), this catalyst is not effective for removing NOx.6,31-33 The challenge of achieving efficient NO removal (e.g., >80%) in the presence of oxygen has thus led to an almost frantic search for both new catalyst systems and different reducing agents. The use of carbon as a reducing agent,6,34-53 catalyst,6,7,54-61 and/or catalyst support31,42,46,47,62-77 offers obvious potential advantages, including (a) very ef(18) Li, Y.; Hall, W. K. J. Catal. 1991, 129, 202. (19) Li, Y.; Armor, J. N. Appl. Catal. B 1992, 1, L21. (20) Hall, W. K.; Valyon, J. Catal. Lett. 1992, 15, 311. (21) Shelef, M. Catal. Lett. 1992, 15, 305. (22) Ogata, A.; Obuchi, A.; Mizuno, K.; Ohi, A.; Ohuchi, H. J. Catal. 1993, 144, 452. (23) Schay, Z.; Guczi, L. Catal. Today 1993, 17, 175. (24) Centi, G.; Nigro, C.; Perathoner, S.; Stella, G. In Environmental Catalysis; Armor, J. N., Ed.; American Chemical Society: Washington, DC, 1994; p 22. (25) Yang, R. T.; Chen, N. Ind. Eng. Chem. Res. 1994, 33, 825. (26) Zhang, Y.; Flytzani-Stephanopoulos, M. In Environmental Catalysis; Armor, J. N., Ed.; American Chemical Society: Washington, DC, 1994; p 7. (27) Zhang, X.; Walters, A. B.; Vannice, M. A. Appl. Catal. B 1994, 4, 237. (28) Saito, K.; Ichihara, S. Catal. Today 1991, 10, 45. (29) Janssen, F.; Meijer, R. Catal. Today 1993, 16, 157. (30) Wood, S. C. Chem. Eng. Prog. 1994, January, 32. (31) Kapteijn, F.; Stegenga, S.; Dekker, N. J. J.; Bijsterbosch, J. W.; Moulijn, J. A. Catal. Today 1993, 16, 273. (32) Zwinkels, M. M.; Ja¨rås, S. G.; Menon, P. G. Catal. Rev.sSci. Eng. 1993, 35, 319. (33) Burch, R.; Millington, P. J.; Walker, A. P. Appl. Catal. B 1994, 4, 65.

© 1996 American Chemical Society

NO Reduction by Activated Carbons

Energy & Fuels, Vol. 10, No. 1, 1996 159

ficient, in situ oxygen scavenging capability, and (b) elimination of the environmentally problematic “slip” of the gaseous reducing agent (e.g., ammonia). The virtues of carbon-based catalysts have thus been scrutinized in considerable detail, especially during the last decade. Accordingly, the mechanisms proposed to account for many accumulated, and some well established,

experimental facts have evolved considerably from a relatively complex scheme proposed several decades ago by Smith et al.36 From the point of view of stoichiometry, the NOcarbon reaction is quite straightforward, especially at low temperatures, when CO2 is the only carbon-containing product:

(34) Levy, J. M.; Chan, L. K.; Sarofim, A. F. In Eighteenth Symposium (International) on Combustion [Proceedings]; The Combustion Institute: Pittsburgh, 1981; p 111. (35) Smith, R. N.; Lesnini, D.; Mooi, J. J. Phys. Chem. 1957, 61, 81. (36) Smith, R. N.; Swinehart, J.; Lesnini, D. J. Phys. Chem. 1959, 63, 544. (37) Kunii, D.; Wu, K. T.; Furusawa, T. Chem. Eng. Sci. 1980, 35, 170. (38) Chan, L. K.; Sarofim, A. F.; Beer, J. M. Combust. Flame 1983, 52, 37. (39) Akhter, M. S.; Chughtai, A. R.; Smith, D. M. J. Phys. Chem. 1984, 88, 5334. (40) Furusawa, T.; Tsunoda, M.; Tsujimura, M.; Adschiri, T. Fuel 1985, 64, 1306. (41) Mochida, I.; Ogaki, M.; Fujitsu, H.; Komatsubara, Y.; Ida, S. Fuel 1985, 64, 1054. (42) Okuhara, T.; Tanaka, K.-I. J. Chem. Soc., Faraday Trans. 1 1986, 82, 3657. (43) Smith, D. M.; Welch, W. F.; Graham, S. M.; Chughtai, A. R.; Wicke, B. G.; Grady, K. A. Appl. Spectrosc. 1988, 42, 674. (44) Suuberg, E. M.; Teng, H.; Calo, J. M. In Twenty-Third Symposium (International) on Combustion [Proceedings]; The Combustion Institute: Pittsburgh, 1990; p 1199. (45) DeGroot, W. F.; Richards, G. N. Carbon 1991, 29, 179. (46) Yamashita, H.; Yamada, H.; Tomita, A. Appl. Catal. 1991, 78, L1. (47) Mochida, I.; Sun, Y. N.; Fujitsu, H.; Kisamori, S.; Kawano, S. Nippon Kagaku Kaishi 1991, 885. (48) Teng, H.; Suuberg, E. M.; Calo, J. M. Energy Fuels 1992, 6, 398. (49) Yamashita, H.; Yamada, H.; Kyotani, T.; Radovic, L. R.; Tomita, A. Energy Fuels 1993, 7, 85. (50) Illań-Go´mez, M. J.; Linares-Solano, A.; Salinas-Martıńez de Lecea, C.; Calo, J. M. Energy Fuels 1993, 7, 146. (51) Kominami, H.; Sawai, K.; Hitomi, M.; Abe, I.; Kera, Y. Nippon Kagaku Kaishi 1994, 582. (52) Suzuki, T.; Kyotani, T.; Tomita, A. Ind. Eng. Chem. Res. 1994, 33, 2840. (53) Tabor, K.; Gutzwiller, L.; Rossi, M. J. J. Phys. Chem. 1994, 98, 6172. (54) Komatsubara, Y.; Ida, S.; Fujitsu, H.; Mochida, I. Fuel 1984, 63, 1738. (55) Kusakabe, K.; Kashima, M.; Morooka, S.; Kato, Y. Fuel 1988, 67, 714. (56) Richter, E. Catal. Today 1990, 7, 93. (57) Mochida, I.; Kawano, S.; Fujitsu, H.; Maeda, T. Nippon Kagaku Kaishi 1992, 275. (58) Ahmed, S. N.; Baldwin, R.; Derbyshire, F.; McEnaney, B.; Stencel, J. Fuel 1993, 72, 287. (59) Lee, J. K.; Suh, D. J.; Park, S.; Park, D. Fuel 1993, 72, 935. (60) Lee, J. K.; Park, T.-J.; Park, D.; Park, S. Ind. Eng. Chem. 1993, 32, 1882. (61) Ku, B. J.; Lee, J. K.; Park, D.; Rhee, H.-K. Ind. Eng. Chem. Res. 1994, 33, 2868. (62) Nozaki, F.; Yamazaki, K.; Inomata, T. Chem. Lett. 1977, 521. (63) Inui, T.; Otowa, T.; Takegami, Y. J. Catal. 1982, 76, 84. (64) Kapteijn, F.; Mierop, A. J. C.; Abbel, G.; Moulijn, J. A. J. Chem. Soc., Chem. Commun. 1984, 1085. (65) Mochida, I.; Yata, K.; Fujitsu, H.; Komatsubara, Y. Bull. Chem. Soc. Jpn. 1985, 58, 900. (66) Otowa, T.; Inui, T. Appl. Catal. 1985, 18, 47. (67) Singoredjo, L.; Slagt, M.; van Wees, J.; Kapteijn, F.; Moulijn, J. A. Catal. Today 1990, 7, 157. (68) Kakuta, N.; Sumiya, S.; Yoshida, K. Catal. Lett. 1991, 11, 71. (69) Grzybek, T.; Papp, H. Appl. Catal. B 1992, 1, 271. (70) Stegenga, S.; Soest, R. v.; Kapteijn, F.; Moulijn, J. A. Appl. Catal. B 1993, 2, 257. (71) Imai, J.; Suzuki, T.; Kaneko, K. Catal. Lett. 1993, 20, 133. (72) Illań-Go´mez, M. J.; Linares-Solano, A.; Radovic, L. R.; SalinasMartıńez de Lecea, C. Energy Fuels 1995, 9, 97. (73) Illań-Go´mez, M. J.; Linares-Solano, A.; Radovic, L. R.; SalinasMartıńez de Lecea, C. Energy Fuels 1995, 9, 104. (74) Illań-Go´mez, M. J.; Linares-Solano, A.; Radovic, L. R.; SalinasMartıńez de Lecea, C. Energy Fuels 1995, 9, 112. (75) Illań-Go´mez, M. J.; Linares-Solano, A.; Radovic, L. R.; SalinasMartıńez de Lecea, C. Energy Fuels 1995, 9, 540. (76) Illań-Go´mez, M. J.; Linares-Solano, A.; Salinas-Martıńez de Lecea, C. Energy Fuels 1995, 9, 976. (77) Imai, J.; Kaneko, K. J. Colloid Interface Sci. 1992, 148, 595.

2NO + C ) CO2 + N2

(1)

But complications readily become apparent when CO or N2O are observed among reaction products, or when NO2 and O2 are considered as additional reactants. We have recently concluded a comprehensive study of both uncatalyzed and catalyzed NO reduction with activated carbons.50,72-76 Both isothermal reactivity and temperature-programmed reaction experiments were performed. The chemisorption of NO on potassiumloaded carbons was analyzed in some detail. The gaseous reactants and products were continuously monitored, both during reaction and using temperatureprogrammed desorption. This allowed us to obtain very informative oxygen and nitrogen balances during and after reactions with different carbons. A comparison of these balances, and of other catalyst characterization data, has allowed us to synthesize much of the information gathered and to gain some new insights into the mechanisms involved. The objective of this final paper in the series is to summarize our current understanding of the mechanism of the NO-carbon reaction, both in the presence and absence of catalysts, as well as to reconcile our findings and conclusions, as much as possible, with those available in the literature. In addition to emphasizing the similarities and/or differences in the effects of different catalysts, we compare NO reduction by carbon with the proposed mechanisms for the selective catalytic reduction (SCR) of NO by both stoichiometric and excess-oxygen reagents (e.g, ammonia or hydrocarbons). Experimental Section The details of the experimental procedures are described elsewhere.50,72-76 Briefly, the kinetics of the NO-carbon reaction were studied at atmospheric pressure in a fixed-bed flow reactor (15 mm, i.d.; ca. 300 mg sample). The reactor effluents were analyzed in detail by gas chromatography (using a Porapak Q 80/100 column and a thermal conductivity detector). This allowed us not only to determine their variation with time, temperature, and nature and quantity of catalyst but also to establish detailed oxygen and nitrogen balances. The average error associated with closure of the material balances was approximately 6%. 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 mL/min); (ii) isothermal reaction at 300-600 °C for a period of 2 h. The various metal-free and metal-loaded carbons were characterized by one or more of the following techniques: (i) physical adsorption of nitrogen (at -196 °C) and carbon dioxide (at 0 °C) in a conventional volumetric apparatus; (ii) chemisorption of NO and CO in a fixed-bed flow reactor connected to a quadrupole mass spectrometer, and of CO2 in a thermogravimetric apparatus; (iii) X-ray diffraction; (iv) X-ray absorption fine structure spectroscopy (XAFS). A chemically activated carbon derived from coal (K-UA1) and a polymer-derived carbon (A) were loaded with alkali,

160

Energy & Fuels, Vol. 10, No. 1, 1996

Figure 1. Temperature-programmed reaction (TPR) profiles of NO reduction for activated carbon and catalyst-loaded carbons. K-UA1, K-UA1(ox)-Ca-1.9, K-UA1-K-4.6, K-UA1-Fe4.7, K-UA1-Cu-3.0, K-UA1-Cr-2.0, K-UA1-Co-3.7, K-UA1-Ni3.5. (All carbons were loaded with metals by impregnation with aqueous nitrate solution, except Ca, which was ion-exchanged onto a carbon that was previously oxidized with HNO3.72-76) alkaline earth, and transition metals mostly by excess-solution impregnation but also, in selected cases, by incipient wetness impregnation and ion exchange. In selected cases, the carbons (K-UA1(ox) and A(ox)) were oxidized with HNO3 prior to ion exchange. In all cases, prior to the NO reduction experiments the samples were subjected to an in situ heat treatment in He, at 50 °C/min to 900 °C for 10 min. Metal catalyst loadings were determined in all cases by atomic absorption spectroscopy. The catalysts are identified by the carbon used, followed by the symbol of the metal used and its loading (in wt %).

Results Figure 1 shows typical TPR profiles for NO reduction with pure and metal-loaded carbons. The extent to which the metals exhibit a catalytic effect is seen to vary in a remarkable way. The temperature sensitivity of NO reduction is especially affected by the catalyst’s presence. Potassium is the most effective catalyst: its presence on the carbon surface is responsible for carbon’s high NO removal capability at low temperatures (∼200 °C) and for the lowest temperature at which 100% NO reduction is achieved (T100 ∼ 500 °C). Cobalt and nickel are similarly effective in the 350-500 °C, even though they exhibit no activity at low temperatures. Copper is inactive at low temperatures ( Cr, Fe > Ni. Figure 2 and Table 1 summarize the results of a more detailed comparison of the effectiveness of calcium and potassium as catalysts of NO reduction with carbon. Potassium is clearly a better catalyst; in particular, in the early reaction stage, before steady state is reached, it exhibits higher activity. The results are also seen to depend on both the nature of the carbon and the temperature level of interest. A common feature, however, is that in all cases, except at the highest temperature, a period of high initial activity is followed by significant deactivation whose severity is inversely proportional to the reaction temperature. At a more fundamental level, the observed phenomena are related to the effectiveness of the catalyst in dissociating NO and its ability to transfer oxygen to the carbon surface (see Discussion). It is interesting to note, however, that Whitman and Ho85 recently concluded, based on very detailed surface science studies of NO adsorption and (80) Jen, H. W.; Gandhi, H. S. In Environmental Catalysis; Armor, J. N., Ed.; American Chemical Society: Washington, DC, 1994; p 53. (81) McKee, D. W. In Chemistry and Physics of Carbon; Walker, P. L., Jr., Thrower, P. A., Eds.; Marcel Dekker: New York, 1981; p 1. (82) Radovic, L. R.; Walker, Jr., P. L.; Jenkins, R. G. J. Catal. 1983, 82, 382. (83) Linares-Solano, A.; Almela-Alarcoń, M.; Salinas-Martıńez de Lecea, C. J. Catal. 1990, 125, 401. (84) Mims, C. A. In Fundamental Issues in Control of Carbon Gasification Reactivity; Lahaye, J., Ehrburger, P., Eds.; Kluwer Academic Publishers: Dordrecht, 1991; p 383.


Energy & Fuels, Vol. 10, No. 1, 1996 161 Table 1. Comparison of NO Reduction Activities of Calcium and Potassiuma sampleb

A300

A400

A500

A600

A(ox)-Ca-3.6 K-UA1(ox)-Ca-1.9 A(ox)-K-4.9 K-UA1(ox)-K-1.9

0 0 72 165

3 4 272 506

244 110 415 683

477 297 434 1031

a Activity (A) at 300-600 °C expressed as µmol NO reduced/ mol metal/s. b Metal-loaded carbons were obtained by ion exchange using acetate solutions.72,74

Figure 2. Isothermal NO reduction activity of activated carbon and catalyst-loaded carbons: (a) K-UA1; (b) K-UA1(ox)-Ca-1.9; (c) K-UA1-K-4.6.

pollution control catalysts. On the other hand, and in agreement with the above-mentioned findings in catalyzed carbon gasification, Sjo¨vall et al.86 found that a monolayer of potassium on a carbon film caused a 104fold increase in carbon oxidation rate in O2; on the basis of work function measurements in a high-vacuum system equipped with an Auger electron spectrometer, they proposed a charge transfer mechanism to explain this catalytic effect and mentioned that such a mechanism could also explain alkali promotion of NO reduction by carbonaceous materials. Figure 3 shows typical gas composition profiles obtained during TPR. These were discussed in detail elsewhere.72-76 Dinitrogen is seen to be the predominant nitrogen-containing product; with the more reactive samples, N2O is also obtained. Carbon dioxide is the predominant oxygen-containing product at low temperatures. It is important to note also that the global reaction stoichiometry (see Introduction) is not obeyed. (This point is discussed in more detail in connection with Figure 4.) In the presence of K, Cu, Co, and Ni, CO2 evolution begins at a lower temperature than for the metal-free carbon; the carbon-oxygen surface complexes that are formed are less stable and more abundant. In contrast, in the presence of Ca and to some extent Fe, CO2 evolution is shifted toward higher temperatures; this suggests the formation of carbonate species at intermediate temperatures. The appearance of CO (which becomes a dominant product at higher temperatures) is not directly related to the NO reduction activity; thus, for example, the iron-loaded carbon is much more reactive than the copper-loaded carbon, yet CO evolution for both samples begins at ca. 650 °C, the same temperature as for the metal-free carbon. In the presence of K, Cr, and Ni, CO evolution begins at a lower temperature than for the metal-free carbon, indicating their destabilizing effect on carbonoxygen surface complexes. Finally, note the appearance of well-defined CO peaks for Fe- and Co-loaded carbons (Figure 3, d and g); this point is also discussed below. Figure 4 shows typical oxygen and nitrogen balances during TPR. In the absence of a catalyst, they are in close agreement, and thus obey the stoichiometric reaction (reaction 1), except at very high temperatures (Figure 4a). Below ca. 600 °C, small discrepancies are observed for the calcium-, copper-, chromium-, cobalt-, and nickel-loaded carbons (Figure 4b,e-h). Large discrepancies are seen for the highly reactive potassiumand iron-loaded carbons (Figure 4c,d). In both cases, at low temperatures, excess nitrogen is observed, while at high temperatures excess oxygen is seen. These observations, together with those discussed in more

reaction on potassium-precovered Rh(100), that addition of alkali metal would not improve the performance of

(85) Whitman, L. J.; Ho, W. J. Chem. Phys. 1988, 89, 7621. (86) Sjo¨vall, P.; Hellsing, B.; Keck, K.-E.; Kasemo, B. J. Vac. Sci. Technol. A 1987, 5, 1065.

162


Illa´ n-Go´ mez et al.

Figure 3. TPR gas composition profiles during NO reduction for activated carbon and catalyst-loaded carbons: (a) K-UA1; (b) K-UA1(ox)-Ca-1.9; (c) K-UA1-K-4.6; (d) K-UA1-Fe-4.7; (e) K-UA1-Cu-3.0; (f) K-UA1-Cr-2.0; (g) K-UA1-Co-3.7; (h) K-UA1-Ni-3.5.

detail elsewhere,72-76 provide important mechanistic clues. They are briefly summarized below; their mechanistic implications are discussed in detail in the next section. It is important to note that, prior to the TPR experiments, all samples were heat-treated in He at 900 °C. Catalyst reduction by the carbon under these conditions was confirmed experimentally in all cases except for Ca

and Cr. Negative oxygen balances in Figure 4, at temperatures above the onset of CO2 evolution, thus indicate the retention of oxygen by the catalyst. The case of Ca is well understood.74 Upon heat treatment it is present on the carbon surface as CaO. The negative oxygen balance between 350 and 600 °C suggests the formation of a surface peroxide, CaO(O), in agreement with the findings in the Ca-catalyzed O2-C reaction.



Figure 4. Oxygen and nitrogen balances during temperature-programmed reduction of NO with activated carbon and catalystimpregnated carbons: (a) K-UA1; (b) K-UA1(ox)-Ca-1.9; (c) K-UA1-K-4.6; (d) K-UA1-Fe-4.7; (e) K-UA1-Cu-3.0; (f) K-UA1-Cr-2.0; (g) K-UA1-Co-3.7; (h) K-UA1-Ni-3.5.

Formation of CaCO3 also takes place in a secondary reaction of CaO with the CO2 formed; this is in agreement both with the delay in CO2 evolution (Figure 3b) and with the carbonate decomposition temperature of ∼600 °C (Figure 4b). In the presence of potassium, the formation of the oxidized catalyst species (KxOy+1)72,73 takes place at the lowest temperatures studied. This

species becomes unstable above 300 °C (Figure 4c); the excess oxygen ratio, O/K, of ∼1.1 (i.e., in comparison with potassium-free carbon) suggests that x ) y. The range of formation of an oxidized iron species is ∼200500 °C (Figure 4d); this species is stable up to ∼770 °C, when its reduction begins. The O/Fe ratio corresponding to the peak observed is ca. 1.0, suggesting the

164


existence of FeO. Similarly, the oxidized cobalt species decomposes at ca. 700 °C (Figure 4g); the O/Co ratio is ca. 1.5, suggesting the existence of Co2O3. While the existence of this oxide has not been demonstrated, oxygen retention by Co3O4 is well-known and this phenomenon would account both for the observed O/Co ratio and for the small oxygen consumption between 100 and 550 °C. Indeed, a more detailed analysis of gas evolution in the low-temperature region shows NO consumption between 50 and 100 °C, which would account for the formation of such an oxidized cobalt species. In the case of Cu-, Cr-, and Ni-catalyzed NO reduction, the oxygen balances follow similar patterns (Figure 4e,f,h), but do not exhibit distinctive features that warrant further comment. Discussion Uncatalyzed NO Reduction by Carbon. From the steadily increasing number of relevant studies, it can be concluded that the effectiveness of carbon as a reducing agent for NO depends both on its surface properties and its inorganic impurities. In their pioneering work, Smith et al.36 used a very pure sugarderived char and studied the kinetics of the NO-C reaction at 450-600 °C. In the work of Kominami et al.,51 some commercial charcoals (for fuel use) achieved high levels of NO reduction at 500 °C. In the recent detailed studies of Suuberg and co-workers,44,48,87,88 the kinetics of NO reduction of a polymer-derived char were studied at 500-800 °C. The very recent work of Suzuki et al.52 with a similar carbon covers a similar carbon reactivity range. Our own results50 support the emerging consensus: relatively disordered, nongraphitic carbons (e.g., activated carbons and chars) free from significant quantities of inorganic impurities (e.g., potassium or iron) are effective in reducing NO at temperatures in excess of 500 °C. One implication of this conclusion is that carbon is less effective in reducing NO than O2. (At comparable partial pressures, typical temperatures for significant gasification of disordered carbons in oxygen are 350-450 °C.89 ) This is in agreement with the findings of DeGroot and Richards45 but conflicts with the recent conclusion of Chu and Schmidt,90 derived from some interesting scanning tunneling microscopy experiments with a graphitic carbon. Another implication is of more practical nature: this reactivity window is outside the temperature range of the convenient locations for installing NO reduction equipment in utility boiler systems, i.e., 350450 °C in the post-economizer section and 100-150 °C in the post-air-preheater section.25 Two solutions to this potential problem are being pursued currently: (a) use of either deliberately added or inherently present catalysts, thus conferring to carbon a dual role of reductant and catalyst support; and (b) NO reduction enhancement in the presence of oxygen. While the latter issue is very interesting, because of its relevance to the increasingly important lean-burn and diesel applications, it will be discussed here only to the extent that it sheds light on the mechanism of both uncatalyzed and catalyzed NO reduction. The former issue is discussed in detail next. (87) Teng, H.; Suuberg, E. M. J. Phys. Chem. 1993, 97, 478. (88) Teng, H.; Suuberg, E. M. Ind. Eng. Chem. Res. 1993, 32, 416. (89) Smith, I. W. Fuel 1978, 57, 409. (90) Chu, X.; Schmidt, L. D. Ind. Eng. Chem. Res. 1993, 32, 1359.


Catalyzed NO Reduction by Carbon. The first report of utilization of carbon in catalytic reduction of NO seems to be the study of Nozaki et al.,62 in which a copper oxide catalyst supported on an activated carbon was found to be effective in NO reduction with ammonia at temperatures as low as 100 °C. No information was provided in this early study on the extent to which the carbon plays the role of reductant as well; on the other hand, it was shown that, at temperatures above 300 °C, practically 100% of the NO reduction is attributable to the catalytic effect of the carbon itself. A number of other catalytic systems have used carbon as a support primarily, with ammonia or some other gaseous species (e.g., CO) being the principal reducing agent.31,64,66,67,69,70,91 However, of primary interest heresand potentially more advantageous from a practical standpointsare catalytic systems in which carbon is used both as the reducing agent and the catalyst support. The first report of utilization of carbon as reductant and catalyst support in NO reduction appears to be the study of Inui et al.,92 in which several weight percent of Ni or Co, with small amount of a rare-earth oxide and Pt, were impregnated onto an activated carbon. These authors reported that complete conversion of NO was achieved at ∼400 °C and high space velocity. Interestingly, however, NO reduction was inhibited in the presence of O2, in agreement with the behavior of many SCR and NO decomposition catalysts. Kapteijn et al.,64 Okuhara and Tanaka,42 Kakuta et al.,68 and Kominami et al.51 subsequently showed that alkalimetal-doped carbons are also effective. Mixed-metal systems (e.g., Zn, Cu, Fe, Ni, and Ce added to potassium-doped carbons,68 or Cu-Cr51) further enhance NO reduction by carbon. Mochida and co-workers47,65 investigated in some detail the catalytic effect of copper and iron salts in NO reduction by activated carbon fibers, that of the latter becoming especially strong in the absence of oxygen. Kaneko and co-workers71,93,94 also found that both NO adsorption and reduction were enhanced in the presence of iron catalysts; in particular, they reported that 80% NO conversion to N2 took place at 200 °C over activated carbon fibers doped with R-FeOOH. The virtues of copper catalysts in particular, both in the presence and absence of oxygen, have been the subject of a detailed study by Tomita and coworkers.46,49 From these studies and our own work it is obvious that catalyzed NO reduction by carbon in the posteconomizer section of a utility boiler is a feasible solution worth pursuing further. The optimization of the catalyst formulation (e.g., with regard to the reactivity of the carbon) will be made easier by the knowledge of the mechanisms involved and the rate-determining steps in the catalytic sequence. These issues are discussed and summarized next. Mechanisms. Four issues are of main interest here, for both uncatalyzed and catalyzed NO reduction: (a) the details of NO adsorption on the surface; (b) the (91) Mehandjiev, D.; Bekyarova, E. J. Colloid Interface Sci. 1994, 166, 476. (92) Inui, T.; Otowa, T.; Takegami, Y. Ind. Eng. Chem. Prod. Res. Dev. 1982, 21, 56. (93) Wang, Z.-M.; Suzuki, T.; Uekawa, N.; Asakura, K.; Kaneko, K. J. Phys. Chem. 1992, 96, 10917. (94) Wang, Z.-M.; Shindo, N.; Otake, Y.; Kaneko, K. Carbon 1994, 32, 515.



routes to CO2 and CO formation; (c) NO reduction enhancement in the presence of O2; and (d) comparisons with the proposed mechanisms for selective catalytic reduction. They are discussed in turn below. In the early work of Smith et al.36 it was concluded, on the basis of first-order NO reduction results with previously O2- or H2-treated carbons, that the first step in the reaction sequence is the formation of carbonoxygen surface complexes as key reaction intermediates:

2NO + 2Cf f 2C(O) + N2

(2a)

Many subsequent studies have confirmed the early report that reaction 2a is facile at temperatures as low as 200 °C; at these temperatures, however, NO reduction was found to cease when the carbon surface became saturated with oxygen, by interesting analogy with catalytic NO decomposition.92 The adsorption of NO was proposed to occur in O-down orientation, which readily explains N2 and C(O) formation and is consistent with the well-known affinity of carbon toward oxygen. On the other hand, there is the N-down proposal of Teng et al.48 based on the work of Zarifyanz et al.,95 in which a loss of paramagnetism was detected in magnetic susceptibility measurements upon irreversible NO adsorption on a freshly cleaved graphite surface. This loss of paramagnetism was similar to the well-known difference between the paramagnetic NO molecule and the diamagnetic behavior of compounds containing an NO group. Of some relevance to this issue is also the study of Yates and Madey,96 who used field-desorption mass spectroscopy and field-emission microscopy to understand the details of NO chemisorption on tungsten. They concluded that the NO molecule is probably bound to W through one end of the molecule as a linear species. They were able to calculate the average dipole moment of an adsorbed NO molecule at full surface coverage, as +0.35 debyes, which implies that NO is adsorbed in the N-down orientation. They were not able to confirm, however, the structural analogy of adsorbed NO with transition-metal nitrosyl compounds, found by infrared spectroscopy, which would further confirm the N-down orientation of adsorbed NO. In a more recent, detailed study of NO adsorption and reduction on carbon, Suuberg and co-workers44,48,87,88 concluded that two different adsorption processes should be distinguished. At low temperatures, NO chemisorption first results indeed in the formation of carbonoxygen surface complexes; subsequent decomposition of these complexes (at higher temperatures) leads to CO2 and CO production. At high temperatures, direct interaction between NO and carbon reactive sites occurs and all the reaction products (e.g., CO2, CO and N2) are immediately formed. Related to the uncertainty about the orientation of adsorbed NO is the important issue of NO dissociation upon adsorption and the subsequent fate of surface nitrogen species. In their elegant isotope-labeling study, Okuhara and Tanaka42 found evidence for the occurrence of the following two reactions in NO reduction over a potassium-loaded mesoporous carbon at low temperatures (25-280 °C), after the carbon had been (95) Zarifyanz, Y. A.; Kiselev, V. F.; Lezhnev, N.; Nikitina, O. Carbon 1967, 5, 127. (96) Yates, Jr., J. T.; Madey, T. E. J. Chem. Phys. 1966, 45, 1623.

heat-treated in vacuum at 470 °C:

2NO f N2O + O(ad)

(3)

NO(ad) + NO f N2O + O(ad)

(4)

Above 280 °C, adsorbed N2O is thought to decompose to N2 and leave behind surface oxygen. Therefore, above 280 °C, the observable elementary reactions are proposed to be

2NO f N2 + 2O(ad) NO(ad) + NO f {2NO(ad) f} N2 + 2O(ad)

(3a) (4a)

They argued further that part of the adsorbed oxygen, known to exist on the surface as a result of the stoichiometric imbalance between N2 and CO2 formation during reaction, exists in the form of adsorbed NO2, as a result of the following reversible reaction:

NO + O(ad) T NO2(ad)

(5)

Nevertheless, the participation of this adsorbed species in the reaction mechanism was thought to be a minor channel for NO reduction by carbon, presumably because of increasing importance of the reverse reaction. These authors argued in favor of nondissociative adsorption of NO, in agreement with reaction 2a. (Note that because the N-O bond must eventually be broken, the distinction between dissociative and nondissociative adsorption reduces to the practical issue of the stability and residence time of surface nitrogen species, as discussed later.) The evidence for this argument came from the finding that NO,16 adsorbed at room temperature, underwent isotopic exchange with preadsorbed O18 and NO18 appeared among the desorption products in TPD. However, this NO18 peak was found not to be “in accordance” with the recombination of N(ad) to form N2 and desorption of O(ad) as CO2, which are signs of dissociative adsorption of NO. Rather, this finding was explained by the occurrence of forward reaction 5, followed by desorption of NO at higher temperatures (reverse reaction 5). In contrast, Suuberg and co-workers44,48,87,88 propose that NO adsorption is a dissociative chemisorption process:

2Cf + NO f C(O) + C(N)

(6)

After gasification of a surface-cleaned, high-purity polymer-derived char in NO at 600 °C, these authors reported significant quantities of N2 in the TPD spectrum and concluded that there is only a small number of sites, created during surface cleaning, that are able to dissociate NO. In their TEM and STM study of NO reduction by freshly cleaved highly oriented pyrolytic graphite, Chu and Schmidt90 observed the formation of a high-meltingpoint solid, (CN)x. They propose that the existence of this material is responsible for the appearance of the above-mentioned high-temperature N2 peak in TPD of NO-oxidized chars.48 Therefore, they also propose the dissociative adsorption of NO on the graphite surface. In a very recent study, Tomita and co-workers52 used XPS to provide evidence for the existence of C(N) species

166



after NO reduction of a surface-cleaned polymer-derived carbon in the presence of O2. Regarding the fate of the NO-derived surface oxygen, i.e., regarding the main routes to CO2 and CO production, it is tempting to conclude that they are no different from those of the O2-C reaction,97-99 i.e.,

2C(O) f CO2 + Cf

(7)

C(O) f CO (+ Cf)

(8)

While this is thought to be a reasonable suggestion based on the venerable Ockham’s razor argument, it requires some justification in light of the existence of other, more complicated proposals.36,38,48 For example, Smith et al.36 proposed reactions 9-11 while Teng et al.48 emphasize the importance of reaction 12.

NO + C(O) + Cf ) C(O‚‚‚ON)C

(9)

NO + C(O‚‚‚ON)C f C(O) + CO2 + N2

(10)

NO + C(O‚‚‚ON)C f CO + CO2 + N2

(11)

NO + C(O) + Cf f CO2 + C(N)

(12)

One argument in favor of reactions 7 and 8 is the wellestablished fact that the CO/CO2 ratio shows similar temperature dependence in both NO-C and O2-C reactions. Also, the finding that surface complex formation has a lower activation energy than surface complex desorption48 is common to both reactions. Finally, the explanation for the reaction’s first-order dependence on NO concentration does not require reactions 9-12; instead, the following sequence is thought to reconcile both the NO dissociation argument and the first-order dependence:

NO + 2Cf f C(O) + C(N)

(2b)

2C(N) f N2 + 2Cf

(2c)

Reaction 2c is thought to occur readily on all but the very active carbon sites (which are produced, for example, during a high-temperature outgassing treatment). Note that reactions 2b and 2c combined yield reaction 2a, thus providing also an explanation for the second-order dependence on NO concentration under certain conditions (e.g., reversible NO chemisorption at low temperatures87 or low-temperature reaction in a closed circulating-gas reactor41). A detailed analysis of the effects of various catalysts on the NO reduction by carbon72-76 does not provide evidence that the basic mechanism presented above requires substantial modification. Alkali, alkalineearth, and transition-metal catalysts appear to enhance NO chemisorption to different extents and intervene with varying degree of effectiveness in the oxidationreduction (redox) cycle which shuttles oxygen from NO, via the catalyst and the carbon surface, to CO2 and CO. This mechanism is analogous to that which is operative (97) Lizzio, A. A.; Jiang, H.; Radovic, L. R. Carbon 1990, 28, 7. (98) Radovic, L. R.; Lizzio, A. A.; Jiang, H. In Fundamental Issues in Control of Carbon Gasification Reactivity; Lahaye J., Ehrburger, P., Eds.; Kluwer Academic Publishers: Dordrecht, 1991; p 235. (99) Walker, Jr., P. L.; Taylor, R. L.; Ranish, J. M. Carbon 1991, 29, 411.

in other carbon gasification reactions.81,84 The analysis of reaction products in TPR experiments clearly reveals three stages, especially in the presence of very active catalysts such as potassium (see Figures 3c and 4c): (i) In the first stage (typically, T e 300 °C), N2 and/or N2O are the only products. Oxygen is retained on the catalyst/carbon surface. (ii) In the second stage, N2 continues to evolve, but CO2 evolution occurs as well and the rate of NO reduction is enhanced. (iii) In the third stage (typically, T > 500 °C), N2 evolution becomes constant and CO becomes the dominant oxygen-containing product. While this NO reduction mechanism is common to all the catalysts studied, the active sites that retain and transfer oxygen to the carbon are different, depending on the nature of the chemical species involved in each case. As discussed previously, the oxygen balances shown in Figure 4 are helpful in suggesting the probable species involved. The proposed redox cycles are CaO/ CaO(O), KxOy/KxOy+1, Fe (or FexOy)/FeO (or FexOy+1), and Co(or CoO or Co3O4)/Co2O3. For the sake of completeness, and admittedly with little experimental evidence provided in this study, we propose that the remaining catalytic cycles are the following: Cu/CuO(or Cu2O), Cr2O3/CrO2, and Ni/NiO. The redox mechanism outlined above is analogous to that proposed in an early study by Mochida et al.65 It is thought to have been illustrated in an especially convincing way for the potassium-catalyzed NO reduction.73 When TPR experiments were performed subsequent to NO chemisorption, the first reaction stage (see above) disappeared; that is, when the catalyst became inactive, as a consequence of its conversion to the oxidized state, negligible NO reduction took place. It is only after CO2 evolution begins that NO reduction occurs on an oxygen-saturated surface. Furthermore, this effect is autocatalytic: the greater the rate of CO2 evolution is, the greater the rate of NO reduction will be (see, for example, Figure 3; see also Figure 9 in ref 73). The perhaps obvious conclusion that a reduced catalyst surface is required for NO reduction to proceed is not only a key component in the mechanism of catalyzed NO reduction; it is also thought to be the key to the understanding of the NO reduction enhancement in the presence of O2, for both catalyzed and uncatalyzed reaction. It was already pointed out, in connection with reaction 2, that even the uncatalyzed reaction ceases when the surface is saturated with carbon-oxygen complexes.36 In both TPR and isothermal kinetic experiments, we showed72,73 that NO chemisorption on K-loaded carbon is less effective on the oxygen-rich surface. Thus the complexes have an inhibiting effect, in a manner analogous to the inhibiting effect of stable C-O complexes in carbon gasification97-100 and of oxygen in catalytic decomposition of NO.13 This is difficult to reconcile with mechanistic proposals such as reactions 9-12 or those of Mochida et al.41 involving direct interaction between NO and carbon-oxygen complexes. In agreement with the recent studies of Tomita and co-workers,49,52 we conclude that the role of oxygen can be summarized simply as follows: (100) Laine, N. R.; Vastola, F. J.; Walker, Jr., P. L. J. Phys. Chem. 1963, 67, 2030.


2Cf + O2 f 2C(O)


(13)

Upon subsequent desorption of CO2 and CO (reactions 7 and 8) a larger number of free reactive sites on the carbon surface are produced; these “nascent” sites obviously have a higher affinity toward NO.49 It is interesting to note in this context the recent study of Mochida et al.101 in which it was concluded that the active sites in the carbon/NO/O2 system are created “by the liberation of CO and CO2 in contrast to the expectation that the oxidation active site is connected to the oxygen functional group”. A similar conclusion was reached recently by Lizzio and de Barr102 in the case of SO2 adsorption on carbon surfaces. Finally, it is interesting to compare the proposed mechanism with that of catalytic reduction of NO by hydrocarbons, an approach that is being vigorously pursued primarily for mobile engine applications. While there appear to be several possible routes,6,13,24,33,80,103-116 depending presumably on the catalyst formulation and the reductant used, one of them is indeed a simple redox sequence analogous to that proposed here. For example, Burch and co-workers.33,107,111 proposed that dissociative adsorption of NO occurs on reduced metallic or metal ion sites in zeolite-based catalysts, releasing N2 into the gas phase and leaving behind adsorbed oxygen atoms; the reductant then regenerates the active sites by removing these oxygen atoms. A similar mechanism has been proposed by Kapteijn et al.31 for NO reduction by CO over Cu/Cr oxide catalysts. It is also intriguing to note that at least one research group105,117 suggests that NO reduction by hydrocarbons occurs through the intermediate formation of an “active coke”. In a recent study, Burch and co-workers, who argue elsewhere against this possibility,111 acknowledge, for the case of NO reduction by propene/O2 over Pt/ alumina catalysts,33 that NO reacts with a carboncontaining species, which they propose is a “molecular fragment formed from propene” and not a “carbonaceous deposit”. Rate-Determining Steps. In agreement with the above-postulated mechanism, the kinetically significant steps in catalyzed NO reduction by carbon are (1) chemisorption of NO on the catalyst surface; (2) transfer of oxygen from the catalytically active sites to the carbon (101) Mochida, I.; Kisamori, S.; Hironaka, M.; Kawano, S.; Matsumura, Y.; Yoshikawa, M. Energy Fuels 1994, 8, 1341. (102) Lizzio, A. A.; de Barr, J. A. Fuel, in press. (103) d’Itri, J. L.; Sachtler, W. M. H. Catal. Lett. 1992, 15, 289. (104) d’Itri, J. L.; Sachtler, W. M. H. Appl. Catal. B 1993, 2, L7. (105) Bennett, C. J.; Bennett, P. S.; Golunski, S. E.; Hayes, J. W.; Walker, A. P. Appl. Catal. A 1992, 86, L1. (106) Blanco, J.; Avila, P.; Fierro, J. L. G. Appl. Catal. A 1993, 96, 331. (107) Burch, R.; Millington, P. J. Appl. Catal. B 1993, 2, 101. (108) Li, Y.; Armor, J. N. Appl. Catal. B 1993, 2, 239. (109) Yogo, K.; Umeno, M.; Watanabe, H.; Kikuchi, E. Catal. Lett. 1993, 19, 131. (110) Yogo, K.; Ihara, M.; Terasaki, I.; Kikuchi, E. Catal. Lett. 1993, 17, 303. (111) Burch, R. Catal. Lett. 1994, 27, 177. (112) Chajar, Z.; Primet, M.; Praliaud, H.; Chevrier, M.; Gauthier, C.; Mathis, F. Catal. Lett. 1994, 28, 33. (113) Chajar, Z.; Primet, M.; Praliaud, H.; Chevrier, M.; Gauthier, C.; Mathis, F. Appl. Catal. B 1994, 4, 199. (114) Gru¨nert, W.; Hayes, N. W.; Joyner, R. W.; Shpiro, E. S.; Siddiqui, M. R. H.; Baeva, G. N. J. Phys. Chem. 1994, 98, 10832. (115) Petunchi, J. O.; Hall, W. K. Appl. Catal. B 1994, 3, 239. (116) Cowan, A. D.; Du¨mpelmann, R.; Cant, N. W. J. Catal. 1995, 151, (117) 356.Ansell, G. P.; Diwell, A. F.; Golunski, S. E.; Hayes, J. W.; Rajaram, R. R.; Truex, T. J.; Walker, A. P. Appl. Catal. B 1993, 2, 81.

Figure 5. Schematic representation of the temperature dependence of the rate-determining step in the catalytic sequence: (a) potassium-catalyzed NO reduction; (b) calciumcatalyzed NO reduction.

reactive sites; and (3) desorption of oxygen from the carbon surface to form CO2 and CO. Therefore, the kinetics of the overall process can be represented as follows:

rate ) k{[O]g - [O]cat} ) k2{[O]cat - [O]c} ) k3[O]c (14) rate ) k1G1 ) k2G2 ) k3G3

(15)

In the above expression the total oxygen concentration difference (between gas-phase concentration and zero) is conveniently divided into three gradients (G), where subscripts g, cat, and c correspond to the gas phase, catalyst surface, and carbon surface, respectively. The concentration of oxygen on the catalyst surface ([O]cat) approaches its equilibrium value to a greater or lesser extent, depending on the temperature, partial pressure of NO, and catalyst affinity toward dissociative NO chemisorption. The coverage of oxygen on the carbon surface ([O]c) is determined by the balance between the oxygen spillover63, 65 from the catalyst surface (enhanced by large catalyst/carbon contact area) and the desorption of CO2 and CO (enhanced by high carbon reactivity). At steady state, the rates of all three steps in the sequence are equal and the rate-determining step (RDS) is the one that has the lowest rate constant or, equivalently, the one that consumes the greatest portion of the overall gradient.118 Thus, for example, if the transfer of oxygen from the catalyst to the carbon and the evolution of CO2 are both facile, the inventory of surface oxygen on the catalyst and carbon surfaces is low, G1 is the largest of the three gradients and NO chemisorption is the RDS. If, on the other hand, the catalyst does not transfer oxygen to the carbon as readily, [O]cat increases, G1 decreases and G2 increases, and oxygen transfer becomes the RDS. The latter situation can arise as a consequence of two factors: (a) high affinity of catalyst toward oxygen, and (b) low catalyst/carbon interfacial (contact) area. The implications of this analysis in the case of potassium- and calcium-catalyzed NO reduction are summarized qualitatively in Figure 5, where both [O]cat and [O]c can exhibit a maximum with temperature as a consequence of the balance between kinetic and thermodynamic factors. For potassium-catalyzed NO (118) Satterfield, C. N. Heterogeneous Catalysis in Industrial Practice, 2nd ed.; McGraw-Hill: New York, 1991; p 554.

168



reduction, it was concluded that G1 > G2 at lower temperatures and G1 < G2 at higher temperatures;72,73 this means that either G1 decreases and G2 increases with increasing temperature or the increase in G1 is smaller than the increase in G2. (The former case is illustrated in Figure 5a.) In contrast, for calciumcatalyzed NO reduction, it was concluded that G1 < G2 at lower temperatures and G1 > G2 at higher temperatures;74 this means that G1 increases and G2 decreases with increasing temperature or, if G1 decreases, that its decrease is smaller than that of G2. (The latter case is shown in Figure 5b.) The qualitative trends shown in Figure 5 are consistent with the facile NO chemisorption on KxOy when compared to CaO (reflected in the large increase in [O]cat,K at low temperatures), as well as with the facile transfer of oxygen from CaO(O) to the carbon surface (reflected in the large increase in [O]c,Ca at low temperatures). Conclusions

FexOy+1 or FeO) to the carbon is crucial for maintaining high steady-state catalytic activity:

S(O) + Cf f C(O) + S

(18)

It is facilitated by the large interfacial area of catalyst/ carbon contact, which in turn is related (sometimes in a complex fashion) to catalyst dispersion on the carbon surface. Upon restoration of the reduced catalytic site, the remaining kinetically significant reactions are the same ones that characterize well the oxygen-carbon reaction:

2C(O) f CO2 + Cf

(7)

C(O) f CO (+Cf)

(8)

The mechanism proposed above is analogous to one of the mechanisms postulated for the selective catalytic reduction by hydrocarbons,13 in which NO first decomposes to N2 and then the hydrocarbon cleans up the adsorbed oxygen from the catalyst surface:

Important details remain to be worked out regarding the use of carbons as commercially viable reducing agents and catalyst supports in NOx reduction. In particular, the relationship between specific catalytic activity, catalyst dispersion, and concentration of catalytically active sites needs to be clarified. But many of the mechanistic aspects appear to be sufficiently clear to serve as an efficient guide in the formulation of novel and effective catalyst systems. Under typical reaction conditions in both catalyzed and uncatalyzed NO reduction on carbon, only the following reactions involving NO seem to be kinetically significant:

It is also analogous to that of alkali- and alkaline-earthcatalyzed gasification of carbon which was recently summarized by Chen and Yang,119 e.g.,

NO + 2S f S(N) + S(O)

(16a)

C(O) T CO

2S(N) f N2 + 2S

(16b)

In the above reactions S denotes a surface site. In the uncatalyzed reaction, this is a carbon reactive site. In catalyzed NO reduction, this is a reduced catalytic site (e.g., KxOy; CaO; FexOy or Fe). The fate of the oxygen surface complexes, S(O), is more important than the fate of the nitrogen surface complexes. If the catalyst is effective in reducing the activation energy of adsorption of NO (e.g., most notably in the case of potassium), the chemisorption process can occur at low temperatures and then it is not accompanied by the formation of surface oxides. Both N2 and N2O can be produced at these low temperatures, the latter probably formed by a reaction such as

S(N) + NO f S + N2O

(17)

and because of the longer half-life of S(N) species at these low temperatures (see Figures 3c,d,g,h). Severe catalyst deactivation is observed, however, under these conditions which are best characterized by oxygen accumulation on the surface. The transfer of oxygen from the oxidized catalytic site (e.g., KxOy+1; CaO(O);

2NO f N2 + 2O(ads) HC (hydrocarbon) + O(ads) f CO2 + H2O

CO2 + S T CO + S(O) S(O) + Cf T C(O) + S

The practical challenges that remain in NO reduction by carbon are now clear. They are related to (i) minimizing carbon consumption (i.e., maximizing the rate of reaction 7 and minimizing that of reaction 8), (ii) maximizing the extent of NO chemisorption (reaction 16) by selecting the catalyst with the highest affinity toward NO (i.e., with an intermediate heat of adsorption, to avoid catalyst deactivation) and maximizing its dispersion, and (iii) maximizing the rate of oxygen transfer (reaction 18) by maximizing the catalyst/carbon contact area. Acknowledgment. This study was made possible by the financial support from DGICYT (project AMB921032-CO2-O2) and OCICARBON (C-23-435), Spain. The thesis grant for M.J.I.G. and an invited research grant to L.R.R. from Generalitat Valenciana, as well as a postdoctoral research grant to M.J.I.G. from the Ministry of Education and Science (Madrid, Spain), are also gratefully acknowledged. EF950066T (119) Chen, S. G.; Yang, R. T. J. Catal. 1992, 138, 12.

NO Reduction by Activated Carbons. 7. Some Mechanistic Aspects of

Recommend Documents